Solenoid Valve Selection

Types of valves, materials of construction

Effective wherever fluid flow has to be controlled automatically, solenoid valves are being used to an increasing degree in the most varied types of plants and equipment. The wide variety of different designs available enables each user to select a valve to specifically suit the application in question.

Solenoid valves are control units which, when electrically
energized or de-energized, either shut off or allow fluid flow. The actuator
takes the form of an electromagnet. When energized, a magnetic field builds up
which pulls a plunger or pivoted armature against the action of a spring. When
de-energized, the plunger or pivoted armature is returned to its original
position by the spring action.

According to the mode of actuation, a distinction is made
between direct-acting valves, internally piloted valves, and externally piloted
valves. A further distinguishing feature is the number of port connections or
the number of flow paths (“ways”).

With a direct-acting solenoid valve, the seat seal is
attached to the solenoid core. In the de-energized condition, a seat orifice is
closed, which opens when the valve is energized.

Two-way valves are shut-off valves with one inlet port and
one outlet port. In the de-energized condition, the core spring, assisted by
the fluid pressure, holds the valve seal on the valve seat to shut off the
flow. When energized, the core and seal are pulled into the solenoid coil and
the valve opens. The electromagnetic force is greater than the combined spring
force and the static and dynamic pressure forces of the medium.

Three-way valves have three port connections and two valve
seats. One valve seal always remains open and the other closed in the
de-energized mode. When the coil is energized, the mode reverses.

Unlike the versions with plunger-type cores,
pivoted-armature valves have all port connections in the valve body. An
isolating diaphragm ensures that the fluid  medium does not come into contact with the coil chamber.
Pivoted-armature valves can be used to obtain any three-way valve operation and
are provided with manual override as a standard feature.

With direct-acting valves, the static pressure forces
increase with increasing orifice diameter which means that the magnetic forces,
required to overcome the pressure forces, become correspondingly larger.
Internally piloted solenoid valves are therefore employed for switching higher
pressures in conjunction with larger orifice sizes; in this case, the
differential fluid pressure performs the main work in opening and closing the
valve.

Internally piloted solenoid valves are fitted with either a
two- or three-way pilot solenoid valve. A diaphragm or a piston provides the
seal for the main valve seat. When the pilot valve is closed, the fluid
pressure builds up on both sides of the diaphragm via a bleed orifice. As long
as there is a pressure differential between the inlet and outlet ports, a
shut-off force is available by virtue of the larger effective area on the top
of the diaphragm. When the pilot valve is opened, the pressure is relieved from
the upper side of the diaphragm. The greater effective net pressure force from
below now raises the diaphragm and opens the valve. In general, internally
piloted valves require a minimum pressure differential to ensure satisfactory
opening and closing.

Internally piloted four-way solenoid valves are used mainly
in hydraulic and pneumatic applications to actuate double-acting cylinders.
These valves have four port connections. When de-energized, the pilot valve
opens at the connection from the pressure inlet to the pilot channel. Both
poppets in the main valve are now pressurized and switch over.

With these types, an independent pilot medium is used to
actuate the valve. In the unpressurized condition, the valve seat is closed. A
three-way solenoid valve, which can be mounted on the actuator, controls the
independent pilot medium. When the solenoid valve is energized, the piston is
raised against the action of the spring and the valve opens. A normally-open
valve version can be obtained if the spring is placed on the opposite side of the
actuator piston. In these cases, the independent pilot medium is connected to
the top of the actuator. Double-acting versions controlled by 4/2-way valves do
not contain any spring.

All materials used in the construction of the valves are
carefully selected according to the varying types of applications. Body
material, seal material, and solenoid material are chosen to optimize
functional reliability, fluid compatibility, service life and cost.

Neutral fluid valve bodies are made of brass and bronze. For
fluids with high temperatures, e.g., steam, corrosion-resistant steel is
available. In addition, polyamide materials are used for economic reasons in
various plastic valves.

All parts of the solenoid actuator which come into contact
with the fluid are made of austenitic corrosion-resistant steel. In this way,
resistance is guaranteed against corrosive attack by neutral or mildly
aggressive media.

The particular mechanical, thermal and chemical conditions
in an application factor in the selection of the seal material. The standard
material for neutral fluids at temperatures up to 194°F is normally Viton.
For higher temperatures, EPDM and PTFE are employed. The PTFE material is
universally resistant to practically all fluids of technical interest.

In the case of vacuum operation, care has to be taken to
ensure that the vacuum is on the outlet side while the higher pressure, i.e.,
atmospheric pressure, is connected to the inlet port.

The flow rate through a valve is determined by the nature of
the design and by the type of flow. The size of valve required for a particular
application is generally established by the Cv rating. This figure is evolved
for standardized units and conditions, i.e., flowrate in GPM and using water at
a temperature of between 40°F and 86°F at a pressure drop of 1 PSI.

The small volumes and relatively high magnetic forces
involved with solenoid valves enable rapid response times to be obtained.
Valves with various response times are available for special applications. The
response time is defined as the time between application of the switching
signal and completion of mechanical opening or closing.

 

This information is from Omega Engineering’s
“Technical Principles of Valves” — a valve selection guide
available on their Web site at www.omega.com. For further information, contact
Omega at 800-872-9436.

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