Piping E - Book
Piping
Contents
I. Preface
II. Forward
III. Dedication
IV. Symbols
V. About the Author
VI.Acknowledgements
1. Introduction
2. Piping Basics
3. Piping Design
4. Flow in Piping
5.Stress analysis of Piping
6. Supports for Piping
7. Construction of Piping
8. Testing of Piping
9. Operation of Piping
10. Maintenance of Piping
A. Bibliography
B. Formulas
C. Conversion factors
D. Appendix
E. Index
12. Loads
 
The piping is subjected to the following loads.
12.1. Self-weight of piping
12.2. Weight of contents
12.3. Weight of insulation
12.4. Weight of refractory
12.5. Weight of inner casing
12.6. Weight of outer casing
12.7. Weight of fittings
12.8. Weight of valves
12.9. Weight of gages
12.10. Weight of instruments
12.11. Weight of attachments
12.12. Weight of equipment
12.13. Weight of devices
12.14. Weight of steam tracers
12.15. Weight of steam traps
12.16. Weight of pumps
12.17. Weight of lugs
12.18. Weight of humans (during maintenance)
12.19. Weight of tools (during maintenance)
12.20. Wind load
12.21. Seismic load (earth-quack load)
12.22. Loads due to fire
12.23. Loads due to floods
12.24. Loads due to rain
12.25. Loads due to snow
12.26. Loads due to ice
12.27. Loads due to waves
12.28. Loads due to tsunami
12.29. Loads due to pressure surges
12.30. Loads due to water hammer
12.31. Loads due to steam hammer
12.32. Loads due to opening of valves
12.33. Loads due to closure of valves
12.34. Impact loads from near-by machines
12.35. Vibration loads
12.36. Shock loads
12.37. Transient loads
12.38. Dynamic loads
12.39. Thermal expansion
12.40. Thermal contraction loads
12.41. Loads due to differential settlement of foundations
12.42. Loads due to equipments displacements
12.43. Loads on buried piping from soil, river, sea, ponds and sediments
12.44. Loads on buried piping from vehicles, trains, air-crafts and space-crafts
12.45. Loads due Bourdons effect
12.46. Loads due to flow of fluids
12.47. Internal pressure from fluids
12.48. External pressure from fluids
12.49. Loads due to change in fluid flow direction
12.50. Loads due to manual & artificial manipulation of valves &apm; devices
13. Load Combinations
   13.1. Sustained Loads
 
(13.1.1) Self-weight of piping, (13.1.2) Weight of contents, (13.1.3) Weight of insulation, (13.1.4) Weight of refractory, (13.1.5) Weight of inner casing, (13.1.6) Weight of outer casing, (13.1.7) Weight of fittings, (13.1.8) Weight of valves, (13.1.9) Weight of gages, (13.1.10) Weight of instruments, (13.1.11) Weight of attachments, (13.1.12) Weight of equipment, (13.1.13) Weight of devices, (13.1.14) Weight of steam tracers, (13.1.15) Weight of steam traps, (13.1.16) Weight of pumps, (13.1.17) Weight of lugs, (13.1.18) Loads due to soil, river, sea and ponds for buried piping, (13.1.19) Loads due to Bourdons effect, (13.1.20) Loads due to flow to fluids laminar and turbulent fluid flows, (13.1.21) Loads due to change in fluid flow direction, (13.1.22) Internal pressure and (13.1.23) External pressure.
  13.2. Operating Loads
 
(13.2.1) All the loads under (13.1), above plus (13.2.2) Thermal expansion loads, (13.2.3) Thermal contraction loads, (13.2.4) Loads due to differential settlement of foundations and (13.2.5) Loads due to equipment movements.
  13.3. Hydraulic Test Loads
 
(13.3.1) All the loads under (13.1), above plus (13.3.2) hydraulic test water weight and (13.3.3) Hydraulic test pressure.
  13.4. Expansion Loads
 
(13.4.1) All the loads under (13.2), above minus loads under (13.1), above.
  13.5. Occasional Loads
 
(13.5.1) All the loads under (13.1), above plus (13.5.2) Weight of humans during shut-down, (13.5.3) Weight of tools during shut-down, (13.5.4) Wind load, (13.5.5) Seismic load (Earth-quake load), (13.5.6) Loads due to fire, (13.5.7) Loads due to flood, (13.5.8) Loads due to rain, (13.5.9) Loads due to snow, (13.5.10) Loads due to ice, (13.5.11) Loads due to waves, (13.5.12) Loads due to Tsunami, (13.5.13) Loads due to pressure surges, (13.5.14) Loads due to water hammer, (13.5.15) Loads due to steam hammer, (13.5.16) Loads due to closure of valves, (13.5.17) Impact loads, (13.5.18) Vibration loads (13.5.19) Shock loads, (13.5.20) Transient loads, (13.5.21) Dynamic loads, (13.5.22) Loads from vehicles, trains, air-crafts and space-crafts for buried piping, (13.5.23) Loads due to opening of valves and (13.5.24) Loads due to manual & artificial manipulations of valves and devices.
14. Allowable Stress
 
