Page 283 - Valve Selection Handbook

270 Valve Selection Handbook

Figure 7-7. Capacity Correction Factor K v due to Viscosity. (Reprinted from API RP
520 Parti, December 1976, Courtesy of American Petroleum Institute.)

Influence of Inlet Pressure Loss on Valve Discharge Capacity

The flow capacity of the discharging 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 in 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 industry commonly neglects the capacity loss
due to an inlet pressure loss of 3% when sizing direct-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 com-
monly 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. In the case of snap-
acting flowing-type pilots, the pressure loss in the pipeline from the
source of pressure to the pilot connection should in no case exceed 3% of