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Forced Draft and Induced Draft Fan Maintenance Guide
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Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.
Technical Report
Forced Draft and Induced Draft Fan Maintenance Guide 1009651
Final Report, December 2004
EPRI Project Manager M. Pugh
EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 •
[email protected] • www.epri.com
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY ELECTRICITY INNOVATION INSTITUTE NEITHER ELECTRICITY INNOVATION INSTITUTE, ANY MEMBER OF ELECTRICITY INNOVATION INSTITUTE, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF ELECTRICITY INNOVATION INSTITUTE OR ANY ELECTRICITY INNOVATION INSTITUTE REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI
ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax). Electricity Innovation Institute and E2I are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2004 Electricity Innovation Institute. All rights reserved.
CITATIONS This report was prepared by EPRI Fossil Maintenance Applications Center (FMAC) 1300 W.T. Harris Blvd. Charlotte, NC 28262 This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Forced Draft and Induced Draft Fan Maintenance Guide, EPRI, Palo Alto, CA: 2004. 1009651.
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REPORT SUMMARY
A continuous flow of air and combustible gases in fossil power plants is required to supply the correct amount of combustion air and to remove the gaseous combustion products. This flow, which passes through ducts, the boiler, heat exchangers, and flues and stacks, is created and sustained by stacks and/or fans. In fossil stations, supply air fans are often referred to as forced draft (FD) fans and are used to push air through the combustion air supply system into the furnace. Some stations also use fans to move the gaseous combustion products through heat exchanger surfaces to the stack. These are often referred to as induced draft (ID) fans. Background Reliability of this equipment is important to plant efficiency and availability, and maintenance of these components becomes an important task for plant personnel. This issue ranked Number 1 in the 2003 FMAC Maintenance Issues Survey and, because of this, FMAC plans to begin a project to produce a guide that will address many of the common problems that members are facing with this equipment. Maintenance issues that are most often cited include bearing and alignment problems, lubrication, vibration problems (resulting from improper balancing or buildup of deposits), and erosion and wear of blades from entrained particles. Also reported are problems with dampers, particularly on flow control dampers. Objectives • To provide information on axial and centrifugal fans used for boiler draft service •
To assist fossil power plant maintenance personnel in troubleshooting and maintaining fans
•
To provide routine and preventive maintenance guidance to assist in improving the reliability of fans
Approach In cooperation with interested FMAC members, a task group of utility engineers, equipment suppliers, and industry experts was formed. This group identified key design and maintenance issues facing plant personnel and provided input that was used in the preparation of the guidance set forth in this report. Experience-proven practices and techniques were identified during this effort and are summarized here for use by all power plant personnel.
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Results This guide provides the user with an understanding of FD and ID fans, including elemental component descriptions, common materials of construction, and typical applications. The scope of the guide includes common applications and criteria for selection, failure modes and troubleshooting guidance, condition monitoring and predictive maintenance techniques, preventive maintenance strategies, recommendations on fan repair and inspection techniques, and good installation practices, including “how to” information on important steps. EPRI Perspective The information contained in this guide represents a significant collection of technical information, including techniques and good practices, related to the maintenance, monitoring, and troubleshooting of this important piece of plant equipment. Industry knowledge from recent experiences and improvements has been included in this report. Assembly of this information provides a single point of reference for power plant personnel, both now and in the future. Through the use of this guide, EPRI members should be able to significantly improve and optimize their existing plant predictive, preventive, and corrective maintenance programs related to this equipment. This will help members achieve increased reliability and availability at a decreased cost. Keywords Plant maintenance Plant operations FD and ID fans Fossil
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ACKNOWLEDGMENTS FMAC would like to acknowledge the following individuals for their contributions during the development of this report. Mark Litzenger
AmerenUE
Alan Parkinson
AmerenUE
Arnold Van Geuns
ESKOM
Scott Hall
Salt River Project
Edward Weeks
Salt River Project
Michael Stewart
AmerenUE
Herman Kleynhans
ESKOM
Ken Leung
Hong Kong Electric
Imraan Dindar
ESKOM
Travis Houn
Great River Energy
Christopher Rauch
AmerenUE
Ray Henry
Sargent and Lundy
Petrus Kruger
ESKOM
Robert Vihnicka
Sargent and Lundy
Bala Gogineni
Sargent and Lundy
Alan Grunsky
EPRI
FMAC also acknowledges the following organizations for permitting the generous use of figures and various materials from their literature and in-house resources and for reviewing and providing valuable comments on this document. Howden Buffalo – John Magill and Robin Flemming Flatwoods – Jim Greenzweigs
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CONTENTS
1 INTRODUCTION ....................................................................................................................1-1 1.1 Purpose ...........................................................................................................................1-1 1.2 Organization ....................................................................................................................1-1 1.3 Key Points .......................................................................................................................1-2 2 GLOSSARY OF TERMS ........................................................................................................2-1 2.1 Terms and Acronyms ......................................................................................................2-1 2.2 Conversions for Units Used in This Report .....................................................................2-3 3 TECHNICAL DESCRIPTION..................................................................................................3-1 3.1 Introduction .....................................................................................................................3-1 3.2 Fan Applications..............................................................................................................3-3 3.2.1 Induced Draft Fans ..................................................................................................3-3 3.2.2 Forced Draft Fans....................................................................................................3-3 3.2.3 Balanced Draft.........................................................................................................3-4 3.2.4 Cold Primary Air Fans .............................................................................................3-5 3.2.5 Hot Primary Air Fans ...............................................................................................3-5 3.2.6 Gas Recirculation Fans ...........................................................................................3-6 3.2.7 Number of Fans.......................................................................................................3-6 3.3 Fan Types .......................................................................................................................3-7 3.3.1 Centrifugal Fans ......................................................................................................3-7 3.3.2 Axial Fans..............................................................................................................3-13 3.4 Fan Drives .....................................................................................................................3-15 3.5 Fan Controls..................................................................................................................3-16 3.5.1 Centrifugal Fan Controls........................................................................................3-16 3.5.1.1 Inlet Vanes .....................................................................................................3-16 3.5.1.2 Inlet Dampers ................................................................................................3-17 3.5.1.3 Two-Speed Motors.........................................................................................3-18
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3.5.1.4 Fluid Drive......................................................................................................3-18 3.5.1.5 Variable-Speed Motors ..................................................................................3-19 3.5.1.6 Turbine-Driven Fans ......................................................................................3-19 3.5.2 Axial Fan Control ...................................................................................................3-20 3.5.2.1 Variable-Pitch Blades.....................................................................................3-20 3.5.2.2 Variable-Speed Drives ...................................................................................3-21 3.5.2.3 Variable Inlet Vanes.......................................................................................3-22 3.5.2.4 Cooling Fans..................................................................................................3-22 3.6 Other Components ........................................................................................................3-22 3.6.1 Bearings ................................................................................................................3-22 3.6.2 Lubrication Systems ..............................................................................................3-23 3.6.3 Turning Gear .........................................................................................................3-24 3.7 Fan Performance...........................................................................................................3-24 3.7.1 Axial Fan Performance ..........................................................................................3-28 3.7.2 Fan Pressure Definition .........................................................................................3-29 3.8 Selection of Fan Type ...................................................................................................3-32 3.9 Fan Requirements.........................................................................................................3-33 3.10 Fan Testing .................................................................................................................3-33 3.11 Operation ....................................................................................................................3-34 3.11.1 Prestart Checks ...................................................................................................3-34 3.11.2 Startup Procedures..............................................................................................3-34 3.11.2.2 Sequence of Steps for Startup.....................................................................3-35 3.11.3 Alarm Conditions to Monitor ................................................................................3-35 3.11.4 Operating Parameters to Monitor ........................................................................3-35 3.11.5 Emergency Actions..............................................................................................3-36 3.11.6 Fan Control..........................................................................................................3-36 3.11.6.1 Electric Motor Restrictions ...........................................................................3-37 3.11.7 Control of Vanes and Dampers ...........................................................................3-38 3.11.8 Fan Outlet Dampers ............................................................................................3-38 3.11.9 Stall Prevention for Axial Fans ............................................................................3-39 3.11.10 Draft Fan Shutdown...........................................................................................3-39 3.11.10.1 Controlled Shutdown..................................................................................3-40 3.11.10.2 Uncontrolled Shutdown..............................................................................3-40 3.11.11 Parallel Fan Operation.......................................................................................3-41
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3.12 National Standards......................................................................................................3-42 4 FAILURE MODES AND EFFECTS ANALYSIS .....................................................................4-1 4.1 Blades .............................................................................................................................4-6 4.2 Bearings ..........................................................................................................................4-7 4.3 Foundations ....................................................................................................................4-8 4.4 Inlet Vanes ......................................................................................................................4-9 4.5 Couplings ......................................................................................................................4-10 4.6 Hydraulic Actuating Mechanism ....................................................................................4-10 4.7 Electric Motors...............................................................................................................4-10 4.8 Hubs ..............................................................................................................................4-11 4.9 Housing .........................................................................................................................4-11 4.10 Turning Gears .............................................................................................................4-12 4.11 Shaft ............................................................................................................................4-12 4.12 Center Plate ................................................................................................................4-13 4.13 Inlet Dampers ..............................................................................................................4-13 4.14 Isolating Dampers .......................................................................................................4-13 4.15 Variable-Speed Drive ..................................................................................................4-14 4.16 Controls .......................................................................................................................4-14 4.17 Ductwork .....................................................................................................................4-14 5 TROUBLESHOOTING ...........................................................................................................5-1 6 CONDITION MONITORING ...................................................................................................6-1 6.1 Vibration Monitoring ........................................................................................................6-1 6.1.1 Parameters ..............................................................................................................6-1 6.1.1.1 Amplitude .........................................................................................................6-2 6.1.1.2 Frequency ........................................................................................................6-2 6.1.1.3 Phase Angle ....................................................................................................6-2 6.1.1.4 Vibration Form .................................................................................................6-2 6.1.1.5 Vibration Mode Shape .....................................................................................6-3 6.1.2 Vibration Analysis ....................................................................................................6-3 6.1.2.1 Amplitude Versus Frequency Analysis ............................................................6-3 6.1.2.2 Real-Time Spectrum Analysis..........................................................................6-4 6.1.2.3 Time Waveform Analysis .................................................................................6-4
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6.1.3 Proximity Probes .....................................................................................................6-5 6.1.4 Velocity Probes........................................................................................................6-6 6.1.5 Accelerometer Probes .............................................................................................6-7 6.1.6 Data Acquisition.......................................................................................................6-8 6.1.6.1 Machine Diagram.............................................................................................6-8 6.1.6.2 Tri-Axial Readings ...........................................................................................6-9 6.2 Oil Analysis ...................................................................................................................6-10 6.3 Nondestructive Examination..........................................................................................6-11 6.4 Infrared Thermography..................................................................................................6-11 6.5 Motor Current Analysis..................................................................................................6-11 7 MAINTENANCE .....................................................................................................................7-1 7.1 Developing a Preventive Maintenance Program .............................................................7-2 7.2 Basic Rules for Conducting Maintenance .......................................................................7-3 7.3 Periodic Maintenance Recommendations.......................................................................7-3 7.4 Component Maintenance ................................................................................................7-5 7.4.1 Bearings ..................................................................................................................7-6 7.4.1.2 Routine Maintenance Recommendations ........................................................7-7 7.4.1.2 Bearing Overhaul .............................................................................................7-8 7.4.2 Lubrication System ................................................................................................7-10 7.4.2.1 Routine Maintenance .....................................................................................7-10 7.4.2.2 Circulating Lube Oil System Overhaul ...........................................................7-10 7.4.3 Couplings...............................................................................................................7-12 7.4.3.1 Routine Maintenance Recommendations ......................................................7-12 7.4.3.2 Coupling Overhaul .........................................................................................7-13 7.4.3.3 Coupling Alignment........................................................................................7-14 7.4.4 Variable Inlet Vanes and Control Dampers ...........................................................7-15 7.4.4.1 Routine Maintenance .....................................................................................7-15 7.4.4.2 Inlet Vane Overhaul .......................................................................................7-15 7.4.5 Centrifugal Fan Wheels .........................................................................................7-16 7.4.5.1 Centrifugal Fan Wheel NDE...........................................................................7-18 7.4.5.2 Blades ............................................................................................................7-18 7.4.5.3 Center Plate/Side Plate .................................................................................7-21 7.4.6 Shaft ......................................................................................................................7-21 7.4.7 Hubs ......................................................................................................................7-22
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7.4.8 Structural Support System.....................................................................................7-22 7.4.8.1 Concrete Foundation .....................................................................................7-22 7.4.8.2 Repairing Concrete Foundations ...................................................................7-22 7.4.8.3 Surface Cleaning ...........................................................................................7-23 7.4.8.4 Crack Repair ..................................................................................................7-23 7.4.8.5 Anchor Bolts...................................................................................................7-24 7.4.8.5.1 Forces Affecting Anchor Bolts................................................................7-24 7.4.8.5.2 Proper Installation ..................................................................................7-24 7.4.9 Housing .................................................................................................................7-25 7.4.9.1 Housing..........................................................................................................7-25 7.4.9.2 Inlet Cones.....................................................................................................7-26 7.4.9.3 Fan Wheel Clearance ....................................................................................7-26 7.4.9.4 Access Plates/Doors......................................................................................7-28 7.4.10 Expansion Joints .................................................................................................7-28 7.4.11 Electric Motors.....................................................................................................7-29 7.4.11.1 Dirt ...............................................................................................................7-29 7.4.11.2 Moisture .......................................................................................................7-30 7.4.11.3 Friction .........................................................................................................7-30 7.4.11.4 Vibration.......................................................................................................7-30 7.4.11.5 Rotor Shaft End Play ...................................................................................7-31 7.4.12 Fluid Drives..........................................................................................................7-32 7.4.12.1 Routine Maintenance ...................................................................................7-32 7.4.12.2 Overhaul Fluid Drive ....................................................................................7-33 7.4.13 Turning Gear .......................................................................................................7-34 7.4.14 Hydraulic Supply System.....................................................................................7-35 7.4.15 Axial Fan Blade Adjustment System ...................................................................7-35 7.4.16 Axial Fan Blade Bearings ....................................................................................7-35 7.4.17 Axial Flow Fan Rotor Overhaul............................................................................7-36 7.5 Fan Wheel Balancing ....................................................................................................7-36 7.5.1 Size of the Balance Weight ...................................................................................7-38 7.5.2 Location of the Balance Weight.............................................................................7-38 7.5.3 Vibration Sensitivity ...............................................................................................7-38 8 SPECIAL MAINTENANCE TASKS........................................................................................8-1 8.1 Extended Shutdown ........................................................................................................8-1
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9 FAN UPGRADE OPTIONS ....................................................................................................9-1 9.1 Reasons for Fan Upgrade ...............................................................................................9-1 9.2 Tipping ............................................................................................................................9-1 9.3 Wheel Replacement ........................................................................................................9-2 9.4 Housing Modifications .....................................................................................................9-3 9.5 Coatings ..........................................................................................................................9-3 10 SAFETY..............................................................................................................................10-1 10.1 Rotating Equipment.....................................................................................................10-1 10.2 Confined Space...........................................................................................................10-1 10.3 Burn Hazards ..............................................................................................................10-1 10.4 Electrical......................................................................................................................10-1 10.5 Operation Testing........................................................................................................10-2 10.6 Cleaning Operations....................................................................................................10-2 10.7 Fan Movement ............................................................................................................10-2 11 TRAINING...........................................................................................................................11-1 12 REFERENCES ...................................................................................................................12-1 A CENTRIFUGAL FAN WHEEL INSPECTION AND REPAIR ................................................ A-1 B KEY POINT SUMMARY ....................................................................................................... B-2 C TRANSLATED TABLE OF CONTENTS .............................................................................. C-1 Français (French) ................................................................................................................. C-2 日本語 (Japanese) ............................................................................................................. C-12 Español (Spanish) ............................................................................................................. C-24
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LIST OF FIGURES Figure 3-1 Boiler Air Flow Schematic.........................................................................................3-2 Figure 3-2 Section of a Centrifugal Fan .....................................................................................3-8 Figure 3-3 Cross-Section View of a Centrifugal Fan..................................................................3-9 Figure 3-4 Centrifugal Fan Components and Accessories ......................................................3-10 Figure 3-5 Typical Rotor with Forward Curved Blades for a Centrifugal Fan...........................3-11 Figure 3-6 Wheel Blade Types and Rotation (Viewed from the Drive End) .............................3-12 Figure 3-7 Centrifugal Fan Rotor Components ........................................................................3-13 Figure 3-8 Two-Stage Axial Fan Assembly..............................................................................3-14 Figure 3-9 Two-Stage Axial Fan Impeller ................................................................................3-14 Figure 3-10 Inlet Vane Control Assembly ................................................................................3-17 Figure 3-11 Variable-Pitch Axial Fan Components ..................................................................3-20 Figure 3-12 Sleeve Bearing Components................................................................................3-23 Figure 3-13 Typical Centrifugal Fan with Variable Inlet Vanes ................................................3-26 Figure 3-14 Typical Centrifugal Fan with Variable Inlet Vanes and Showing System Curve................................................................................................................................3-27 Figure 3-15 Typical Centrifugal Fan with Speed Control .........................................................3-28 Figure 3-16 Performance Field for Variable-Pitch Axial Flow Fan ...........................................3-29 Figure 3-17 Fan Pressure Definitions ......................................................................................3-30 Figure 3-18 Fan Correction for Inlet Density ............................................................................3-31 Figure 4-1 Centrifugal Fan Housing Components ...................................................................4-12 Figure 7-1 Centrifugal Fan Wheel............................................................................................7-17 Figure 7-2 Wear and Erosion Protective Accessories .............................................................7-20 Figure 7-3 Cross-Section View of a Fan Illustrating Clearance Requirements Between the Wheel Inlet and the Inlet Bell .....................................................................................7-26 Figure 7-4 Enlarged View of Figure 7-3 ...................................................................................7-27 Figure 7-5 Basic Block Diagram of a Hydraulic Regulating System ........................................7-32 Figure 9-1 Airfoil Blade with the Blade Tipped...........................................................................9-2 Figure A-1 Blade Map Example ................................................................................................ A-6 Figure A-2 Blade Map Example ................................................................................................ A-7 Figure A-3 Blade Map Example ................................................................................................ A-8 Figure A-4 Example of Undercut and Overlap ........................................................................ A-11 Figure A-5 Hand-Held Yoke Probe with Articulated Legs ....................................................... A-15
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Figure A-6 Orientation of an Articulated Yoke Probe to Produce Flux Lines at Right Angles ............................................................................................................................. A-16 Figure A-7 Indications Are Consecutively Numbered and Circled .......................................... A-19 Figure A-8 Marking Multiple Indications.................................................................................. A-19
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LIST OF TABLES Table 4-1 Summary of Centrifugal Fan Problem Areas .............................................................4-2 Table 4-2 Summary of Axial Fan Problem Areas.......................................................................4-3 Table 4-3 FD Fan Failure Data for U.S. Fossil Plants from 1982 Through 1995 (NERC/GADS Data) ...........................................................................................................4-4 Table 5-1 Fan Troubleshooting..................................................................................................5-1 Table 5-2 Bearing Troubleshooting............................................................................................5-3 Table 5-3 Lubrication System Troubleshooting .........................................................................5-4 Table 5-4 Hydraulic System Troubleshooting ............................................................................5-5 Table 5-5 Troubleshooting Noise Level .....................................................................................5-6 Table 5-6 Troubleshooting Fluid Drive.......................................................................................5-7 Table 5-7 Fan Performance Troubleshooting ............................................................................5-9 Table 7-1 Surveillance and Preventive Maintenance Frequencies ............................................7-3 Table A-1 Summary of Inspection Practices for Centrifugal Fan Wheels ................................. A-3 Table A-2 Percent of Fans Undergoing NDE .......................................................................... A-13 Table A-3 Typical Materials in Power Plant Fans ................................................................... A-25
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1 INTRODUCTION
Fired steam generators (that is, boilers) require large draft fans to move air into the furnace and to remove the combustion products. Draft fans are primary auxiliaries that support boiler operation for all types of fuel and firing methods. Draft fans are typically used in four principal applications: forced draft (FD), primary air (PA), induced draft (ID), and gas recirculation (GR). Other applications include booster fans and mill exhausters. This guide addresses FD and ID fans, which are the largest and most common applications. The maintenance recommendations for these fans are easily adapted for most other draft fan applications.
1.1 Purpose This FD and ID fan maintenance guide provides information on axial and centrifugal fans used for boiler draft service and is intended to assist fossil power plant maintenance personnel in troubleshooting and maintaining fans. A discussion of fan characteristics and components that serves as a reference for understanding the basics of fan performance and mechanical construction is provided. Routine and preventive maintenance guidance is provided to aid in improving the reliability of fans. A troubleshooting guide assists in diagnosing problems that have been encountered in various fan failure reports. Data for this guide were obtained from direct experience in fossil plants, industry surveys on failure reports, vendor input, and reviews of plant literature and industry documentation.
1.2 Organization The organization of this guide is as follows: Section 1 is an introduction and discussion of the guide’s purpose and organization. Section 2 is a glossary of items, definitions, and acronyms used in this guide. Section 3 provides technical description, basic discussion of fan performance characteristics, and the mechanical components. Section 4 provides an analysis of failure modes and effects as well as failure data. Section 5 provides guidance for troubleshooting and corrective actions. Section 6 is a discussion of condition monitoring. 1-1
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Section 7 provides guidance for preventive maintenance. Section 8 includes recommendations for special maintenance tasks. Section 9 presents upgrade options. Section 10 provides recommendations for safety. Section 11 provides training recommendations. Section 11 is a list of references. Appendix A discusses centrifugal fan wheel inspection and repair. Appendix B provides a list of all the key points indicated in the guide.
1.3 Key Points Throughout this guide, key information is summarized in “Key Points.” Key Points are boldlettered boxes that succinctly restate information covered in detail in the surrounding text, making the key points easier to locate. The primary intent of a Key Point is to emphasize information that will allow individuals to take action for the benefit of their plant. The information included in these Key Points was selected by Fossil Maintenance Applications Center (FMAC) personnel, the consultants, and utility personnel that prepared this guide. The Key Points are organized according to three categories: Operation and Maintenance (O&M) Costs, Technical, and Human Performance. Each category has an identifying icon, as shown below, to draw attention to it when quickly reviewing the guide. Appendix B contains a listing of all of the key points in each category. The listing restates each key point and provides reference to its location in the body of the report. By reviewing this listing, users of this guide can determine whether they have taken advantage of key information that the writers of the guide believe would benefit their plants. Key O&M Cost Point Emphasizes information that will result in reduced purchase, operating, or maintenance costs.
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Key Technical Point Targets information that will lead to improved equipment reliability. Key Human Performance Point Denotes information that requires personnel action or consideration in order to prevent injury or damage or ease completion of the task.
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2 GLOSSARY OF TERMS
2.1 Terms and Acronyms ABMA
American Boiler Manufacturers Association
acfm
actual cubic feet per minute
AMCA
Air Movement and Control Association, Inc.
ASM
American Society for Metals
ASTM
American Society for Testing and Materials
AWS
American Welding Society
BTS
Blade tip speed
cfm
cubic feet per minute
EPRI
Electric Power Research Institute
FD
Forced draft
FMAC
Fossil Maintenance Applications Center
fpm
feet per minute
FSP
Fan static pressure
FTP
Fan total pressure
FVP
Fan velocity pressure
GADS
Generating Availability Data System
GR
Gas recirculating
HAZ
Heat-affected zone 2-1
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hp
Horsepower
ID
Induced draft
ksi
kips per square inch
MT
Magnetic particle testing
MTBF
Mean time between failures
NDE
Nondestructive examination
NEMA
National Electrical Manufacturers Association
NERC
North American Electric Reliability Council
NFPA
National Fire Protection Association
O&M
Operation and maintenance
PA
Primary air
PM
Preventive maintenance
psi
pound(s) per square inch
psig
pound(s) per square inch, gauge
PWHT
Post-weld heat treatment
SCR
Selective catalytic reduction
SMAW
Shielded metal arc welding
SPR
Static pressure rise
SSS
Structural support system
UT
Ultrasonic test
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VT
Visual test
WG
Water gauge
WMP
Wet fluorescent magnetic particle (test)
WR2
Moment of inertia
2.2 Conversions for Units Used in This Report °C
=
(°F – 32) x 5/9
1 hp
=
746 W
1 lb
=
0.45 kg
1 lb/ft3 =
16 kg/m3
1 psi(g)=
6.9 kPa
1 in.
25.4 mm
=
1 fpm =
0.30 mpm
1 ft
=
0.3 m
1 in2
=
6.45 cm2
1 ft-lb =
1.35 joules
1 ksi
6.9 MPa
=
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3 TECHNICAL DESCRIPTION
3.1 Introduction A fan is a device that produces a flow of gas by the movement of a surface. As used in this guide, a fan is defined as a turbo machine with a rotating impeller enclosed in a casing. Fans are similar to compressors; the difference is that fans create a flow of gas whereas compressors increase the pressure of the gas. To increase a flow, fans must increase the pressure of the gas and compressors must create a flow. In the past, there were specific criteria defining the difference between fans and compressors. For example, the 1946 edition of ASME PTC-11, “Performance Test Code for Fans,” defines a fan as providing a compression ratio of 1.1 or a density change of 7%. ISO 5801 defines the upper limit of fans as a pressure increase of 120 inches Wg (30 kPa). ASME PTC-10, “Performance Test Code on Compressors and Exhausters,” states that compressors are usually intended to produce considerable density change. The choice of whether a device is a fan or a compressor is not regulated or standardized. Draft fans provide one or a combination of the following functions in a boiler: •
Supply air required for combustion
•
Remove products of combustion
•
Deliver fuel to the burners
•
Circulate the gases for better heat transfer
In addition, the fan selected must be adequate for the required duty with regard to air volume, static pressure, horsepower, and noise. Discussion of these topics follows. Figure 3-1 is a typical schematic of air and gas flow through a boiler.
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Figure 3-1 Boiler Air Flow Schematic
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3.2 Fan Applications 3.2.1 Induced Draft Fans ID fans move the combustion flue gas through the boiler, air heater, and precipitator or the baghouse, scrubber, and chimney to the atmosphere. They are a major component of a fossilfired plant and typically consume approximately 2% of the gross electrical output. ID fans have the largest design margins of any major equipment in a fossil-fueled power plant. The margins are typically 15% on flow, 30% on head, and 25°F on temperature. The large margins are intended to allow for the following: •
Uncertainty in determining system requirements
•
Allowance for wear
•
Operating flexibility
•
Allowance for pluggage and leakage
•
Air infiltration
Even with these large margins, it is not uncommon for the ID fans to be the limiting factor on the output of a coal-fired unit. ID fans are included in the top 25 causes of fossil plant outages and are responsible for approximately 2% of the total outages of fossil-fired units. The temperature of gas to be handled by the ID fan is based on the calculated unit performance at maximum boiler load. Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation. Key Technical Point Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation.
3.2.2 Forced Draft Fans FD fans provide combustion air for boilers. The FD fan inlet is open to the atmosphere and discharges through air preheating coils, an air heater into the boiler windbox, and finally through the burners into the furnace. In pulverized coal-fired boilers, approximately one-third of the combustion air is PA that is used to transport the pulverized coal to the burners. Some boiler designs use PA fans, which may take suction from the atmosphere and operate in parallel with the FD fans or may take suction from the FD fan discharge and operate in series with the FD fans. The PA fan application is similar to the FD fan; therefore, the description, problem area, and maintenance requirements described for 3-3
EPRI Licensed Material Technical Description
FD fans are essentially the same for PA fans. Other boiler designs use mill exhausters that take the air and pulverized coal mixture from the mill outlet and transport the mixture to the burners. Because of the erosive nature of the pulverizer coal-air mixture, mill exhausters have a very different application than the FD, ID, or PA fans and are not addressed in this guide. For pressurized units without ID fans, the FD fan is sized for the entire system to the stack or to the pollution control system. FD fans for coal-fired plants rank close behind ID fans as the cause of outages. The causes of FD fan failures are similar to those for ID fans. The FD fans for a coal-fired plant consume approximately 0.7% of the gross electrical output. The design margins on FD fans are typically smaller than the margins on ID fans but still larger than on other major equipment. Margins of 15% on flow and 30% on head at the maximum expected ambient temperature are common. FD fans are normally equipped with sound trunks (inlet boxes) for noise attenuation. When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration. An alternative method of noise attenuation is using a fan room. This involves the use of open inlet FD fans located in a specially designed room with acoustical baffles for air entry. Key Technical Point When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration.
3.2.3 Balanced Draft The balanced draft system uses both an FD fan system and an ID fan system to move air through the boiler. FD fans on a balanced draft boiler must have the necessary volume output of air required for combustion, plus air heater losses and discharge pressure high enough to equal the total resistance of air ducts, air heater, burners, and any other resistance between fan discharge and the furnace. ID fans in a balanced draft boiler move the gaseous products of combustion over convection heating surfaces, pollution control system(s), plus the gas passages between the furnace and stack.
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The weight of gas to be handled by the ID fan is the sum of the following: •
Theoretical air for combustion
•
Excess air required at burner
•
Infiltration
•
Leakage air-to-gas through the air heater
The draft to be provided by the fan is determined by losses through the following boiler components: •
Furnace
•
Boiler and superheater
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Economizer
•
Selective catalytic reduction (SCR)
•
Air heater
•
Precipitator or baghouse
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Ductwork
•
Flue gas desulfurization system (scrubber)
•
Stack
For fan design, safety margins are added to the net weight requirement, net draft requirement, and gas temperature. 3.2.4 Cold Primary Air Fans Cold primary air fans take ambient air and discharge it through the air heater, where the air is heated up to 650ºF (the actual temperature depends on the moisture content of the coal), and then into the pulverizers—where it is used to dry, heat, and convey the pulverized coal to the burners. This system is used on large boilers where fans are installed in parallel in order to service a bank of pulverizers. 3.2.5 Hot Primary Air Fans Hot primary air fans take heated air from the air heater and blow it into the pulverizers where it is used to dry, heat, and convey the pulverized coal to the burners.
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3.2.6 Gas Recirculation Fans A gas recirculation system performs either or both of the following functions: •
Controls steam temperature over a wide boiler load range. To accomplish steam temperature control, a portion of the flue gas from the economizer outlet is introduced in the lower part of the furnace by means of a suitable fan and ducts. This is known as gas recirculation.
•
Controls furnace gas temperature when a portion of the flue gas from the economizer outlet is recirculated to the furnace outlet. This is called gas tempering and may be used to control NOX.
The volume requirements of the gas recirculating fan are determined by the amount of recirculation necessary to obtain the required steam temperature. Maximum flow can occur at either full boiler load or some reduced boiler load point, depending on boiler design. The gas recirculation fan must be sized so its pressure capability will always exceed the pressure differential developed by the boiler; otherwise, backflow of high-temperature furnace gas will result through the fan, with serious consequences. Radial tip blade fans (see Figure 3-6) can be applied for gas recirculation duty, but the straight blade fan may be needed where high concentrations of fly ash will be encountered, depending on the ash properties. Inlet dampers are the principal means of accomplishing volume control. 3.2.7 Number of Fans One of the major design decisions for a fan system is the number of fans. The factors to be considered in selecting the number of fans are initial cost, operation and maintenance (O&M) costs, flexibility of operation, and reliability. When evaluating initial cost, the cost of motors, ductwork, insulation, control equipment, electrical equipment, and foundations must be considered in addition to the cost of the fans. The fewest number of fans usually results in the lowest initial cost. Key O&M Cost Point The fewest number of fans usually results in the lowest initial cost. Operating cost usually decreases as the number of fans increases. Fans usually have their highest efficiency near their design points. At lower loads, some of the fans can be shut off in a system with more fans. The remaining fans will then operate closer to their design points and, therefore, more efficiently. An important parameter for evaluating operating cost is the projected loading schedule for the generating unit. The variation in operating cost with the number of fans will be less for a unit that operates at or near full load most of the time than for a unit that operates at lower loads. Maintenance cost increases as the number of fans increases.
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It is difficult to assign cost values to the differences in reliability and flexibility of operation with different numbers of fans. It is also difficult to assign a cost value to plant arrangement. Plant arrangements can be improved by reducing the number of fans. For these reasons, the selection of the number of fans is not a straightforward economic evaluation. In addition to the factors discussed above, practical aspects should also be considered. For centrifugal fans, because of fan size limitations, the maximum unit size for which two ID fans can be used is approximately 700 MW. The same number of FD and ID fans is usually selected to simplify operation of the fans. The trend in the power industry has been to use two FD and two ID fans up to approximately 700 MW. Above this size, it is not practical to build ID fans that are large enough to use two fans. In recent years, this concept has been challenged, and units with one FD and one ID fan have been built up to a limit of 500 MW. Obviously, the cost for one fan and motor is less than for two, but the bulk of the savings comes from the reduced number of ducts as well as capital costs. The number of ID fans can have an effect on unit availability. Many owners believe that two fans will provide better availability than one. However, with two fans, the probability of a fan failure is roughly twice that for one fan, but the impact of a failure is approximately one-half. Thus, the equivalent availability is about the same. There is not a large enough database of boilers with single FD and ID fans to verify this probability, but statistical studies (using the NERC-GADS database) of boiler feed pumps verify this theory. A boiler with one full-size boiler feed pump has more full forced outages than units with two 50% capacity pumps but fewer forced deratings. The overall equivalent availability of one full-size feed pump is slightly higher than two half-size pumps. On large units with four ID fans, the unit will probably be capable of operating at full load with three fans under normal operating conditions. Thus, the unit will essentially have an installed spare, which should result in improved availability.
3.3 Fan Types Fans are a type of turbo machinery that transfers energy to air in order to increase pressure to induce flow. They are usually classified by the direction of flow through the impeller. The two types used in boiler draft applications are centrifugal fans, where the flow is radially outward, and axial fans, where the flow is axial along the fan shaft. Centrifugal fans are sometimes referred to as radial fans. 3.3.1 Centrifugal Fans The centrifugal fan produces movement by throwing air off the blades in a radial direction by means of centrifugal force. The air or gas enters the inlet boxes and travels through the inlet vanes into the spinning fan wheel and then discharges through the scroll outlet, which is perpendicular to the fan shaft. Centrifugal fans are available with and without inlet boxes and with single or double inlets. The flow through the fan is controlled with inlet dampers or variable-pitch inlet vanes. Figure 3-2 is a photograph of a cut-out section of a centrifugal fan. 3-7
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Figure 3-2 Section of a Centrifugal Fan (Courtesy of Howden Buffalo, Inc.)
Figures 3-3 and 3-4 illustrate the cross-sectional view of various parts of a typical centrifugal fan.
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Figure 3-3 Cross-Section View of a Centrifugal Fan
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Figure 3-4 Centrifugal Fan Components and Accessories (Courtesy of Howden Buffalo, Inc.)
A typical rotor for a centrifugal fan is shown in Figure 3-5.
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Figure 3-5 Typical Rotor with Forward Curved Blades for a Centrifugal Fan (Courtesy of Howden Buffalo, Inc.)
Centrifugal fans are available with the following various blade profiles, as shown in Figure 3-6: •
Airfoil backward inclined
•
Flat bladed backward inclined
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Radial tip
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Straight radial
•
Forward curved
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Figure 3-6 Wheel Blade Types and Rotation (Viewed from the Drive End)
In most power plant applications where the fans will handle clean air or clean gas, the highly efficient backward inclined airfoils are the preferred design. In an application where the fan is subject to erosion due to heavy dust loading, a straight radial type fan provides erosion resistance, but at the expense of efficiency. 3-12
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The airfoil centrifugal fan is the most frequently applied fan for forced draft duty because of its inherently high efficiency, low noise, stable performance, and steep rising pressure curve. These features ensure good operation, particularly when fans are operating in parallel. Forced draft service tends to be a good application for axial fans. The parts of a typical centrifugal fan rotor are shown in Figure 3-7.
