Descripción: Provides guidance for the design of pumping systems for water and wastewater treatment plants and water con...
BEST PRACTICE DESIGN GUIDE FOR PUMPING SYSTEMS Version 1.0 October 2011
OCT 2011
BEST PRACTICE DESIGN GUIDE FOR PUMPING SYSTEMS VERSION 1.0 Date: October 2011
Quality Assurance Statement
Office Address
DEN-1
Prepared by
Timur Ayvaz (see list of contributors)
Reviewed by
Tino Senon, George Tey
Approved for issue by
Tino Senon
Revision Schedule Rev No.
Date
Description
Prepared By
Reviewed By
Page | Chapter 1-1
Approved By
Disclaimer This document contains information from MWH which may be confidential or proprietary. Any unauthorized use of the information contained herein is strictly prohibited and MWH shall not be liable for any use outside the intended and approved purpose.
OCT 2011
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Page | Chapter 1-1
OCT 2011
1.
INTRODUCTION The design of pumping systems is an important component of MWH’s core business in the municipal water and wastewater industry. Pumping systems convey a variety of liquids into, and out of, treatment plants as well as conveyance systems. It is the responsibility of the pump system design engineer to fully understand the system hydraulics and operating scenarios required by the project. Although many textbooks, reference guides, standards and other publications describing how to design pumping systems exist, most of these publications focus on either general or very specific pumping applications. In addition, some of these publications reflect the authors’ experiences and opinions, but may not necessarily apply to project design applications. This Best Practices Design Guide ("the Guide") provides the design engineer with the relevant information and MWH Best Practices required selecting and sizing the right pump for the required application. The MWH Best Practice Guide was originally developed to document best practices in early 1990 by Tino Senon, Julian Strassle, and Brian Stone under the direction of Dewey Dickson, JMM’s Engineering Director. The first edition was completed and distributed in the Americas but later withdrawn because of some issues in Chapter 1, Hydraulic Considerations. Soon after that, there were several attempts to finish the Guide but were not successful. The 2011 Edition of this Best Practice Guide should be considered the first release and it is a compilation of experience in the US and overseas in designing small, medium and large size pump stations with an aggregate total connected load in excess of 2,000,000 horsepower. In 2008, MWH became a Hydraulic Institute Standards (HIS) Partner and has provided valuable time to review and provide input to the Standards. The Engineer should use the HIS as a reference. This Guide would never been completed without contribution of the following MWH Staff: Atul Yadav
Michael Arroyo
Chris Michalos
Rich Atoulikian
David Baar
Shane Ramcharansingh
David Sudibyo
Steve Hyland
Ed Pascua
Steven Hinman
George Tey
Sushrut Joshi
Jagannath Hosmani
Tim Ayvaz
Jakub Adidjaja
Constantino Senon
Lou Yaussi
1.1.
Scope The purpose of the Guide is to provide guidance for the design of pumping systems for water and wastewater treatment plants and water conveyance systems. It focuses on eight areas: development of the design criteria, sizing and section of pumps, system and efficiency, construction optimization, ancillary systems, interdisciplinary coordination, specification development, and safety. This Guide also includes the following:
MWH Standard P&ID’s for pumping systems
Pump data sheets
Best practices for pumping system layouts
Reference Attachments Page | Chapter 1-1
OCT 2011
Attachment No. A – Pump Station Design Procedure Flow Chart Attachment No. B – Typical Treatment Plant Process Flow Diagrams (C.1, C.2, C.3 and C.4) Attachment No. C – Design of Trench Type Wet Wells by Sanks Attachment No. D – CFD Modelling of Proposed Pump Station by Constantino Senon Attachment No. E - Design Worksheets Attachment No. F - Example Drawings
o Not included in this version of the Guide are valve selection, electrical schematics and conduit drawings, Input/Output (I/O) lists, and valve and auxiliary equipment lists. This information is currently being developed and will be included in future editions. The Guide is applies only to fluids (liquids) typically encountered in water and wastewater treatment facilities, including potable and non-potable water and dilute water/chemical mixtures (for the pumping of chemical solutions, refer to MWH Best Practice Design Guides for Chemical Feed System).
1.2.
Usage
1.2.1.
How to Use this Guide It is impossible for one guide to cover all aspects of pump station design. The Guide is a starting point for the design of pumping system projects designed by MWH Americas, and combines best industry practices with MWH experience. Refer to Attachment A – Pump Station Design Procedure Flow Chart. In addition to this Guide, the design engineer should consider project-specific requirements, MWH quality control, client input and manufacturer’s recommendations. Furthermore, the design engineer must be cognizant of and carefully review requirements mandated by local codes, the governing regulatory agency having jurisdiction and client preferences. Where compliance with such requirements appears to be in conflict with the Guide, the MWH Chief Mechanical Engineer and the MWH Water or Wastewater National Practice Leader must be consulted to reconcile those differences. Each project has its own unique and specific requirements, which may require customization of the Guide's recommended practices. The design engineer should always verify the client’s operational, performance criteria and review them with the design team throughout the design process. Customization of design may be required in order for the pumping system to meet the required operational performance as well as the client’s expectations. However, the design engineer should not deviate from the Guide without the review and acceptance of the MWH Chief Mechanical Engineer or an approved alternative reviewer. The design engineer should provide the Chief Mechanical Engineer with a list of any deviations on the modification form located in the Appendix G. Although an attempt is made to discuss hydraulic theory, the Guide assumes the design engineer has a fundamental understanding of hydraulics and design. Basic hydraulics, pump characteristics and pump theories are included in the Hydraulic Institute Standards as reference.
1.2.2.
Design Philosophy Regardless of the level at which an engineer becomes involved in the design of a pumping station, whether it be at the conceptual, pre-design or final design phase, it is important to have as complete a picture as possible of the entire system. The Guide attempts to discuss the major topics relevant at all phases of design. Topics include recommendations relative to consultation with the client to determine preferences that may affect the design of equipment. This process involves review of pertinent data, referral to standard designs, manuals and previous designs, discussions with accompany experts and site reconnaissance. Once all data is collected, the engineer should then determine the magnitude and principal features of the pumping station; such as location, capacity, Page | Chapter 1-2
OCT 2011 suction and discharge conditions (including transmission pipeline diameters and lengths), power requirements, and type of pumps (with reasonable alternatives). In addition, all available data regarding the system should be obtained. These data allows the design engineer to determine the magnitude of the total capacity, power requirements, type and number of pumps, type of driving units, and other major features sufficient for preparation of preliminary design. Data collection and calculations involved at all stages of the investigation and design should be summarized and recorded. Final calculations (civil, mechanical, structural, electrical, controls, surge, and cost estimates) should be checked and well documented. A project file should be maintained and records of all computations, memos and letters should be kept in accordance with the project and MWH standards. It is MWH’s policy that pumping systems be as simple and maintenance free as possible, equipment and materials shall be selected to be long-lasting and, in general, to employ designs that have a long record of success. Innovative designs and equipment selection is not discouraged when there are compelling reasons, such as a , project specific requirement, client’s requirements or significant cost savings, but with the knowledge that there may be limited risks involved. In which case, the Engineer must seek the assistance of the Chief Mechanical Engineer or his designated pump station design specialist and exercise due diligence in working with the pump manufacturer to insure that all aspects of design have been addressed to make sure that the pumping system work. Table 1-1 indicates the typical scope of design services relative to the size of the pumps at the station. Table 1-1 Typical Scope of Design Services Relative to Pump Station Size Description
Typical Scope of Design Services
Fractional horsepower pumps (less than 1.0 HP (0.75 kW) such as sump pumps, sample pumps, small lift stations, utility water pumps, etc.
Utilize package system as much as possible. Determine flow, head, horsepower and controls requirement. Select pumps and manufacturer. Establish foot print and show single line piping layout. Normally no detail layout is required other than reference to standard details. Indicate in schematics, if appropriate. Use standard specifications Utilize standard designs if available. Show simple outline layout of equipment and single line piping details. Utilize standard specifications. Proceed on the basis of pre-design then detail design. Utilize Criteria Committee Meeting review (CCM) as discussed in the Delivery Framework. The designs may require additional services such as geo tech, surge analysis, vibration analysis, architectural input and constructability review. Design team should include the Mechanical Chief Engineer or his designated pump station design specialist. All design discipline shall be involved especially electrical and I & C as early as the predesign phase. Alternative layout studies consultation and extensive coordination with equipment manufacturers and substantial (electrical and mechanical) input.
Water and wastewater pumps 2 to 40 hp (1.5 to 30 kW)1 Water and wastewater pumping facilities 50 to 1000 HP (40 to 750 kW)
Water and wastewater pumping facilities with installed total capacity in excess of 1000 HP (750 kW)
1
Low lift pumps (head less than 60 ft (20 m)) require careful review of the system head loss calculation hydraulics. Include velocity head through the pump column in the pump TDH. All pump station hydraulics shall be supported by a system head curve with pump curves superimposed over the system curve.
Page | Chapter 1-3
OCT 2011
1.2.3.
Abbreviations and Definitions Pump nomenclature, abbreviations and definitions as used by the Hydraulic Institute Standards are provided in Appendix F.
1.2.4.
Symbols and Specification References The MWH General Drawing Sheets included pump symbols and can be accessed from the CAD Drafting Standards in the Delivery Framework. The MWH Guide Specifications for pumping equipment is provided in the Delivery Framework arranged by pump type.
1.2.5.
Glossary Appendix E includes commonly used terms through this guide and within the Water and Waste-water Industries.
1.3.
Codes and Standards The codes and standards listed in our Guide Specifications are available through MWH's IHS subscription service as indicated in the Delivery Framework. In addition, the design engineer should become knowledgeable of project-specific and local codes and ordinances within the jurisdiction of the project.
1.4.
References The Guide is based on the references listed below. The design guide attempts to summarize the main discussions in the references; however the design engineer is encouraged to become familiar with these references. In order to determine if there is a copy available in your local office, please contact your supervisor. For further information regarding the delivery process, please see the delivery framework at the following link: http://design-framework/
1.4.1.
Industry
Hydraulic Institute Standards, Parsippany, New Jersey
Internal Flow Systems 2nd Edition, D.S. Miller (2009)
Pumping Station Design, Robert L. Sanks, et al. (1989), Butterworths, Stancham, Massachusetts
Cameron Hydraulic Data, 18th edition, Liberty Corner, New Jersey
Pump Handbook, 4th edition, Igor J. Karassik, et a.l, McGraw-Hill Book Company, New York, New York
Design of Trench-Type Wet Wells for Pumping Stations, Robert Sanks, reviewed by Senon (May 2008 Pumps & Systems magazine)
Computation Fluid Dynamic Modeling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006
Machinery Malfunction Diagnosis and Correction, Robert Eisenman, Hewlett-Packard Professional Books
Friction Factors for Non-Newtonian Fluids (Sludge), Design of Municipal Wastewater Treatment Plants, 4th Ed., WEF Manual of Practice 8, ASCE and Report on Engineering Practice No.76, Volume 3, Chapter 8: Solids Storage and Transport
Flow of Fluids Through Valves, Fittings and Pipes Crane Technical Paper No. 410. Page | Chapter 1-4
OCT 2011
1.4.2.
MWH Contacts NAME
TELEPHONE
E-MAIL
Constantino (Tino) Senon
Cell 425 421-6842 Direct 360 387-7851
[email protected]
Timur Ayvaz
Cell 713 501-6784 Direct 303 291-2124
[email protected]
George Tey
Direct 626 568-6259
[email protected]
Page | Chapter 1-5
OCT 2011
2.
DESIGN CRITERIA DEVELOPMENT
2.1.
General The following information is presented as a standard methodology in order to ensure consistent and accurate MWH designs. Information is presented in a sequence typically encountered during a design project.
2.1.1.
Client Preferences With every new project, MWH employees have the opportunity to work with multiple clients throughout the world. Each client has a unique perspective of their environment. As consultants we must listen to our Clients in order to ensure our Clients’ success. Many times, Clients have very specific preferences regarding the type of design, equipment used or how the system is controlled. These preferences could be based on a new direction the client is interested in pursuing, past experiences or even the level of local vendor support. In developing the design criteria, it is essential to first determine the Client’s preferences with regards to the project.
2.1.2.
Site Constraints A design engineer must also understand the site details surrounding the new or existing pump station. In some instances, there may be limitations influencing the design of the station. These constraints could range from geotechnical information, such as settlement, seismic requirements, flood elevation or high ground water to neighbors on adjacent properties. The design engineer should always be mindful of the environment the pump station is being constructed.
2.2.
Fluid Properties Determining the process fluids properties to define the fluid service and pump materials is the first step in developing the pump station design criteria. The properties of fluids which are of fundamental importance to the subject of the Guide are discussed in the following sections. These properties are specific gravity (based on density or specific weight), viscosity, temperature and corrosion and erosion potential.
2.2.1.
Specific Gravity The density of a substance is a measure of the concentration of matter, and is expressed in terms of mass per volume. The specific gravity is a term used to compare the density of a substance with the density of water. Because the density of liquids depends on temperature, the temperature of the liquid in question as well as the reference temperature of water should be stated in giving precise values of specific gravity. When dealing with fluids other than water, identifying the specific gravity is essential as it directly affects the pump station power demand. The power input (horsepower) of the pump is directly proportional to the specific gravity (S.G.) of the fluid. Therefore, if the horsepower for a pump conveying water (S.G. =1.0) is 100 HP, then a liquid with a specific gravity of 1.2 requires 120 HP. The following are two reference tables for the design engineer. Table 2-1 indicates the density of water with respect to temperatures. Table 2-2 identifies the specific gravity of typical fluids the design engineer may come across. More detailed information is available in the Cameron Hydraulic Data reference book.
Page | Chapter 2-1
OCT 2011
Table 2-1: Density of Water at Various Temperatures Temperature °F
Density (slugs/ft3)
Specific Weight (lb/ft3)
32
1.94
62.41
40
1.94
62.43
50
1.94
62.41
60
1.94
62.37
70
1.94
62.31
80
1.93
62.22
90
1.93
62.12
100
1.93
62.00
120
1.92
61.71
140
1.91
61.38
160
1.90
60.99
180
1.88
60.57
200
1.87
60.11
212
1.86
59.81
Table 2-2: Specific Gravity of Some Liquids at 60°F
2.2.2
Liquid
Specific Gravity
Gasoline
0.66-0.74
Kerosene
0.78-0.82
Sea Water
1.03
SAE Oils
0.88-0.94
100% Glycerin
1.26
Ethyl Alcohol
0.79
40% Caustic Soda
1.43
Mercury
13.57
Water
1.00
Viscosity Another major factor affecting pump behavior and system response is the fluid's viscosity, which is the fluid's resistance to shearing. Fluids differ from solids by continuing to deform in the presence of a shearing stress; when a shearing stress causes a liquid to flow, it continues to flow as long as the shear stress acts on it. Therefore, viscosity can also be defined as a fluid's resistance to flow. A general formula developed by Isaac Newton is:
μ Where = the shear stress exerted by the fluid (its "drag") µ = the fluid's viscosity (a constant of proportionality) = the velocity gradient perpendicular to the direction of shear
Page | Chapter 2-2
OCT 2011 Fluids which behave in the above manner are called Newtonian fluids, and continue to behave this way no matter how fast it is stirred or mixed. With a non-Newtonian fluid, on the other hand, stirring can leave a "hole" that gradually fills in over time, or cause the fluid to become thinner. The viscosity of a Newtonian fluid depends only on temperature, pressure and the chemical composition of the fluid. Therefore, for a given substance and pressure, these fluids have a straightline slope when plotting viscosity against temperature on the viscosity charts included in standard references. Non-Newtonian fluids are classified as thixotropic, dilatent, or rheopectic, depending on how the viscosity changes with respect to the rate of shear (see Figure 2-1). Of particular importance in wastewater engineering are thixotropic fluids, which include thick sludges and some chemical precipitants. A thixotropic fluid is one in which the viscosity decreases as the rate of shear increases (a characteristic of ketchup, difficult to start, but once started it is difficult to stop – shaking the bottle first "pre-shears" the ketchup, making it easier to pour).
