1.5 times the operating pressure.

Regards,

1.5 P St/S but Min 1.5P.

It really depends on where the piping is (relative to population) and how hazardous the fluids are.  In dry natural gas pipelines outside of population centers, DOT allows 1.1 in the US.  You have to get inside a plant fence, have really nasty fluids, or be in a pretty dense population center before you are required to go to 1.5.

Since you're talking about B31.3, you will certainly end up at the 1.5 number, but B31.8 allows lower multiples on pipelines and compressor/pumping stations.

Usually 1.5 x the maximum allowable working pressure unless limited to a lower pressure by a relief valve; then it can be 1.5 x the relief valve set pressure.

The more you learn, the less you are certain of.

Note that minimum hydrotest pressure is 1.5 times the DESIGN pressure (not operating pressure or max. operating pressure).  You may be required to compensate for temperature difference per Equation 24 of ASME B31.3.  We write into our hydrotest spec that minimum hydrotest pressure will be 7.0 bar g, since anything less on a conventional piping system doesn't provide a real test of the joints or system.

Minor point, but MAWP usually is the Design Pressure; maximum ALLOWABLE operating pressure is different from the maximum operating pressure.  Both MAWP and DESIGN pressure have been used for vessel nameplates.  I would agree, however,  that using Design pressure is less confusing.

The more you learn, the less you are certain of.

Just to confuse things a bit. I have been involved in a project where the Design Pressure was slightly higher than the MAWP (by 10%). The origins of that was that the relief valves were set at 100% MAWP and hence the Design Pressure was set at the 100% value. The hydrotest pressure was then defined as 125% of Design Pressure (this was for an offshore pipeline rather than plant piping).

Sorry, slippery keyboard. 100% MAWP should have read 110% MAWP.

Yes, MAWP and Design really can get confusing. Another case that often occurs.  In the original specification of a plant vessel the DESIGN pressure is what the engineer specifies in the spec.  The fabricator may build the vessel where it is good for more than the specified design (extal wall thickness available because the fabricator just happen to have it in his yard - for example).  In this case the fabricator could do one of two things:  1) stamp it as MAWP or DESIGN pressure equal to what you specified (100 psig for example) OR 2) call you and stamp it for what its actually good for (maybe 135 psig, for example).

AS for your case, I'm not certain I would agree with that, but I don't know enough about the installation to say you can't do it.  The pipe code does allow for greater than design pressure to occur under temporary pressure excursions.

The more you learn, the less you are certain of.

Yes. I should have clarified that the 10% uplift was to allow for surge transients.