Figure 3-7 Centrifugal Fan Rotor Components
3.3.2 Axial Fans Axial fans produce movement of air along their axis or in an axial direction. Axial fans are offered in single- and two-stage models. They are available with fixed-pitch and variable-pitch blading. The variable-pitch fan is more sophisticated and efficient than the fixed-pitch fan. Figures 3-8 and 3-9 illustrate sectional views through a two-stage axial fan. Air or gas enters through a single inlet box where it makes a 90-degree turn through straightening vanes, passes through the impellers, and exits through the diffuser section.
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Figure 3-8 Two-Stage Axial Fan Assembly
Figure 3-9 Two-Stage Axial Fan Impeller
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Where higher pressures are necessary, two-stage fans may be required. Two-stage fans are offered for large-capacity induced draft service. A brief description of each of the main components of the axial fan follows. Rotor Hub. The rotor hub is the major component of an axial fan. The hub serves as the retaining ring for the blades and their bearings and as the housing and protective cover for the blade-actuating mechanism. Blade Shaft Bearings. Blade shaft bearings are used to transfer the centrifugal force from the blades to the fan hub. Main Shaft and Main Shaft Bearings. In the axial fan, the main shaft does not carry large loads over long spans between bearings as in a centrifugal fan. The rotating masses are low, and the bearings are located close to the rotor. The shafts, therefore, are small in diameter and relatively inexpensive in comparison to the large-diameter, long, forged steel shafts required for centrifugal fans. Blades. Various materials are used for axial fan blades. For forced draft service, relatively inexpensive cast aluminum blades are generally used. For induced draft service, where high wear resistance is required, ductile iron, cast steel, cast steel with hardened surfaces, or forged aluminum blades with stainless steel inserts are available. Blade material selection is influenced by the blade velocity and the size and hardness of the dust particles. In induced draft service, if the fans are operated for long periods with precipitators performing at low efficiencies, excessive blade wear will occur no matter what blade material is used. This is true for either type of fan. In the axial fan, however, a worn set of blades can be replaced in a few shifts. In the centrifugal fan, days of welding and balancing may be required to repair the fan. Axial flow fans are more complicated than centrifugal fans but have the advantage, in most cases, of higher efficiency over a wider load range.
3.4 Fan Drives Electric motors are normally used for fan drives because they are less expensive and more efficient than any other type of drive. For fans of more than a few horsepower, squirrel-cage induction motors are more widely used. This type of motor is relatively inexpensive, reliable, and highly efficient over a wide load range. It is frequently used in large sizes with a hydraulic coupling for variable-speed installations. Two-speed ac electric motors can be used in conjunction with inlet vanes to obtain slightly higher efficiencies at lower loads. This arrangement has a higher initial cost and is less reliable than a single-speed motor and inlet vanes. Variable-speed motors are the optimum type of drive
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for centrifugal fans. They allow the fan to operate near its peak efficiency over the entire load range; controllability is good, and fan erosion is substantially reduced at lower speeds. Recent developments in variable-speed motors make them an excellent choice for centrifugal fans, although the high initial cost of variable-speed motors is not justified in all applications. Steam turbines have higher initial and maintenance costs and are less reliable but do provide a variable-speed drive. Steam turbines are not competitive with electric motors for fan drives.
3.5 Fan Controls 3.5.1 Centrifugal Fan Controls As the load on a boiler varies, the pressure and flow requirements from the fans in the system vary. The most widely used methods to control centrifugal fans are by means of inlet vanes and variable-speed drives. 3.5.1.1 Inlet Vanes Inlet vanes introduce a swirl to the flow entering a fan. This changes the angle of attack between the flow and the fan blade and effectively changes the fan characteristics. Inlet vane control has a low initial cost, is a simple method of control, and is very common for ID fans. Figure 3-10 is an illustration of an inlet vane control assembly. The major disadvantage of inlet vanes is poor efficiency at lower loads. Inlet vanes are subject to erosion if ash concentrations are high. The vane linkage and bearings can bind or become damaged if they are located in the gas stream. Therefore, these components should be located outside the fan inlet housing, where inspection and maintenance can be performed without entering the fan. The duct connections at the fan inlet or outlet should be flexible to isolate the fan from duct expansion and vibration. The duct should be separately anchored and not supported by the fan.
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Figure 3-10 Inlet Vane Control Assembly
Inlet vane control is more efficient than inlet damper control because inlet vanes use part of the pressure head loss to accelerate the incoming gas in the direction of wheel rotation. To optimize system performance, an operator should perform the following: •
Operate with all dampers fully open in order to limit pressure head losses. (Vane position should be set based on the load requirement.)
•
Watch for control or mechanical problems that will cause vane flutter.
•
Manually position the inlet vane control during initial startup to avoid pressure excursions.
•
Ensure that shut-off dampers are closed tightly to provide air/gas seal before startup; this limits the inertia acceleration load on the motor.
3.5.1.2 Inlet Dampers Inlet dampers control air flow by introducing a swirl in the flow and pressure drop. Inlet dampers have a low initial cost, are simple, and are not as prone to erosion as inlet vanes. The control linkage on inlet dampers is simpler than that for inlet vanes and can be located completely outside the duct. However, system pressure pulsations are more common with inlet dampers than inlet vanes. Inlet dampers can create vortices in the inlet boxes or around the fan shaft. The biggest disadvantage of inlet dampers is their low efficiency at low loads.
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3.5.1.3 Two-Speed Motors The fan selection with two-speed motors is the same as with inlet vane control or inlet damper control. The fans for two-speed motors are often sized so that the fan can operate on low speed at full load and at normal operating temperature. The high speed provides the design margin. Operators at some plants with two-speed ID fan motors do not change the motor speed while the unit is operating. This is usually based on their past experience, when the unit has tripped because of furnace pressure excursions that occur when speeds are changed. This problem can be overcome by a careful design of the control system and requires a review of the damper or vane control response, the allowable furnace pressure limits, and the time it takes for the motor to change speed. 3.5.1.4 Fluid Drive Fluid drive is a method of varying the fan speed for flow control. The fan selection is essentially the same as the inlet damper alternative, except that a fluid drive is located between the motor and the fan to control the fan speed. Inlet dampers are typically used in addition to the fluid drive to increase the speed of response to avoid furnace pressure excursions during transients. The use of the dampers for control during normal operation is typical but can be eliminated in most installations. Using speed control with the dampers full open can result in a significant power savings (200 hp on a 6000-hp fan) with only minor modifications. Speed control allows the fan to operate near peak efficiency over the entire load range. However, the fluid drive has a maximum efficiency of approximately 95%, and it decreases at lower speeds. The combined efficiency of the fan and fluid drive is slightly lower than inlet vane control at full load, but it is higher at lower loads. The major disadvantage of a fluid drive is the high initial cost, which is approximately the same as that of the fan. Fluid drive control provides better overall efficiency than inlet vane configuration. To optimize system performance for a fluid drive system, the operator should perform the following: 1. Start the motor under a no-load condition (that is, ensure that the fluid coupling is empty and the inlet vane or damper is closed). This will limit the inertia acceleration load on the motor by accelerating the inertia of the fan to its running speed at a controllable rate. 2. Verify that all shut-off dampers (that is, non-control dampers) are fully open before startup to prevent pressure head losses. 3. Do not use louver dampers or inlet vanes as flow control devices to throttle the fan output volume in a system that was backfitted with a fluid drive. This will cause energy losses. The control system should be tuned to allow a variable-speed drive to have control with the dampers in the fully open position for a given flow range. 4. Watch for control or mechanical problems that could cause the system to hunt from one point to another.
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3.5.1.5 Variable-Speed Motors Variable-speed motors are directly connected to the fan. The speed of the motor is continuously variable from approximately 10% up to the full speed. Synchronous or induction motors can be used with variable frequency drives, and the frequency of the power to the motor is controlled by an electronic system. The incoming ac power is converted to adjustable voltage dc power by a thyristor. The adjustable dc power is connected to an inverter, which converts it to an adjustable ac power output. Speed control is the optimum method of controlling centrifugal fans. The system resistance for ID service, and most FD fan service, is essentially a square curve. Because fan efficiency is essentially constant over similar flow-head squared curves, a variable-speed fan can operate near its best efficiency over the entire load range. Also, the control is stable down to essentially zero flow. Fans with variable-speed motors do not require a turning gear because the main motor can operate at the turning gear speed for extended periods. 3.5.1.6 Turbine-Driven Fans Turbine-driven ID fans have been studied by architect-engineers and turbine manufacturers, but very few have been installed. The advantage of turbine-driven fans is the improved plant heat rate. Based on previous studies, the capital cost of turbine-driven fans is considerably more than the cost for centrifugal fans with variable-speed motors. In most cases, turbine-driven ID fans do not result in a fuel cost savings over ID fans with variable-speed motors. The efficiency of a turbine drive is typically 81%, which is less than the main turbine efficiency of 88%. For a motor-driven fan, the efficiencies of the main generator (98%), transmission (98%), and motor (95%) result in an overall efficiency approximately the same as that of turbine-driven fans. The National Fire Protection Association (NFPA) standards for explosion prevention require that a master fuel trip shall not trip the fans. To meet this requirement, an auxiliary steam source, such as a cross connection to a second unit or an auxiliary boiler on continuous standby, would be required with turbine-driven fans. The costs for this auxiliary source would have to be added to the estimated costs above. Turbine drives are less reliable than motor drives. Because the availability of variable-speed motors—including the associated electrical equipment—is higher than that of turbine drives, turbine drives are not recommended for ID fans.
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3.5.2 Axial Fan Control Axial flow fans can be controlled by using variable-pitch blades, inlet vanes, or variable-speed drives. In a variable-pitch axial fan, blade adjustment levers are located within the hub and are actuated hydraulically. Variable-pitch axial fans respond quickly and smoothly to system demand changes. Variable-speed axial fans are not normally used because variable-pitch axial fans are lower in cost and achieve similar efficiency. 3.5.2.1 Variable-Pitch Blades Axial fans can be controlled by varying the blade pitch or speed or by using variable inlet vanes. Either varying the blade pitch or using variable inlet vanes controls the flow by operating on the same principle as do variable inlet vanes on a centrifugal fan. Varying the blade pitch is more efficient than using variable inlet vanes because the flow resistance of the vanes is absent. Variable-pitch blades can provide efficiency as high as that of variable-speed control over most of the load range for a lower initial cost. Variable-pitch blades are the most common method of control; variable inlet vanes are used occasionally, and variable-speed control is rare. Figure 3-11 shows the components of a variable-pitch axial fan.
Figure 3-11 Variable-Pitch Axial Fan Components
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The most common blade actuation system for variable-pitch axial fans is a hydraulic system with a rotating union between the rotating and stationary parts. A critical design area of variable-pitch axial fans is the blade thrust bearings. These bearings experience very little movement and loads in excess of 100,000 pounds. Also, the shaft rotation acts as a centrifuge that separates particles out of the bearing lubricant and separates the lubricant if it is a mixture of elements of different densities. Ball bearings are accepted as the best type of bearing for this application. The impeller blade adjustment system is made up of a stationary servomechanism, rotating seal, and rotating piston rod. Alignment between the servomechanism and rotating piston rod is maintained by antifriction-type bearings. This unit provides a transition between the non-rotating regulating levers and the rotating piston rod; and the servomechanism’s pilot valve transforms a mechanical input signal from the regulating lever to a hydraulic signal. This hydraulic signal, in turn, is received by the hydraulic cylinder in the rotor assembly. The impeller blade adjustment system provides interface between the lever assembly and the rotor assembly. The impeller blade adjustment system receives its input signals from the boiler control system through an electric actuator (located outside the axial fan housing) and the hydraulic supply system. A clevis arm provides a mechanical link between the lever and the servo. Lever motion is translated into the clevis arm moving axially. The clevis arm’s motion causes three oil ports inside the servo to close or open in a specific sequence in order to allow oil to flow to or from the hydraulic supply system. Oil movement determines which side of the hydraulic cylinder is under pressure, ultimately resulting in movement of the blades to an ordered position. In some applications, a combined lube oil/hydraulic supply system is used to provide oil that both lubricates main fan bearings and provides pressurized hydraulic oil to serve as the working medium to vary the blade pitch. Basic components of the hydraulic oil system include primary and secondary pumps, heat exchangers (either air- or water-cooled type), reservoir, instrumentation (such as pressure switches, temperature switches, gauges, and alarms), supportive piping, and isolation valves. 3.5.2.2 Variable-Speed Drives It is possible to have fixed pitch blades with an axial fan and control the fan using a variablespeed drive. Although the arrangement would be slightly more efficient at low flow rates than a variable-pitch blade axial flow fan, the cost would be higher. Variable-speed axial flow fans are therefore rarely used in boiler draft applications.
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3.5.2.3 Variable Inlet Vanes This alternative has fixed blades and variable inlet vanes. The inlet vanes change the angle of attachment between the flow and the blades, similar to the effect of inlet vanes on a centrifugal fan. The design is less complicated than the variable-pitch blade design but is also considerably less efficient. Fixed blade axial fans have most of the disadvantages of variable-pitch blade axial fans without the advantage of high efficiency. 3.5.2.4 Cooling Fans All the bearings (that is, main shaft bearings and blade bearings) in an axial flow fan are inside a fairing inside the fan. For an ID fan or any other hot air or gas service, an external cooling fan is used to provide cool, clean air to pressurize and cool the main shaft bearings and vibration probes.
3.6 Other Components 3.6.1 Bearings Two general types of bearings used in draft fan applications are rolling contact and sliding contact. Both types, depending on the application, can be designed to support axial or radial loads. Both centrifugal and axial fans can use either ball or roller bearings; however, ball and roller bearings are more common on axial fans. Sliding contact bearings are more common on centrifugal fans. Ball and roller bearings consist of four major components: •
Outer race
•
Inner race
•
Rolling elements
•
Spacer for the rolling elements
A sliding contact type bearing known as a journal or sleeve bearing is used extensively on centrifugal draft fans and some axial fans. This type of bearing is made up of four parts: journal, upper and lower sleeves, and (where required) thrust collars and oil rings. A journal bearing can be further categorized as a fixed or floating type, depending on whether the axial movement of the shaft is allowed. Fixed bearings can have either one or two thrust collars. Floating-type bearings are used to allow for thermal growth in center-hung fans and are installed at the opposite end of the fan motor. A typical sleeve bearing is shown in Figure 3-12.
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Figure 3-12 Sleeve Bearing Components
3.6.2 Lubrication Systems Various techniques are used to provide oil to fan and motor bearings. Static lubrication in which each bearing has a fixed supply of oil in their sumps is very common. This method is simple and very cost-effective; however, it relies on operator vigilance to detect low oil levels or poor oil quality. Use of temperature sensors to provide remote warning to the control room operators, in case of a hot bearing, offers added protection for this method. A gear pump attached to the input shaft of the driver is a second method used to provide lube oil to fan and motor bearings. A third method, the use of fluid drives to supply oil to the bearings, is also used on some fans. A fourth method involves the use of a dedicated circulating lube oil system. Two variations of this system involve the use of a forced lubricating oil system in which oil is supplied to the bearings under pressure. The second variation involves the use of a circulating oil system, which supplies oil to the bearing sump; oil rings then move the oil to the bearing surface. The latter variation is the one used most often.
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3.6.3 Turning Gear Turning gears are sometimes installed with ID or gas recirculating fans where exposure to hightemperature flue gas (while the fan is idle) could result in warping or thermal growth of fan internals. Once a fan is stopped and the inlet and outlet dampers are closed, a fixed volume of hot flue gases remains inside the fan housing. As these hot gases cool down, a natural temperature gradient forms. Consequently, the fan rotor cools unevenly. The result is thermal distortion, which causes the fan shaft to bow upwards in the center. It is interesting to note that many mistakenly believe that the shaft sags under the weight of the wheel. A thermally distorted fan wheel will cause vibration when the fan is restarted because the fan is out of balance due to the distortion. Power stations may choose any one of three solutions to resolve vibration caused by thermal growth. One approach involves starting the fan and accepting the accompanying vibrations. Over a period of time, the fan rotor geometry is restored as a consequence of the rotor becoming evenly heated once again. The second approach involves the operator’s initiating a series of start-stop cycles to allow the fan wheel temperature to balance out. As with the first approach, high vibration becomes an accepted condition. This, however, in addition to exposing the fans to low cycle stresses, makes this option even less attractive. The third approach involves the installation of a turning gear. A turning gear consists of a small motor, a reduction gear, and an overrunning clutch. The turning gear is used to keep the fan rotor turning at a slow speed (approximately 60 to 90 rpm) when the fan is hot but not operating. This provides an optimum solution to prevent thermal distortion. The design speed of the turning gear is critical. Most centrifugal fans have sleeve bearings that have a minimum speed. Below the minimum speed, the oil film between the journal and the sleeve is not adequate to prevent metal-to-metal contact, and the bearing will be damaged. The turning gear should be designed to operate above the minimum bearing speed (usually 60 to 90 rpm). The bearing manufacturer should be consulted to determine the minimum speed. Key Technical Point The design speed of the turning gear is critical. Most centrifugal fans have sleeve bearings that have a minimum speed. Below the minimum speed, the oil film between the journal and the sleeve is not adequate to prevent metal-to-metal contact, and the bearing will be damaged.
3.7 Fan Performance Because boiler draft fans are among the highest auxiliary power consumers in the plant, the performance of the fans is important for efficient plant performance. Deficiencies in fan performance can cause a load limitation on plant output. In many coal-fired plants, the ID fans are the limiting factor on plant electrical output. Although the ID fans may be the apparent cause of a load limit, in many cases the root cause is high air heater leakage, air heater pluggage, high gas temperatures, precipitator infiltration, or something similar.
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Key Technical Point In many coal-fired plants, the ID fans are the limiting factor on plant electrical output. Although the ID fans may be the apparent cause of a load limit, in many cases the root cause is high air heater leakage, air heater pluggage, high gas temperatures, precipitator infiltration, or something similar. As with most other turbo machinery, the performance of fans is best illustrated by a curve of pressure versus flow. Fan curves are usually plotted as pressure in inches of water gauge ("Wg) versus volumetric flow in actual cubic feet per minute (acfm). Note that most curves are labeled as cfm, where it is understood that cfm is acfm. Because fan performance depends on inlet density, the fan curve should specify the density. Some European fan suppliers plot curves in head—in feet (meters) of fluid (rather than pressure, in inches of water)—versus volumetric flow. These curves are essentially independent of inlet density, similar to a centrifugal pump curve that is plotted as head versus volumetric flow. Key Technical Point Note that most curves are labeled as cfm, where it is understood that cfm is acfm. Because fan performance depends on inlet density, the fan curve should specify the density. Figure 3-13 charts the performance field for a typical centrifugal fan with inlet vane control. It is a customary design practice to specify a test block condition (which contains flow) and head margin above the expected operating requirements at full unit load. Both centrifugal and axial fans must be capable of meeting conditions well beyond the expected conditions at full plant operating load. When using these curves, care must be taken not to confuse inlet vane position in degrees and percent open. Fan manufacturers usually present their curves in terms of vane angle, with 90 degrees being the full open position. Many boiler controls identify inlet vane position in terms of percent open, with 100% being full open. Because control room and actuator position indicators may not accurately indicate actual vane positions, the actual vane position should be verified.
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Figure 3-13 Typical Centrifugal Fan with Variable Inlet Vanes
Key Technical Point Fan manufacturers usually present their curves in terms of vane angle, with 90 degrees being the full open position. Many boiler controls identify inlet vane position in terms of percent open, with 100% being full open. Figure 3-14 is the same fan curve as Figure 3-13 with the system curve superimposed.
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EPRI Licensed Material Technical Description
Figure 3-14 Typical Centrifugal Fan with Variable Inlet Vanes and Showing System Curve
Note that at the minimum flow (25% per NFPA), the inlet vanes would need to be less than 15 degrees. The inlet vanes are designed for flow control and not to isolate the fan, and—with the vanes fully closed—the performance will be approximately the same as with the vanes 15 degrees open. Thus, controllability at this low vane opening may be a problem. Most large boilers have two 50% capacity FD and ID fans; single-fan operation at low flow rates usually provides better controllability.
Key Technical Point The inlet vanes are designed for flow control and not to isolate the fan, and—with the vanes fully closed—the performance will be approximately the same as with the vanes 15 degrees open. Thus, controllability at this low vane opening may be a problem.
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EPRI Licensed Material Technical Description
Figure 3-15 is a typical curve for a centrifugal fan with speed control (applicable to a fluid drive or a variable frequency drive). The system curve is also shown in this figure. The minimum flow (25%) is achieved at 200 rpm. A stable speed of 200 rpm could be a problem with a fluid drive that has a maximum speed of 900 rpm. With variable frequency drives, a minimum speed of 200 rpm should not be a problem.
Figure 3-15 Typical Centrifugal Fan with Speed Control
A centrifugal fan’s most efficient area of operation is near the full-load condition in the test block area. Because the lines of constant efficiency run approximately perpendicular to the system resistance line, as load drops, the efficiency of a centrifugal fan also drops rapidly. A centrifugal fan sized for a test block condition will not operate in its most efficient region under normal conditions. For a typical centrifugal fan using inlet vane control, efficiency at the test block condition may be as high as 88% but will be only 70–75% at the 100% unit load condition. At 50% unit load, fan efficiency may be as low as 25%. A centrifugal fan with a variable-speed drive operates near its peak efficiency at all loads. 3.7.1 Axial Fan Performance A typical performance field for a variable-pitch axial fan is shown in Figure 3-16. The boiler resistance curve and test block condition are the same as those used in the previous example for a centrifugal fan. For axial flow fans, maximum operating efficiencies occur below the stall line, which represents the maximum capability of the fan. This makes it possible to select a fan that operates near its optimal efficiency at the expected full-load condition. 3-28
EPRI Licensed Material Technical Description
Figure 3-16 Performance Field for Variable-Pitch Axial Flow Fan
Furthermore, for axial fans, the areas of constant efficiency run approximately parallel to the boiler resistance line. As load decreases, efficiency does not drop off as drastically as it does with a centrifugal fan. Thus, at 50% load, efficiency may remain as high as 65%—more than double the efficiency of a centrifugal fan with inlet vane control. Higher operating efficiencies and the resulting fuel savings are the most significant factors favoring axial fans. 3.7.2 Fan Pressure Definition The fan pressure should be defined on the fan curve. Possible definitions are fan static pressure (FSP), static pressure rise (SPR), or fan total pressure (FTP). These are defined as follows: •
FSP = SP2 – TP1
•
SPR = SP2 – SP1
•
FTP = TP2 – TP1 3-29
EPRI Licensed Material Technical Description
Where the subscripts 1 and 2 refer to the fan inlet and outlet, SP is static pressure, and TP is total pressure. Note that fan static pressure is not the same as static pressure rise. These definitions are illustrated in Figure 3-17. Key Technical Point Note that fan static pressure is not the same as static pressure rise.
Figure 3-17 Fan Pressure Definitions
3-30
EPRI Licensed Material Technical Description
Fan curves are useful in evaluating actual fan performance, which is usually measured in terms of pressure rise across the fan and volumetric flow rate. The measured volumetric flow can be applied directly to the fan curve (assuming that the units are the same). The pressure rise across the fan must be converted to the density on the fan curve. Key Technical Point The pressure rise across the fan must be converted to the density on the fan curve. Figure 3-18 is an example of correcting the fan performance to the operating conditions. It shows a typical fan curve based on an inlet density of 0.075 lb/ft3. It also shows an operating point based on measured data. Due to a temperature difference, the density at the measured point is 0.0696 lb/ft3. Comparing the operating point to the fan curve appears to show that the fan is not performing as designed. However, correcting the fan curve to the actual density at the operating point shows that the fan is actually performing better than design.
Figure 3-18 Fan Correction for Inlet Density
3-31
EPRI Licensed Material Technical Description
The procedure for correcting a fan curve to the operating density is to correct the fan head as follows:
H 2 = H1
ρ2 ρ1
Eq. 3-1
Where: H2 = fan pressure at density 2, "Wg H1 = fan pressure at density 1, "Wg ρ1 = density 1, lb/ft3 ρ2 = density 2, lb/ft3 To convert the fan curve, use Equation 3-1 at several flow rates and plot the new curve. Note that the volumetric flow rate (cfm) for point 1 and point 2 is the same. A fan is a constant-volume machine, regardless of density.
3.8 Selection of Fan Type The following are some of the considerations in selecting the type of fan to be used:
•
Volume/Pressure Characteristics. As the flow through an axial fan decreases, the output pressure decreases. The characteristic curve for a centrifugal fan shows an increase in static pressure as flow decreases. This feature results in the maximum pressure capability of a centrifugal fan that is substantially higher than that required to satisfy the test block condition. This higher maximum pressure capability can affect the design pressure of ductwork, precipitators, and other components in the draft system.
•
Erosion/Corrosion. In general, a centrifugal fan has much greater capability to withstand erosion than axial fans. Protective liners and nose and tip plates are easily applied. Protective nose pieces and coatings are available for axial fan blades, but generally, axial fans are best suited for air or clean gas applications.
•
Mass and WR2 (moment of inertia). Axial fans weigh less than centrifugal fans and have fewer massive rotating elements, which reduces foundation mass requirements. The lower weight results in a lower WR2, which tends to reduce the cost of the drive motor.
•
Evaluation. If a decision is made to consider axial fans, bids should be taken for both centrifugal and axial fans. The final decision can then be based on the results of a comprehensive economic evaluation.
The costs are lowest for fans with variable-speed motors because inlet vanes are not required. Axial fans have a higher initial cost. Two-speed and variable-speed motors are considerably more expensive than constant-speed motors. Axial fans allow a simpler duct arrangement, have 3-32
EPRI Licensed Material Technical Description
more moving parts, and require more maintenance than centrifugal fans. Variable-speed centrifugal fans are the most efficient arrangement, resulting in the lowest auxiliary power consumption and, therefore, the lowest fuel cost. The appropriate fans should be included based on an evaluation of the economic parameters. Note that the selection can be affected by the plant arrangement.
3.9 Fan Requirements A fan specification should give the fan bidders all of the pertinent information regarding performance, service, arrangement, and basis of evaluation when required, so that the bidders can offer the proper fan for the service. The fan’s capability must be specified and must include rate of mass flow, temperature, elevation, density, pressure at inlet, and pressure at discharge. Fan capacity is usually rated as a volume rate of flow and is specified in cfm at inlet conditions. It should be noted that fan performance is based on conditions at the inlet. Key Technical Point Fan performance is based on conditions at the inlet. The fan must provide the air or gas with sufficient energy to overcome the losses encountered through the system. The usual method of specifying this energy requirement is to stipulate the static pressure of the fan in inches of water at a certain volumetric requirement. If fan total pressure is specified, a clear statement of the fact should also be given to avoid giving the impression that total static pressure is being specified. In either case, the distribution of pressure between the inlet and outlet should also be specified. The fan static pressure is expressed in terms of the sum of the total pressure losses of the system, the difference between the total pressure at the system exit and the system entrance, and the fan velocity pressure. Fan velocity pressure (FVP) is defined as the velocity pressure corresponding to the average velocity through the fan outlet (see Figure 3-17). Many times, the fan velocity pressure is unknown because the fan has not been selected. Ignoring FVP builds a margin of safety into the stated pressure requirement.
3.10 Fan Testing A good fan test requires test instrumentation and controlled conditions. Data collected from existing plant instrumentation can be used to identify possible problems but cannot be used to quantify the magnitude of a performance problem. Plant instrumentation is not accurate enough to conduct a good fan test. The codes and standards that specify field performance testing of fans are ASME PTC-11, Fans; AMCA 203, Field Performance Tests; and AMCA 803, Site Performance Test Standard. These codes provide guidance on the instruments and methods used to conduct high-quality field performance tests on fans. 3-33
EPRI Licensed Material Technical Description
The actual operating point can be determined from measurements of static pressure at the fan inlet and outlet and the calculated air or gas flow. The air or gas flow can be calculated from measurements of oxygen and boiler load. Although these calculations are only approximate, they are usually adequate for identifying fan performance problems. Before the actual operating point is compared to the fan design performance, adjustments to actual operating conditions must be made. Fan performance is affected by inlet vane position (blade position for variable-pitch axial fans), fan speed, and fan inlet density.
3.11 Operation A well-operated system improves efficiency and reduces auxiliary power consumption. To optimize draft fan system performance, operators must thoroughly understand the capabilities and limitations of their specific fan system. The startup of any piece of major equipment requires following specific steps, which can be divided into two phases: 1. Prestart checks 2. Startup procedures 3.11.1 Prestart Checks Prestart checks involve a detailed system walk through to verify that the fan will operate safely. During prestart checks, the operator should perform the following tasks: Verify that supportive auxiliary systems and monitoring equipment are operating correctly. For example, check for adequate lube oil and cooling water, proper temperature range of the lube oil and cooling water, and proper damper position. Perform safety-related checks to verify that all DANGER, WARNING, CAUTION, and DO NOT OPERATE tags have been properly cleared; all safety-related interlocks, alarms, and similar operational interlocks have been cleared; and all personnel have been cleared from the fan and ductwork. 3.11.2 Startup Procedures Startup procedures should consist of a well-defined sequence of steps that ensure fan safety and that prevent boiler explosions and implosions. In writing the startup procedures for a draft fan, the operating engineer should take into account the station’s practices and regulations, the level of automation, the number of personnel on the shift, and the manufacturer’s recommendations. The startup procedures, in addition to the controls and interlocks, should follow the requirements of the current version of NFPA 85 [1].
3-34
EPRI Licensed Material Technical Description
Key Human Performance Point The startup procedures, in addition to the controls and interlocks, should follow the requirements of the current version of NFPA 85. 3.11.2.2 Sequence of Steps for Startup The procedure for starting up a draft fan should include the following:
•
Sequence of all operator actions required for startup
•
Alarm conditions to monitor
•
Operating parameters to monitor
•
Emergency actions
•
Fan control requirements
•
Electric motor restrictions
•
Requirement for the control of inlet vanes
•
Fan outlet dampers requirements
•
Individual responsibilities
•
Procedures to prevent a stall
3.11.3 Alarm Conditions to Monitor Draft fan alarms are either executive or advisory. Executive alarms give operating personnel audible and visual warnings of a dangerous condition, such as excessive vibration, loss of lube oil, or a hot bearing. Through safety interlocks, an executive alarm condition may initiate a shutdown of a fan. The automatic boiler control system would then respond to maintain a safe condition in the furnace. Advisory alarms are similar to executive alarms except that operating personnel must initiate the corrective action. A high differential pressure on the discharge filter of a circulating lube oil system is an example of an advisory alarm. 3.11.4 Operating Parameters to Monitor The operating parameters that affect the safety of the equipment and personnel should be monitored.
3-35
EPRI Licensed Material Technical Description
Recommended parameters to monitor for operation include the following:
•
Motor winding temperature
•
Bearing oil temperature
•
Bearing temperature
•
Motor amperes
•
Damper position
•
Fan vibration levels
•
Flue gas temperature (if applicable)
Additional parameters and analysis are discussed in the section on condition based monitoring. 3.11.5 Emergency Actions The startup procedures should contain cautions and warnings to remind operators of problems that could develop. In draft fan emergencies where personnel or equipment safety are in danger, shutting down the fan should be a standard operating procedure. Examples of such circumstances include the following:
•
High vibration levels or excursions
•
Temperature excursions
•
Electrical fires and oil leaks
•
Hot bearings and loss of fan control
•
Fan stall
•
Loss of damper or vane control
•
Major flow unbalance on double inlet fans
3.11.6 Fan Control Before placing control of the fan in the furnace automatic combustion control system, prestart checks verifying positive control of the fan should be complete. These checks include the following:
•
Verifying that the control damper/inlet guide vane is operating correctly and that local position indicators agree with the remote indicators located in the control room
•
Test operating the fluid drive control (if applicable)
•
Verifying that the pitch control (for axial fans only) is operating correctly
•
Verifying that the dampers are synchronized (for double inlet fans)
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EPRI Licensed Material Technical Description
3.11.6.1 Electric Motor Restrictions During startup, it is important not to exceed the electric motor’s duty cycle, especially for an ID fan being started up with cold air. Frequent starts that exceed the duty for which the motor was designed form local hot spots or raise the operating temperature above the allowable temperature rating for the motor insulation. Such a condition could reduce the expected operating life of the motor or cause a premature failure of the motor’s insulation. Ensuring that the design duty cycle of a motor is incorporated into a written procedure for starting a draft fan will help in preventing damage to the motor. The criteria used to establish the minimum number of starts for a large motor (that is, 500 hp and greater) are provided in NEMA Standard MG-l, section 20.43:
•
Two starts in succession, coasting to rest between starts, with the motor initially at ambient temperature.
•
One start with the motor initially at a temperature not exceeding its rated load operating temperature.
Specific manufacturer’s requirements for starting operations after a motor has undergone a cycle of two cold starts and one hot start can be found in the manufacturer’s operating instruction manual or on the motor’s starting plate. Requirements that should be incorporated include the following:
•
Minimum time the motor must run before it is shut down
•
Length of time the motor must be at a standstill before additional starts are attempted
•
Maximum operating temperature of the motor windings
•
Number of total starts per day that should not be exceeded
Another consideration involves operating the draft fan at or above the motor’s maximum rated current level. A loss in fan efficiency could cause the motor to be operated above its rating. According to NEMA Standard MG-l, motors are to be designed to run at a maximum horsepower and full-load current without exceeding a specified temperature rise. It is normal design practice for a utility power plant motor to have a design margin of 15% for horsepower for the following reasons:
•
To allow margin for a demand increase from the driven equipment under unusual or infrequent operating conditions
•
To prolong the operating life of the motor in the event of unusual or infrequent operating conditions
3-37
EPRI Licensed Material Technical Description
The life of a motor is originally determined by the effective life of its insulation. The numbers of starts and operating temperature of the motor directly influence a motor’s operational life. By operating the motor under normal conditions at less than rated horsepower (and therefore less than rated maximum current), the motor operating temperature is kept below its maximum operating temperature. 3.11.7 Control of Vanes and Dampers The startup procedures should specify the position of the inlet and outlet dampers and vanes during startup. The degree of automation will determine what the operator should do and how the control system performs. For systems that require the operator to initiate all actions, startup procedures should address the sequence of opening and closing the dampers and vanes, the time limits between each step, the requirements for a visual verification of the damper position, and indications that a vane or damper has failed to open. 3.11.8 Fan Outlet Dampers Most fan manufacturers recommend starting constant-speed fans with the outlet dampers closed. A closed outlet damper reduces the starting time and load on the motor. The torque required at any given speed can be three to four times higher with an open versus closed outlet damper. Some plants have found that the required torque and startup time for a draft fan system configured with both inlet vanes and dampers and outlet dampers may not differ significantly if the outlet dampers are kept open and the inlet control devices are closed. Outlet dampers are typically high maintenance items—especially on ID fans—and may be required only for startup and fan isolation during maintenance. Some stations have found outlet dampers unnecessary, but this depends on the fan design, the motor capabilities, and the design and condition of the inlet vanes or inlet dampers. The best practice is to follow the fan manufacturer’s recommendation. If a change in damper operation is desired, data on startup times and motor current should be collected and discussed with the fan and motor suppliers. Key O&M Cost Point The best practice is to follow the fan manufacturer’s recommendation. If a change in damper operation is desired, data on startup times and motor current should be collected and discussed with the suppliers of the fan and motor.
