Description; Specifications; Educational Services; Principle Of Operation - Emerson Fisher 785C Instruction Manual

Instruction Manual
D104483X012

Description

The 785C product lines encompass medium to large spring-return single-acting and double-acting piston actuators
that provide accurate, high thrust output for short to long travel applications. These actuators are designed for use
with a variety of medium to large Fisher sliding-stem control valves.
The single-acting actuators feature an internal bias spring that forces the actuator piston rod to extend or retract upon
a loss of supply pressure, thereby ensuring a fail-closed or fail-open mode of operation. This effectively eliminates the
need for a trip valve and volume tank in most constructions.

Specifications

Specifications for the 785C line of piston actuators are shown in tables 1 to 8. Refer to the nameplate affixed to the
actuator yoke for specifications specific to individual constructions.

Educational Services

Emerson Automation Solutions
Educational Services - Registration
Phone: 1-800-338-8158
E-mail: education@emerson.com
emerson.com/mytraining

Principle of Operation

785C piston actuators utilize a pneumatically controlled piston that moves inside of a cylinder to generate thrust. A
seal contained on the circumference of the piston provides a seal between the top and bottom of the cylinder,
preventing supply pressure leakage.
785C single-acting or double-acting spring-return piston actuators utilize a spring below the piston (outside the
pressurized chamber) that will drive the piston rod upon a loss of supply pressure. This fail action will result in forcing
an attached control valve to either fail-open or fail-closed. The fail action of 785C double-acting springless piston
actuators relies on pneumatic system such as system with Fisher 377 trip valve.
From an equilibrium state, the actuator operates by reacting to a force unbalance that is created by change of supply
pressure on one side of the piston for single-acting, or a change of differential pressure between each side of the
piston for double-acting. This moves the piston up or down, and results in a repositioning of the attached control
valve. Travel can be adjusted using travel limits within a valve positioner, which limit the travel range of the actuator.
The option of a hard travel stopper is also available, consult your
override does not have the ability to act as a hard travel stop.
An optional manual override (top mounted handwheel or side mounted handpump) is capable of extending or
retracting the actuator manually and can be engaged at any position from full open to full close. The top-mounted
handwheel utilizes an engagement lever that couples the handwheel and piston rod. The handpump has a hydraulic
cylinder attached to the piston rod. This enables the handpump to operate the actuator manually, unless the
handpump is set to bypass position.
Emerson sales office for details. The optional manual
785C Actuator
May 2022
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