ASME 31.3 [ Process Plant Piping System Design ]
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Overview of Process Plant Piping System Design
Participant’s Guide
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Overview of Process Plant Piping System Design By: Vincent A. Carucci Carmagen Engineering, Inc.
Copyright © 2000 by
All Rights Reserved
TABLE OF CONTENTS PART 1:
PARTICIPANT NOTES ..............................................................................3
PART 2:
BACKGROUND MATERIAL .................................................................................... 73
I. II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
XI.
Introduction ....................................................................................................................... 73 General ............................................................................................................................. 73 A. What is a piping system .......................................................................................... 73 B. Scope of ASME B31.3............................................................................................. 73 Material selection considerations...................................................................................... 75 A. Strength................................................................................................................... 75 B. Corrosion Resistance .............................................................................................. 77 C. Material Fracture Toughness .................................................................................. 77 D. Fabricability ............................................................................................................. 78 E. Availability and Cost ................................................................................................ 78 Piping Components........................................................................................................... 79 A. Fittings, Flanges, and Gaskets................................................................................ 79 B. Flange Rating .......................................................................................................... 85 Sample Problem 1 - Determine Flange Rating ................................................................. 88 Solution ............................................................................................................................. 88 Valves ............................................................................................................................... 89 A. Valve Functions....................................................................................................... 89 B. Primary Valve Types ............................................................................................... 90 C. Valve Selection Process ......................................................................................... 98 Exercise 1 – Determine Required Flange Rating ............................................................. 99 Design ............................................................................................................................. 100 A. Design Conditions ................................................................................................. 100 B. Loads and Stresses............................................................................................... 101 C. Pressure Design of Components .......................................................................... 105 Sample Problem 2 - Determine Pipe wall thickness ....................................................... 110 Sample Problem 3 .......................................................................................................... 116 Exercise 2: Determine Required Pipe Wall Thickness .................................................. 121 System Design ................................................................................................................ 122 A. Layout Considerations .......................................................................................... 122 B. Pipe Supports and Restraints ............................................................................... 123 C. Piping Flexibility..................................................................................................... 129 D. Required Design Information for Piping Stress Analysis ...................................... 132 E. Criteria for Allowable Equipment Nozzle Loads .................................................... 132 F. When Should A Computer Analysis Be Used ....................................................... 134 G. Design Considerations for Piping System Stress Analysis ................................... 134 Fabrication, Assembly, and Erection .............................................................................. 140 A. Welding and Heat Treatment ................................................................................ 140 B. Assembly and Erection.......................................................................................... 144 Quality Control ................................................................................................................ 151 A. Inspection .............................................................................................................. 151 B. Testing................................................................................................................... 154 Other Considerations ...................................................................................................... 156 A. Nonmetallic Piping................................................................................................. 156 B. Category M Fluid Service...................................................................................... 157 C. High Pressure Piping............................................................................................. 158 Summary......................................................................................................................... 160
Part 1: Participant Notes
3
OVERVIEW OF PROCESS PLANT PIPING SYSTEM DESIGN By: Vincent A. Carucci Carmagen Engineering, Inc.
1
Notes:
Piping System Piping system: conveys fluid between locations Piping system includes: • Pipe • Fittings (e.g. elbows, reducers, branch connections, etc.) • Flanges, gaskets, bolting • Valves • Pipe supports 2
Notes:
4
ASME B31.3 • Provides requirements for: – Design – Materials – Fabrication
– Erection – Inspection – Testing
• For process plants including – – – –
Petroleum refineries Chemical plants Pharmaceutical plants Textile plants
– Paper plants – Semiconductor plants – Cryogenic plants
3
Notes:
Scope of ASME B31.3 • Piping and piping components, all fluid services: – Raw, intermediate, and finished chemicals – Petroleum products – Gas, steam, air, and water – Fluidized solids – Refrigerants – Cryogenic fluids
• Interconnections within packaged equipment • Scope exclusions specified 4
Notes:
5
Strength • • • • • •
Yield and Tensile Strength Creep Strength Fatigue Strength Alloy Content Material Grain size Steel Production Process
5
Notes:
Stress - Strain Diagram B
S A
C
E
6
Notes:
6
Corrosion Resistance • Deterioration of metal by chemical or electrochemical action • Most important factor to consider • Corrosion allowance added thickness • Alloying increases corrosion resistance
7
Notes:
Piping System Corrosion General or Uniform Corrosion
Uniform metal loss. May be combined with erosion if high-velocity fluids, or moving fluids containing abrasives.
Pitting Corrosion
Localized metal loss randomly located on material surface. Occurs most often in stagnant areas or areas of low-flow velocity.
Galvanic Corrosion
Occurs when two dissimilar metals contact each other in corrosive electrolytic environment. Anodic metal develops deep pits or grooves as current flows from it to cathodic metal.
Crevice Corrosion Localized corrosion similar to pitting. Occurs at places such as gaskets, lap joints, and bolts where crevice exists. Concentration Cell Corrosion
Occurs when different concentration of either a corrosive fluid or dissolved oxygen contacts areas of same metal. Usually associated with stagnant fluid.
Graphitic Corrosion
Occurs in cast iron exposed to salt water or weak acids. Reduces iron in cast iron, and leaves graphite in place. Result is extremely soft material with no metal loss.
8
Notes:
7
Material Toughness • Energy necessary to initiate and propagate a crack • Decreases as temperature decreases • Factors affecting fracture toughness include: – Chemical composition or alloying elements – Heat treatment – Grain size 9
Notes:
Fabricability • Ease of construction • Material must be weldable • Common shapes and forms include: – Seamless pipe – Plate welded pipe – Wrought or forged elbows, tees, reducers, crosses – Forged flanges, couplings, valves – Cast valves 10
Notes:
8
Availability and Cost • Consider economics • Compare acceptable options based on: – Availability – Relative cost
11
Notes:
Pipe Fittings • Produce change in geometry – – – –
Modify flow direction Bring pipes together Alter pipe diameter Terminate pipe
12
Notes:
9
Elbow and Return
90°
45°
180° Return
Figure 4.1
13
Notes:
Tee
Reducing Outlet Tee
Cross Tee Figure 4.2
14
Notes:
10
Reducer
Concentric
Eccentric
Figure 4.3 15
Notes:
Welding Outlet Fitting
16
Figure 4.4
Notes:
11
Cap
Figure 4.5 17
Notes:
Lap-joint Stub End Note square corner
R R Enlarged Section of Lap
18
Figure 4.6
Notes:
12
Typical Flange Assembly Flange
Bolting
Gasket
Figure 4.7
19
Notes:
Types of Flange Attachment and Facing Flange Attachment Types
Flange Facing Types
Threaded Flanges
Flat Faced
Socket-Welded Flanges Blind Flanges
Raised Face
Slip-On Flanges Lapped Flanges
Ring Joint
Weld Neck Flanges
20
Table 4.1
Notes:
13
Flange Facing Types
Figure 4.8
21
Notes:
Gaskets • • • •
Resilient material Inserted between flanges Compressed by bolts to create seal Commonly used types – Sheet – Spiral wound – Solid metal ring
22
Notes:
14
Flange Rating Class • Based on ASME B16.5 • Acceptable pressure/temperature combinations • Seven classes (150, 300, 400, 600, 900, 1,500, 2,500) • Flange strength increases with class number • Material and design temperature combinations without pressure indicated not acceptable 23
Notes:
Material Specification List
24
Table 4.2
Notes:
15
Pressure - Temperature Ratings Material Group No. Classes Temp., °F -20 to 100 200 300 400 500 600 650 700 750 800 850 900 950 1000
1.9
1.8 150 235 220 215 200 170 140 125 110 95 80 65 50 35 20
300 620 570 555 555 555 555 555 545 515 510 485 450 320 215
400 825 765 745 740 740 740 740 725 685 675 650 600 425 290
150 290 260 230 200 170 140 125 110 95 80 65 50 35 20
300 750 750 720 695 695 605 590 570 530 510 485 450 320 215
1.10 400 1000 1000 965 885 805 785 785 710 675 650 600 425 290 190
150 290 260 230 200 170 140 125 110 95 80 65 50 35 20
300 750 750 730 705 665 605 590 570 530 510 485 450 375 260
400 1000 1000 970 940 885 805 785 755 710 675 650 600 505 345
Table 4.3
25
Notes:
Sample Problem 1 Flange Rating New piping system to be installed at existing plant. Determine required flange class. • Pipe Material: • Design Temperature: • Design Pressure:
1 1 Cr − 1 Mo 4 2
700°F 500 psig
26
Notes:
16
Sample Problem 1 Solution • Determine Material Group Number (Fig. 4.2) Group Number = 1.9 • Find allowable design pressure at intersection of design temperature and Group No. Check Class 150. – Allowable pressure = 110 psig < design pressure – Move to next higher class and repeat steps
• For Class 300, allowable pressure = 570 psig • Required flange Class: 300 27
Notes:
Valves • Functions – Block flow – Throttle flow – Prevent flow reversal
28
Notes:
17
Full Port Gate Valve 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Handwheel Nut Handwheel Stem Nut Yoke Yoke Bolting Stem Gland Flange Gland Gland Bolts or Gland Eye-bolts and nuts Gland Lug Bolts and Nuts Stem Packing Plug Lantern Ring Backseat Bushing Bonnet Bonnet Gasket Bonnet Bolts and Nuts Gate Seat Ring Body One-Piece Gland (Alternate) Valve Port
Figure 5.1
29
Notes:
Globe Valve • • • • •
Most economic for throttling flow Can be hand-controlled Provides “tight” shutoff Not suitable for scraping or rodding Too costly for on/off block operations
30
Notes:
18
Check Valve • • • •
Prevents flow reversal Does not completely shut off reverse flow Available in all sizes, ratings, materials Valve type selection determined by – Size limitations – Cost – Availability – Service
31
Notes:
Swing Check Valve Cap Pin
Seat Ring Hinge Flow Direction
Disc Body
32
Figure 5.2
Notes:
19
Ball Check Valve
Figure 5.3
33
Notes:
Lift Check Valve Seat Ring Piston Flow Direction
34
Figure 5.4
Notes:
20
Wafer Check Valve
Figure 5.5
35
Notes:
Ball Valve No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
36
Part Names Body Body Cap Ball Body Seal Gasket Seat Stem Gland Flange Stem Packing Gland Follower Thrust Bearing Thrust Washer Indicator Stop Snap Ring Gland Bolt Stem Bearing Body Stud Bolt & Nuts Gland Cover Gland Cover Bolts Handle
Figure 5.6
Notes:
21
Plug Valve Wedge
Molded-In Resilient Seal
Sealing Slip
Figure 5.7
37
Notes:
Valve Selection Process General procedure for valve selection. 1. Identify design information including pressure and temperature, valve function, material, etc. 2. Identify potentially appropriate valve types and components based on application and function (i.e., block, throttle, or reverse flow prevention). 38
Notes:
22
Valve Selection Process, cont’d 3. Determine valve application requirements (i.e., design or service limitations). 4. Finalize valve selection. Check factors to consider if two or more valves are suitable. 5. Provide full technical description specifying type, material, flange rating, etc. 39
Notes:
Exercise 1 - Determine Required Flange Rating • Pipe:
1 1 Cr − 1 Mo 4 2
• Flanges: • Design Temperature: • Design Pressure:
A-182 Gr. F11 900°F 375 psig
40
Notes:
23
Exercise 1 - Solution 1. Identify material specification of flange A-182 Gr, F11 2. Determine Material Group No. (Table 4.2) Group 1.9 3. Determine class using Table 4.3 with design temperature and Material Group No. – The lowest Class for design pressure of 375 psig is Class 300. – Class 300 has 450 psig maximum pressure at 900°F 41
Notes:
Design Conditions • General – Normal operating conditions – Design conditions
• Design pressure and temperature – Identify connected equipment and associated design conditions – Consider contingent conditions – Consider flow direction – Verify conditions with process engineer 42
Notes:
24
Loading Conditions Principal pipe load types • Sustained loads – Act on system all or most of time – Consist of pressure and total weight load
• Thermal expansion loads – Caused by thermal displacements – Result from restrained movement
• Occasional loads 43
– Act for short portion of operating time – Seismic and/or dynamic loading
Notes:
Stresses Produced By Internal Pressure Sl
Sc P t
44
Sl
=
Longitudinal Stress
Sc
=
Circumferential (Hoop) Stress
t
=
Wall Thickness
P
=
Internal Pressure
Figure 6.1
Notes:
25
Stress Categorization • Primary Stresses – Direct – Shear – Bending
• Secondary stresses – Act across pipe wall thickness – Cause local yielding and minor distortions – Not a source of direct failure 45
Notes:
Stress Categorization, cont’d • Peak stresses – More localized – Rapidly decrease within short distance of origin – Occur where stress concentrations and fatigue failure might occur – Significance equivalent to secondary stresses – Do not cause significant distortion
46
Notes:
26
Allowable Stresses Function of – Material properties – Temperature – Safety factors
Established to avoid: – General collapse or excessive distortion from sustained loads – Localized fatigue failure from thermal expansion loads – Collapse or distortion from occasional loads 47
Notes:
B31.3 Allowable Stresses in Tension Basic Allowable Stress S, ksi. At Metal Temperature, °F. °° Material
Spec. No/Grade
100
200
300
400
500
Carbon Steel
A 106
B
20.0
20.0
20.0
20.0
18.9
17.3
16.5
10.8
6.5
2.5
1.0
C - ½Mo
A 335
P1
18.3
18.3
17.5
16.9
16.3
15.7
15.1
13.5
12.7
4.
2.4
P11
20.0
1¼ - ½Mo
A 335
600
700
800
900
1000 1100
1200
1300
1400
1500
18.7
18.0
17.5
17.2
16.7
15.6
15.0
12.8
6.3
2.8
1.2
18Cr - 8Ni pipe
A 312
TP304 20.0
20.0
20.0
18.7
17.5
16.4
16.0
15.2
14.6
13.8
9.7
6.0
3.7
2.3
1.4
16Cr - 12Ni-2Mo pipe
A 312
TP316 20.0
20.0
20.0
19.3
17.9
17.0
16.3
15.9
15.5
15.3
12.4
7.4
4.1
2.3
1.3
Table 6.1 48
Notes:
27
Pipe Thickness Required For Internal Pressure •
t=
PD 2 (SE + PY )
P = Design pressure, psig D = Pipe outside diameter, in. S = Allowable stress in tension, psi E = Longitudinal-joint quality factor Y = Wall thickness correction factor
• •
t m = t + CA t nom =
tm 0.875
49
Notes:
Spec. No.
Class (or Type)
Description
Ej
Carbon Steel API 5L
... ... ...
A 53
Type S Type E Type F
A 106
...
Seamless pipe Electric resistance welded pipe Electric fusion welded pipe, double butt, straight or spiral seam Furnace butt welded
1.00 0.85 0.95
Seamless pipe Electric resistance welded pipe Furnace butt welded pipe
1.00 0.85 0.60
Seamless pipe
1.00
Low and Intermediate Alloy Steel A 333
... ...
Seamless pipe Electric resistance welded pipe
1.00 0.85
A 335
...
Seamless pipe
A 312
... ... ...
Seamless pipe Electric fusion welded pipe, double butt seam Electric fusion welded pipe, single butt seam
1.00 0.85 0.80
1.00
Electric fusion welded pipe, 100% radiographed Electric fusion welded pipe, spot radiographed Electric fusion welded pipe, double butt seam
1.00 0.90 0.85
Stainless Steel
A 358
1, 3, 4 5 2
B 161
...