3-38
EPRI Licensed Material Technical Description
3.11.9 Stall Prevention for Axial Fans The aerodynamic term stall is often used to describe a phenomenon that can occur in an axial flow fan. Under certain pressure and flow combinations, the gas flow cannot accommodate the guiding surface of the fan blade, and flow separation occurs. Pronounced separation results in circulator flow within the fan and a significant reduction in flow through the fan as well as pressure rise across the fan. An abrupt change in flow and pressure will have an adverse effect on boiler operation and can cause a unit trip. In addition, stall can damage fan blades. To prevent a fan stall, operators must understand the fan curves of the fans they are operating. The startup procedures should describe how to prevent fan stalls. Operators must understand how they can prevent fan stalls by monitoring the operating values of static pressure, airflow, and blade position on axial fans. The procedures should show the fan curves and identify points on the curve that, if exceeded, will stall the fan. The procedures should also describe the following indications of a fan stall:
•
Abnormal flow volume (pulsations) and power consumption (motor amps displaying abnormal fluctuations)
•
High vibration levels
•
Failure of the variable-pitch blades to move on command
•
Loud, abnormal noise levels
The stall line is usually identified on the performance curve (head versus flow) for axial fans. The fan control system should monitor the head and flow and give the operator a stall warning so that the operator can take action before a stall occurs. Key Human Performance Point The fan control system should monitor the head and flow and give the operator a stall warning so that the operator can take action before a stall occurs.
3.11.10 Draft Fan Shutdown The draft fan shutdown procedure should be developed around the safety of personnel and equipment. There are two types of draft fan shutdown, referred to here as a controlled and uncontrolled shutdown.
3-39
EPRI Licensed Material Technical Description
3.11.10.1 Controlled Shutdown A controlled shutdown of a fan occurs in a logical and orderly manner. During shutdown, operators will perform the following:
•
Reduce firing rate demands
•
Stop the associated fan(s)
•
Perform fan post-shutdown-related checks
Written procedures for a controlled shutdown consist of the following:
•
Sequence of steps required to safely shut down the draft fan
•
Post-shutdown checks
3.11.10.2 Uncontrolled Shutdown An uncontrolled shutdown is characterized by a loss of one or more draft fans. An operator error or a problem that went undetected by the monitoring system could result in an uncontrolled shutdown. The following conditions could cause an uncontrolled shutdown:
•
Control system upsets and/or failures
•
High vibration level
•
Uncontrolled hot bearing
•
Loss of lube oil
•
Electrical fire at the motor controller or fan motor
•
Fires in ductwork and/or fan housing (GR or PA fan)
•
Loss of electrical power
Direct consequences of an unexpected loss of a draft fan include the following:
•
Reduced unit load capability
•
Loss of a draft fan cascading into a boiler explosion or implosion (this occurs if boiler controls do not function properly)
•
Loss of a unit in case its pair of ID or FD fans is shut down unexpectedly
A written procedure for an uncontrolled shutdown of a draft fan should incorporate adequate measures to ensure boiler safety.
3-40
EPRI Licensed Material Technical Description
Boiler safety is addressed through a system of mandatory safety interlocks that protect against an unstable operating condition. As with any mechanical or electrical system, this safety interlock system can provide only a limited level of assurance in preventing conditions that might lead to a boiler explosion or implosion. Two critical factors must be recognized about the reliability of automatic safety systems:
•
The effectiveness of the system depends on how well it is maintained.
•
No safety system is 100% reliable.
Written procedures and training on the system requirements and operator responsibilities in the event of a fan loss provide an additional margin of safety. NFPA 85 [1] discusses conditions resulting from the loss of a draft fan that may lead to a boiler explosion or implosion. The following summarizes the required safety system interlock responses from NFPA 85 [1] in the event of an uncontrolled shutdown of draft fan(s):
•
•
Response to the loss of one draft fan (either one ID or one FD) –
Close dampers of the affected fan unless it is the last FD or ID fan in service.
–
If the unit’s automatic safety interlock system is designed to start, stop, or trip ID and FD fans in pairs, trip the corresponding paired fan; close dampers of these fans unless they are the last ones in service.
Response to the loss of all FD or all ID fans: –
Trip the corresponding ID or FD fans.
–
Close dampers of all fans (to avoid any pressure excursion during fans coasting down).
–
Open all dampers after the required time delay (to provide appropriate natural ventilation of the boiler).
3.11.11 Parallel Fan Operation When two or more fans are brought into parallel operation, the operator must ensure that the airflow from the fans is balanced. An improper fan paralleling operation can lead to serious consequences, such as the following:
•
High vibration levels, stalled fan(s), airflow pulsations in the ductwork, pressure excursions to the boiler, or damage to the electric motor.
•
Potential for losing both fans. This is particularly serious if the unit loses all of its FD or ID fans because this may lead to furnace damage and will trip the master fuel valve, resulting in the loss of the unit.
When paralleling two fans, the operator should observe the following:
•
Verify that the second fan to be brought on-line is mechanically ready.
•
In the case of axial fans, bring the on-line fan to a pressure below the fan’s stall point; this often requires backing down the unit. 3-41
EPRI Licensed Material Technical Description
•
Bring the second fan up to speed.
•
Adjust flow control devices of the second fan to match those of the on-line fan.
•
Confirm that the load of the on-line fan decreases accordingly as the second fan is brought on-line.
•
Place the second fan in automatic and confirm that the system balances both fans.
An on-line monitoring program should be designed to do the following:
•
Recognize a change in the equipment’s normal operating conditions or parameters.
•
Provide a method to warn operators when changes in the operating conditions and parameters have exceeded safe limits.
The following are the primary objectives of an on-line monitoring program:
•
Reliably assesses the fan’s mechanical condition and displays the results in a format that is user-friendly to O&M personnel.
•
Quickly and accurately recognizes any changes in the fan’s operating parameters and the significance of the changes.
•
Assesses and displays the severity of a problem in a user-friendly format that requires no additional interpretation.
•
Provides a means to initiate a shutdown of the fan in the event that the problem is determined to be severe.
•
Maintains data files for long-term trending and machinery history.
3.12 National Standards The Air Moving and Conditioning Association, Inc. (AMCA) has developed a standard for laboratory testing of fans: AMCA 210. This standard provides rules for testing fans under laboratory conditions to provide rating information.
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EPRI Licensed Material
4 FAILURE MODES AND EFFECTS ANALYSIS
A failure modes and failure causes review is discussed in this section. The data presented here were obtained from NERC/GADS and other sources. The failure data presented here were supplemented with information obtained in a survey on FD/ID fans sent to select fossil plants and with information obtained in follow-up interviews from those surveys. The most common problem areas for centrifugal ID fans are blades, bearings, and foundations, which account for over 50% of all problems. Other problem areas include inlet vanes, motors, dampers, hubs, shafts, and controls. The most common direct causes of these problems are erosion and vibration. Bearing problems can be caused by either a design problem or improper maintenance and operation. The major cause of foundation problems is improper design, and the impact is high fan vibration. Over 50% of the problems with axial ID fans were with blades, shaft bearings, and blade thrust bearings. Other problem areas include the hydraulic supply unit, blade adjusting mechanism, and the shaft. As with centrifugal fans, axial fans have problems with erosion of blades and the main shaft bearings. However, the biggest problem area with axial fans is related to the variable-pitch blades: 33% of the problems were due to either the hydraulic supply unit, blade thrust bearings, regulating arm, or blade adjusting mechanism. Axial fans have less massive rotors and shorter bearing spans than centrifugal fans. Therefore, the bearing loads are less, which should increase availability. However, axial fans have much higher loads on the thrust bearings. The major difference between axial and centrifugal fans that can affect availability is the control mechanism. The control mechanism for centrifugal fans—whether it is inlet vanes, inlet dampers, or fluid drives—is much less complicated than the blade adjusting mechanism for axial fans. There were nearly twice as many problems with blade positioning systems in axial fans. EPRI report CS-1693, Failure Cause Analysis – Fans, documents the results of the survey performed in 1981 on the causes of fan failures in fossil plants. The data in Tables 4-1 and 4-2 were taken from this report and show the failure causes and the failure rates of the FD and ID fans. The failure data shown in Tables 4-3 and 4-4 for 1982 through 1995 were taken from NERC/GADS. The failure mechanisms of the individual components are discussed next.
4-1
EPRI Licensed Material Failure Modes and Effects Analysis
Table 4-1 Summary of Centrifugal Fan Problem Areas Problem Area
ID Fan %
FD Fan %
Foundation
9.3
0.1
Pedestals
1.7
0.6
Bearings
9.4
11.3
Shaft
1.6
0.1
Hub
2.4
0.9
Center plate/side plate
2.1
1.6
Blades
13.3
1.7
Housing/boxes
4.9
0.9
Inlet dampers
2.0
0.3
Inlet vanes
4.6
0.4
Dampers (isolation)
0.6
1.6
Coupling
0.6
0.4
Variable-speed drive
1.9
2.0
Motor/turbine
2.4
2.3
Turning gear
0.9
0
Controls
1.6
0.9
Ductwork
3.6
1.4
No problems
9.6
16.8
4-2
EPRI Licensed Material Failure Modes and Effects Analysis
Table 4-2 Summary of Axial Fan Problem Areas Problem Area
ID Fan %
FD Fan %
0
0
Internal bearing supports
4.8
0
Shaft bearings
19
0
Shaft
7.1
0
Hub
0
0
Hydraulic actuator
0
4.8
Blade adjustment mechanism
4.8
4.8
Regulating arm
4.8
0
Blade shaft
0
0
Blades
19
0
Hydraulic supply unit
9.5
4.8
No problems
2.4
4.8
Foundation
4-3
EPRI Licensed Material Failure Modes and Effects Analysis
Table 4-3 FD Fan Failure Data for U.S. Fossil Plants from 1982 Through 1995 (NERC/GADS Data) FD Fans Number of Units: 1343 Description
Total Occurrences
No. of Forced Outages
Forced Outages No. of Forced Forced Outage – Outage per Forced Deratings, Hours Lost per Unit Year Deratings Occurrence Occurrence per Unit Year
Equivalent Derated Hours Lost per Occurrence
Actual Derated Hours Lost per Occurrence
No of Parallel Components
Mean Time Between Failures (hrs)
Mean Down Time (hrs)
FD fans
4798
355
0.0213
4443
0.2667
23.707
9.30
18.60
2
40,000
18.97
FD fan lubrication system
674
57
0.0034
617
0.0370
16.233
4.921
9.84
2
287,000
10.38
FD fan motors
1732
139
0.0083
1593
0.0956
59.037
25.043
50.09
2
112,000
50.80
FD fan motors – variable-speed
177
43
0.0026
134
0.0080
8.852
9.592
19.18
2
1,095,000
16.67
FD fan drives (other than motor)
323
34
0.0020
289
0.0173
13.462
7.25
14.50
2
600,000
14.39
FD fan controls
1622
357
0.0214
1265
0.0759
6.613
3.04
6.08
2
119,000
6.20
Other FD fan problems 2189
222
0.0133
1967
0.1181
9.776
6.426
12.85
2
89,000
12.54
Total
1207
0.0725
10308
0.6188
18.987
17,000
20.1
4-4
11515
20.199
EPRI Licensed Material Failure Modes and Effects Analysis
ID Fans Number of Units: 872
Description
Total Occurrences
No. of Forced Outages
Forced Outages – Outage per Unit Year
No. of Forced Deratings
Forced Forced Outage Deratings, Hours Lost per Occurence Occurence per Unit Year
Equivalent Derated Hours Lost per Occurrence
Actual Derated Hours Lost per Occurrence
No of Parallel Components
Mean Time Between Failures (hrs)
Mean Down Time (hrs)
ID fans
6600
359
0.0341
6241
0.5934
19.6
7.50
14.99
2
18,000
15.24
ID fan lubrication systems
479
45
0.0043
434
0.0413
7.61
5.429
10.86
2
254,000
10.55
ID fan fouling
492
21
0.0020
471
0.0448
21.511
5.918
11.84
2
247,000
12.25
ID fan motors and 2263 drives
236
0.0224
2027
0.1927
39.077
18.04
36.09
2
54,000
36.40
ID fan motors – variable-speed
302
56
0.0053
246
0.0234
7.268
8.27
16.54
2
402,000
14.82
Induced draft fan controls
2269
676
0.0643
1593
0.1515
7.715
3.83
7.66
2
54,000
7.68
Other ID fan problems
4325
402
0.0382
3923
0.3730
20.033
6.94
13.88
2
28,000
14.45
Total
16730
1795
0.1707
14935
1.4199
17.119
7,000
16.6
16.585
4-5
EPRI Licensed Material Failure Modes and Effects Analysis
4.1 Blades Blade erosion is a significant failure mechanism for both centrifugal and axial fans. While erosion of ID fans is the major problem, erosion has also been reported on FD fans. Key Human Performance Point Erosion is a significant failure mechanism for both centrifugal and axial fans. While erosion of ID fans is the major problem, erosion has also been reported on FD fans.
The following primary factors affect erosion from fly ash:
•
Fly ash concentration
•
Ash particle size
•
Fan tip speed
•
Fan type
•
Blade type
The ID fan erosion rate is determined by a number of variables, including the following:
•
The fuel ash content and the ash constituents. The chemical composition of the ash and the size of the particles affect the rate, type, and location of the erosion.
•
The fuel firing rate. A low Btu fuel requires a higher firing rate with consequent higher ash flow to the fans.
•
The actual efficiency of the ash-collection equipment ahead of the fan. Deterioration of this equipment with age along with overloading due to the use of higher-ash coal has a significant effect on fan erosion.
•
Distribution of the ash in the gas stream as it enters the fan. The duct configuration leading up to the fan may tend to concentrate the ash on one side of the fan or at one side of the inlet boxes.
•
The action of the flow profile through the fan, which may further concentrate the ash.
•
The blade shape, which may present a profile that is more susceptible to erosion effects, for example, blade tips that create a strong turning effect on the gas flow.
•
Blade velocity relative to the ash/gas velocity. Assuming the same total flow, a smaller, higher-speed rotor will erode faster than a larger, lower-speed rotor.
•
Blade material. Common blade materials have a small impact on erosion, but erosion liners have a large impact.
4-6
EPRI Licensed Material Failure Modes and Effects Analysis
Continual surveillance of vibration, inspection (when indicated), and good maintenance or repair have the best chance to succeed. The personnel accountable for a fan outage should be responsible for proper surveillance, inspection, cleaning, and repair. The best method for preventing ID fan erosion is to collect the fly ash before it reaches the fan. Erosion rates are directly proportional to fly ash concentrations. Stratification of ash due to gravity or duct configuration can create high, localized concentration and localized erosion and should be considered in addition to average concentrations. Baghouses have higher collection efficiencies and are less likely to experience excursions than electrostatic precipitators; therefore, they reduce the erosion potential for ID fans. The primary erosion area for airfoil centrifugal fans and axial fans is the leading edge of the blades. The gas flow in a centrifugal fan must make a 90° turn inside the fan. Because the inertia of the ash particles prevents them from turning as quickly as the gas, the particles and erosion will be concentrated at the junction of the blades and the fan center plate. As the size of the ash particles increases, this effect will increase; thus, center plate erosion will increase as will erosion of the trailing edge of the blade. The erosion rate varies approximately with the cube (or higher) of the velocity of impact. Thus, fan tip speed relative to the gas speed is a significant factor. A direct comparison between the tip speeds of axial and centrifugal fans is not valid. The leading edge of centrifugal fan blades is toward the inside diameter and has a lower velocity than the periphery of the wheel, whereas the leading edge of an axial fan blade extends to the periphery. Reduced erosion rates are a significant benefit of variable-speed fans (because erosion rates will vary approximately with the cube of fan speed). Variable-speed fans are generally capable of meeting the full-load system requirements at 90% speed, considering the design margins used. Therefore, at full load, the fan erosion rate of variable-speed fans should be only 73% as great as the erosion rate of constant-speed fans. The difference is even larger at lower loads. At 50% load, the erosion rate of a variable-speed fan should be only 12% of that of a constant-speed fan. If high particle loading is expected, a single-thickness-blade centrifugal fan is the best choice. Properly protected hollow airfoil blades have a relatively high resistance to erosion. However, erosion of hollow airflow blades can cause a hole in a blade and fill the interior of the blade with fly ash. This can cause fan imbalance and vibration problems. A related failure mechanism is the buildup of particle on fan blades. This causes fan imbalance and vibration and may require the fan to be shut down for repair and cleaning.
4.2 Bearings According to two EPRI reports, Failure Cause Analysis – Fans, CS-1693 [2] and Electric Motor Predictive and Preventive Maintenance Guide, NP-7502 [3], bearing failures are a major contributing factor to fan outages. Main shaft bearings are a leading cause of centrifugal fan problems and are prominent in the list of axial fan problems. Nearly all centrifugal fans use sleeve bearings, while axial fans are typically supplied with ball/roller types. 4-7
EPRI Licensed Material Failure Modes and Effects Analysis
The main shaft bearings support not only the static load of the rotor, but also the dynamic load due to vibration. Prolonged operation at high vibration levels puts a severe strain on the bearings, mostly due to manufacturing and maintenance deficiencies. Most bearing failures are a result of loss of oil, contaminated oil, loss of cooling, high level vibration, or maintenance deficiencies. The failure modes include the following:
•
Loss of oil
•
Insufficient oil flow
•
Loss of cooling
•
Loose thrust collars
•
Incorrect clearance
•
Contaminated oil
•
Oil leakage
Bearing failure can lead to high vibration, which can result in the damage of various subcomponents such as blades, rotors, shafts, fan housings, ductwork, and flexible couplings. Successful bearing maintenance includes proper lubrication and lube oil quality, correct alignment of rotating elements, trend analysis, and adherence to regular inspection intervals. Section 5 of this report addresses troubleshooting procedures. Anti-friction ball bearings or roller bearings are used for the main shaft bearing on axial fans. When used, sleeve bearings are similar to those for centrifugal fans except that they are built to have greater capability to sustain thrust.
4.3 Foundations Larger fans require more substantial foundations. These foundations should be level, rigid, and of sufficient mass for the equipment. The supports must be rigid enough to ensure permanent alignment and to prevent excessive vibrations. The minimum frequency of any foundation part should be 25–50% higher than the fan speed. When the fan is mounted on the foundation, the fan shaft should be level at the coupling end, and shims should be used as support points before the bolts are tightened. To obtain a level shaft at the coupling end, it may be necessary to raise the outboard end of the shaft end due to the sag in the shaft. This procedure will prevent any distortion or twisting of equipment and any possible rubbing of rotating parts [4]. The three principal causes of crack development in concrete machinery foundations are improper foundation design, poor construction practices, and poor operating conditions. This section addresses the latter two causes. There are two types of foundation cracking, and the first type occurs during the curing process. As concrete cures, it normally shrinks between 0.02% and 0.03%. The interaction of the reinforcing steel with the concrete during shrinkage produces tension forces in the concrete and 4-8
EPRI Licensed Material Failure Modes and Effects Analysis
compression forces in the steel. As a result of this reaction, cracking (referred to as curing cracks) develop in the concrete. Although these cracks are not considered detrimental to the structural integrity of the foundation, if left unchecked, they can provide “avenues” for oil and water to penetrate into the concrete foundation. Water can expand and contract during temperature changes, causing further crack damage. Oil penetration may, over time, cause degradation of the concrete and introduce a degree of difficulty in the repair process. The second type of cracking affects the structural integrity of the fan foundation. This cracking is caused by dynamic forces such as those produced by excess fan vibration. Two possible conditions may appear with this form of cracking: first, the cracking progresses throughout the concrete block, producing segments of various sizes; second, the segments may move. This movement may cause further crack progression, potentially leading to structural failure of the foundation and the potential for serious damage to the fan.
4.4 Inlet Vanes The inlet vane assembly includes sleeves, rods, shafts, center ring, operating levers and linkage, gears, and tabs. The failure modes for inlet vanes include the following:
•
Binding
•
Wear of control linkage
•
Vane bearing failure
•
Breakage of vanes
•
Wear of control gears, levers, and bearings
•
Erosion
•
Insufficient clearance to fan inlet cone to allow maintenance and lubrication
•
Weak control arms and linkage
In addition, breakage of dampers results in the loss of fans when broken parts are drawn into the rotor.
4-9
EPRI Licensed Material Failure Modes and Effects Analysis
4.5 Couplings Coupling problems usually result from insufficient or loss of lubrication and improper alignment. Coupling/shaft interaction produces tortional vibration problems. The failure modes for couplings include the following:
•
Seizing
•
Sheared stub shaft
•
Broken flexible element or gear teeth
More details are provided in Flexible Shaft Couplings Maintenance Guide, EPRI report 1007910 [5].
4.6 Hydraulic Actuating Mechanism The hydraulic actuating mechanism on axial flow fans is the system that is used to vary the blade angle and the control mechanism for the fan. The following failure modes apply to this mechanism:
•
Oil leaks
•
Contaminated oil
•
Failure of the servomechanism
4.7 Electric Motors Ninety percent of motor failures and problems are the result of four basic causes: dirt, moisture, vibration, and friction. The primary damage includes the following:
•
Overheating due to undersizing
•
Damage to the insulation, resulting in its inability to hold the voltage
•
Moisture intrusion combined with dirt, causing a sticky mass to form inside the motor
•
Loose connections
•
Wires or winding shorting
•
Loose windings
•
Insulation breakdown
•
Bearing failure
•
Breakage of hold-down pads
•
Misalignment
4-10
EPRI Licensed Material Failure Modes and Effects Analysis
More details can be found in Electric Motor Predictive and Preventive Maintenance Guide, EPRI report NP-7502 [3].
4.8 Hubs Examples of problems reported include the following:
•
Loose fit on shaft
•
Failure of rivets or bolts that connect the hub to center plate
•
Cracks or inclusions on faulty castings
•
Insufficient stiffness
•
Erosion
•
Differential thermal expansion of hub and shaft, which allows hub to loosen
4.9 Housing Examples of problems that a housing subsystem may be subject to include the following:
•
Cracking and breakage due to insufficient bracing and/or welding
•
Bolt failures due to high vibration levels
•
Inlet cones having erosion, corrosion, cracking, or breaking
•
Improper clearance between the fan wheel and housing
•
Improperly maintained access doors and/or plates
•
Damaged slide plates
•
Seal rubs as a result of misalignment or improper setting
•
Rubber liner that came loose
•
Liner pieces that broke off and fell into the rotor
•
Corrosion or erosion
Recognition of the cause of the trouble, problem, or failure will usually be a major step toward solving it. Corrective measures may include alteration to the system, modifications to the fan outlet and/or inlet connections, or adjustments to the fan. Excessive speed beyond manufacturer recommendations may cause catastrophic impeller failure. Identification of a problem associated directly with the fan may require the assistance of the fan manufacturer. Figure 4-1 illustrates centrifugal fan housing components.
4-11
EPRI Licensed Material Failure Modes and Effects Analysis
Figure 4-1 Centrifugal Fan Housing Components
4.10 Turning Gears Problems with turning gears may result from insufficient capacity to maintain proper speed to provide lubrication.
4.11 Shaft The shaft problems include the following:
•
Cracks at change of section
•
Incorrect machining, which causes stress-raisers and subsequent cracking
•
Natural frequency too close to running frequency
•
Improper welding or metal buildup, resulting in cracks or failure
•
Out-of-round at bearing journals
•
Diameter too small, causing bowing and tortional problems
•
Shaft dropped and damaged during transit
4-12
EPRI Licensed Material Failure Modes and Effects Analysis
4.12 Center Plate Center plate problems include the following:
•
Cracks
•
Oversize holes, which result in loose rivets or bolts
•
Weld flaws and cracks
•
Insufficient weld penetration
•
High stress levels
•
Thin material, resulting in flexing and cracking
•
Erosion
4.13 Inlet Dampers Inlet damper problems include the following:
•
Cracking
•
Breaking
•
Binding
•
Leaves loosened on shaft, resulting in insufficient control
•
Insufficient clearance to discharge duct to allow maintenance and lubrication
•
Weak control arms and linkage
•
Breakage of dampers, resulting in loss of fan when parts were drawn into the rotor
4.14 Isolating Dampers Isolating damper problems include the following:
•
Binding
•
Breaking
•
Cracking
•
Insufficient sealing
•
Leaves loose on shaft
•
Bearing failures
4-13
EPRI Licensed Material Failure Modes and Effects Analysis
4.15 Variable-Speed Drive Hydraulic couplings are subject to instability, which results in fan speed surging.
4.16 Controls Problems with controls subsystems include the following:
•
Frozen air lines
•
Vanes that opened too quickly
•
Lack of bearing-temperature sensors
•
Vibration sensors mounted vertically rather than horizontally
•
Proximity vibration sensors that are found unreliable
•
Failure of damper torque operator
4.17 Ductwork Problems with ductwork include the following:
•
Insufficient bracing
•
Uneven distribution of fly ash and/or gas to the fans
•
Excessive system resistance
•
Insufficient bracing of stationary vanes
•
Ductwork misalignment with housing
•
Corrosion
•
Expansion joint failure
4-14
EPRI Licensed Material
5 TROUBLESHOOTING
Troubleshooting and industry experience help identify probable causes and provide suitable corrective actions. Tables 5-1 through 5-7 provide potential problems, possible causes, and corrective actions. Many of these problems apply to both centrifugal and axial flow fans. Problems that are unique to one type of fan have been identified, but virtually all of the potential problems apply to both FD and ID fans. Table 5-1 Fan Troubleshooting Problem High vibration
Possible Cause
Corrective Action
Deposits on blades
Clean the entire rotor.
Cracked blade welds
Repair the welds (see Appendix B).
Missing balance weights
Rebalance the fan.
Ash inside hollow blades
Cut openings in blades, remove ash, and repair holes.
Loose bearing or pedestal bolts
Check the torque of bolts.
Loose foundation bolts
Check the torque of bolts.
Cracked pedestal welds
Inspect and repair the welds.
Coupling/shaft misalignment (typical symptom is a vibration that is 2x running speed)
Check alignment.
Damaged bearing(s)
Check oil for contamination, water, and particles; inspect bearings; check bearing alignment; and check bearing temperatures.
Damaged thrust collars
Inspect thrust collars and alignment.
Fan wheel moving on shaft
Inspect hub bolts and hub-to-shaft fit.
Foundation problems
Inspect the foundation for cracks; inspect grout.
5-1
EPRI Licensed Material Troubleshooting
Table 5-1 Fan Troubleshooting (continued) Problem High vibration (cont.)
Possible Cause
Corrective Action
Fan wheel distortion
Verify whether the fan has experienced extreme temperature transient; inspect the wheel for distortion.
Imbalance in inlet vanes
Verify that the inlet vanes at both inlets are synchronized.
Motor problem
Check motor vibration, check motor bearings, verify that the motor is running within its magnetic center, and check motor bearing temperatures.
Unknown
Balance the fan.
Air/gas distribution: ducts, fan housing, expansion joint leaks (increase the hp required)
Excessive wear; improper installation
Reinstall the seal according to the proper procedure.
Variable-speed drive concerns
Leakage
Repair leaks.
Sensors/instrumentation reading
Improper operation
Verify that the variable-speed drive has been tuned to deliver the ordered input.
Electric motor alignment (increases the hp required)
Incorrect readings
Recalibrate sensors.
Fan wheel clearance (excessive clearance will cause recirculation to develop)
Dirty filters, inadequate lubrication of bearings
Check and realign the motor.
Shaft
Excessive clearance between the inlet ring of the fan wheel and the housing inlet cone
Check clearances between the fan and the housing.
Horsepower too high
Bent, undersized
Verify and replace the shaft.
Fan size or type not the best for the application
Check the fan type and design.
Low density
Verify air or gas temperature.
Point of operation on curve
Determine the actual fan operating point.
Seal wear (seals that have excessive wear permit air/gas stream leakage)
Check fan assembly and adjustment, including inlet vanes and inlet cones.
5-2
EPRI Licensed Material Troubleshooting
Table 5-1 Fan Troubleshooting (continued) Problem
Possible Cause
Corrective Action
Horsepower too high (cont.)
Incorrect motor selection
Verify motor selection/size per application.
Fan will not start
Blown fuses
Inspect fuses.
Impeller touching housing
Inspect and verify access.
Wrong voltage
Verify voltage for application.
Wrong fan rotation
Check blade angles in relation to fan rotation.
Inlet or outlet obstructions
Remove obstructions.
Air volume too small
Table 5-2 Bearing Troubleshooting Problem High bearing temperature
Possible Cause Inadequate cooling
Correction Action Verify water flow and temperature (self-contained bearing cooling). Verify oil flow and temperature (circulating lube oil system); verify that pumps are operating and that filters are not blocked.
Erroneous data
Verify that thermocouple/RTO is operating correctly; a portable thermometer may often be used to check the bearing operating temperature against fixed sensor.
Lube oil contaminated
Verify that the lube oil is clean and of sufficient amount.
Oil ring damage
Verify that the oil ring is operating correctly.
Heat flingers damaged
Confirm that heat flingers (if applicable) are not damaged or dirty.
Bearing alignment
Check bearing alignment.
Bearing damage
Open and inspect the bearing: 1. Check for excessive or insufficient clearance. 2. Wipe the bearing surfaces.
5-3
EPRI Licensed Material Troubleshooting
Table 5-2 Bearing Troubleshooting (continued) Problem High bearing temperature (cont.)
Possible Cause
Correction Action
Fan shaft alignment
Verify that the fan shaft is not distorted or misaligned.
Cooling fan problem (axial flow fan)
Verify that the cooling fan is operating properly.
External heat source
Verify that no externally applied heat source is present, such as high ambient temperature.
High vibration
See guidance on high vibration previously in this table.
Table 5-3 Lubrication System Troubleshooting Problem No lube oil flow
Low oil pressure
Noise/vibration
5-4
Possible Cause
Corrective Action
Low oil level
Add oil.
Pumps not operating
Repair pumps.
Dirty filter
Clean or replace the filter.
Incorrect system lineup
Review the valve lineup.
Heat exchanger pluggage
Inspect the heat exchanger.
Dirty filter
Clean or replace the filter.
Pump capacity
Examine pumps.
Incorrect line
Verify proper valve positions.
Oil viscosity
Check oil temperature and type.
Pressure switches
Verify set points and operation.
Relief valve
Verify set points and position.
Flow orifice
Verify size of the flow orifice.
Pump operation
Check the pump alignment for damage.
Relief valve
Verify position of the relief valve.
Cooling fan
Examine operation of the cooling fan.
Oil viscosity
Check temperature and oil type.
EPRI Licensed Material Troubleshooting
Table 5-3 Lubrication System Troubleshooting (continued) Problem Oil contamination
High oil temperature
Possible Cause
Corrective Action
Cooling water leaks
Examine and test heat exchangers.
Filter
Verify the integrity of filters.
Storage tank
Verify that the oil source is clean.
Heat exchanger
Inspect heat exchangers.
Cooling fan (air cooled)
Verify fan operation.
Cooling water
Check temperature and flow rate of cooling water
Heater
Check set point and operating of heaters.
High bearing temperature
Inspect the bearing and bearing clearance to identify heat generation.
Table 5-4 Hydraulic System Troubleshooting Problem Hydraulic oil leakage in the fan diffuser section (axial flow fan)
Hydraulic oil leakage in intermediate section (axial flow fan)
Possible Cause
Corrective Action
Leakage in rotating union
Replace the union.
Leakage in oil line
Replace the line.
Leakage in hydraulic cylinder
Replace the cylinder.
Cooling air fan stopped
Start the fan.
Cooling air duct blocked
Clear the duct.
5-5
EPRI Licensed Material Troubleshooting
Table 5-5 Troubleshooting Noise Level Problem Noise
5-6
Possible Cause
Corrective Action
Insufficient bearing clearance
Adjust or replace the bearing.
Damaged bearing surface
Replace the bearing or liner.
Inadequate lubrication
Add oil or check the lube oil system.
Worn or damage coupling
Replace the coupling.
Coupling alignment; poor wheel balance
Realign the coupling; balance the wheel.
Damper or inlet vanes failed close
Inspect and repair the dampers or vanes.
Stall
Review the fan operating point.
Fan wheel rub
Inspect clearance between the wheel and housing.
Motor noise
See Electric Motor Predictive and Preventive Maintenance Guide [3]
Impeller hitting inlet
Realign the impeller.
Loose motor bolts
Tighten the bolts.
Wrong pulley size
Replace the pulley.
Defective bearings
Replace the bearings.
Bent or undersized shaft
Replace the shaft.
EPRI Licensed Material Troubleshooting
Table 5-6 Troubleshooting Fluid Drive Problem High fluid drive oil temperature
Possible Cause Inadequate cooling
Corrective Action 1. Verify that the ambient temperature of the cooling water is within design limits. 2. Verify that there are no restrictions in the water lines: check drain points to confirm water supply to various points. 3. Verify that the water side of the heat exchanger is clean and unobstructed. 4. Vent the cooler of any air pockets that may have formed if the system was disassembled for maintenance. 5. Verify that cooling system is properly aligned; confirm that all valves are functioning properly. 6. Check system strainers/filters for dirt or debris buildup. 7. Vent system lines to remove any air pockets. 8. Conduct a visual inspection of the quality of hydraulic oil; check for water or dirt contamination. 9. Inspect the cooling water system for proper pressure.
5-7
EPRI Licensed Material Troubleshooting
Table 5-6 Troubleshooting Fluid Drive (continued) Problem High fluid drive oil temperature (cont.)
Possible Cause Poor oil circulation
Corrective Action 1. Check filters: shift or clean as required. 2. Verify that the hydraulic oil system is aligned for operation. 3. Inspect all piping, valves, and filter assemblies for leakage. 4. Verify that all designated valves are opened correctly, and inspect oil side of the heat exchanger for clogs or buildup of dirt. Restrictions at the pump suction point may indicate a buildup of sludge in the oil reservoir. 5. Inspect pumps for proper operation; low discharge pressure may indicate a worn impeller. Check for dirty suction strainers. 6. Verify that relief valves are properly set and not opening prematurely. 7. Vent oil side to remove any air pockets that may have formed if the system was opened.
Low fluid drive oil pressure
5-8
Incorrect oil viscosity or contamination
Verify that the proper oil is used, and analyze oil quality.
Oil heaters operating
Shut off heaters.
Low oil level
Verify oil level.
Bearing damage or wear
Inspect bearings.
High oil temperature
See corrective actions for high oil temperature in Table 5-3.
Plugged strainers
Clean strainers.
Damaged oil pumps
Inspect the pump for damage or internal debris.
Relief valves open
Verify that relief valves are functioning properly.
EPRI Licensed Material Troubleshooting
Table 5-6 Troubleshooting Fluid Drive (continued) Problem High fluid drive vibration
Fluid drive oil leaks
Possible Cause
Corrective Action
Loose bolts
Verify that all foundation and machinery hold-down bolts are properly tightened; inspect for cracked bolt heads. Ensure that fasteners on the steel housing are properly tightened. Inspect all peripheral connections and equipment for damaged or loose fittings.
Alignment
Check the alignment between the motor and fluid drive and between the fluid drive and fan.
Bearing damage
Inspect fluid drive bearings.
Labyrinth seal
Inspect seals for damage.
Piping and valves
Check for leaks.
Input/output shafts
Check for scoring on the shafts.
Table 5-7 Fan Performance Troubleshooting Problem Poor fan performance
Possible Cause
Corrective Action
Inlet vanes
Verify that inlet vanes are responding to the controls. Verify that the inlet vanes on double-inlet fans are synchronized.
Fan speed control
Verify that speed control (if equipped) is responding properly.
Uneven air flow
For double-inlet fans, verify that the flow is even on both inlets (within 5%).
System conditions
Evaluate that the actual fan performance correct for actual conditions.
Wheel to housing clearance
Verify that the clearance between the wheel and housing is within recommended limits.
Fan damage
Inspect fan blades for damage.
5-9
EPRI Licensed Material Troubleshooting
Table 5-7 Fan Performance Troubleshooting (continued) Problem
Possible Cause
Corrective Action
Turning gear high oil temperature
Oil contaminated
Analyze oil, and check viscosity.
