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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.