Technical Guide
DAN-LIQ-TG-44-rev0813
November 2013
22
Model 770 Minimum Differential Pressure Control (N.C.)
Opens on increasing differential pressure
The Model 770 valve is normally closed and throttles toward an open position on increasing differential pressure. It is a regulating
or positioning type valve that does not require any outside power source to operate.
The pilot control is normally closed. It is an adjustable spring loaded variable orifice in the Z-port. The pilot is piston operated,
spring biased (loaded) with a differential pressure sensing chamber connected to the high and low pressure sources.
The Model 770 valve is used for:
1. Pump differential pressure control
2. Bypass differential pressure control for pumps, strainers, filters, etc.
3. Vapor pressure control on LPG, NH3 and similar products.
= Inlet Pressure
= Outlet Pressure
= Pilot Spring Force plus P4
CLOSED POSITION - The pilot is closed. The differential pressure between (P1) and (P4) is less
than the pilot spring setting, indicating the pump is not running or sufficient differential pressure
(P1 minus P4) is not available to overcome the pilot spring setting. Pilot is closed. Y-port (P3) to
Z-port (P2) is closed. X-port (P1) and Y-port (P3) pressures are balanced. The main valve spring,
being the differential force, closes the piston and keeps it seated.
X
Z
Y
Figure 4-12
Sensing
Chamber
Line (Hi)
Needle Valve/Strainer
(Closing/Sensitivity Control)
Differential Pilot (N.C.)
Pump
P1
P3
P2
P4
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