Cla-Val CRD-L - Manual (Page 2)

Velocity Delta from
Set Point Pressure
Set Point,fps
Falloff psi
3 ⁄
4
"–1"
5.0 6.0
7.5 9.5
10.0 12.5
Reduced Pressure Falloff
Unlike pilot controlled pressure reducing valves, direct
acting valves are subject to "reduced pressure falloff"
(RPF). Reduced pressure falloff is the decrease in
downstream regulated pressure that occurs when the flow
increases. When the demand for flow increases, the valve
must open wider and wider to permit the flow. The only
way the valve can open is for the spring force to be greater
than the hydraulic force under the diaphragm (the force
trying to close the valve). The downstream pressure
therefore, must "fall off" or decrease before the spring can
open the valve. All spring actuated direct acting valves
have similar operating characteristics.
Noise and Velocity Guidelines
Noise in water piping systems can sometimes be attributed
to high velocities of water through the valve seat. In
general, as the water velocity increases, the noise
produced by the installation will increase.
Where noise levels are important, such as residences,
hospitals, or schools, pipeline velocities should be in the
range of 5 to 10 fps. The chart below shows velocity and
the corresponding reduced pressure falloff. If these values
for falloff are not exceeded, the CRDL will produce the
least amount of noise.
Sizing
Step One
Determine the following from the application:
1. Inlet pressure and desired outlet pressure
2. Maximum and minimum flow rate
3. Allowable reduced pressure falloff or
maximum velocity based on acceptable noise level
Step Two
Determine the pressure differential across the valve by
subtracting the desired outlet pressure from the inlet pressure. If
there will be any fluctuations in the inlet pressure, calculate both
high and low differentials. At all times the differential must be at
least 14.5 psi. When the differential is greater than 100 psi, use
two valves in series.
Step Three
Determine the valve size by using the Valve Capacity Charts on
the next page. Start by referring to the valve capacity chart for
the system's maximum flow rate. Locate the value for the
maximum allowable reduced pressure falloff for your
application. Select the valve size with a RPF value that is less
than the maximum RPF.
If flow demand fluctuates beyond the capacity of one valve, use
two or more Model CRDL’s in parallel. Size one valve to handle
the low flow, and the other valve(s) to handle the higher flows.
Set the low flow valve to approximately 3-4 psi higher than the
other valve(s).
Valve Operation
Flow Condition No Flow Condition
Velocity Guide Chart
When flow begins, the pressure on the underside of the
diaphragm will be lower than the set-point of the spring
causing the diaphragm to move the valve seat away from
the valve seal allowing flow to occur. As the flow increases
downstream, the pressure acting on the spring pushes the
diaphragm and the valve seat away from the valve seal to
regulate outlet pressure to desired value. When there is no flow, the downstream pressure increases and
acts against the under side of the diaphragm, pulling the valve
seat up against the valve seal to close the valve.