Typically, you could test to 1.5 times the maximum flange rating at ambient temperatures. For carbon steel A105 flanges, this is 1.5 x 740 = 1125 psig.
The catch here is that on large diameter piping, you could go beyond yield if you use this rule indiscrimately. You should check piping stress in this case. The code advises us for this check.
Your question is posed the wrong way......there is no one hydro test pressure for piping constructed with a certain "class" of flanges. A lot will depend on the diameter of the piping system
Hydrotest pressures for piping systems depend on the "maximum sustained operating pressure" that can be expected for that particular system. This maximum pressure is called the "design pressure" in ASME B31.1 and B31.3.
This pressure might be determined for that particular system by a PRV setting, a control system, the dead-head pressure of a pump etc.
You could have schedule 40 OR schedule 80 piping in a system using Class 300 flanges. You could also have sensitive components (expansion bellows or pump seals)that could not sustain a 1.5 X hydrotest pressure.
When process plants are designed, a "Line List" os commonly developed for each segment of a piping system. This usefull list defines the piping class, design pressure, hydrotest pressure, materials used, piping contents and insulation required. You obviously do not have such a list...
I suggest that you develop a design pressure for the system, evaluate components, calculate min wall ( as per approriate piping code) THEN test the system...
While searching for something else I found this thread.
Hiliter is correct, and so is MJC.
At our site, for new piping we predominantly do a flange rating based test for piping up to 400# rating and within the nominal pipe sizes that our plant has at this rating. This ensures that the line is tested for all possible design pressure conditions suitable for the pressure rating. One set of calcs was done to ensure that yield wouldn't be exceeded.
For tie-ins to existing piping where the hydro is unknown or doubtful, or new piping outside the above guidelines, the ANSI B31.3 calc is the go.
Also, the design code used for teh piping will affect teh hydrostatic pressure- some new codes which have higher allowable stresses require a 1.3 times design pressure as teh hydrotest pressure.
