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Watts Water Technologies, Inc.
The difference between the downstream pressure P2 measured with no flow and the same pressure measured at
the generic flow rate Q represents the pressure drop DP of the pressure reducing valve. It depends on the flow
rate, as indicated by the pressure drop diagrams. If you want the downstream pressure never to exceed a given
value P2, you must regulate this to value P2 with no flow. At flow Q, the downstream pressure will be less than
value P2 by an amount equivalent to the pressure drops DP. If the pressure reducing valve is installed to
guarantee that the downstream pressure reaches a given value P2 at a given flow rate Q, this pressure must be
regulated to value P2 + DP with no flow. At flow rate Q, the downstream pressure will be equivalent to P2.
The criterion for choosing the pressure reducing valve consists in determining the diameter in such a way that the speed of
the fluid does not reach excessively high values, at the rated flow rate, causing excessive pressure drop and outflow noise
transmitted to the distribution network. The flow rate-speed diagrams help you to choose the diameter of the pressure
reducing valve in the case of liquids (see water) or gases at pressures of 8-10 bar (see air).
Example 1 (cavitation)
Pressure reducing valve with:
Inlet pressure
P1=14 bar
Outlet pressure P2=3 bar
From the cavitation diagram it can be seen that the pressure reducing valve is constantly working in the red zone.
To avoid rapid deterioration, two pressure reducing valves could be used, one connected upstream to the other.
Upstream pressure reducing valve: pressure front from 14 to 6 bar (green zone).
Downstream pressure reducing valve: pressure front from 6 to 3 bar (green zone).
Example 2 (flow rate)
DRV/N pressure reducing valve with:
Inlet pressure (minimum) P1=8 bar
Outlet pressure
P2=4 bar
Maximum flow rate Q = 50 l/min
From the flow rate-speed diagram, it can be deduced that a 20 or 25 diameter should be used.
From the pressure drop diagram, the following two cases emerge:
DRV20/N Q = 50 l/min Dp=1.1 bar
DRV25/N Q = 50 l/min Dp=0.68 bar
Setting
Sizing
Exemples of sizing
