Actuators; Control Valves; Pneumatic; Spring Ranges - Honeywell Pneumatic Controls Engineering Manual

Actuators and Final Control Elements

Actuators

General
Pneumatic actuators position damper blades and valve stems.
A damper actuator typically mounts on ductwork or on the
damper frame and uses a push rod and crank arm to position
the damper blades (rotary action). A valve actuator mounts on
the valve body and positions the valve stem directly (linear
action) for a globe valve or rotary action via linkage for a
butterfly valve. Valve actuator strokes typically are between
one-quarter and one and one-half inch. Damper actuator
strokes range from one to four inches (longer in special
applications). In commercial pneumatic actuators, air pressure
positions the actuator in one direction and a spring returns it the
other direction.
Valve actuators are direct or reverse acting. Damper actuators
are direct acting only. A direct-acting actuator extends on an
increase in branchline pressure and retracts on a decrease in
pressure. A reverse-acting actuator retracts on an increase in
branchline pressure and extends on a decrease in pressure.
Pneumatic valve and damper actuator assemblies are termed
"normally open" or "normally closed." The normal position is the
one assumed upon zero actuator air pressure. Three-way
valves have both normally open (N.O.) and normally closed
(N.C.) ports.

SPRING RANGES

Springs used in valve and damper actuators determine the start
pressure and pressure change required for full movement of the
actuator from open to closed, or from closed to open. Actuators
designed for special applications can move through the full
range, open to closed or closed to open, on a limited change in
pressure from the controller. Such actuators can provide a
simple form of sequence control (e.g., operating heating and
cooling valves from a single thermostat). Typical spring
pressure ranges are 2-7 psi, 8-12 psi, and 3-13 psi.

Control Valves

Single-seated globe valves (Fig. 28) are used where tight close-
off is required. The valve body can be either direct acting or
reverse acting. A direct-acting valve body allows flow with the
stem up, while a reverse-acting valve body shuts off flow with
the stem up. The combination of valve body and actuator
(called the valve assembly) determines the normal valve stem
position.
The position maintained by the valve stem depends on the
balance of forces acting on it:
— Force F1 from the air pressure on the diaphragm
— Opposing force F2 from the actuator spring
— Controlled-medium force F3 acting on the valve disc and
plug due to the difference between inlet and outlet pressures
An increase in controller branchline pressure increases force
F1, (Fig. 28A), moving the diaphragm down and positions the
valve stem toward closed until it has moved far enough that the
sum of the spring force F2 and the controlled-medium force F3
increases balance the increased force F1 on the diaphragm.
Conversely, a decrease in controller branchline air pressure in
the diaphragm chamber of a direct-acting actuator decreases
force F1, allowing forces F2 and F3 to push the diaphragm
upward and move the valve stem toward the open position.
138 customer.honeywell.com
BRANCH
LINE
FLOW
A.
NORMALLY OPEN VALVE ASSEMBLY (DIRECT-ACTING
VALVE BODY AND DIRECT-ACTING ACTUATOR)
BRANCH
LINE
FLOW
B.
NORMALLY CLOSED VALVE ASSEMBLY (DIRECT-ACTING
VALVE BODY AND REVERSE-ACTING ACTUATOR)
BRANCH
LINE
C.
NORMALLY CLOSED VALVE ASSEMBLY (REVERSE-ACTING
VALVE BODY AND DIRECT-ACTING ACTUATOR)
Fig. 28. Single-Seated Valves.
In Figure 28B, branchline pressure is applied on the bottom
surface of the diaphragm. An increase in air pressure in the
diaphragm chamber increases force F1 causing the actuator
diaphragm to move upward and open the valve. Motion
continues until the increase in pressure on the diaphragm plus
the controlled-medium force F3 is balanced by the increase in
spring compression (force F2). On a decrease in air pressure in
the diaphragm chamber, the compressed spring moves the
diaphragm down toward its normal position and the valve stem
toward closed. A normally closed valve assembly usually has a
lower close-off rating against the pressure of the controlled
medium than a normally open valve because the spring force
F2 is the only force available to close the valve.
In Figure 28C, an increase in branchline pressure in the
actuator increases force F1 causing the diaphragm to move
downward and open the valve. Motion continues until the
increase in pressure on the diaphragm (force F1) plus the
controlled-medium force F3 is balanced by the increase in
spring compression (force F2). On a decrease in air pressure in
the diaphragm chamber, the compressed-spring pressure
moves the diaphragm up and the valve stem moves toward the
closed position.
70-6925
F1
F2
F3
F1
(F2
)
F3
F1
F2
F3
FLOW
C2613