Piping Stress Analysis is simply creating the load cases required for analysis and study the impact of the same on the behavior of the critical piping systems. A load case can be defined as a set of loads (Weight, Pressure, Temperature, External Forces, Displacements, etc) and boundary conditions for defining a particular loading condition. So Stress Analysis can not be thought without proper load case creation. Sometimes these load cases are mentioned in the piping stress analysis design basis. In this article, we will learn the basic load cases that are required for stress analysis activity.
Objectives of Pipe Stress Analysis:
The main objectives of stress analysis are to ensure:
- 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.)
- Operational Integrity (Limiting nozzle loads of the connected equipment within allowable values, Avoiding leakage at joints, Limiting sagging & displacement within allowable values.)
- Optimal Design (Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.)
Notations Used for Load Cases in Caesar II
To meet these objectives several load cases are required during stress analysis. In this article we will use the following notations for building load cases:
- WW=water filled weight of the piping/pipeline system,
- HP=Hydrotest Pressure,
- W=weight of pipe including content and insulation,
- P1=Internal Design pressure,
- T1=Maximum Design temperature,
- T2=Maximum Operating temperature,
- T3= Minimum Design temperature,
- WIN1, WIN2, WIN3, WIN4: wind loads acting in some specific direction,
- U1, U2, U3, U4: uniform (seismic) loads acting in some specific direction.
Basic Load Cases for Caesar II Pipe Stress Analysis:
For Stress analysis in Caesar II, Various Load case combinations are used which serves several purposes. While analysis at a minimum the stress check is required for the below- mentioned cases:
a. Hydrotesting case:
Piping/ Pipeline systems are normally hydro tested (sometimes pneumatic tested) before the actual operation to ensure the absence of leakage. Water is used as the testing medium. So during this situation pipe will be subjected to water weight and hydro-test pressure.
Accordingly, our first load case will be as mentioned below
b. Operating and ALT Sustained load cases:
When operation starts working fluid will flow through the piping at a temperature and pressure. Alt Sustained cases are used as Hot Sustained cases which means sustained stress that the system carries during operation. So accordingly our operating load cases will be as mentioned below:
|2||W+T1+P1||OPE||For operating temperature case at maximum design temperature|
|3||W+P1||SUS||Alt Sustained case based on operating case 1 (T1)|
|4||W+T2+P1||OPE||For a maximum system operating temperature case|
|5||W+P1||SUS||Alt Sustained case based on operating case 1 (T2)|
|6||W+T3+P1||OPE||For minimum system temperature case|
|7||W+P1||SUS||Alt Sustained case based on operating case 1 (T3)|
c. Sustained Load 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:
d. Occasional Load Cases:
Piping may be subjected to occasional wind and seismic forces. So to check stresses in those situations we have to build the following load cases:
|9||W+T2+P1+WIN1||OPE||Considering wind from +X direction|
|10||W+T2+P1+WIN2||OPE||Considering wind from -X direction|
|11||W+T2+P1+WIN3||OPE||Considering wind from +Z direction|
|12||W+T2+P1+WIN4||OPE||Considering wind from -Z direction|
|13||W+T2+P1+U1||OPE||Considering seismic from +X direction|
|14||W+T2+P1-U1||OPE||Considering seismic from -X direction|
|15||W+T2+P1+U2||OPE||Considering seismic from +Z direction|
|16||W+T2+P1-U2||OPE||Considering seismic from -Z direction|
While stress analysis the above load cases form load case 9 to load case 16 is generated only to check loads at node points. Figure 1 shows typical load cases that should be generated during stress analysis
To find occasional stresses we need to add pure occasional cases with sustained load and then compare with code allowable values. The following sets of load cases are built for that purpose.
|17||L9-L4||OCC||Pure wind from +X direction|
|18||L10-L4||OCC||Pure wind from -X direction|
|19||L11-L4||OCC||Pure wind from +Z direction|
|20||L12-L4||OCC||Pure wind from -Z direction|
|21||L13-L4||OCC||Pure seismic from +X direction|
|22||L14-L4||OCC||Pure seismic from -X direction|
|23||L15-L4||OCC||Pure seismic from +Z direction|
|24||L16-L4||OCC||Pure seismic from -Z direction|
Load cases from 25 to 32 will be used for checking occasional stresses with respect to code ASME B 31.3 allowable (=1.33 times Sh value from code). Use scalar combination for load cases 25 to 32 above and algebraic combination for others as shown in Fig. 2 attached below:
e. Expansion Cases:
Following load-cases are required for checking expansion stress range as per code
The above load cases (from 33 to 36) are used to check expansion stress range
The above-mentioned load cases are the minimum required load cases to analyze any stress system. Out of the above load cases, the load cases mentioned in point numbers 1, 3, 5, 7, 8 and 25-36 are used for stress check. And load cases mentioned in point numbers 1, 2, 4 and 6 to 16 are used for checking restraint forces, displacements, and nozzle load checking.
Few additional load cases may be required for PSV connected systems, systems having surge or slug forces and 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 the Caesar II spreadsheet (Will be discussed in my future posts).
If the stress system involves the 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.
Better Engineering Practices for Stress Analysis
It is a better practice to keep:
- Hydro and sustained stresses below 60% of code allowable
- Expansion and occasional stresses below 80% of code allowable
- Sustained and Hydrotest sagging below 10 mm for process lines and below 3 mm for steam, two-phase, flare lines and free-draining lines.
- Design/Maximum displacement below 75 mm for unit piping and below 200 mm in rack piping.
Some more resources for you!