B 514
...
Welded pipe
0.80
B 675
All
Welded pipe
0.80
Nickel and Nickel Alloy
50
Seamless pipe and tube
1.00
Table 6.2
Notes:
28
Temperature, °F 950
1000
1050
1100
1150 & up
Ferritic Steels
0.4
0.5
0.7
0.7
0.7
0.7
Austenitic Steels
0.4
0.4
0.4
0.4
0.5
0.7
Other Ductile Metals
0.4
0.4
0.4
0.4
0.4
0.4
Cast iron
0.0
...
...
...
...
...
Materials
900 & lower
Table 6.3 51
Notes:
Curved and Mitered Pipe • Curved pipe – Elbows or bends – Same thickness as straight pipe
• Mitered bend – Straight pipe sections welded together – Often used in large diameter pipe – May require larger thickness • Function of number of welds, conditions, size 52
Notes:
29
Sample Problem 2 Determine Pipe Wall Thickness Design temperature: 650°F Design pressure: 1,380 psig. Pipe outside diameter: 14 in. Material: ASTM A335, Gr. P11 ( 1 14 Cr − 12 Mo ), seamless Corrosion allowance: 0.0625 in. 53
Notes:
Sample Problem 2 - Solution t=
PD 2(SE + PY)
t=
1,380 × 14 2[(16,200 × 1) + (1,380 × 0.4 )]
t = 0.577 in.
54
Notes:
30
Sample Problem 2 Solution, cont’d tm = t + c = 0.577 + 0.0625 = 0.6395 in. t nom =
0.6395 = 0.731 in. 0.875
55
Notes:
Welded Branch Connection Db
Tb Reinforcement Zone Limits
Nom. Thk.
c
tb
A3
A3
L4
Reinforcement Zone Limits
Mill Tol.
A4
A4 A1
Tr
Th Dh Nom. Thk.
c
th
Mill Tol.
d1
A2
A2
d2
d2
β
Pipe C
56
Figure 6.2
Notes:
31
Reinforcement Area d1 =
Db − 2(Tb − c) sin β
d1 = Effective length removed from run pipe, in. Db = Branch outside diameter, in. Tb = Minimum branch thickness, in. c = Corrosion allowance, in. β = Acute angle between branch and header 57
Notes:
Required Reinforcement Area Required reinforcement area, A1: A 1 = t h d1(2 − sin β)
Where: th = Minimum required header thickness, in.
58
Notes:
32
Reinforcement Pad • Provides additional reinforcement • Usually more economical than increasing wall thickness • Selection variables – Material – Outside diameter – Wall thickness
æ (D − Db ) ö A 4 = çç p Tr è sin β 59
Notes:
Sample Problem 3 • Pipe material: Seamless, A 106/Gr. B for branch and header, S = 16,500 psi • Design conditions: 550 psig @ 700°F • c = 0.0625 in. • Mill tolerance: 12.5%
60
Notes:
33
Sample Problem 3, cont’d • Nominal Pipe Thicknesses:
Header: 0.562 in. Branch: 0.375 in.
• Required Pipe Thicknesses:
Header: 0.395 in. Branch: 0.263 in.
• Branch connection at 90° angle 61
Notes:
Sample Problem 3 - Solution d1 = d1 =
Db − 2(Tb − c) sin β
16 − 2 (0.375 × 0.875 − 0.0625 ) = 15.469 in. sin 90°
A1 = thd1(2 − sinβ) A1 = 0.395 × 15.469 (2 − sin90°) = 6.11in.2 62
Notes:
34
Sample Problem 3 Solution, cont’d • Calculate excess area available in header, A2.
A 2 = (2d2−d1)(Th−th−c ) d2 = d1 = 15.469 in. < Dh = 24 in. A2 = (2 × 15.469 - 15.469) (0.875 × 0.562 0.395 - 0.0625) A2 = 0.53 in.2 63
Notes:
Sample Problem 3 Solution, cont’d •
Calculate excess area available in branch,
•
A3. A3 =
2L 4(Tb − tb−c ) sinβ
L 4 = 2.5 (0.875 × 0.375 − 0.0625 ) = 0.664 in. A3 =
2 × 0.664 (0.875 × 0.375 − 0.263 − 0.0625 ) = 0.003 in.2 sin 90°
64
Notes:
35
Sample Problem 3 Solution, cont’d • Calculate other excess area available, A4. A4 = 0.
• Total Available Area: AT = A2 + A3 + A4 AT = 0.53 + 0.003 + 0 = 0.533 in.2 available reinforcement. AT < A1
∴ Pad needed 65
Notes:
Sample Problem 3 Solution, cont’d • Reinforcement pad: A106, Gr. B, 0.562 in. thick • Recalculate Available Reinforcement L41 = 2.5 (Th - c) = 2.5 (0.875 × 0.562 - 0.0625) = 1.073 in. L42 = 2.5 (Tb - c) + Tr = 2.5 (0.875 × 0.375 - 0.0625) + 0.562 (0.875) = 1.16 in
66
Notes:
36
Sample Problem 3 Solution, cont’d Therefore, L4 = 1.073 in. A3 =
2L 4 (Tb − t b − c) sin β
A3 =
2 × 1.073 (0.875 × 0.375 − 0.263 − 0.0625 ) sin90 o
A 3 = 0.005 in.2 (vs. the 0.003 in.2 previously calculated ) A T = A 2 + A 3 + A 4 = 0.53 + 0.005 + 0 = 0.535 in.2
67
Notes:
Sample Problem 3 Solution, cont’d
• Calculate additional reinforcement required and pad dimensions: A4 = 6.11 - 0.535 = 5.575 in.2 Pad diameter, Dp is: Tr = 0.562 (0.875) = 0.492 in. Dp =
A 4 Db 5.575 + = + 16 = 27.3 Tr sin β 0.492
Since 2d2 > Dp, pad diameter is acceptable 68
Notes:
37
Exercise 2 - Determine Required Pipe Wall Thickness • • • • • • •
Design Temperature: 260°F Design Pressure: 150 psig Pipe OD: 30 in. Pipe material: A 106, Gr. B seamless Corrosion allowance: 0.125 Mill tolerance: 12.5% Thickness for internal pressure and nominal thickness?
69
Notes:
Exercise 2 - Solution • From Tables 6.1, 6.2, and 6.3 obtain values: – S = 20,000 psi – E = 1.0 – Y = 0.4 • Thickness calculation: t=
PD 150 × 30 = 2(SE + PY ) 2[(20,000 × 1.0 ) + (150 × 0.04 )]
t = 0.112 in. 70
Notes:
38
Exercise 2 - Solution, cont’d • Corrosion allowance calculation: t m = t + CA = 0.112 + 0.125 t = 0.237 in.
• Mill tolerance calculation: tm 0.237 = 0.875 0.875 = 0.271 in.
t nom = t nom 71
Notes:
Layout Considerations • Operational – Operating and control points easily reached
• Maintenance – Ample clearance for maintenance equipment – Room for equipment removal – Sufficient space for access to supports
• Safety – Consider personnel safety – Access to fire fighting equipment 72
Notes:
39
Pipe Supports and Restraints • Supports – Absorb system weight – Reduce: + longitudinal pipe stress + pipe sag + end point reaction loads
• Restraints – Control or direct thermal movement due to: + thermal expansion + imposed loads
73
Notes:
Support and Restraint Selection Factors • • • • • •
Weight load Available attachment clearance Availability of structural steel Direction of loads and/or movement Design temperature Vertical thermal movement at supports
74
Notes:
40
Rigid Supports
Shoe
Dummy Support
75
Base Adjustable Support
Saddle
Trunnion
Figure 7.1
Notes:
Hangers
76
Figure 7.2
Notes:
41
Flexible Supports Load and Deflection Scale
Small Change in Effective Lever Arm
Large Change in Effective Lever Arm Relatively Constant Load Typical Variable-Load Spring Support
Typical Constant-Load Spring Support Mechanism
Figure 7.3
77
Notes:
Restraints • Control, limit, redirect thermal movement – Reduce thermal stress – Reduce loads on equipment connections
• Absorb imposed loads – Wind – Earthquake – Slug flow – Water hammer – Flow induced-vibration 78
Notes:
42
Restraints, cont’d • Restraint Selection – Direction of pipe movement – Location of restraint point – Magnitude of load
79
Notes:
Anchors and Guides • Anchor – Full fixation – Permits very limited (if any) translation or rotation
• Guide – Permits movement along pipe axis – Prevents lateral movement – May permit pipe rotation 80
Notes:
43
Restraints - Anchors
Anchor
Anchor
Partial Anchor
Figure 7.4
81
Notes:
Restraints - Guides
Guide
Guide
x
Vertical Guide 82
Guide
Figure 7.5
Notes:
44
Piping Flexibility • Inadequate flexibility – Leaky flanges – Fatigue failure – Excessive maintenance – Operations problems – Damaged equipment
• System must accommodate thermal movement 83
Notes:
Flexibility Analysis • Considers layout, support, restraint • Ensures thermal stresses and reaction loads are within allowable limits • Anticipates stresses due to:
84
– Elevated design temperatures + Increases pipe thermal stress and reaction loads + Reduces material strength – Pipe movement – Supports and restraints
Notes:
45
Flexibility Analysis, cont’d • Evaluates loads imposed on equipment • Determines imposed loads on piping system and associated structures • Loads compared to industry standards – Based on tables – Calculated
85
Notes:
Design Factors • Layout • Component design details • Fluid service • Connected equipment type • Operating scenarios
• Pipe diameter, thickness • Design temperature and pressure • End-point movements • Existing structural steel locations • Special design considerations
86
Notes:
46
Equipment Nozzle Load Standards and Parameters Equipment Item
Parameters Used To Determine Acceptable Loads
Industry Standard
Centrifugal Pumps
API 610
Nozzle size
Centrifugal Compressors
API 617, 1.85 times
Nozzle size, material
Air-Cooled Heat Exchangers
API 661
NEMA SM-23 allowable Nozzle size
Pressure Vessels, Shell- ASME Code Section and-Tube Heat VIII, WRC 107, Exchanger Nozzles WRC 297
Nozzle size, thickness, reinforcement details, vessel/exchanger diameter, and wall thickness. Stress analysis required.
Tank Nozzles
API 650
Nozzle size, tank diameter, height, shell thickness, nozzle elevation.
Steam Turbines
NEMA SM-23
Nozzle size
Table 7.1
87
Notes:
Computer Analysis • Used to perform detailed piping stress analysis • Can perform numerous analyses • Accurately completes unique and difficult functions
88
– Time-history analyses – Seismic and wind motion – Support motion – Finite element analysis – Animation effects
Notes:
47
Computer Analysis Guidelines Type Of Piping General piping
Pipe Size, NPS
Maximum Differential Flexibility Temp.
≥4
≥ 400°F
≥8
≥ 300°F
≥ 12
≥ 200°F
≥ 20
any
For rotating equipment
≥3
Any
For air-fin heat exchangers
≥4
Any
For tankage
≥ 12
Any
Table 7.2
89
Notes:
Piping Flexibility Temperature • Analysis based on largest temperature difference imposed by normal and abnormal operating conditions • Results give: – Largest pipe stress range – Largest reaction loads on connections, supports, and restraints
• Extent of analysis depends on situation 90
Notes:
48
Normal Temperature Conditions To Consider Stable Operation
Temperature range expected for most of time plant is in operation. Margin above operating temperature (i.e., use of design temperature rather than operating temperature) allows for process flexibility.
Startup and Shutdown
Determine if heating or cooling cycles pose flexibility problems. For example, if tower is heated while attached piping remains cold, piping flexibility should be checked.
Regeneration and Decoking Piping
Spared Equipment
Design for normal operation, regeneration, or decoking, and switching from one service to the other. An example is furnace decoking. Requires multiple analyses to evaluate expected temperature variations, for no flow in some of piping, and for switching from one piece of equipment to another. Common example is piping for two or more pumps with one or more spares.
Table 7.3
91
Notes:
Abnormal Temperature Conditions To Consider Loss of Cooling Medium Flow
Temperature changes due to loss of cooling medium flow should be considered. Includes pipe that is normally at ambient temperature but can be blocked in, while subject to solar radiation.
Most on-site equipment and lines, and many off-site lines, are freed of gas or air by using steam. For 125 psig steam, 300°F is typically used for metal temperature. Piping connected to equipment which Steamout for Air will be steamed out, especially piping connected to or Gas Freeing upper parts of towers, should be checked for tower at 300°F and piping at ambient plus 50°F. This may govern flexibility of lines connected to towers that operate at less than 300°F or that have a smaller temperature variation from top to bottom. If process flow can be stopped while heat is still being No Process Flow applied, flexibility should be checked for maximum While Heating metal temperature. Such situations can occur with Continues steam tracing and steam jacketing.
