Self-actuated valves use the system
Figure 35 Solenoid Actuated Valve
fluid to position the valve. Relief
valves, and steam traps are
examples of self-actuated valves.
All of these valves use some
characteristic of the system fluid to
actuate the valve. No source of
power outside the system fluid
energy is necessary for operation
of these valves.
Solenoid Actuated Valves
Solenoid actuated valves provide
for automatic open-close valve
Figure 35. Most solenoid actuated
valves also have a manual override
that permits manual positioning of
the valve for as long as the
override is manually positioned.
Solenoids position the valve by
attracting a magnetic slug attached
to the valve stem.
solenoid valves, spring pressure
acts against the motion of the slug
when power is applied to the
solenoid. These valves can be arranged such that power to the solenoid either opens or closes
the valve. When power to the solenoid is removed, the spring returns the valve to the opposite
position. Two solenoids can be used to provide for both opening and closing by applying power
to the appropriate solenoid.
Single solenoid valves are termed fail open or fail closed depending on the position of the valve
with the solenoid de-energized. Fail open solenoid valves are opened by spring pressure and
closed by energizing the solenoid. Fail closed solenoid valves are closed by spring pressure and
opened by energizing the solenoid. Double solenoid valves typically fail "as is." That is, the
valve position does not change when both solenoids are de-energized.
One application of solenoid valves is in air systems such as those used to supply air to pneumatic
valve actuators. The solenoid valves are used to control the air supply to the pneumatic actuator
and thus the position of the pneumatic actuated valve.