VISCOSITY
Newtonian
thixotropic
SHEAR RATE Figure 2-1: Newtonian versus Thixotropic Materials The "pumpability" of a thickened sludge, dewatered sludge or a chemical precipitant depends on many factors and should be assessed by specialists in this type of pumping. In some cases, a representative sample of the fluid is sent to the laboratory for testing. Based on the test, the pumping design criteria can be established especially for mine slurry and sludge. Another good reference is Moyno Pumps by Robbins Meyers Company. They have compiled actual test data of different Newtonian fluids especially for sludge. In fact Moyno will test any sludge for a minimal fee. For this type of analysis, the design engineer should seek assistance from the Chief Mechanical Engineer. Viscosity is expressed as either dynamic or kinematic viscosity. The kinematic viscosity is the ratio of dynamic viscosity to density:
μ
Where = kinematic viscosity µ = dynamic viscosity = density As with specific gravity the viscosities effect on the pump performance. There are many guidelines and tables published to predict pump performance when pumping highly viscous solutions. The design engineer should seek assistance from the Chief Mechanical Engineer when dealing with viscous solutions. The following Figure predicts the affect of a viscous solution on the performance of a pump. The effect on pump performance is presented in the form of percentage changed as compared to pumping water. Page | Chapter 2-3
OCT 2011
Figure 2-2: Viscosity Corrections for Large Pumps Obtained from Cameron Hydraulic Data, 19th Edition
2.2.3.
Temperature The previous sections discuss the impact of specific gravity and viscosity on the pumping system. These properties are not always constant; temperature affects both the density (specific gravity) and viscosity of the pumped fluid. A design engineer must incorporate the affects of temperature in the hydraulic calculations especially if the process covers a wide range of temperatures greater than 68 F. Furthermore, temperatures may directly impact the Net Positive Suction Head (NPSH) for the pumps. (NPSH is discussed in greater detail in section 2.6.4). Hydraulically, for optimum pump performance, the pump requires a minimum amount of pressure at the eye of the impeller. This minimum pressure at the eye of the impeller is affected by the temperature of the solution. At higher temperatures, the vapor pressure of the fluid increases thereby decreasing the overall inlet pressure. The design engineer should always consider the fluid temperatures and vapor pressure when determining the NPSH available in a system. Temperature can also affect pump efficiency. Usually, pumps are tested at the factory using water at ambient temperature. If the test water temperature is higher or lower than the ambient temperature of approximately 50 F, the pump efficiency shall be corrected to ambient temperature. Conversely, if the test pressure is lower than the water temperature at field condition such as for hot water circulating service. The test efficiency should also be corrected to field condition. Refer to the Hydraulic Institute Standards, Rotodynamics Acceptance Test Criteria. When designing a system, certain materials or components are temperature dependent. The MWH Guide Specifications provides guidance in the form of Notes to Specifier when dealing with special considerations for high temperature applications. For example, Section 43 10 50, Piping General provides guidance on the selection of gaskets, couplings, connectors and other piping components for various temperature conditions.
Page | Chapter 2-4
OCT 2011
2.2.4.
Corrosion and Erosion Considerations The discussion presented in the previous sections considers the fluid property’s effect on the hydraulics of the pumping system. Corrosion and erosion are a fluid characteristic with no effect affect on the hydraulics, but if not considered may be detrimental to the life of pumps, valves and piping. The effects of corrosion and erosion should always be considered when dealing with fluids other than potable water. Corrosion is an undesirable degradation of material resulting from a chemical or physical reaction with the environment. Erosion is the deterioration of metals buffeted by the entrained solids in a corrosive medium. The corrosive or erosive potential of a service would dictate the materials of construction, hardness and ductility of material and special liners such as rubber are required. Figures below show an example of corrosion and erosion on the pump impeller.
Figure 2-3: Corrosion on Pump Impeller
Figure 2-4: Erosion on Pump Impeller The following is a brief list of potential corrosive services the design engineer may encounter. Note this list is a small excerpt of various corrosive services defined in the Pump Handbook by Igor Karrassik. When designing a pump station with a fluid containing corrosive constituents such as the Colorado River Water, water known to be corrosive, or fluids other than water, a sample must be taken and tested. Results should be reviewed by the corrosion engineer and the pump manufacturer Page | Chapter 2-5
OCT 2011 for proper material selection of pump components. An example of water constituent analysis is included in the MWH Guide Specifications for Pumps.
Common Corrosive Applications o
Sea Water
o
Water with high sulfides (hydrogen sulfide in wastewater)
o
Chemical Acids and bases with lower and higher pH
The following is commonly used terms the design engineer should be familiar with.
Erosion Corrosion – The deterioration of metals buffeted by the entrained solids in a corrosive medium. This corrosion also depends on flow angle of attack of liquid relative to the component
Abrasive wear – Erosion of any material as a result of the following suspended solids characteristics o
Solid concentration
o
Solid size and mass
o
Solids shape
o
Solid hardness
o
Relative velocity between solids and surface
Abrasive material – Suspended solids which contribute to abrasive wear such as grit and sand.
Cavitation erosion – Pumps experiencing inadequate NPSH margin, air entrainment, freesurface and sub-surface vortices are susceptible to cavitation erosion. Cavitation erosion is the degradation of the material surface due to cavitation. Pump materials resistance to cavitation erosion in increasing order are as follows: o
Cast iron (least resistant)
o
Bronze
o
Cast steel
o
Manganese bronze
o
Monel
o
400 Series stainless steel
o
300 Series stainless steel
o
Nickel-aluminum bronze
o
Ni-resist ductile iron ( Ni-Hard) (most resistant)
Corrosion Fatigue – Related to the endurance stress of material based on cyclic reversal of load applications.
Galvanic Corrosion – Galvanic corrosion occurs when two dissimilar materials are in contact or electrically connected in a corrosive medium. Corrosion of less noble material is accelerated and corrosion of more noble material is decreased.
Graphitization – In the presence of an electrolyte, a galvanic cell exists between the cast or ductile iron and the graphite particles. In the galvanic cell of iron and graphite, iron becomes the anode and the graphite becomes the cathode. A galvanic current flows from the iron to graphite; therefore, the iron goes into solution resulting in gradual depletion of iron until only graphite remains. While the casting appears sound on the outside, pieces may be broken off with the fingers.
Page | Chapter 2-6
OCT 2011
Concentration cell, or crevice, corrosion – When an electric current flows between two areas causing a localized attack. This usually occurs where water is stagnant, such as threads, gasket surfaces, holes, crevices, surface deposits and in the underside of bolts and rivet heads. When concentration of corrosion occurs, the concentration of metal ions or oxygen in the stagnant area is different from the concentration in the main body of the liquid.
Selective Leaching – Removal of one element of material from solid alloy in a corrosive medium such as the process of dezincification, dealuminumification, and graphitization. For example where a certain water source is known to have dezincification characteristics, low zinc bronze is normally recommended
Intergranular – Materials can look sound on the surface but intergranular corrosion can progress to a point that the material literally disintegrates. Intergranular corrosion of austenitic stainless steel occurs as a result of carbides precipitating out the grain boundaries during slow cooling of the casting.
Buried piping – Corrosive soils are a factor when designing buried piping systems. The geotechnical reports shall consider the overall corrosive properties of the soils, and recommend a method to mitigate its affect on the buried piping.
Corrosive Constituents in Municipal type Wastewater Systems When designing a wastewater pump station, the design engineer shall pay special attention to any air/wastewater surfaces. Anaerobic sulfate-reducing bacteria (such as Desulfovibrio) thrive in wastewater. This bacteria utilizes the oxygen in sulfate (commonly found in wastewater) to create hydrogen sulfide, which escapes from the wastewater to the atmosphere above. At that point, an aerobic bacteria (Thiobacillus) converts the hydrogen sulfide to sulfuric acid. Sulfuric acid is extremely corrosive whether it is concrete, steel or ductile iron. Process design should minimize hydraulic jumps, turbulence which could cause off-gassing of H2S. If off-gassing cannot be prevented by alternate design, materials resistant to H2S shall be specified such as 316 stainless steel. Iron Reducing Bacteria found in Deep Wells Iron reducing bacteria have been found in deep wells which corrode pumps components made of cast iron, ductile iron, carbon steel or even 304 stainless steel. Recommend using 316 stainless steel. During development of deep wells, a water sample shall be tested for its corrosivity and look for the presence of iron reducing bacteria as well.
Page | Chapter 2-7
OCT 2011
Table 2-3 Galvanic Series of Metals and Alloys
>>
Sacrificial Anodes Magnesium Magnesium alloys Zinc Aluminum 2S Cadmium Aluminum 17ST Steel or iron Chromium stainless steel, 400 series (active) Austenitic nickel or nickel-copper cast iron alloy 18-8 Chromium-nickel stainless steel, Type 304 (active) 18-8-3 Chromium-nickel-molybdenum stainless steel, Type 316 (active) Lead-tin solders Lead Tin Nickel (active) Nickel-base alloy (active) Nickel-molybdenum-chromium-iron alloy (active) Brasses Copper Bronzes Copper-nickel alloy Nickel-copper alloy Silver solder Nickel (passive) Nickel-base alloy (passive) 18-8 Chromium-nickel stainless steel, Type 304 (passive) Chromium stainless steel, 400 series (passive) 18-8 Chromium-nickel-molybdenum stainless steel, Type 316 (passive) 18-8-3 Chromium-nickel-molybdenum stainless steel, Type 316 (passive) Nickel-moly Silver Graphite Gold Platinum Protected End (cathodic, or most noble)
Reprinted by courtesy of The International Nickel Company Inc.,67 Wall St., New York, NY Page | Chapter 2-8
OCT 2011
2.3.
Pump Station Configuration Unfortunately, no single pump station configuration fits every application. The design engineer is urged to review the pump station examples provided in this Guide as a starting point. If the required features do not closely resemble the examples, contact the Chief Mechanical Engineer or any of the persons listed in the contact list for any other examples. The type of pump station configuration varies greatly depending on different factors such as hydraulic considerations, Client’s preference, and the type of pumps, the control and maintenance expectations and the size of the station. The following sections provide a brief overview of the various types of pump station configurations available.
2.3.1.
Suction Configuration Depending on the hydraulics of the specific application, a variety of pump suction configurations may be used.
2.3.1.1.
Wet wells are typically used to provide storage volume or a hydraulic break. Typical pump stations which use wet wells are sewage lift station, treated water high service pump station, raw water booster pump station or submersible sewage pump station.
A “can” (also called a barrel) is typically used for a vertical turbine pump connected to an adjacent forebay or reservoir.
Piping manifold suction headers are typically used for horizontal centrifugal pumps stations connected to an adjacent forebay or reservoir. This configuration may also be used for inline booster pump stations. (Inline booster pump station is usually discouraged because it complicates the surge protection and controls)
Wet Well A wet well is a below-grade structure (above grade is possible, but not typical) of a pumping station. It is the structure into which the liquid flows from, and where the pumps draw water. The wet well serves three purposes. First, it creates a hydraulic break minimizing the effects of upstream system on the current system. The free water surface is allowed to rise and fall buffering the system from any fluctuations in flow and pressure. Second, it provides storage volume to allow constant speed pumps to start and stop without exceeding the number of starts required for a certain size motor. Third, it provides adequate submergence above the suction bell of pump to prevent formation of vortices and provide adequate Net Positive Suction Head (NPSH). Fourth, it provides free-board to allow the level to rise during upset or emergency operation without overflowing. Additionally, the wet well shall be configured to preclude formations of free-surface or subsurface vortices or prerotations that can be carried through the pump suction volute. At a minimum the wet well design shall meet the flow distribution based on the accepted criteria recommended by the Hydraulic Institute’s Intake Design Standard. These recommendations mitigate adverse hydraulic phenomenon that may occur in the pump station wet well. In summary, the geometry of the wet well, operation of the pumps, and the depth of water in the sump influence the approach flow hydrodynamics and can result in the following adverse hydraulic phenomena (Sweeney and Rockwell 1982):
Pre-swirl of flow approaching the pump impeller
Free surface vortex formation
Spatial asymmetry of the flow approaching the pump impeller
Temporal fluctuation (turbulence) in the flow approaching the pump impeller
Air Entrainment
Page | Chapter 2-9
OCT 2011 The following section describes typical wet well configurations used by MWH. The appendix includes example drawings of these configurations.
Rectangular with flat bottom used for clean water application such as treated water high service pump stations, and potable water storage reservoirs.
Rectangular with hopper bottom used for solids bearing fluids such as raw sewage, grit chamber, sludge, raw water, etc.
Rectangular with sloped bottom and flow distribution inlet channel for sewage and sludge pump stations. Refer to Wet Well Design Guide for Large Submersible Pumps by ITT Flygt Pump.
Open-trench Type wet well with an Ogee weir for liquids bearing solids especially raw unscreened municipal sewage.
For design guides, refer to the following references:
ANSI 9.6-1998 HI Pump Intake,
Design of Trench-Type Wet Wells for Pumping Stations, Robert Sanks, reviewed by Constantino Senon (May 2008 Pumps & Systems magazine)
Computational Fluid Dynamic Modelling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006
MWH exceptions and improvements to HI recommendations based on lessons learned from Durham and Tacoma IPS. For list of exceptions, refer to Computational Fluid Dynamic Modeling of a Proposed Influent Pump Station, Wicklein, Sweeney, Senon, et al., WEFTEC 2006. For detailed drawings, refer to example drawings from Durham and Tacoma IPS.
Use minimum submergence S=4x suction bell diameter “D” or greater.
Use long radius suction elbow with flare bottom.
Provide flow straightening device downstream of the suction elbow
Use long radius reducing elbow to connect the horizontal suction pipe to the pump suction.
When is Physical Hydraulic Model Study Required? A physical hydraulic model study can be expensive, but in many cases will provide insight into specific hydraulic issues that may adversely affect the pump station. Based on ANSI/HI 9.8-1998, the Hydraulic Institute Standards recommend physical model testing if one or more of the following features exist in the project:
Sump or piping geometry deviates from the intake design standards.
Non-uniform or non-symmetric approach flow to the pump sump exists.
Pumps have flows greater than 2520 l/s (40,000 gpm) per pump or the total suction flow with all pumps running would be greater than 6310 l/s (100,000 gpm).
Pumps with an open bottom barrel or riser arrangement with flow greater than 315 l/s (5,000 gpm) per pump.
Proper pump operation is critical and pump repair, remediation of a poor design, and the impacts of inadequate performance or pump failure all together would cost more than ten times the cost of model study.
When is the Computational Fluid Dynamics (CFD) Study Required? The computational fluid dynamics study could be performed as a pre-requisite to the physical model study for the following reasons:
The CFD model is less expensive as the physical model study.
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OCT 2011
The CFD model can be used to optimize the wet well design for least cost. The model can be revised easily in the computer and operational flow scenarios can be simulated without the need to modify fabricated physical model.
Although the result of the CFD model have not been endorsed by HI as a substitute to the physical model because of the accuracy of the software available in the market, MWH experience indicate that if the result of the CFD model is gauged against the HI acceptance criteria used for physical model, the result of CFD model could be conclusive in lieu of conducting physical model study if approved by the Senior CFD modeller and Chief Mechanical Engineer. The approval criteria shall be based from previous experience of similar wet well configuration that had been proven to work.
An example of the CFD Study of the wet well configuration that demonstrate non-conformance to the HI acceptance criteria is shown in Figure 2-5 below. This wet well was an existing wet well designed by another consulting engineering firm and it is in the process of being modified by MWH using physical model test.
Figure 2-5 Example CFD Study of a Wet Well Configuration with Flow Distribution Deficiency
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OCT 2011 MWH has taken exception to the HI guidelines regarding when to perform a CFD or physical model study under the following circumstances. If the wet well design is identical in flow, wet well configuration with a pump station that has been previously modeled, constructed and have a record of successful operating experience such as the Durham IPS and City of Tacoma IPS, etc., a physical hydraulic model test may not be required. Please contact the Chief Mechanical Engineer for the additional list of projects. Example of a physical model is shown in Figure 2-6.