Turning gear high vibration
Alignment
Check alignment between fan and turning gear.
Balance
Check the balance of the coupling and clutch.
Loose bolts
Check torque of hold-down bolts.
Fan surge
If fan is operating with inlet vanes less than 30% open, the fan may be in an unstable range. Changes in the system or operation may be the easiest solution. Fans with inlet box dampers may become unstable when the dampers are less than 50% open.
Parallel operation
Multiple fans operating in parallel can be unstable, depending on the shape of the system curve and the fan curves.
Operating point
Measure the fan flow and head, and plot the operating point to verify that the fan is not operating in the unstable region.
Stall
Verify the actual operating point of the fan versus the stall line.
Pressure pulsations (centrifugal)
Pressure pulsations (axial)
5-10
EPRI Licensed Material
6 CONDITION MONITORING
Condition monitoring is the use of advanced technologies to determine equipment condition and, potentially, predict failure. It includes technologies such as the following:
•
Vibration measurement and analysis
•
Oil analysis
•
Nondestructive examination (NDE)
•
Infrared thermography
•
Motor current analysis
The goal of condition monitoring is to identify changes in the condition of the fan, motor, or auxiliary that could indicate some potential failure. Physical characteristics are measured, recorded, and analyzed so that trends can be identified. Key Technical Point The goal of condition monitoring is to identify changes in the condition of the fan, motor, or auxiliary that could indicate some potential failure.
6.1 Vibration Monitoring Vibration monitoring consists of acquiring and analyzing specific machine operating parameters. 6.1.1 Parameters The following parameters can be used to form a database for draft fans:
•
Amplitude
•
Frequency
•
Phase angle
•
Vibration form
•
Vibration mode shape
6-1
EPRI Licensed Material Condition Monitoring
6.1.1.1 Amplitude Amplitude can be expressed in terms of displacement, velocity, or acceleration and provides an indication of severity by measuring how smooth or rough the draft fan is operating. Equipment that is operating within the manufacturer’s recommended limits will have a stable amplitude reading. A change in this reading indicates a change in the condition of the machine; this change indicates a need for further investigation. 6.1.1.2 Frequency The frequency of vibration is expressed in either cycles-per-second, cycles-per-minute, or as multiples of the rpm. Examples of these multiples include the following:
•
1 x rpm: vibration frequency is the same as the machine’s rpm
•
1/2 x rpm: vibration frequency is one-half the machine’s rpm
•
0.43 x rpm: vibration frequency is 43% of the machine’s rpm
Vibrations occurring at frequencies that are a direct multiple (for example, lx, 2x, 4x) of the machine’s rpm are termed synchronous or harmonic. Vibrations occurring at frequencies that are an integer fraction (for example, l/2X, 1/3X) of the machine’s rpm are termed subsynchronous. Nonsynchronous vibrations occur at frequencies other than direct multiples of the machine’s speed. Machine problems will most often occur at low vibration frequencies, typically less than 4 times (4X) the running speed. Frequency should not be used as a measure of the problem’s severity, unless roller and ball bearings are involved. Although, in certain cases, specific frequencies can be linked to specific problems (for example, unbalance and misalignment), this does not mean that there is a direct correlation between problems and vibration frequencies. 6.1.1.3 Phase Angle The phase angle provides a reference measure of movement of a specific point on the shaft or rotor. This point can be a high spot located on a shaft or a concentration of uneven weight that may have collected on the fan rotor. The measurement is taken relative to either another moving point or to a fixed point such as a transducer. The phase measurement or angle is expressed in degrees. Accurate phase angle measurement plays an important role in balancing fan rotors and in analyzing the mode shape of the vibration. 6.1.1.4 Vibration Form Vibration form is the actual vibration displayed as a wave pattern. The wave pattern generated will represent shaft motion. Short-lived, transient types of vibrations are best analyzed through observing their wave form characteristics (such as shape, amplitude, and pattern) on an oscilloscope. This provides the ability to “see” what the fan is doing at any moment. 6-2
EPRI Licensed Material Condition Monitoring
Vibration form can be displayed as a time-based presentation or as an orbital presentation (which is seldom used in draft fan applications). Time-base presentation uses inputs from a displacement transducer displayed on an oscilloscope in the time-base mode. An orbital presentation uses the input from two probes spaced 90° apart from each other in the X-Y mode of an oscilloscope. This latter method allows operating engineers to observe the centerline motion of the shaft. As an example, if the probes were mounted on a bearing housing, the display would show the movement of the shaft centerline relative to the bearing. 6.1.1.5 Vibration Mode Shape Vibration mode shape is obtained by recording vibration amplitude and phase values at many points on the structure of the entire machine, including fan bearings, driver, and foundation. The vibration mode of a draft fan provides a means of confirming resonance conditions, locating nodal points (that is, points of minimum amplitude), and identifying points of structural weakness. By conducting casing measurements along a drive train, problems such as pipe resonance, structural resonance, or loose/cracked foundations can be determined. 6.1.2 Vibration Analysis Vibration analysis can be accomplished through a variety of available techniques. These techniques offer methods to obtain and display vibration data:
•
Amplitude versus frequency
•
Real-time spectrum analysis
•
Time waveform
6.1.2.1 Amplitude Versus Frequency Analysis The vibration amplitude-versus-frequency analysis method is considered to be the most useful. Over 85% of mechanical problems occurring on rotating equipment can be identified using this method. This technique has applications for both continuous monitoring/machinery protection and for equipment diagnostic checks. Key Technical Point The vibration amplitude-versus-frequency analysis method is considered to be the most useful. Over 85% of mechanical problems occurring on rotating equipment can be identified using this method. Primary equipment needed to conduct this test includes a vibration-pickup (probe) and vibration analyzer. Use of an X-Y recorder provides the added feature of automatically producing a plot of frequency-versus-vibration amplitude. An additional option available on some vibration analyzers is an automatic frequency tuner, which eliminates the need for operators to manually 6-3
EPRI Licensed Material Condition Monitoring
tune through the frequency spectrum. Using an automatic frequency tuner also provides extended troubleshooting capability. It reduces the element of human error by eliminating the chance of missing significant vibration frequencies, and it reduces the actual analysis time by eliminating time spent on fine-tuning each significant frequency. The following are additional recommendations to follow when using the amplitude-versusfrequency technique:
•
Take vibration readings along the horizontal, vertical, and axial directions at each bearing.
•
Select an amplitude range setting on the vibration analyzer sufficient for the maximum vibration signature in order to obtain data that are plotted on the same range.
•
Select a single amplitude range setting sufficient for the entire analysis.
•
Obtain an overall “filter out” reading (in each of the three positions) at each bearing.
6.1.2.2 Real-Time Spectrum Analysis This method is most effective when the vibration is not steady state or transient. Use of a realtime spectrum analyzer allows O&M personnel to “capture” and analyze vibration signatures. Two features available with real-time analyzers are the “hold” and “peak hold.” The “hold” control is operator-initiated when the transient frequency reaches its maximum amplitude. This action stores the transient signal into the analyzer’s memory for future analysis. The “hold” feature provides a means for manually capturing the transient signal. This method will work if the following conditions are met:
•
The operator is fully aware of when the transient signal will occur.
•
The transient signal occurs slowly enough for the operator to depress the “hold” button.
The “peak hold” feature provides a method to capture and store a transient signal in situations that do not meet the above two conditions. If this feature is used, the operator must set the trigger level, which specifies the percentage of the signal’s amplitude used to begin the process. For example, if the operator chooses 50%, detection circuits will look for an amplitude level exceeding this value from incoming signals. Once this 50% criterion is met, the circuit will trigger and automatically capture an incoming transient signal. 6.1.2.3 Time Waveform Analysis Time waveform analysis uses an oscilloscope to provide a time display of vibration amplitude. The oscilloscope can be set up to receive an input vibration signal either directly from the transducer or from a real-time spectrum analyzer. Either method allows operators to analyze vibration quickly and easily. Data obtained are not filtered, thereby providing a true measure of maximum amplitude present. Using an oscilloscope can also provide an excellent means for observing and evaluating transient vibration signals that may be present because of fan pulsations or control problems.
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Essential to any on-line monitoring system is the choice of sensors (that is, transducers) to be used. For vibration measurement, there are three types of transducers available: proximity (noncontact), velocity, and accelerometer probes. 6.1.3 Proximity Probes A proximity probe is a transducer used for vibration and position measurement. Physical contact with the object being measured (for example, the shaft) for vibration or position is not required. Proximity probes are used to measure the relative movement between the shaft and bearing or bearing housing. Principal components include the probe, pickup, connecting cable, and driver. Inside the probe is a coil that receives high-frequency current from the driver. As this current passes through the coil, a magnetic field is established. Conductive material, such as a steel shaft, brought in proximity to the coil due to vibration or axial motion will “cut” the lines of the magnetic flux, thereby setting up eddy currents on the conductive material’s surface. Stronger eddy currents are established as the shaft gets closer to the probe and weakens the magnetic field around it. The strength of the magnetic field is directly related to the level of equipment vibration. In practice, the field strength is monitored by detection circuitry located in the driver. Output from the driver is either in the form of a dc signal or a combination of a dc and ac signal. When no vibration is present, a dc voltage directly proportional to the distance (that is, gap) between the shaft and probe is transmitted from the driver. As vibration levels develop, a dc signal proportional to the average gap and an ac signal proportional to vibration amplitude are generated. There are three applications for systems using proximity probes:
•
Radial vibration
•
Axial position, phase reference
•
Rotating speed reference
A proximity probe offers the following advantages:
•
Measures the motion of the shaft (which is the primary source of machine vibrations) in terms of displacement
•
Makes no contact with the shaft
•
Has no moving parts and is small
•
Has an excellent frequency response
•
Is easily calibrated
•
Provides accurate low-frequency amplitude and phase angle information
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A proximity probe has the following disadvantages:
•
Requires an external power source
•
Is sensitive to certain shaft materials
•
Is sensitive to shaft mechanical and electrical run out
•
May not detect looseness in support components
•
Provides no useful information for fans with heavy rotors
6.1.4 Velocity Probes Operation of a velocity probe is based on the movement of a conductor (in this case, the coil) through a magnetic field. The amount of voltage that is induced in the coil will be proportional to the relative velocity between it and the magnetic field. Unlike a displacement probe, a velocity probe requires no external power source to operate. A velocity pickup consists of six principal parts: pickup case, wire coil, damper, mass, springs, and permanent magnet. The pickup case provides a structure to house the remaining components. The wire coil is wrapped around the mass that in turn is suspended between the permanent magnet by the springs and damper. The permanent magnet is attached to the pickup case and provides a magnetic field around the suspended coil. In certain applications, velocity probes and their cables can be susceptible to magnetic interference. The magnetic interference may be caused by the alternating magnetic field that is generated around large ac motors. Without proper shielding, these fields can induce a voltage in the pickup or cable; consequently, an erroneous vibration signal is generated. If this condition is suspected, the magnetic field’s intensity and presence can be determined. To perform this test, personnel will need a vibration analyzer connected to a portable velocity probe. The probe should be held steady by its cable near the permanently installed velocity probe, and the suspended probe should not come in contact with the operating equipment. During draft fan operation, a reading on the analyzer indicates that a strong magnetic field is causing interference. To alleviate this problem, a magnetic shield should be installed around the fixed velocity probe.
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A velocity probe offers the following advantages:
•
The probe is mounted on the external housing of the machine component; it gives a strong signal in mid-frequency ranges.
•
No external power supply is required.
•
The signal can be electronically integrated to provide a displacement signal.
A velocity probe has the following disadvantages:
•
The probe is relatively large and heavy.
•
The probe is difficult to calibrate and requires a shaker table.
•
The probe has mechanical moving parts that can be expected to wear out.
•
The probe may pick up outside magnetic interference.
•
The probe measures only dynamic motion.
•
The probe has a narrow frequency response; amplitude and phase errors can be generated at low frequencies.
•
Because the probe is manufactured as a unit, a fault in any of the components will require replacement of the entire unit.
•
The probe is susceptible to cross-axis sensitivity at high vibration amplitudes.
6.1.5 Accelerometer Probes The internals of an accelerometer probe are made up of four components: mounting stud, frame, piezoelectric disks, and mass. The mounting stud provides a means for attaching the probe assembly to the equipment being monitored. The frame assembly houses the mass and the piezoelectric disks; these last two components form the heart of an accelerometer probe. Piezoelectric material can generate an electric charge when it undergoes mechanical stress, which can be compressive or tensile. Disks of piezoelectric material are rigidly “sandwiched” between the frame and the mass. In the event of vibration, these disks undergo a series of compressive and tensile reactions. These in turn produce an electric signal proportional to the magnitude of the force imparted to the mass. Because the amount of mass is known, the signal generated represents the acceleration of the mass. Accelerometers are small, lightweight, and rugged. A principal advantage of this type of probe is its ability to operate over a wide frequency range. This capability makes an accelerometer well suited for monitoring high-frequency vibration that can develop in anti-friction bearings or gears. Accelerometers are unaffected by magnetic fields that may originate from nearby electric machinery. They require no special shielding or other provisions. 6-7
EPRI Licensed Material Condition Monitoring
Accelerometer probes offer the following advantages:
•
Easy to install
•
Useful for high-frequency measurements above 2 kHz, highly reliable, no moving parts, and relatively lightweight
•
Available for high-temperature applications
Accelerometer probes have the following disadvantages:
•
Can be influenced by vibration transmitted to the fan housing by the surrounding environment.
•
Transducer fault will require replacement of the entire unit.
•
Difficult to calibrate, poor signal-to-noise ratio, double integration to displacement is susceptible to noise problems, requires external power source, and usually requires filtering in the monitor.
6.1.6 Data Acquisition Accurate data acquisition is the basis for a successful vibration-monitoring program. It is essential to adopt a systematic approach in obtaining and recording the data for use in the analysis process. For draft fans equipped with installed vibration sensors, obtaining data means monitoring and recording vibration levels displayed on the gages or output devices provided. Monitoring programs using portable equipment either to supplement existing installed vibration sensors or to check equipment not outfitted with an installed monitoring system will require personnel to identify specific locations on which to place the probe. These locations should be identified both on the equipment or component and on the forms used to record the data. This procedure will help ensure that consistent and reliable information is obtained. Use of color-coded marks (such as “red dots”) provides operators with an easily seen reference point from which to take vibration readings. Stenciling the equipment with identification marks such as “FD 1 EAST” to distinguish between the various fans at the site will also help prevent confusion—a prime consideration because introducing incorrect data into trending and equipment history files will produce erroneous results. 6.1.6.1 Machine Diagram An important part of the data acquisition process is the machine diagram. A diagram of this type forms a link between the actual physical location on the equipment or component and the record used to log the results. To be effective, the diagram should show all the essential elements. For a draft fan, these elements should include the driver (motor), driven unit (fan), bearings, coupling
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fluid drive (if applicable), and installed vibration sensors. Additional information such as motor rpm, type of bearings (sleeve or antifriction), pickup points, date, name of persons performing the check, and space to enter vibration amplitudes and frequencies should also be incorporated into the form. 6.1.6.2 Tri-Axial Readings Recommended practice indicates that readings should be taken in the horizontal, vertical, and axial directions at each bearing housing. The three-axis approach can assist the maintenance organization in distinguishing between various mechanical problems such as imbalance and misalignment. For example, both of these problems are manifested as an increase in vibration. The frequency of this vibration level is typically lx rpm for both misalignment and imbalance. With the exception of overhung rotors, an out-of-balance fan will have high vibration levels in the radial directions (that is, horizontal and vertical); vibration levels observed in the axial direction would be significantly less. Misalignment, however, typically produces high vibration levels in all three directions. Analyzing horizontal and vertical readings can also provide insight into the condition of the equipment. For horizontally mounted draft fans with a floating outboard bearing, the vibration levels in the horizontal direction will be higher than those in the vertical direction. This difference is considered normal for rigidly mounted fans where the vertical stiffness is greater than the horizontal stiffness. A deviation from this condition may indicate loose equipment holddown bolts, damaged grout layers and/or foundation, wiped bearings, or excessive bearing clearance. The basic parameters that will be monitored are bearing temperature, vibration level, flue gas temperature, and motor amperage. This system package can be used as a guideline for determining the amount of protection needed. Primary features of this system include the following:
•
A remote monitoring capability that allows control room operators to monitor equipment operating parameters.
•
A configuration with safety interlocks that provides audible and visual indications to operators if an operating parameter has exceeded a defined safe limit.
•
A safety interlock system that can initiate a shutdown of a draft fan if a condition develops that is dangerous to equipment or personnel.
Additional parameters and/or features such as lube oil flow and gas stream temperatures (for inside a duct) may also be added if the system configuration requires it. The decision of whether to elect a continuous demand display or an operator demand display setup should be based on the type of fan. For example, fans subjected to blade erosion or buildup are suggested candidates for a system that can be continuously monitored. Fans operating in a clean gas stream are less likely to develop damage from erosion; therefore, they require less stringent monitoring.
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EPRI Licensed Material Condition Monitoring
The following additional procedures could complement the monitoring system:
•
Regularly scheduled operational checks of the equipment, including monthly vibration checks using portable equipment that can provide a check on the installed transducers.
•
Accurate equipment operating logs that can be used for data trending.
6.2 Oil Analysis Oil samples taken for oil analysis purposes are also appropriate for out-of-service checks. A basic spectro-chemical analysis can provide maintenance personnel with the following information:
•
Particle count
•
Viscosity
•
Total acid/base measure
•
Condition of oil additives
This information can be used to determine whether it is necessary to open and inspect a bearing for wear and damage as well as provide a “snapshot” picture of the lube oil’s quality. However, because there are not any recognized industry standards for what is acceptable in lubricating oil, the oil analysis should be used to identify changes and trends. Oil analysis tests are often offered free or for a nominal charge by the lubricant supplier as part of the overall service provided. If the physical design of the fan prevents ready access to the bearings, the pre-startup checks should be accomplished during the out-of-service maintenance period before “closing up” the fan for operation. Key O&M Cost Point Oil analysis tests are often offered free or for a nominal charge by the lubricant supplier as part of the overall service provided.
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EPRI Licensed Material Condition Monitoring
6.3 Nondestructive Examination Nondestructive examination of the rotating components of large fans is important to identify problem before catastrophic failures occur. Details are provided in Appendix B.
6.4 Infrared Thermography Infrared thermography is typically used to identify problems with electric motor leads. It is not used on the fans.
6.5 Motor Current Analysis Condition monitoring of the electric motors is addressed in EPRI report NP-7502, Electric Motor Predictive and Preventive Maintenance Guide [3].
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7 MAINTENANCE
Fans require periodic inspection and maintenance to ensure their mechanical and aerodynamic integrity for reliable operation. This section identifies the recommended maintenance tasks and their frequencies. Because each application could be different, the experience at a given plant should be reviewed and these frequencies adjusted. The major maintenance areas for centrifugal fans are the blade liners, main shaft bearings, and inlet vane or inlet damper linkages. Repair or replacement of the blade liners requires the most specialized labor. The liners can often be repaired by welding instead of being replaced. Either case requires balancing the fan wheel. The main shaft bearing requires the same type of maintenance as that required for the bearings for most other large rotating equipment. Maintenance of inlet vane or control damper linkage presents no special problems if the linkage is properly designed. Key Technical Point The major maintenance areas for centrifugal fans are the blade liners, main shaft bearings, and inlet vane or inlet damper linkages. Axial fans require considerably more maintenance than centrifugal fans. The maintenance areas include blade bearings, main shaft bearings, the hydraulic blade positioning system, and blade replacement. The blade bearings are subjected to high loads and require frequent maintenance. Hydraulic blade positioners have been a source of problems for some axial fan installations. Some utilities send the hydraulic actuators back to the manufacturer for rebuilding rather than repairing them. Blades on axial fans are designed to be replaced. Key Technical Point Axial fans require considerably more maintenance than centrifugal fans. Previous surveys and studies indicated that some plants have had availability problems with axial flow fans—and the availability has a strong correlation with maintenance practices. Stations that follow the manufacturer’s recommendations and rebuilt the axial fan rotors have high availability; those that do not have had problems. It was found that the units that follow the recommended maintenance had axial fan availability similar to that for centrifugal fans.
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EPRI Licensed Material Maintenance
Actual maintenance will depend on the design of the fans, actual operating conditions, and the owner’s philosophy on preventive maintenance. Maintenance inspections at specified frequencies must be performed on draft fans and on their supportive auxiliaries in order to maintain a high standard of safe and reliable operation. This section provides checks for specific fan components. Recommendations are provided for routine maintenance and for overhauls. The frequency of the activities varies with the application, and suggested frequencies are provided for some activities.
7.1 Developing a Preventive Maintenance Program The evolution from a fix-when-fail philosophy to a preventive maintenance (PM) philosophy begins with developing a list of maintenance checks for each fan and its supportive auxiliary systems. This list of checks is based on manufacturer’s recommendations, operational experience, and the power plant’s trained manpower assets. Knowing the capabilities of the maintenance department helps determine which maintenance checks can be accomplished by inhouse staff and which must be contracted out. Recommendations from the manufacturer and operational experience can provide valuable insight in determining not only what checks should be done, but also how often they should be done. Conversely, inspection intervals should be increased for equipment for which maintenance checks show no signs of wear. Key O&M Cost Point Recommendations from the manufacturer and operational experience can provide valuable insight in determining not only what checks should be done, but also how often they should be done. Warning signs, such as excessive equipment failure after maintenance has been completed, could be indications of inadequately trained personnel and/or poor quality control standards. The philosophy of “if it works, don’t fix it” should be considered in any maintenance program. Opening and inspecting components such as bearings or actuators can lead to additional problems and equipment downtime. A balance must be attained by performing PM but avoiding opening and inspecting equipment if no problems (such as high temperature or pressure) are present. This balance is achieved by carefully determining an adequate time period (that is, frequency) between checks. Note that this does not in any way suggest that basic sound operating engineering practices—such as ensuring adequate lubrication, clean lubrication, daily visual inspection of the equipment, and clean equipment—should be ignored. Key O&M Cost Point A balance must be attained by performing PM but avoiding opening and inspecting equipment if no problems (such as high temperature or pressure) are present. 7-2
EPRI Licensed Material Maintenance
Once this list of checks is finalized, the next step is to develop written formatted PM tasks. Each PM task should provide an itemized listing of what is required to accomplish the specific maintenance action. Actual procedures to conduct a specific maintenance action may also be incorporated on this PM task sheet. For example, a well-thought-out PM task should include information such as the trades involved in the check; related additional checks that could be accomplished simultaneously, special tools required to accomplish the job, person-hours required, technical manuals and drawings required, and data readings to be taken. A database incorporated into a PM task can provide a ready reference for personnel to use during equipment maintenance periods. In certain cases, a “pictorial map” could also be attached to a task sheet to provide standard locations to take measurements.
7.2 Basic Rules for Conducting Maintenance To avoid unnecessary injury to personnel or damage to the equipment, sound basic maintenance rules should be practiced at all times:
•
Keep the area around the disassembled equipment clean.
•
Take inventory of the tools brought in and out of a job.
•
Use the correct tool for the job specified.
•
Do not take shortcuts when safety is involved.
•
Follow proper tag-out and valve-out procedures.
•
Keep the shop supervisor informed of the progress and any problems encountered.
•
For PM actions, verify that the required parts are available before starting the job.
7.3 Periodic Maintenance Recommendations Table 7-1 presents typical surveillance and PM frequencies. The actual tasks and frequencies for a specific plant should be reviewed based on the plant-specific experience. Table 7-1 Surveillance and Preventive Maintenance Frequencies Task
Operator Rounds (each shift)
Check bearing oil temperature
X
Check bearing oil levels
X
Check bearing oil pressure
X
Inspect bearing lube oil and cooling system
X
Six Month
Annual (minor overhaul)
Four Years (major overhaul)
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Table 7-1 Surveillance and Preventive Maintenance Frequencies (continued) Task
Operator Rounds (each shift)
Check hydraulic oil unit, flue gas fan, and cooling air fan
X
Inspect lube oil and hydraulic lines for leakage
X
Check fan bearing temperature
X
Analyze vibration
Six Month
Annual (minor overhaul)
X
Inspect and overhaul main shaft bearings
X
Inspect and overhaul coupling
X
Lubricate impeller blade bearings to ensure that the same amount of grease is injected into all blade bearings to avoid unbalance
X
Replace filters on hydraulic unit
X
Check lube oil quality, and change oil if required (change at least every two years) Check hydraulic oil quality, and change oil if required (change at least every two years)
Visual
Lab analysis Axial with variablepitch blades
Check all instrumentation on hydraulic unit
X
Check all instruments for temperature and vibration monitoring
X
Check pitch control system
Axial with variablepitch blades
Check fan for wear and erosion
X
Inspect and lubricate variable inlet vanes
X
Perform visual inspections of wheel for cracks
Centrifugal (see Appendix B)
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Four Years (major overhaul)
EPRI Licensed Material Maintenance
Table 7-1 Surveillance and Preventive Maintenance Frequencies (continued) Task
Operator Rounds (each shift)
Six Month
Four Years (major overhaul)
Annual (minor overhaul)
Inspect housing and wheel-to-inletcone clearances
X
Inspect foundation, pedestals, and anchor bolts
X
Inspect expansion joints
X
Inspect turning gear
X
Inspect main drive motor
X
Inspect fluid drive
X
Replace blade bearings
Axial with variablepitch blades
Replace control studs
Axial with variablepitch blades
Replace slide bearings
Axial with variablepitch blades
Replace rotating union
Axial with variablepitch blades
Replace shaft bearings
Roller bearings only
Rebuild variable inlet vanes
X
For extended shutdowns (shutdowns greater than one month), the following maintenance tasks should be performed:
•
Start hydraulic unit once every two weeks, and run it for 30 minutes.
•
Inspect and clean flue gas fan, and check for wear of impeller blades.
7.4 Component Maintenance The following subsections describe the maintenance tasks for the components and accessories of centrifugal and axial fans.
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EPRI Licensed Material Maintenance
7.4.1 Bearings Ensuring that the lubricant is at the correct level, clean, and of good quality is essential to prolonging the operational life of a bearing. An inadequate amount of oil being supplied to a bearing will result in its operating at higher-than-normal temperatures. High-temperature conditions can cause the oil to break down and the bearing to be damaged by excessive friction levels. Similarly, excessive amounts of oil or grease can cause the bearing to overheat as well. Levels recommended by the manufacturer should be adhered to. Static lubricating oil systems offer a particular challenge to a maintenance department. If a lowlevel condition is discovered during operation, personnel should proceed with caution when adding oil to the affected bearing while the fan is in operation. Relatively colder oil that is added while the shaft is rotating can disrupt the oil film that has been formed. If this condition occurs, metal-to-metal contact can be expected, with resulting damage to the bearing. Key Technical Point If oil in a self-contained sump is below the normal level, adding relatively cold oil may disrupt the oil film. Shut down the fan before adding oil. The quality of the oil being used is best determined by taking an oil sample. Visual inspections taken before fan startup can assure operators that the oil quality is satisfactory. Periodic oil samples taken while the fan is in operation are recommended for bearings lubricated by a circulating oil system. Taking an oil sample on a static system during fan operation is not recommended. Use of a drain valve installed in place of a drain plug can provide some measure of control if personnel consider it necessary to sample oil during fan operation. If a drain valve is used, it is good practice to install a cup or plug to seal the discharge line from dirt and to act as a safeguard to prevent draining of the system if the valve is accidentally opened. Two primary concerns are that the oil film may be disrupted as the oil is being replenished with colder oil and that the sump may run dry while the fan is in operation. Do not mix oils in the lubrication system or bearing housing. Chemical additives in different oils can cause a breakdown in the viscosity, cooling, or bearing lubrication. Key Technical Point Do not mix oils in the lubrication system or bearing housing. Chemical additives in different oils can cause a breakdown in the viscosity, cooling, or bearing lubrication. There are four general indicators for impending bearing failure: vibration, excessive noise, steady increase in bearing operating temperature over time, and/or a lube oil sample contaminated with babbitt or water.
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EPRI Licensed Material Maintenance
7.4.1.2 Routine Maintenance Recommendations Routine maintenance should include the following:
•
Verifying that oil is at the proper level
•
Visually inspecting the seals for leakage
•
Checking the bearing temperature and ensuring that heaters (if installed) are placed in operation
•
Adjusting the cooling water flow to maintain correct temperature
It should be noted that the recommended minimum temperature varies for each manufacturer. For bearings cooled by a circulating oil system, the following tasks should be performed:
•
For bearing fitted with inspection covers, verify lube oil flow through the bearings, check for proper operation of oil rings, and visually observe oil circulation.
•
Wipe the inspection cover thoroughly before opening to prevent any dirt or debris from entering the bearing system. Bleed any air pockets from the system by “cracking open” a vent valve.
•
Inspecting the bearing cooling systems:
•
–
Verify alignment and start the system.
–
Inspect for leaks.
–
Verify that flow has been established.
–
For a closed system, bleed any air pockets from the system by cracking open a system vent valve.
Drawing an oil sample and conduct a visual inspection for the following: –
Water contamination
–
Particulate contamination (for example, dirt or metallic)
Note: When drawing an oil sample, ensure that the sample bottle is clean and dry; if not, it should be wiped with a lint-free rag.
•
Ensure that the area around the sample petcock/sample plug is clean before opening.
•
For systems having sample lines, adequately flush the line to remove any dirt or condensation that may have collected inside the line.
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EPRI Licensed Material Maintenance
7.4.1.2 Bearing Overhaul Worn components should be replaced as required. The replacement criteria are directly related to physical damage caused by cuts, heat, corrosion, or other factors. The following provides a list of components most susceptible to wear:
•
Gaskets
•
Packing rings
•
Seals (including any auxiliary seals)
•
Seal springs
•
Laminated shims
•
Garter springs
•
Wave springs
An inspection should be made of the babbitt surface for surface scoring or wiping (axial scoring is unacceptable; circumferential scoring is not to exceed the manufacturer’s recommendations), noting the following:
•
Fatigue cracking
•
Corrosion
•
“Black scab” or “wire wool” damage
•
Pitting due to electrical discharge
•
Overheating
•
Uneven wear
•
Fretting
•
Inadequate lubrication
During normal open and inspect periods, maintenance personnel can check tightness by applying hand pressure at the ends and horizontal joint of the babbitt. If oil is observed to permeate out, then the babbitt is loose. A dye penetrant test could also be used. The sleeve bore inner diameter should be checked. If bearing sleeves must be replaced, the rust preservative used on the new sleeves must be verified as being compatible with the lubricating oil. If it is not, then any rust preservative applied to the new sleeves must be thoroughly removed with a manufacturer-approved solvent.
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EPRI Licensed Material Maintenance
An inspection should be made of the shaft journal for ridges, grooves, and/or sharp edges, and, if they are present, they should be removed with an approved abrasive cloth. The shaft journal should have a finish range specified by the manufacturer. Thrust collars should be inspected for axial and radial run out. On a thrust bearing, the face-toface dimension is critical. A check should be made that the passes are perpendicular to the face. Clearance checks should be made to verify the following:
•
Total indicated run out clearance between the sleeve thrust face and the thrust collar
•
Distance from the shaft to the bearing housing seal groove/recess
•
Clearance between outer diameter of the seal and inner diameter of the bearing housing
•
Clearance between the inner face of the oil conveyor disk and the orifice partition
•
Clearance between the scoop and guide assembly
Some bearings require a specific housing-to-bearing clearance, which must be set. The following alignment checks should be performed:
•
Inspect shaft journal, sleeves, thrust collars, and interior of bearing housings for corrosion and dirt. Clean surfaces with an approved cleaning solvent, such as mineral spirits or kerosene.
•
Inspect water cavities and oil cavities for dirt and corrosion; clean as required.
•
Inspect oil rings and oil conveyor for damage or looseness.
•
Inspect threads of various bolts and pipe nipples for burrs and corrosion.
•
Inspect sensor cavities for dirt and corrosion.
•
Replenish oil to the bearing sump
•
Enter accomplished maintenance actions on equipment PM records.
Maintenance will vary with the complexity of this system. Specific components requiring various degrees of maintenance include the following:
•
Pumps and motors
•
Gauges, pressure relief valves, pressure switches, temperature switches, and motor controllers
•
Piping and valves
•
Heat exchanger
•
Cooling fan
•
Sump
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EPRI Licensed Material Maintenance
7.4.2 Lubrication System 7.4.2.1 Routine Maintenance Some routine maintenance tasks should be completed to verify that the lube oil is at the proper level in the sump and at the required temperature. To control the temperature through the heaters or cooling system (which is either air or water), it should be verified that the cooling fans are running (if installed). The cooling water flow should be checked, and correct heater operation should be verified. Discharge filters and pump suction strainers should be cleaned and inspected. In addition, all electrical connections (such as cables for pumps, sensors, and so on) should be checked for cracks or looseness, and all control panel access doors should be checked to verify that they are closed and properly secured. External fasteners should be checked for the pumps, fan motor, heat exchanger (air/water cooled type), and pressure switches. In the event of a low-pressure alarm, it should be verified that the standby pump will start by checking for signs of leaks in the following:
•
Supply/return piping and flexible hoses
•
Filter and strainer assemblies
•
Pump suction and discharge liner
•
Inlet/outlet of heat exchangers (if installed), including both water and oil sides
•
Valves and sight glasses
Any unusual conditions of noise or vibration should be noted. The differential pressure across filter elements should be checked—and shifted, inspected, and cleaned as required. 7.4.2.2 Circulating Lube Oil System Overhaul The calibration and inspection of all system instrumentation and sensors should include the following:
•
Pressure switches
•
Thermostats
•
Relief valves
•
Liquid-level switches
Pump operation and alarms at given system test pressures should be verified.
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EPRI Licensed Material Maintenance
The inspection of a water-cooled heat exchanger should include the following:
•
Open and inspect heat exchanger interior; remove sludge and scale deposits.
•
If leakage is suspected, check for tube leakage (cracked tubes, damaged tube joints).
In the event of low-discharge pressure, the pump should be opened and the impeller surfaces and wear ring, as well as the foot valve, should be inspected. When poor oil quality is suspected, the sump should be drained and cleaned; all door gaskets should be inspected and replaced as necessary. Before re-closing the sump, it is important to ensure that all rags have been removed. In the maintenance of all electrical components, the following tasks should be performed:
•
Conduct continuity checks on electric motors and heater coils.
•
Inspect all cables for cracking or other damage.
•
Open, inspect, and clean all control and relay boxes.
•
Verify that all electric motors have been connected properly by checking component rotation.
All exterior surfaces should be inspected and cleaned; all corrosion should be removed; and priming and painting should be performed, as required. An inspection and cleaning of cooling fan blades and air-cooled heat exchanger surfaces should also be performed. All filters and strainers should be inspected and cleaned. The location of filters is very important. Filters installed only at the suction side of the oil pump do well in protecting the pump. However, pump wear or damage can pass to the bearings if oil is not final-filtered before it enters the bearing. The optimum system has both suction filters and final filters. These final filters should be installed as close to the bearings as possible. All system components and lines should be primed and vented.
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7.4.3 Couplings Couplings provide a means of connecting the prime mover (that is, electric motor or steam turbine) to the designated load—the draft fan rotor. There are two types of couplings: flexible and rigid. Flexible couplings are used extensively in draft fan service. Depending on the design and service requirements, flexible couplings can provide the following benefits:
•
Provide protection for slight misalignment caused by thermal expansion and contraction between the prime mover and its load
•
Lessen vibrational torque to reduce noise and absorb any torsional oscillation that may be generated during a transient condition (startup, shutdown, and speed changes associated with a variable-speed motor).
