Pneumatic valve actuators have an
advantage of simple construction
requiring little maintenance, and
quick valve response time to
changes in the control signal.
Hydraulic Valve Actuators:
Hydraulic valve actuators usually
have a si,ple design feature with
minimum mechanical parts.
Hydraulic valve actuators convert
fluid pressure into linear motion,
rotary motion or both.
They are also used on both quarter-
turn valves and linear valves. In
the case of quarter-turn valves, the
hydraulic fluid provides the thrust
which is mechanically converted to
rotary motion to adjust the valve.
While for linear valves, the pressure
of the hydraulic fluid acts on the
piston to provide the thrust in a
linear motion. This is particularly for
gate or globe valves etc. Hydraulic
valve actuators are used especially
in situations where a large stem
thrust is required, such as the steam
supply in turbines or the movement
of large valves in chimney flues.
In a situation where very large
valves are to be actuated, it is often
adviceable to install the actuators
on mechanical gearboxes in order
to provide an increased outputs
(torque).
There are different types of
hydraulic valve actuators that
convert linear motion to rotary
motion. Firstly, the diaphragm
actuators are basically used with
linear motion valves, but they are
also applicable with rotary motion
valves, but with a linear to rotary
motion linkage.
Secondly, the lever and link
actuators transfers the linear
motion of a piston cylinder or
diaphragm to rotary motion.
Thirdly, the rack-and-pinion
actuators transfers the linear
motion of a piston cylinder to
rotary motion. The other type of
linear motion to rotary motion
actuator is the scotch yoke
actuators. Finally, for safety reasons
most hydraulic actuators are
provided with fail safe features of
either “fail-open”, “fail-close”, or “fail-
stay put”.
Issue 32 PECM
151