Hydro Test

Because to do otherwise would mean that the vessel might undergo catastrophic and irreversible self-disassembly with attendant risks to life and limb:

it is just effect in the case of high temperature service in which the maximum allowable stress will be reduced then the value of hydrostatic test will be reduced.

in high temperature rating the construction particulars of the matel has a different behaivour from its in normal temperature.

to avoid the effect of creep at high temperature service you have to exchange the metal with another one which has higher resistance against the high temerature specially at 400C or you can use the same metal but at lower maximum allowable stress as per ASME sec II part D.