Hydrostatic Test on Fixtures

[HotStab]

Please, help me to understand the extension that a hydrostatic test shall be applied in equipment that has mechanical connections indirectly collaborating with pressure resistance.
As far as I know, the flange bolting submitted to hydrostatic pressure tests does not need be the same to be shipped with or assembled in the equipment. But, in a more generic definition, regarding to fixtures (bolting, threads and mechanical connections) that, directly or indirectly, collaborate to hold pressure load:
Is it required that all these fixtures be submitted to hydrostatic test with the equipment?
Is it acceptable that higher strength fixtures are used exclusively for hydrostatic tests?
Is it acceptable that the fixtures used in hydrostatic test are replaced after test if damaged?
Is it acceptable that additional external devices are used to absorb part of the stress on these fixtures during the hydro test?
Thanks in advance.

 

[gr2vessels]

HCKS,
I'll try to follow your list:
1. The hydrotest of a pressure equipment is specified in the applicable code and requires the equipment to be outfitted with every component subjected to pressure containment or to be provided during the test with a suitable closure element (like blind covers on nozzles) for each and every opening, up to the limit specified in the code (like the flange face of the process connectios). Yes, that includes all the suitable bolting nominated in the design calculations.
2. Yes, you can use for the hydrotest purpose removable, temporary fixtures with higher strength than used for the pressure containment equipment. Eg., you can use custom designed blind for temporary cover of a pressure vessel opening, during the hydrotest, provided it is suitable and compatible with the opening facing.
3. The damages fixtures during the hydrotest (or for any other reason) must be discarded and not used, until/unless re-assessed for suitability and the damages suitably repaired. You must replace the damaged flange bolting used during hudrotest, but you don't have to replace the undamaged bolts, because you must have been using the "operating" bolts anyway. Also, you have to replace the gasket in any joint you opened after hydrotest, if the gasket is part of the equipment, not a test accessory.
4. Generally speaking, it is acceptable to use external restraining devices, designed and properly engineered to absorb the potential energy of the hydrotest. Eg., for the pressure testing of expansion joints, is common to use a fixture to restrain the movement, since you are testing the pressure containment only. However, you cannot use restraining devices to prevent the elastic expansion of the pressurised elements, that will defeat the whole purpose of the hydrotest.