Control Valves - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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 15-50 kPa,
55-85 kPa, and 20-90 kPa.

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.
BRANCH
F1
LINE
F2
FLOW
F3
A.
NORMALLY OPEN VALVE ASSEMBLY (DIRECT-ACTING
VALVE BODY AND DIRECT-ACTING ACTUATOR)
BRANCH
F1
LINE
(F2
FLOW
F3
B.
NORMALLY CLOSED VALVE ASSEMBLY (DIRECT-ACTING
VALVE BODY AND REVERSE-ACTING ACTUATOR)
BRANCH
F1
LINE
F2
C.
NORMALLY CLOSED VALVE ASSEMBLY (REVERSE-ACTING
VALVE BODY AND DIRECT-ACTING ACTUATOR)
Fig. 28. Single-Seated Valves.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
)
F3
FLOW
C2613
PNEUMATIC CONTROL FUNDAMENTALS
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.
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.
In a double-seated valve (Fig. 29), the controlled agent flows
between the two seats. This placement balances the inlet
pressures between the two discs of the plug assembly and
reduces the actuator force needed to position the plug assembly.
Double-seated valves generally do not provide tight close-off
because one disc may seat before the other and prevent the
other disc from seating tightly.
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