When evaluating the load and potential overpressuring of equipment in accordance with ASME B31.3, please pay attention to:
Internal relief valves on diaphragm metering pumps are not acceptable in place of external relief valves unless all of the following criteria are met:
- Pump capacity: less than 10 gpm (38 L/min)
- Pump service: non-corrosive or non-toxic media
- Internal valve setting: 250 psig (1700 kPa gauge) and under
When the above conditions cannot be met, an external relief valve should be located on the pump discharge. That allows internal relief valves on positive displacement pumps consistent with three API documents on pumps and adds additional requirements to limit the size of the system and risk from internal fluid. Internal relief valves are more difficult to test and, therefore, maintain than external relief valves.