There are four types of flexible couplings used with draft fans: grid, gear, elastomeric sleeve and elements, and disk. These couplings can be divided into two broad categories:
•
Sealed lubricated (grid and gear types)
•
Non-lubricated (rubber sleeve, rubber elements, and disk)
Gear and grid type couplings are designed with limited end float to prevent motor rotor axial movement that can cause damage to the motor. Coupling maintenance is discussed in more detail in EPRI report 1007910, Flexible Shaft Couplings Maintenance Guide [5]. 7.4.3.1 Routine Maintenance Recommendations Some routine maintenance recommendations include the following:
•
Verifying that the coupling has the correct type and amount of grease (grease-lubricated couplings only)
•
Ensuring that the fasteners and grease fittings are properly tightened
•
Visually inspecting the area around the coupling for:
•
–
Leaks
–
Rags or other obstructions
Ensuring that the coupling guard is installed
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These checks are performed on fans having couplings that are readily accessible for a pre-start inspection.
•
Inspecting for cracked disks (disk-type coupling)
•
Visually inspecting for unusual noise or vibration
•
Visually checking for collection of rubber-like dust directly below the coupling (elastomeric sleeve/element type only)
7.4.3.2 Coupling Overhaul For a gear-type coupling, an overhaul would include the following tasks:
•
Disassembling and inspecting the coupling in accordance with the manufacturer’s guidelines
•
Thoroughly removing and cleaning old grease from the coupling and inspecting the following areas for signs of wear or fatigue cracks: –
Teeth
–
Grid
–
Fasteners
–
O-rings, gaskets, and grease seals
•
Obtaining a sample of the grease for testing
•
Reassembling the unit Key Technical Point Verify that components used to restrict motor shaft movement are installed.
•
Replenishing grease
•
Performing alignment checks: –
Parallel alignment
–
Angular alignment
Note that the gap spacer is in place and correctly installed (for limited end float coupling designs).
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For a disk-type coupling, an overhaul would include the following tasks:
•
Conducting a visual inspection for cracks developing in the disk(s).
•
Disassembling the coupling, checking the extent of the disk cracks; and replacing any damaged disk.
Note: The order or number of the shims in a disk pack coupling must not be changed.
•
Replacing the fasteners if the locking feature is in doubt.
Note: The locking feature of these fasteners becomes compromised when they are removed and reinstalled a certain number of times. As an example, Rexnord recommends replacing the fasteners after they have been removed and reinstalled between 7 and 10 times. Key Human Performance Point The locking feature of these fasteners becomes compromised when they are removed and reinstalled a certain number of times.
•
Performing alignment checks: –
Parallel alignment
–
Angular alignment
7.4.3.3 Coupling Alignment Misalignment can result in high fan vibrations. For fan installations that have high vibration sensitivity, the coupling can be aligned to a tighter tolerance, which reduces vibration. A tighter tolerance on coupling alignment can be achieved by using a computerized alignment device. It is important to verify that the thrust load of the fan is not imposed on the motor thrust bearing. This requires knowledge of the magnetic center of the motor. For more detailed information on alignment, refer to EPRI report TR-112449, Shaft Alignment Guide [6]. Key Human Performance Point It is important to verify that the thrust load of the fan is not imposed on the motor thrust bearing. This requires knowledge of the magnetic center of the motor.
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7.4.4 Variable Inlet Vanes and Control Dampers On centrifugal fans, the variable inlet vanes require the most maintenance. 7.4.4.1 Routine Maintenance Some routine maintenance recommendations include the following:
•
Inspection of the linkage/lever assembly.
•
Inspection of the fittings and locking bolts for signs of wear.
•
For double inlet configuration, verification that the dampers are synchronized. (This is accomplished by establishing a reference dimension, actuating the vanes, and comparing the two sides for balance.)
•
Checking the hub linkage pins for wear. –
Remove dirt-infested grease from the area around the grease fittings, and lubricate the grease fittings.
–
Ensure that lubrication intervals are not longer than one year.
–
Note that some vane designs have more than one grease fitting per vane, and some vane designs are lubricant-free with self-lubricant bushings.
7.4.4.2 Inlet Vane Overhaul To overhaul the inlet vane, the following tasks should be performed: 1. Remove old grease and dirt from the linkage assembly; replenish the grease. 2. Consult the manufacturer for an approved cleaning method before conducting this step. Steam clean, water wash, and sandblast the blades to remove any dirt buildup. 3. Conduct a visual inspection of the following items, checking for corrosion, erosion, and cracking: –
Blades
–
Damper seals
–
Frame assembly
–
Actuator
–
Linkage assembly
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4. Disassemble the damper bearings and inspect and clean the roller bearings, paying particular attention to the following items: –
Broken or cracked rings
–
Dented shields and seals
–
Cracked or broken separators
–
Broken or cracked balls or rollers
–
Flaked or spalled areas on balls, rollers, or raceways
–
Brownish-blue or blue-black discoloration caused by overheating
–
Indented, brinelled, or etched raceways
If any of these conditions exist, the bearing should be replaced. 5. Inspect the blade shafts: –
Clean and inspect bearing mounting points; remove any dirt, burrs, or other extraneous items.
–
Check for bowed shaft.
6. Reassemble the components in accordance with manufacturer’s instructions: –
Ensure that all fasteners are in good condition; check for damaged threads.
–
Replace packing/seals.
7. Manually cycle the dampers; observe operation of the linkage and blades. Note any binding or noise. 8. Verify that dampers are synchronized. Establish a reference dimension, actuate the vanes, and compare the two sides for balance. 9. Set mechanical and electrical limits. 7.4.5 Centrifugal Fan Wheels The fan wheel is the most critical area of the fan in terms of structural reliability. Inspections should be conducted at regularly scheduled intervals and whenever the opportunities arise (termed inspections of opportunity). A more detailed description of fan wheel inspection and weld repair is provided in Appendix A. However, as an overview of recommended maintenance, particular attention is suggested for the structural welds that connect the blades, center plate, and side plates as a functional unit. The integrity of these welds is critical. Damage (manifested in the form of cracking or corrosive wear) can be partial or total. Unchecked damage may result in an increase in vibration levels with additional wheel damage, which, subsequently, may increase in severity to catastrophic failure with a serious potential for injury to personnel.
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Key Human Performance Point Damage (manifested in the form of cracking or corrosive wear) can be partial or total. Unchecked damage may result in an increase in vibration levels with additional wheel damage, which subsequently may increase in severity to catastrophic failure with a serious potential for injury to personnel. A typical centrifugal fan rotor (wheel) is shown in Figure 7-1 and comprises the following components:
•
Blade
•
Center plate
•
Side plate
•
Shaft
•
Hub
The fan wheel consists of the blades, center plate, and side plates. The primary goal of the maintenance program is to maintain a high state of material upkeep and availability of these components.
Figure 7-1 Centrifugal Fan Wheel
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7.4.5.1 Centrifugal Fan Wheel NDE Large centrifugal fan wheels are highly stressed rotating equipment. The design stress may be as high as 80% of yield, and the material may be a quenched and tempered high-strength material such as ASTM A514 or A517, with a yield strength of 100,000 psi. Many large centrifugal fans have had cracks, and there have been a few mechanical failures. It is important to periodically conduct NDE (such as wet fluorescent magnetic particle) of welds and to properly repair any cracks. The techniques for examining and repairing welds are covered in Appendix A. Key Human Performance Point Large centrifugal fan wheels are highly stressed rotating equipment. The design stress may be as high as 80% of yield, and the material may be a quenched and tempered high-strength material such as ASTM A514 or A517, with a yield strength of 100,000 psi. Many large centrifugal fans have had cracks, and there have been a few mechanical failures.
7.4.5.2 Blades Several problem areas associated with centrifugal fan blades include the following:
•
Erosion with subsequent imbalance
•
Loss or deterioration of blade liners due to erosion
•
Failed bolts or poor or failed welds
•
Cracking
•
Buildup of fly ash inside airfoil blades
These problems can be readily detected and corrected in their early stages if proper cleaning and inspection (both visual and nondestructive methods) are conducted regularly. Regular cleaning of fan wheel blades subjected to erosive or corrosive environments is highly recommended to avoid fly ash or corrosion buildup. Successful cleaning methods include steam cleaning, high-pressure water, or abrasive blast cleaning. Blasting with frozen CO2 pellets has been used for cleaning, with the benefit of eliminating the need to remove the blast medium from the fan. Care should be exercised when cleaning the blades with either steam or high-pressure water. To prevent thermal shock to the blades, personnel should defer cleaning until the fan blades have adequately cooled. The temperature difference between the blades and the cleaning medium should not exceed the manufacturer’s recommendations. A second consideration concerning all three techniques deals with “filling” the hollow sections of the airfoil blades with
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either water or grit or sand, thereby causing imbalance. To avoid this situation, a visual inspection is recommended before any cleaning is undertaken. Direct impingement of areas noted to have large cracks or surface separation should be avoided. Cleaning should be done using wire brushes or scrapers to minimize particles from entering the blade interior. Key O&M Cost Point Blasting with frozen CO2 pellets has been used for cleaning, with the benefit of eliminating the need to remove the blast medium from the fan. Buildup of fly ash inside airfoil blades is caused by erosive wear providing a path for the gas stream to enter the blade interior. Correction of this situation (should it occur) involves two steps: 1. Confirming that the cause of the vibration is from fly ash buildup inside the blades 2. Removing the fly ash from the blade interior The first step involves the detection of a high vibration level followed by pinpointing which blades are affected. High vibration can be detected through daily monitoring and trending of the vibration levels. Once a high vibration condition is reached, diagnostic equipment allows maintenance personnel to determine the exact cause and location. Once the blade(s) has been identified, the next step is to remove the fly ash. This step will require a section of the blade skin to be removed to allow personnel to access the blade interior. If this is required, reviewing technical data—such as drawings that show how the blade is constructed and the applicable welding procedure used to determine blade material—is recommended before any work is begun. After the section(s) has been removed, maintenance personnel may be required to use a metal rod to mechanically dislodge fly ash that has hardened. This material should be “broken up” as best as possible and removed with a vacuum. Some utilities recommend using a high-pressure water jet to fully remove fly ash in difficult-to-reach places. Rotating the blade 180° will allow for the water to adequately drain out. Using highpressure air to remove remaining pockets of water should also be considered. Any holes due to erosion or made to remove ash from hollow blades should be repaired. The fan manufacturer should be consulted for recommendations for cutting holes and repairing holes. One method of protecting centrifugal ID fan blades from erosion is to use protective liners and solid nose pieces, as shown in Figure 7-2. The liner should cover the nose of the blade and the full length of the blade adjacent to the center plate. Liners should not be added without evaluating the impact of the additional weight on the stresses in the fan wheel and the additional starting load on the motor. The fan manufacturer should be consulted to evaluate the effect of liners on the stresses on the fan.
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Figure 7-2 Wear and Erosion Protective Accessories
Liners can create a problem with NDE of the fan welds. Also, cracks in liners can propagate into the blade material. The capability to perform NDE should be considered when the material of the liners is selected. Using the same material as the fan blades may not provide the same erosion resistance as a hardened material but will avoid the NDE and crack propagation problems. Many plants have had success with flame spray coatings (usually with 45% chrome) on ID fans and gas recirculation fans. Coating fan blades to improve erosion resistance has met with varying degrees of success. Coatings can affect the physical properties of the base materials of the fan. Cracks in coatings can propagate into the fan members. Tests using proposed coatings and fan structural material should be performed and evaluated before the coatings are actually used. Key Technical Point Coatings can affect the physical properties of the base materials of the fan. Cracks in coatings can propagate into the fan members. Tests using proposed coatings and fan structural material should be performed and evaluated before the coatings are actually used.
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Coatings create a problem with NDE of the fan welds. The coating in the area of the welds may need to be removed for NDE. 7.4.5.3 Center Plate/Side Plate Principal problems associated with these components include the following:
•
Erosion
•
Weld flaws and cracks
•
Insufficient weld penetration
•
Loose rivets or bolts caused by oversized holes (found on older fans)
An inspection program that encompasses both visual (VT) and nondestructive evaluation (NDE) inspections will greatly assist maintenance personnel in detecting these problems. Consideration should be given to increasing the frequency of the inspection and performing a detailed NDE inspection for fans that exhibit cracking problems that continue to increase in severity. If replacement wheels are being considered, acceptability standards for the welds should be given a high priority before being written into the specifications. A standard that requires no linear indications and welds that are contour ground will lessen problems associated with interpretation and subjectivity during future inspections. For older fans where insufficient weld penetration is suspected, an ultrasonic test could be used to identify problem areas. 7.4.6 Shaft Results from previous EPRI reports [2, 3] as well as surveys conducted in support of this project indicate that shafts continue to have limited problems. Operationally induced problems range from bowing to scored journal surfaces. Cracking may sometimes develop at the step change of a section as well. If cracking is suspected, either from surface indications noted during a visual inspection or from an increase in vibration levels, follow-up NDE inspections are recommended to confirm or eliminate the possibility of cracks at the step. Technology exists to determine the presence of cracks elsewhere on the shaft. The practicality and economics involved in work of this scope together with low stress levels placed on a fan shaft and few, if any, incidents of cracked shafts being reported make this maintenance action difficult to justify. A cracked shaft may develop as a second-order vibration. Unfortunately, this condition may also indicate a bowed shaft, misalignment, or looseness. However, inspection of cracks on a fan shaft is recommended in the case of catastrophic wheel failure or serious misalignment or imbalance. An additional consideration in shaft maintenance involves the presence of corrosion in areas outside the gas stream. This condition may develop typically with ID or GR fans in the event of leaks around the shaft/housing seal. Replacement of the seal, along with removal of the corrosion, is advised when practical. 7-21
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7.4.7 Hubs Although feedback from the surveys for this project indicated no problems with this component, a regular visual inspection of hubs is recommended to examine for erosion or damaged fasteners. This inspection could easily be incorporated into the maintenance program for the entire wheel. An ultrasonic test (UT) can effectively be used to supplement a VT inspection, checking a hub for cracks, inclusions, cracked fasteners, and erosive wear. A UT examination of the hub should be a low priority, however, and is recommended only once every 10 years unless a problem is suspected or the VT identifies potential problems. 7.4.8 Structural Support System The structural support system (SSS) is a five-component system comprising a concrete foundation, a grout layer, anchor bolts, pedestal, and sole plate/base plate. Maintenance of these components is often nonexistent. However, the SSS is no different from any other supportive auxiliary system and should therefore be incorporated into a PM program. 7.4.8.1 Concrete Foundation An annual visual inspection of the foundation is recommended. 7.4.8.2 Repairing Concrete Foundations Assuming that the soil base has been properly stabilized, the repair of concrete foundations encompasses two areas:
•
Surface cleaning
•
Crack repair
To gain maximum effectiveness, each of these maintenance actions should include both prevention and detection. Prevention is a two-tiered process. Early detection through regular inspections will provide a decisive step toward prevention of major problems and the major repairs that follow. Investigating and determining the cause of the casualty is the second part of a PM action. Maintenance personnel should avoid hasty repairs without first addressing the question: “What caused this problem?” This is especially true in the case of concrete foundation repair. If the foundation continues to crack, hidden problems such as voids in a concrete foundation or an unstable soil base should be considered for investigation. The correction or repair of a concrete foundation involves the choice between a permanent fix (which may require the erection of a new foundation) and a semi-permanent fix used as a “stop gap measure” to keep the fan on-line until a scheduled outage or until the needed logistics are brought together to effect more permanent repairs.
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7.4.8.3 Surface Cleaning Over time, oil may degrade the structural integrity of concrete by causing a physical breakdown in the matrix structure of the concrete. This breakdown process is not immediate and may take years to develop into a serious problem; however, it is one problem that can certainly be prevented and corrected. Steam cleaning provides one possible solution for concrete surfaces that have been subjected to oil spillage over a period of years. 7.4.8.4 Crack Repair Curing cracks do not have any appreciable depth or pattern; structural cracks, however, are large and deep. If structural cracks are not addressed, they may continue to develop in severity. As the foundation continues to degrade, operators and maintenance personnel can expect equipment misalignment, bearing problems, and an increase in vibration amplitude. Early detection is critical to limiting damage to both the foundation and the draft fan. Because anchor bolts are stress risers, structural cracks will appear most often in the immediate vicinity of these bolts. Proper installation of these anchor bolts and correction of equipment vibration offer the best courses of action to take in avoiding this problem. Before actual repairs can begin, several factors, primarily the following, should be considered:
•
Choice of the correct grout
•
Severity of the damage
•
Urgency to bring the fan back on line
•
Access to the damaged area
The effectiveness of a grout is based on its ability to act as an adhesive and to bond the damaged area into a structurally sound joint. Determining which grout resin system to use is an important factor in the repair process. Variables such as pot life (curing time), non-shrink capability, ability to bond through oil films, viscosity, and wetability all factor into the effectiveness of the grout to penetrate the crack and bond the two cracked segments into a cohesive unit. The extent of damage is the final determining factor in deciding whether a foundation can be repaired. Experienced personnel in the field of concrete foundation repair should be consulted to assist in the decision process. Once it has been determined that the foundation can be repaired, the need for the draft fan provides an important input into the decision to conduct temporary repairs or defer the maintenance action until a scheduled outage.
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7.4.8.5 Anchor Bolts An anchor bolt functions as a clamp to provide a secure means for holding the base plate to the concrete foundation. This clamping effect is accomplished through a spring-type action between the anchor bolts and the concrete foundation/grout layer system. As the bolt nuts are tightened, the bolt itself begins to stretch. This stretching phenomenon plays an important part in the successful application and performance of the anchor bolt. As each anchor bolt is tightened, the bolts become a system of springs that will collectively apply a clamping or compressive effect around the entire base plate. The net effect achieved is a secure base on which the draft fan can operate. 7.4.8.5.1 Forces Affecting Anchor Bolts
During the operational life of the draft fan, anchor bolts will be subjected to three loading conditions:
•
Initial preload: Load developed when the anchor bolt is first tightened.
•
Residual preload: The load remaining after all anchor bolts have been properly tightened.
•
Working load: The load to which the anchor bolt is subjected while the draft fan is in operation.
If properly installed and tightened, the residual pre-load on the bolts should be less than any load caused by operation. 7.4.8.5.2 Proper Installation
Critical to achieving a good installation is the need to properly isolate the anchor bolt, and there are two areas that require special attention to ensure anchor bolt isolation. First, foam insulation in the area around the grout where the bolt penetrates can provide adequate isolation. Failure to accomplish this allows the grout to bond to the bolt. Should this occur, the bolt would be limited to stretch in a short section. Two consequences of this condition are the following:
•
Loose bolts after fan startup with subsequent cracking of the grout around the bolt.
•
Inability of the anchor bolt to flex in case the base plate moves laterally.
Second, the area between the sleeve and the anchor bolt requires isolation. AMCA 202, “Fan Application Manual” [7], suggests filling the sleeve with a pliable material such as silicone rubber. If the sleeve is filled with grout or other rigid material, extremely high stresses can develop in the foundation by preventing the anchor bolts from developing the spring action. As a direct consequence, cracking can occur in the foundation in the area around the affected anchor bolt(s). There are three principal checks associated with these components. Maintenance is based on “good housekeeping” practices and consists of surface cleaning, corrosion prevention, and inspecting hold-down bolts for proper tightness. 7-24
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An oil-soaked surface can easily mask small leaks and provides a slip hazard to personnel required to work in the affected area. It also provides a mechanism for oil to migrate down to the grout and the concrete foundation, encourages an attitude that accepts leaks or standing oil as normal station procedure, and presents a potential fire hazard. Conversely, a clean surface provides a visual baseline for personnel to quickly detect leaks, prevents oil from reaching the grout and concrete, and allows inspections of bearing housings or hold-down bolts to be accomplished without danger of injury to personnel due to a slip hazard or a hand injury resulting from a wrench slipping off the bolt head. Corrosion prevention begins with the initial installation. Surfaces that will be in contact with the grout layer should be coated with a primer that is compatible with the grout being used. Base metal, blisters, rusted surfaces, and oil- or grease-soaked surfaces are all unacceptable. Failure to heed this precaution will prevent adequate bonding between the base plate/sole plate and the layer of grout, which will lead to the grout cracking over time. Inadequate corrosion prevention and unchecked corrosion can have serious consequences to machinery using cementatious grout with metallic filings. If corrosion develops at the metalgrout interface, it will continue and spread to the metallic filings. The pressure caused by the corrosive forces can be of such magnitude as to cause misalignment of the machinery and cracking of the concrete foundation. 7.4.9 Housing Housing problems can be detected in their early development stages through a regular visual inspection program. A visual inspection will be the primary element of a maintenance program supporting this subsystem. Additional recommended maintenance actions include cleaning and repair. Frequently, the need for proper maintenance of the fan housing is either deferred or not fully understood by station personnel. If these components do not receive adequate care, station personnel can expect minor problems to escalate into major problems, requiring additional capital and person-hours to correct. If the fan housing cracks either at the welds or at various points, the following actions are recommended:
•
Check the quality of the original welds through a visual and NDE inspection.
•
Check the quality of the weld repair procedure that was used.
7.4.9.1 Housing Housing may undergo erosion due to ash in the flue gas. The erosion can be controlled by liner plates welded to the housing.
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Corrosion can result from air leakage around shaft seals; corrosion of the housing from the outside can be a problem if the insulation and lagging allow rain to penetrate and collect next to the housing. 7.4.9.2 Inlet Cones Inlet cones that operate in an erosive environment can erode or crack. Use of protective coatings or hard-surfacing techniques offers protection, but these are susceptible to wear as well. 7.4.9.3 Fan Wheel Clearance Although this particular topic is covered in more detail in the fan wheel maintenance section, it is important to understand the effects of each part on the whole system. Improper fan wheel clearance, for example, can have a negative impact on overall fan performance (see Figures 7-3 and 7-4). A simple adjustment of the fan-wheel-to-inlet-cone clearance can affect fan performance by 5% or more.
Figure 7-3 Cross-Section View of a Fan Illustrating Clearance Requirements Between the Wheel Inlet and the Inlet Bell
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Figure 7-4 Enlarged View of Figure 7-3
Key O&M Cost Point A simple adjustment of the fan-wheel-to-inlet-cone clearance can affect fan performance by 5% or more.
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7.4.9.4 Access Plates/Doors Specific maintenance actions are required to ensure that the access plate can be used when needed and to prevent the door from acting as a large crack or leak. This condition can cause the following:
•
Escape of corrosive or erosive gases into the environment.
•
Negative effect on fan performance by not providing a gas-tight surface.
•
Escape of corrosive gases that “attack” the surrounding lagging.
•
Leakage of air on the ID and GR fan inlet, causing reduced capacity and corrosion.
To prevent these conditions from occurring, the following maintenance actions are recommended:
•
Proper securing of the hold-down bolts.
•
Replacement of missing or damaged hold-down bolts.
•
Inspection and replacement of door gaskets.
•
Reinstallation of insulation covers over the access plates at the completion of the work. (Failure to reinstall an insulation cover will cause cold air to condense and initiate a corrosive attack in the immediate area.)
7.4.10 Expansion Joints Although expansion joints are not a direct fan component, they are a relatively high-maintenance component related to fans. The routine maintenance recommendations for expansion joints consist of the following:
•
Inspect joints for cracks and holes.
•
Check for fly ash buildup in the folds of the expansion joints.
•
Verify that the dust shields are in place and that the space behind the shields is not filled with ash.
•
Visually check expansion joints for uneven expansion or contraction.
•
Conduct cold checks to verify proper alignment between the fan housing and joints.
Note: If painting is in progress, ensure that painters do not paint fabric expansion joints. The overhaul of expansion joints consists of replacement. Expansion joints typically last 15–20 years.
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7.4.11 Electric Motors Problems with motors can be identified early and corrected through a maintenance program that is structured around motor cleanliness, lubrication, and routine inspections. These three elements will form the necessary foundation for an effective electric motor maintenance program that will lead to dependable and economical operation. 7.4.11.1 Dirt Dirt is a common element in any generating station. Controlling this problem is a 24-hour a day job. Electric motor ventilating spaces will be restricted over time if dirt is allowed to build up. This problem will directly impact the ability of the motor to cool itself, and consequences of this problem (if it is allowed to go unchecked) include the following:
•
Breakdown of motor insulation
•
Increase in auxiliary power consumption
•
Potential for abrasion and wear of motor internal components
An effective cleaning program requires regularly cleaning the exterior of the electric motor, regularly cleaning the motor’s filter assemblies, and cleaning the motor internals. Components such as access panels and covers also play an important role in keeping dirt outside. Verifying that gasket material and dust seals are in good condition, together with properly installing the access panels, will pay valuable “maintenance dividends” in the battle to keep dirt out. Use of a pre-filter assembly has an added advantage of allowing personnel to clean the filters without requiring the motor to be shut down. Cleaning should be performed during every scheduled boiler shutdown. Use of clean, dry, compressed air is effective in removing dry, loose dust and particles. Air pressure at 30 psig can be effective in blowing out a motor. Considerations when using compressed air include the following:
•
Blow out any accumulation of water in the air line and hose before using it.
•
Consult the manufacturer on recommended air pressure; pressure exceeding the recommended value can drive abrasive particles into the insulation and puncture it.
•
Use recommended safety equipment when blowing out the motor; safety goggles, respirators, and hearing protection are highly recommended.
•
Install a suction blower or similar device at the opposite end to remove dirt-laden air.
Additional options for removing dirt include the use of clean lint-free rags and vacuum cleaners. Lint will adhere to the insulation, resulting in an increase in dirt collection. Lint is also particularly damaging on high-voltage insulation because it causes corona discharge to concentrate in one area.
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7.4.11.2 Moisture In all of these cases, simple anti-moisture precautions such as the following can be taken to avoid motor damage:
•
Protecting motors opened for maintenance against moisture by using space heaters, coverings, and, when feasible, reinstalling access panels.
•
Identifying and removing sources of moisture in and around the motor: –
Verify that all piping is properly insulated.
–
Correct all leaks.
–
Install protective coverings over motors when required; ensure that these coverings do not restrict air movement.
7.4.11.3 Friction To avoid damage caused by friction, the motor manufacturer’s lubricating instructions should be followed, and the proper type of lubricant in the proper quantities and intervals suggested should be used. Verifying the proper quantity of lubricant is vital. Excess amounts of lubricant can be just as damaging as insufficient lubricant. Too much grease can promote friction and heat and can leak onto stationary windings and rotating elements. This, in turn, can cause overheating and deterioration of the insulation, resulting in eventual grounds and shorts. 7.4.11.4 Vibration Excessive vibration can damage electrical connections, loosen fasteners, promote frictional wear, and cause portions of the metallic structure to develop cracks. Checks to avoid vibration damage include the following:
•
Verifying correct alignment between the motor and fan.
•
Inspecting the foundation for cracks.
•
Opening and inspecting bearings when heavy wear or damage is suspected.
•
Verifying that machinery hold-down bolts are installed and properly torqued.
•
Inspecting the motor bearings at regular intervals during operation; personnel should check for signs of rapid heating and unusual noise.
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7.4.11.5 Rotor Shaft End Play The rotor shaft assembly of induction motors will have a certain amount of end play designed into it. End play refers to the axial distance through which the motor’s shaft is free to move when it is uncoupled from the load. This freedom of movement occurs because of tolerances that inevitably occur in the design and manufacturing process. These tolerances include machining variations, bearing design requirements to prevent binding, and allowances for any thermal growth of the rotor shaft assembly during motor operation. Induction motors designed with sleeve bearings have a larger end play than those that use roller bearings. Typical values given for large motors with sleeve bearings are 1/4 to 1/2 inch. Motors outfitted with roller bearings may have an end play range from 1/32 to 1/8 inch. If a bearing locking arrangement is used, this range will be even smaller. Because induction motors with sleeve bearings do not have any locking arrangement, the rotor shaft assembly is free to “float.” This does not become a problem while the motor is in operation and is not subjected to any external forces. Under these conditions, the rotor shaft assembly will align itself to the magnetic center of the applied field. Endwise restraint that limits the movement of the rotor shaft is achieved through one of the thrust bearings. The forces of these bearings are designed to withstand momentary thrust that may develop during the starting or stopping of the motor. Damage from continuous thrust occurs when external forces are applied that prevent the rotor shaft from seeking its magnetic center. External forces can result from coupling misalignment or locking. When a coupling is misaligned, asymmetrical forces between the hubs develop. This imbalance may cause the load and motor coupling halves to move apart. This separation is limited by the thrust bearings on the load and motor. Continuous thrust resulting in damage to the motor bearing thrust faces is likely to occur. Limited end float couplings will prevent motor damage to the motor journal bearings by excessive axial movement of the motor rotor. External forces may also develop from a coupling locking. If a coupling is worn or poorly lubricated, torque transmission through the coupling gear teeth sets up high friction, which resists endwise movement (in and out or side-to-side movement of the coupling gear teeth or grids). This condition could prevent the motor shaft from seeking its magnetic center and could subject the thrust faces of the motor bearings to continuous thrust. Figure 7-5 provides a basic block diagram of a hydraulic regulating system used with variablepitch axial draft fans. Note that the boiler combustion control system and rotor assembly are added to provide an overview of the system. This auxiliary system provides a means to adjust the blade pitch upon demand for an increase or decrease in airflow from the boiler’s combustion control system. The following three major subsystems make up a hydraulic regulating system:
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Hydraulic supply system
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Impeller blade adjustment system
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Regulating lever assembly 7-31
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Figure 7-5 Basic Block Diagram of a Hydraulic Regulating System
7.4.12 Fluid Drives The maintenance recommendations for fluid drives are similar to those for the circulating lube oil system. In addition to controlling the fan by varying the speed, the fluid drive may also provide cooling for the oil for the fan bearings. 7.4.12.1 Routine Maintenance Routine maintenance for fluid drives consists of the following tasks: 1. Inspect filters and strainers for cleanliness. 2. Draw an oil sample and conduct a visual inspection, following the same procedure as that used for fan bearings. 3. Annually, conduct a lab analysis of the oil. 4. Verify that the system has the proper oil level. 7-32
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5. Inspect for leaks, and wipe up standing oil. 6. Verify that the hydraulic oil is at the specified operating temperature. 7. Inspect the foundation fasteners for proper torque setting. 8. For fluid drives equipped with electric pumps, check for proper operation. 9. Inspect for unusual noise and vibration. 10. Conduct vibration monitoring. 7.4.12.2 Overhaul Fluid Drive Fluid drives are specialized rotating equipment. The manufacturer or other specialists should be used for an overhaul that includes disassembly of the fluid drive. The scope of a typical overhaul includes the following: 1. Disassemble the fluid drive unit and check for wear or damage on the following components: –
Impeller and runner
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Pillow blocks
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Journal bearings
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Thrust bearings
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Scoop tube and scoop slide to guide
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Control rod mechanism
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Labyrinth seals
2. Replace gaskets. 3. Clean out hydraulic oil sump. 4. Conduct a dye penetrant test on the impeller and runner. 5. Inspect valves, fittings, flexible hoses, and piping (including supports) for wear or damage. 6. Inspect the relief valve for proper operation. 7. Inspect internal fasteners for damage or wear and proper torque value. 8. Overhaul the oil supply pump: –
Disassemble the pump; inspect gears and bearings for signs of wear or damage.
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Check alignment of the drive shaft and pump shaft.
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Check the condition of all fasteners.
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Open and inspect the filters and strainers, and clean or replace as required.
9. Check the condition of the foundation; inspect for signs of corrosion, excessive cracking, and deterioration.
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10. Check alignment of fluid drive shafts with fan rotor and motor shafts. 11. Inspect and clean flexible couplings (see Section 7.4.3, “Couplings”). 12. Calibrate vibration and temperature sensors if installed. 13. Flush oil lines with clean hydraulic oil. 7.4.13 Turning Gear The maintenance recommendation for a turning gear consists of the following: 1. Observe for any unusual noise, vibration, odor, or arcing. 2. Inspect electric conduit connections for wear, damage, or looseness. 3. Inspect the following components for signs of leaking lubricant: –
Worm gear reducer
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Clutch
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Flexible coupling
4. Draw an oil sample from the worm gear reducer and conduct a visual inspection, similar to the one for fan bearings. 5. If the worm gear is to be idle for an extended time, do the following: –
Flush the housing with oil containing a vapor phase corrosion inhibitor.
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Replace the breather cap with a pipe plug.
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Ensure that the breather cap is attached to the unit by means of a chain for reinstallation when required.
The scope of an overhaul typically includes the following: 1.
Worm gear: –
Clean and inspect the shaft bearings
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Clean and inspect the motor couplings
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Measure the axial play of the worm shaft
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Replace the oil seal
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Check contact of the gear teeth
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Clean all disassembled parts, and protect them from dirt and moisture
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Replace all seals, shims, and gaskets
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2. Overrunning clutch: –
Thoroughly drain and clean internal parts
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nspect the primary pawls and primary pawl springs for signs of wear
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Inspect ball and roller bearing surfaces and sliding surfaces for signs of wear
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Inspect oil seals for signs of wear
Note: When required, ensure correct angular position of internal components. –
Inspect profile and bore of bearing surfaces
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Inspect stop pins
3. Coupling: The maintenance for the coupling between the fan shaft and the turning gear is similar to that for the coupling between the fan and main motor drive. 7.4.14 Hydraulic Supply System Specific maintenance for this system has the same requirements as a circulating lube oil system. The hydraulic oil system should be inspected similar to the lube oil system (that is, for oil level, temperature, and pressure). As with the lube oil, the hydraulic oil should be analyzed by a laboratory. 7.4.15 Axial Fan Blade Adjustment System The blade adjustment system on an axial fan consists of a hydraulic servo and linkage within the fan rotor. These parts should be inspected and overhauled during every minor outage. The servo is one of the highest maintenance items on an axial fan. 7.4.16 Axial Fan Blade Bearings The blades on an axial fan with variable-pitch blades have bearings that allow the blades to rotate along the blade axis and take the thrust due to the centrifugal load of the blade. These are typically grease-lubricated ball bearings. These bearings should be lubricated using the manufacturer’s recommended grease, and manufacturers typically recommend lubrication at least once a year. Note: The same amount of lubricant should be added to each blade bearing to prevent impeller imbalance. A special injector may be required to add a measured amount of lubricant to each blade bearing.
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7.4.17 Axial Flow Fan Rotor Overhaul Axial fan rotor overhaul is important to maintain high reliability of the fans. Previous studies have shown that there is a significant difference in reliability if the overhauls are not performed. The overhaul should be performed according to the manufacturer’s recommendation, which is typically every four to five years. Some plants ship a spare rotor to the manufacturer’s shop for overhaul; other plants overhaul spare rotors on site. If the overhaul is performed on site, the manufacturer’s service technician should be consulted. The scope of the overhaul usually includes the following:
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Replace the blade bearings
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Replace the blade positioning arms
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Replace the main shaft bearings
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Replace the rotating union
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Inspect and test the blade positioning control system
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Inspect all parts for wear, corrosion, and erosion
Axial fan blades are more prone to erosion than centrifugal fan blades. Some studies indicate that hollow-blade airfoil centrifugal fans can tolerate three times the particle loading that an axial fan can tolerate. However, the axial fan blades are easier to replace. Axial fan blades should be designed to be relatively insensitive to erosion with respect to performance deterioration and structural integrity.
7.5 Fan Wheel Balancing Fan wheel imbalance is cited as the most common cause of vibration; however, this should not deter station personnel from conducting additional checks. Troubleshooting a high-vibration condition can be a complicated process that involves eliminating one possibility at a time. Prematurely deciding that wheel imbalance is the source of a fan’s high vibration may temporarily eliminate the vibration. However, the vibration will resurface (after a period of time depending on its severity) with damage to the affected component (such as bearing or coupling) having subsequently increased. Once it has been determined that the wheel is out-of-balance, the next step is to undertake corrective action. The first step is to clean the entire wheel in an effort to determine whether that will eliminate the imbalance condition. If the imbalance condition persists, the wheel should be balanced by adding weights at the appropriate spots. Adding weights can be necessary in cases where erosive wear of the wheel surfaces has taken place.
