Pressure switches are used in various applications that require an adjustable, pressure-actuated electrical switch to make or break an electrical circuit at a predetermined pressure. An electrical circuit may be used to actuate an electrically controlled valve or control an electric motor starter or a signal light. Figure 5-10 shows a pressure switch. Liquid, under pressure, enters chamber A. If the pressure exceeds the adjusted pressure setting of the spring behind ball 1, the ball is unseated. The liquid flows into chamber B and moves piston 2 to the right, actuating the limit to make or break an electrical circuit.
When pressure in chamber A falls below the setting of the spring behind ball 1, the spring reseats ball 1. The liquid in chamber B is throttled past valve 3 and ball 4 because of the action of the spring behind piston 2. The time required for the limit switch to return to its normal position is determined by valve 3’s setting.
A pressure-control valve may limit or regulate pressure, create a particular pressure condition required for control, or cause actuators to operate in a specific order. All pure pressure-control valves operate in a condition approaching hydraulic balance. Usually the balance is very simple: pressure is effective on one side or end of a ball, poppet, or spool and is opposed by a spring. In operation, a valve takes a position where hydraulic pressure balances a spring force. Since spring force varies with compression, distance and pressure also can vary. Pressure-control valves are said to be infinite positioning. This means that they can take a position anywhere between two finite flow conditions, which changes a large volume of flow to a small volume, or pass no flow.
Most pressure-control valves are classified as normally closed. This means that flow to a valve’s inlet port is blocked from an outlet port until there is enough pressure to cause an unbalanced operation. In normally open valves, free flow occurs through the valves until they begin to operate in balance. Flow is partially restricted or cut off. Pressure override is a characteristic of normally closed-pressure controls when they are operating in balance. Because the force of a compression spring increases as it lowers, pressure when the valves first crack is less than when they are passing a large volume or full flow. The difference between a full flow and cracking pressure is called override.
a. Relief Valves.
b. Pressure-Reducing Valves.
c. Sequence Valves.
d. Counterbalance Valves.
e. Pressure Switches.
A pressure switch is an instrument that automatically senses a change in pressure and opens or closes an electrical switching element, when a predetermined pressure point is reached. A pressure-sensing element is that part of a pressure switch that moves due to the change in pressure. There are oasically three types of sensing elements commonly used in pressure switches:
1. Diaphragm: This model (Figure 6.44) can operate from vacuum pressure up to a pressure of 10.5 kg/cm2 (150 psi). It consists of a weld-sealed metal diaphragm acting directly on a snap action switch.
2. Bourdon tube-type sensing element: This model (Figure 6.45) can operate with pressures ranging from 3.5 kg/cm2 (50psi) to 1265 kg/cm2 (18 000 psi). It has a weldsealed bourdon tube acting on a snap action switch.
3. Sealed piston-type sensing element: This type of sensing element can operate with pressures ranging from 1 kg/cm^ (15 psi) to 844kg/cm^ (12 000 psi). It consists of an O-ring-type-sealed piston direct acting on a snap action switch (Figure 6.46).
The electrical switching element in a pressure switch, opens and closes an electrical circuit in response to the actuating force received from the pressure-sensing element.
There are two types of switching elements:
1. Normally open
2. Normally closed.
A normally open switching element is one in which the current can flow through the switching element only when it is actuated. The plunger pin is held down by a snap action leaf spring and force must be applied to the plunger pin to close the circuit. This is done by an electrical coil which generates an electromagnetic field, when current flows through it. In a normally closed switch, current flows through the switching element until the element is actuated, at which point it opens and breaks the current flow.