Speed of Power Actuators
Plant safety considerations dictate valve speeds for certain safety-related valves. Where a system
must be very quickly isolated or opened, very fast valve actuation is required. Where the
opening of a valve results in injection of relatively cold water to a hot system, slower opening
is necessary to minimize thermal shock. Engineering design selects the actuator for safety-
related valves based upon speed and power requirements and availability of energy to the
In general, fastest actuation is provided by hydraulic, pneumatic, and solenoid actuators.
However, solenoids are not practical for large valves because their size and power requirements
would be excessive. Also, hydraulic and pneumatic actuators require a system for providing
hydraulic or pneumatic energy. The speed of actuation in either case can be set by installing
appropriately sized orifices in the hydraulic or pneumatic lines. In certain cases, the valve is
closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the
Electrical motors provide relatively fast actuation. Actual valve speed is set by the combination
of motor speed and gear ratio. This combination can be selected to provide full valve travel
within a range from about two seconds to several seconds.
Valve Position Indication
Operators require indication of the position of certain valves to permit knowledgeable operation
of the plant. For such valves, remote valve position indication is provided in the form of
position lights that indicate if valves are open or closed. Remote valve position indication
circuits use a position detector that senses stem and disk position or actuator position. One type
of position detector is the mechanical limit switch, which is physically operated by valve
Another type is magnetic switches or transformers that sense movement of their magnetic cores,
which are physically operated by valve movement.
Local valve position indication refers to some visually discernable characteristic of the valve that
indicates valve position. Rising stem valve position is indicated by the stem position. Nonrising
stem valves sometimes have small mechanical pointers that are operated by the valve actuator
simultaneously with valve operation. Power actuated valves typically have a mechanical pointer
that provides local valve position indication. On the other hand, some valves do not have any
feature for position indication.