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Table 7.4
Notes:
49
Extent of Analysis • Extent depends on situation • Analyze right combination of conditions • Not necessary to include system sections that are irrelevant to analysis results
93
Notes:
Modifying System Design • • • •
Provide more offsets or bends Use more expansion loops Install expansion joints Locate restraints to: – Minimize thermal and friction loads – Redirect thermal expansion
• Use spring supports to reduce large vertical thermal loads • Use Teflon bearing pads to reduce friction loads 94
Notes:
50
System Design Considerations • Pump systems – Operating vs. spared pumps
• Heat traced piping systems – Heat tracing + Reduces liquid viscosity + Prevents condensate accumulation – Tracing on with process off 95
Notes:
System Design Considerations, cont’d • Atmospheric storage tank – Movement at nozzles – Tank settlement
• Friction loads at supports and restraints – Can act as anchors or restraints – May cause high pipe stresses or reaction loads
• Air-cooled heat exchangers – Consider header box and bundle movement 96
Notes:
51
Tank Nozzle SHELL
NOZZLE
BOTTOM
Figure 7.6
97
Notes:
Welding • • • •
Welding is primary way of joining pipe Provides safety and reliability Qualified welding procedure and welders Butt welds used for: – Pipe ends – Butt-weld-type flanges or fittings to pipe ends – Edges of formed plate
98
Notes:
52
Butt-Welded Joint Designs Equal Thickness
(a) Standard End Preparation of Pipe
(b) Standard End Preparation of Butt-Welding Fittings and Optional End Preparation of Pipe 7/8 in. and Thinner
(c) Suggested End Preparation, Pipe and Fittings Over 7/8 in. Thickness
Figure 8.1
99
Notes:
Butt-Welded Joint Designs Unequal Thickness 3/32 in. max. (a)
(b)
(c)
(d)
100
Figure 8.2
Notes:
53
Fillet Welds
Figure 8.3
101
Notes:
Weld Preparation • Welder and equipment must be qualified • Internal and external surfaces must be clean and free of paint, oil, rust, scale, etc. • Ends must be: – Suitably shaped for material, wall thickness, welding process – Smooth with no slag from oxygen or arc cutting 102
Notes:
54
Preheating • Minimizes detrimental effects of: – High temperature – Severe thermal gradients
• Benefits include: – Dries metal and removes surface moisture – Reduces temperature difference between base metal and weld – Helps maintain molten weld pool – Helps drive off absorbed gases 103
Notes:
Postweld Heat Treatment (PWHT) • Primarily for stress relief – Only reason considered in B31.3
• Averts or relieves detrimental effects – Residual stresses + Shrinkage during cooldown + Bending or forming processes – High temperature – Severe thermal gradients 104
Notes:
55
Postweld Heat Treatment (PWHT), cont’d • Other reasons for PWHT to be specified by user – Process considerations – Restore corrosion resistance of normal grades of stainless steel – Prevent caustic embrittlement of carbon steel – Reduce weld hardness
105
Notes:
Storage and Handling • Store piping on mounds or sleepers • Stacking not too high • Store fittings and valves in shipping crates or on racks • End protectors firmly attached • Lift lined and coated pipes and fittings with fabric or rubber covered slings and padding 106
Notes:
56
Pipe Fitup and Tolerances • Good fitup essential – Sound weld – Minimize loads
• Dimensional tolerances • Flange tolerances
107
Notes:
Pipe Alignment Load Sensitive Equipment • Special care and tighter tolerances needed • Piping should start at nozzle flange – Initial section loosely bolted – Gaskets used during fabrication to be replaced
• Succeeding pipe sections bolted on • Field welds to join piping located near machine 108
Notes:
57
Load Sensitive Equipment, cont’d • Spring supports locked in cold position during installation and adjusted in locked position later • Final bolt tensioning follows initial alignment of nozzle flanges • Final nozzle alignment and component flange boltup should be completed after replacing any sections removed 109
Notes:
Load Sensitive Equipment, cont’d • More stringent limits for piping > NPS 3 • Prevent ingress of debris during construction
110
Notes:
58
Flange Joint Assembly • Primary factors – Selection – Design – Preparation – Inspection – Installation
• Identify and control causes of leakage
111
Notes:
Flange Preparation, Inspection, and Installation • • • • • •
Redo damaged surfaces Clean faces Align flanges Lubricate threads and nuts Place gasket properly Use proper flange boltup procedure
112
Notes:
59
“Criss-Cross” Bolt-tightening Sequence
Figure 8.4
113
Notes:
Causes of Flange Leakage • • • • • • • •
Uneven bolt stress Improper flange alignment Improper gasket centering Dirty or damaged flange faces Excessive loads at flange locations Thermal shock Improper gasket size or material Improper flange facing
114
Notes:
60
Inspection • Defect identification • Weld inspection – Technique – Weld type – Anticipated type of defect – Location of weld – Pipe material
115
Notes:
Typical Weld Imperfections Lack of Fusion Between Weld Bead and Base Metal
a) Side Wall Lack of Fusion
b) Lack of Fusion Between Adjacent Passes
Incomplete Filling at Root on One Side Only
c) Incomplete Penetration Due to Internal Misalignment
Incomplete Filling at Root
d) Incomplete Penetration of Weld Groove External Undercut
Root Bead Fused to Both Inside Surfaces but Center of Root Slightly Below Inside Surface of Pipe (Not Incomplete Penetration)
Internal Undercut
e) Concave Root Surface (Suck-Up)
f) Undercut
g) Excess External Reinforcement
116
Figure 9.1
Notes:
61
Weld Inspection Guidelines Type of Inspection Visual
Radiography
Magnetic Particle
Liquid Penetrant
Ultrasonic
Situation/Weld Type All welds.
Defect •
Minor structural welds.
•
Cracks.
•
Slag inclusions.
•
Butt welds.
•
Gas pockets.
•
Girth welds.
•
Slag inclusions.
•
Miter groove welds.
•
Incomplete penetration.
•
Ferromagnetic materials.
•
Cracks.
For flaws up to 6 mm (1/4 in.) beneath the surface.
•
Porosity.
•
•
Lack of fusion.
•
Ferrous and nonferrous materials.
•
Cracks. Seams.
•
Intermediate weld passes.
• •
Porosity.
•
Weld root pass.
•
Folds.
•
Simple and inexpensive.
•
Inclusions.
•
Shrinkage.
Confirms high weld quality in pressurecontaining joints.
•
Surface defects.
•
Laminations.
•
Slag inclusions in thick plates.
•
Subsurface flaws.
Table 9.1
117
Notes:
Testing • Pressure test system to demonstrate integrity • Hydrostatic test unless pneumatic approved for special cases • Hydrostatic test pressure – ≥ 1½ times design pressure
118
Notes:
62
Testing, cont’d – For design temperature > test temperature:
PT =
1. 5 P S T S
ST/S must be ≤ 6.5 PT P ST S
= Minimum hydrostatic test pressure, psig = Internal design pressure, psig = Allowable stress at test temperature, psi = Allowable stress at design temperature, psi
119
Notes:
Testing, cont’d • Pneumatic test at 1.1P • Instrument take-off piping and sampling piping strength tested with connected equipment
120
Notes:
63
Nonmetallic Piping • Thermoplastic Piping – Can be repeatedly softened and hardened by increasing and decreasing temperature
• Reinforced Thermosetting Resin Piping (RTR) – Fabricated from resin which can be treated to become infusible or insoluble
121
Notes:
Nonmetallic Piping, cont’d • No allowances for pressure or temperature variations above design conditions • Most severe coincident pressure and temperature conditions determine design conditions
122
Notes:
64
Nonmetallic Piping, cont’d • Designed to prevent movement from causing: – Failure at supports – Leakage at joints – Detrimental stresses or distortions
• Stress-strain relationship inapplicable
123
Notes:
Nonmetallic Piping, cont’d • Flexibility and support requirement same as for piping in normal fluid service. In addition: – Piping must be supported, guided, anchored to prevent damage. – Point loads and narrow contact areas avoided – Padding placed between piping and supports – Valves and load transmitting equipment supported independently to prevent excessive loads. 124
Notes:
65
Nonmetallic Piping, cont’d • Thermoplastics not used in flammable service, and safeguarded in most fluid services. • Joined by bonding
125
Notes:
Category M Fluid Service Category M Fluid • Significant potential for personnel exposure • Single exposure to small quantity can cause irreversible harm to breathing or skin. 126
Notes:
66
Category M Fluid Service, cont’d • Requirements same as for piping in normal fluid service. In addition: – Design, layout, and operation conducted with minimal impact and shock loads. – Detrimental vibration, pulsation, resonance effects to be avoided or minimized. – No pressure-temperature variation allowances.
127
Notes:
Category M Fluid Service, cont’d – Most severe coincident pressure-temperature conditions determine design temperature and pressure. – All fabrication and joints visually examined. – Sensitive leak test required in addition to other required testing.
128
Notes:
67
Category M Fluid Service, cont’d • Following may not be used – Miter bends not designated as fittings, fabricated laps, nonmetallic fabricated branch connections. – Nonmetallic valves and specialty components. – Threaded nonmetallic flanges. – Expanded, threaded, caulked joints.
129
Notes:
High Pressure Piping • Ambient effects on design conditions – Pressure reduction based on cooling of gas or vapor – Increased pressure due to heating of a static fluid – Moisture condensation
130
Notes:
68
High Pressure Piping, cont’d • Other considerations – Dynamic effects – Weight effects – Thermal expansion and contraction effects – Support, anchor, and terminal movement
131
Notes:
High Pressure Piping, cont’d • Testing – Each system hydrostatically or pneumatically leak tested – Each weld and piping component tested – Post installation pressure test at 110% of design pressure if pre-installation test was performed
• Examination 132
– Generally more extensive than normal fluid service
Notes:
69
Summary • Process plant piping much more than just pipe • ASME B31.3 covers process plant piping • Covers design, materials, fabrication, erection, inspection, and testing • Course provided overview of requirements
133
Notes:
70
Part 2: Background Material
71
OVERVIEW OF PROCESS PLANT PIPING SYSTEM DESIGN Carmagen Engineering, Inc.
72
I.
INTRODUCTION
This course provides an overview of process plant piping system design. It discusses requirements contained in ASME B31.3, Process Piping, plus additional requirements and guidelines based on common industry practice. The information contained in this course is readily applicable to on-the-job applications, and prepares participants to take more extensive courses if appropriate.
II.
GENERAL A.
What is a piping system A piping system conveys fluid from one location to another. Within a process plant, the locations are typically one or more equipment items (e.g., pumps, pressure vessels, heat exchangers, process heaters, etc.), or individual process plants that are within the boundary of a process facility. A piping system consists of: •
Pipe sections
•
Fittings (e.g., elbows, reducers, branch connections, etc.)
•
Flanges, gaskets, and bolting
•
Valves
•
Pipe supports and restraints
Each individual component plus the overall system must be designed for the specified design conditions. B.
Scope of ASME B31.3 ASME B31.3 specifies the design, materials, fabrication, erection, inspection, and testing requirements for process plant piping systems. Process plants include petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related process plants and terminals.
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ASME B31.3 applies to piping and piping components that are used for all fluid services, not just hydrocarbon services. These include the following: •
Raw, intermediate, and finished chemicals.
•
Petroleum products.
•
Gas, steam, air, and water.
•
Fluidized solids.
•
Refrigerants.
•
Cryogenic fluids.
The scope also includes piping that interconnects pieces or stages within a packaged-equipment assembly. The following are excluded from the scope of ASME B31.3: •
Piping systems for internal gauge pressures at or above zero but less than 15 psi, provided that the fluid is nonflammable, nontoxic, and not damaging to human tissue, and its design temperature is from -20°F through 366°F.
•
Power boilers that are designed in accordance with the ASME Boiler and Pressure Vessel Code Section I and external boiler piping that must conform to ASME B31.1.
•
Tubes, tube headers, crossovers, and manifolds that are located inside a fired heater enclosure.
•
Pressure vessels, heat exchangers, pumps, compressors, and other fluid-handling or processing equipment. This includes both internal piping and connections for external piping.
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III.
MATERIAL SELECTION CONSIDERATIONS
Piping system material selection considerations are discussed below. A.
Strength A material's strength is defined by its yield, tensile, creep, and fatigue strengths. Alloy content, material grain size, and the steel production process are factors that affect material strength. 1.0
Yield and Tensile Strength A stress-strain diagram that is produced from a standard tensile test (Figure 3.1) illustrates the yield and tensile strengths. As the stress in a material increases, its deformation also increases. The yield strength is the stress that is required to produce permanent deformation in the material (Point A in Figure 3.1). If the stress is further increased, the permanent deformation continues to increase until the material fails. The maximum stress that the material attains is the tensile strength (Point B in Figure 3.1). If a large amount of strain occurs in going from Point A to Point C, the rupture point, the material is said to be ductile. Steel is an example of a ductile material. If the strain in going from Point A to Point C is small, the material is brittle. Gray cast iron is an example of a brittle material. B
S A
C
E Typical Stress-Strain Diagram for Steel Figure 3.1
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2.0
Creep Strength Below about 750°F for a given stress, the strain in most materials remains constant with time. Above this temperature, even with constant stress, the strain in the material will increase with time. This behavior is known as creep. The creep strength, like the yield and tensile strengths, varies with temperature. For a particular temperature, the creep strength of a material is the minimum stress that will rupture the material during a specified period of time. The temperature at which creep strength begins to be a factor is a function of material chemistry. For alloy materials (i.e., not carbon steel) creep strength becomes a consideration at temperatures higher than 750°F.
3.0
Fatigue Strength The term “fatigue” refers to the situation where a specimen breaks under a load that it has previously withstood for a length of time, or breaks during a load cycle that it has previously withstood several times. The first type of fatigue is called “static,” and the second type is called “cyclic.” Examples of static fatigue are: creep fracture and stress corrosion cracking. Static fatigue will not be discussed further in this course. One analogy to cyclic fatigue is the bending of a paper clip. The initial bending beyond a certain point causes the paper clip to yield (i.e., permanently deform) but not break. The clip could be bent back and forth several more times and still not break. However after a sufficient number of bending (i.e., load) cycles, the paper clip will break under this repetitive loading. Purely elastic deformation (i.e., without yielding) cannot cause a cyclic fatigue failure. The fatigue strength of a material under cyclic loading can then be defined as the ability to withstand repetitive loading without failure. The number of cycles to failure of a material decreases as the stress resulting from the applied load increases.
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B.
Corrosion Resistance Corrosion of materials involves deterioration of the metal by chemical or electrochemical attack. Corrosion resistance is usually the single most important factor that influences pipe material selection. Table 3.1 summarizes the typical types of piping system corrosion.
General or Uniform Corrosion
Characterized by uniform metal loss over entire surface of material. May be combined with erosion if material is exposed to high-velocity fluids, or moving fluids that contain abrasive materials.
Pitting Corrosion
Form of localized metal loss randomly located on material surface. Occurs most often in stagnant areas or areas of low-flow velocity.
Galvanic Corrosion
Occurs when two dissimilar metals contact each other in corrosive electrolytic environment. The anodic metal develops deep pits or grooves as a current flows from it to the cathodic metal.
Crevice Corrosion
Localized corrosion similar to pitting. Occurs at places such as gaskets, lap joints, and bolts, where a crevice can exist.
Concentration Cell Corrosion
Occurs when different concentration of either corrosive fluid or dissolved oxygen contacts areas of same metal. Usually associated with stagnant fluid.
Graphitic Corrosion Occurs in cast iron exposed to salt water or weak acids. Reduces iron in the cast iron and leaves the graphite in place. Result is extremely soft material with no metal loss.
Typical Types of Piping System Corrosion Table 3.1 For process plant piping systems in corrosive service, corrosion protection is usually achieved by using alloys that resist corrosion. The most common alloys used for this purpose are chromium and nickel. Low-alloy steels with a chromium content of 1¼% to 9% and stainless steels are used in corrosive environments. C.
Material Fracture Toughness One way to characterize the fracture behavior of a material is the amount of energy necessary to initiate and propagate a crack at a given temperature. This is the material's fracture toughness, which 77
decreases as the temperature decreases. Tough materials require a relatively large amount of energy to initiate and propagate a crack. The impact energy required to fracture a material sample at a given temperature can be measured by standard Charpy V-notch tests. Various factors other than temperature affect the fracture toughness of a material. These include the following: •
Chemical composition or alloying elements.
•
Heat treatment.
•
Grain size.
The major chemical elements that affect a material's fracture toughness are carbon, manganese, nickel, oxygen, sulfur, and molybdenum. High carbon content, or excessive amounts of oxygen, sulfur, or molybdenum, hurts fracture toughness. The addition of manganese or nickel improves fracture toughness. D.
Fabricability A material must be available in the shapes or forms that are required, and it typically must be weldable. In piping systems, some common shapes and forms include the following:
E.
•
Seamless pipe.
•
Plate that is used for welded pipe.
•
Wrought or forged elbows, tees, reducers, and crosses.
•
Forged flanges, couplings, and valves.
•
Cast valves.
Availability and Cost The last factors that affect piping material selection are availability and cost. Where there is more than one technically acceptable material, the final selection must consider what is readily available and what are the relative costs of the acceptable options. For example, the use of carbon steel with a large corrosion allowance could be more expensive than using a low-alloy material with a smaller corrosion allowance.
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IV.
PIPING COMPONENTS A.
Fittings, Flanges, and Gaskets 1.0
Pipe Fittings Fittings are used to make some change in the geometry of a piping system. This change could include: •
Modifying the flow direction.
•
Bringing two or more pipes together.
•
Altering the pipe diameter.
•
Terminating a pipe.
The most common types of fittings are elbows, tees, reducers, welding outlets, pipe caps, and lap joint stub ends. These are illustrated in Figures 4.1 through 4.6. Fittings may be attached to pipe by threading, socket welding, or butt welding. An elbow or return (Figure 4.1) changes the direction of a pipe run. Standard elbows change the direction by either 45° or 90°. Returns change the direction by 180°.