Figure 2-6 Example Physical Model What is Model Performance Acceptance Criteria The Hydraulic Institute Standards established criteria for evaluating performance of pump station designs through the use of physical model studies. ANSI/HI 9.8-1988 details the physical modeling procedures and the interpretation of the results. The following is a list of minimum performance criteria for a physical model:
Free surface and subsurface vortices entering the pump must be less severe than vortices with coherent (dye) cores (free surface vortices of Type 3 and sub-surface vortices of Type 2 – from HI 9.856). Dye core vortices may be acceptable only if they occur for less than 10% of the time or only for infrequent pump operating conditions.
Swirl angles, both short-term and long term as defined in the Standard.
Time-averaged velocities at points in the throat of the bell or at the pump suction in a piping system shall be within 10% of the cross-sectional area average velocity as defined in the Standard.
For special case pumps with double suction impellers, distribution of flow at the pump suction flange shall provide equal flows to each side of the pump within 3% of the total flow.
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OCT 2011 2.3.1.2.
Design Considerations The wet well or forebay volume should be designed with adequate storage to prevent frequent starting and stopping of the pump. This starting and stopping is called cycling of the pumps. The maximum number of allowable starts is typically dependent on the characteristics of the electric motors and typically ranges between 6 for large motors and 15 for small motors. The design engineer is responsible for contacting the pump/motor manufacturer to obtain the minimum cycle time. Furthermore, the wet-well should be sized to allow for the pump starting sequence. The starting sequence usually takes between one to three minutes, depending on the required opening and closing time of the pump control valves. The opening and closing delays may be field adjusted to prevent extended operation of the pumps between shut off and operating duty point. The starting and stopping times for pumping units equipped with check valves, is usually less than a minute. The wet well should be sized to provide adequate storage during this time period. For multiple-speed pumps, the available storage volume in the wet well does not need to be as conservative. As flow rate is controlled by the speed of the pump, the pump does not need to start against a closed valve. The pumps can start, and increase speed to immediately contribute flow into the system. One design criteria often overlooked is the storage volume required in the event of a power out-age. With a constant flow rate entering the pump station wet well, a disruption in local power will immediately be reflected with a rise in the water surface elevation as in the case of booster pumps in series. In this example, it is impossible to provide storage for an extended power outage. Therefore, the SCADA system shall be configured such that in the event of power failure in a downstream pump station, the upstream pump station shall be signaled to stop. In collection system applications, the flow can be allowed to back-up into the system, otherwise the wet well should be designed with adequate storage volume or overflow potential during a power outage. The design engineer shall review the local codes and client’s preferences regarding the design power outage duration. Most state regulatory agencies in the US include maximum retention time in the wet well design criteria, when pumping wastewater. The intent is to minimize the potential for the development of septic conditions and resultant odors. A maximum retention time of 10 minutes, at average design flow rates is often quoted. Unfortunately, this requirement may conflict with the need for adequate volume to prevent short-cycling of the pumps. In such cases, multiple pumps or variable-speed pumps should be considered to reduce the required volume. Furthermore, in addition to minimizing retention times, odors can be minimized if the lowest liquid level in the well is set above the sloping portion of the wet well. This can be accomplished by making this level the stop point for the lead pump in the sequence. For Sizing of Wet well or sump Volume, refer to HI Pump Intake Design, Appendix B
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OCT 2011
Figure 2-7: Hydraulic Institute Wet Well Volume Calculation Procedure
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OCT 2011 When designing the wet well, the design engineer shall consider the following:
2.3.2.
Provide an opening in the deck with adequate clearance to allow removal of any pump components or piping from the wet well.
The wet well shall be provided with an air vent sized to release or admit outside air due to the rise and fall in water levels. Area of vent is typically equal to at least half of the inlet pipes area this dimension is a minimum, the required dimension may be larger. For se-wage or sludge pump station, vent pipe shall be connected to the foul air scrubber
Provide a hatchway for access to the wet well. Hatchway size to be at least 4 ft by 4 ft with appropriately sized safety net or equivalent safety system.
Permanent ladders shall NOT be included in the wet well due to corrosion and the potential safety concerns.
Address Confined Space requirement and fire and safety requirement per NFPA 820. Consult the MWH HVAC Lead Engineer.
Dry Well or Dry Pit As mentioned previously, pumps draw water from the wet well. Locating the pump adjacent to the wet well minimizes the suction losses. The pump would need to be installed below the water surface elevation. To accomplish this, a second structure is installed adjacent to the wet well. This below grade structure, called a dry well, contains the pumps, drive shafts, valves and piping. For this configuration, there is no liquid surrounding the pump and valves, therefore the equipment are accessible for maintenance. This maintenance accessibility is the main advantage of the dry well configuration.
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OCT 2011
Figure 2-8: Typical Dry Well Configurations
2.3.3.
Submersible Pump Station Another type of pump station configuration typically used for sewage lift stations is a submersible pump station. A submersible pump station does not have a dry well. The pump and piping is located within the wet well. The pump and motors is specially designed for submergence in the water or wastewater.
Figure 2-9: Typical Submersible Pump Station Page | Chapter 2-16
OCT 2011 The advantage of this type of station is cost. The overall building footprint is much smaller than a dry pit station. Furthermore, the entire pump station, excluding electrical panels and SCADA system, can be located below grade, which might have its advantages if the pump station is located in a visually sensitive area. Unless otherwise preferred by the Client, MWH’s preference regarding when to utilize a submersible configuration versus a dry well configuration is related to the pump horsepower. For pumping stations equipped with pumps sized up to 700 HP the installation costs of a submersible pumping system is normally less than the dry well pumping systems because the pumps can be installed without the need of building a dry pit. However for larger installations with pumps larger than1, 000 hp, a dry-pit pumping system is preferred by most Clients because it is easier to inspect and maintain. Dry pit pumping systems; however have a risk of being flooded. The clients’ policies and/or preferences should be given careful consideration. For small manhole type sewage lift stations, submersible pumping system offer advantages over drypit systems because they are less expensive to build and they are also available from the manufacturer as a package unit. The manufacturers are Smith and Loveless and Gorman-Rupp. Submersible pump stations can be installed underground, below streets, or in a limited space by the roadside with the motor control panel mounted above ground.
2.3.4.
Vertical Turbine Installations Vertical turbine pumps are typically suspended from the structure, with only the pump unit (bowl assembly) submerged below the water surface. The motor is typically installed above a wet well or above ground and supported by the discharge head. Vertical turbine pumps can also be installed inside the can or barrel. Barrel mounted vertical turbine pumps are highly sensitive to the intake hydraulics. The vertical turbine pumps are classified into three types based on their specific speed; bowl assembly and impellers.
Radial and Francis-Vane turbine impeller [Ns: 500 to 4000] – enclosed or open type, multi-stage, low flow high head
Mixed flow impeller [Ns: 4500 to 8000] –enclosed or open type, maximum two stage, medium flow, medium head
Axial/propeller flow impeller [Ns: 8000 to 15000] – open type, maximum single stage, high flow, low head
Figure 2-10: Typical Vertical Turbine Pump Installation Vertical turbine pumps are sensitive to intake design configuration, including submergence over the 1st stage impeller, spacing between two adjacent pumps or the wall; and spacing from the bottom of Page | Chapter 2-17
OCT 2011 the pump to the floor. Complying with these requirements mitigates vortices, and a wet well designed to establish uniform flow velocity distribution at the suction bell. Uneven velocity distribution, compounded by insufficient submergence can result in the formation of vortices which may introduce air into the pump suction causing a reduction in capacity, unbalanced impeller loading, rough operation or impeller damage due to cavitation. For vertical turbine pump intake design guidelines, refer to the Hydraulic Institute Standards. For submergence over the bell requirements, refer to the pump manufacturer’s published performance curves and pump data. Design Considerations When mounting vertical turbine above a wet well, the design engineer shall consider the following requirements:
The top structure of the wet well shall be designed for the maximum down thrust generated from pumping the liquid. The down thrust generated by the pump is absorbed by the motor top bearing and transferred to the structure. The pump thrust information is available from the pump manufacturer. Thrust factor is normally indicated on the pump performance data sheet in terms of pounds per foot of head.
The top deck should be designed so that the natural frequency is at least two times the maximum speed of the pump. The design engineer shall coordinate design with the structural engineer.
Pump pads should be designed integral with the top of the wet well deck.
Location of piping and valves for access needs to be coordinated with the tank and nearby area.
For pump stations with flow rates in excess of 5,000 gpm, provide isolation baffles between the pumps.
Provide adequate submergence over the suction bell to prevent vortexing at low water level, create NPSH available greater than what is required by the pump, avoid cycling on/ off pump operation and free board above high water level. Provide NPSH margin of at least 5 feet absolute. Minimum submergence shall be equal to or greater than the pump manufacturer’s recommendation.
Note to Design Engineer: The terms minimum submergence and NPSHr refer to two separate items. Minimum submergence is the minimum water level required regardless of NPSHr. It is the Design Engineer’s responsibility to ensure that requirements are met.
Provide a combination air release and vacuum valve mounted on the discharge pipe located between the pump discharge and the check valve. Size air valve using APCO valve calculator.
Provide opening in deck with adequate clearance to allow removal of pump assembly. Use the maximum diameter of the column pipe flange, bowl assembly or suction bell whichever is the largest.
Wet well shall be provided with an air vent sized to release or admit outside air due to the rise and fall in water levels. The area of vent is usually equal to at least half of the area of inlet pipe. Provide a hatchway for access to the wet well. Hatchway size to be at least 4 ft by 4 ft with appropriately sized safety net or equivalent safety system. Design engineer shall also consider potential debris removal when sizing and locating hatches.
For wastewater pump stations, permanent ladders shall NOT be included due to corrosion and create a potential safety concerns.
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OCT 2011
When using a vertical turbine installed in a barrel/can, the design engineer shall consider the following requirements:
The annular velocity between the inside diameter of the barrel and the pump shall be between 3 to 5 fps. The annular velocity shall be calculated using the maximum flow for each pump. This size should be confirmed with the manufacturer requirements for the selected model.
MWH’s exception to HI design criteria. The vertical distance from the centerline of the inlet pipe to the barrel to the suction bell shall be at a minimum 3 to 4 times the diameter of the barrel instead of 2 times the diameter of the barrel per HI. MWH exceptions are annotated in Figure 212.
Pump foundation or inertia base should be designed with a mass equal to or greater than 4 times the weight of the motor or adequate to support the pump and motor assembly whichever is the largest. Pump foundations shall be isolated from the concrete area or floor of the building. This design will limit transferring vibrations to the slab or building. Any exceptions shall be brought to the Chief Mechanical Engineer attention.
Calculate hydraulic grade line at the pump suction with friction loss based from the maximum pump flow. The hydraulic grade line at the centerline of the pump shall be at least: o
One diameter higher than the crown of the inlet pipe
o
Net Positive Suction Head Available (NPSHa) referenced to the datum of the pump shall be calculated using the hydraulic grade line inside the barrel, including friction loss through the annular space between the pump and the barrel. Figure 2-11: Excerpt from Hydraulic Institute Intake Design Various Vertical Turbine Intakes
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OCT 2011
Figure 2-12: Excerpt from Hydraulic Institute Intake Design Vertical Turbine Can/Barrel Pumps
Figure 2-13: Suction and Discharge Piping for Vertical Turbine Can/Barrel Pumps
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OCT 2011
2.3.5.
Hydraulic Institute Self-Cleaning Wet Well (Trench Type) The self cleaning wet well design can be used for wastewater or solids bearing fluids with end suction, non-clog pumps. Vertical turbine or submersible type pumps have been used by other consultant and they have been known to have problems with flow distribution and stringy materials depositing around the pump column and slide rails and cables in the case of submersible pumps especially for unscreened and wider screen spacing. Trench type wet wells were developed based on the philosophy that variable speed pumps do not require significant wet well storage volumes. The speed of the pump can be adjusted using a Variable Frequency Drive (VFD) to maintain a constant water level (water entering the wet well equals the water leaving the wet well) thereby minimizing the time the fluid is in the wet well. Over the years, this concept has progressed through much iteration in the design. The most current design shown in Figure 2-12 is obtained from the Hydraulic Institute Intake Design Guidelines. MWH exceptions are annotated within the figure.
Figure 2-14: Open Trench Type Wet Well, Hydraulic Institute Standards, ANSI/HI 9.8-1998 The two main benefits of the open trench wet well design are minimizing wet well storage volume and the ability to convey solids without the need to have varying water surface elevations for pump control. This feature results in a compact pump station design. With regards to the ability to convey solids, the operational guidelines for this type of wet well includes a cleaning cycle in which the water surface elevation is lowered. The influent flow cascades down the “ogee” weir at the entrance of the Page | Chapter 2-21
OCT 2011 wet well. The cascading flow accelerates, with the flow velocity reaching a scouring velocity removing settlement and debris from the bottom of the wet well. The solids become suspended, enter the pump and are conveyed downstream. Self cleaning wet well design should be provided for sewage pumping stations as indicated within Hydraulic Institute Standards. MWH’s previous experience has taken exception to the minimum submergence over the suction bell of 2 times pump bell diameter. The design engineer shall use 4 times pump bell diameter for the minimum submergence. In the three models performed, submergence of 2D or as calculated using Froude’s equation, any turbulence in the water surface is carried to the pump suction bell. However using a submergence of 4 times D, when the wet well was drawn down to the operating low water level, the flow entering the suction bell was not affected by any turbulence in the surface.
2.3.6.
Forebay / Reservoir Pump Station Wet wells are typically used to create hydraulic breaks between two separate systems. In water distribution or conveyance systems, the wet wells or reservoirs are required to handle large variations in flow so that the pumps can be controlled by level and/or flow. The wet well or reservoir shall be designed with a storage volume for the following:
The storage volume at the bottom of the wet well or reservoir shall provide adequate submergence over the pump suction bell or adequate NPSH margin over the pump datum.
The storage volume at the middle shall provide adequate level or dead band to control the pumps preventing it from cycling motor that the number of starts per hour as recommended by the motor manufacturer.
The top storage volume shall provide free board to prevent the wet well or reservoir from over flowing in the event of abnormal operation. The volume shall also be adequate to allow w all of the pumps to stop during abnormal condition without overflowing.
As a result, it becomes advantageous to store large volumes of water upstream of the pump station. The pump station must rely on a forebay or reservoir as the source of water for the pump station. The forebays or reservoirs are typically above ground concrete structures used for storage of large volumes of water. The forebay/reservoir pump station arrangement can be used with horizontal end suction, split case pumps or vertical turbine pumps. The design engineer shall use MWH Best Practices for designing reservoirs.
FOREBAY
PUMP STATION
Figure 2-15: Aurora Forebay/Reservoir and Pump Station Page | Chapter 2-22
OCT 2011
2.3.7.
Industry Standards and Guidelines The following sections identify commonly used design guides in the water/wastewater industry. The design engineer should be familiar with these standards.
2.3.7.1.
Hydraulic Institute Intake Design Guide The Hydraulic Institute Standards were established to promote the continued growth and well-being of pump manufacturers and further the interests of the public in the areas of pumping systems. It is a collection of best practices when designing pumping systems. The guide covers topics ranging from pump placement in wet wells to maximum and minimum velocities in the flow stream. The use of the HI Standard is voluntary and the Design Engineer (Engineer of Record) is responsible for his design and therefore shall use his or her engineering judgment in using this Standard. As a minimum, MWH designs shall use the Hydraulic Institute Intake Design Guidelines where ever applicable to the project. The design engineer should be very familiar with the technical content in the guide.
2.3.7.2.