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Cleaning the entire wheel is recommended to eliminate an out-of-balance condition. The major advantage of this procedure over spot cleaning is that welding a balance weight on the wheel may not be required. In fact, stringent welding procedures and welding-induced cracking and errors can be avoided altogether. Arbitrarily adding weights without first completely cleaning the wheel can result in a fan wheel that is “covered” with balance weights. This added weight may induce stresses for which the wheel was not originally designed. Based on the operating environment, ID or GR fans are considered more susceptible to wheel imbalance than “clean air” fans. An imbalance condition on an ID fan could be caused by any of the following conditions:
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Fly ash buildup on the blade surface and/or blade interior
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Cracked blades
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Loss of balance weight(s)
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Loose rivets on blades
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Erosion/corrosion of wheel surfaces
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Partial loss of the erosion liner
To avoid vibration problems, it is important to perform vibration trending. Vibration levels taken once every eight hours and trended over time will assist personnel in identifying an imbalance condition in its early stages. Both amplitude and phase measurements are required to balance a draft fan in the field. A variety of instrumentation exists to obtain these two parameters. Automatic balancers are available that provide step-by-step visual instructions to guide the maintenance team through the balancing procedure. This equipment can indicate the amount and location of the correction weight to be added, thus eliminating the need for manual vector diagrams. If an automatic balance is not available, use of a vibration analyzer together with either an oscilloscope or phase meter is suggested. Balancing a fan wheel in the field should be approached in a methodical manner. It is not simply a matter of stopping the fan and attaching a weight. The following steps are presented for use as a check list before actually balancing a fan wheel: 1. Verify that the fan and motor alignments are within recommended tolerances. 2. Ensure that all hold-down bolts are securely tightened. 3. Remove any accumulated buildup that is present on the fan wheel. 4. Verify that all instrumentation (for example, balances, vibration analyzers, and probes) is calibrated and functioning correctly. 5. Conduct a visual inspection of the fan wheel for indications of cracks developing.
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A balancing procedure consists of four steps: 1. Determine the size of the balance weight. 2. Establish the location of the balance weight. 3. Establish a correct welding procedure. 4. Conduct a nondestructive examination of the welds. In all four steps, it is recommended that the fan manufacturer’s requirements and recommendations be thoroughly reviewed and understood. 7.5.1 Size of the Balance Weight The dimensions of a balance weight may have specific limitations and restrictions. The thickness of the balance weight should be equal to or less than the thickness of the side plate, and the balance weight should be rectangular. Consult the manufacturer about any specific length-towidth ratios that may apply. 7.5.2 Location of the Balance Weight The optimum position for balance weights is centered between the blades. Manufacturers may specify minimum distance from blades and the outside diameter of the impeller. Welding a balance weight in an area over the top edge of the blade is not recommended because doing so may result in serious damage to the structural welds that join a side plate and blade. Consult the manufacturer for any restrictions on the minimum separation distance between balance weights. It is important to follow the proper weld procedure, including heat treating, for attaching balance weights to the wheel. Improper welding of balance weights can cause cracks in the attachment weld that can propagate into the base metal of the fan. Key Technical Point Improper welding of balance weights can cause cracks in the attachment weld that can propagate into the base metal of the fan.
7.5.3 Vibration Sensitivity When fans are balanced, the change in vibration and the balance weights added should be recorded. The vibration sensitivity of the fan should also be calculated and recorded. Vibration sensitivity is usually defined in ounces/mil, which is the balance weight in ounces, added at the wheel outer diameter to change the vibration by one mil. The range of typical sensitivity is 8–12 oz/mil. A value less than 8 oz/mil indicates the fan has a high vibration sensitivity and will probably require frequent balancing. 7-38
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8 SPECIAL MAINTENANCE TASKS
8.1 Extended Shutdown During extended shutdown of the fan, the lube oil and hydraulic oil systems should be started at least once every two weeks to maintain an oil film on all surfaces. For axial fans, the impeller should be rotated a few turns to ensure proper lubrication of the bearings. If the fan is shut down for a short time (that is, less than a few weeks), it is better to keep the lube oil and hydraulic oil systems in operation.
8.2 Weld Nondestructive Examination Failures of centrifugal fans, including catastrophic failures as well as cracking problems that have required replacement, have been a chronic industry problem over the past 20 years. The problem is now under control but still requires a significant effort on the part of owners to maintain their fans. NDE of the fans’ welds is a time consuming and expensive process, and the proper inspection including documentation is important. This subject is covered in detail in Appendix B.
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9 FAN UPGRADE OPTIONS
9.1 Reasons for Fan Upgrade ID and FD fans are often cited as the load-limiting factor on fossil-fired units. In some cases, the fan performance may be poorer than design because of wear, housing clearances, or other problems as described in Section 5, “Troubleshooting.” However, the root cause may be due to poor performance of other equipment such as air heater leakage or pluggage, higher-than-design excess air, or higher-than-design flue gas temperatures. Although the fan may not be the root cause of the problem, upgrading the fans may be the most practical solution. In addition to performance problems, fan capacity may need to be increased due to plant modification such as the addition of new pollution control equipment. If additional fan capacity is needed, there are fan upgrades that can be implemented that require only a fraction of the cost and outage time of complete new fans. Key O&M Cost Point If additional fan capacity is needed, there are fan upgrades that can be implemented that require only a fraction of the cost and outage time of complete new fans.
9.2 Tipping The capacity of centrifugal fans with airfoil blades can be increased by tipping the blades. Tipping refers to extending the trailing edge of the fan blades, either with a straight extension or an angle. (Figure 9-1 is a sketch of tipping a fan blade.) As a general rule, tipping will increase the fan head by 10%, the flow by 5%, and the horsepower requirement by 15%. The performance improvement is a “rule of thumb”; actual performance will depend on the fan and is best determined by the fan manufacturer. The cost of tipping is approximately $10,000 per fan. Note that the existing motors may not have the capability to support the required increase in power or inertia.
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Figure 9-1 Airfoil Blade with the Blade Tipped
Replacing the blades with blades with a longer cord can sometimes increase the performance of axial flow fans. The manufacturer would need to review the rotor design to determine whether this alteration is practical. Blade tipping of centrifugal fans or blade replacement of axial flow fans should be investigated if a small increase in fan performance is needed.
9.3 Wheel Replacement The replacement of the wheel in an existing centrifugal fan housing with either a larger diameter wheel or a wheel operating at a higher speed can be a cost-effective alternative to replacing the entire fan or adding booster fans. In most cases, this will require a new motor. A similar type of modification can be made to an axial flow fan by replacing the blades.
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9.4 Housing Modifications In some cases, the housing of a centrifugal fan with airfoil blades can be modified to accommodate a larger wheel or improve performance. One of the critical dimensions is the distance between the wheel and the cutoff. This dimension should be approximately 10% of the wheel diameter. In some cases, the housing has been modified to increase the dimension (when it was less than 10%), resulting in a slight increase in fan performance and a decrease in fan noise. Any housing modifications should be verified with the manufacturer and may require a model study to quantify the impact.
9.5 Coatings Erosion of ID fan blades is a major issue that causes unscheduled downtime as the fly ash constantly bombards the hollow airfoil of the ID fan blades, eventually blowing holes in the blades. The erosion may be caused by burning lower grade coal and/or burning at a reduced temperature. The blades may be coated with a hard surface coating, one of which available in the market is tungsten carbide. There is a conflict between erosion protection and weld inspection. Coatings can be effective in erosion protection but inhibit NDE of fan welds. The coating has to be removed in order to conduct effective magnetic particle examination of the welds.
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10 SAFETY
Operating, inspecting, and maintaining fans and accessories exposes personnel to various safety hazards. Many of the safety hazards associated with fans are present elsewhere in the plant and are therefore not addressed in this guide. All personnel that operate, inspect, or maintain fans should be trained in the plant safety procedures.
10.1 Rotating Equipment Fans are rotating equipment, and appropriate tag-out procedures should be followed before any inspection or maintenance is started. Key Human Performance Point All personnel that operate, inspect, or maintain fans should be trained in the plant safety procedures.
10.2 Confined Space Fan housings are confined spaces that may be filled with harmful gases. Confined space entry procedures, including air checks, should be followed before entry.
10.3 Burn Hazards The lubrication oil of all fans and motors provides lubrication and cooling of the bearings, and the temperature of the oil may be high enough to present a burn hazard. ID fans and GR fans operate at elevated temperatures and may retain heat for a long period after being shut down.
10.4 Electrical The fan motor and fan auxiliaries are electrical and therefore present the risk of electrical shock. Before work on any electrical equipment is performed, the equipment should be de-energized and properly tagged out.
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10.5 Operation Testing Some testing and inspection, such as vibration and performance testing, require the fans to be running during the tests. Special precautions must be observed while performing these tests.
10.6 Cleaning Operations Respiratory health is the primary concern when personnel are involved in abrasive blast cleaning operations. Use of sand abrasives carries with it a specific health hazard termed silicosis. Silicosis is a disease of the lungs resulting from prolonged breathing of very fine particles of silica. The effects of this disease can be permanently disabling or fatal. While the use of grit abrasives does not carry a risk for a specific disease, it nonetheless exposes an individual to some risk. A correct and fully functioning air supply and hood are essential and must be used at all times by personnel engaged in this operation; heavy duty gloves, a helmet or hood, and upper body covering should be standard items as well. A nozzle equipped with a “dead-man” feature should also be included in any type of blast cleaning operations. An air nozzle operating at 100 psi that gets away from an operator could cause serious injury to personnel as well as damage to fan internals. Additional safety-related equipment includes a replacement hood lens (because they tend to become frosted after a period of time) and an ample supply of filters for air supply systems that are outfitted with them. Fans can suck in loose material and discharge it as dangerous projectiles. Large fans can also be dangerous to personnel. Access doors should never be opened with the fan running. On the downstream side, releasing the door may result in explosive opening. On the upstream side, the inflow may be sufficient to suck in materials such as tools and clothing. Key Human Performance Point Fans can suck in loose material and discharge it as dangerous projectiles. Large fans can also be dangerous to personnel.
10.7 Fan Movement Even when a fan is locked out electrically, it may be subject to “windmilling.” Therefore, the impeller should be secured to physically restrict rotational movement. When personnel need to climb on the wheel to perform work or an inspection, the wheel should be mechanically secured. Key Human Performance Point Even when a fan is locked out electrically, it may be subject to “windmilling.”
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Fans contain various rotating components such as fan wheels, shafts, couplings, and shaft coupling wheels—all of which are potential hazards. Therefore, guards should be provided at exposed fan inlets and outlets as well as over the couplings and other equipment. Fans should not be operated above their recommended speeds. Excessive speed may result in catastrophic impeller failure. Impeller rotation should be verified to be correct and not installed backwards. Before a fan is started, it should be ensured that all personnel have exited the fan housing, inlet boxes, inlets, and discharge ductwork. In addition, all material and tools should be removed from the housing.
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11 TRAINING
Formal training programs on boiler draft fans are not readily available. The best source of training is documents on fans. This document provides basic information on the design and operation of fans. More detailed information on fans and their applications can be found in other references. The most comprehensive book on fans is Fan Engineering. The first edition was published by the Buffalo Forge Company in 1914. The ninth edition is available in hard copy or on CD from Howden Buffalo. Fan Engineering covers all types and sizes of fans, including HVAC fans, as well as boiler draft fans.
Steam, Its Generation, and Use (first edition published in 1879; 40th edition available from Babcock and Wilcox) includes a section on fans related to boilers.
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12 REFERENCES 1. NFPA 85, Boiler and Combustion System Hazard Code, 2004. 2. Failure Cause Analysis – Fans. EPRI, Palo Alto, CA: 1981. CS-1693. 3. Electric Motor Predictive and Preventive Maintenance Guide. EPRI, Palo Alto, CA: 1992. NP-7502. 4. AMCA 803, Site Performance Test Standard. 5. Flexible Shaft Couplings Maintenance Guide. EPRI, Palo Alto, CA: 2003. 1007910. 6. Shaft Alignment Guide. EPRI, Palo Alto, CA: 1999. TR-112449. 7. AMCA 202, Fan Application Manual. 8. AMCA 203, Field Performance Measurements. 9. Frank P. Bleier, Fan Handbook: Selection, Application, and Design. TWI Press, Terra Haute, IN 1997. General Reference
Operation and Maintenance Guidelines for Draft Fans, EPRI, Palo Alto, CA: 1993. TR-101698.
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A CENTRIFUGAL FAN WHEEL INSPECTION AND REPAIR
Introduction This section provides guidelines for conducting field inspections of centrifugal fan wheels. An inspection program for fan wheels is a logical extension of any preventive maintenance strategy for a draft fan. A maintenance department can realize two benefits from a well-designed and properly implemented fan inspection program. First, it offers personnel a way to identify cracking or erosion problems for early corrective action, before the damage escalates into major problems with a potential for catastrophic failure. Second, it can help in establishing a database for either a newly installed wheel or one that has been in operation. A database is a good starting point for maintenance personnel before they begin inspecting or conducting repairs. Welldocumented records along with blade maps and photographs can help a station’s maintenance department begin a material history file on the fan wheel. A material history file provides the following:
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A training tool for new personnel to use as a guideline.
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A consistent and reliable way to track surface irregularities that have not been designated as cracks but should be monitored for indications of growth.
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A central listing of information such as chemical composition, hardness, and heat treatment of the metals used to construct the fan wheel; engineering drawing numbers and their locations; electrodes and rods used in the original fabrication and/or repair; modifications made to the fan during its operational life; and contractors used at various times (for example, to clean, repair, and inspect).
•
A record of cracking and erosion problems that can easily be adapted to a trend analysis program. A trend analysis program can serve the following functions: –
A form of input into the procurement of new fan wheels and blades
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A way to identify unique problem areas associated with the fan wheel
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A way to assist in establishing an accurate cycle for fan wheel inspections
There are specific maintenance actions that should encompass a fan wheel inspection. For example, before repairs can begin, it is important to know the exact chemical makeup of the material used in constructing the particular fan wheel component. This particular check need only be done once during the operational life of the fan.
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Inspection Areas Before beginning an inspection, maintenance personnel should consult and review the fan manufacturer’s recommendations on areas to inspect, records of how the fan was constructed, and records of previous fan wheel inspections. Review of previous blade maps in particular will provide invaluable information on alerting personnel to potential problem areas identified by crack patterns reappearing. Recommended areas to be inspected during visual and magnetic particle examinations include the following:
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The entire length of every weld joining the fan blades to the center plate and side plate.
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Splice welds on the side and center plates.
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Welds attaching stiffeners to center plates, hubs, and/or shafts.
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Welds in fabricated hubs.
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Welds attaching balance weights to the side plates or center plate.
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Pad welds used in place of balance weights.
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Seal welds of bolted-on erosion liners. These welds are not structural; however, cracks beginning in these seal welds may propagate into the structural weld or component. These cracks may also form an area on which erosive gas streams may concentrate.
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Cracks in erosion liners made of some hardened material are common but are normally not considered to be detrimental to the fan operation.
Survey Results Survey results involving 256 fans from participating utilities provided information on inspection practices currently being used. Table A-1 provides a summary of the survey results and key points noted.
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Table A-1 Summary of Inspection Practices for Centrifugal Fan Wheels
Visual inspections (VT)
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75% of the fans undergo a visual inspection.
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37% of the total 256 fans have annually scheduled visual inspections.
Nondestructive examinations (NDE)
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65.6% of the fans undergo an NDE inspection.
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41.8% have scheduled annual NDE inspections.
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32.0% undergo a wet-magnetic particle inspection.
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Blade tip speed (BTS) greater than 25,000 fpm
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18.7% of the fans were reported having a BTS greater than 25,000 fpm.
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89.5% of the total 48 fans in this category undergo visual inspections.
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39.5% of the visual inspections are conducted annually.
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79.2% have scheduled NDE inspections.
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29.2% of the NDE inspections are conducted annually.
Number of start/stop cycles greater than 100
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5.8% of the fans were reported having start/stop cycles greater than 100 per year.
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26% of the fans having start/stop cycles greater than 100 also have a BTS greater than 25,000 fpm.
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Only four fans (two FD and two ID) were reported having both a start/stop cycle greater than 100 per year and a BTS greater than 25,000 fpm. The ID fans undergo a 6-month inspection frequency (visual and wet-magnetic particle) while the two FD fans fall under a 24-month inspection cycle.
Scheduled Inspection An average of 70% of the 256 fans receives a regularly scheduled fan wheel inspection of some type. Inspection Techniques VT and wet-magnetic particle are the two most common types of inspection techniques utilized. Inspection Cycle A 12-month period is the most common cycle employed in fan wheel inspections. Other Inspections An average of 50% of the fans that undergo a wheel inspection receives both an annual VT and magnetic particle inspection. Component Marking Procedures Procedures must be established to identify specific blades and/or specific areas of a hub, center plate, and side plates. Identification will allow personnel to document and track crack propagation and to establish a permanent means for component identification. The standard should be consistent with the fan manufacturer’s numbering system, if one exists. Implementing this type of standard will allow for consistency in documentation. It is suggested that each utility, rather than each station, develop a single-unified identification standard for all of their fans.
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It is recommended that the permanent identification marks be centered on the trailing edge of each blade. This area is readily accessible, allowing marks to be easily identified for future inspections. Care should be exercised to avoid creating a stress concentration when marking the blades. The numbering direction could coincide with the fan’s rotation. Top/front and bottom/back of a blade should be identified as an individual observes a fan blade moving toward him. The surface seen first should be the top/front of the blade. Letter identifiers such as “T” for top could be included with the number to further clarify blade identification. Various methods may be used to individually mark blades, such as using a low impact punch to stamp a number on the blade (or other designated location) or using the hub-to-shaft keyway as a standard reference mark. With this technique, the nose of the blade that is closest to the key is always blade Number 1. Blade numbers could increase against fan rotation. Paint or other such markers will eventually wear off during operation or during abrasive blast cleaning operations. Mapping Procedures Mapping all inspection areas is recommended. Diagrams for a blade and center plate are shown in Figures A-1 through A-3. Note that the diagrams do not have to be complex in order to be effective. Accuracy in indicating crack location is essential. Note that Figure A-3 has a grid superimposed that could easily be adapted for pinpoint cracks. A pattern of cracks that continues to repeat itself in a specific area would be cause for concern.
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Figure A-1 Blade Map Example
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Figure A-2 Blade Map Example
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Figure A-3 Blade Map Example
Health Hazards and Safety The fan casing is normally the barrier between the work area and the outside environment. The casing must have adequate dust-tight lighting and ventilation to control the level of dust generation. Ventilation hoses can be placed above or directly across from the area being worked on in order to move the dust away from maintenance personnel. Refer to Section 10 for additional information. Surface Cleaning Surface cleaning is one of the prerequisites for an accurate magnetic particle test. Cleaning provides a surface that is free of dirt, fly ash deposits, and rust, thereby preventing any cracks from being masked. Two methods used in surface cleaning are water blasting and dry blasting. Dry blasting appears to be the most prevalent method used for surface cleaning. One California utility reports successful results using water blasting. Fans are cleaned using glass beads entrained in a 30,000-psi water stream. Reduction in potential respiratory hazards and easy disposal of the residual are two reported benefits of this method.
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Dry blast cleaning is one method used to prepare fan internals for both magnetic and visual inspections. This technique employs several different types of abrasive materials, of which grit and sand are the two most common. No matter what type of abrasive material is used, the principle behind dry blast cleaning remains the same: propel abrasive material in a well-defined stream with sufficient force to dislodge any buildup on the fan surfaces. Grit abrasive material consists of angular metallic particles, such as cast steel, that have been crushed and hardened. This type of material offers several advantages over traditional sand abrasives:
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Less time to clean a fan wheel
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Less breakdown of the abrasive
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Less dust abrasives
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Recycling of grit abrasives (in most cases, they can be “recycled” for reuse 200 times more than conventional sand abrasives)
The disadvantage of using this type of material is that it can remove a layer of material from the fan component.
Sand abrasive is a term used to cover a wide spectrum of nonmetallic abrasives as well as ordinary silica sand. Both natural (for example, flint quartz, silica, slag, and garnet) and manmade (aluminum oxide and silicon carbide) nonmetallic abrasives exist. As would be expected, natural abrasives have the lowest initial cost, followed by man-made abrasives, which are less costly than metallic abrasives. Because sand abrasives are susceptible to breakdown, it is recommended that they be used only once. Reuse of an abrasive that has broken down can result in the material becoming lodged or embedded into crevices that may already exist in the weld, increasing the potential for masking any problem areas. The key to good results is the type of blast nozzle used. According to the American Society for Metals (ASM), the principal materials used in manufacturing a blast nozzle are hard iron, boron carbide, aluminum oxide, and tungsten carbide. ASM recommends purchasing a nozzle with a boron carbide liner encased in a steel jacket. This type of nozzle provides the longest wear per dollar of initial cost. The operational life of a boron carbide nozzle is much longer because of its ability to retain its internal contour. As a result, less air is consumed and a more uniform abrasive velocity and stream contour are maintained. A nozzle should be replaced when its outlet has worn to 1.5 times its original size. There are two abrasive blast techniques that can be used by a cleaning crew. Brush blasting refers to a relatively quick cleaning operation in which the nozzle tip is held between 3 and 4 feet away from the work surface. Brush blasting can be used for rapidly cleaning areas not having a build up of deposits and on surfaces having a protective coating. In the latter case, before any
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
abrasive surface cleaning can take place, the personnel actually doing the work should clearly understand where the coating has been applied and what type of surface finish they will see on the coating after applying the abrasives. Extreme care should be exercised by the crews to avoid “blasting away” the protective coating. As a second recommendation, blade maps should have these coated areas clearly marked to avoid any confusion. The second cleaning technique is often referred to as strip blasting. With this method, personnel should maintain a distance of 9 to 10 inches from the nozzle tip to the work surface. Increasing this distance will reduce the abrasive effect of the material. A shorter distance tends to produce more dust when nonmetallic abrasives are used and tends to cause impact damage and layer removal to the work surface when metallic abrasives are used. This technique is used to clean down to a white metal finish in preparation for conducting NDE tests on welds. When cleaning fans with hollow airfoil blades, crews should avoid directing the abrasive stream on suspected crack areas. There is a potential for “filling” the blade with abrasive material, causing an imbalance condition. To avoid this problem, these suspect areas should be cleaned by hand with a wire brush. When carrying out abrasive cleaning operations, special care must be given to fan internals that could easily be damaged from high-velocity particles impacting them. Heat fingers and shaft seals can be protected by securely wrapping them in a rubber or urethane cover. Operators should exercise caution when working around these components. Surface preparation consists of two parts: maintaining a clean surface once it has been sandblasted and eliminating any surface discontinuities (such as undercutting, overlap, and lack of fusion) that may mask or give false indications. Coordination between the abrasive cleaning team and the station’s NDE inspectors is a must in a geographical area or seasonal period where there is a high level of moisture in the air. This condition will cause corrosion to develop on freshly cleaned fan internals. The time lag between cleaning and inspecting should be kept to a minimum. Better coordination between the abrasive cleaning team and NDE inspectors and the use of electric space heaters inside the fan casing will help reduce moisture and dampness levels. Closing access doors and dampers will also aid in reducing the flow of moisture-laden air into the fan. Visual Inspection Qualified personnel classified as Level II or higher (or Level I personnel under the direct supervision of Level II or III personnel) should conduct the visual inspection of the rotating elements, weld joints, blade surfaces, and hubs. A visual inspection should check for visible cracks and surface discontinuities. It is recommended that any surface discontinuities be removed before conducting a magnetic particle test because these areas can produce false A-10
EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
indications. Often, experienced personnel can successfully excavate the area in question without disturbing the surface to the point where a weld repair would be required. One utility reported that an overlap condition obscured serious underbead cracking. Specific surface discontinuities that should be removed include undercut and overlap (see Figure A-4) and erosion cutting.
Figure A-4 Example of Undercut and Overlap
If both manpower and funding are available, consideration should be given to blend or contour grinding the weld joints. Blend grinding involves the use of a rotary file to remove surface irregularities. One utility takes this process a step further by requiring fan welds to be contour ground. This technique requires the weld to be ground down to a concave shape. Contour grinding, while time consuming and costly, can improve the fatigue strength of the weld by providing a surface that is free from discontinuities. Implementing contour grinding into a fan maintenance program can be accomplished in one of four ways:
•
Establish a general policy that all welds on existing fans shall be contour ground.
•
Require future fan wheel specifications to include contour grinding all welds.
•
Contour grind welds that have been repaired during an outage period.
•
Phase-in contour grinding requirements over a period of years by reworking a fixed amount of fan wheels per year.
Both blend and contour grinding require the use of a rotary file with a grinding surface attachment. Improper use of this tool can result in undersizing the welds, cutting into the base metal, and creating sharp notches in the weld, which will lead to cracking and induce heat cracks in the weld joint. Heat cracks can be avoided by not using excessive pressure (that is, attempting to remove too much material too quickly). Carbide blades will change to a red-hot color if excessive pressure is applied. After blend grinding, scratches can be removed using a belt sander. A-11
EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Although this process is time consuming, it will eliminate false indications of cracks and will result in a higher confidence in the examination. A second option is to use a flapper wheel of emery paper. This generates less heat, making it less likely to produce cracking. Using this tool, however, is extremely time consuming when compared to using a grinder. Once the grinding procedure has been completed, the area to be inspected is wiped down with a lint-free rag. Residual dust and particles from abrasives and grinding operations should be removed before conducting a magnetic particle test. Inspection for any missed problem areas also takes place at this time. A different approach involves the use of experienced NDE personnel to determine whether the indication is a crack or a surface discontinuity based on NDE results. If it is determined to be a potential crack, it is then documented on a map. If subsequent inspections indicate no growth, the indication is downgraded to a surface discontinuity, and no further action is taken unless the test group recommends that the discontinuity be removed. The following points should be noted:
•
Consistent and accurate mapping techniques should be used to make this procedure effective.
•
Even with experienced personnel, there is an element of subjectivity that becomes difficult to control unless an acceptance standard is developed and adhered to.
•
Certain surface discontinuities can lead to cracks.
Chemical Analysis Test The first step in implementing a fan inspection program is verifying the chemistries and heat treatment process applied to the materials used in the construction of the fan wheel components. Although this may appear to be an unnecessary recommendation, experience has shown it to be important on older fans. Several utilities that have analyzed their fans have found the materials to fall outside the ASTM limits. Some catastrophic failures have been attributed at least in part to materials outside the specified limits. An accurate record of the fan wheel’s metallurgical properties can provide the following:
•
Verification that the actual material, hardness, and heat treatment concur with those specified in the original design.
•
Identification of potential problems that may develop with respect to the conditions (such as temperature or weather) under which the fan operates.
•
Verification that current weld repair procedures are correct.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Developing this database for each installed fan at any given power station can be a difficult and costly task. One utility has taken two paths to chemical verification:
•
For existing fans without reliable documentation on file, the utility initiated its own independent chemical analysis testing program. This was a one-time test requirement to obtain a database for each fan, and material samples were taken during normal scheduled outage periods.
•
For new fan wheels being constructed, a quality assurance program was organized to monitor and verify the materials specified. A certified material test report was required from the fan wheel manufacturer for records purposes.
If the utility elects to conduct a chemical analysis test, care should be exercised as to where the samples are taken. Consulting the fan wheel manufacturer before taking any samples is highly recommended to avoid inadvertently damaging the wheel. Portable equipment is now available that allows the chemical analysis of fan components without the removal of material samples. Equipment cost is expensive; however, fan repair is minimized because no samples need to be removed. Magnetic Particle Testing Magnetic particle testing is an effective method for conducting a nondestructive examination to detect cracks on the surface of ferromagnetic materials. The two types of magnetic particle testing are wet particle and dry particle. Input received from the surveys provides the following breakdown (see Table A-2): Table A-2 Percent of Fans Undergoing NDE
Method
% Stations Using the Method
Wet Magnetic particle
32%
Dry magnetic particle
13.7%
Dye penetrant
19.9%
Note that the figures above are straight percentages and do not reflect draft fans that are inspected using two or more of the above methods. Of the 256 draft fans involved in the survey, 45.7% of them received a magnetic particle examination. The wet-magnetic method is more sensitive to fine surface defects than the dry method, making it the preferred method when inspecting for surface cracks. The dry magnetic particle method, however, is considered more sensitive for the detection of subsurface discontinuities. Use of a yoke probe with ac/pulsating dc capability in conjunction with a wet fluorescent magnetic particle (WMP) test offers the best capability to utilities for examining fan wheels for cracks. The WMP medium is available in either a prepared or mixed bath. Prepared baths are A-13
EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
available in aerosol cans. Mixed baths must be prepared by station personnel before use and are considerably less expensive than prepared baths. Additional equipment requirements include a Pie gage to check the magnetic field strength, a black light, and a black light meter. Specific reasons for choosing a wet fluorescent magnetic particle system and yoke probes are listed below:
•
A WMP test is more sensitive and produces more rapid results than a dry particle test.
•
While prods can develop a stronger field, yoke probes will not produce arc burns on the working surface.
•
A yoke probe having both ac and pulsed dc options gives the station more capability. A magnetic field set up with dc power can detect cracks below the surface; this becomes especially useful when inspecting fan hubs for cracks or cracks starting at the roots of the welds.
Pre-Examination Requirements There are several requirements that must be satisfied before conducting a magnetic particle examination:
•
The surface to be examined must be adequately cleaned.
•
When using wet fluorescent particles, the examination must be performed in a darkened area.
•
Designated personnel responsible for carrying out the test must have their eyes adjusted to the dark. This can normally be accomplished by being inside the darkened area for at least five minutes before conducting the test.
•
The black light should be warmed for a minimum of five minutes before use.
•
The intensity of the black light should be measured using a black light meter. A minimum of 800 microwatts/cm2 on the surface being checked is recommended for best results.
•
The black light intensity should be measured at least once every eight hours and whenever shifting to a different fan.
•
The yoke must be calibrated annually.
•
Each ac electromagnetic yoke must have a lifting power of 10 lb at the maximum pole spacing that will be used.
•
Each dc electromagnetic yoke must have lifting power of at least 40 lb at the maximum pole spacing that will be used.
•
Before using a weight for the first time, it must be verified that it has been checked with an accurate scale.
•
A grid must be laid out on the areas to be inspected. The size of the grid should be less than the maximum spacing of the articulated arm to ensure that the entire area is inspected. Chalk or paint markers are typically used to mark the grid spacing on the work.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Examination Procedure This procedure describes the necessary steps to conduct a magnetic particle examination using a yoke with articulated legs (see Figure A-5). This type of yoke provides the inspector with the flexibility needed to accommodate surfaces offset from one another because of curvature or orientation. In addition to being articulated, the legs are also adjustable. The maximum spacing between the legs is 8 inches. Exceeding this limit will result in a weakened magnetic field.
Figure A-5 Hand-Held Yoke Probe with Articulated Legs
After the area to be inspected has been cleaned and prepared, a final check for any residual dust should be conducted before applying the wet fluorescent particle medium to the surface. Once this has been accomplished, the strength and direction of the magnetic field should be verified. A yoke within its calibration period is not a guarantee that sufficient field strength is being generated. The magnetic particle indicator will provide the inspector with a simple indication of direction and sufficient or insufficient field strength. An indicator should be used once per fan to check for proper field strength. Suitable field strength is indicated by a clearly defined line of magnetic particles forming across the copper face of the indicator while the magnetizing force is applied. The direction of the field coincides with the orientation of the formed line. Field direction plays an important role in detecting the presence of cracks. Because of this, it is necessary to examine each area at least twice. As a rule of thumb, the lines of flux in each check should be perpendicular to one another. The weld line can easily be used as a reference to achieve this A-15
EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
condition. For example, with the weld line shown in Figure A-6, orienting the yoke to first straddle the line and then placing both yoke legs on the weld will produce flux lines at right angles. In this case, surface cracks that develop parallel or perpendicular (longitudinal or cross cracks) to the weld can be detected by experienced personnel.
Figure A-6 Orientation of an Articulated Yoke Probe to Produce Flux Lines at Right Angles
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Additional recommendations concerning this technique include the following:
•
The base of the legs should be flush to the surface of the material.
•
When “straddling” the weld line, ensure that the centerline of the yoke coincides with the weld.
•
Each check should have sufficient overlap from the previous check to ensure 100% coverage of the area. Moving the yoke 3 inches to either side of the original position will provide adequate coverage.
The procedure described above uses what is known as the “continuous” method. This method requires inspectors to apply the wet fluorescent medium while the magnetic field is being applied. Follow-Up Examinations A follow-up examination is recommended to include two additional wet-magnetic particle inspections. The purpose of each of these exams is to act as a quality check on accomplished maintenance action. Specifically, a follow-up exam is suggested after the removal of a crack or surface discontinuity to verify that the entire damaged area has in fact been removed before beginning weld repairs. The second time a follow-up exam should be done is after completion of the field weld repairs. Care should be taken by the individuals conducting this examination to ensure that the proper waiting period between completion of the repair and actual inspection is adhered to. Failure to follow the required waiting period can lead to false results. ASTM A514 steels, for example, require a 48-hour waiting period before conducting a final wet magnetic particle examination (and visual examination). DC Coil Wraps This technique was reported in use by one California utility. It consists of wrapping the fan wheel with coils of wire that are energized with dc current. The utility reports that this method provides a significant time savings when compared to using a yoke and results in no loss in sensitivity when used in conjunction with a wet florescent magnetic particle inspection. Wrapping the fan shaft will produce a field in only one direction. A comprehensive examination using this technique would require placing a coil in several positions on the wheel. Marking Indications A standardized marking procedure for fan wheel inspections provides an efficient way for station personnel and contractors involved in the work to communicate. A standardized marking procedure is particularly important when plant maintenance engineers and NDE personnel are assigned to different shifts.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Simplicity and consistency are the key features of a well-thought-out marking system. Simplicity is vital to ensure that personnel spend minimal time and effort when working with the system. Consistency implies standardization; once a system is developed, it should be followed. Use of unauthorized modifications will only cause unnecessary confusion and lost time. The following recommended practices were observed to work effectively in the field (see Figures A-7 and A-8).
•
Use a yellow crayon or grease pencil to mark indications.
•
Number indications consecutively, and circle each number.
•
Annotate multiple indications within a 6-in2 area with a single number and the letters MULT (for multiple indications); place this annotation underneath the corresponding number.
•
Write a red letter “W” to indicate that the indication has been ground and cleared and is ready to be welded.
•
Use the red letters “OK” to show that the indication cleared with minor grinding and that welding is not required.
•
Place a yellow check (“√”) at an indication where the cracks have been ground out to indicate that the area is ready for inspection.
•
Write a red letter “R” to indicate that the completed weld repair has failed the NDE inspection and must be ground out for re-inspection.
•
Write a red letter “P” to indicate that the completed weld repair has passed the NDE inspection.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Figure A-7 Indications Are Consecutively Numbered and Circled
Figure A-8 Marking Multiple Indications
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Interpreting the Indications Wet fluorescent magnetic particle inspection offers the maintenance department a highly effective method of detecting potential problems on a fan wheel. The following points should be noted:
•
The training and experience level of the individuals conducting the test are absolutely critical. AMCA 803, Site Performance Test Standard [4] lists the personnel qualifications needed to conduct and interpret test results.
•
A written and approved fan inspection guideline must be developed for use by the individuals conducting the WMP inspection. If current, these guidelines can provide consistency by defining the acceptance standards to be used during the inspection.
•
As a minimum, utilities are recommended to adapt acceptance standards from American Welding Society (AWS) standard D 14.6-81, “Specification for Welding of Rotating Elements of Equipment,” for magnetic particle inspection of welds, which states the following: –
Cracks, laps, and fissures are not acceptable.