90°
45°
180° Return Elbow and Return Figure 4.1 79
A tee (Figure 4.2) provides for the intersection of three sections of pipe. •
A straight tee has equal diameters for both the run and branch pipe connections.
•
A reducing-outlet tee has a branch diameter which is smaller in size than the run diameter.
•
A cross permits the intersection of four sections of pipe and is rarely seen in process plants.
Tee Figure 4.2 A reducer (illustrated in Figure 4.3) changes the diameter in a straight section of pipe. The centerlines of the large and small diameter ends coincide in a concentric reducer, whereas they are offset in an eccentric type.
Concentric
Eccentric Reducer Figure 4.3
A welding outlet fitting, or integrally reinforced branch connection (Figure 4.4) has all the reinforcement required to strengthen the opening contained within the fitting itself.
80
Typical Integrally Reinforced Branch Connection Figure 4.4 A pipe cap (Figure 4.5) closes off the end of a pipe section. The wall thickness of a butt-welded pipe cap will typically be identical to that of the adjacent pipe section.
Cap Figure 4.5 A lap-joint stub end (Figure 4.6) is used in conjunction with lap-joint flanges.
Note square corner
R R Enlarged Section of Lap Lap-Joint Stub End Figure 4.6
81
2.0
Flanges A flange connects a pipe section to a piece of equipment, valve, or another pipe such that relatively simple disassembly is possible. Disassembly may be required for maintenance, inspection, or operational reasons. Figure 4.7 shows a typical flange assembly. Flanges are normally used for pipe sizes above NPS 1½.
Flange
Bolting
Gasket
Typical Flange Assembly Figure 4.7 A flange type is specified by stating the type of attachment and the type of face. The type of attachment defines how the flange is connected to a pipe section or piece of
82
equipment (e.g., welded). The type of flange face or facing defines the geometry of the flange surface that contacts the gasket. Table 4.1 summarizes the types of flange attachments and faces. Figure 4.8 illustrates flange facing types.
Flange Attachment Types
Flange Facing Types
Threaded Flanges
Flat Faced
Socket-Welded Flanges Blind Flanges
Raised Face
Slip-On Flanges Lapped Flanges
Ring Joint
Weld Neck Flanges Types of Flange Attachment and Facing Table 4.1
83
Flange Facing Types Figure 4.8
84
3.0
Gaskets A gasket is a resilient material that is inserted between the flanges and seated against the portion of the flanges called the “face” or “facing”. The gasket provides the seal between the fluid in the pipe and the outside, and thus prevents leakage. Bolts compress the gasket to achieve the seal and hold the flanges together against pressure and other loadings. The three gasket types typically used in pipe flanges for process plant applications are:
B.
•
Sheet.
•
Spiral wound.
•
Solid metal ring.
Flange Rating ASME B16.5, Pipe Flanges and Flanged Fittings, provides steel flange dimensional details for standard pipe sizes through NPS 24. Specification of an ASME B16.5 flange involves selection of the correct material and flange "Class." The paragraphs that follow discuss the flange class specification process in general terms. Flange material specifications are listed in Table 1A in ASME B16.5 (excerpted in Table 4.2). The material specifications are grouped within Material Group Numbers. For example, if the piping is fabricated from carbon steel, the ASTM A105 material specification is often used. ASTM A105 material is in Material Group No. 1.1. Refer to ASME B16.5 for additional acceptable material specifications and corresponding Material Group Numbers.
85
ASME B16.5, Table 1A, Material Specification List (Excerpt) Table 4.2 After the Material Group has been determined, the next step is to select the appropriate Class. The Class is determined by using pressure/temperature rating tables, the Material Group, design metal temperature, and design pressure. Selecting the Class sets all the detailed dimensions for flanges and flanged fittings. The objective is to select the lowest Class that is appropriate for the specified design conditions. Table 2 of ASME B16.5 provides the information that is necessary to select the appropriate flange Class for the specified design conditions. ASME B16.5 has seven classes: Class 150, 300, 400, 600, 900, 1,500, and 2,500. Each Class specifies the design pressure and temperature combinations that are acceptable for a flange with that designation. As the number of the Class increases, the strength of the flange increases for a given Material Group. A higher flange Class can withstand higher pressure and temperature combinations. Table 4.3 is an excerpt from Table 2 of ASME B16.5 and shows some of the temperature and pressure ratings for several Material Groups. Material and design temperature combinations that do not have a pressure indicated are not acceptable. Specifying the flange size, material, and class completes most of what is necessary for selecting an ASME B16.5 flange. The flange type, facing, bolting material, and gasket type and material must be
86
added to complete the flange selection process. Discussion of these other factors is beyond the scope of this course.
Material Group No. Classes Temp., °F -20 to 100 200 300 400 500 600 650 700 750 800 850 900 950 1000
1.8 150 235 220 215 200 170 140 125 110 95 80 65 50 35 20
300 620 570 555 555 555 555 555 545 515 510 485 450 320 215
1.9 400
150
825 765 745 740 740 740 740 725 685 675 650 600 425 290
300
290 260 230 200 170 140 125 110 95 80 65 50 35 20
750 750 720 695 695 605 590 570 530 510 485 450 320 215
1.10 400 1000 1000 965 885 805 785 785 710 675 650 600 425 290 190
150 290 260 230 200 170 140 125 110 95 80 65 50 35 20
300 750 750 730 705 665 605 590 570 530 510 485 450 375 260
400 1000 1000 970 940 885 805 785 755 710 675 650 600 505 345
ASME B16.5, Pressure-Temperature Ratings (Excerpt) Table 4.3
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SAMPLE PROBLEM 1 - DETERMINE FLANGE RATING A new piping system will be installed at an existing plant. It is necessary to determine the ASME class that is required for the flanges. The following design information is provided: •
Pipe Material: 1¼ Cr – ½ Mo.
•
Design Temperature: 700°F.
•
Design Pressure: 500 psig.
SOLUTION Determine the Material Group Number for the flanges by referring to ASME Table 1A (excerpted in Table 4.2). Find the 1¼ Cr – ½ Mo material in the Nominal Designation Steel column. The material specification for forged flanges would be A182 Gr. F11, and the corresponding material Group Number is 1.9. Refer to Table 2 for Class 150 (excerpted in Table 4.3). Read the allowable design pressure at the intersection of the 700°F design temperature and Material Group 1.9. This is only 110 psig and is not enough for this service. Now check Class 300 and do the same thing. The allowable pressure in this case is 570 psig, which is acceptable. The required flange Class is 300.
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V.
VALVES A.
Valve Functions The possible valve functions must be known before being able to select the appropriate valve type for a particular application. Fluid flows through a pipe, and valves are used to control the flow. A valve may be used to block flow, throttle flow, or prevent flow reversal. 1.0
Blocking Flow The block-flow function provides completely on or completely off flow control of a fluid, generally without throttling or variable control capability. It might be necessary to block flow to take equipment out of service for maintenance while the rest of the unit remains in operation, or to separate two portions of a single system to accommodate various operating scenarios.
2.0
Throttling Flow Throttling may increase or decrease the amount of fluid flowing in the system and can also help control pressure within the system. It might be necessary to throttle flow to regulate the filling rate of a pressure vessel, or to control unit operating pressure levels.
3.0
Preventing Flow Reversal It might be necessary to automatically prevent fluid from reversing its direction during sudden pressure changes or system upsets. Preventing reverse flow might be necessary to avoid damage to a pump or a compressor, or to automatically prevent backflow into the upstream part of the system due to process reasons.
89
B.
Primary Valve Types 1.0
Gate Valve Most valves in process plants function as block valves. About 75% of all valves in process plants are gate valves. The gate valve is an optimum engineering and economic choice for on or off service. The gate valve is not suitable to throttle flow because it will pass the maximum possible flow while it is only partially open. Figure 5.1 illustrates a typical full-port gate valve.
90
1.
Handwheel Nut
2.
Handwheel
3.
Stem Nut
4.
Yoke
5.
Yoke Bolting
6.
Stem
7.
Gland Flange
8.
Gland
9.
Gland Bolts or Gland-Eye Bolts and Nuts
10. Gland Lug Bolts and Nuts 11. Stem Packing 12. Plug 13. Lantern Ring 14. Backseat Bushing 15. Bonnet 16. Bonnet Gasket 17. Bonnet Bolts and Nuts 18. Gate 19. Seat Ring 20. Body 21. One-Piece Gland (Alternate) 22. Valve Port
Full-Port Gate Valve Figure 5.1 2.0
Globe Valve The globe valve is the type most commonly used to throttle flow in a process plant. In the smaller sizes, they are 91
typically used as hand-control valves. In larger sizes, applications are limited primarily to bypasses at control valve stations. They provide relatively tight shutoff in control valve bypasses during normal operations; they serve as temporary flow controllers when control valves must be taken out of service. Because all globe valve patterns involve a change in flow direction, they are not suitable for piping systems that require scraping or rodding. Globe valves are rarely used for strictly on/off block valve operations because conventional gate valves adequately serve that function at a lower cost and a much lower pressure drop. 3.0
Check Valve Check valves prevent flow reversal. Typical check valve applications are in pump and compressor discharge piping and other systems that require protection against backflow. Valves which contain a disc or discs that swing out of the flow passage area usually create a lower pressure drop in the system than those which contain a ball or piston element. These latter elements remain in the flowstream and the port configurations frequently include an angular change in flow direction. For all process designs, the intended purpose of check valves is to prevent gross flow reversal, not to effect complete leakage-free, pressure-tight shutoff of reverse flow. The selection of a particular check valve type generally depends on size, cost, availability, and service. Ball and lift check valves are usually the choice for sizes NPS 2 and smaller, while swing check and plate check valves are used in the larger sizes. 3.1
Swing Check Valve The main components of a swing check valve (Figure 5.2) are the body, disc, cap, seat ring, disc hinge, and pin. The disc is hinged at the top and closes against a seat in the valve body opening. It swings freely in an arc from the fully closed position to one that provides unobstructed flow. The valve is kept open by the flow, and disc seating is accomplished by gravity and/or flow reversal. 92
Cap Pin Seat Ring Hinge Flow Direction
Disc Body
Swing Check Valve Figure 5.2 3.2
Ball Check Valve The ball check valve utilizes a ball to prevent flow reversal (Figure 5.3). The basic types are the straight-through- and globe-type (90° change in direction, similar to a typical globe valve body). Ball check valves are available in sizes NPS ½ through 2 in all ratings and materials used in process plants. Their low cost usually makes them the first choice for valves sized NPS 2 and smaller, provided the pressure drop is not a concern.
93
Ball Check Valve Figure 5.3 3.3
Lift Check Valve A lift check valve (Figure 5.4) usually depends on gravity for operation. Under forward flow, a piston or disc is lifted off the seat by the fluid while being retained in the valve by guides. On reverse flow, the piston or disc is forced against the seat to block further flow. Some lift check valves utilize spring loading to assure positive seating. Lift check valves employing the disc- or piston-type mechanism are available in sizes from NPS ½ through 2 in all ratings and materials used in process plants. They are most commonly used in the higher ASME B16.5 ratings (Class 300 and greater), and where tighter shutoff is required. Valves of this type should only be used in clean services.
94
Seat Ring Piston Flow Direction
Lift Check Valve Figure 5.4 3.4
Wafer Check Valve The wafer body or flangeless valve is a valve body without flanges (Figure 5.5). Valves of this type are placed between pipe flanges and held in place by the compressive force between the flanges and transmitted through the gaskets. The lug-wafer (or single-flanged) valve is also shown in Figure 5.5. Valves of this type are mounted between pipe flanges and are held in place by cap screws, machine bolts, or stud bolts which thread into the valve body.
95
Figure 5.5 3.5
Ball Valve Ball valves (Figure 5.6) usually function as block valves. Ball valves are well suited for conditions where quick on/off and/or bubble-tight shut-off is required. The pressure/temperature ratings for ball valve soft seats above ambient temperatures are usually lower than the ASME ratings for steel valves. This is because of the lower physical properties of the soft-seat materials. Soft-sealed ball valves are not normally used for throttling service because the softseats are subject to erosion or distortion/displacement caused by fluid flow when the valve is in the partially open position.
96
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Part Names Body Body Cap Ball Body Seal Gasket Seat Stem Gland Flange Stem Packing Gland Follower Thrust Bearing Thrust Washer Indicator Stop Snap Ring Gland Bolt Stem Bearing Body Stud Bolt & Nuts Gland Cover Gland Cover Bolts Handle
Ball Valve Figure 5.6 3.6
Plug Valve Plug valves (Figure 5.7) usually function as block valves. They are well suited for conditions where quick on/off and/or bubble-tight shutoff is required. The soft-seal-types may have lower temperature/pressure ratings than the ASME ratings for steel valves because of the lesser physical properties of the soft-seat materials. Soft-seal plug valves are not normally used for throttling service since the soft seals are subject to erosion or distortion/displacement caused by fluid flow when the valve is partially open.
97
Wedge
Molded-In Resilient Seal
Sealing Slip
Plug Valve Figure 5.7 C.
Valve Selection Process The steps that follow provide a general procedure for selecting valves and valve components. 1. Identify the necessary design information. This includes design pressure and temperature, valve function, material, etc. 2. Identify potentially appropriate valve types (i.e., ball, butterfly, check, etc.) and components based on application and function (i.e., block, throttle, or reverse flow prevention). 3. Determine valve application requirements (i.e., design or service limitations). 4. Finalize valve selection. Check which factors need consideration if two or more valves are suitable. 5. Provide a full technical description. This is done by specifying the valve type, material, flange rating, etc. 98
Exercise 1 – Determine Required Flange Rating For the piping system described below, determine the required flange rating (or Class) in accordance with ASME B16.5. Pipe:
1¼ Cr – ½ Mo
Flanges:
A - 182 Gr. F11
Design Temperature:
900°F
Design Pressure:
375 psig
99
VI.
DESIGN A.
Design Conditions 1.0
General Normal operating conditions are those expected to occur during normal operation, excluding failure of any operating device, operator error, and the occasional, short-term variations stated in the applicable code. Startup and controlled shutdown of plants and similar foreseeable events are included within normal operation. Design conditions are those which govern the design and selection of piping components, and are based on the most severe conditions expected to occur in service. A suitable margin is used between the normal operating and design conditions to account for normal operating variations. ASME B31.3 does not specify what margins should be used between operating and design conditions; suitable margins are determined by the user based on his experience.
2.0
Determining Design Pressure and Temperature The design pressure and temperature are used to calculate the required thickness of pipe and other design details. The design temperature is used to determine the material basic allowable stress and other design requirements. The values for design pressure and temperature are based on process requirements. Piping system design conditions generally are determined based on the design conditions of the equipment to which the piping is attached. Determining the piping design conditions consists of: 1. Identifying the equipment to which the piping system is attached. 2. Determining the design pressure and design temperature for the equipment.
100
3. Considering contingent design conditions, such as upsets not protected by pressure-relieving devices. 4. Considering the direction of flow between the equipment. 5. Verifying the values with the process engineer. B.
Loads and Stresses 1.0
Classification of Loading Conditions Pipe loads are classified into three principal types: sustained loads, thermal expansion loads, and occasional loads. Sustained loads are those that act on the piping system during all or most of its operating time. Sustained loads consist of two main categories: pressure and weight. The pressure load (caused by the design pressure) usually refers to internal pressure, although some piping systems may also be designed for external pressure. Design pressure is defined as the maximum sustained pressure that a piping system must contain without exceeding its allowable stress limits. Design pressure is normally the governing factor in determining the minimum required pipe wall thickness. As shown in Figure 6.1, internal pressure produces both circumferential (i.e., hoop) stress and longitudinal stress in the pipe wall.
