The fiow capacity of the dischargmg valve responds to the pressure at the valve inlet, which is the set pressure minus inlet pressure loss.
In the case of gas flow at sonic velocity, the inlet pressure loss reduces the mass flow rate m direct proportion to the inlet pressure loss. In the case of subsonic gas flow and liquid flow, the change in mass flow depends on the change of pressure difference across the nozzle.
From observation, the mdustry commonly neglects the capacity loss due to an mlet pressure loss of 3% when sizing duect-acting pressure relief valves, and application codes do not appear to offer guidance at this point. In most practical applications however, the selected nozzle size is bigger than the calculated size by an amount that may compensate partly or fully for the capacity loss due to flow resistance in the inlet pipe.
The same capacity loss due to inlet piping pressure loss is also commonly neglected when sizing pilot-operated pressure relief valves. Any capacity loss due to pressure loss in the inlet piping to the main valve of more than 3% however should be taken into account. h the case of snap-acting flowing-type pilots, the pressure loss in the pipeline from the source of pressure to the pilot cormection should in no case exceed 30/o of the set pressure to permit stable valve operation at a blowdown of not lower than 5%. Consult manufacturer.
ball valve
