Valve

With a non-compressible fluid, as with a hydrotest on anything else. Water is a good fluid to choose.

Hydro: From Greek Hydra - means water A prefix added to the start of a word. Indicates that "water" modifies the word. Created to expand meanings. Can be used with many words to form new words.

Test : To examine or Try - (one of the meanings)
To put to the prrof; to prove the truth, genuineness

Hence in hydrotest - you prove the genuineness of the valve using water.

But you mean Hydrostatic Pressure test of the valve

for that you please find the

a) Type and condition of service

b) The applicable codes.

Since you are from petrochems, I feel that API will be applicable to you. Go through API standards for valves, (eg API6D, also a few others are there or you may have to go for ASME if that is your application)

The details of Hydrostatc tests as well as other tests are spelt out in these standards.