Model 770 Differential Vapor Pressure Control (N.c.) - Emerson Daniel 700 Series Technical Manual

Technical Guide
DAN-LIQ-TG-44-rev0813
November 2013

Model 770 Differential Vapor Pressure Control (N.C.)

Opens on increasing differential pressure
Typical for LPG, NH3 or similar products
The Model 770 as illustrated is identical to the previously described 770 valve except it is shown as a vapor pressure
control valve for products having high flash points such as: butane, propane, anhydrous ammonia or other products with
similar characteristics.
When metering these products, the pressure at the meter must be higher than the vapor pressure of the product, otherwise it
turns into a gaseous state resulting in meter damage due to overspeed plus inaccurate measurement and possible pump
damage. The differential pressure between the meter inlet pressure and the vapor pressure (P4) is generally 10 to 30 psid, to
assure the product is in a liquid state when it passes through the meter.
CLOSED POSITION - The pilot is closed. Vapor pressure (P4) plus pilot spring setting is greater
than the line pressure (P1), 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 become balanced. The main valve
spring being the differential force, closes the piston and keeps it sealed.
P4
Vapor Pressure
Liquid
= Inlet Pressure
= Outlet Pressure
= Pilot Spring Force plus P4
= Product Pressure
Sense Line (LO)
Needle Valve/Strainer
(Closing/Sensitivity Control)
Flow Meter
Figure 4-17
P3
X
P1
25
Differential Pilot
(N.C.)
Y
Z
P2