–
Indications in excess of 0.25 inches (regardless of direction) are unacceptable.
–
Indications less than 0.015635 (1/64) inches in length are considered non-relevant.
Note: The acceptance criteria listed above are recommended for fans currently in service. For new fans, several utilities have specified that they are to be constructed with a more stringent requirement of zero linear indications. Ultrasonic Testing From the surveys received, ultrasonic testing (UT) is not widely used. Detection of cracks or voids below the surface is one advantage of UT inspections. Visual or magnetic particle tests offer no real capability in this area. Fan Handbook: Selection, Application, and Design [9] provides current standards and practices for UT inspections. Utilities that employed this technique used it primarily for thickness measurements of fan surfaces (including housings) that operate in an erosive environment. A second application involved the use of UT to inspect for voids in fan hubs manufactured from cast iron or cast steel. This second application was a onetime test. Inspection Frequency The final step in implementing an inspection program is to determine the interval for conducting visual and nondestructive examinations. Careful analysis in deciding this inspection interval is essential. If the time period between inspections is excessive, undetected cracking or erosion could increase in severity.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
If cracks are allowed to go undetected, the potential for serious and costly damage to the fan wheel is clear. The key is early detection. Uncorrected damage can escalate into a catastrophic failure of the fan with the overall operational availability of the unit being jeopardized as well. Early detection is achieved by conducting regular inspections and by carefully noting any upward swings or trends in vibration levels while the draft fan is in operation. Vibration monitoring plays an important role in the process of determining an adequate inspection interval. While high vibration levels can have other causes (such as misalignment or an imbalance condition), vibration due to cracking on the wheel is possible and should not be overlooked. Only through a process of elimination can the cause of vibration be identified and corrected. Therefore, in cases where high vibration levels persist after repeated attempts and after the alignment and wheel balance have been checked, it is advisable to conduct an NDE inspection of the fan wheel. Developing an accurate assessment of the inspection frequency needed requires the maintenance department to collect and analyze the key factors that have an effect on the operational health of the fan wheel. The following factors need to be considered:
•
What does the manufacturer of the fan wheel recommend?
•
What were the results from the previous inspections?
•
What is the operating environment of the fan?
•
Are there sufficient funds in the maintenance budget to support an inspection program?
•
How often has the fan wheel been repaired?
•
Has the fan wheel ever undergone an NDE or VT inspection?
•
What is the inspection cycle currently in use? Is it effective?
•
Who will conduct the inspection? Does the power station have its own manpower assets sufficient to conduct an inspection of the fan wheel?
Manufacturer’s Input Both the manufacturer’s own suggested inspection frequency as well as the feedback it can provide to the utility on current inspection practices are extremely valuable. An exchange of information can be beneficial to both groups. Previous Inspection Results Reviewing blade maps and other related maintenance records from previous inspections provides a mechanism to check for trends. Specific trends that should be cause for concern include the following:
•
Cracks that redeveloped in the same location, suggesting a high-fatigue area or a poor weld repair.
•
Cracks that are increasing in size and/or number. A-21
EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
In either of the above cases, it is strongly recommended that the time between inspections be reduced and that the fan wheel be replaced. Operating Equipment The operating environment of a draft fan is probably the most important factor to consider. The following are specific operating conditions that could lead to fatigue cracks:
•
Start/stop cycles greater than 100 per year.
•
Blade tip speed greater than 25,000 fpm.
•
Temporary excursions by the fan.
Responses to the following questions may also indicate conditions that could lead to fatigue cracks:
•
How well do the isolation dampers and sealing air systems work to protect the fan?
•
Are thermocouples installed to warn operators of a temperature excursion (and are they accurate)?
•
Is there erosion wear caused by fly ash?
•
How well do the precipitators operate? How well are they maintained?
•
Is the surface hardening system in good condition? Are the wear plates in good condition?
•
Is there vibration?
•
Has the fan been operated for long periods with inlet vanes or dampers closed?
Repairs As the number of weld repairs to a fan wheel increases, the fatigue life remaining on the wheel decreases. Because of the lead time involved in manufacturing a new wheel (unless a spare wheel is kept on hand), it is important to take into account the amount of repair work that should be done, to consider reducing the number of start/stop cycles that the wheel currently undergoes, and to reduce the inspection interval to allow for more inspections with subsequent repair work. Initial Inspection If the fan wheel has never undergone an NDE or VT inspection, one should be scheduled as soon as possible. The condition of the fan wheel will determine the schedule of the next inspection. Available Manpower Assets It is easier to schedule inspections when an inspection team is always available. Coordination becomes slightly more difficult when a private contractor or a team that is rotated between stations within the utility’s network is used.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Determining a Cycle for NDE Inspections Input was received from private contractors specializing in NDE inspections, fan manufacturers, and participating utilities. The fan manufacturers and NDE specialists contacted favored annual NDE inspections. Feedback from utilities indicated a wide variety of inspection cycles currently in use. Of particular interest is the group of utilities employing inspection cycles that ranged from 18 to 60 months. These included inspection of fans with blade tip speeds both above and below 25,000 fpm. Generating stations using these extended intervals reported successful results (that is, low incidence of cracks). This indicates that, for given situations, NDE inspection cycles may be extended beyond a one-year period. The following were common features of fans having NDE inspection intervals greater than 12 months:
•
Low number of annual start/stop cycles (typically less than 20 per year).
•
Most fans were monitored for high-vibration levels (vibration monitoring ranged from continuous to every six months).
•
New fan wheels with proper design, low hydrogen weld rod, and proper heat treatment.
For fans meeting the conditions listed above, a 48-month interval is recommended for an NDE inspection cycle. This time period was chosen based on successful results reported by several utilities. An NDE inspection conducted every 12 months or less is recommended for the following conditions:
•
High number of annual start/stop cycles (100 or greater).
•
Persistent high vibration/temperature excursions.
•
Fan wheels identified by the manufacturer as having a high risk of cracking because of their design.
Visual Inspection Cycle A visual inspection is its own unique program and should not be viewed as an initial preparation for a magnetic particle examination. This fact becomes especially evident when NDE inspections extend beyond a year. Annual visual inspections are highly recommended. This interval is chosen based on the following:
•
Feedback from the utilities suggests that annual visual inspection cycles are beneficial.
•
Visual inspections, while not as sensitive as magnetic particle examinations, can still provide valuable information on the condition of the fan wheel. A well-trained individual can quickly identify and check for erosion, gross cracking, and blade damage.
In addition to an annual VT, it is recommended that maintenance engineers conduct VT inspections whenever opportunities arise due to an unscheduled outage. These “inspections of opportunity” can pay large maintenance dividends to a fan wheel’s operational life span. Constant vigilance provides an additional margin of safety to both the operators and the equipment.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Repairs Before beginning repairs, maintenance personnel should be aware of four important considerations. First, fan construction varies from manufacturer to manufacturer. Different operating environments as well as different design philosophies can result in a unique construction for each fan. Second, there are a variety of materials that can be used in the construction of a wheel. Table A-3 provides a sample list of various materials used in the manufacture of blades, center plates, side plates, and hubs. The ASTM A242 material shown in Table A-3 has been used on gas recirculation fans. When drawings show this material, repair should be approached cautiously. This is a low alloy steel specification that does not specify the type or amount of alloy added. The potential trouble area pertains to the high phosphorus allowed by Type 1 in this material specification. As a rule, the high-strength/low-alloy steels have relatively poor low-temperature impact strength. After a few years of service at temperatures above 650ºF, the high phosphorus alloy material is very susceptible to temper brittleness. This does not affect the tensile properties but causes a shift in the ductile-to-brittle transition curve at higher temperatures. Thus, the room temperature impact strength can decrease to values less than 10 ft-lb. If cracks are present, catastrophic failure can occur. A third consideration is defective or incorrect material (see Section 5). In some cases, utilities have discovered material that did not meet the ASTM specifications called for in the design specifications. In other instances, materials different from those shown on the design drawings were found to have been used. These discrepancies can include chemical specifications, Charpy V-notch impact values, or quenching and tempering processes. If this is suspected, a sample from the material should be chemically analyzed. The first requirement in any weld repair is to determine the material type and grade used in the wheel’s construction. Generic terms such as carbon steel, forged steel, or boiler plate should not be accepted as accurate information, because fan wheels can be made of several materials. For example, the center plate could be constructed of a low-alloy carbon steel such as A441, while the blades could be made from A514, which is a high yield strength material. The fourth consideration is that individuals performing the actual weld repair must be fully qualified as required in either ASME Section IX or AWS D1.1 Section 5 parts A, B, and C.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Table A-3 Typical Materials in Power Plant Fans
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Field Weld Repairs and Procedures Weld repair demands attention to detail. This is especially true for high strength steels subject to high fatigue stresses. When repair is necessary because of cracks, it is important that all cracks are removed before repairing. There are seven fundamental rules for weld repair:
•
Make sure that the electrode strength is consistent with design requirements.
•
Make sure that the weld is not a source of any undue hardening or softening of the parent metal.
•
Know the material type and grade of the parent metal.
•
Use the specified electrodes.
•
Follow all instructions concerning preheat, interpass temperatures, and post-heat.
•
Know the handling procedures of the electrodes specified for the job.
•
Follow the prescribed weld procedure for the job.
Review of several repair procedures written by utilities and fan manufacturers indicates a common ground shared by all. Before any welding is attempted to repair a fan, the welder should be familiar with the welding guidelines described in the following paragraphs. Surface Preparation Before welding, surfaces must be clean and dry. Rust is hygroscopic; its removal is an absolute necessity—especially when welding the A514/A517 materials. Grease and oil are also sources of hydrogen that must be removed. The welding procedure should show a sketch of the minimum slope angles of the cavity walls (when a deep cavity results from defect removal). If there is insufficient slope or a sharp corner at the base of the cavity, incomplete fusion may occur. Welding Technique If possible, the repairs should be made in the flat position. For fillet welds, this will likely be impossible; therefore, try to maneuver the fan wheel in such a way that the fillet weld can be made in the horizontal position. For the A514/A517 materials, welds should be made using stringer beads (that is, no weaving of the electrode) to minimize the heat input. One fan manufacturer recommends 60 kilojoules-inch maximum heat input.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Preheat Preheat should be at least 200ºF minimum with a maximum interpass temperature of 400ºF for the quenched and tempered materials. The reason for the higher preheat is to drive off moisture that is likely to be present in the fan installation area. Under certain conditions, such as deep and/or long repair welds that cannot be completed without interruption, it is recommended that either the preheat temperature or a 300ºF post-heat temperature be maintained for about 8 hours. After this time, the weld should be allowed to cool slowly. This can be accomplished by resuming welding the next day and reapplying the preheat. Shielded metal arc welding (SMAW) is the common welding process used for fan repairs. The SMAW electrode that matches A514/A517 materials in strength is E-l 1018-M. To keep welding current (and thus heat input low), the electrode size should not exceed 5/32 inches in diameter. The fan manufacturer should be consulted regarding the use of electrodes lower in strength, such as the E9018-M, which is generally used for tee or corner joints. Use of the E-7018-Al electrode is possible when the higher strength electrode weld repairs result in cracking. The alloy pickup from the base metal can produce welds of about 100 ksi in strength from the E7018-A1. The E7018-A1 electrodes should be baked for 1 hour at 700–800ºF to reduce their moisture content. Portable rod ovens should be used for all electrodes on the A514/A517 materials. Post-Weld Heat Treatment The vanadium containing grades of A514/A517, such as Grade F, are subject to stress relief cracking and should not be subjected to a post-weld heat treatment (PWHT). For the other highstrength grades, the fan manufacturer performs PWHT on the entire wheel. However, a local PWHT may not be practical. For example, high thermal stresses may result from the temperature gradients during PWHT. These could cause cracking problems. One method of eliminating the hard heat-affected zone (HAZ) in all of the A514/A517 materials is to use the temper bead method. With this method, any weld bead deposited on base metal is tempered by the deposit of another bead on top of it that does not contact the base metal. The temper beads may even be removed by grinding, if the contour is improved. One Midwest utility avoids PWHT but performs contour grinding at the toes of the T-joint welds and then has the entire weld shot peened. This is done primarily to improve the fatigue life. Another Midwest utility improves the fatigue life by hollow grinding the T-joint fillet welds (that is, the fillet welds are ground concave). For fans built using structural steel materials, PWHT is not performed. Welder Qualification A welder should be qualified in accordance with either ASME Section IX or AWS Dl. 1. The welder must be qualified for the position(s) he will be welding and for an F-4 electrode. This means that a welder qualified for welding carbon steels such as a A36 with an E-7018 electrode is also qualified to weld the quenched and tempered fan materials with the E-1 1018-M, E-9018M, etc. However, a welder who has never welded high strength materials before could have
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
problems, and it would be advisable that the individual be given some training about the necessity of not deviating from the welding procedure or the electrode storage and handling requirements. For example, the welder should know that even for tack welds or temporary welds, the required preheat must be used. Generic Problems Certain fans have excessive stresses because of their design. Repairing fatigue cracks in such fans will be ongoing until the wheels are replaced. Field Welding Requirements Feedback from several utilities indicates that major weld repairs can be accomplished in the field. Establishing and maintaining a suitable environment is essential to the success of repairs performed on quenched and tempered steels. To maintain a minimum ambient temperature of 50ºF during cold weather, use of portable electric heaters in conjunction with an airlock system is recommended. Use of plastic bulkheads to seal both the inlet and discharge ducts is suggested to reduce drafts. Erecting a temporary plywood vestibule around the opened manway will further help control the ambient temperature inside the fan housing. Use of two doors will help seal the fan housing from a rush of cold air as personnel enter and leave the fan housing. Post-Weld Repair Inspection Criteria There are two types of cracking that can occur when making weld repairs. First, cracking can start during the repair welding process. Such cracks are discovered visually and frequently occur in the HAZ immediately adjacent to the weld. If the welding is interrupted for a significant time, a visual examination should be performed before welding is resumed. Hydrogen-delayed cracking is the second type of cracking that can develop and surface after weld repairs. The chances of hydrogen cracking can be greatly reduced by adhering to correct electrode handling procedures, by using electrodes that have a low-hydrogen coating, and by slowly cooling the welded area to allow hydrogen to escape into the atmosphere. However, in view of the potential for catastrophic failure if any cracking goes undetected, a visual and magnetic particle test should be conducted. For ASTM A514 and A517 steels, a 48-hour waiting period is required before conducting a final VT or wet magnetic particle examination. The wet magnetic particle test is recommended because it is more sensitive to finding very fine cracks. Specific information and pass/fail criteria can be found in the AWS standard Dl.l-1990.
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EPRI Licensed Material Centrifugal Fan Wheel Inspection and Repair
Visual Pass/Fail Criteria The passing criteria for a visual test are provided below:
•
The weld shall have no cracks.
•
Thorough fusion shall exist between adjacent layers of weld metal and between weld metal and base metal.
•
All craters shall be filled to the cross-section of the weld.
•
Weld profiles shall be in accordance with AWS 14.6 or Dl.1.
•
Undercut shall be no more than 0.031 inch (0.1 mm) deep.
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EPRI Licensed Material
B KEY POINT SUMMARY
Key O&M Cost Point Emphasizes information that will result in reduced purchase, operating, or maintenance costs.
Section
Page
Key O&M Cost Point
3.2.7
3-6
The fewest number of fans usually results in the lowest initial cost.
3.11.8
3-38
The best practice is to follow the fan manufacturer’s recommendation. If a change in damper operation is desired, data on startup times and motor current should be collected and discussed with the suppliers of the fan and motor.
6.2
6-10
Oil analysis tests are often offered free or for a nominal charge by the lubricant supplier as part of the overall service provided.
7.1
7-2
Recommendations from the manufacturer and operational experience can provide valuable insight in determining not only what checks should be done, but also how often they should be done.
7.1
7-2
A balance must be attained by performing PM but avoiding opening and inspecting equipment if no problems (such as high temperature or pressure) are present.
7.4.5.2
7-19
Blasting with frozen CO2 pellets has been used for cleaning, with the benefit of eliminating the need to remove the blast medium from the fan.
7.4.9.3
7-27
A simple adjustment of the fan-wheel-to-inlet-cone clearance can affect fan performance by 5% or more.
9.1
9-1
If additional fan capacity is needed, there are fan upgrades that can be implemented that require only a fraction of the cost and outage time of complete new fans.
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EPRI Licensed Material Key Point Summary
Key Technical Point Targets information that will lead to improved equipment reliability.
Section
Page
3.2.1
3-3
Temperature affects fan performance, and thus, a margin on temperature is included to allow for variations in operation.
3.2.2
3-4
When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration.
3.6.3
3-24
The design speed of the turning gear is critical. Most centrifugal fans have sleeve bearings that have a minimum speed. Below the minimum speed, the oil film between the journal and the sleeve is not adequate to prevent metal-to-metal contact, and the bearing will be damaged.
3.7
3-25
In many coal-fired plants, the ID fans are the limiting factor on plant electrical output. Although the ID fans may be the apparent cause of a load limit, in many cases the root cause is high air heater leakage, air heater pluggage, high gas temperatures, precipitator infiltration, or something similar.
3.7
3-25
Note that most curves are labeled as cfm, where it is understood that cfm is acfm. Because fan performance depends on inlet density, the fan curve should specify the density.
3.7
3-26
Fan manufacturers usually present their curves in terms of vane angle, with 90 degrees being the full open position. Many boiler controls identify inlet vane position in terms of percent open, with 100% being full open.
3.7
3-27
The inlet vanes are designed for flow control and not to isolate the fan, and—with the vanes fully closed—the performance will be approximately the same as with the vanes 15 degrees open. Thus, controllability at this low vane opening may be a problem.
3.7.2
3-30
Note that fan static pressure is not the same as static pressure rise.
3.7.2
3-31
The pressure rise across the fan must be converted to the density on the fan curve.
3.8
3-33
Fan performance is based on conditions at the inlet.
6
6-1
The goal of condition monitoring is to identify changes in the condition of the fan, motor, or auxiliary that could indicate some potential failure.
6.1.2.1
6-3
The vibration amplitude-versus-frequency analysis method is considered to be the most useful. Over 85% of mechanical problems occurring on rotating equipment can be identified using this method.
7
7-1
The major maintenance areas for centrifugal fans are the blade liners, main shaft bearings, and inlet vane or inlet damper linkages.
7
7-1
Axial fans require considerably more maintenance than centrifugal fans.
7.4.1
7-6
If oil in a self-contained sump is below the normal level, adding relatively cold oil may disrupt the oil film. Shut down the fan before adding oil.
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Key Technical Point
EPRI Licensed Material Key Point Summary Section
Page
Key Technical Point
7.4.1
7-6
Do not mix oils in the lubrication system or bearing housing. Chemical additives in different oils can cause a breakdown in the viscosity, cooling, or bearing lubrication.
7.4.3.2
7-13
Verify that components used to restrict motor shaft movement are installed.
7.4.5.2
7-20
Coatings can affect the physical properties of the base materials of the fan. Cracks in coatings can propagate into the fan members. Tests using proposed coatings and fan structural material should be performed and evaluated before the coatings are actually used.
7.5.2
7-38
Improper welding of balance weights can cause cracks in the attachment weld that can propagate into the base metal of the fan.
Key Human Performance Point Denotes information that requires personnel action or consideration in order to prevent injury or damage or ease completion of the task.
Section
Page
Key Human Performance Point
3.11.2
3-35
The startup procedures, in addition to the controls and interlocks, should follow the requirements of the current version of NFPA 85.
3.11.9
3-39
The fan control system should monitor the head and flow and give the operator a stall warning so that the operator can take action before a stall occurs.
4.1
4-6
Erosion is a significant failure mechanism for both centrifugal and axial fans. While erosion of ID fans is the major problem, erosion has also been reported on FD fans.
7.4.3.2
7-14
The locking feature of these fasteners becomes compromised when they are removed and reinstalled a certain number of times.
7.4.3.3
7-14
It is important to verify that the thrust load of the fan is not imposed on the motor thrust bearing. This requires knowledge of the magnetic center of the motor.
7.4.5
7-17
Damage (manifested in the form of cracking or corrosive wear) can be partial or total. Unchecked damage may result in an increase in vibration levels with additional wheel damage, which subsequently may increase in severity to catastrophic failure with a serious potential for injury to personnel.
7.4.5.1
7-18
Large centrifugal fan wheels are highly stressed rotating equipment. The design stress may be as high as 80% of yield, and the material may be a quenched and tempered high-strength material such as ASTM A514 or A517, with a yield strength of 100,000 psi. Many large centrifugal fans have had cracks, and there have been a few mechanical failures.
10.1
10-1
All personnel that operate, inspect, or maintain fans should be trained in the plant safety procedures.
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EPRI Licensed Material Key Point Summary
Section
Page
10.6
10-2
Fans can suck in loose material and discharge it as dangerous projectiles. Large fans can also be dangerous to personnel.
10.7
10-2
Even when a fan is locked out electrically, it may be subject to “windmilling.”
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Key Human Performance Point
EPRI Licensed Material
C TRANSLATED TABLE OF CONTENTS
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE, (IV) THAT ANY TRANSLATION FROM THE ENGLISH-LANGUAGE ORIGINAL OF THIS DOCUMENT IS WITHOUT ERROR; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. THE TRANSLATION OF THIS DOCUMENT FROM THE ENGLISH-LANGUAGE ORIGINAL HAS BEEN PREPARED WITH LIMITED BUDGETARY RESOURCES BY OR ON BEHALF OF EPRI. IT IS PROVIDED FOR REFERENCE PURPOSES ONLY AND EPRI DISCLAIMS ALL RESPONSIBILITY FOR ITS ACCURACY. THE ENGLISH-LANGUAGE ORIGINAL SHOULD BE CONSULTED TO CROSS-CHECK TERMS AND STATEMENTS IN THE TRANSLATION. ORGANIZATION(S) THAT PREPARED THIS REPORT EPRI
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RESUME Objectifs • Fournir des informations sur les ventilateurs axiaux et centrifuges utilisés pour les chaudières •
Aider le personnel de maintenance des centrales thermiques dans la détection des pannes des ventilateurs
•
Fournir des conseils de maintenance courante et préventive pour améliorer la fiabilité des ventilateurs
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TABLE DES MATIERES
1 INTRODUCTION .................................................................................................................. 1-1 1.1 Objectifs ......................................................................................................................... 1-1 1.2 Organismes ................................................................................................................... 1-1 1.3 Points clés ..................................................................................................................... 1-2 2 GLOSSAIRE.......................................................................................................................... 2-1 2.1 Termes et acronymes .................................................................................................... 2-1 2.2 Conversions des unités utilisées ............................................................................... 2 ou 3 3 DESCRIPTION TECHNIQUE ............................................................................................... 3-1 3.1 Introduction ................................................................................................................... 3-1 3.2 Applications des Ventilateurs ......................................................................................... 3-3 3.2.1 Ventilateur à circulation induite............................................................................... 3-3 3.2.2 Ventilateur à circulation forcée ............................................................................... 3-3 3.2.3 Equilibre ................................................................................................................. 3-4 3.2.4 Air primaire froid ventilateur ................................................................................... 3-5 3.2.5 Air primaire chaud ventilateur ................................................................................ 3-5 3.2.6 Ventilateurs de recyclage de gaz .......................................................................... 3-6 3.2.7 Quantité deventilateurs .......................................................................................... 3-6 3.3 Types ............................................................................................................................ 3-7 3.3.1 Ventilateurs centrifuges ......................................................................................... 3-7 3.3.2 Ventilateurs axiaux .............................................................................................. 3-13 3.4 Entraînements des Ventilateurs ................................................................................... 3-15 3.5 Commande des Ventilateurs ....................................................................................... 3-16 3.5.1 Commandes centrifuge de ventilateur ................................................................. 3-16 3.5.1.1 Vanne d’admission ...................................................................................... 3-16 3.5.1.2 Amortisseurs d’admission ............................................................................. 3-17 3.5.1.3 Moteurs à deux vitesses .............................................................................. 3-18
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3.5.1.4 À couplage fluide ......................................................................................... 3-18 3.5.1.5 moteurs à Vitesse Variable ........................................................................... 3-19 3.5.1.6 Ventilateurs à Turbine................................................................................... 3-19 3.5.2 Commande axiale de ventilateur ......................................................................... 3-20 3.5.2.1 Hélice à pas variable ................................................................................... 3-20 3.5.2.2 entraînements à Vitesse Variable ................................................................. 3-21 3.5.2.3 Admission variable ....................................................................................... 3-22 3.5.2.4 Ventilateurs de refroidissemnt ..................................................................... 3-22 3.6 Autres composants ...................................................................................................... 3-22 3.6.1 Roulements ......................................................................................................... 3-22 3.6.2 Systèmes de lubrification ..................................................................................... 3-23 3.6.3 Trains de rotation ................................................................................................. 3-24 3.7 Ventilateurs, performance ........................................................................................... 3-24 3.7.1 Performance de ventilateur axial .......................................................................... 3-28 3.7.2 Définition de pression .......................................................................................... 3-29 3.8 Sélection du type de ventilateur .................................................................................. 3-32 3.9 Besoins ........................................................................................................................ 3-33 3.10 Tests .......................................................................................................................... 3-33 3.11 Fonctionnement ......................................................................................................... 3-34 3.11.1 Tests de prédémarrage ..................................................................................... 3-34 3.11.2 Procédures de démarrage ................................................................................. 3-34 3.11.2.2 Séquence des phases pour la mise en route ............................................. 3-35 3.11.3 Conditions d'alarme ............................................................................................ 3-35 3.11.4 Paramètres d'emploi .......................................................................................... 3-35 3.11.5 Mesures d'urgence ............................................................................................ 3-36 3.11.6 Commande ........................................................................................................ 3-36 3.11.6.1 Restrictions de moteur électrique .............................................................. 3-37 3.11.7 Commande d’aubes et d'amortisseurs .............................................................. 3-38 3.11.8 Amortisseurs de sortie ....................................................................................... 3-38 3.11.9 Mesure de prévention pour les ventilateurs axiaux ........................................... 3-39 3.11.10 Arrêt de ventilateur .......................................................................................... 3-39 3.11.10.1 Arrêt contrôlé ........................................................................................... 3-40 3.11.10.2 Arrêt incontrôlé ......................................................................................... 3-40 3.11.11 Fonctionnement en parallèle de ventilateur ..................................................... 3-41 3.12 Normes nationales .................................................................................................... 3-42
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4 MODES DE DEFAILLANCE ET ANALYSES D'EFFETS .................................................... 4-1 4.1 Pales .............................................................................................................................. 4-6 4.2 Roulements ....................................................................................................................4-7 4.3 Fondations .................................................................................................................... 4-8 4.4 Aubes d’admission ......................................................................................................... 4-9 4.5 Accouplements ............................................................................................................ 4-10 4.6 Méchanisme de commande hydraulique ..................................................................... 4-10 4.7 Moteurs électriques ..................................................................................................... 4-10 4.8 Pivots ...........................................................................................................................4-11 4.9 Capots .......................................................................................................................... 4-11 4.10 Axes en mouvement .................................................................................................. 4-12 4.11 Arbre ..........................................................................................................................4-12 4.12 Plaque centrale ......................................................................................................... 4-13 4.13 Amortisseurs d’entrée ............................................................................................... 4-13 4.14 Amortisseurs d’isolement ........................................................................................... 4-13 4.15 Entraînement à Variable-Vitesse................................................................................ 4-14 4.16 Commandes .............................................................................................................. 4-14 4.17 Canalisation ............................................................................................................... 4-14 5 DETECTIONS DES PANNES .............................................................................................. 5-1 6 CONTROLE DE CONDITION................................................................................................ 6-1 6.1 Contrôle de vibration ..................................................................................................... 6-1 6.1.1 Paramètres ............................................................................................................ 6-1 6.1.1.1 amplitudes ..................................................................................................... 6-2 6.1.1.2 Fréquence ...................................................................................................... 6-2 6.1.1.3 Angle de phase .............................................................................................. 6-2 6.1.1.4 Forme de vibration ......................................................................................... 6-2 6.1.1.5 Forme du mode de vibration .......................................................................... 6-3 6.1.2 Analyse de vibration .............................................................................................. 6-3 6.1.2.1 Amplitudes en fonction de l'analyse de fréquence ......................................... 6-3 6.1.2.2 Analyse du spectre du temps réel................................................................... 6-4 6.1.2.3 Analyse de la forme d'onde ............................................................................ 6-4 6.1.3 Sonde de proximité................................................................................................. 6-5 6.1.4 Sondes de vitesse ................................................................................................. 6-6
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6.1.5 Accéléromètrs ........................................................................................................ 6-7 6.1.6 Acquisition de données ......................................................................................... 6-8 6.1.6.1 Tableau de machine ...................................................................................... 6-8 6.1.6.2 Les relevés à trois axes .................................................................................. 6-9 6.2 Analyse d’huile ............................................................................................................ 6-10 6.3 Inspection non destructive ........................................................................................... 6-11 6.4 Thermographie infrarouge ........................................................................................... 6-11 6.5 Analyse du courant moteur ......................................................................................... 6-11 7 MAINTENANCE .................................................................................................................... 7-1 7.1 Développement d’un programme de maintenance préventive....................................... 7-2 7.2 Règles fondamentales pour la conduite de maintenance .............................................. 7-3 7.3 Recommandations de maintenance périodique ............................................................. 7-3 7.4 Maintenance de composants ......................................................................................... 7-5 7.4.1 Roulements ........................................................................................................... 7-6 7.4.1.2 Recommandations courantes de maintenance .............................................. 7-7 7.4.1.2 Changement de roulements............................................................................ 7-8 7.4.2 Système de lubrification ...................................................................................... 7-10 7.4.2.1 Maintenance courante ................................................................................. 7-10 7.4.2.2 Révision du système de circulation d'huile ................................................... 7-10 7.4.3 Accouplements .................................................................................................... 7-12 7.4.3.1 Recommandations courantes de maintenance ............................................ 7-12 7.4.3.2 Changement ................................................................................................ 7-13 7.4.3.3 Alignement d'accouplements ....................................................................... 7-14 7.4.4 Aubes d’admission et amortisseurs ..................................................................... 7-15 7.4.4.1 Maintenance courante ................................................................................. 7-15 7.4.4.2 Révision des aubes ..................................................................................... 7-15 7.4.5 Galets de ventilateur centrifuges ......................................................................... 7-16 7.4.5.1 Galet de ventilateur centrifuge NDE ............................................................ 7-18 7.4.5.2 aube ............................................................................................................. 7-18 7.4.5.3 Plaquecentrale/plaque latérale .................................................................... 7-21 7.4.6 Arbre .................................................................................................................... 7-21 7.4.7 Pivots ................................................................................................................... 7-22 7.4.8 Système structural de support ............................................................................. 7-22 7.4.8.1 Fondation en béton ...................................................................................... 7-22
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7.4.8.2 Réparantion des fondations en béton ........................................................... 7-22 7.4.8.3 Nettoyage extérieur ..................................................................................... 7-23 7.4.8.4 Réparation de fissures ................................................................................. 7-23 7.4.8.5 Boulons d'anchrage ..................................................................................... 7-24 7.4.8.5.1 Forces affectant des boulons d'anchrage ............................................ 7-24 7.4.8.5.2 Installation appropriée .......................................................................... 7-24 7.4.9 Capot ................................................................................................................... 7-25 7.4.9.1 Capot ........................................................................................................... 7-25 7.4.9.2 Cônes .......................................................................................................... 7-26 7.4.9.3 Jeu de galet de ventilateur ........................................................................... 7-26 7.4.9.4 Plaques/trappes d'Access ............................................................................ 7-28 7.4.10 Joints de dilatation ............................................................................................. 7-28 7.4.11 Moteurs électriques ........................................................................................... 7-29 7.4.11.1 Saletés ....................................................................................................... 7-29 7.4.11.2 Humidité ..................................................................................................... 7-30 7.4.11.3 Friction ....................................................................................................... 7-30 7.4.11.4 Vibration ..................................................................................................... 7-30 7.4.11.5 Jeu d'extrémité d'arbre de rotor ................................................................. 7-31 7.4.12 Fluide d’entraînement......................................................................................... 7-32 7.4.12.1 Maintenance courante ............................................................................... 7-32 7.4.12.2 Révision du couplage fluide ....................................................................... 7-33 7.4.13 Trains de rotation ............................................................................................... 7-34 7.4.14 Circuit d'alimentation hydraulique ...................................................................... 7-35 7.4.15 Système axial de réglage de la pale de ventilateur ........................................... 7-35 7.4.16 Roulements axiaux de la pale de ventilateur ..................................................... 7-35 7.4.17 Révision du rotor de ventilateur à écoulement axial ........................................... 7-36 7.5 Équilibrage des galets de ventilateur .......................................................................... 7-36 7.5.1 taille du contrepoids ............................................................................................. 7-38 7.5.2 Emplacement du contrepoids .............................................................................. 7-38 7.5.3 Sensibilité de vibration ......................................................................................... 7-38 8 TACHES SPECIALES DE MAINTENANCE ........................................................................ 8-1 8.1 Arrêt prolongé ................................................................................................................ 8-1 8.2 Inspection non destructive de soudure .......................................................................... 8-1
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9 OPTIONS DE MISE A NIVEAU DU VENTILATEUR ........................................................... 9-1 9.1 Raisons de mise à niveau du ventilateur ....................................................................... 9-1 9.2 Conseils ..........................................................................................................................9-1 9.3 Remontage de galet ...................................................................................................... 9-2 9.4 Modifications du logement.............................................................................................. 9-3 9.5 Revêtements .................................................................................................................. 9-3 10 SECURITE .........................................................................................................................10-1 10.1 Matériel tournant ....................................................................................................... 10-1 10.2 L'espace confiné ....................................................................................................... 10-1 10.3 Risques de brûlure .................................................................................................... 10-1 10.4 Risques électriques .................................................................................................... 10-1 10.5 Contrôle de fonctionnement ...................................................................................... 10-2 10.6 Nettoyage .................................................................................................................. 10-2 10.7 Ventilateurs en mouvement ....................................................................................... 10-2 11 FORMATION ..................................................................................................................... 11-1 12 REFERENCES ................................................................................................................. 12-1 A INSPECTION ET DEPANNAGE DES VENTILATEUR ...................................................... A-1 B RESUME DES POINTS CLES............................................................................................. B-1
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LISTE DE FIGURES Figure 3-1 schéma de circulation d'air de chaudière ................................................................ 3-2 Figure 3-2 d'un ventilateur centrifuge ....................................................................................... 3-8 Figure 3-3 vue en coupe d'un ventilateur centrifuge ................................................................ 3-9 Figure 3-4 composants et accessoires centrifuges de ventilateur ......................................... 3-10 Figure 3-5 rotor avec pales incurvées..................................................................................... 3-11 Figure 3-6 types de pales (vues de l'extrémité d'entraînement) ............................................ 3-12 Figure3-7 composants centrifuges de rotor de ventilateur...................................................... 3-13 Figure 3-8 ventilateur axial à deux étages .............................................................................. 3-14 Figure 3-9 turbine axiale à deux étages.................................................................................. 3-14 Figure 3-10 commande de pales ............................................................................................ 3-17 Figure 3-11 Pas variable......................................................................................................... 3-20 Figure 3-12 palier à manchon ................................................................................................. 3-23 Figure 3-13 ventilateur centrifuge avec les pales variables ................................................... 3-26 Figure 3-14 ventilateur centrifuge avec les pales variables et indicateur................................ 3-27 Figure 3-15 ventilateur centrifuge avec de commande de vitesse ......................................... 3-28 Figure 3-16 Domaine de performance pour ventilateur à écoulement axial .......................... 3-29 Figure 3-17 définitions de pression de ventilateur .................................................................. 3-30 Figure 3-18 Réglage d’aubage en fonction de la densité d’admission .................................. 3-31 Figure 4-1 Composants de capot de ventilateur centrifuge ................................................... 4-12 Figure 7-1 galet de ventilateur centrifuge ............................................................................... 7-17 Figure 7-2 accessoires protecteurs d'usure et d'érosion......................................................... 7-20 Figure 7-3 vue en coupe d'un ventilateur illustrant les conditions de jeu................................ 7-26 Figure 7-4 vue agrandie du schéma précedent ..................................................................... 7-27 Figure 7-5 schéma fonctionnel fondamental d'un système de régulation hydraulique .......... 7-32 Figure 9-1 Aube à spoiler ......................................................................................................... 9-2 Figure A-1 exemple d’aube...................................................................................................... A-6 Figure A-2 exemple d’aube...................................................................................................... A-7 Figure A-3 exemple d’aube...................................................................................................... A-8 Figure A-4 exemple de dégagement et de superposition ..................................................... A-11 Figure A-5 sonde manuelle avec jambe articulée ................................................................. A-15 Figure A-6 orientation d'une sonde de chape articulée pour produire des lignes de flux perpendiculaires..........................................................................................................… A-16
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Figure A-7 Les indications sont à la suite numérotées et entourées .................................... A-19 Figure A-8 marquage des signes multiples ........................................................................... A-19
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LISTE DE TABLEAUX Tableaux 4-1 Résumé des domaines problématiques de ventilateurs centrifuges.................. 4-2 Tableaux 4-2 Résumé des domaines problématiques de ventilateurs axiaux .......................... 4-3 Tableaux 4-3 Caractéristiques de défaut de ventilateur pour les centrales des États-Unis de 1982 à 1995 (NERC/VAGABONDE) ............................................................................4-4 Tableaux 51 Détection des pannes du ventilateur.................................................................... 5-1 Tableaux 5-2 Détection des pannes de roulement ................................................................... 5-3 Tableaux 5-3 Détection des pannes de système de lubrification.............................................. 5-4 Tableaux 5-4Détection des pannes du circuit hydraulique ...................................................... 5-5 Tableaux 5-5 Niveau de bruit de la détection des pannes........................................................ 5-6 Tableaux 5-6 Détection des pannes du couplage fluide .......................................................... 5-7 Tableaux 5-7 Détection des pannes de performance du ventilateur ........................................ 5-9 Tableaux 7-1 Surveillance et les fréquences préventives de maintenance ............................. 7-3 Tableaux A-1 Résumé des pratiques en matière d'inspection pour les galets de ventilateur centrifuges ..................................................................................................... A-3 Tableaux A-2 Pourcentage des ventilateurs subissant des NDE ......................................... A-13 Tableaux A-3 Matériaux types des ventilateurs d'usine électriques ..................................... A-25
