Automatic valves are commonly considered as control valves
that operate in conjunction with an automatic controller or device
control the fluid flow. The “control valve” as used
here actually consists of a valve body and an actuator. The valve
body and actuator
may be designed so that the actuator is removable and/or replaceable,
or the actuator may be an integral part of the valve body. This
section covers the most common types of valve actuators and control
valves with the following classifications:
• Two-way bodies (single- and double-seated)
• Three-way bodies (mixing and diverting)
• Butterfly arrangements (two- and three-way)
The valve actuator converts the controller’s output, such
electric or pneumatic signal, into the rotary or linear action
by the valve (stem), which changes the control variable (flow).
Actuators cover a wide range of sizes, types, output capabilities,
Sizes. Actuators range in physical size from small solenoid or
clock motor self-operated types, to large pneumatic actuators
100 to 200 in2 of effective area.
Types. The most common types of actuators used on automatic
valve applications are solenoid, thermostatic radiator, pneumatic,
electric motor, electronic, and electrohydraulic.
Output (Force) Capabilities. Although the smallest actuators,
designed for unitary commercial HVAC and residential control
applications, are capable of only a very small output, larger
pneumatic or electrohydraulic actuators are capable of great force.
overall force ranges from a few ounces to over 0.5 ton of force.
Pneumatic or diaphragm valve actuators are available with diaphragm
sizes ranging from 3 to 200 in2. The design consists of a
flexible diaphragm clamped between an upper and a lower housing.
On direct-acting actuators, the upper housing and diaphragm create
a sealed chamber (Figure 9). A spring opposing the diaphragm force
is positioned between the diaphragm and the lower housing.
Increasing air pressure on the diaphragm pushes the valve stem
down and overcomes the force of the load spring to close a direct
acting valve. Springs are designated by the air pressure change
required to open or close the valve. A 5 lb spring requires a
control pressure change at the actuator to operate the valve.
valves have an adjustable spring feature; others are fixed. Springs
for commercial control valves usually have ±10% tolerance,
5 lb spring setting is 5 psi ± 0.5 psi. Two valves in a
control may be
sequenced simply with adjustable actuator springs.
Reverse-acting valves may use a direct-acting actuator if they
have reverse-acting valve bodies; otherwise, the actuator must
reverse-acting and constructed with a sealed chamber between the
lower housing and the diaphragm.
The valve close-off point shifts as the supply and/or the differential
pressure increases across a single-seated valve due to the fixed
areas of the actuator and the valve seat. The manufacturer’s
off rating tables need to be consulted to determine if the actuator
of an adequate size or if a larger actuator or a pneumatic positioner
relay is required.
A pneumatic positioner relay may be added to the actuator to
provide additional force to close or open an automatic control
valve (Figure 9). Sometimes called positive positioners or pilot
positioners, pneumatic positioners are basically high-capacity
relays that add air pressure to or exhaust air pressure from the
actuator in relation to the stroke position of the actuator. Their
application is limited by the supply air pressure available and
by the actuator’s spring.
Electric actuators usually consist of a double-wound electric
motor coupled to a gear train and an output shaft connected to
valve stem with a cam or rack-and-pinion gear linkage (Figure
For valve actuation, the motor shaft typically drives through
rotation. The use of damper actuators with 90° full stroke
rapidly increasing in valve control applications. Gear trains
are coupled internally to the electric actuators to provide a
of valve stroke to increase operating torque and to reduce overshooting
of valve movement. Gear trains can be fitted with limit
switches, auxiliary potentiometers, etc., to provide position
indication and feedback for additional system control functions.
In many instances, a linkage is required to convert rotary motion
to the linear motion required to operate a control valve (except
and butterfly valves). Electric valve actuators operate with two
position, floating, proportional electric, and electronic control
systems. Actuators usually operate with a 24 V (ac) low-voltage
control circuit. Actuator time to rotate (or drive full stroke)
30 s to 4 min, with 60 s being most common.
Electric valve actuators may have a spring return, which returns
the valve to a normal position in case of power failure, or it
powered with an electric relay and auxiliary power source. Since
motor must constantly drive in one direction against the return
spring, spring return electric valve actuators generally have
approximately one-third of the torque output of non-spring return
Hydraulic actuators combine characteristics of electric and pneumatic
actuators. In essence, hydraulic actuators consist of a sealed
housing containing the hydraulic fluid, pump, and some type of
metering or control apparatus to provide pressure control across
a piston or piston/diaphragm. A coil controlled by a low- to medium
level dc voltage usually activates the pressure control apparatus.
A solenoid valve is an electromechanical control element that
opens or closes a valve on the energization of a solenoid coil.
Solenoid valves are used to control the flow of hot or chilled
water and steam and range in size from 1/8 to 2 in. pipe size.
Solenoid actuators themselves are two-position control devices
and are available for operation in a wide range of alternating
current voltages (both 50 and 60 Hz) as well as direct current.
Thermostatic Radiator Valves
Thermostatic radiator valves are self-operated and do not require
external energy. They control room or space temperature by modulating
the flow of hot water or steam through free-standing radiators,
convectors, or baseboard heating units. Thermostatic radiator
valves are available for a variety of installation requirements
with remote-mounted sensors or integral-mounted sensor and remote
or integral set point adjustment
Control of Automatic Valves
Computer-based control of automatic control valves is replacing
older technologies and provides many benefits, including speed,
accuracy, and data communication. However, care must be exercised
in selecting the value of control loop parameters such as loop
speed and dead band (allowable set-point deviation). High loop
speed coupled with zero dead band can cause the valve-actuator
seek a new control position with each control loop cycle unless
actuator itself has some type of built-in protection against this.
example, a 1 s control loop with zero dead band could result in
30,000,000 repositions (corrections) in 1 year of service.
Computer-based control systems should be tuned to provide the
minimum acceptable level of response and accuracy required for
application in order to achieve maximum valve and actuator service
Two-Way Valves (Single- and Double-Seated)
In a two-way automatic valve, the fluid enters the inlet port
and exits the outlet port either at full or reduced volume, depending
on the position of the stem and the disk in the valve.
In the single-seated valve, one seat and one plug-disk close
against the stream. The style of the plug-disk varies depending
the requirements of the designer and the application.
The double-seated valve is a special application of the two-way
valve with two seats, plugs, and disks. It is generally applied
where the close-off pressure is too high for the single-seated
Three-way valves either mix or divert streams of fluid
The three-way mixing valve blends two streams into one common
stream based on the position of the valve plug in relation to
upper and lower seats of the valve. A common use is to mix chilled
or hot water. The valve controls the temperature of the single
leaving the valve.
The three-way diverting or bypass valve takes one stream of
fluid and splits it into two streams for temperature control.
limited applications, such as a cooling tower control, a diverting
bypass valve must be used in place of a mixing valve. In most
a mixing valve can perform the same function as a diverting or
bypass valve if the companion actuator has a very high spring
Otherwise, water hammer or noise may occur when operating near
Special-purpose valve bodies may be used on occasion. One type
of four-way valve is used to allow separate circulation in the
loop and a heated zone. Another type of four-way valve body is
as a changeover refrigeration valve in heat pump systems to reverse
the evaporator to a condenser function.
Float valves are used to supply water to a tank or reservoir
serve as a boiler feed valve to maintain an operating water level
the float level location (Figure 14).