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Valves are employed throughout industry to control the flow of both liquids and gasses. Many valves are manually operated by turning a handle, but in industrial applications, electrically operated valves are generally used. Electrically operated valves can be placed near the equipment they operate, which helps to minimize the amount of piping required. Also, electrically operated valves can be controlled from remote locations by running a pair of wires from the control station to the valve.
Solenoid valves contain two distinct parts, the electrical part and the valve part. The electrical part consists of a coil of wire that supplies an electromagnetic field that operates the plunger or core. When the solenoid coil is energized, the plunger is drawn into the coil, opening or closing the valve. Solenoid valves can be opened or closed when de-energized. If the valve is normally closed, it will open when the solenoid is energized. The plunger will return to its normal position when the solenoid is de-energized. Most valves contain a spring that re-seats the valve when de-energized.
Some valves are normally open and will close when energized. They will return to their normal open state when the solenoid is de-energized.
Although solenoid valves are very similar to the solenoid used to operate relays and contacts, the symbol used to represent a solenoid valve is often drawn differently. Relay and contactor coils are generally represented by a circle. Solenoid valves are often rep resented by the symbol shown in Fgr. 1.
Two-Way Solenoid Valves
Two-way valves are used to control the flow of liquids or gasses. They are digital devices in that they are completely on or completely off. They don't have the ability to control the rate of flow. Two-way valves have an inlet and an outlet and are connected directly into the pipe line. The inlet and outlet shouldn't be re versed, because the valve is designed is such a manner that the pressure of the inlet liquid or gas is used to help maintain a seal (Fgr. 2). The valve contains a wedge-shaped disc that seats against a wedge-shaped seat. The inlet pressure forces the disc against the seat to help provide a more secure seal. If the valve is reversed, the inlet pressure tries to force the disc up against the spring. If the pressure is great enough, it can cause the valve to leak.
Four-Way Solenoid Valves
Four-way valves are generally used to control the air supplied to double acting cylinders (Fgr. 3).
When the valve is de-energized, one side of the cylinder is open to atmospheric pressure, and the other side is supplied by line pressure. When the solenoid coil is energized, the valve permits the high pressure side to exhaust to the atmosphere and the side that was previously open to the atmosphere to be supplied by line pressure. The piston inside the cylinder will move back and forth in accord with the solenoid being energized or de-energized. The speed of the piston's movement is determined by the amount of air pressure, the surface area of the piston, and the amount of force the load places against the piston.
Motor Operated Valves
Motor operated valves (MOVs) are used extensively in industries where the control of liquids or gasses is required. Pipe line companies and the petrol-chemical industry are just two examples of these types of industries. Motor operated valves are valves that employ an electric motor to open or close the valve (Fgr. 4).
There are generally two sections to the control system for MOVs, local and remote. The local controls are housed with the valve at the field location and the remote controls are housed in a control room some distance away.
The control system is basically a forward/reverse control with the addition of a special limit switch that detects when the valve is open or closed and a torque switch that can be used to ensure that the valve is tightly seated (Fgr. 5). It is common practice to use the limit switch (Fgr. 6) to determine when the valve is completely open, and the torque switch (Fgr. 7) to determine when the valve is closed.
The schematic for an MOV is shown in Fgr. 8.
The schematic is drawn to assume that the valve is in the open position, and all limit switches are drawn to reflect this condition.
This control circuit for an MOV is of particular interest because a two-wire circuit's used to control the opening and closing of the valve from a remote location. This two-wire circuit consists mainly of an 80 volt transformer, relay coils K1A, K1B, K2A, and K2B, and push buttons. Two-wire control is accomplished by converting the 80 volts AC into half-wave rectified DC with a voltage of 36 volts.
80 VAC x 0.9 (RMS to Average) = 72 VDC (Full-Wave) 72 VDC (Full-Wave)/2 = 36 VDC (Half-Wave)
With the valve in the open position, normally closed limit switch LSC connected in series with coil K2B is closed and normally closed limit switch LSO connected in series with coil K2A is held open. This permits a current path through coil K2B and diode D4 to the coils of K1A and K1B. At this point, current cannot flow through coil K1A because diode Dl is reverse biased. Current can flow through coil K1B and diode D2, however. Coils K2A and K2B have a voltage rating of 36 VDC, and coils K1A and K1B have a voltage rating of 24 VDC. Since coils K1B and K2B are connected in series, the voltage drops across both these coils must equal the applied voltage of 36 VDC. The coil resistances are such that 24 VDC is dropped across coil K1B and 12 VDC is dropped across coil K2B.
Since coil K1B has a voltage rating of 24 VDC, it energizes and closes a contact to turn on the OPEN indicator light. Coil K2B has a voltage rating of 36 VDC.
The 12 VDC applied to it's not enough to energize it, so its contacts remain in their normal position.
Now assume that the CLOSE push button is pressed at the remote location. This short-circuits coil K1B, which causes the entire 36 VDC to be applied across coil K2B. When coil K2B energizes, C contactor energizes and the motor begins closing the valve. As the valve closes, limit switch LSC connected in series with coil K2B opens, breaking the current path through coils K1B and K2B.
When the valve reaches the closed position, limit switch LSO connected in series with coil K2A closes.
A current path now exists through coils K2A, diode D3, coil K1A, and diode D1. Relay K1A energizes and turns on the CLOSED indicator light. Although the torque switch is generally used to stop the motor when the valve is closed, the limit switch is adjusted to indicate that the valve is in the closed position.
1. What is meant by the statement that solenoid valves are digital devices?
2. Why is it important that the inlet and outlet ports on a two-way valve not be reversed?
3. What type of valve is generally used to supply air pressure to a double acting cylinder?
4. What two sections are generally used to operate motor operated valves?
5. What type of switch is generally used to ensure that a motor operated valve is tightly seated?
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