101
Sl
Sc P t
Sl
=
Longitudinal Stress
Sc
=
Circumferential (Hoop) Stress
t
=
Wall Thickness
P
=
Internal Pressure
Stresses Produced By Internal Pressure Figure 6.1 The weight refers to the total design weight load. The total weight load includes the weight of the pipe, the fluid in the pipe, fittings, insulation, internal lining, valves, valve operators, flanges, supports and any other concentrated loads. The weight loads produce a longitudinal stress in the pipe wall. A piping system will expand or contract due to changes in its operating temperature. Thermal expansion loads are created when the free expansion and contraction of the piping is prevented at its end points by connected equipment, or prevented at intermediate points by supports and/or restraints that are installed. The resulting loads cause thermal stresses in the pipe. Increasing the restraint in a system increases the loading and results in higher thermal expansion stresses. Another cause of pipe thermal loads can be from the thermal expansion of equipment at 102
pipe-to-equipment nozzle attachment points, causing displacements in the piping system. The third type of loading comes from occasional loads. Occasional loads act during a small percentage of the system’s operating time. Occasional loads involve seismic and/or dynamic loading. The degree of seismic loading that must be considered varies with geographic location and is defined by a seismic zone (Ref. ANSI/ASCE 7). Dynamic loads may be caused by safety-relief valve discharges, valve operation (both opening and closing), steam/water hammer, surge due to pump start-up and shutdown, and wind loads. 2.0
Stress Categorization To evaluate the stresses in a piping system, it is necessary to distinguish among primary, secondary, and peak stresses.
3.0
•
Primary stresses are the direct, shear, or bending stresses generated by the loading.
•
Secondary stresses are those acting across the pipe wall thickness due to a differential radial deflection of the pipe wall. Secondary stresses cause local yielding and minor distortions. Secondary stresses, unlike primary stresses, are not a source of direct failure from a single load application.
•
Peak stresses are more localized stresses which die away rapidly within a short distance from their origin. Peak stresses occur in areas such as welds, fittings, branch connections, and other piping components where stress concentrations and possible fatigue failure might occur. Peak stresses are considered equivalent in significance to secondary stresses, but they do not cause any significant distortion.
Allowable Stresses The basic allowable stress is a function of material properties, temperature, and safety factors. The basic allowable stress provides an upper limit for the actual stresses.
103
•
Allowable stresses for sustained loads are established to prevent general collapse or excessive distortion of the piping system.
•
Allowable stresses for thermal expansion loads are established to prevent a localized fatigue failure.
•
Allowable stresses for occasional loads are established to prevent wind and earthquake type loads from collapsing or distorting the piping system.
Actual stresses are calculated for the following load cases: •
Sustained loads
•
Occasional loads
•
Stress range due to differential thermal expansion
The piping system is designed such that the calculated stresses are no larger than the appropriate allowable stresses. Table 6.1 (excerpted from ASME B31.3 Table A-1) lists basic allowable stresses in tension versus temperature for several materials.
104
Basic Allowable Stress S, ksi. At Metal Temperature, °F. Material
Spec. No/Grade
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Carbon Steel
A 106
B
20.0
20.0
20.0
20.0
18.9
17.3
16.5
10.8
6.5
2.5
1.0
C - ½Mo
A 335
P1
18.3
18.3
17.5
16.9
16.3
15.7
15.1
13.5
12.7
4.
2.4
1¼ - ½Mo
A 335
P11
20.0
18.7
18.0
17.5
17.2
16.7
15.6
15.0
12.8
6.3
2.8
18Cr - 8Ni pipe
A 312
TP304
20.0
20.0
20.0
18.7
17.5
16.4
16.0
15.2
14.6
13.8
9.7
6.0
3.7
2.3
1.4
16Cr - 12Ni-2Mo pipe
A 312
TP316
20.0
20.0
20.0
19.3
17.9
17.0
16.3
15.9
15.5
15.3
12.4
7.4
4.1
2.3
1.3
1.2
ASME B31.3, Table A-1 (Excerpt), Basic Allowable Stresses in Tension for Metal Table 6.1 C.
Pressure Design of Components 1.0
General Two different types of pressure may be imposed on a piping system: external or internal. Most piping systems need only be designed for internal pressure. Some piping systems may be subject to a negative pressure or vacuum condition during operation (e.g., process vacuum conditions, steamout, underwater lines, etc.) and must be designed for external pressure. This section only discusses the internal pressure design of straight sections of pipe. Refer to ASME B31.3 for design requirements for external pressure.
2.0
Required Wall Thickness for Internal Pressure of Straight Pipe The required wall thickness for internal pressure is calculated using the following equation: t=
1500
PD 2 (SE + PY )
Where: t
= Required thickness for internal pressure, in.
P
= Internal design pressure, psig
105
S
= Allowable stress in tension (Table 6.1), psi
E
= Longitudinal-joint quality factor (Table 6.2)
Y
= Wall thickness correction factor (Table 6.3)
The longitudinal-joint quality factor is based on: •
Whether the pipe is seamless or has a welded longitudinal seam
•
The pipe material and welding process (if welded pipe)
The wall thickness correction factor is based on the type of steel and the design temperature.
106
Spec. No.
Class (or Type)
Description
Ej
Carbon Steel API 5L
... ... ...
Seamless pipe Electric resistance welded pipe Electric fusion welded pipe, double butt, straight or spiral seam Furnace butt welded
1.00 0.85 0.95
A 53
Type S Type E Type F
Seamless pipe Electric resistance welded pipe Furnace butt welded pipe
1.00 0.85 0.60
A 106
...
Seamless pipe
1.00
Low and Intermediate Alloy Steel A 333
... ...
Seamless pipe Electric resistance welded pipe
1.00 0.85
A 335
...
Seamless pipe
1.00
Stainless Steel A 312
... ... ...
Seamless pipe Electric fusion welded pipe, double butt seam Electric fusion welded pipe, single butt seam
1.00 0.85 0.80
A 358
1, 3, 4 5 2
Electric fusion welded pipe, 100% radiographed Electric fusion welded pipe, spot radiographed Electric fusion welded pipe, double butt seam
1.00 0.90 0.85
Nickel and Nickel Alloy B 161
...
Seamless pipe and tube
1.00
B 514
...
Welded pipe
0.80
B 675
All
Welded pipe
0.80
ASME B31.3, Table A-1B (Excerpt), Basic Quality Factors for Longitudinal Weld Joints, Ej Table 6.2
107
Temperature, °F Materials
900 & lower
950
1000
1050
1100
1150 & up
Ferritic Steels
0.4
0.5
0.7
0.7
0.7
0.7
Austenitic Steels
0.4
0.4
0.4
0.4
0.5
0.7
Other Ductile Metals
0.4
0.4
0.4
0.4
0.4
0.4
Cast iron
0.0
...
...
...
...
...
ASME B31.3, Table 304.1.1 (Excerpt), Values of Coefficient Y Table 6.3 Two additional thickness allowances must be considered to determine the final required pipe wall thickness: corrosion allowance and mill tolerance. Corrosion allowance (CA) is an additional thickness that is added to account for wall thinning and wear that can occur in service. The corrosion allowance is based on experience and data for the particular pipe material and fluid service. Thus: tm = t + CA Where: tm
=
Total minimum required wall thickness, in.
Mill tolerance accounts for the difference between the actual manufactured pipe wall thickness and the “nominal” wall thickness specified in the relevant pipe dimensional standard. The typical pipe mill tolerance is 12.5%. This means that the as-supplied pipe wall thickness can be up to 12.5% thinner than the nominal thickness and still meet its specification requirements. Use the following equation to determine the minimum required nominal thickness to order:
108
t nom =
tm 0.875
Where: tnom = Minimum required nominal pipe wall thickness, in. Each pipe size has several standard nominal thicknesses that are available. The nominal pipe thickness that is specified for a system must be selected from those readily available and be at least equal to tnom. 3.0
Curved and Mitered Pipe Segments The minimum required thickness of curved pipe (elbows or bends) is the same as that required for straight pipe sections. A mitered bed is fabricated by welding straight pipe sections together to produce the direction change. A mitered bend is generally less expensive than a wrought elbow for large pipe sizes (over ~ NPS 24). The minimum required thickness for a miter may be greater that that of the connected straight pipe sections, depending on the number of miter welds, design conditions, size, etc. Refer to ASME B31.3 for thickness calculation requirements.
109
SAMPLE PROBLEM 2 - DETERMINE PIPE WALL THICKNESS A piping system must be modified to add a new, spare heat exchanger. You have been assigned the responsibility to determine the required wall thickness for the pipe from the heat exchanger to several pumps. The piping system will have a design temperature of 650°F. The design pressure is 1,380 psig. The pipe outside diameter is 14 in. The material is ASTM A335, Gr. P11 (1¼ Cr – ½ Mo), seamless. Corrosion allowance is 0.0625 in. What is the minimum required thickness for this pipe?
SOLUTION The following equation applies: t=
PD 2 (SE + PY )
Based on the given information: P
=
1,380 psig.
D
=
14 in.
For the A335, Gr. P 11 material: S
= 16,150 psi. [Table A-1 of ASME B31.3 at 650°F
E
= 1.0 [Table A-1B of ASME B31.3]
Y
=
0.4 [Table 304.1.1 of ASME B31.3, since the material is ferritic and the temperature is below 900oF.
Since all the required parameters have now been determined, the required internal pressure thickness may be calculated as follows:
110
1,380 × 14 2 [(16,150 × 1) + (1,380 × 0.4)] t = 0.577 in.
t=
In this case, a 0.0625 in. corrosion allowance has been specified. Therefore:
4.0
tm
=
t + c = 0.577 + 0.0625
tm
=
0.6395 in.
tnom =
0.6395 = 0.731 in. 0.875
Branch Reinforcement Requirements A pipe with a branch connection is weakened by the required opening. Unless the wall thickness of the pipe is sufficiently greater than that required to sustain the pressure, additional reinforcement must be provided. ASME B31.3 contain rules for determining the required reinforcement for both welded and extruded outlet-type branch connections. Branch connections can also be made using forged or wrought fittings (i.e., tees, laterals, crosses, couplings, or half-couplings), or an integrally reinforced branch connection. Reinforcement calculations are not required for forged or wrought type branch connections because they have adequate inherent reinforcement and have been designed and tested to meet ASME B31.3 requirements. This section discusses only branch connections that are fabricated by welding a branch pipe to the run pipe.
111
4.1
Area Removed By Branch Connection A volume of metal is removed from a pipe wall when a hole is cut in it for a branch connection. However, a simplification is made when evaluating branch reinforcement requirements. An imaginary plane is passed through the branch and run pipes, and the intersection is viewed in crosssection. The removed volume of pipe wall is then looked at as an area (see Figure 6.2). Db
Tb Reinforcement Zone Limits
Nom. Thk.
c
tb
Mill Tol.
A3
A3
L4
Reinforcement Zone Limits
A4
A4 A1
Tr Th Dh Nom. Thk.
c
th
d1
A2
Mill Tol.
A2
d2
d2
β
Pipe C
Welded Branch Connection Figure 6.2 4.2
Limits of Reinforcement Zone The reinforcing zone is the region where credit may be taken for any reinforcement that is present. The branch connection must have adequate reinforcement to compensate for the weakening caused by cutting a hole in the run pipe. This reinforcement:
112
•
Must be located reasonably close to the opening to provide any practical benefit.
•
May be located in the branch pipe, the run pipe, or both.
Additional material located outside of this zone is not effective for reinforcement. 4.3
Branch Connection Reinforcement Branch connection reinforcement located within the reinforcement zone may come from one or more of the following sources. •
Excess thickness available in the branch or header pipe.
•
Additional reinforcement added in the form of a pad, ring, saddle, or weld metal.
If excess thicknesses in the branch and header pipes do not provide enough reinforcement, additional metal may be added. 4.4
Reinforcement Area The required reinforcement area is based on the metal area removed. This is calculated using: d1 =
D b − 2(Tb − c ) sin β
Where: d1
=
Effective length removed from the run pipe, in.
Db
=
Branch outside diameter, in.
Tb
=
Minimum branch thickness, in.
c
=
Corrosion allowance, in.
β
=
Acute angle between branch and header
113
The required reinforcement area, A1, is then calculated using : A1 = t h d1(2 − sin β)
Where: th 4.5
=
Minimum required header thickness, in.
Reinforcement Pad Additional branch reinforcement is needed when the required area exceeds the available area, and may be provided by locally increasing the thickness of either the header or branch pipe. However, it is usually more economical to provide a reinforcement pad to supply the additional reinforcement. There are three variables to select in designing the reinforcement pad: •
Material
•
Outside diameter
•
Wall thickness
To calculate the area of the reinforcement pad, A4, the following equation is used:
æ (D p − D b ) ö A 4 = çç Tr è sin β Where: D01p = Outside diameter of the pad, in. Db =
Outside diameter of the branch, in.
Tr =
Pad thickness, in.
β=
The acute angle between the branch and header pipes.
114
The pad must be large enough to provide the additional reinforcement needed and be within the reinforcement zone. The pad material is generally equivalent to that of the pipe. The following Sample Problem illustrates the branch reinforcement calculation procedure.
115
SAMPLE PROBLEM 3 A new steam turbine is being installed within a process plant. This will require a new NPS 16 steam supply line to be connected to an existing NPS 24 distribution header. The following design information has been determined:
•
Pipe material - Seamless, A 106/Gr. B for both the branch and header.
•
Design temperature
-
700°F
•
Design pressure
-
550 psig
•
Allowable stress
-
16,500 psi.
•
Corrosion allowance
-
0.0625 in.
•
Mill tolerance
-
12.5%
•
Nominal Pipe Thicknesses
-
Header: 0.562 in. Branch: 0.375 in.
•
Required Pipe Thicknesses for Pressure
-
Header: 0.395 in. Branch: 0.263 in.
•
The branch connection is made on top of the header at a 90° angle, and does not penetrate a header weld.
Determine if additional reinforcement is required for this branch connection. If it is, size the reinforcing pad, neglecting the area of any welds. Assume that the pad material is equal to the header material, and that its thickness equals the header thickness.
SOLUTION See Figure 6.2 for the relevant nomenclature.
•
The required thicknesses for pressure were given.
•
Next, the value for the effective length removed from the run pipe, d1, must be calculated. This equals the corroded inside diameter of the branch connection after accounting for mill tolerance (i.e., the actual pipe wall thickness may be up to 12.5% less than the nominal thickness).
d1 =
Db − 2(Tb − c ) sin β
116
d1 =
16 − 2(0.375 × 0.875 − 0.0625 ) sin 90°
d1 = 15.469 in.
•
Now the required reinforcement area, A1, may be calculated. A1 = t h d1 (2 − sinβi A1 = 0.395 × 15.469 (2 − sin90°) A1 = 6.11in.2
The available reinforcement areas in the header and branch pipe are now calculated. This is determined using any “excess” thickness available in the header and branch that is not necessary to withstand the pressure (or other) loads. Disregard any contribution from nozzle attachment welds since this is minimal.
•
Calculate the excess area available in the header, A2.
A2 = (2d2 − d1)(Th − th − c ) First determine d2 which is the greater of d1, or,
(Tb−c ) + (Th−c ) +
d1 2 , but less than the header diameter, Dh
(0.875 × 0.375 − 0.0625 ) + (0.875 × 0.562 − 0.0625 ) + 15.469 2
= 8.43 in.