The allowable stress for piping at working temperature (As per the Power Piping code ASME B31.1) is given below. For safety, the following shall be satisfied: Induced Stress < Allowable Stress. The following six conditions shall be satisfied for safety of the piping.
1. Ssust < Sh
2. Shyro < 0.9 x Scy
3. Socct < 1.2 Sh
4. Sexpn < f (1.25 Sh + 1.25 Sc)
5. Shear stress < 0.8 x Sh
6. Bearing stress < 1.6 x Sh
Where,
Ssust = Induced stress due to sustained loads, MPa
Shyro = Induced stress due to hydraulic test at shop or field, MPa
Socct = Induced stress due to occasional loads like Wind, Seismic, Wave, etc., MPa
Sexpn = Induced stress due to the expansion loads, MPa
Sh = Allowable stress at working temperature, MPa
Sc = Allowable stress in cold condition (at 21°C), MPa
Scy = Yield stress for the piping material at ambient temperature (21°C), MPa
f = A factor to take care of fatigue load cycles
   
  = 1.0 for 7,000 fatigue load cycles (maximum)
  = 0.9 for 7,000 to 14,000 fatigue load cycles
  = 0.8 for 14,000 to 22,000 fatigue load cycles
  = 0.9 for 22,000 to 45,000 fatigue load cycles
  = 0.9 for 45,000 to 100,000 fatigue load cycles
  = 0.5 for the number of fatigue load cycles = 100,00 or more
Sh is the least of the following:
(a) Scy/1.5, (b) Shy/1.5, (c) Scu/3.5, (d) Shu x 1.1/3.5, (e) 0.67 x Sravg, (f) Srmin/1.25 & (g) Scrp
Where,
Scy = Yield stress for the piping material at ambient temperature (21°C), MPa
Shy = Yield stress for the piping material at working temperature, MPa
Scu = Ultimate tensile stress for the piping material at ambient temperature (21°C), MPa
Shu = Ultimate tensile stress for the piping material at working temperature, MPa
SRavg = Average stress at working temperature for creep rupture in 100,000 hours, MPa
SRmin = Minimum stress at working temperature foe creep rupture in 100,000 hours, MPa
Scrp = Stress at working temperature to produce a creep strain of 0.01% in 1,000 hours, MPa
Sc is corresponding to Sh, where the working temperature is taken as 21°C. The values of Sh and Sc are tabulated in ASME B 31.1, for various materials and various working temperatures. There are no limits on stresses induced during the normal operation of the piping. There are no explicit limits on the deformation of piping. The induced stress can be computed using any reliable method. The hydraulic test shall be conducted at shop or field, at a temperature not less than 21°C. Piping at cryogenic temperatures is not covered by ASME B31.1. Vibration requirements are non-mandatory.
15. Deformation
 
The piping deforms due to loads. The deformation at any point has two values. One deformation is the linear deformation. The other deformation is the angular deformation. The linear deformation can be resolved into three linear deformations in any three mutually perpendicular directions. Similarly, the angular deformation can be resolved into three angular deformations in any three mutually perpendicular directions. the deformation of the pipe and the piping components can be computed by manual or computer-based methods. The induced deformation is to be limited to the allowable. The allowable deformation as per various codes, standards and regulations are different. The suggested limit on piping deformation, in the vertically downward direction 9linear deformation) is 2.54 mm (0.1 inch), as per the Power Piping code ASME: B31.1. The allowable deformation, in the vertically downward direction for tubes, used in some of the boiler industry is not more than 6.35 mm (0.25 inch). The allowable deformation in the vertically downward direction for structures (as per IS: 800) is Span / 325. There are no limits on the angular deformations for pipes. Some of the piping supports makers limit the angular deformation of the piping hanger rods to half a Degree to the vertical direction (half cone angle). The Exercise given below gives the application of these concepts to a commonly used piping.
Exercise: 15.1 Linear Deformation of Piping
Pipe size: NPS4 Schedule 80
Pipe outside diameter: 114.3 mm
Pipe nominal thickness: 12.7 mm
Pipe material: ASTM A 106 Grade B
Pipe working pressure: 120 kg / sq cm (g)
Pipe working metal temperature: 350 Degree C
Pipe allowable deformation (ASME: B31.1): 2.54 mm (0.1 inch) suggested
Pipe allowable deformation (Boiler): 6.35 mm (0.25 inch) suggested
Pipe support span: 3,250 mm (assumed)
Pipe allowable deformation (IS: 800): Span / 325 = 3,250 / 325 = 10.0 mm suggested
Conclusion:
The allowable deformations as per different codes are different.
16. Vibration
 
Piping is subjected to vibration due to loads varying with time. In the stress analysis of piping the following six assumptions are generally made:
   1. Linear behavior
   2. Elastic material
   3. Homogeneous solids
   4. Isotropic material property
   5. Steady state loading
   6. Static piping
Even-though none of the above indicated six assumptions correct, these assumptions lead to simplified design and stress analysis. Out-of the six simplifying assumptions, the assumption that the piping is Static can be improved with Dynamic stress analysis. In the Dynamic stress analysis, the loading varies with time. In comparison to the Dynamic stress analysis, the piping can be analyzed considering Transient behavior. Even-though the words Dynamic and Transient are used inter-changeably in the normal usage of the English language, these words are worlds-apart, in the Technical terminology. Dynamic behavior indicates the consideration of Mechanical Vibration. The Transient behavior indicates Thermal Transients. The popularly used piping stress analysis computer programs like CAESAR II and CAEPIPE can perform Dynamic stress analysis. The Transient behavior can be analyzed using computer programs applicable for checking the fluid flow, fluid distribution and pumping power. The following loads are Dynamic in nature:
   a) Wind load
   b) Seismic load (Earth-quack load)
   c) Pressure surge
   d) Water hammer
   e) Steam hammer
   f) Sudden opening and / or closing of valves
The methods used for vibration analysis are numerous. The exact method to be used for a particular application is to be decided by the Stress Analyst. The code on Power Piping - ASME B31.1 doesnt mandate the use of Dynamic stress analysis.