The main objectives of stress analysis is to ensure
A. Structural Integrity (Design adequacy for the pressure of the carrying fluid,Failure against various loading in the life cycle and Limiting stresses below code allowable.)
B. Operational Integrity (Limiting nozzle loads of the connected equipment within allowable values, Avoiding leakage at joints, Limiting sagging & displacement within allowable values.)
C. Optimal Design (Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.)
To meet these objectives several load cases are required during stress analysis. This article will guide all the beginners with the methodology to build several load cases which will be required for stress analysis.
In this article we will use following notations for building load cases:
WW=water filled weight of piping system,
W=weight of pipe including content and insulation,
P1=Internal Design pressure,
T3= Minimum temperature,
WIN1, WIN2, WIN3 AND WIN4: wind loads acting in some specific direction,
U1, U2, U3 AND U4: uniform (seismic) loads acting in some specific direction.
While analysis at a minimum the stress check is required for the below mentioned cases:
a. Hydrotesting case: Pipelines are normally hydrotested before actual operation to ensure absence of leakage. Water is used as the testing medium. So during this situation pipe will be subjected to water weight and hydrotest pressure.
Accordingly our first load case in Caesar II will be as mentioned below
1. WW+HP HYD
b. Operating case: When operation starts working fluid will flow through the piping at a temperature and pressure. So accordingly our operating load cases will be as mentioned below:
2. W+T1+P1 OPE for operating temperature case
3. W+T2+P1 OPE for maximum system temperature case
4. W+T3+P1 OPE for minimum system temperature case
c. Sustained Case: Sustained loads will exist throughout the plant operation. Weight and pressure are known as sustained loads. So our sustained load case will be as follows:
5. W+P1 SUS
d. Occasional Cases: Piping may be subjected to occassional wind and seismic forces. So to check stresses in those situations we have to build the
following load cases:
6. W+T1+P1+WIN1 OPE Considering wind from +X direction
7. W+T1+P1+WIN2 OPE Considering wind from -X direction
8. W+T1+P1+WIN3 OPE Considering wind from +Z direction
9. W+T1+P1+WIN4 OPE Considering wind from -Z direction
10. W+T1+P1+U1 OPE Considering seismic from +X direction
11. W+T1+P1-U1 OPE Considering seismic from -X direction
12 W+T1+P1+U2 OPE Considering seismic from +Z direction
13 W+T1+P1-U2 OPE Considering seismic from -Z direction
While stress analysis the above load cases form load case 6 to load case 13 is generated only to check loads at node points.
To find occasional stresses we need to add pure occassional cases with sustained load and then compare with code allowable values. Following sets of load cases are built for that purpose.
14. L6-L2 OCC Pure wind from +X direction
15. L7-L2 OCC Pure wind from -X direction
16. L8-L2 OCC Pure wind from +Z direction
17. L9-L2 OCC Pure wind from -Z direction
18. L10-L2 OCC Pure seismic from +X direction
19. L11-L2 OCC Pure seismic from -X direction
20. L12-L2 OCC Pure seismic from +Z direction
21. L13-L2 OCC Pure seismic from -Z direction
22. L14+L5 OCC Pure wind+Sustained
23. L15+L5 OCC Pure wind+Sustained
24. L16+L5 OCC Pure wind+Sustained
25. L17+L5 OCC Pure wind+Sustained
26. L18+L5 OCC Pure seismic+Sustained
27. L19+L5 OCC Pure seismic+Sustained
28. L20+L5 OCC Pure seismic+Sustained
29. L21+L5 OCC Pure seismic+Sustained
Load cases from 22 to 29 will be used for checking occasional stresses with respect to code B 31.3 allowable (=1.33 times Sh value from code). Use scalar combination for load cases 22 to 29 above and algebraic combination for others as shown in figure attached below:
e. Expansion Case: Following load cases are required for checking expansion stress range as per code
30. L2-L5 EXP
31. L3-L5 EXP
32. L4-L5 EXP
33. L3-L4 EXP for complete stress range
The above load cases (from 30 to 33) are used to check expansion stress
The above mentioned load cases are minimum required load cases to analysis any stress system. Out of the above load cases the load cases mentioned in point number 1, 5, and 22-33 are used for stress check. And load cases mentioned in point number 1 to 13 are used for checking restraint forces, displacements and nozzle load checking.
Few additional load cases may be required for PSV connected systems, Rotary equipment connected systems.
Seismic and Wind analysis may not be required every time. So those load cases can be deleted if the piping system does not fall under the purview of seismic and wind analysis by project specification. However to perform wind and seismic analysis proper related data must have to be entered in Caesar II spreadsheet (Will be discussed in my future posts).
If the stress system involves use of imposed displacements (D) and forces (F) then those have to be added with the above load cases in the form of D1, D2 or F1, F2 as applicable.
It is a better practice to keep
1. Hydro and sustained stresses below 60% of code allowable
2. Expansion and occasional stresses below 80% of code allowable
3. Sustained sagging below 10 mm for process lines and below 3 mm for steam, two phase and flare lines
4. Design/Maximum displacement below 75 mm for unit piping and below 200 mm in rack piping.