Hydrotest of Vessels

Most (but not all) vessel codes require vessels to be hydrostatically tested at 1.5 x MAWP (maximum allowable working pressure). Various other piping codes may require a different test pressure. If higher, this may require that pressure vassels and relief valve be isolated from the piping when performing the pipe test. For example, in most of the U.S., ammonia refrigeration high-side vessels are designed for 250 psig, but high-side piping must be nitrogen-tested at 300 psig. (At least it used to be that way, but this may since have been updated.)

This whole business depends on local code requirements.