∴ d2 = d1 = 15.469 in., which is less than the header diameter of 24 in. A2 = (2 x 15.469 - 15.469) (0.875 x 0.562 - 0.395 - 0.0625) A2 = 0.53 in.2
•
Calculate the excess area available in the branch, A3.
117
A3 =
2L4(Tb − tb−c ) sin β
First determine L4. L4= 2.5 (Th−c ) or 2.5 (Tb−c ) + Tr, whichever is smaller . Since Tr = 0 (i.e., no reinforcing pad initially) and Th is greater than Tb, L4 is based on the second equation. L4 =
2.5 (0.875 x 0.375 - 0.0625)
L4 =
0.664 in.
A3 =
2 × 0.664 (0.875 × 0.375 − 0.263 − 0.0625 ) sin 90°
A3 = 0.003 in.2
•
Calculate other excess area that may be available, A4. There is no reinforcing pad and the area contribution from the branch weld is being disregarded. Therefore, A4 = 0.
•
Total Available Area: The total available reinforcement area, AT, is calculated by adding the contributions from each source. AT =
A2 + A3 + A4
AT =
0.53 + 0.003 + 0
AT =
0.533 in.2 available reinforcement.
The available total reinforcement of 0.533 in.2 is obviously much less than the required reinforcement area of 6.11 in.2. Therefore, a reinforcing pad is required. The reinforcement pad will now be sized.
118
A106, Gr. B material will be used for the reinforcement pad. Its thickness is set to be equal to the header nominal thickness of 0.562 in.
•
Recalculate Available Reinforcement: Now that a reinforcing pad is being used, the available reinforcement in the branch must be recalculated since the height of the reinforcement zone in the branch pipe will change slightly. L41 =
2.5 (Th - c)
L41 =
2.5 (0.875 × 0.562 - 0.0625)
L41 =
1.073 in.
L42 =
2.5 (Tb - c) + Tr
L42 =
2.5 (0.875 × 0.375 - 0.0625) + 0.562 (0.875)
L42 =
1.16 in.
Therefore, L4 = 1.073 in. A3 =
2L 4 (Tb − t b − c) sin β
A3 =
2 × 1.073 (0.875 × 0.375 − 0.263 − 0.0625) sin 90 o A 3 = 0.005 in.2 (vs. the 0.003 in.2 previously calculated) A T = A 2 +A 3 + A 4 A T = 0.53 + 0.005 + 0 A T = 0.535 in.2 available reinforcement
•
Calculate additional reinforcement required and the pad dimensions: The required reinforcement area is 6.11in.2, and the available area is 0.535 in.2. Therefore, the additional reinforcement area to be provided in the pad, A4, is:
119
•
A4 =
6.11 - 0.535
A4 =
5.575 in.2
Determine the diameter of the pad, Dp. Tr = 0.562 (0.875) = 0.492 in. Db = 16 in. Dp =
A4 Db + Tr sin β
5.575 + 16 0.492 D p = 27.3 in. Dp =
The pad diameter must be at least 27.3 in. to provide adequate reinforcement. Since 2d2 = 30.938 in., this pad diameter is within the reinforcement zone along the header and is acceptable. The following approach of calculating the required pad width, Lr, may be used as an alternative to calculating the pad diameter. 0.5 A4 Tr 0.5 × 5.575 Lr = 0.492 L r = 5.66 in. Lr =
120
EXERCISE 2:
DETERMINE REQUIRED PIPE WALL THICKNESS
A new project is being considered to transport 48° API crude oil in a carbon steel pipe between two areas within a tank farm. The fluid being transported will have a design temperature of 260°F. The system design pressure is 150 psig, the pipe outside diameter is 30 in., and the pipe being used is A 106, Gr. B seamless pipe. A corrosion allowance of 1/8 in. has been specified for the pipe. All piping within the tank farm is designed in accordance with ASME B31.3. Assume there is a 12.5% mill tolerance. a.
What is the thickness required for internal pressure?
b.
What is the minimum required nominal wall thickness?
Use Table 6.1 along with Tables 6.2 and 6.3 for the necessary information.
121
VII.
SYSTEM DESIGN A.
Layout Considerations Operational, maintenance, and safety considerations influence the layout of a piping system. These factors must be recognized when designing the layout and spacing of piping and equipment. This section discusses how these factors influence piping layout. 1.0
Operations Requirements Operating and control points (e.g., valves, flanges, instruments, sample points, drains, and vents) should be located so that they can be used safely and easily. For example, valves must be located so that they can be reached. There must be enough clearance above and below the pipe to perform basic operations on valves and flanges. There must also be enough lateral space to access valves, sample points, vessel flanges, and other equipment that may require operator attention.
2.0
Maintenance Requirements The piping system must be laid out so that its components can be inspected, repaired, or replaced with minimum difficulty. There must be ample clearance for maintenance equipment (e.g., cranes) and for vehicles (e.g., trucks). Access must be provided so supports can be maintained. There must be enough space to access and remove large pieces of equipment if they require maintenance.
•
Access near rotating equipment is important because cranes must reach the equipment when removal or realignment is required.
•
Heat exchanger bundles must be pulled out for cleaning.
•
Large valves must be removed to repair or replace their seats.
122
• 3.0
Rotating equipment requires frequent monitoring and maintenance.
Safety Considerations Piping layout must consider the safety of personnel near the pipe. This specifically includes access for fire fighting equipment and fire prevention. Fire fighting equipment needs clearance to access major pieces of equipment (e.g., heat exchangers, vessels, and tankage). Pipeways must be routed and designed to provide the necessary clearances. There must be enough space beneath pipeways for people to walk and work. Firewater piping must be routed so that it would not be damaged by piping containing hazardous fluids that could rupture.
B.
Pipe Supports and Restraints A piping system needs supports and restraints because of the various loads that are imposed upon it. Supports absorb system weight and reduce longitudinal pipe stress, pipe sag, and end point reaction loads. Restraints control or direct the thermal movement of a piping system. The control of thermal movement may be necessary either to keep pipe thermal expansion stresses within allowable limits, or to limit the loads that are imposed on connected equipment. Selection of a specific type of support or restraint to use in a particular situation depends on such factors as:
•
Load to be supported or absorbed.
•
Clearance available for attachment to pipe.
•
Availability of nearby structural steel that is already there.
•
Direction of loads to be absorbed or movement to be restrained.
•
Design temperature.
•
Need to permit vertical thermal movement at a support.
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1.0
Rigid Supports Rigid supports are used in situations where weight support is needed and no provision to permit vertical thermal displacement is required. A rigid support always will prevent vertical movement downward, will sometimes prevent vertical thermal movement upward, and will permit lateral movement and rotation. See Figure 7.1.
Shoe
Base Adjustable Support
Saddle
Dummy Support
Trunnion
Rigid Supports Figure 7.1 Hangers are a type of rigid support. They support pipe from structural steel or other facilities that are located above the pipe and carry piping weight loads in tension. Pipe hangers are typically
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one or more structural steel rods bolted to a pipe attachment and to the overhead member. A hanger rod is designed to move freely both parallel and perpendicular to the pipe axis, and not restrict thermal expansion in these directions. A hanger will prevent movement both down and up. See Figure 7.2.
Hangers Figure 7.2 2.0
Flexible Supports Flexible or resilient supports allow the piping system to move in all three directions while still supporting the required weight load. Weight is supported by the use of a coil spring having an appropriate stiffness to carry the applied weight
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load. Since the spring is resilient, it permits vertical thermal movement while still carrying the weight. This type of support is used in situations where support must be provided at a particular location, and vertical thermal expansion must also be permitted. There are two basic types of flexible supports: variable load and constant-load-type. In the variable-load type flexible support, the amount of vertical load exerted by the support changes as a result of the pipe thermal movement (which compresses or extends the spring). The amount of vertical load exerted by a constant-load type support does not change throughout its movement range. See Figure 7.3.
Load and Deflection Scale
Small Change in Effective Lever Arm
Large Change in Effective Lever Arm Relatively Constant Load Typical Variable-Load Spring Support
Typical Constant-Load Spring Support Mechanism
Flexible Supports Figure 7.3 3.0
Typical Restraints and Anchors 3.1
Restraints Restraints have two primary purposes in a piping system.
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•
Restraints control, limit, or redirect the unrestricted thermal movement of a pipe. They are used to either reduce the thermal stress in the pipe or the loads exerted by the pipe on equipment connections.
•
Restraints absorb loads imposed on the pipe by other conditions such as wind, earthquake, slug flow, water hammer, or flow-induced vibration. Excessive loads could result in high pipe stress or equipment reaction loads, or cause flange leakage.
There are several different types of restraints that may be used. The selection of which type to use and its specific design details depends primarily on the direction of pipe movement that must be restrained, the location of the restraint point, and the magnitude of the load that must be absorbed. It is also possible to restrain more than one direction at one location in a piping system, or to combine a restraint with a support. 3.2
Anchors An anchor is a special type of restraint that stops movement in all three directions. Anchors provide full fixation of the pipe, permitting very limited, if any, translation or rotation. An anchor is used in situations where it is necessary to totally isolate one section of a piping system from another from the standpoint of load and deflection. A total anchor that eliminates all translation and rotation at one location is not used as commonly as one or more restraints that act at a single location. A directional anchor which restrains the line only in its axial direction is more commonly used. Figure 7.4 provides several examples of anchors.
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Anchor
Anchor
Partial Anchor
Restraints/Anchors Figure 7.4 3.3
Guides A guide is a particular type of restraint that permits movement along the pipe axis while preventing lateral movement. Depending on the particular guide details employed, pipe rotation may or may not be restricted. Common situations where guides are used are in long pipe runs on a pipe rack to control thermal movement and prevent buckling, and in straight pipe runs down the side of a tower to prevent wind-induced movement and control thermal expansion. See Figure 7.5.
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Guide
Guide
x
Guide
Vertical Guide
Examples of Guides Figure 7.5 C.
Piping Flexibility Piping must have sufficient flexibility to accommodate thermal expansion (or contraction) effects. Piping systems must be designed to ensure that they do not fail because of thermal stresses or produce excessive forces and moments at connected equipment. If a system does not provide adequate flexibility, the results can be leaky flanges, fatigue failure of the pipe, excessive maintenance, operations problems, and damaged equipment.
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A structure that is subject to a change in temperature will change in dimensions. If these thermal movements are allowed to occur without any restraint whatsoever, no pipe stresses or reaction loads result. However, in real systems, stresses are developed in the pipe and moments and forces are imposed on the connected equipment and at supports and restraints installed in the system. The basic problem is to determine the internal pipe stresses and the external loads, and then decide if they are acceptable. A thermal flexibility analysis is done to ensure that the piping system is laid out, supported, and restrained such that the thermal stresses in the pipe and the loads on the end points are within allowable limits. 1.0
Rationale for Piping Flexibility and Support Design Support and flexibility design is a combination of art and science with multiple factors to consider and usually more than one way to design the system. It requires knowledge of how the operating and design conditions of a piping system influence its overall design, and the supports and restraints required for the system. A piping system can be described as an irregular structural frame in space because of its relatively slender proportions when compared to structural steel systems. Elevated design temperatures or various operating scenarios may cause sufficient pipe thermal stress or reduce material strength such that supplementary structural assistance to support the piping system is required. It is also often necessary to limit the pipe movement at specific locations to protect sensitive equipment, control vibration, or to resist external forces (e.g., wind, earthquake, or shock loading). Attention must also be paid to pipe support/restraint design details to ensure that localized stresses in the pipe wall are kept within allowable limits. In those situations, design details that spread the applied load over a wider portion of the pipe surface are used. Planning for pipe supports and restraints should be done simultaneously with establishing possible layout configurations to achieve the most cost-effective design.
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2.0
Approaches to Design Due to the complexity of the piping flexibility and support design process, there is no single procedure or design method applicable for all situations. The following is one way to approach the problem.
•
Examine the layout and operation of the piping system to identify: -
Layout geometry.
-
Pipe diameter and thickness, and locations of any changes in these parameters.
-
Piping component design details such as branch connection details and type of elbows used (i.e., long radius or short radius).
-
Design temperature and pressure.
-
Fluid service, including its potential danger.
-
End-point movements.
-
Type of connected equipment (i.e., rotating or fixed).
-
Locations of existing structural steel.
-
Relevant operating scenarios.
-
Special design considerations (e.g., wind, vibrationprone services, orientation of loads).
•
Determine the potential effects of those conditions (e.g., thermal movements, loads, and stresses).
•
Determine the types of support or restraint required and their approximate locations.
•
Determine if the situation warrants a detailed computer analysis.
•
If required, identify which conditions apply for the analysis and utilize an appropriate computer program.
•
Interpret the results of the analysis.
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D.
Required Design Information for Piping Stress Analysis Detailed piping stress analysis is done using a computer program such as Caesar II, Simflex, or Triflex. Such programs have the capability to consider any combination of pipe geometry, support, restraint, and load conditions. However, several things must be considered:
•
Applicable design conditions and operating scenarios for the piping system.
•
Allowable stresses from ASME B31.3.
•
Load limitations, if any, on connected equipment.
•
Extent of analysis required to identify most severe case.
Design conditions that must be known to perform a detailed pipe stress analysis are listed below:
•
Layout geometry of the piping system.
•
Pipe diameter and wall thickness.
•
Design temperature and pressure.
•
Fluid service, including whether it is dangerous.
•
End-point movements.
•
Type of connected equipment.
•
Structural steel located in the vicinity.
•
Special design considerations and load cases.
Another consideration is the number of cycles that the system will undergo during its design life. This influences piping flexibility design because the allowable flexibility stress is based on fatigue failure. All ASME B31.3 piping systems are designed for a minimum of 7000 cycles. Systems that will undergo more than 7000 operating cycles during their design life are designed using a reduced allowable stress basis. E.
Criteria for Allowable Equipment Nozzle Loads A poorly designed piping system can cause damage to the equipment it is connected to, whether the equipment is a rotating
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type (e.g., pump or compressor) or stationary type (e.g., pressure vessel or heat exchanger). Rotating equipment is the more sensitive with respect to imposed piping loads because of the moving parts and small clearances involved in its design. Excessive piping loads imposed on rotating equipment can cause damage, poor operation, and/or maintenance problems at levels well below those that would cause pipe or equipment stress concerns. Loads that are imposed by the piping system on connected equipment are determined from the results of the piping flexibility analysis. These loads are then compared to allowable values based on industry standards for particular types of equipment to determine if they are acceptable. The allowable values can sometimes be read from a table contained in the applicable industry standard. Other times, the allowable loads or the equipment stresses that they cause must be calculated. Equipment vendors will sometimes have allowable load criteria that must be considered. Table 7.1 summarizes industry standards that apply to equipment nozzle load evaluations, and the parameters that are used to determine the allowable loads.
Equipment Item
Industry Standard
Parameters Used To Determine Acceptable Loads
Centrifugal Pumps
API-610
Nozzle size
Centrifugal Compressors
API-617, 1.85 times NEMA SM-23 allowable
Nozzle size, material
Air-Cooled Heat Exchangers
API-661
Nozzle size
Pressure Vessels, Shelland-Tube Heat Exchanger Nozzles
ASME Code Section VIII, WRC-107, WRC-297
Nozzle size, thickness, reinforcement details, vessel/exchanger diameter, and wall thickness. Stress analysis required.
