Emerson Fisher 3710 Instruction Manual page 21

Instruction Manual
D101728X012
Figure 13. Mounting the Positioner Base Plate with Purge Tube
LONG‐BLADED SCREW
DRIVER THROUGH VENT
W6073
The positioner does not have an external vent connection. A unit exhausts actuator pressure through a screened hole
(vent opening, figure 12) located in the positioner base plate. Do not restrict the vent opening. Restricting the vent
opening can produce a pressure buildup in the positioner case and degrade positioner performance.
A purge tube (key 58, figure 13) is available as an option when mounting the positioner on selected Fisher actuators.
One end of the purge tube with an O‐ring (key 59) installs in the actuator cover plate. The other end of the purge tube
with an O‐ring installs in the vent opening in the positioner base plate. When the purge tube is used, it relays actuator
exhaust pressure from the positioner into the actuator housing.
Within the actuator and positioner assembly, exhaust pressure takes the following course:
D The actuator exhaust pressure vents from the actuator through the positioner spool valve into the positioner
case.
D The exhaust pressure then exits the positioner case through the vent opening.
D The purge tube carries the exhaust pressure into the actuator housing.
D The exhaust pressure then purges the actuator housing and dissipates from the housing.
Most Fisher actuators do not have vent openings. The exhaust pressure simply dissipates from within the actuator
housing. However, the 1051, 1052, and 1061 actuators feature an operational screened vent connection when a
metal access plate is specified for the actuator housing.
When using the purge option, remember: The actuator and positioner assembly is not airtight. However, if you use the
optional purge tube to connect the positioner to the actuator and the actuator has a vent opening, leave open the 1/4
NPT vent connection in the actuator housing to prevent pressure buildup in the positioner case.
PURGE TUBE
(KEY 58)
O‐RINGS (KEY 59)
3710 and 3720 Positioners
September 2017
21