|
 Actuators
are the "push" a system needs to get started. They are used for dozens of
applications. Today we're going to be talking about the specific application of
valve actuation (that almost sounded poetic!). I have a special place in my
heart for valves since I worked on the recent update of the Valve Area on
GlobalSpec (check it out here) and a valve would be
just a device with holes if it didn't have an actuator to help. Valve actuators
mount on valves and, in response to a signal, move a valve to a desired
position using an outside power source. The right valve actuator will increase
uptime, reduce maintenance costs, and increase plant safety.
Valve motion and operation style are important
specifications to consider when selecting a valve and its actuator.
There
are two basic operating styles for valve actuators.
- Start/stop valves, also known as on/off or isolating devices, limit actuator motion
to preset open and closed positions.
- Throttling or control devices provide controllable
motion so that valves can be throttled as necessary. This type of actuator is
paired with a positioner so the actuator can move to the required position
accurately.
Then one needs to consider how the valve moves,
typically this is in a rotational or linear manner.
- Rotary motion valves (rotary valves) such as ball, plug, and butterfly valves
rotate a quarter-turn or more from open to close.
- Linear motion valves (linear valves) such as gate, globe, diaphragm, pinch and
angle-style valves have a sliding stem design that pushes the closure element
open or closed. The valve stem may rise during rotation, or may rise without
rotation.
Actuation Method
There
are several basic types of valve actuators: manual, electric, pneumatic, and
hydraulic.
|
Manual valve actuators
|
Manual
valve actuators do not require an outside power source. They use a handwheel
or lever to drive a series of gears whose ratio results in a higher output
torque compared to the input (manual) torque.
Image Credit: Samson |
|
Electric valve actuators
|
Electric valve
actuators use a single-phase or three-phase
alternating current (AC) or direct current (DC) motor to drive a combination
of gears to generate the desired torque level.
Image
Credit: Valve World |
|
Pneumatic valve actuators |
Pneumatic
valve actuators adjust valve position by
converting air pressure into linear or rotary motion.
Image
Credit: Nuclear Power Training |
|
Electrohydraulic valve actuators |
Electrohydraulic
valve actuators and hydraulic valve actuators
convert fluid pressure supply into linear or rotary motion. |
The
specifications for rotary and linear valve actuators are different. Rotary
actuators produce motion or torque in one direction to cause rotation. A
linear actuator is an assembly that creates motion and force along a straight
line. Linear actuators use an external energy source and various
methods to achieve this motion. Mechanical, hydraulic, pneumatic, and electric
actuators can be designed as linear actuators. Hydraulic and pneumatic actuators
inherently produce linear motion, while other types provide linear motion from
rotating motors.
|
Actuator
Type |
Advantages |
Disadvantages |
|
Mechanical |
Cheap.
Repeatable. No power source required. Self-contained. Identical behavior
extending or retracting. |
Manual
operation only. No automation. |
|
Electro-mechanical |
Cheap.
Repeatable. Operation can be automated. Self-contained. Identical behavior
extending or retracting. DC or stepping motors. Position feedback
possible. Can be remotely controlled. |
Many
moving parts prone to wear. |
|
Linear
motor |
Simple
design. Minimal moving parts. High speeds possible. Self-contained. Identical
behavior extending or retracting. |
Low
force. |
|
Piezoelectric |
Very
small motions possible. |
Requires
position feedback to be repeatable. Short travel. Low speed. High voltages
required. Expensive. Good in compression only, not in tension. |
|
Hydraulic |
Very
high forces possible. |
Can
leak. Requires position feedback for repeatability. External hydraulic pump
required. Some designs perform well in compression only. |
|
Pneumatic |
Strong,
light, simple, fast. |
Precise
position control impossible except at full stops |
Chart
Credit: Wikipedia.
Actuators
can vary in operating speed. The speed should be selected based on the
speed and power requirements of the system and availability of energy to
the actuator.
- Fast-acting actuators are best used when a system must be quickly isolated or
opened. Fast action is provided by hydraulic, pneumatic, and solenoid
actuators.
- Slow-acting actuators are best used when cold water is injected into a hot
system or slower opening is needed.
Failsafe
Method
There
are several fail-safe methods for valve actuators. Failsafe means that the device
can open or close the valve in case of power failure, or in case of loss of control
signal. Double-acting actuators, those that need an actuation method to move
(i.e. air to open, air to close), will fail in their last position if there is
a loss of power, while a spring return design will return to its initial
position when there is a loss of power.
Spring
return actuators are often chosen for fail-safe critical requirements since
operators can select whether the valve would be left open or closed in the case
of a power failure. Spring return is common in pneumatic actuators and not
widely available for electric actuators.
Actuators provide the power and the push that valves need to
perform in a system. There are many choices, so make sure you take a look at
GlobalSpec's How
to Select Valve Actuator Guide for more information on types of
actuators and the specifications you need.
| 
