Hydraulic Relief Valves
Relief valves are the most common type of pressure-control valves. The relief valves’ function may vary, depending on a system’s needs. They can provide overload protection for circuit components or limit the force or torque exerted by a linear actuator or rotary motor.
The internal design of all relief valves is basically similar. The valves consist of two sections: a body section containing a piston that is retained on its seat by a spring(s), depending on the model, and a cover or pilot-valve section that hydraulically controls a body piston’s movement. The adjusting screw adjusts this control within the range of the valves.
Valves that provide emergency overload protection do not operate as often since other valve types are used to load and unload a pump. However, relief valves should be cleaned regularly by reducing their pressure adjustments to flush out any possible sludge deposits that may accumulate. Operating under reduced pressure will clean out sludge deposits and ensure that the valves operate properly after the pressure is adjusted to its prescribed setting.
(1) Simple Type. Figure 5-2 shows a simple-type relief valve. This valve is installed so that one port is connected to the pressure line or the inlet and the other port to the reservoir. The ball is held on its seat by thrust of the spring, which can be changed by turning the adjusting screw. When pressure at the valve’s inlet is insufficient to overcome spring force, the ball remains on its seat and the valve is closed, preventing flow through it. When pressure at the valve’s inlet exceeds the adjusted spring force, the ball is forced off its seat and the valve is opened. Liquid flows from the pressure line through the valve to the reservoir. This diversion of flow prevents further pressure increase in the pressure line. When pressure decreases below the valve’s setting, the spring reseats the ball and the valve is again closed.
The pressure at which a valve first begins to pass flow is the cracking pressure of a valve. The pressure at which a valve passes its full-rated capacity is the full-flow pressure of a valve. Because of spring rate, a full-flow pressure is higher than a cracking pressure. This condition is referred to as pressure override. A disadvantage of a simple-type relief valve is its relatively high-pressure override at its rated capacity.
(2) Compound Type. Figure 5-3 shows a compound type relief valve. Passage C is used to keep the piston in hydraulic balance when the valve’s inlet pressure is less than its setting (diagram A). The valve setting is determined by an adjusted thrust of spring 3 against poppet 4. When pressure at the valve’s inlet reaches the valve’s setting, pressure in passage D also rises to overcome the thrust of spring 3. When flow through passage C creates a sufficient pressure drop to overcome the thrust of spring 2, the piston is raised off its seat (diagram B). This allows flow to pass through the discharge port to the reservoir and prevents further rise in pressure.
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