Allowable Stress for Pressure Relieving Reaction Load vs Occasional load

Piping systems be protected against over-pressuring by Pressure-relieving devices like pressure safety valve, pressure relief valve, … by realizing extra pressure (popping up) according to the system operating philosophy.

Regarding variation of service fluid pressure and velocity between the upstream and downstream of PSV, during popping up PSVs exert considerable reaction force over the system. During stress analysis of PSV involved systems, we have to consider this reaction force.

Also according to Appendix II B31.1, Pressure loads acting on the safety valve installation are important from two main considerations. The first consideration is that the pressure acting on the walls of the safety valve installation can cause membrane stresses which could result in rupture of the pressure-retaining parts. The second consideration is that the pressure effects associated with discharge can cause high loads acting on the system which creates bending moments throughout the piping system.

This article’s subject is an investigation of system allowable stress associated with PSV/PRV reaction force and PSV/PRV reaction force calculation is out of this writing goal. To learn about the PSV reaction force calculation and stress analysis philosophy kindly click here.

ASME B31.3 and Caesar II approach for occasional load and pressure-relieving load

Some companies consider PSV/PRV reaction load like other occasional loads e.g. seismic loads/wind loads because it acts occasionally and according to clause 302.3.6 part (1) of ASME B31.3 (Refer to Fig. 1), use of 1.33 times the basic allowable stress provided in Table A-1/A-1M.

Limits of Calculated Stresses due to Occasional Loads as per ASME B31.3
Fig. 1: Limits of Calculated Stresses due to Occasional Loads as per ASME B31.3

In the Caesar II equivalent static method, the PRV reaction force is entered in the input spreadsheet and proper load cases are prepared after adding those forces to simulate the behavior. But in the load cases, the OCC load Factor column is left blank and Caesar II by default use 1.33 as the occasional load factor similar to other occasional stress categories. Therefore, while calculating the allowable stress for PRV pop-up case, Caesar II software multiplies the allowable stress with 1.33 for the calculation of OCC allowable stress. Refer to Fig. 2 that shows a Caesar II snapshot for the same.

Occasional allowable stress when OCC load factor is left blank
Fig. 2: Occasional allowable stress when OCC load factor is left blank

But note that the factor 1.33 is for those occasional loads which occur into the piping design (operating) limitation but pressure reliving device duty is keeping away system from over pressurizing and does not operate since pressure is below design (set pressure) condition.

paragraph 302.2.4 part 2 Involves terms either or both of temperature and pressure variate from design condition. According to part (2) of 302.2.4 (B31.3), when the variation is self-limiting (e.g., due to a pressure-relieving event), and lasts no more than 50 h at any one time and not more than 500 h/y, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20%. Hence when Caesar II users leave the OCC load Factor blank for PSV/PRV load case, the relevant allowable load be calculated about 11% more than the limiting of 302.2.4. Refer to Fig. 3 below.

Allowances for Pressure and Temperature Variation
Fig. 3: Allowances for Pressure and Temperature Variation per ASME B31.3

So, as per my understanding, it can be concluded that occasional loads like wind and earthquake, maybe as much as 1.33 times the basic allowable stress but pressure relieving loads like PSV/PRV discharge reaction loads should be limited to an OCC factor of 1.2. It is, therefore, mandatory for Caesar II users to fill the OCC load Factor for PSV reaction forces as 1.2. Refer to Fig. 4 below.

Occasional allowable stress when OCC load factor is filled
Fig. 4: Occasional allowable stress when OCC load factor is filled

I wish to know the inputs from other pipe stress engineers on the above subject on whether my understanding is right or I am missing somewhere. Kindly provide your inputs in the comments section.

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Amir Jeddipour

A piping stress engineer with 7+ years’ experience currently working at EIED.

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