Flygt Design Recommendation Guides The design engineer is not always required to develop a wet well design from scratch. Not only are guidelines a useful resource, but under certain circumstances templates are available to aid the design engineer. ITT Flygt is a pump manufacturer who invested a significant amount of time in developing, verifying by Computation Fluid Dynamic (CFD) Models and physical model testing wet well designs. Flygt has developed design guides and templates for large submersible and axial flow pump installations. Although HI has not endorsed the Flygt design guides, these are industry accepted design methods. The HI Standard Committee has agreed to include it as a reference attached to the appendix of the Hydraulic Institute Standards. MWH has used the Flygt design guide in many successful wastewater installation. The Flygt guide includes a table which relates the individual pump capacities to various dimensions in the wet well. The two templates developed by Flygt are Pump Stations with Large Submersible Centrifugal Pumps and PL Pump Station Design Guidelines. The following is a link to the Flygt website: http://www.flygtus.com/
2.3.7.2.1.1. Pumping Stations with Large Submersible Centrifugal Pumps
This design guide identifies key dimensions for the wet well, and correlates these dimensions to the individual pump capacity. The design engineer should review footnotes in the design guide concerning appropriateness of use. There are known limitations of the design with regards to overall capacity and number of pumps. The Flygt pump station design guide utilizes a baffle region in the wet well with ports aligned with each individual pump. The influent line conveys flow into a baffled region, directing it downward along the finished floor towards the pump intakes. The intent on the design is not only to align the flow through the ports with the centerline of the pumps and mitigate short circuiting, but to increase the velocity along the finished floor. The higher velocities along the bottom of the wet well scour solids from the floor. In some instances, the design may call for the ability to take half of the wet well off line for cleaning or maintenance. In this type of situation, it is necessary to connect two adjacent wet wells using a slide gate. The main advantage of this configuration is to allow one wet well to be taken off line while other station remains fully operational. If the design calls for two wet wells to be joined, the connection point shall be in the baffle area of the wet well or upstream of the baffle area. Positioning the slide gate in the baffle area, or upstream of the baffle, minimizes the extent of cross flow at the pump intake. For any deviations from the design guide, no matter how minor, the design engineer shall consult the MWH Chief Mechanical Engineer. The concern is small changes (which may seem minor), could potentially have hydraulic ramifications to the design. For example, the port openings at the base of the baffle should have a velocity for 7 to 10 ft/sec and, subsequently, large head losses. If the design Page | Chapter 2-23
OCT 2011 engineer is concerned the velocity is too high, he or she may incorporate larger openings. To the design engineer, it may be a small change, however hydraulically the high head loss is beneficial and helps to ensure even flow distribution between the ports. Increasing the size of the ports is actually detrimental to the pump station. MWH has in-house capabilities to perform Computational Fluid Dynamic (CFD) simulations. Any wet well design which deviates from a recognized wet well design guideline shall be verified using CFD Modeling. The design engineer shall seek advice from the Chief Mechanical Engineer and Hydraulic Specialist if a CFD Model is appropriate. Figure 2-16. Includes images of the various configurations included in the Flygt design guide.
Figure 2-16: Excerpt from Flygt Design Guide Pump Stations with Large Submersible Centrifugal Pumps
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OCT 2011
The radial type wet well shown to the left has been used in the past in wastewater systems. To our knowledge, the pump station worked well. The main complaint was that this design was difficult to isolation individual flow to one side. If this configuration is to be used, a dual wet well is recommended.
Figure 2-17: Excerpt from Flygt Design Guide Pump Stations with Large Submersible Centrifugal Pumps 2.3.7.2.2.
Propeller (PL) Pump Station Design General Principles
In addition to the design guide for large submersible pumps, Flygt also publishes a design guide for submersible propeller pumps. The submersible propeller pump is a high flow – low head vertically suspended pump. The PL designation is a Flygt sales code for: P: Multi-blade propeller pump with bowl assembly, bell-mouth and outlet cone, for large capacity pumping of clean liquids. L: Semi-permanent vertical installation in large diameter discharge column made of steel or concrete. Typically this type of arrangement is used for flood control applications. The pump and motor are inserted into a pipe or caisson and submerged below the water surface. The discharge from the propeller is conveyed over the exterior of the motor, and then vertically to the discharge fitting at the surface.
Figure 2-18: Submersible Propeller Pump Propeller pumps are more sensitive to inlet flow disturbances relative to other pumps. Ideally the flow at the pump inlet should be uniform and steady without swirl, vortices or entrained air. To aid the design engineer in creating ideal hydraulics at the suction of the pump, Flygt developed the PL Pump Station Design General Principles. The recommendations presented in the design guide utilize established principles of hydraulic design obtained from the Hydraulic Institute Standards. The guide Page | Chapter 2-25
OCT 2011 also includes design information based on model and full scale tests, specific to propeller pumps, conducted by Flygt. The guide provides the design engineer a reliable hydraulic configuration for this type of pump. CAUTION: The design engineer shall request Flygt to review the design. If possible, request Flygt to verify by CFD simulation at no or minimal cost. CFD Model can also be prepared by our MWH specialist. The Flygt design is typically suited for their pumps. If an or equal is required on the project, the design engineer must indicate on the drawing that the pump manufacturer shall verify the intake design configuration and recommend to the engineer any modifications required to suit the or equal pump. The Flygt guide is a combination of narratives describing the various wet well configurations and dimensional drawings. The following text and images were obtained from the Flygt guide. The exact dimensions for each configuration are not included. The design engineer shall refer to the Flygt PL Pump Station Design General Principles for more information. The designs are divided into three configurations “A”, “B” and “C”. Configuration “A” – Standard Open Sump This configuration is the simplest to build and is often the first alternative considered. However, as it requires more submergence and possibly a longer approach than other configurations, the total cost of the station may be higher than other options.
Figure 2-19: Configuration A with Plain Intake or Vortex Cone and Swirl Plate
Figure 2-20: Configuration A with Intake Modifications for Asymmetrical Flows Configuration “B” – Compact Closed Intake This type of configuration is typically constructed of concrete or steel. The geometric features, like the curvature of the front wall, the corner fillets and the benching at the back wall, have been developed to allow smooth acceleration and turning as the flow enters the pump. Page | Chapter 2-26
OCT 2011
Figure 2-21: Configuration B With Concrete Construction Configuration “C” – A Closed Intake of the Draft-Tube Type This type of configuration utilizes a draft-tube intake, (also called a formed suction intake) and can be constructed of either steel or concrete. The intake reduces inconsistencies and swirl in the approaching flow. This intake is more effective than Configuration “B” because the sloping front wall is designed to minimize stagnation of the surface flow. The geometrical features of this intake provide for smooth acceleration and turning as the flow enters the pump. The minimum inlet submergence should not be less than 1xD.
Figure 2-22: Configuration C Closed Intake with Draft Tube
2.3.8.
Hydraulic Intake Design Intakes are considered to be an integral part of “pumping facilities”. Intakes vary in design, requiring special knowledge and expertise. A comprehensive discussion of intake layouts is beyond the scope of this Guide. This section is meant only to introduce the topic to the design engineer. Intakes include necessary structures located within a lake, reservoir, river, stream, canal or other body of water. The structures are inlet points for the pump station, and serve to minimize hydraulic disturbance and/or attempt to prohibit debris from entering the suction pipe. Pumping equipment may be mounted directly on the intake structure or may be located some distance away on dry land. When intakes provide a supply to remotely located pumps, the conveyance between the intake and pumps must be designed for gravity flow.
Page | Chapter 2-27
OCT 2011 In large bodies of water the actual intake may be located some distance from the shore. The intake should be located at a sufficient distance from shore (or edge of the reservoir) to assure adequate submergence under all anticipated seasonal and cyclic conditions. Frequently water quality considerations or ice influence the depth and/or distance from the shore. Potential damage of submerged intakes (by vandalism and anchors) needs to be considered if the area is used for recreational activities. The simplest intake consists of a screened pipe or concrete box, which supplies a pipeline extending to the pumping stations. Where the body of water is deep (more than 30 ft), and may experience seasonal turnover, a tower may be constructed with valve or gated intake ports to enable selection of the best quality water. The tower may also serve as the suction sump for vertical pumps. The latter requires a bridge, floats or other means for the discharge piping and electrical equipment to reach shore. In some lake intakes, fish screens are required. If fish screens are required, consult our fish screens expert in the MWH US Seattle Office. Example of a lake intake design at Lake Meade, Lake Powell and Morse Lake is shown in Figure 2-23.
Figure 2-23: Example Lake Intake Design
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OCT 2011
2.4.
Preliminary Design, Equipment and Piping Layout The design engineer shall follow the design and quality review requirement in the MWH Delivery Framework. In summary, the delivery framework consists of a list of activities and tasks required to implement a project from the preliminary design phase through detailed design and construction document development. The delivery framework requires the design engineer to follow the steps in chronological order to enhance efficiency, accuracy, avoid rework to meet quality, budget and schedule. Once the Client and MWH have selected a specific pump station configuration, the design engineer shall develop a preliminary piping layout. The preliminary piping layout shall be thoroughly developed as it directs further work by Structural, Architectural and Civil disciplines with respect to the overall building size. Future changes in the piping layout could result in a ripple effect through other disciplines forcing hours of additional rework. As a minimum, the design engineer shall comply with the piping layout guidelines identified in the Hydraulic Institute Standards and within this guide. The first step in defining the piping layout is to determine the total number of pumps. The number of pumping units and operating range of each pump is partially defined by the minimum and maximum capacities required by the pump station. The combination of pumps in the pump system must be capable of covering the entire range of required operational conditions. The Engineer shall identify the minimum pump station flow and the maximum flow at full build out. Providing a system which covers this range is a critical component of the design criteria. If the lower flow and head range is beyond the operating range of the main pump(s), a small “jockey” pump may be required. The jockey pump would operate at low flow and low head conditions. The design point of the jockey pump should be determined such that the flow and head range will slightly overlap with the operational envelope of the first large system pump. Once the quantity of pumps is defined, the design engineer shall proceed with designing the piping layout. The design engineer shall determine the following as part of the preliminary piping layout.
2.4.1.
Flow, TDH, horsepower, speed
System head curve and pump curve operating envelope
Type and number of pumps
Define characteristics of fluids, temperature, specific gravity, and fluids constituents
Identify corrosivity of fluid
Pump, valves and piping material selection
The system boundary for the piping layout.
Size of the suction and discharge piping.
Pressure rating of the piping and valves
Pipe materials of construction
Types of valves
Select ancillary equipment such as air compressors, HVAC, electrical rooms
Hydraulics Before commencing on hydraulic calculations, basic criteria should be summarized. The design engineer must know the full range of conditions that will be expected of the pumping facility. The following criteria should be obtained and summarized;
Maximum design capacity including the estimated seasonal, daily (or hourly rates) and other aspects of flow patterns.
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OCT 2011
The minimum required plant capacity including whether or not such flows need to be intermittent or continuous and the estimated percentage of the time that minimum or low flows may persist.
Future and/or estimated ultimate flows.
Where applicable, determine the pump station flow for which the pumps will operate most of the time or exceedence flow. For example, a sewage pump station in a collection system is to be designed for 200 MGD, but the 90% exceedence flow would be approximately 24 MGD.
The range of pressure or water levels that may be available on the suction side of the facility
The discharge conditions including the range of pressure and/or levels, that may exist at discharge locations
Details of existing (or proposed) discharge piping including materials, pressure rating, age, condition, diameters, layout lengths, valves and fittings.
Figure 2-24: Example Exceedance Curve
In most cases, the degree of accuracy required for computation of friction losses (and static heads) in pumping systems needs to be in the range of plus or minus 2 percent. If losses thru valves and fittings are not computed, include an allowance for minor losses and velocity head in the range of 2 to 4 percent of calculated pipeline friction, depending on the complexity of the discharge piping. For low lift applications (total pumping heads of less than 60 ft) minor losses (including velocity heads) may be a significant portion of total pumping head and need to be individually calculated. If information on discharge system dynamic losses is available from actual field tests and/or other hydraulic analyses, it should be scrutinized and if representative utilized in preference to computed data. It is important to note, that total dynamic head estimates are frequency over-estimated. A rare or extreme condition should not be the basis of design. There is a tendency to use conservative friction factors in computing dynamic losses in pumping systems, particularly transmission mains. Overly conservative assumptions should be avoided because they may result in misapplication of pumping equipment. Always use realistic and applicable assumptions and confirm with in-house advice. Use hydraulic data that you are familiar with. Hazen-Williams, Manning and Darcy-Weisbach/Colebrook formulas are all applicable. For water and wastewater friction loss calculation, use Hazen-Williams equation with C-factor per AWWA M-11. For viscous fluids such as chemical solutions, water and wastewater with velocity greater than 10 fps, use Darcy-Weisbach/Colebrook equation. 2.4.1.1.
Pump Station Capacity Considerations The design capacity of pumping facilities should be based on detailed investigations and projections. For most municipal applications, the station infrastructure such as buildings, embedded pipes and Page | Chapter 2-30
OCT 2011 electrical conduits will have a design life of more than 50 to 75 years and equipment can be replaced every 25 years. It may be appropriate to design the facility so that it could be expanded to meet anticipated demands of a 50 to 75-year planning period. Wastewater pumping stations and certain water applications may need to be designed to meet projected peak hour demands. Other water pump stations may be designed to meet estimated maximum-day demands. It is necessary to be familiar with the rationale of how the proposed plant capacity was determined and to be sure that it is appropriate and objective. It is also necessary to know what changes may be proposed on the discharge side of the pumping station in the future. Future pipeline improvements within a distribution system, for example, can significantly influence head (and capacity) of a pump station. The design criteria for the pump station should be shown on the design drawings. The criteria should show initial and future pump station capacity, number of units at each stage of development, key piping diameters, nominal velocity in suction and discharge piping, maximum operating pressure and motor ratings. For storm water pumping stations there are significant risks and exposure. All assumptions that were made to estimate peak flows need to be confirmed and indicated on a design criteria sheet that is part of the contract drawings. Key criteria should be listed such as:
Design storm frequency
Rainfall intensity
Watershed area
Runoff coefficient
Percentage impervious area
Storage basin capacity
Basin operating levels
If there is uncertainty due to such variables as rainfall intensity or demand forecasting assumptions, such uncertainties should be exposed and discussed with the client. Frequently the agreed solution is to provide for staged construction. The design criteria and other data on the drawings should clearly indicate if future stages were assumed. The criteria sheet should be signed by a responsible person representing the client. Be cautious in adopting past designs and criteria, for both wastewater and treated (potable) water. In the past, temporary overflows from wet wells or forebays, due to short term power out-ages may have been acceptable. Currently in the US, however, releases of untreated wastewater (or even disinfected potable water) due to negligence, including deficient designs can result in legal action and criminal charges. It is imperative that all foreseeable conditions and events should be considered to ensure that inadequate station capacity or design deficiencies will not result in releases. Once the design capacity of a pump station is established, subsequent steps should ensure that the capacity can be delivered by the proposed pump station with at least one pumping unit off line. For water pumping stations that directly serve a distribution area, the capacity determination (on whether the pump station capacity should be based on peak hour or maximum day projections) is related to the capacity of on-line service reservoirs and/or elevated storage. It will be necessary to confirm that there is sufficient storage to provide for peak hour demands and fire requirements.
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OCT 2011 2.4.1.2.