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レポートの概要 目的 •
ボイラー・ドラフト・サービス(供給空気、排気空気)に使用する軸流および遠心フ ァンに関する情報を提供すること。
•
問題解決およびファンの保全にあたる火力発電所の所員を助けること。
•
日常および予防保全のガイドをファンの信頼性の向上のため提供すること
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目次
1 はじめに................................................................................................................................. 1-1 1.1 目的 ................................................................................................................................ 1-1 1.2 構成 ................................................................................................................................ 1-1 1.3 キーポイント.................................................................................................................. 1-2 2 用語集 .................................................................................................................................... 2-1 2.1 用語および頭辞語........................................................................................................... 2-1 2.2 このレポートで使用される単位系の換算....................................................................... 2-3 3 技術的な記述 ......................................................................................................................... 3-1 3.1 はじめに ......................................................................................................................... 3-1 3.2 ファンのアプリケーション ............................................................................................ 3-3 3.2.1 吸出し送風機 .......................................................................................................... 3-3 3.2.2 押込送風機.............................................................................................................. 3-3 3.2.3 平衡通風 ................................................................................................................. 3-4 3.2.4 冷温一次空気ファン ............................................................................................... 3-5 3.2.5 高温一次空気ファン ............................................................................................... 3-5 3.2.6 ガスの再循環ファン ............................................................................................... 3-6 3.2.7 ファンの番号 .......................................................................................................... 3-6 3.3 ファンのタイプ .............................................................................................................. 3-7 3.3.1 遠心ファン.............................................................................................................. 3-7
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3.3.2 軸流ファン............................................................................................................ 3-13 3.4 ファン駆動 ................................................................................................................... 3-15 3.5 ファン制御 ................................................................................................................... 3-16 3.5.1 遠心ファン制御 .................................................................................................... 3-16 3.5.1.1 入口のベーン ................................................................................................ 3-16 3.5.1.2 入口のダンパー ............................................................................................ 3-17 3.5.1.3 二段変速式モーター ..................................................................................... 3-18 3.5.1.4 流体駆動 ....................................................................................................... 3-18 3.5.1.5 可変速度モーター ........................................................................................ 3-19 3.5.1.6 タービン駆動ファン...................................................................................... 3-19 3.5.2 軸流ファン制御 .................................................................................................... 3-20 3.5.2.1 可変ピッチブレード...................................................................................... 3-20 3.5.2.2 可変速度駆動機構 ......................................................................................... 3-21 3.5.2.3 可変入口ベーン............................................................................................. 3-22 3.5.2.4 冷却ファン .................................................................................................... 3-22 3.6 他の部品 ....................................................................................................................... 3-22 3.6.1 ベアリング............................................................................................................ 3-22 3.6.2 潤滑油システム .................................................................................................... 3-23 3.6.3 回転ギヤ ............................................................................................................... 3-24 3.7 ファンパフォーマンス ................................................................................................. 3-24 3.7.1 軸流ファンパフォーマンス .................................................................................. 3-28 3.7.2 ファン圧力の定義................................................................................................. 3-29 3.8 ファンタイプの選択 ..................................................................................................... 3-32 3.9 ファンの要求事項......................................................................................................... 3-33 3.10 ファンテスト.............................................................................................................. 3-33
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3.11 運転 ............................................................................................................................ 3-34 3.11.1 運転前チェック .................................................................................................. 3-34 3.11.2 起動手順 ............................................................................................................. 3-34 3.11.2.2 開始のためのステップのシーケンス .......................................................... 3-35 3.11.3 モニタへのアラーム条件 .................................................................................... 3-35 3.11.4 モニタへのオペレーティングパラメータ........................................................... 3-35 3.11.5 緊急アクション .................................................................................................. 3-36 3.11.6 ファン制御.......................................................................................................... 3-36 3.11.6.1 電動機の制限 .............................................................................................. 3-37 3.11.7 ベーンおよびダンパーの制御............................................................................. 3-38 3.11.8 ファン出口のダンパー ....................................................................................... 3-38 3.11.9 軸流ファンのためのストールの防止 .................................................................. 3-39 3.11.10 ドラフトファン停止 ......................................................................................... 3-39 3.11.10.1 必要とされる停止 ..................................................................................... 3-40 3.11.10.2 それ以外の停止......................................................................................... 3-40 3.11.11 並行ファン運転 ................................................................................................ 3-41 3.12 国の基準 ..................................................................................................................... 3-42 4 故障モードおよび効果の分析 ................................................................................................ 4-1 4.1 ブレード ......................................................................................................................... 4-6 4.2 ベアリング ..................................................................................................................... 4-7 4.3 基礎 ................................................................................................................................ 4-8 4.4 入口ベーン ..................................................................................................................... 4-9 4.5 カップリング................................................................................................................ 4-10 4.6 油圧作動メカニズム ..................................................................................................... 4-10 4.7 電動機 .......................................................................................................................... 4-10
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4.8 ハブ .............................................................................................................................. 4-11 4.9 ハウジング ................................................................................................................... 4-11 4.10 回転ギヤ ..................................................................................................................... 4-12 4.11 シャフト ..................................................................................................................... 4-12 4.12 センタープレート....................................................................................................... 4-13 4.13 入口のダンパー ......................................................................................................... 4-13 4.14 ダンパーの隔離 .......................................................................................................... 4-13 4.15 可変速度駆動機構....................................................................................................... 4-14 4.16 制御 ............................................................................................................................ 4-14 4.17 ダクト作業 ................................................................................................................. 4-14 5 問題解決................................................................................................................................. 5-1 6 コンディションモニター ...................................................................................................... 6-1 6.1 振動モニタリング........................................................................................................... 6-1 6.1.1 パラメータ.............................................................................................................. 6-1 6.1.1.1 振幅(アンプ).................................................................................................... 6-2 6.1.1.2 振動数 ............................................................................................................. 6-2 6.1.1.3 位相角 ............................................................................................................. 6-2 6.1.1.4 振動形式 ......................................................................................................... 6-2 6.1.1.5 振動モードの形............................................................................................... 6-3 6.1.2 振動解析 ................................................................................................................. 6-3 6.1.2.1 振幅対周波数解析 ........................................................................................... 6-3 6.1.2.2 リアルタイムのスペクトル解析...................................................................... 6-4 6.1.2.3 時間波形解析 .................................................................................................. 6-4 6.1.3 近接度のプローブ................................................................................................... 6-5 6.1.4 速度のプローブ ...................................................................................................... 6-6
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6.1.5 加速度のプローブ................................................................................................... 6-7 6.1.6 データ収集.............................................................................................................. 6-8 6.1.6.1 機械図表 ......................................................................................................... 6-8 6.1.6.2 三軸の 読み ..................................................................................................... 6-9 6.2 油の分析 ....................................................................................................................... 6-10 6.3 非破壊的な検査 ............................................................................................................ 6-11 6.4 赤外線サーモグラフィー ............................................................................................. 6-11 6.5 モーター電流の分析 ..................................................................................................... 6-11 7 保全........................................................................................................................................ 7-1 7.1 予防保全プログラムの構築 ............................................................................................ 7-2 7.2 保全を行なうための基本ルール ..................................................................................... 7-3 7.3 定期的な保全の提言 ....................................................................................................... 7-3 7.4 機器の保全 .................................................................................................................... 7-5 7.4.1 ベアリング.............................................................................................................. 7-6 7.4.1.2 日常的保全の提言 ........................................................................................... 7-7 7.4.1.2 ベアリングの分解 ........................................................................................... 7-8 7.4.2 潤滑油システム .................................................................................................... 7-10 7.4.2.1 日常的保全 ................................................................................................... 7-10 7.4.2.2 循環潤滑油システム分解 .............................................................................. 7-10 7.4.3 カップリング ........................................................................................................ 7-12 7.4.3.1 日常的保全の提言 ......................................................................................... 7-12 7.4.3.2 カップリングの分解...................................................................................... 7-13 7.4.3.3 カップリングのアラインメント.................................................................... 7-14 7.4.4 可変的な入口ベーンおよび制御ダンパー ............................................................ 7-15 7.4.4.1 日常的保全 ................................................................................................... 7-15
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7.4.4.2 入口ベーンの分解 ......................................................................................... 7-15 7.4.5 遠心ファンウィール ............................................................................................. 7-16 7.4.5.1 遠心ファンウィールのNDE ......................................................................... 7-18 7.4.5.2 ブレード ....................................................................................................... 7-18 7.4.5.3 センタープレート/サイドプレート ............................................................... 7-21 7.4.6 シャフト ............................................................................................................... 7-21 7.4.7 ハブ ...................................................................................................................... 7-22 7.4.8 構造サポートシステム.......................................................................................... 7-22 7.4.8.1 コンクリートの基礎...................................................................................... 7-22 7.4.8.2 コンクリートの基礎の修理........................................................................... 7-22 7.4.8.3 表面のクリーニング...................................................................................... 7-23 7.4.8.4 ひび修理 ....................................................................................................... 7-23 7.4.8.5 アンカー・ボルト ......................................................................................... 7-24 7.4.8.5.1 アンカー・ボルトに加わる力 ............................................................... 7-24 7.4.8.5.2 適切な据付け......................................................................................... 7-24 7.4.9 ハウジング............................................................................................................ 7-25 7.4.9.1 ハウジング .................................................................................................... 7-25 7.4.9.2 入口のコーン ................................................................................................ 7-26 7.4.9.3 ファンウィールのクリアランス.................................................................... 7-26 7.4.9.4 アクセスプレート/ドア ................................................................................. 7-28 7.4.10 伸縮継手 ............................................................................................................. 7-28 7.4.11 電動機................................................................................................................. 7-29 7.4.11.1 ほこり ......................................................................................................... 7-29 7.4.11.2 湿気............................................................................................................. 7-30 7.4.11.3 摩擦............................................................................................................. 7-30
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7.4.11.4 振動............................................................................................................. 7-30 7.4.11.5 ローターシャフトのエンドプレー ............................................................. 7-31 7.4.12 流体継手 ............................................................................................................. 7-32 7.4.12.1 日常的保全 ................................................................................................. 7-32 7.4.12.2 流体継手の分解........................................................................................... 7-33 7.4.13 ターニングギヤ .................................................................................................. 7-34 7.4.14 油圧供給方式 ...................................................................................................... 7-35 7.4.15 軸流ファン・ブレードの調節システム .............................................................. 7-35 7.4.16 軸流ファン・ブレードベアリング...................................................................... 7-35 7.4.17 軸流ファン回転子の分解 .................................................................................... 7-36 7.5 ファンウィールのバランス .......................................................................................... 7-36 7.5.1 バランス重量のサイズ.......................................................................................... 7-38 7.5.2 バランス重量の位置 ............................................................................................. 7-38 7.5.3 振動感度 ............................................................................................................... 7-38 8 特別な保全タスク .................................................................................................................. 8-1 8.1 延長された停止期間 ....................................................................................................... 8-1 8.2 溶接非破壊的試験........................................................................................................... 8-1 9 ファンアップグレードオプション ......................................................................................... 9-1 9.1 ファンアップグレードの理由......................................................................................... 9-1 9.2 ティッピング.................................................................................................................. 9-1 9.3 ウィールの取替 .............................................................................................................. 9-2 9.4 ハウジングの改造........................................................................................................... 9-3 9.5 コーティング.................................................................................................................. 9-3 10 安全................................................................................................................................ 10-1年
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10.1 回転装置 ................................................................................................................. 10-1年 10.2 閉所 ........................................................................................................................ 10-1年 10.3 火災の危険 ............................................................................................................. 10-1年 10.4 電気 ........................................................................................................................ 10-1年 10.5 運転のテスト.......................................................................................................... 10-2年 10.6 清浄作業 ................................................................................................................. 10-2年 10.7 ファンの動き.......................................................................................................... 10-2年 11トレーニング .................................................................................................................. 11-1年 12 参照................................................................................................................................ 12-1年 A 遠心ファンウィール点検および修理.................................................................................... A-1 Bキーポイントの概要 .............................................................................................................. B-1
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図のリスト 図3-1 ボイラー空気流の図........................................................................................................ 3-2 図3-2 遠心ファンの断面図........................................................................................................ 3-8 図3-3 遠心ファンの断面図........................................................................................................ 3-9 図3-4 遠心ファン機器およびアクセサリ ................................................................................ 3-10 図3-5 遠心ファンの前方に曲げられたブレードが付いた典型的な回転子 .............................. 3-11 図3-6 ウィールのブレードのタイプおよび回転(駆動端からの予想図) .................................. 3-12 図3-7 遠心ファン回転子の部品............................................................................................... 3-13 図3-8 二段式軸流ファンアセンブリ ....................................................................................... 3-14 図3-9 二段式軸流ファンインペラー ...................................................................................... 3-14 図3-10 入口ベーンの制御装置 ................................................................................................ 3-17 図3-11 可変ピッチの軸流ファン構成部品 .............................................................................. 3-20 図3-12 スリーブ軸受けの構成部品 ......................................................................................... 3-23 図3-13 可変の入口ベーンを持つ典型的な遠心ファン ............................................................ 3-26 図3-14 可変の入口ベーンおよび典型的な遠心ファンとシステムカーブ................................ 3-27 図3-15 速度制御を持つ典型的な遠心ファン........................................................................... 3-28 図3-16 可変ピッチの軸流ファンのためのヘッド-流量の図 .................................................... 3-29 図3-17 ファンの圧力定義 ....................................................................................................... 3-30 図3-18 入口流の密度に応じたファン性能の訂正 ................................................................... 3-31 図4-1 遠心ファンハウジングの構成部品 ................................................................................ 4-12 図7-1 遠心ファンウィール...................................................................................................... 7-17 図7-2 摩耗およびエロージョンの保護のアクセサリ .............................................................. 7-20 図7-3 ウィールの入口とインレットベルの間のクリアランスを示すファンの断面図 ........... 7-26
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図7-4 図7-3の拡大図
.............................................................................................7-27
図7-5 油圧調整システムのの基本的なブロックダイヤグラム ................................................ 7-32 図9-1 ティッピングされたブレードが付いている翼ブレード .................................................. 9-2 図A-1 ブレードのマップの例................................................................................................... A-6 図A-2 ブレードのマップの例................................................................................................... A-7 図A-3 ブレードのマップの例................................................................................................... A-8 図A-4 アンダーカットとオーバーラッブの例........................................................................ A-11 図A-5 連結された足を持つ手持ち型のヨークのプローブ...................................................... A-15 図A-6 フラックスラインを直角にするためのカップリングされたヨークのプローブの方向 ............… A-16
図A-7 指示は連続的に番号が付けられ、丸で囲まれている。 .............................................. A-19 図A-8 複数の指示................................................................................................................... A-19
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表のリスト 表4-1 遠心ファン問題の概要 .................................................................................................... 4-2 表4-2 軸流ファン問題の概要 .................................................................................................... 4-3 表4-3 1982年から1995年までの米国の化石プラントのFDファン不良データを集めたもの。 (NERC/GADSデータ )........................................................................................................4-4 表5-1 問題解決 ファン ........................................................................................................... 5-1 表5-2 問題解決 ベアリング .................................................................................................... 5-3 表5-3 問題解決 潤滑油システム ............................................................................................. 5-4 表5-4 問題解決 油圧装置........................................................................................................ 5-5 表5-5 問題解決 騒音レベル .................................................................................................... 5-6 表5-6 問題解決 流体駆動........................................................................................................ 5-7 表5-7 問題解決 ファンパフォーマンス .................................................................................. 5-9 表7-1 監視および予防保全の頻度 ............................................................................................. 7-3 表A-1 遠心ファンウィールの点検方法の概要.......................................................................... A-3 表A-2 NDEを受けるファンのパーセント .............................................................................. A-13 表A-3 発電所の典型的なファンの材料 .................................................................................. A-25
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RESUMEN DEL REPORTE Objetivos • Para ofrecer información en los ventiladores axial y centrífugos usados para servicio de corriente de aire de caldera •
Para asistir a el personal de mantenimiento de la central eléctrica fósil en localización de averías y mantenimiento de ventiladores
•
Para ofrecer guía en mantenimiento rutinario y preventivo para asistir en mejorar la confiabilidad de ventiladores
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CONTENIDO
1 INTRODUCCIÓN ....................................................................................................................1-1 1.1 Propósito .........................................................................................................................1-1 1.2 Organización ...................................................................................................................1-1 1.3 Puntos Claves .................................................................................................................1-2 2 GLOSARIO DE TÉRMINOS ...................................................................................................2-1 2.1 Términos y Siglas............................................................................................................2-1 2.2 Conversiones para Unidades Usadas en Este Reporte..................................................2-3 3 DESCRIPCIÓN TÉCNICA ......................................................................................................3-1 3.1 Introducción.....................................................................................................................3-1 3.2 Aplicaciones de Ventilación.............................................................................................3-3 3.2.1 Ventiladores de Corriente de Aire Inducido.............................................................3-3 3.2.2 Ventiladores de Corriente de Aire Forzado .............................................................3-3 3.2.3 Corriente de Aire Balanceada .................................................................................3-4 3.2.4 Ventiladores de Aire Primario Frió ..........................................................................3-5 3.2.5 Ventiladores de Aire Primario Caliente....................................................................3-5 3.2.6 Ventiladores de Recirculación de Gas ....................................................................3-6 3.2.7 Números de Ventiladores ........................................................................................3-6 3.3 Tipos de Ventiladores......................................................................................................3-7 3.3.1 Ventiladores Centrifugo ...........................................................................................3-7 3.3.2 Ventiladores Axiales ..............................................................................................3-13 3.4 Impulsores de Ventiladores ..........................................................................................3-15 3.5 Controles de Ventilador.................................................................................................3-16 3.5.1 Controles de Ventilador Centrifugo .......................................................................3-16 3.5.1.1 Aletas de Entrada ..........................................................................................3-16 3.5.1.2 Amortiguadores de Entrada ...........................................................................3-17 3.5.1.3 Motores de Dos-Velocidades.........................................................................3-18
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3.5.1.4 Conductor de Fluido ......................................................................................3-18 3.5.1.5 Motores de Velocidad-Variable......................................................................3-19 3.5.1.6 Ventiladores Impulsados con Turbina............................................................3-19 3.5.2 Control de Ventilador Axial ....................................................................................3-20 3.5.2.1 Alabes de Pendiente-Variable .......................................................................3-20 3.5.2.2 Conductores de Velocidad-Variables.............................................................3-21 3.5.2.3 Aletas de Entrada Variables ..........................................................................3-22 3.5.2.4 Ventiladores de Enfriamiento.........................................................................3-22 3.6 Otros Componentes ......................................................................................................3-22 3.6.1 Cojinetes................................................................................................................3-22 3.6.2 Sistemas de Lubricación .......................................................................................3-23 3.6.3 Motor de Girar ......................................................................................................3-24 3.7 Funcionamiento del Ventilador......................................................................................3-24 3.7.1 Funcionamiento del Ventilador Axial .....................................................................3-28 3.7.2 Definición de la Presión del Ventilador..................................................................3-29 3.8 Selección de Tipo de Ventilador....................................................................................3-32 3.9 Requisitos del Ventilador...............................................................................................3-33 3.10 Pruebas del Ventilador ................................................................................................3-33 3.11 Operación....................................................................................................................3-34 3.11.1 Verificaciones de Prearranque ............................................................................3-34 3.11.2 Procedimientos de Arranque ...............................................................................3-34 3.11.2.2 Secuencias de Pasos para Arranque ..........................................................3-35 3.11.3 Condiciones de Alarma para Monitor ..................................................................3-35 3.11.4 Parámetros de Operación Para Monitor..............................................................3-35 3.11.5 Acciones de Emergencia.....................................................................................3-36 3.11.6 Control del Ventilador ..........................................................................................3-36 3.11.6.1 Restricciones del Motor Eléctrico.................................................................3-37 3.11.7 Control de Aletas y Amortiguadores....................................................................3-38 3.11.8 Amortiguadores de Salida de Ventilador .............................................................3-38 3.11.9 Prevención de Ahogue para Ventilador Axial .....................................................3-39 3.11.10 Apague de Ventilador de Corriente de Aire.......................................................3-39 3.11.10.1 Parada Controlada.....................................................................................3-40 3.11.10.2 Parada Incontrolable .................................................................................3-40 3.11.11 Operación de Ventilador Paralelo......................................................................3-41 3.12 Normas Nacionales .....................................................................................................3-42
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4 MODOS DE FALLO Y ANÁLISIS DE EFECTOS .................................................................4-1 4.1 Alabes .............................................................................................................................4-6 4.2 Cojinetes .........................................................................................................................4-7 4.3 Fundaciones....................................................................................................................4-8 4.4 Aleta de Entrada..............................................................................................................4-9 4.5 Acoplamientos...............................................................................................................4-10 4.6 Mecanismo que se Actúa Hidráulico .............................................................................4-10 4.7 Motores Eléctricos.........................................................................................................4-10 4.8 Bujes .............................................................................................................................4-11 4.9 Alojamiento ...................................................................................................................4-11 4.10 Engranajes de Giro .....................................................................................................4-12 4.11 Eje ...............................................................................................................................4-12 4.12 Placa Central...............................................................................................................4-13 4.13 Amortiguadores de Entrada ........................................................................................4-13 4.14 Amortiguadores Aislados ............................................................................................4-13 4.15 Impulso de Velocidad-Variable....................................................................................4-14 4.16 Controles .....................................................................................................................4-14 4.17 Canalización................................................................................................................4-14 5 LOCALIZANDO AVERÍAS.....................................................................................................5-1 6 VIGILANCIA DE CONDICION................................................................................................6-1 6.1 Vigilancia de Vibración ....................................................................................................6-1 6.1.1 Parámetros ..............................................................................................................6-1 6.1.1.1 Amplitud...........................................................................................................6-2 6.1.1.2 Frecuencia .......................................................................................................6-2 6.1.1.3 Angulo de Fase................................................................................................6-2 6.1.1.4 Forma de Vibración..........................................................................................6-2 6.1.1.5 Forma de Modo de Vibración...........................................................................6-3 6.1.2 Análisis de Vibración ..............................................................................................6-3 6.1.2.1 Amplitudes Contra Análisis de Frecuencia ......................................................6-3 6.1.2.2 Análisis del Espectro del Tiempo Real ............................................................6-4 6.1.2.3 Análisis de Forma de Onda de Tiempo ...........................................................6-4 6.1.3 Probetas de Proximidad ..........................................................................................6-5 6.1.4 Probetas de Velocidad ............................................................................................6-6
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6.1.5 Probetas del Acelerómetro ......................................................................................6-7 6.1.6 Adquisición de Data.................................................................................................6-8 6.1.6.1 Diagrama de Maquina .....................................................................................6-8 6.1.6.2 Lecturas de Tres-Axial .....................................................................................6-9 6.2 Análisis de Aceite .........................................................................................................6-10 6.3 Examinación Indestructiva ............................................................................................6-11 6.4 Termografía Infrarrojo ...................................................................................................6-11 6.5 Análisis de Corriente de Motor ......................................................................................6-11 7 MANTENIMIENTO..................................................................................................................7-1 7.1 Desarrollando un Programa de Mantenimiento Preventivo.............................................7-2 7.2 Reglas Básicas para Mantenimiento Conductivo............................................................7-3 7.3 Recomendaciones de Mantenimiento Periódica .............................................................7-3 7.4 Mantenimiento de Componente ......................................................................................7-5 7.4.1 Cojinetes..................................................................................................................7-6 7.4.1.2 Recomendaciones de Mantenimiento Rutinario ..............................................7-7 7.4.1.2 Revisión de Cojinete ........................................................................................7-8 7.4.2 Sistema de Lubricación ........................................................................................7-10 7.4.2.1 Mantenimiento de Rutina ...............................................................................7-10 7.4.2.2 Revisión de Sistema de Aceite de Lubricación Circulando............................7-10 7.4.3 Acoplamientos .......................................................................................................7-12 7.4.3.1 Recomendaciones de Mantenimiento Rutinario ............................................7-12 7.4.3.2 Revisión de Acoplamiento .............................................................................7-13 7.4.3.3 Alineamiento de Acoplamiento ......................................................................7-14 7.4.4 Aletas de Entrada Variable y Amortiguadores de Control .....................................7-15 7.4.4.1 Mantenimiento de Rutina ...............................................................................7-15 7.4.4.2 Revisión de Aletas de Entrada.......................................................................7-15 7.4.5 Ruedas de Ventilador Centrifugo ..........................................................................7-16 7.4.5.1 Rueda de Ventilador Centrifugo NDE ............................................................7-18 7.4.5.2 Alabes ............................................................................................................7-18 7.4.5.3 Placa Central/Placa de Lado .........................................................................7-21 7.4.6 Eje .........................................................................................................................7-21 7.4.7 Buje .......................................................................................................................7-22 7.4.8 Sistema de Soporte Estructural.............................................................................7-22 7.4.8.1 Fundación de Concreto..................................................................................7-22
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7.4.8.2 Reparando Fundaciones de Concreto ...........................................................7-22 7.4.8.3 Limpieza de Superficie...................................................................................7-23 7.4.8.4 Reparación de Grieta.....................................................................................7-23 7.4.8.5 Tornillos de Anclaje .......................................................................................7-24 7.4.8.5.1 Fuerzas Afectando los Tornillos de Anclaje ...........................................7-24 7.4.8.5.2 Instalación Apropiada.............................................................................7-24 7.4.9 Alojamiento............................................................................................................7-25 7.4.9.1 Alojamiento ....................................................................................................7-25 7.4.9.2 Conos de Entrada ..........................................................................................7-26 7.4.9.3 Huelgo de Rueda de Ventilador.....................................................................7-26 7.4.9.4 Placas de Acceso/Puertas .............................................................................7-28 7.4.10 Juntas de Expansión ...........................................................................................7-28 7.4.11 Motores Eléctricos ...............................................................................................7-29 7.4.11.1 Suciedad .................................................................................................... 7-29 7.4.11.2 Humedad ................................................................................................... 7-30 7.4.11.3 Fricción ...................................................................................................... 7-30 7.4.11.4 Vibración .................................................................................................... 7-30 7.4.11.5 Juego de Extremo de la Varilla del Rotor .................................................. 7-31 7.4.12 Conductores de Fluidos.......................................................................................7-32 7.4.12.1 Mantenimiento de Rutina .............................................................................7-32 7.4.12.2 Conductores de Fluidos Revisados .............................................................7-33 7.4.13 Engranajes de Giro..............................................................................................7-34 7.4.14 Sistema de Suministro Hidráulico........................................................................7-35 7.4.15 Sistema de Ajuste de Alabe de Ventilador Axial .................................................7-35 7.4.16 Cojinete de Alabe de Ventilador Axial .................................................................7-35 7.4.17 Revisión del Rotor de Ventilador de Flujo Axial ..................................................7-36 7.5 Balanceo de Rueda de Ventilador.................................................................................7-36 7.5.1 Tamaño de el Peso Balanceado ...........................................................................7-38 7.5.2 Localización de el Peso Balanceado.....................................................................7-38 7.5.3 Sensitividad de Vibración .....................................................................................7-38 8 TAREAS DE MANTENIMIENTO ESPECIALES ....................................................................8-1 8.1 Pare Extendido................................................................................................................8-1 8.2 Examinación Indestructivas de Soldadura ......................................................................8-1
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9 OPCIONES DE MEJORA DEL VENTILADOR ......................................................................9-1 9.1 Razones de Mejora de Ventilador ..................................................................................9-1 9.2 Inclinación .......................................................................................................................9-1 9.3 Cambio de Ruedas..........................................................................................................9-2 9.4 Modificaciones de Alojamiento........................................................................................9-3 9.5 Revestimientos................................................................................................................9-3 10 SEGURIDAD ......................................................................................................................10-1 10.1 Rotación de Equipos ...................................................................................................10-1 10.2 Espacio Limitado .........................................................................................................10-1 10.3 Peligro de Quemaduras ..............................................................................................10-1 10.4 Eléctrica ......................................................................................................................10-1 10.5 Prueba de Operación ..................................................................................................10-2 10.6 Operaciones de Limpieza............................................................................................10-2 10.7 Movimientos de Ventilador .........................................................................................10-2 11 ENTRENAMIENTO.............................................................................................................11-1 12 REFERENCIAS ..................................................................................................................12-1 A INSPECCION Y REPARCIÓN DE RUEDA DE VENTILADOR CENTRIFUGO ................... A-1 B RESUMEN DE PUNTO CLAVE............................................................................................ B-1
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LISTA DE FIGURAS Figura 3-1 Esquemática de Flujo de Aire de Caldera ................................................................3-2 Figura 3-2 Sección de el Ventilador Centrifugo .........................................................................3-8 Figura 3-3 Vista de la Sección Transversal de un Ventilador Centrífugo ..................................3-9 Figura 3-4 Componentes de Ventilador Centrífugo y Accesorios ............................................3-10 Figura 3-5 Rotor Típico con Aletas Curveadas Hacia Delante para un Ventilador Centrifugo.........................................................................................................................3-11 Figura 3-6 Tipos de Aletas de Rueda y Rotación (Vista del Extremo de la Impulsión)............3-12 Figura 3-7 Componentes de Rotor de Ventilador Centrifugo...................................................3-13 Figura 3-8 Asamblea de Ventilador Axial de Dos-Etapas........................................................3-14 Figura 3-9 Impulsor de Ventilador Axial de Dos-Etapas ..........................................................3-14 Figura 3-10 Asamblea de Control de Aleta de Entrada ...........................................................3-17 Figura 3-11 Componentes de Ventilador Axial de Pendiente-Variable....................................3-20 Figura 3-12 Componentes de Cojinete de Manguito ...............................................................3-23 Figura 3-13 Ventiladores Centrífugo Típico con Aletas de Entrada Variable ..........................3-26 Figura 3-14 Ventiladores Centrífugo Típico con Aletas de Entrada Variable y Enseñando Curvas de Sistema .........................................................................................................3-27 Figura 3-15 Ventiladores Centrífugo Típico con Control de Velocidad....................................3-28 Figura 3-16 Campo de Funcionamiento para Ventilador de Flujo Axial de PendienteVariable ............................................................................................................................3-29 Figura 3-17 Definiciones de Presión de Ventilador..................................................................3-30 Figura 3-18 Corrección de Ventilador para Densidad de Entrada ...........................................3-31 Figura 4-1 Componentes de Alojamiento de Ventilador Centrifugo.........................................4-12 Figura 7-1 Rueda de Ventilador Centrífugo .............................................................................7-17 Figura 7-2 Accesorios de Protección de Desgaste y Erosión ..................................................7-20 Figure 7-3 Vista de la Sección Transversal de un Ventilador que Demuestra Requerimientos de Espacio Libre En Medio de la Entrada de Rueda y Entrada de Campana..........................................................................................................................7-26 Figura 7-4 Visión Aumentada de la Figura 7-3 ........................................................................7-27 Figura 7-5 Diagrama de Bloque Básico de un Sistema de Regulación Hidráulica ..................7-32 Figura 9-1 Paleta de Plano Aerodinámico con Paleta Inclinada................................................9-2 Figura A-1 Ejemplo de Mapa de Paleta .................................................................................... A-6 Figura A-2 Ejemplo de Mapa de Paleta .................................................................................... A-7 Figura A-3 Ejemplo de Mapa de Paleta .................................................................................... A-8
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Figura A-4 Ejemplo de Undercut y Solapar ............................................................................ A-11 Figura A-5 Probeta de Horquilla Agarrada a Mano para Piernas Articuladas ........................ A-15 Figura A-6 Orientación de un Probeta de Horquilla Articulado para Producir Flujo de Líneas en Ángulos Derechos .......................................................................................... A-16 Figura A-7 Indicaciones Están Consecutivamente Numeradas y En circuladas .................... A-19 Figura A-8 Marcando Indicaciones Múltiples .......................................................................... A-19
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LISTA DE TABLAS Tabla 4-1 Resumen de Áreas de Problema de Ventiladores Centrífugos .................................4-2 Tabla 4-2 Resumen de Áreas de Problemas de Ventiladores Axial ..........................................4-3 Tabla 4-3 FD Data de Falla de Ventiladores para Plantas Fósiles en los EE.UU. desde 1982 Hasta 1995 (NERC/GADS Data) ..............................................................................4-4 Tabla 5-1 Localización de Averías de Ventiladores ...................................................................5-1 Tabla 5-2 Localización de Averías de Cojinetes........................................................................5-3 Tabla 5-3 Localización de Averias de Sistemas de Lubricación................................................5-4 Tabla 5-4 Localización de Averías de Sistemas Hidráulicos .....................................................5-5 Tabla 5-5 Localización de Averías de Nivel de Sonido ............................................................5-6 Tabla 5-6 Localización de Averías de Conducto de Fluido........................................................5-7 Tabla 5-7 Localización de Averías Funcionamiento de Ventilador ............................................5-9 Tabla 7-1 Frecuencias de Vigilancia y Mantenimiento Preventivo ............................................7-3 Tabla A-1 Resumen de Practicas de Inspección para Ruedas de Ventiladores Centrífugos........................................................................................................................ A-3 Tabla A-2 Porcentaje de Ventiladores que Experimentan NDE ............................................. A-13 Tabla A-3 Materiales Típicos en Ventiladores de Planta de Energía ..................................... A-25
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