Tank Nozzles
API-650
Nozzle size, tank diameter, height, shell thickness, nozzle elevation.
Steam Turbines
NEMA SM-23
Nozzle size
Equipment Nozzle Load Standards and Parameters Table 7.1
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F.
When Should A Computer Analysis Be Used Computer programs can perform numerous analyses with many different combinations of design conditions and system geometries. They can perform many functions that would be difficult for a piping analyst to do “by hand.” Computers can also perform unique functions that would be difficult or impossible to do by hand or other methods with sufficient accuracy. Even though hand calculations can be used in many situations, a computer program can often be used to finalize and optimize the final design. A computer analysis should also be used when there are several operating combinations to be considered and other methods would be inadequate or too time consuming, when greater accuracy is required due to the nature of the system, and for complicated piping systems. Computer programs are also very useful for analyzing the stresses and loads at piping components such as valves, branches, and bends. A piping system designer should remember that a computer program only gives quantitative guidelines, to which they must apply common sense and judgement. The guidelines listed in Table 7.2 may be used to help determine when a computer analysis should be performed:
Type Of Piping
Pipe Size, NPS
General piping
For rotating equipment For air-fin heat exchangers For tankage
≥4 ≥8 ≥ 12 ≥ 20 ≥3 ≥4 ≥ 12
Maximum Differential Flexibility Temp. ≥ 400°F ≥ 300°F ≥ 200°F any Any Any Any
Computer Analysis Guidelines Table 7.2 G.
Design Considerations for Piping System Stress Analysis The following paragraphs discuss several design considerations in piping system stress analysis.
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1.0
Piping Flexibility Temperature Flexibility analysis should be made for the largest temperature difference that may be imposed on the pipe by normal and abnormal operating conditions. This results in the largest pipe stress range to be considered in fatigue failure evaluation, and the largest reaction loads imposed on equipment end connections, supports, and restraints. Tables 7.3 and 7.4 provide guidelines to determine the temperatures to consider in a flexibility analysis. Note that more than one of these items might require consideration in a particular system and lead to the need for multiple computer calculations to identify the case that governs the system design.
Stable Operation
Startup and Shutdown
Gives the temperature range expected for most of the time a plant is in operation. Some margin above equipment operating temperature (i.e., use of the design temperature rather than operating temperature) allows for process flexibility. Must be examined to determine if the heating or cooling cycles pose flexibility problems. For example, if a tower is heated while some attached piping remains cold, the piping flexibility should be checked for that case.
Regeneration and Decoking Piping
Must be designed for normal operation, regeneration, or decoking, and switching from one service to the other. An example is the decoking of furnaces.
Spared Equipment
Requires multiple analyses to determine if the piping is adequate for the expected variations of temperature, for no flow in some of the piping, and for switching from one piece of equipment to another. A common example is the piping for two or more pumps with one or more spares.
Normal Temperature Conditions To Consider Table 7.3
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Loss of Cooling Medium Flow
Temperature changes due to a loss of cooling medium flow should be considered. This includes pipe that is normally at ambient temperature but can be blocked in, while subject to solar radiation.
Steamout for Air or Gas Freeing
Most on-site equipment and lines, and many off-site lines, are freed of gas or air by the use of steam. For 125 psig steam, 300°F is typically used for the metal temperature. Piping connected to equipment which will be steamed out, especially piping connected to the upper parts of towers, should be checked for the tower at 300°F and the piping at ambient plus 50°F. This situation may govern the flexibility of lines connected to towers that operate at less than 300°F or that have a smaller temperature variation from top to bottom.
No Process Flow While Heating Continues
If process flow can be stopped while heat is still being applied, the piping flexibility should be checked for the maximum metal temperature. Such situations can occur with steam tracing and steam jacketing.
Abnormal Temperature Conditions To Consider Table 7.4 Metal temperatures that govern the flexibility design of a piping system are not necessarily the ones associated with the most severe coincident pressure and temperature which govern the wall thickness of the pipe. Piping flexibility depends only on the temperature. Therefore, a condition of high temperature and low pressure may govern the piping flexibility design while the wall thickness is based on a higher pressure but a lower temperature. Pipe thermal movement is caused by a temperature change from the piping installation temperature (i.e., the ambient temperature). Piping analysis computer programs typically include a “default” ambient temperature (commonly 70°F). Then, all thermal movements and resulting thermal stresses are calculated based on the difference between the specified pipe design temperature and the default ambient temperature. A realistic ambient installation temperature (typically lower than 70°F) must be used for the specific plant site to accurately calculate the maximum thermal stress range and reaction loads.
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2.0
Extent of Analysis The extent of a piping system analysis depends on the situation. The overall purpose of the analysis is to provide enough flexibility for the system. The engineer must analyze the right combination of operating conditions to determine where, and if, additional flexibility is needed to reduce pipe stresses or loads at end points. The engineer must also decide if it is desirable and acceptable to not include portions of a large, complex system in the analysis to simplify the modeling. For example, including an NPS 4 branch run in the model of a NPS 24 main system may not be necessary. Judicious installation of anchors or other restraints in a large system could also help simplify the modeling by separating the system into sections. Use the following steps to develop the piping design:
3.0
•
Define line size, wall thickness, material, number of temperature cycles, layout, maximum differential temperature, and any alternative operating scenarios.
•
Determine conditions of end-point restraint and movements.
•
Locate intermediate points of restraint and define any limitations that they impose on piping movement.
•
Select a suitable analysis method and calculate the loads and stresses.
•
Compare the results with the allowable stress range for thermal expansion stresses, the allowable stress at design temperature for weight-plus-pressure stresses, and the applicable load criteria for connected equipment.
Modifying System Design The initial piping system layout may not be satisfactory for thermal flexibility stresses or loads on connected equipment. The following guidelines may help the situation.
•
Provide more offsets or bends, or use more expansion loops within the same space. These make the system more flexible and reduce the thermal stresses.
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4.0
•
Install expansion joints. However, this approach should be the exception rather than the rule. Expansion joints represent a "weak link" in a piping system. They may affect the life of the system since they are more susceptible to damage than pipe, and can create maintenance and operational problems. Thus, the use of expansion joints should only be considered as a last resort. One situation where expansion joints must be used is where pressure drop or other process requirements dictate the use of relatively straight pipe runs (e.g., fluidized solids transfer lines).
•
Strategically locate restraints to minimize thermal and friction loads at equipment. Restraints could also be used to direct pipe thermal expansion into a section of the system that has more inherent flexibility to absorb it.
•
Use spring supports if large vertical thermal movements are expected, or if thermal expansion causes pipe to lift off fixed supports. Avoid fixed supports that result in large thermal stresses.
•
Use Teflon bearing pads at supports for large-diameter pipe or other large weight loads if friction loads are excessive on equipment connections or structural members.
System Design Considerations Each type of piping system has particular factors that must be considered when performing a detailed analysis. For example:
•
Pump systems will often be installed with spared pumps. Thus, various scenarios of operating vs. spared pump(s) must be considered since portions of the system near the pumps will be hot while other portions are cold.
•
Piping systems are sometimes heat traced. This might be done either to reduce liquid viscosity to allow the necessary flow, or to prevent condensate accumulation. The condition with the process flow off while the heat tracing remains on must also be considered since the pipe metal temperature for this case may be higher than the normal design temperature.
•
Piping systems connected to atmospheric storage tanks must be designed considering movement that occurs at
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the tank nozzle. When the tank is filled with liquid, the shell will bulge outward and the nozzle will rotate down due to this shell bulging (see Figure 7.7). Over a period of time, the tank may also settle down into its foundation with respect to the pipe. Because of these expected tank movements, it is often necessary to use a flexible-type pipe support located near the tank nozzle to ensure that the tank nozzle is not overloaded.
SHELL
NOZZLE
BOTTOM
Tank Nozzle Figure 7.7
•
It may be necessary to consider pipe frictional effects at support points. If large enough, friction loads can restrict pipe movement and cause unexpectedly high pipe stresses or end point reaction loads. Typical situations where it may be necessary to consider friction loads are for long horizontal pipe runs, or where large concentrated weight loads are supported near equipment nozzles.
•
The most common configuration for air-cooled heat exchanger piping uses short, straight sections of pipe to connect the manifold to the exchanger nozzles. The manifold is located directly above or below the exchanger header box. The heat exchanger tube bundle is allowed to move laterally to accommodate the thermal expansion of the pipe manifold. The flexibility analysis should include the restraining effect of friction from movement of the exchanger bundle, which will resist lateral movement of the bundle.
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VIII. FABRICATION, ASSEMBLY, AND ERECTION Individual sections of pipe must be fabricated into convenient sections (i.e., spool pieces). Individual spool pieces are then assembled and erected in the field. A.
Welding and Heat Treatment Welding is one of the primary ways of joining pipe. Welded joints represent the ultimate in safety and reliability. All design codes call for welding to be carried out using a qualified procedure and welders. Included in the welding procedure are: base-metal specification, electrode type and material, joint preparation (i.e., geometry), weld position (e.g., vertical, overhead, etc.), welding process (including whether it is manual or automatic), techniques, electrical details, preheat and interpass temperatures, and postweld heat treatment (PWHT) requirements. 1.0
Butt-Welds Butt-welds are made between two components whose edges are in close proximity. Butt-welded joints in piping systems are primarily of the single-V configuration and are welded from the pipe outside surface. The joint preparation and the procedure that is used ensure that there is complete fusion between the edges of the components being joined. Joint designs shown in Figure 8.1 are typically used for ends of equal thickness. The transition between ends of unequal thickness may be accomplished by taper grinding the thicker pipe to match the thinner, or by using weld metal to provide a smooth transition as shown in Figure 8.2. Butt-welds are always used to weld pipe ends together, to weld butt-weldtype flanges or fittings to pipe ends, or to weld the edges of formed plate together when plate is used to manufacture pipe.
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(a) Standard End Preparation of Pipe
(b) Standard End Preparation of Butt-Welding Fittings and Optional End Preparation of Pipe 7/8 in. and Thinner
(c) Suggested End Preparation, Pipe and Fittings Over 7/8 in. Thickness
Butt-Welded Joint Designs Equal Thickness Figure 8.1
3/32 in. max. (a)
(b)
(c)
(d) Butt-Welded Joint Design Unequal Thickness Figure 8.2 2.0
Fillet Weld The fillet weld generally requires no special joint preparation. It is an angular weld bead that joins components normally positioned at a 90° angle to each other. The size of a fillet weld is stated as a leg length of the largest inscribed right isosceles triangle. In piping systems, fillet welds are only used for slip-on flanges, socket welds, and for welding
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attachments to piping components (e.g., reinforcing pads, supports, etc.). See Figure 8.3.
Fillet Welds Figure 8.3
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3.0
Welding Preparation Steps The following outlines the overall steps that are required for welding.
4.0
•
The individuals and equipment executing the welding procedure must be confirmed to be qualified to produce acceptable results.
•
Internal and external surfaces to be welded shall be clean and free from paint, oil, rust, scale, or other material that would be detrimental to either the weld or base metal when heat is applied.
•
The ends of the components to be welded must be set to the correct geometric shape suitable for the materials, wall thickness, and welding process involved.
Preheating Preheating is used, along with heat treatment, to minimize the detrimental effects of high temperature and severe thermal gradients that are inherent in welding. The following identifies the benefits of preheating:
5.0
•
Dries the metal and removes surface moisture which could result in weld porosity.
•
Reduces the temperature difference between the base metal and the weld to reduce the cooling rate of the weldment. This lowers the weld hardness and reduces cooling/shrinkage stresses.
•
Helps maintain the weld pool molten longer to permit maximum separation of impurities.
•
Helps drive off absorbed gases (e.g., hydrogen) which could contribute to weld porosity.
Postweld Heat Treatment (PWHT) PWHT averts or relieves the detrimental effects of high temperature and severe temperature gradients that are inherent in welding, and relieves residual stresses that are created by bending and forming. Specific heat treatment temperature and procedure requirements are specified in
143
ASME B31.3 based on the pipe material and wall thickness being joined. The following summarizes the principal reasons for PWHT:
B.
•
Stress relief is the most common reason for specifying PWHT. This is the only consideration for the PWHT requirements specified in ASME B31.3. Other reasons for PWHT (e.g., due to process considerations) must be specified by the user or contractor. Residual stresses will remain in the pipe and result from shrinkage as the weld and adjacent pipe metal cool down from elevated welding temperatures. Residual stresses will also remain after bending or forming processes. If these residual stresses are too high, they can lead to premature failure of the pipe.
•
After welding the normal grades of stainless steels (i.e., those that are not stabilized with alloy additions), the material must be heat treated to restore its maximum corrosion resistance.
•
PWHT is required to prevent caustic embrittlement of welded carbon steel pipe that handles alkaline solutions. Caustic embrittlement is a form of stress corrosion where the residual stresses due to welding are sufficient to cause failure.
•
PWHT is sometimes necessary to reduce weld hardness in certain materials. Minimizing weld hardness reduces the tendency to crack, especially in certain process environments (e.g., wet H2S).
Assembly and Erection Additional piping fabrication requirements must be considered. Several of these are discussed below. 1.0
Storage and Handling Improper handling and storage of pipe materials and welding filler metals can cause damage and result in poor construction quality and failures during operation.
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2.0
•
Pipe should not be stored directly on the ground to help prevent rainwater accumulation around the pipe, which could result in corrosion.
•
Pipe should not be stacked so high that pipes or their coatings may be damaged.
•
Fittings and valves should be stored in shipping crates or on racks to provide protection until used.
•
End protectors should be firmly attached to prevent damage to weld bevels, flange faces, threads, or socketweld ends.
•
Lined and coated pipes and fittings should be lifted with wide fabric or rubber-covered slings and padding to prevent damage.
Pipe Fitup and Tolerances Good joint fitup is essential to making a sound weld and minimizing the loads imposed on the piping system and connected equipment. Depending on the welding process used, a slight mismatch may be permissible.
•
Pipe fitup for welded joints shall be as required by the welding procedure.
•
The tolerance for axial dimensions, face-to-face, centerto-face, and location of attachments should be ±1/8 in. maximum.
•
Flattening of bends, measured as the difference between the largest and smallest outside diameter at any crosssection, should not exceed 5% of the nominal diameter of the pipe (3% at the ends).
•
Lateral translation of branches and connections from centerline of run should not exceed ±1/16 in.
•
Flange bolt holes shall straddle the centerlines. Rotation of flanges, measured as the offset between elevation of bolt holes on opposite sides of a flange centerline, should not exceed ±1/16 in.
•
The tilt of flanges measured at the periphery across any diameter should not exceed 1/32 in. from the square position. Use of a 1/64 in. tolerance is often necessary for flanges at load-sensitive equipment.
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3.0
Alignment of Pipe Attached to Load-Sensitive Equipment Special care must be taken for load-sensitive equipment, especially rotating equipment. Specifically, in attaching pipe to rotating equipment, the installation should avoid putting excessive forces and moments on the machinery nozzles which could result in misalignment.
•
Installation of piping that is connected to rotating equipment should preferably start at the machine nozzle flange. This will reduce the possibility of having a large mismatch between the pipe and machine flanges if pipe installation is begun from the opposite end of the system.
•
Bolt on succeeding pipe sections as appropriate up to the first support. Adjust this support as required to just contact the pipe at its bearing point. Proceed to any other adjacent supports which should be similarly adjusted.
•
One or more field welds are typically used to join the piping nearest to the machine with the rest of the system. The number and location of these field welds are determined such that they will permit final position adjustments to achieve acceptable flange alignment at the machine nozzle.