Total Dynamic Head Total dynamic head under a given flow is the sum of
Static lift
Friction head
Velocity head
Static head is the difference in elevation between the water level (or equivalent hydraulic grade line) on the suction side of a pump and the level on the discharge side. It is a measured or estimated under zero flow (static) conditions. Friction head is the sum of losses due to friction in suction and discharge piping and valves and fittings. For most applications it needs only be computed for one flow condition and at other flow conditions it can be estimated by assuming that total friction is proportional to flow squared. Velocity head is the energy required to move liquid. It is equivalent to V2/2g. It usually is a minor consideration but can be significant in low-lift pumping applications that are why velocity head should always be included in the calculation. Normally kinetic energy (or velocity head) is lost when flows are discharged into a larger pipe or into a reservoir. The kinetic energy can be recovered with gradual velocity reduction. If the velocity head is a significant factor (more than say 5 percent of TDH) which may be the case in low lift applications, particular attention should be given to hydraulic computations and all minor losses should be computed. TDH is the steady state head that will need to be developed by pumping equipment under operating conditions. TDH does not include temporary surges or increased head conditions (due to starting and stopping and associated valve operation) that may prevail for less than 30 seconds. In the US, TDH is normally quoted in feet of water of head. For SI units, the correct term for head (pressure) is kilo Pascal (kPa) but in practice it is often quoted in meters. (1 meter of head is equivalent to ten kPa) One reason that head is quoted in linear units (feet or meter) instead of pressure units is that head produced by a pump may be quoted linear units without correction for specific gravity. Thus, head is interchangeable for pump performance at different specific gravities. Whenever there is an application using sea water (or liquids other than fresh water), the head developed by the pump must be multiplied by specific gravity in order to compute pressure and energy requirements. 2.4.1.3.
System Head Curves In order to establish typical pump operating conditions, it is advisable to graphically represent the total range of pumping conditions with system head curves. Once such curves are prepared, they can be overlaid with pump performance curves. See Section 5.1 for methodology for creating a system curve.
2.4.1.4.
Major Pump Stations For major pump stations (in excess of 10,000 HP connected loads) factors which may influence the number or units include:
Limitations of size for commercially available pumps. Large pumps and motor are available up to 30,000 horsepower
Impact on power grid of starting large motors based on motor starting current
Matching power demands with generation capacity and other pumping stations.
A review of major large pump stations with capacities to 5,000 cfs for the US Bureau of Reclamation, California Aqueduct Pump Stations by State Water Resources Board, Central Arizona Water Project, Southern Nevada Water Authority and MWD of Southern California, indicates that the number of pumps ranges from 2 to nine units.
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OCT 2011 2.4.1.5.
Medium Size Pump Stations For medium size plans, say 500 to 10000 HP there may not be size limitations on commercially available equipment but starting electrical loads may influence the maximum motor size. Other factors which may influence the number of units include:
Starting and stopping frequency due to forebay size or delivery side storage limitations
Client’s standards, or requirements for more than one standby unit
Provisions for future expansion
Delivery to more than one water system pressure zone
The unit costs of design and construction are increased when more than one-size (of pumping unit) is specified. Not only are pump layouts and specifications affected by more than one size of pumps but mechanical appurtenances (including valves and pipe fittings) and electrical (motors, starters ad controls) details are also more complicated. The client’s requirements for stocking spare parts and other operating considerations, such as maintenance manuals, servicing and records, may be increased. In the long run, the perceived advantage of providing smaller pumps “for in-between flows,” may be lost. Unless there are specific limitations on electrical loads or frequency of stopping and starting (due to inlet or discharge considerations) the “one size” layout is preferable. 2.4.1.6.
Small Pump Stations Pumping stations with total installed capacity of less than 500 HP are the most numerous applications for water and sewage. For sewage pumping plants, there is a need to be more conservative than for most water applications. It is common practice to provide two standby pumps for sewage pumps and it also may be the clients’ preference to provide two sizes of units (one size for normal dry weather flows and one size for peak wet weather flows). Thus, the minimum number of units for sewage plants may be three (or four if the client so directs) For water applications, it is necessary to consider if the facility that is being proposed is the sole source of water for the area being served or if there are other pumping or gravity sources available. If the facility is the sole source of water and it serves an area with limited storage (less than 6 hours) or high fire risk (industrial and/or heavy commercial) then two standby units could be provided. For stations servicing residential areas and areas with adequate storage (capable of furnishing peak hour demands, fire flow and emergency provisions) a simple 2-unit stations (one duty and one standby unit) may be applicable.
2.4.1.7.
Standby Units and Standby Power Supply The number of standby pumping units is usually based from the Clients preference or as mandated by the EPA or other applicable government agency to meet the reliability design criteria. Standby unit is defined as the additional pump unit(s) in addition to the minimum number of unit required to deliver the maximum pump station capacity with one largest unit out of service. The capacity of the standby pump shall be equal to the largest pump unit. Several factors enter into an analysis of the risk of power interruption (vulnerability) associated with the pumping design. The risk factors that need to be evaluated include: equipment reliability, electrical demand patterns, power interruption frequency and available forebay storage or emergency storage. It is seldom possible to compute and accurate or comparable risk factor. Historical data is generally not representative of future interruptions because the causes of past interruptions may have been partially corrected. Also future conditions may not have been experienced in the past. Unless it is a contract requirement, risk analysis should be avoided and the rationale for standby units (and power supplies) should be evaluated based on the following considerations.
In the case of water systems, most short-durations power interruptions are not a serious concern because of the provision of system storage. Most water pumping stations therefore, do not include standby power provisions but do include an electrically-driven standby pumping unit with Page | Chapter 2-33
OCT 2011 a capacity equivalent to the largest unit. Generally, the standby unit is identical and interchangeable with other units and there are provisions in the control system for any pump to be designated as the ‘lead pump” and any unit to be designated the standby unit.
2.4.1.8.
Power interruption is more of a concern with sewage pumping plants because of accidental sewage overflow to the nearby river or ocean. Also the risks associated with maintenance on pumps are significantly higher with sewage pumps, as compared to water pumps. If the client has adopted policies with respect to standby units and/or standby power, they should be accepted, unless it is apparent that they are not adequate. In general, two standby pumps should be considered for sewage pumping stations.
Net Positive Suction Head (NPSH) The NPSH available (NPSHa) in the system is the total suction head in feet of liquid being pumped (absolute measured at the pump centerline or impeller eye) less the absolute vapor pressure (in feet) of liquid being pumped. Please refer to the Cameron Hydraulic Book or the Hydraulic Institute Standards. NPSHa = (144/w)(Pa-Pvp) + hs Where; Pa = atmospheric pressure, psia Pvp = vapor pressure of liquid in psia W = specific weight of liquid in pounds per cubic foot The NPSH required (NPSHr) by the pump is usually indicated on the pump data sheet or pump curve and it is referred to at the pumps’ best efficiency point. The information must be obtained from the pump manufacturer. NPSH characteristics are particular to each pump. It is important for pump station design engineers to understand the relationship between the impeller design versus, head, capacity, speed and suction condition which determine the shape of the pump curve. Refer to the Hydraulic Institute Standards for additional information regarding the relationship between NPSHr and Suction Specific Speed “S”. S=rpm(gpm)0.5/(NPSHr)0.75 Where: S = specific speed rpm = speed NPSHr = feet absolute The recommended suction specific speed value should be limited to 8,500. Higher or lower values maybe used depending on liquid properties, intake design, impeller suction capability, materials of construction and application experience. If higher specific speed is to be used, consult the pump manufacturer. The data from one pump model or impeller type cannot be extrapolated to a larger mode or to a high speed application. The only safe assumptions that can be made are:
NPSH margin shall not be less than 5 feet ( NPSHa=/>NPSHr +5 ft)
If higher specific speed is to be used, consult the pump manufacturer
Figure 2-25 is typical of information on NPSH that may be made available by pump manufacturers. NPSH requirement are often shown on the lower right hands side of pump performance curves and generally have a separate vertical head scale as shown.
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OCT 2011
Figure 2-25 Typical Manufacturer’s Performance Curve Showing Required NPSH
The following facts should be noted regarding NPSH:
Required NPSH is always inversely related to pumping head but is not otherwise related.
The NPSH data is generally only shown for performance on the right side of the performance curve. It may be assumed that requirements on the left of the curve will not be critical.
Required NPSH is normally applicable to all impeller trims shown on pump performance curves.
The NPSH data is applicable only at the speed shown. Small increases in speed (i.e. from indication speeds to synchronous speeds) may cause significant changes in requirements.
The data is not transferable from one pump manufacturer (or one pump model) to another, even though other performance characteristics appear almost identical.
In practice, not less than three NPSH curves from different manufacturers should be obtained for each pump size application. If all curves indicate that the required NPSH, under maximum flow conditions is less than 30 ft than normal submergence requirements will be applicable (except for warm or volatile fluids). At elevations above 1,000 feet above sea level adjust barometric pressure for elevation change when calculating for NPSHa. The higher the site elevation, the higher the required submergence. If during design for a certain application, it is determined that NPSH may be a problem, it may be necessary to consider the following alternatives
The greatest risk of cavitation occurs when a pump operates at lower than anticipated discharge pressures and the operating point moves to the far right of the performance curve. Design criteria should always be reviewed to determine if the original selected design head is too high. In practice many cavitation problems are due to over-estimation of the head requirement and consequent operation to the right of the performance curve, in the zone of high required NPSH.
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OCT 2011
NPSH requirements are influenced significantly by pump speed. If required NPSH adversely influences design, it may be necessary to consider lower speed pumps.
An obvious solution would appear to be to increase adequate available NPSH at the suction of the pump by increasing submergence of the pump impeller and/or reducing friction and/or velocity head losses on the suction side of the pump. Such solutions however may not be feasible.
In some instances, it may be possible to minimize cavitation problems in existing installation by changing impellers or changing metallurgy of impellers and/or pump suction components but such potential solutions should never be relied upon during the design stage or consult the pump manufacturer.
2.4.2.
Material Considerations Welded steel is typically used for pump station piping. However, certain clients and regional practice may dictate the use of ductile iron or PVC pipe and fittings. In such instances, the primary concern is the cost and space requirements for flanges. Welded steel pipe is normally devoid of flanges, except where connections to flanged valves and other flanged equipment are required. However, it is sometimes convenient to provide a flanged elbow to facilitate removal of a valve and avoid the use of a mechanical couplings or dismantling joints. (i.e. Victaulic) Steel pipe and fittings have many advantages over ductile iron and/or PVC for pump station applications because it can be fabricated to suit. It is normally structurally superior and can be fabricated to conform to most configurations. Steel pipe is almost always prefabricated in a shop, using shop drawings prepared by the manufacturer and reviewed by the design engineer. Included with the prefabricated pieces and fittings are makeup pieces, usually in the form of butt straps, to facilitate mating up continuously welded pipe. Elbows, tees, wyes and specialty fittings are fabricated from previously rolled and testing straight pipe. Outlets on the suction and discharge header are reinforced to comply with design standards shown in AWWA Steel Pipe Manual M-11. Flanges are usually furnished loose for field welding. Steel pipe and fittings are generally furnished with cement mortar lining and mortar coating for buried installations, and mortar lining with no coating for exposed use. On-site buried pipe should be consistent with adjacent pipelines and coating systems and corrosion protection should be coordinated with adjacent pipeline design and practice. To aid the design engineer, MWH has developed a Piping Schedule based on experience and industry standards. The Pipe Schedule correlates the process and the typical pipe and valve materials used for a specific process. The design engineer should always callout the pipe size, fluid and pipe material using the MWH Standard Tagging system as indicated in the Piping Schedule. The Piping Schedule is included in Appendix B and is available on Delivery Framework. The station piping is usually lined with material compatible with the fluid handles and to protect the pipe from corrosion or erosion. The design engineer has a variety of options available with regards to pipe lining. The lining should be selected based on the process and specific properties of the fluid. Ideally the lining should protect the interior of the pipe and extend its useful life. The lining material and its thickness are based on the characteristics of the liquid being pumped; there-fore, the design engineer needs to confirm what liquid(s) are transported through the piping system prior to making this selection. The following pipe lining materials are commonly available:
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OCT 2011
Table 2-4: Commonly Used Pipeline Materials Lining Material
2.4.3.
Service
Mortar Lined
General raw water, treated water; reclaimed water; raw sewage, wastewater
Epoxy
General raw water, treated water; reclaimed water; raw sewage, wastewater
Glass Lined
raw/primary sludge; scum; thickened sludge; digested sludge dewatered sludge;
Polyurethane
Raw sewage up to aeration basins; any piping systems that are intermittently wet and dry
System Boundary for the Piping Layout Design work is typically divided based on specific disciplines. Some of the disciplines have clear dividing lines with regards to the extent of the work, while others are less clear. Mechanical and civil tend to overlap in responsibility when it comes to buried piping. The interface point between mechanical should be clearly decided before the start of the design. Typically, the interface between mechanical and civil occurs at approximately 3 ft beyond the exterior of the building wall at a buried pipe coupling. Where differential settlement between the building and the yard piping due to type of soil condition, the piping system shall be provided with two sleeve couplings spaced at least 5 to 10 feet between centers to allow the pipe to articulate and adjust for differential settlement. Where the differential settlement exceeds the capability of the sleeve coupling, ball type coupling by EBAA shall be used. The amount of differential settlement shall be recommended by the Geotechnical Engineer.
2.4.4.
Sizing the Suction and Discharge Piping In development of the piping layout, the design engineer shall determine the size of the suction and discharge piping. For purposes of this discussion, suction and discharge piping refers to the piping immediately upstream and downstream of the pump. The terms suction and discharge header refer to manifolded piping where flow streams from multiple pumps are combined. The suction and discharge pipe sizes for each pump are usually dictated by the flow stream velocity within the pipe when only one pump is operating. The flow where the system head curve intersects with the pump performance curve is usually the worst case flow scenario. The suction and discharge header shall be determined using the pump station firm capacity. Firm capacity is the maximum capacity of the pumps station with the largest pump out of service. The design engineer shall also evaluate the full range of flows including minimum flows and future flows. Based on the process, a minimum flow rate may be required to keep solids suspended in the flow stream. Furthermore, higher flows in the future may be the controlling factor when sizing the suction and discharge pipe sizes.
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OCT 2011
Figure 2-26: Typical Suction and Discharge Piping 2.4.4.1.
Suction Piping Suction Pipe Size In order to optimize the suction piping, the design engineer shall select a size which results in a maximum flow velocity between 2 and 5 feet per second (fps) at maximum pump flow rate. Typically suction side velocity is low to minimize the impact on the NPSHa. High velocities induce suction side losses which reduce the overall NPSHa. Furthermore, high velocities on the suction side could result in non-uniform velocity distributions entering the pump impeller, creating impeller imbalance and vibration issues. Table 2-5: Recommended Suction Side Velocities Special Applications
Recommended Minimum Suction Side Velocity
Solids Handling
>2.5 ft/sec
Slurries
>5 ft/sec
Mining Slurries
Representative sample should be sent to a testing facility to determine the carrying velocity, settlement velocity, shear, specific gravity, corrosivity, and friction factor
Suction Pipe Length Suction piping must have a straight approach length leading into the pump greater than 5 times the pipe diameter. Straightening vanes may be provided to reduce the approach length, however, the design engineer must consult with the Chief Mechanical Engineer before using straightening vanes in a project.
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OCT 2011
Figure 2-27: Suction Piping Configuration Excerpt from Hydraulic Institute Intake Design Standard Suction Pipe Fittings Provide suction piping with eccentric reducers adjacent to the pump. The reducers shall be mounted with the straight section on top, to preclude the trapping of air inside the pipe. (If an eccentric reducer cannot be provided, DO NOT provide an automatic air vacuum release valve because the air valve can leak and admit air into the suction piping – rather, provide a manually operated ball valve for venting air during initial startup.) Eccentric reducers may be mounted directly on the suction flange of the pump. Suction Valves The design engineer shall also review the design, for any features which would create an uneven flow distribution in the pump. For example, horizontal split-case double-suction centrifugal pumps include a horizontal suction nozzle baffle. If the pump experiences an uneven flow with the same orientation as the suction baffle, one side of the baffle receives more flow than the other. This uneven flow distribution is detrimental to the pump performance. The source of an uneven flow could be a butterfly valve on the suction piping. As flow passes through a partially open butterfly valve, it is directed towards one side of the downstream pipe creating an unbalanced flow distribution. If the unbalanced flow distribution is aligned with the suction baffle, more flow enters one side of the baffle as opposed to the other side. As a result, the unbalanced flow distribution enters the eye of the impeller. To maintain an even flow distribution into the pump, the suction isolation butterfly valve should be installed with the shaft in the vertical position perpendicular to the pump suction nozzle baffle. The uneven flow distribution is guided downward allowing an equal amount of flow to enter each baffle area.
