markzu,
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...
My opinion only.....
MJC