My understanding of MAWP in ASME BPVC Sec VIII Div 1 code is that it is the maximum pressure a pressure vessel should be subjected to. If that is the case, then why should the hydrotest pressure be kept at 1.3 times MAWP . Would'nt it damage the vessel, since it is exceeding MAWP. Can anybody please clarify on the same.
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A Mechanical engineer, or some one who works in materials/reliability would be better suited to answer this, but I believe the reasoning to be the a combination of safety factor (ensuring fittings, gaskets, etc are all going to withstand the pressure load) in addition to the difference between peak acute strength vs sustained or cyclic loads. MAWP stands for Maximum Allowable Working Pressure, and is talking about the maximum pressure the vessel can see on a sustained, or cyclic & repeating basis. Note that metals are all subject to fatigue, especially under cyclic loads. For many materials, this means listing the MAWP as some number that is significantly lower than say, absolute yield strength, knowing that over time the effective yield strength will be reduced.
The EXACT ratio (1.3, 1.5, 2.0 etc) and the application will depend on what materials it's holding, the type of environment it is in, and especially the actual materials of construction and that material's physical & wear characteristics. This ratio takes into account that new piping/vessels/equipment are all considerably stronger than the MAWP, which is defined as the limit the vessel will see during it's service/working life, and will benefit in terms of ensuring sound construction by going above the MAWP (I.E. if a vessels holds up against 275 PSI, you know it will suffice for 150 PSI)
There may be additional reasoning behind this, but from a practical perspective this is my interpretation in industry.
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1$\begingroup$ Good answer! To add a little more information, ASME B&PVC Section II lists the maximum allowable stresses for all acceptable materials at various temperatures. Those allowable stresses are calculated based on the actual strength of the materials, and a safety factor. Since 1999 the safety factor has been 3.5 - i.e. the actual strength of the material is 3.5 times higher than the maximum allowable stress. So when a pressure tests is performed at 1.5 times the MAWP, there is still plenty of margin left in the design and it won't damage anything. $\endgroup$– MarkCommented Dec 29, 2020 at 23:46