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OCT 2011
Figure 2-28: Vertical Turbine Intake Flange Excerpt from Floway Pumps Turbine Pump Handbook Vertical Turbine Pump Barrel/Can Installation Vertical turbine pumps in a “barrel or can” installation, the suction piping are connected to the barrel. Flow enters the barrel, and is guided downward entering the suction bell of the bowl assembly. Where the suction inlet is connected above the suction bell, the barrel inside diameter should be sized for a velocity in the annular space between the ID of the barrel and the OD of the pump bell, bowl or flanged column, (whichever is greater) does not to exceed 3 ft/sec, based on the maximum flow of the pump. For small and medium sized pumps (200 to 2000 gpm) barrel velocities to 5 ft/sec may be applicable. For large applications, refer to pump manufacturer. Mixed flow and propeller pumps should not be barrel mounted due to the high capacities confined to the barrel. The Hydraulic Institute Guidelines for Intake Design Standards identifies specific dimensional criteria for the suction piping associated with vertical turbine barrel/can installations. This criteria describes the suction pipe approach length and the inlet location relative to the pump suction bell. With regards to the approach length, the horizontal suction piping must have straight approach length (i.e., no flow disturbances such as valves) greater than 5 times the suction pipe diameter. As stated previously, the intent of the approach length is to establish a uniform velocity distribution at the inlet to the barrel. The second dimensional criteria indentified in HIS are the distance between the barrel/can inlet and the pump suction bell. MWH exception to HI, the vertical distance between the centerline of the suction piping and the pump suction bell must be a minimum of 3 to 4 times the pump "can" inside diameter instead of 2D. Refer to the Hydraulic Institute Standards for additional requirements, however HIS requires only twice the can diameter based on MWH experience, additional distance is required in order to allow the flow to straighten before it reaches the suction bell. The pump “can” or “barrel” must be furnished with air/ vacuum valve in accordance with MWH Standard Detail M-119.
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OCT 2011 2.4.4.2.
Discharge Piping The velocity limitations at the suction piping are primarily due to the inlet conditions of the pump. On the discharge side, however, the flow velocities can be higher. The discharge pipe diameter is dictated by balancing the most economical pipe size with the effects of high velocity flow. At a specific flow rate, as the pipe diameter is reduced, the flow velocity increases thereby increasing the head loss through the pipe. It is normally economical to design on-site discharge piping for higher velocities because of the higher average unit costs of the more complex piping and more numerous valves and fittings appurtenant to pumping stations. Velocities leaving pumping units are frequently in the range of 10 ft/sec and it is practical and economical to pass such flows through valves and manifolds before the velocity head is dissipated. The off-site piping is often designed with velocities between 6 and 10 ft/sec. For water conveyance piping longer than one mile, a pump-pipeline economic analysis study shall be performed to determine present worth versus pipe size. A copy of an example pump-pipeline economic analysis study is shown in Figure 2-29.
Pumped Pipeline System Economic Analysis 10
$50,000,000 $45,000,000
9
$40,000,000 8 $35,000,000
Annual Cost ($)
6
$25,000,000 $20,000,000
5
$15,000,000 4 $10,000,000 3
$5,000,000
Water Velocity
7
$30,000,000
2
$60
62
64
66
68
70
72
74
76
78
80
82
Pipe Diameter Total Annual Cost
Annual Pumping Cost
Annual Pipe Cost
Water Velocity
Figure 2-29: Example Economic Analysis of Pump and Pipeline system The following table is a guide for velocities to be utilized for water and wastewater pumping plants. Detailed hydraulic investigations and consultation with equipment manufacturers may result in adjustment to the listed velocities. Reducing the velocities indicated is safe but may result in a less economical design. Increasing the velocities by more than 10 percent, however, should not be undertaken without careful investigation and approval from the Chief Mechanical Engineer.
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OCT 2011
Table 2-6: Summary of Recommended Maximum Pipe Velocities Piping Segment Description
Ft/sec
m/sec
On-site suction delivery pipe
5.0
1.52
Suction pipe to individual pump units or pump barrels
5.0
1.52
Nominal velocity through suction valve (90% minimum port area)
5.0
1.52
Maximum actual downward velocity in the vertical pump barrel.
3.0
0.91
Net velocity in vertical pump discharge column after reduction of column shaft or inclosing tube
10.0
3.05
Discharge pipe from each pump Nominal velocity through discharge
8.0
2.44
Valves (90% minimum port area)
8.0
2.44
Discharge manifold
10.0
3.05
Fire Pump Manifold
12.0
3.66
Bypass or return flow piping
20.0
6.09
Maximum nominal flow through a butterfly valve
16.0
4.88
* Based on nominal capacity of pumping facilities or the design capacity of individual units as Applicable.
Figure 2-30: Example Photograph of Discharge Piping 2.4.4.3.
Pumps without Suction Piping Submersible non-clog centrifugal pumps and vertical turbine pumps which are directly taking suction from the wet well do not require suction piping. Wet well suction configuration shall be designed in accordance with the Hydraulic Institute Standards.
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OCT 2011
2.4.5.
Pressure Rating of the Piping and Valves The maximum allowable design pressure of the pipe flanges or valve shall be based on the design head or shut-off head of the pump whichever is larger. The maximum transient pressure is kept less than 125 percent of the pipe design pressure by providing proper surge protection devices. The piping system, valves and flanges are tested 125 percent of the maximum allowable pressure rating to allow for transient pressure rating. This information is used to determine the pipe wall thickness and the class of flanges used on the equipment and fittings. To mate flanges of different standards and ratings requires that both flanges have pressure ratings equal or above the maximum anticipated line pressure Table 2-7: Summary of Flange Pressure Ratings and Fit Up AWWA Flanges
Mate to ANSI Flanges
Class B (86 psi, steel) Nominal sizes from 6” - 96”
Class 125 lb Std., Cast Iron
Class B (86 psi, steel) Nominal sizes from 6” - 96”
Class 25 lb. Std., Cast Iron
Class B (86 psi, steel) Nominal sizes from 6” - 24”
Class 150 lb. Std., Steel
Class D (175 & 150 psi, steel) Nominal pipe sizes 6” - 96”)
Class 125 lb. Std., Cast Iron 6” – 12” inclusive 175 psi; larger sizes 150 psi Class 150 lb. Std., Steel 6” – 12” inclusive 175 psi; larger sizes 150 psi Class 125 lb. Std., Cast Iron 6” – 12” inclusive 175 psi; larger sizes 150 psi Class 150 lb. Std., Steel
Class D (175 & 150 psi, steel) Nominal pipe sizes 6” - 24”) Class E (275 & 150 psi, steel) Nominal pipe sizes 1” - 96”) Class E (275 & 150 psi, steel) Nominal pipe sizes 1/2” - 24”)
The design engineer must identify the maximum system pressure during normal and abnormal operation for a specific section of pipe. A good starting point for determining the maximum pressure rating of the piping is to determine the shutoff head for the pumps. The shutoff head is the maximum pressure created by the pump at zero flow. Shut off head is a differential pressure; therefore the suction pressure should be added to the shutoff head in order to determine the total discharge pressure of the pump. This discharge pressure represents the highest pressure generated by the pump. In some cases, it may be necessary to increase the pressure rating of the suction piping. Pump systems are typically equipped with check valves on the discharge side of the pump. In the event check valve fails, the valve allows the system pressure to migrate through the pump to the suction piping. The suction isolation valve, flanges, piping, sleeve (or Victaulic-style) coupling, and suction nozzle must be rated at the pressure rating of the discharge piping. Harness or restrained joints must be designed for a maximum pressure equal to the discharge piping. The design engineer must also account for transient conditions. In the event of a hydraulic surge, the pressures in the piping can dramatically increase. Hydraulic Transients are discussed in further detail in Section 2.9. The design engineer should consult the Senior Hydraulic Specialist to determine a realistic maximum pressure for the piping.
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OCT 2011
2.4.6.
Flexible Couplings and Dismantling Joints When designing the piping layout, the engineer, shall include features considered good practice when it comes to maintenance and operations. The piping system shall be designed for flexibility and ease of disassembly. It is typically good practice to include dismantling joints upstream and downstream of a pump to facilitate removal of the pump. Dismantling joints are flanged adaptors or sleeve couplings or grooved mechanical joints. This flexible connection or expansion joints such as rubber or stainless steel bellows shall be allowed only for high temperature applications such as in the hot water or digester sludge pumping application. The use of flexible bellows connection at the suction and discharge have been misapplied in many situation because the contractor relies on the flexibility of the joints to make up for the misalignment between the pump and the piping system instead of aligning the piping to the pipe in the first place. The end result was the bellows was forced to connect to the pump, causing the pump to misalign. The pump suction and discharge nozzles are not designed to support the piping system loads. Significant loads on the suction and discharge piping may distort the pump casing (or discharge head) applying unnecessary stresses to the casing. In some cases, exceeding the allowable nozzle load has been associated with excess vibration. The design engineer should also consider the maintenance aspects of the design. Pump and valves need to be removed for periodic maintenance. During removal of a valve, the operators pull the piping apart axially to separate the flanges. To assist in this process, the piping should be designed with dismantling joints. A dismantling joint is a type of pipe coupling specially designed with a small gap. The operators are able to collapse the dismantling joint creating a gap which can be used to move the piping axially. The design engineer should be familiar with the various types of dismantling joints and pipe couplings. The design intent may suggest one coupling over another. The following is a list of typical coupling types.
Flexible Coupling
Dismantling Joint
Sleeve Coupling
Victaulic Coupling
Figure 2-31: Examples of Various Pipe Coupling Fittings
2.4.7.
Bridge Cranes Limitations to Piping Layout When utilizing a bridge crane, the design engineer shall verify the full range of motion for the lifting hook, in both the vertical and horizontal planes. The range of motion (and lifting capability) of bridge crane is limited by the physical constraints of the trolley and hoist. The design engineer shall verify that equipment and valves are located within the lifting range of the bridge crane. This verification is critical to finalizing the overall building dimensions. Any errors in determining the range and lifting capability may lead to costly rework of the structural design.
Page | Chapter 2-44
OCT 2011
Vertical Range The design engineer shall verify each piece of equipment can be lifted and moved to various locations in the pump station. The lifted components must be able to clear the height of railing and adjacent equipment and piping. The most common mistake in determining the high hook elevation, is not accounting for the lifting straps or chains. The bridge crane hook is never connected directly to the equipment. Typically lifting strap or spreader bar is used. The design engineer shall determine how the equipment shall be lifted (straps or spreader bar) and include these dimensions in determining the high hook elevation. Special spreader bar can be specified for large vertical motor and vertical turbine pump removal. The design engineer must pay special attention when dealing with vertical turbine pumps. Due to the overall length of the pump, it may not be possible to remove the pump as one unit. In an enclosed building, it may be necessary to disassemble the pump as it is being removed from the wet well. Initially, the discharge head and motor are disassembled and removed. Next, the remaining column and bowl assembly is lifted out of the water until he next joint is visible. The intermediate column pieces may be as long as 20 feet unless otherwise stated in the project specifications. The bridge crane high hook elevation must be high enough to allow this range of lifting. OSHA requires minimum clearance of 3 inch from the top of the trolley assembly or any component of the crane and the bottom of the roof support structure and 2 inch on the sides. Provide a minimum clearance of 18 inches on the top and 12 inches on the sides to allow for any variation of dimensions between specified manufacturers. Horizontal Range The design engineer must also verify the horizontal lifting range. When the bridge crane is installed, the hoist and trolley have a limited range of motion. The design engineer shall review the bridge crane manufacturer’s drawings to determine the full range of horizontal motion for the bridge crane hook. The design engineer shall include this lifting zone on the pump station plan drawings as a separate layer than can be turned on and off.
Figure 2-32: Typical Bridge Crane
2.4.8.
Pipe Differential Settlement Where differential settlement of buried piping is anticipated and cannot be prevented, means must be provided to prevent damage to the pump and/or stresses on the pump or appurtenant fittings. This can be accomplished in two ways: 1. By providing at least two flexible couplings at a minimum spacing of 10 ft. The couplings nearest the point of maximum shear) adjacent to the wall of a structure) should be located no more than two feet from that point. Coupling manufacturer’s data indicate the maximum allowable deflection
Page | Chapter 2-45
OCT 2011 per coupling and assed upon the estimated differential settlement (if available) the minimum spacing of coupling can be estimated. 2. By providing articulated piping at the interface where settlement is expected to be the greatest. This is usually accomplished with two offset 90 degree elbows and mechanical-type ductile iron fittings to allow for rotation of the elbows. Groove joints are not allowed for buried application because they are prone to leakage. 3. By providing ball type joints where differential settlement is in excess of what the sleeve coupling or mechanical type DIP fittings can absorb. Whether for installation and removal of appurtenances, for differential settlement or for ease of joining two pieces of pipe, couplings should be kept to a minimum practicable number. They should be placed where needed but should not be placed indiscriminately, as they can be a source of leaks. Couplings most commonly used in pumping station piping are mechanical or grooved-type (Victaulic) and sleeve-type (Dresser or Baker). The mechanical-type can move axially only a fraction of an inch while sleeve-type units are designed to allow for considerably more movement. A harness set on such couplings may be necessary if additional forces can be developed.
2.4.9.
Thrust Restraint and Pipe Anchorage All pressure pipes should be axially restrained in order to contain hydraulic thrust internally within the pipe. Piping connections such as welded, flange, sleeve coupling with harness bolts, groove type coupling such as Victaulic couplings and DIP mechanical joints with thrust restraints are among the few method of providing thrust restraint. Residual thrust external to the elbows and bends can either be supported using thrust blocks or thrust supports with forces transferred to the ground. The pump should not be used as an anchor for piping. Pumps should be isolated for the piping system using expansion joints for high temperature applications. Site piping must be adequately anchored and isolated in a manner that prevents forces being transmitted to the pump and appurtenant fittings. Anchorage of such piping by concrete thrust blocks or wall flanges is normal practice. A flexible coupling between such anchor and the pump may be necessary to ensure that pipe forces can be relieved and to ensure that the pump and adjacent valves can be removed. Continuously joined and buried pipe may be self-anchored if the length of pipe and type of soils are suitable. The required length of joined pipe necessary to provide anchorage is proportional to the maximum operating pressure. Other design factors include: diameter, soil types, coating type, depth of cover. The length of pipe required for anchorage should be determined based on soils report data and reviewed by an expert in anchorage. Refer to AWWA M-11 for design guideline. High temperature piping shall be designed by calculating the thermal expansion of the pipe per unit length. The piping system shall be supported using pipe anchors, fixed and sliding supports strategically located along the piping alignment to direct the expansion away from the pipe anchor and contained within the expansion joints. Expansion loops is another alternate means of absorbing thermal expansion of piping. For more information, refer to Crane Piping handbook for pipe supports. Pump supports shall be per pump manufacturer recommendation. It is recommended that the pump be relieved with any external thrust associated with the piping system. Consult the pump manufacturer as to the maximum allowable nozzle load to be imparted by the piping system.
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OCT 2011
2.5.
Preliminary Valve Selection Once the piping layout concept is defined, the design engineer shall select the appropriate valves for the system. The type of valves used at the pump station is of significant importance as they must not only be applicable to the process, but work with the pump station control scheme. Typical valves used in pump stations are isolation, check, pump control, air and vacuum valves. Depending on the system operational requirement more specialized valves can be utilized including pressure reducing, pressure relief, pressure sustaining, energy dissipation and other specialty valves. The following sections briefly discuss the various types of valves.
2.5.1.
