Hydraulic Relief Valves

Hydraulic relief valves are one of pressure control valve type. Hydraulic relief valve has many functions such as giving overload protection or to limit the force/torque on hydraulic rotary actuator.

Hydraulic relief valve consist of to two main sections:
1. Body section to contain a piston and spring to retain the piston on its seat.
2. Pilot valve section which provides controls to the piston movement and adjusting screw to set up its range.

Hydraulic relief valve which functioned as overload protection will only operate to load or unload the hydraulic pump. This kind of valve doesn’t work so often but it’s still need to be cleaned regularly. The cleaning method is by reducing the pressure adjustment so the accumulated sludge deposit can be flushed.

Hydraulic relief valve types:

1. Simple Type Hydraulic Relief Valve.

Simple Type Hydraulic Relief Valve

Simple Type Hydraulic Relief Valve

This valve consists of two ports, one port for pressure line and the other port to the hydraulic reservoir. The ball inside its body held by the spring, the thrust of spring can be adjusted by turning the adjusting screw. The ball will remain on its position if the inlet pressure insufficient to push the spring and the valve is closed without any flow. When the inlet pressure high enough to push the spring, the ball will leave its position and the valve will be opened. The hydraulic oil will flow to the reservoir through the valve and it will hold the system pressure from further increasing. If the pressure decreases below valve’s setting, the valve will close again.

The cracking pressure is the pressure at which the valve begins to open. The full-flow pressure is the pressure at which the valve passes hydraulic oil on maximum capacity. The full flow pressure is always higher than the cranking pressure due to spring rate.

2. Compound Type Hydraulic Relief Valve.

Compound Type Hydraulic Relief Valve

Compound Type Hydraulic Relief Valve

Inside its body there is a small passage – C – to hold the piston if the inlet pressure less than its setting. When the inlet pressure reaches the setting pressure, the pressure on passage D will push the poppet allowing hydraulic oil to flow through passage C. The flow on passage C will create pressure drop to overcome the spring 2 and raised the piston. This condition will allow hydraulic oil to flow from inlet to the discharge port to stop pressure increasing.

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