Compound Pressure Relief Valve
A pilot-operated pressure-relief valve consists of a small pilot relief valve and main relief valve as shown in Fig. 1.4. It operates in a two-stage process:
1. The pilot relief valve opens when a preset maximum pressure is reached.
2. When the pilot relief valve opens, it makes the main relief valve open.
The pilot-operated pressure-relief valve has a pressure port that is connected to the pump line and the tank port is connected to the tank. The pilot relief valve is a poppet type. The main relief valve consists of a piston and a stem. The main relief piston has an orifice drilled through it. The piston has equal areas exposed to pressure on top and bottom and is in a balanced condition due to equal force acting on both the sides. It remains stationary in the closed position. The piston has a light bias spring to ensure that it stays closed. When the pressure is less than that of relief valve setting, the pump flow goes to the system. If the pressure in the system becomes high enough, it moves the pilot poppet off its seat. A small amount of flow begins to go through the pilot line back to the tank. Once flow begins through the piston orifice and pilot line, a pressure drop is induced across the piston due to the restriction of the piston orifice. This pressure drop then causes the piston and stem to lift off their seats and the flow goes directly from the pressure port to the tank.
The advantages of pilot-operated pressure-relief valves over direct-acting pressure-relief valves are as follows:
1. Pilot-operated pressure-relief valves are usually smaller than direct-acting pressure-relief valves for the same flow and pressure settings.
2. They have a wider range for the maximum pressure settings than direct-acting pressure-relief valves.
3. They can be operated using a remote while direct-acting pressure-relief valves cannot.
Graphic symbol of a pressure-relief valve is shown in Fig. 1.5. The symbol shows that the valve is normally closed (the arrow is offline). On one side of the valve, pressure is fed in (the dashed line) to try to open the valve, while on the other side, the spring tries to keep it adjustable, allowing the adjustment of pressure level at which the relief valve opens. The arrow through the spring signifies that it is adjustable, allowing the adjustment of pressure level at which the relief valve opens.
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