Isolation Valve Considerations Isolation valves are typically located both upstream and downstream of the pump in fully open or fully closed valve positions. If a pump is to be taken off line, the upstream and downstream valves are closed. Otherwise the valves are fully open. The most common type of isolation valve is the butterfly valve. Butterfly valves are preferred for water applications, while raw sewage and storm water applications require full port gate valves, eccentric plug valves, bonneted knife gates or other appropriate valves. The reason for the distinction is butterfly valves do not have a full-port opening and it is not suitable for fluids with stringy materials. The disc (sealing element) rotates about the centerline of the valve in the center of the flow stream. Therefore, the disc has the potential to catch stringy material and debris commonly found in raw sewage and storm water. Butterfly valves are also preferred for isolation applications in large water transmission and distribution pipelines operating at less than 150 psi. High pressure butterfly valves rated from 250 to 400 psi are available in triple offset configuration. In buried pipelines, valves 24-inch and larger should be installed in a vault for ease of repair and/or removal. When dealing with flow meters, full port valves are required to minimize flow disturbance through the meters, affecting its accuracy. At a minimum, a single isolation valve should normally be in-stalled on the discharge header, downstream of the flow meter, to allow removal of the meter without dewatering the entire pipeline.
2.5.2.
Check Valve Considerations Check valves shall be installed at each pump discharge to prevent the flow from draining back through the pump. There are a number of different types of check valves available to prevent flow reversal, such as globe type silent check, slanting disc, and swing check with air or hydraulic cushion-type. For small and medium sized potable water applications, slanted disc check valves are most common. For sewage applications, swing check valves with external weight or springs are applicable. MWH does not recommend the use of double door check valves because they have been found to fail during surge conditions. These valves typically do not have any type of the external visual confirmation that the valve is open. As a result, not only is it impossible to tell what position the valve is in, but if there was a valve failure, there would be no way to know. Furthermore, the quick closing response of these valves has been known to create slamming issues. During a surge event, the slamming may be great enough to break the valve.
2.5.3.
Control Valve Considerations Automatically controlled valves are typically used in lieu of the conventional check valve when the surge analysis requires controlled opening and closing in order to alleviate surge pressures. It functions like a check valve but it offers an adjustable opening and closing periods. These are known as pump control valves. They are designed to be fully closed when the pump starts and, through an operator, slowly open over a pre-determined time period. When the pump is required to stop, the controller calls for the valve to slowly close before the pump actually stops. Control valves shall be capable of closing automatically after a power failure event using stored energy such as an air or nitrogen accumulator. Thus, through the control valves, the system transitions from no flow to full Page | Chapter 2-47
OCT 2011 flow and back. Pump control valves may be actuated by diaphragm, pneumatic or hydraulic piston operators. Valves used for pump control should have good throttling characteristics, such as globe, plug, and cone or ball configurations. Typically valves on discharge piping are designed for a velocity of approximately 8 ft/sec. When required, globe pattern valves, globe pattern valves may be utilized as combined pump control and check valves. Under such applications, normal velocities through such valves should be less than 12 ft/sec.
Figure 2-33: Example Control Valve Installation 2.5.3.1.
Butterfly Valve The butterfly valve is a quarter turn rotary valve that controls flow and pressure by rotating a circular disc supported on a shaft to a circular seat. The AWWA C504 valve is equipped with a rubber seat. For pressures higher than 250 psi and beyond the rating of AWWA butterfly valves, triple offset butterfly valves are recommended. The rubber seat is not conducive to high velocities and sustained throttling and may fail in a low angled throttled position ( 2.33 x v 2/2g where v = velocity at Intake Design (ANSI/HI 9.8LONGITUDINAL SECTION top of ramp (2D min), r2 > 1.25D, 45° MWH takes tangent between r1 and r2 . 2007) [1] reproduced in exception to HI, 3. 1.2 m/s (4 ft/s) max wet pit pumps, Sequent depth Figure 1. They were 1.0 m/s (3 ft/s) max dry pit pumps and recommends 30° 4. > 45° smooth surface (plastic lining) developed by years of both 0.5D min a submergence of 5. > 60° concrete surface model and prototype testing. Fillet- 45° 6. S > (1+2.3F )D 4D 0.25D Vanes 7. See Appendix D for details and tutorials However, some Cone Figure 1. Dimensions of open trench-type wet wells for wastwater. Taken from American recommendations are still National Standard for Pump Intake Design, ANSI/HI 9.8-2007 occasionally ignored, and the result is usually a flawed no indication of poor or unacceptable pump product that will not give satisfactory performance. performance. The feature that makes the trench-type wet well so attractive to those wastewater operators who have used one is its ability for D
1
If the disparity between peak wet weather flow and average dry weather flow is large, energy use can be decreased and flow matching improved by installing more pumps and in two different sizes so as to place most of the expected average dry weather flow rates in the pumps’ Preferred Operating Region. Using more pumps, however, increases capital and maintenance costs, and decreasing the size of the last pump adversely affects cleaning. Decisions here depend on the artfulness of the designer.
The purpose of this paper is to assist designers by giving the reasons for the recommendations in ANSI/HI 9.8. CHOOSING PUMPS This subject is covered thoroughly in both second and third editions of Pumping Station Design, Chapter 12 [2, 3]. In addition, some considerations specific to trench-type wet wells are given below. Types of Pumps Any type of water or wastewater pump (dry pit centrifugal, vertical turbine solids handling, or both dry pit and wet pit submersible solids handling pumps) can be used in trench-type wet wells. Dry pit pumps can be used by installing a flare for the suction bell followed immediately by an elbow (preferably long radius) and a horizontal pipe to the dry well. Submersible pull-up pumps can be used by casting a recess for the discharge elbow in the side of the trench; after the elbow and discharge pipe are placed, the recess can be filled with lean concrete. Column pumps are ideal in trenches.
Effect of number of pumps on cleaning Cleaning is accomplished by dewatering the wet well with the last main pump, so the pump operates under the severe service conditions of inadequate submergence and inadequate net positive suction head. It is therefore desirable to reduce running time during cleaning as much as possible. If the wet well is cleaned when inflow rate to the pumping station is, say, half the capacity of the last pump, then the net dewatering flow rate is also about half the last pump's capacity. If the pumping station contains three pumps (one is a standby), the net dewatering flow rate would be about 25 percent of the total station capacity. Increasing the number of pumps to four reduces the dewatering rate to about 17 percent of the station capacity, and, of course, reduces the scouring potential as well. The result is increased running time for the pump and reduced effectiveness of cleaning. Keep the number of pumps to a minimum commensurate with other considerations such as providing for minimum flows. Analyze each set of pumps using Trench2.0 [4], a program for calculating velocity and depth of water, Froude number, and sequent depth along the trench at pumpdown. Velocities less than about 7 ft/s and Froude numbers less than 3.5 are not very effective. For long wet wells, velocities can be increased by three ways: (1) widening flow splitters and fillets to increase the hydraulic radius, (2) covering the bottom with plastic
Number of pumps An ideal number is three; two pumps to carry the maximum load plus a standby. If they are variable speed units, the lowest flow rate that can be pumped is usually about 25 percent (half the capacity of a single pump) of the station capacity. Check with the manufacturer; some machines can pump at lesser capacities. All centrifugal pumps have a lower limit, however, and below the tolerable flow rate for a single pump, the pump must be turned off and on as with constant speed pumps. That may be a problem for downstream processes. Sedimentation basins are upset by sudden changes in incoming flow. Other processes, such as chlorination and de-chlorination may not meet requirements at all with sudden changes in flow rates. Confer with the treatment plant designer. 2
better), and (3) shedding of stringy material by making vane noses smooth, round, and inclined less than 45 degrees to the streamlines. As cast iron should not be welded, vanes can be bolted to cast iron flares used as suction bells. Of course, vanes can be welded in fabricated steel suction bells. For clear water applications, the vanes can A A extend the full width of the flare. Four vanes work Plan fairly well, but more are better. Similar to the wastewater vane Section A-A application, Figure 3. Straightening vanes in a suction bell for clear water vanes can be bolted to cast iron flares used as suction bells. Vanes in fabricated steel bells can be welded to the bell.
coatings or linings, and (3) sloping the floor beginning at the point where the velocity is too low. Use a safety factor, because, as friction factors are only estimates, hydraulic calculations are never precise. For pumping clean water, there is no need to clean the wet well, hence no need for the ramp, and no need to limit the number of pumps except for considerations of cost and maintenance effort. NORMAL PUMPING Water should flow into the wet well from a straight pipe (or channel) at least 8 pipe diameters long and coaxial with the trench to prevent deleterious cross-currents in the basin. The incoming current flows above the trench to the end wall, dives and flows upstream along the bottom of the trench. The flow that passes pump intakes joins the incoming flow near the top of the ramp. Swirling The narrow trench tends to keep currents evenly distributed, but swirling is sometimes greater than allowed in ANSI/HI 9.8. Although not mentioned in that publication, swirling can be controlled by adding straightening vanes (Figures 2 and 3) in the suction bell or in the horizontal pipe between the suction elbow and a dry pit pump. For wastewater applications, the vane design should allow for: (1) a A A sphere passageway of at least 3 in. and at Plan least equivalent to the pump's passageway , (2) at least four vanes Section A-A (six are Figure 2. Straightening vanes in a suction bell 3"
M
in
Entrance Baffle Another means for improving performance, especially for reducing swirling and changing a "good pump environment" to an "ideal pump environment" is to reduce incoming currents with a baffle. Almost any baffle that intercepts the incoming current is beneficial, but a large, vertical rectangular baffle that forces flow under it as well as around its sides is superb. A horizontal baffle is easier to install, but it can be a rag catcher and it forces all the current to go over it or under it and not around the sides. Vertical baffles (see Figure 4) were found in model tests to be more effective than horizontal ones in eliminating swirling, and in a prototype, rags can slide down and off of them. The best width was 5/6 Dp, which for this 18 Mgal/d wet well just happens to equal D. The best bottom elevation was D/2 below 3
the inlet invert, and the best location was 60 percent of the distance from the end of the pipe to the centerline of the first pump. The baffle can, however, be moved back and forth 5'-0"
Vortices Strong vortices form at the trench floor under suction bells and at the trench walls about 0.28 D below the bells. As vortices tend to cause vibration and cavitation, they should either be eliminated or at least attenuated. Otherwise, impellers and casings should be made of material more resistant than cast iron. Addition of nickel to cast iron is a partial palliative, but there are other metals far more cavitation (but not vibration) resistant. Side wall vortices can be eliminated by fillets sloped 45 degrees if sufficiently high. The ANSI/HI 9.8 recommendations for upstream suction bells are D/2 for floor clearance and 3/8 D for fillet height thereby placing the top of the fillet D/8 below the bell rim--a safety factor of about 2. Floor vortices can be virtually eliminated by a flow splitter with 45-degree sides and also 3/8 D high. Flow splitters with base width equal to height (side slopes of 63.4 degrees) are the steepest that can keep floor vortices under reasonable control. A slope of 45 degrees is preferable if it leaves room for workers' feet during installation. In clear water applications, cones under suction bells are also effective at eliminating floor vortices. The diameter of the cone must be twice the floor clearance and the apex must be in the plane of the bell rim. Four, or preferably six, vanes can reduce swirling to much less than the maximum allowed in ANSI/HI 9.8. Some hydraulic model testing experts greatly prefer flow splitters instead of cones. Trenches narrower than 38 inches are physically too confining for workers. If there are two duty pumps in a trench, that difficulty essentially precludes the use of fillets and flow splitters for capacities much less than about 10 Mg/d. Just omit flow splitters and fillets for smaller pumping stations and use materials more resistant to cavitation than gray cast iron. Many trench-type stations that have no fillets or flow splitters but do have nickel in the iron have operated very satisfactorily for many years.
10'-0"
Railing Walkway
Pipe or box beam
5'-0"
Ø30"
HWL 2'-11"
Inlet baffle 1'-0.5" 5'-2.5"
2'-1" Ø2'-1"
1'-0.5" (3" Min)
1'-0.5" 9.4"
9.4" 4'-2"
Figure 4. Cross-section of 18-Mgal/d wet well with baffle for reducing incoming currents. Walkway makes it easy to access pump intakes and to wash grease off sidewalls.
with only minor effects. The baffle should be a thin (say, 4 in.) box section, because a simple plate would probably flutter. It can be supported in many different ways. The way shown in Figure 4 is to install a box or pipe beam (that can resist both bending and torsion) above high water level (HWL). Another way is by a beam above HWL and another (or even the roof) above that. Another way is by means of a beam above HWL and another at the bottom of the baffle, but the lower beam would prevent shedding stringy material. The structure should be stiff enough to resist vibration due to von Karman vortices. Use stainless steel and fill box sections with concrete to resist microbial corrosion, which attacks even stainless steel in stagnant wastewater. The need for and the design of baffles should be established by hydraulic model testing.
4
ft or more with a velocity of 12 ft/s or more and if the Froude number at the end of the wet well were at least 3. The cleaning performance indicated by Figure 5 (depth 0.27 ft, velocity 18 ft/s at foot of ramp and Froude number 6.2 at end of trench) is superb.
Uneven Distribution of Throat Velocities ANSI/HI 9.8 limits the variation in throat velocities to 10 percent. Uneven and fluctuating throat velocities in the suction bell have not been a problem in trench-type wet wells with column, wet pit submersible, horizontal dry pit, or vertical dry pit pumps when the vertical dry pit pump is preceded by a long-radius reducing elbow wherein the exit velocity is at least twice the inlet velocity.
Cleaning Time The time can be calculated by estimating the volume to be discharged and the net flow rate into and out of the basin. The volume in the sewer pipe is that between the drawdown curve and the original depth. If 18 Mgal/d (27.9 ft3/s) fills a 3.0 ft pipe, 7.8 ft3/s fills it to a depth of 1.25 ft. (See Figure B-5 in Pumping Station Design [3].) Critical depth is 0.88 ft from UnifCrit2.2 [5], so drawdown at the end of the pipe is 1.25 - 0.88 = 0.37 ft. If the drawdown curve is roughly 600 ft long at the end of pump-down, the volume, V1, in a parabolic wedge is
SELF-CLEANING The wet well is cleaned by dewatering it rapidly (pump-down) with the last pump. Choose a time when the inflow is about half the capacity of the last pump. As the water level falls below the top of the ramp, a hydraulic jump is formed. As the water level continues to fall and the hydraulic jump approaches the foot of the ramp, the currents wash floating material to the last pump where it is entrained in the fluid and pumped out. As the jump progresses along the floor, it suspends all remaining debris and the currents wash the debris into the pump. Consider, for example, a wet well featuring three equal variable-speed pumps (two duty pumps) with 25-in. suction bells having an entrance velocity of 4 ft/s--an 18 Mgal/d facility. A single duty pump would have a capacity of about 10 Mgal/d (15.5 ft3/s). Assume cleaning occurs when the inflow is 7.8 ft3/s. At the bottom of the ramp, the velocity is about 17 ft/s from Figure 5, and the depth of flow is only 0.3 ft. The flow splitter ends at Node 15 and the average water depth drops accordingly. The Froude number never falls much below 6 and the sequent depth (height of jump) remains above 1.3 ft. As the last bell should be no higher than D/2 below the sequent depth to prevent loss of prime, the bell should be at an elevation no higher than 1.3 - 25/ (2x12) = 0.26 ft above the upstream floor. It should also be D/4 or 0.52 ft above the floor below, so the floor must be lowered by 0.52 - 0.26 = 0.26 ft. Cleaning would be adequate if, at the foot of the ramp, the depth of water were 0.1
V1, Approximate volume of drawdown, 600 x 3 x 0.37 x (1/3) ≈ 220 ft3 V2, Volume above trench, 3 1.85(4.17+8.0)(1/2)21.04 ≈ 240 ft V3, Top of trench to 0.3 ft above floor, 4.97 x 4.17 x 17 ≈ 350 ft3 V4, Empty two column 2 (18/12) π/4 x 2 x 30 ≈ 110 ft3
pumps
Pumping at 15.5 ft3/s with an inflow of 7.8 ft3/s gives a net discharge-pumping rate of 7.7 ft3/s. The volume in the wet well is 220 + 240 + about half of 350 = 635 ft3. The time required to pump this amount is 635/7.7 = 82 s. The pump probably loses about 15 percent of its capacity at low submergences, so the rest of the water (110 + 350/2 = 285 ft3) is discharged at a net flow rate of 15.5 x 0.85 7.8 = 5.4 ft3/s. This part takes about 53 s. The total time is about 2.3 minutes. If two pumps are used to discharge the 635 ft3, the net pumping rate is 27.9 - 7.8 = 27.1 ft3/s, and the 82 s shrinks to 23 s for a net cleanout time of about 1.3 minutes.
