Hydraulic Test Pressure

........is to prove that it is very unlikely the vessel will rupture or leak at the working pressure while containing the working fluid over the insurance liability interval.

It isn't always 1.5 times! We have been required by Lloyd's to conduct tests at 4 times. This is very unusual but it does happen.

As PW said it's because.....! The reason that it is undesirable to exceed 1.5 times is due to hydrostatic stretch. It would be a bad thing to continue testing (say) a diving cylinder on a yearly basis to excessive and largely unnecessary 'stretch' although the diver may end up with more air in the cylinder to breath if the stretch became permanent! In this case, the cylinder would fail if the measured stretch exceeded specified limits!

It should also be remembered that a time parameter should be included in the hydrostatic test. This also gets complicated by having e.g. a time at 1.5 times pressure followed by a longer time at MWP.

Each case needs to cover different requirements!

<...hydrostatic stretch...>

Of course, the vessel is designed to be operating at no more than 1/1.5 times the yield stress of each component, isn't it?

Isn't it?

Emphatic Yes! If it is a vessel (as in a reservoir). Other 'component' type bits maybe not!

Phew!