The ASME Boiler and Pressure Vessel Code details all the various calculations necessary to determine the MAWP of any pressure vessel.
API 675 says that the metering pump has to be tested at 1.5 times the MAWP (Maximum Allowable Working pressure) but not less than 20psig (API 675 4.3.2.1)
The Maximum Allowable Working Pressure (MAWP) is used to rate components, steam drums, piping, tubes, boilers and any other pressure vessel in a hydraulic system. The reason this is so important can be seen easily in steam systems. The temperature that a fluid boils increases as pressure increases and decreases if pressure decreases. What this means is if you are running a boiler at 200 psi the water will boil around 387 degrees F. If it is creating steam this is the minimum temperature. If the boiler cracks and the pressure drops to atmospheric pressure, water would boil at 212 degrees F. This creates superheating or heating passed the boiling point. This will create Flash steam and cause an explosion. The MAWP rating is to decrease the chances of this happening.