5
Details… Assumptions… Web Address… The Spreadsheet… Version 2.0 Posted on web January 3, 2003. Date: January 31, 2007 Project Title: 18 MGD Trench-Type Self-Cleaning Wet Well Client: N/A Location: NA Job No.: N/A Calculation by: Sanks Remarks: Save this worksheet unaltered to use as the default. Program developed by Dr. Joel Cahoon, Montana State University. Access: http://www.coe.montana Uniform Flow Depth in the Circular Inlet Channel Section A - B Section B - C b= 4.17 ft b= 4.17 ft 7.80 cfs Flow Rate = bf = bf = 2.61 ft 2.61 ft 0.0015 ft/ft Slope = zf = bs = 1.56 ft 1.00 3.00 ft Diameter = zf = zs =
1.00
yf =
0.78 ft
Ramp Height (ft) =
5.90
Flow Area =
ys =
0.78 ft
Upper Radius (ft) =
4.20
Wetted Perimeter =
3.87 ft 0.59 ft
yf = nconcrete =
1.00
Manning's n =
0.78 ft
nsplitter =
0.009
Lower Radius (ft) =
2.60
Hydraulic Radius =
nconcrete =
0.011
Length 1 (ft) = Length 2 (ft) =
8.90 4.20
Velocity = r.h.s. =
EL1 = EL2 = r1 = r2 = L1 = L2 =
5.9 0.0 4.2 2.6 8.9 4.2
ft ft ft ft ft ft
Vertical Lines for L1 and L2:
0.012 1.08 ft 2 2.30 ft
Flow Depth =
0.011
i 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Bed Elevation Data Scroll down for diagram.
Author…
3.39 ft/sec 0.0000 cfs
21.82 21.82
0.00 5.90
17.62 17.62
0.00 5.90
8.72 8.72
0.00 5.90
x(i) 0.00 0.82 1.61 2.33 2.97 3.95 4.92 5.90 6.88 7.39 7.87 8.32 8.72 11.68 14.65 18.67 19.72 20.77 21.82
y(i) 5.90 5.82 5.58 5.19 4.67 3.69 2.72 1.74 0.76 0.44 0.20 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00
7.00 Distance from Elevation
Run Run
Vertical Flow
Water Surface
Mean
Froude
Depth Elevation
Velocity
Number
Energy
F
E, (ft)
1.11 1.48 2.08 2.75 3.48 4.51 5.36 6.10 6.76 6.97 7.07 7.10 7.07 6.67 6.30 6.76 6.58 6.41 6.24
6.95 6.95 6.94 6.93 6.91 6.85 6.74 6.59 6.39 6.33 6.24 6.15 6.08 5.60 5.17 4.58 4.42 4.27 4.13
Normal Flow
6.00
Control
Head
Depth
Node
x, (ft)
z, (ft)
yv, (ft)
y, (ft)
y, (ft) V, (ft/sec)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
0.00 0.82 1.61 2.33 2.97 3.95 4.92 5.90 6.88 7.39 7.87 8.32 8.72 11.68 14.65 18.67 19.72 20.77 21.82
5.90 5.82 5.58 5.19 4.67 3.69 2.72 1.74 0.76 0.44 0.20 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.73 0.64 0.57 0.55 0.57 0.49 0.45 0.42 0.39 0.39 0.33 0.29 0.27 0.28 0.29 0.18 0.18 0.18 0.19
0.73 0.63 0.53 0.46 0.40 0.35 0.32 0.29 0.28 0.27 0.27 0.27 0.27 0.28 0.29 0.18 0.18 0.18 0.19
6.63 6.46 6.15 5.74 5.24 4.19 3.16 2.16 1.16 0.82 0.52 0.34 0.27 0.28 0.29 0.18 0.18 0.18 0.19
Channel Floor
Water Surface
4.27 5.36 7.00 8.72 10.47 12.78 14.59 16.12 17.45 17.86 18.04 18.10 18.04 17.26 16.53 15.70 15.41 15.13 14.85
Energy Grade Line
Sequent
5.00
Depth y2, (ft)
4.00 3.00 2.00 1.00 0.00 0
5
10
15
20
25
1.76 1.72 1.69 1.42 1.40 1.39 1.38
Sequent Depth
8.00 7.00
Elevation (ft)
6.00 5.00 4.00 3.00 2.00
Figure 6 Trench 2.0 Bed Characteristics and definitions for the trench of figure 6
1.00 0.00 0
5
10
15
20
25
Easy access to all wall areas with the water jet helps to facilitate removing grease. The jet must be close to the wall to be effective, so attach a nozzle to the end of a long tube to make a water "lance" that can reach to within a yard or so of the surface to be washed. Access hatches make washing possible, but they are a nuisance. A better solution is an inside walkway the full length of the wet well as shown in Figure 4, but the entire space must then be well-ventilated and the vented air treated, all of which results in additional cost. However, ventilation and treatment protects concrete from corrosion and does allow easy access (not readily
Distance from Control (ft)
Figure 5. Trench2.0. Flow along the trench at pump-down. From www.coe.montana.edu/ce/joelc/wetwell, available free on the internet.
Grease Grease accumulates on walls between high and low water levels and must be occasionally removed. It clings tightly to concrete and may have to be scraped off. It can be more readily washed off walls coated or lined with plastic with a water jet of about 25 to 30 gal/min at a nozzle Pitot pressure of about 90 lb/in.2. Although expensive, coating is well worth the cost. 6
but they are small, and the deleterious effect is not pronounced.
achieved otherwise) to the suction bells for clearing out trash or for other needs, so the walkway can be justified in many pumping stations.
Caveat 3. Elevation of last pump intake Pumps in confined trenches often lose prime when the suction bell submergence is less than D/2. So place the bell rim at least D/2 below the sequent depth (given in Figure 5), and drop the floor under it to give a floor clearance of D/4. At greater floor clearances, larger grit and small stones may not be picked up by inlet velocities around 4 ft/s. ANSI/I 9.8 allows inlet velocities between 3 and 8 ft/s with a recommendation of 5.5 ft/s for medium-size pumping stations. One manufacturer designs solids handling column pumps for 3.5 to about 5 ft/s, and the authors recommend these lower velocities to prevent large, heavy trash such as parts of bricks or concrete blocks from being sucked into the pump and damaging it. (Include a rock trap upstream or just remove large, heavy trash manually from the trench as necessary.) Submersible pumps are designed for very high inlet velocities; so if the last pump is a submersible, add a suction nozzle with a flare to reduce the inlet velocity to 5 ft/s or less.
CAVEATS There are some caveats that must be heeded for a trench-type wet well to be completely successful. Caveat 1. Flow Splitter on Ramp To retain the energy developed by the ramp and to obtain a swift flow of water along the floor during cleaning (as in Figure 5), the flow splitter must begin at the top of the ramp and continue without interruption to some point between the last two pumps. (See the section entitled "Model" for an extensive discussion of flow splitters.) At super-critical velocities, any type of obstruction saps energy, may send a jet of water flying, and certainly reduces cleaning capability. If the flow splitter begins at the base of the ramp, the water--now at very high velocity--strikes this obstruction, jumps off the floor, probably impacts the first pump bell, loses nearly all of its energy, and never regains it. Quick cleaning is prevented. The trench can still be cleaned, but now it is by turbulence and attrition only. That process is slow, and the pump must run for many minutes under severe conditions As with flow splitters, the fillets also must begin at the top of the ramp. They should extend to the back wall to prevent the formation of sidewall vortices at the last pump.
Caveat 4. Cone Under Last Pump Because the last pump has a floor clearance of only D/4 (so as to scour grit effectively) and any flow splitter must clear the rim by 3 in. to pass solids, there is not enough room for an effective flow splitter in moderate-size pumping stations. Consequently, a cone under the last suction bell is a logical substitute for the flow splitter. See Figure 7. During pump-down for cleaning, water has a strong tendency to circulate between the last pump and the end wall. That circulation results in an upstream current on one side of the trench that keeps the hydraulic jump far upstream. The circulation can be so strong that it can go under the suction bell (even though the pump is running) and travel upstream. To prevent this
Caveat 2. Water Guide If the water from the inlet pipe is allowed to spread wider than the trench itself, some of it runs up on the sloping wall above the trench, where it slows and then falls back into the main stream at low velocity and disrupts the main flow. To prevent this occurrence, raise the sides of the trench near the top of the ramp to form a "water guide" that keeps all the water confined to the width of the trench. Water may run up on the fillets, 7
into the bottom of a ramp largely destroyed the energy of the flowing water and caused downstream flow to become sub-critical, a machinist at Montana State University suggested making the curved portions of flow splitters and fillets of a two-component casting compound [6]. The one chosen is very strong but flexible. Pour a slight excess into a wooden mold coated with floor wax. If the nose is gently tapered, one end of the mold can be suitably tapered to form a continuous nose. As soon as the compound cures enough to allow it, trim off the excess with a sharp knife. After final cure, warm the flow splitter with a bathroom heater and bend it over curves cut into wood with a band saw. The compound takes a permanent set upon cooling, but it does relax a little, so make the curves a little (say, 10 percent) sharper than the ramp curves. Fasten the pieces to the ramp with rubber cement, or screws, or both. Straight sections of fillets and flow splitters can be more easily made of wood well painted to resist moisture change and warping. With model flow splitter and fillets installed all the way up the ramp, the improvement in cleaning was dramatic. Velocities were very high and held up well to the end of the trench. But when the flow splitter was changed to end at the toe of the ramp, the energy and high velocity were destroyed. The downstream flow was turbulent but of sub-critical velocity that would not scour quickly. There is just no comparison between the performances of the two designs. Therefore, both flow splitter and fillets must extend to the top of the ramp for adequate cleaning potential.
occurrence, a fore-and-aft vane on the cone is needed as shown in Figure 7. a) 3" min (Notch apex of flow splitter if needed for 3" clearance under bell rim) Flow 2.5D Min 0.75D splitter Water surface See D sequent depth note a) Anti-rotation baffles 0.5D 0.38D Vanes Cone
0.25D
Figure 7. Construction at last pump intake.
Caveat 5. Anti-Rotation Baffle The tendency of water to circulate behind the last pump has been described in Caveat 4. To complete the suppression of flow between pump and end wall, an antirotation baffle (or barrier) is also required. Allow a minimum of construction clearance between the pump column and the baffle, but note that column pumps move slightly when pumping. Consult the manufacturer to find how much to allow for that movement. Caveat 6. End Wall During normal pumping, the stagnant water behind the last pump tends to form a surface vortex. Such vortices take the form of the letter "J". If the wall is moved close to the pump (ANSI/HI 9.8 recommends 0.75 D from the pump centerline), it intersects the "J" and the vortex does not organize. Sloping back walls outward allows the creation of a vortex. FORMING FLOW SPLITTERS AND FILLETS In the early stages of development, there were problems of how to design a flow splitter or fillets on a curving ramp--at that time not an easy task with either models or prototypes.
Other Means of Fabricating Model Flow Splitters and Fillets. Although casting compound is by far the most satisfactory material when installed, its use is involved and time-consuming. One way is to make the flow splitter and fillets entirely of wood. Cut it into thin cross-sections over curves, and set the sections in bathtub caulking compound such as Dap. After the
Models After a model test in a commercial laboratory showed that a flow splitter merging 8
cut with tin snips. These are glued to the plastic strip with Dap or super glue at intervals of three to six inches as appropriate. These are illustrated in Figure 8. Plastic sheets (for models and steel plates for prototypes) over ramp curves must be cut to the proper curvature. If the ramp curve has a radius of r, the formula for the radius, R, of a flat sheet to fit it is
Dap cures, coat the entire unit with Dap to waterproof it and fill the cracks. The result is ugly and rough but reasonably quick and easy.
R = r/Sinα, equation 1, where α is the angle between ramp and sheet. Flow Splitter Noses. A flow splitter must have some kind of nose. One of the easiest to make and best in performance tapers linearly from full size where it joins the prismatic flow splitter to zero at the top of the ramp. If β is the angle between the centerline of the ramp and the contact between nose and ramp (see Plan in Figure 9), the formula for the radius of curvature becomes
(a). Background: fillet of casting compound. Middle: 1D long (α =63o, β ≈17o) flow splitter nose of segmented wood followed by thin plastic sheet with triangular spreaders. Foreground: thin plastic sheet on supports of balsa wood triangles.
R = r/(Sin α Cos β), equation 2 The nose of a flow splitter separates 1.96D the Plan, developed incoming A flow into two A 45° streams B 1.96D that must have B R = 2.5D enough Longitudinal section depth and velocity max = 63.4° everywher preferred = 45° Section B-B Section A-A e to wash Figure 9. An excellent flow splitter nose debris off the ramp. Tests of inflow from horizontal pipe (low fluid velocity) and from approach pipe (high fluid velocity) were made at flow rates of 50 and 75 percent of the last pump's capacity. The best of several noses tried is shown in Figures 8(b) and 9. The apex angle (2 α ) is constant and
(b). 2D long (α =45 o, β ≈11 o) flow splitter nose of plastic sheet fastened with duct tape. Figure 8. Construction of model fillets and flow splitters.
One quick, easy way is to make flow splitter and fillets of thin (40 mil) plastic. It can be cut with tin snips and has about the right amount of stiffness. The strips for a flow splitter can be joined at the apex and to the floor with either Dap or long, narrow strips of duct tape pressed down firmly. Supports for fillets can be triangular pieces of plastic or thin (1/8 in.) balsa wood, which can also be 9
316L, or 347. Straight sections can be bent as shown in Figure 11a and held by 5/8- in. anchors either cast in the concrete or set in two-component adhesive in drilled holes. Alternatively, the construction shown in Figure 11b can be used. Along ramp curves, the plates must be cut to the radii given in Equation 1 (or 2 for noses), and then they must be flexed and stitch-welded at the apex. Either the type of construction in Figure 11b can be used, or the detail in Figure 11a can be followed if the plates are cut off a little below the floor and tabs are fastened to them for the bolts.
the nose height tapers linearly over a length of approximately 2D from the bottom of the upper ramp curve to the top of the ramp. A very long (4.3 D) nose was very good, and a very short (D long) one was adequate. Other Model Materials. Still another way is to cut fillets and flow splitters from plastic foam with a knife, table saw, or band saw, bend them to follow the contour of the ramp, and fasten them with rubber cement or even duct tape. One suitable material is Ethafoam [7], a polystyrene closed-cell, relatively rigid, strong foam. It holds a curvature if warmed and bent while cooling.
Alternate: bend plate at 3/8" radius
Choice. If the model floor can be removed for convenience in constructing fillets and flow splitters, the use of thin plastic sheet material is as easy and quick as any method. If not, the use of foam on ramp curves and either foam or wood for straight sections is easier. Either kind of flow splitters and fillets is easy to remove and replace--an advantage for demonstrating their value.
1-1/2" holes at 4'-0" C-C for pumpcrete
1/4" Type 304L S.S. Pumpcrete fill
Epoxy grout
2" 5/8" S.S.bolt x 8" at 12" C-C a. Detail for flow splitter straight sections
Prototype Fillets and Flow splitters Prototype fillets are easily made with shotcrete anchored into the corners with dowels (and with a rebar or two running the full length), screeded, and troweled smooth. If greater smoothness is needed as in long wet wells, coat the concrete with epoxy or line it with PVC. See Figure 10.
63.4°< 2"
3/4"x 4" (or as required by ogee curve) type 304L S.S. plate, at 2'-0" C-C max
Pumpcrete fill
