Figure 18-5 shows a pressure-compensated pump with a direct-acting relief valve to protect it against overpressure. Pressure spikes often occur in pressure-compensated pump circuits with high flow or fast cycling. When the pump must compensate rapidly or often from full flow to no flow, the resulting overpressure drastically shortens pump life.
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In Figure 18-5, the pump would be at low pressure and full flow when cylinder CYL3 extends rapidly. When the cylinder stops, fluid requirement is zero, but pump flow is still 40 gpm. As pressure builds, the pump finally starts compensating at about or psi. It is still producing 40 gpm &#; with no place for the oil to go. Without a relief valve in the circuit, system pressure spikes during each cycle can reach four to ten times the compensator setting. Pressure spikes damage the pump and piping after a few hours of operation. The faster the cycle, the more quickly shock damage from pressure spikes causes problems.
A relief valve, installed in Figure 18-5, reduces pressure spikes to protect the system. When the pump shifts to no flow, excess flow goes to tank through the relief valve. When the pump reaches compensator pressure, the relief valve closes. (For another and better way to reduce pressure spikes and protect a pressure-compensated pump from rapid cycling, see Chapter 1, Figures 17-19.)
Set the relief valve in a pressure-compensated pump circuit at 150 to 200 psi higher than the pump compensator. With relief pressure below compensator setting, pump flow goes to tank and makes heat. With relief pressure set at compensator pressure, the relief valve starts dumping when the pump starts compensating. When the relief valve passes fluid, the pump sees a pressure drop, and starts flowing again. The resultant pressure drop allows the relief valve to close and the dump/flow cycle starts again. After a few hours of this erratic operation, the pump fails.
Adding a solenoid valve to the vent port of a pilot-operated relief valve makes an effective unloading valve. Figure 18-6 shows a fixed-displacement pump supplying three cylinders. There is no power to the solenoid on the relief valve with the cylinders idle, so pump flow goes to tank at low pressure. Energizing a solenoid on the relief valve and one cylinder&#;s directional valve causes an action. Energizing both solenoids at the same time sends pump flow to the cylinder until reaching maximum relief pressure. A solenoid relief valve always has a slight delay before blocking flow to tank after energizing the solenoid. The delay is in milliseconds so it usually is only noticeable on very fast cycles.
A pressure relief valve is a hydraulic component designed to limit pressure in one or more locations of a hydraulic system. A pressure relief valve, also known as a relief valve, is constructed with a ball, poppet or spool against a spring and installed into a cavity or ported body.
Every hydraulic system and circuit requires a pressure relief valve to act as either the primary or secondary safety protection device. These valves play an important part in stopping the pressure from getting too high if something in the circuit fails. Without this protection, pressure could build up and could cause costly, irreparable damage or even physical harm to people near the hydraulic system.
Hydraulic systems require a power source to operate, such as electric motor or diesel engine. However, a pressure relief valve must be capable of operating at all times, even during a period of power failure. Therefore, the sole source of power for the pressure relief valve is the hydraulic fluid. Reliability is also vital to the prevention of catastrophic failure, so the design of a relief valve is kept as simple and concise as possible.
As soon as a condition occurs that causes the pressure in a hydraulic system to equal that of the relief valve setting it allows oil to pass through it to the reservoir. The pressure relief valve opens at a predetermined set pressure, however, if the downstream pressure is acting on top of the valve poppet then the actual pressure at which the relief valve opens increases. This is why multi-stage relief valves often have external pilot drains to ensure the pressure is level and stable.
Relief valves are not all the same, but each relief valve comprises of essentially three functional elements:
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Typically, a pressure relief valve uses a spring loaded &#;poppet&#; as the valve element. A poppet is a cone shaped object that sits within an oppositely machined seat. When the pressure exerts a greater force on the spring to which it is loaded, the poppet moves away from the seat and allows fluid to pass through to the outlet port.
When high flows are required 2 stage relief valves are the optimal choice. The 2 stage design uses a small pilot relief that essentially shifts and in turn allows a large poppet to release oil from the high pressure side to the low.
Typically a mechanical spring, the reference force element exerts a force on the sensing element which closes the valve. The reference spring can often be adjusted by the operator to change the relief pressure set point.
As we have noted previously, relief valves are installed to limit the pressure in an entire hydraulic system by diverting pressurised flow to the reservoir. Without controlling or limiting these pressurised forces, the fluid components and equipment could be damaged. Damage to hydraulic systems can be extremely costly in failed components and system down time. Hydraulic system failure through too much pressure could also have the potential to be physically dangerous to those nearby.
Hydraulic relief valves avoid these hazards.
It is critical that relief valves are designed with the correct materials compatible with many different types of process fluid. There are three types of pressure relief valve currently used in a typical circuit.
A spring loaded relief valve comprises a valve mounted in the hydraulic system, a poppet held against the valve to prevent the valve from lifting under normal operating conditions, a spring to keep the poppet closed and a body (&#;manifold&#;) to contain the operating elements.
The piston area vented to the atmosphere is exactly equal to the piston area exposed to the backpressure. The poppet is vented to ensure that the pressure area will always be exposed to the atmospheric pressure. This provides protection and minimises the effects of excessive pressure drop.
These pressure relief valves comprise the main valve with a piston or a disk operated by a diaphragm and a pilot. When the predetermined pressure point has been reached the pilot actuates to close the system fluid to the piston chamber while also venting the piston chamber. This series of events causes the valve to open.
Posted by admin in category Hydraulic Valves Advice on Wednesday, 27th July
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