Hi, I remember during my refinery days, we used to hyrdo test at 1.5 times the design pressure (not operating pressure), for 24 hours, regardless of the material or size. This was done for heat exchangers, pressure vessels, etc. This was the standard used at our company, properly documented and referenced with interanal code numbers.

I'm pretty sure this was based on ASTM or ASME standard, just can't remember which one in particular.

As the first replier specified the pressure should be 1.5 of design pressure which in your case is 150 Psig and the test time after removing air from the piping system by means of air vents and after preliminary test. should be 4 hours plus the time that engineer may require to assure that no air pocket is affecting the system.Probably you could find it in hydrostatic test requirement of mechanical section of country building codes standards(which may vary from country to country).

Holding the test for 24 hours on large volume test allows for temperature stabilazation over a days temperature range. If weather conditions cause a large change in temperature, during hold, there may result a corresponding change in test pressure, this may be a factor in determining where the pressure went when no leaks are found.

ASME B31.1

Chapter VI Examination, Inspection, And Testing

137.4.5 Required Hydrostatic Test Pressure

The hydrostatic test pressure shall not be less than 1.5 times the design pressure.

The pressure shall be continuously maintained for a minimum time of 10 minutes and then may be reduced to the design pressure for the amount of time necessary to conduct the examination.

ASME B31.3 "Process Piping" paragraph 345.2.2(a) " Examination for Leaks. A leak test shall be maintained for at least 10 min, and all joints and connections shall be examined for leaks."

http://catalog.asme.org/Codes/PrintBook/B313_2006_Process_Piping.cfm