•
Spring supports should be locked in their cold position during pipe installation.
•
All spring supports will be adjusted in the locked position just until they contact their respective support points. If spring-support adjustment is insufficient, modifications to associated structural members or shimming will be required.
•
Final bolt tensioning of component flanges close to the machinery should be done after initial alignment of nozzle flanges.
•
Piping that requires any sections to be removed for flushing after completing field welds should have final nozzle alignment and component flange boltup completed after replacing flushed sections.
•
For piping over NPS 3 connected to machinery, flange alignment must be within more stringent limits than is
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specified for general piping systems. More stringent limits are required to minimize the loads that are imposed by flange boltup.
• Precautions should be taken to prevent ingress of debris into machine internals during construction of connecting pipework. 4.0
Flange Joint Assembly Flange joint assembly procedures directly affect the ability of the flange to be leak-tight in service. In many low-pressure, low-temperature, and/or nonflammable services, many rules of good flanged joint design and makeup can and have been violated with no adverse consequences. However, it is dangerous to break these rules in critical, high-temperature services since the results can be serious leakage problems with consequent fires. The primary factors for successfully making up a flanged joint and controlling leakage are the following:
•
Proper selection and design of the flanged joint.
•
Proper preparation, inspection, and installation of the flanged joint.
•
Identifying and controlling the causes of leakage.
Flanged joint assembly and leakage control are discussed below. 5.0
Flange Preparation, Inspection, and Installation The following discusses the primary steps that are required to achieve a properly assembled flanged joint.
•
Redo Damaged Surfaces. Warped or badly corroded flanges should be replaced or refaced. Reface flanges with tool marks or scratches across the gasket seating surface.
•
Clean Faces. All gasket and flange surfaces should be clean. Remove all burrs, rust, and dirt from flange faces with scrapers or wire brushes.
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•
Align flanges. Flanges at rest should be within the alignment tolerances previously discussed, with the flanges practically mating before the bolts are installed. Bringing the flanges into alignment should not leave any residual stresses in the piping system. Residual stresses could lead to flange leakage in service or overload problems in systems that are connected to load-sensitive equipment. This becomes more important with increasing pipe diameter, as the residual stress increases with increasing diameter for the same amount of misalignment.
•
Lubricate Threads and Nuts. Lubricate the bolt threads and the nut faces where they will contact the flange. Lubrication helps increase the amount of bolt load that goes into tightening the flange rather than into overcoming friction.
•
Place Gasket Properly. The gasket must be centered on the flange faces to achieve a reliable joint, but holding the gasket in place can be a problem. If something must be used to hold the gasket, a high-temperature grease may be used sparingly in systems that operate at less than 200°F. No grease, paste, or adhesive should be used to hold gaskets for systems operating at 200°F or more. The high temperature causes these materials to burn off, which could damage the gasket and cause leakage. Thin cellophane tape may be used on the outside edges of a gasket, but never on the seating surfaces. Tape on the seating surfaces will deform the gasket during joint assembly, burn out at operating temperature, and thus provide a leakage path. Centering rings on spiral-wound gaskets help by allowing the gasket to be supported in the proper position by a few bolts while the other bolts are inserted. Sheet gaskets should be cut so that their outside diameter corresponds to the bolt position, again to help centering.
•
Use Proper Flange Boltup Procedure. Flanges may be made up using a wrench and hammer, an impact wrench, a torque wrench, or a stud tensioner. The most important aspects of a proper boltup procedure, regardless of method, are to:
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6.0
-
Use a "criss-cross" pattern bolt-tightening sequence, as is used when bolting a wheel onto a car. This approach helps to achieve a uniform bolt load around the flange. See Figure 8.4.
-
Use at least three rounds of tightening around the flange, increasing the applied load in each round, with two rounds at the maximum load. This approach helps achieve uniform bolt load around the flange circumference.
-
For the most critical high-temperature or highpressure flanges, use a method that permits measuring the applied load (i.e., torque wrench or stud tensioner). In this way, there is greater assurance that uniform bolt load is achieved. For such applications, a maximum stud stress during boltup of 40-50,000 psi is the normal target.
Causes of Flange Leakage Most of the primary causes of flange leakage are directly related to poor inspection or installation. These are summarized below:
•
Uneven Bolt Stress. An incorrect boltup procedure or limited working space near one side of a flange can leave some bolts loose while others crush the gasket. This is especially troublesome in high-temperature services, when the heavily loaded bolts relax during operation.
•
Improper Flange Alignment. Improper flange alignment, especially nonparallel faces, causes uneven gasket compression, local crushing, and subsequent leakage.
•
Improper Gasket Centering. If a gasket is off-center, it will be unevenly compressed and more prone to leakage.
•
Dirty or Damaged Flange Faces. Dirt, scale, scratches, protrusions, or weld spatter on gasket seating surfaces provide leakage paths or can cause uneven gasket compression that results in leakage.
•
Excessive Loads in the Piping System at Flange Locations. Excessive piping system forces and moments at flanges can distort them and cause leaks. Common causes of this are inadequate flexibility, using excessive
149
force to align flanges, and improper location of supports or restraints.
•
Thermal Shock. Rapid temperature fluctuations can cause flanges to deform temporarily and leak.
•
Improper Gasket Size or Material. Using the wrong gasket size or material can cause leakage.
•
Improper Flange Facing. A rougher flange-surface finish than specified for spiral-wound gaskets can result in leakage.
Typical "Criss-Cross" Bolt-Tightening Sequence Figure 8.4
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IX.
QUALITY CONTROL A.
Inspection Prior to initial operation, each piping installation, including individual components and overall workmanship, shall be examined. The following requirements are based on ASME B31.3. Defects must be identified before a piping system can be tested or go into operation. Defect identification is especially important in welded areas. A good weld starts with a proper design and is executed using a qualified procedure and welder. However, the quality that is achieved in a particular instance may not be acceptable for a variety of reasons. The method of weld examination needed to ensure that welds of acceptable quality are achieved must be specified. Not all welds are inspected in the same manner. Determining the proper type of weld inspection is a function of technique, weld type, anticipated type of defect, location of weld, and pipe material. The following are common weld defects (illustrated in Figure 9.1):
•
Lack of fusion between adjacent weld passes.
•
Lack of fusion between weld bead and base metal.
•
Incomplete penetration due to internal misalignment.
•
Incomplete penetration of weld groove.
•
Concave root surface.
•
Undercut.
•
Excess external reinforcement.
•
Cracks.
Table 9.1 summarizes the primary weld inspection methods, where they are typically used, and the types of defects they can locate.
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Typical Weld Imperfections Figure 9.1
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Type of Inspection Visual
Situation/Weld Type All welds
Radiography
Magnetic Particle
Liquid Penetrant
Ultrasonic
Defect •
Minor structural welds
•
Cracks
•
Slag inclusions
•
Butt welds
•
Gas pockets
•
Girth welds
•
Slag inclusions
•
Miter groove welds
•
Incomplete penetration
•
Ferromagnetic materials
•
Cracks
•
For flaws up to ¼ in. beneath the surface
•
Porosity
•
Lack of fusion
•
Ferrous and nonferrous materials
•
Cracks
•
•
Seams
Intermediate weld passes
•
•
Porosity
Weld root pass
•
•
Folds
Simple and inexpensive
•
Inclusions
•
Shrinkage
•
Surface defects
•
Laminations
•
Slag inclusions in thick plates
•
Subsurface flaws
Confirms high weld quality in pressure-containing joints
Guidelines for Weld Inspection Table 9.1 The following inspection guidelines also apply:
•
ASME B31.3 specifies weld examination requirements and acceptance criteria based on fluid service category (i.e., Normal, Severe Cyclic Conditions, and Category D fluid services).
•
For P-Nos. 3, 4, and 5 materials, examination shall be performed after heat treatment. Thus, any defects caused by heat treatment will be present.
•
For a welded branch connection, the examination of and any necessary repairs to the pressure-containing weld shall be completed before any reinforcing pad or saddle is added. Thus, the reinforcement will not prevent inspection and repair.
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B.
•
At least 5% of all fabrication shall be visually examined.
•
100% of fabrication for longitudinal welds, except in components made in accordance with a listed specification, shall be visually inspected.
•
Random visual examination of the assembly of threaded, bolted, and other joints.
•
Random visual examination during the erection of piping.
•
Not less than 5% of circumferential butt- and miter-groove welds shall be examined fully by random radiography or random ultrasonic examination.
•
Not less than 5% of all brazed joints shall be examined, by inprocess examination.
•
Piping in severe cyclical service requires additional examination.
Testing The piping system must be pressure tested after it has been completely fabricated, erected, and inspected. The pressure test demonstrates the mechanical integrity of the system before it is placed into operation. The following highlights several test requirements.
•
A hydrostatic test must be used unless otherwise approved for special situations.
•
The hydrostatic test pressure at any point in a metallic piping system shall be as follows: a) Not less than 1½ times the design pressure. b) For design temperatures that are above the test temperature, the minimum test pressure shall be calculated as follows, except that the value of ST/S shall not exceed 6.5: PT =
1.5 PS T S
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Where:
c)
PT
=
P
=
Internal design pressure, psig
ST
=
Allowable stress at test temperature, psi
S
=
Minimum hydrostatic test pressure, psig
Allowable stress at design temperature, psi
If the test pressure as defined above would produce a stress in excess of the yield strength at test temperature, the test pressure may be reduced to the maximum pressure that will not exceed the yield strength at test temperature.
•
Pneumatic strength tests, when approved, shall be conducted at 110% of the design pressure.
•
Instrument take-off piping and sampling system piping, up to the first block valve, shall be strength tested with the piping or equipment to which it is connected.
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X.
OTHER CONSIDERATIONS A.
Nonmetallic Piping The following highlights several aspects of nonmetallic piping design. Refer to ASME B31.3 for additional details. Examples of nonmetallic piping include:
•
Thermoplastic Piping. Piping fabricated from a plastic which is capable of being repeatedly softened by an increase of temperature and hardened by a decrease of temperature.
•
Reinforced Thermosetting Resin Piping (RTR). Piping fabricated from a resin capable of being changed into a substantially infusible or insoluble product when cured at room temperature, or by application of heat, or by chemical means.
Some differences in the design of nonmetallic piping vs. metallic piping in normal fluid service include:
•
Allowances for variations of pressure or temperature, or both, above design conditions are not permitted. The most severe conditions of coincident pressure and temperature will be used to determine design conditions.
•
Piping systems shall be designed to prevent thermal expansion or contraction, pressure expansion, or movement of piping supports and terminals from causing: -
Failure of piping supports from overstrain or fatigue.
-
Leakage at joints.
-
Detrimental stresses or distortions in connected equipment.
•
The stress-strain behavior of most nonmetals differs considerably from that of metals. Therefore, the assumptions that stresses throughout the piping system can be predicted from strains, or that displacement strains will produce proportional stress because of fully elastic behavior of the piping materials, are generally not valid.
•
In addition to the requirements of flexibility and support for metallic piping in normal fluid service:
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B.
-
Nonmetallic piping shall be supported, guided, and anchored to prevent damage to the piping.
-
Point loads and narrow areas of contact between piping and supports shall be avoided.
-
Suitable padding shall be placed between piping and supports where piping damage may occur.
-
Valves and equipment that would transmit excessive loads to the piping shall be independently supported.
•
Thermoplastics shall not be used in flammable fluid service above ground and shall be safeguarded when used in most fluid services.
•
Nonmetallic piping is joined by bonding. Bonding can be achieved through many methods including adhesive, wrapping, heat fusion, hot gas welding, and solvent cementing.
Category M Fluid Service The following highlights several aspects of Category M fluid service. Refer to ASME B31.3 for additional details. Category M defines a fluid service in which the potential for personnel exposure is judged to be significant, and a single exposure to a very small quantity of the toxic fluid can cause irreversible harm to breathing or points of bodily contact. The following highlights several provisions, in addition to those specified for normal fluid service, that apply to Category M Fluid Service.
•
Design, layout, and operation of piping shall be conducted to minimize impact and shock loads.
•
Conditions which could lead to detrimental vibration, pulsation, or resonance effects should be avoided or minimized.
•
No allowances may be made for pressure-temperature variations. The coincident pressure-temperature conditions requiring the greatest wall thickness or the highest component rating will determine design temperature and pressure.
•
All fabrication, as well as all threaded, bolted, and other mechanical joints, shall be visually examined.
•
A sensitive leak test in addition to the required leak test must be included.
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•
C.
The following may not be used: -
Miter bends not designated as fittings, fabricated laps, and nonmetallic fabricated branch connections
-
Nonmetallic valves and specialty components
-
Threaded nonmetallic flanges
-
Expanded, threaded, and caulked joints
High Pressure Piping The following highlights several aspects of high pressure piping design. Refer to ASME B31.3 for additional details. Design Conditions and Criteria Piping is generally considered to be high pressure if it has a pressure over that allowed by Class 2500 for the specific design temperature and Material Group. However, there are no specific pressure limitations for the application of the rules for high pressure piping. In most cases, the design pressure of each component in a high pressure piping system must be at least equal to the pressure at the most severe condition of coincident internal or external pressure and temperature expected during service. The design temperature of each component in a piping system is the temperature at which, under the coincident pressure, the greatest thickness or highest component rating is required. Consideration must be given to the ambient effects on a piping system.
•
The cooling of a gas or vapor may reduce the pressure sufficiently to create a vacuum.
•
The heating of a static fluid in a piping component causes a pressure increase.
•
Moisture condensation can result in atmospheric icing when piping system design minimum temperature is less than 32°F.
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In any case, the design must allow the system to either withstand or provide some type of relief from the ambient effects. Other effects to consider include:
2.0
•
Dynamic Effects (e.g., impact, wind, earthquake, vibration, discharge reactions).
•
Weight Effects (e.g., live loads, dead loads).
•
Thermal Expansion and Contraction Effects.
•
Effects of Support, Anchor, and Terminal Movements.
•
Allowable stresses.
•
Wall thickness calculation requirements.
•
No allowance for pressure above the design pressure permitted.
•
Particular fabrication details not permitted (e.g., miters).
Examination While the examination of High Pressure Piping is very similar to that of piping in normal fluid service, it must be more extensive. For example, in normal fluid service, a sample selected at random per the inspector's judgement is sufficient to make a determination as to the acceptability of the material. In high pressure piping, 100% of the material and components must be examined. Also, only 5% of the fabrication must be examined for normal fluid service, whereas 100% of fabrication must be examined in high pressure piping.
3.0
Testing Prior to initial operation, each piping system shall be either hydrostatically or pneumatically leak tested. Each weld and each piping component (except bolting and individual gaskets to be used during final assembly) shall be tested. If the testing is done on the equipment prior to installation, an additional test of the installed piping system shall be conducted at a pressure not less than 110% of the design pressure. If the initial testing is done on the installed piping, then the additional test is not necessary.
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XI.
SUMMARY
A process plant piping system includes much more than just straight sections of pipe. It also includes fittings, flange assemblies, valves, pipe supports, and restraints. ASME B31.3 specifies the design, materials, fabrication, erection, inspection, and testing requirements for process plant piping systems. This course provided an overview of process plant piping system requirements, including items that are not explicitly included in B31.3 (e.g., valve selection and design, flexibility analysis guidelines, equipment nozzle load requirements, etc.). Participants can use this information on their jobs, and are prepared to take more extensive courses if appropriate.
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