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.
Hydraulic relief valves limit system pressure to protect components. The valves also limit the hydraulic system&#;s maximum output force. All hydraulic system relief valves work by balancing the hydraulic force with an adjustable spring force. Whenever the relief valve opens in response to a predetermined pressure, it releases heat. A properly adjusted valve will enable the system to operate correctly, while controlling the amount of heat generated.
If valves are not set properly or monitored carefully, the hydraulic system can malfunction and suffer damage.
Below we describe the steps involved in setting hydraulic system relief valves and the signs that a valve needs replacing.Below we describe the steps involved in setting hydraulic system relief valves and the signs that a valve needs replacing.If a relief valve is set but is releasing pressure before a system reaches maximum pressure, or if it&#;s constantly leaking or chattering, something may be wrong with the system.
A properly maintained pressure relief valve can stay in service for up to 30 years. If you&#;ve been testing your valves regularly, it&#;s likely there&#;s something else in your system to blame.
That said, pressure relief valves can and do fail. It&#;s important to learn the signs of a possible failure in order to quickly solve the problem and keep your facility safe.That said, pressure relief valves can and do fail. It&#;s important to learn the signs of a possible failure in order to quickly solve the problem and keep your facility safe.
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Here are 3 signs of pressure relief valve failure to watch out for.
If your system is not reaching the right pressure, it could be a sign of pressure relief valve failure. In some cases, this could be fixable. If the valve was calibrated to the wrong set pressure, it could simply be releasing early. This can happen with changes to your facility&#;s plant design. In some cases, technicians forget to recalibrate pressure relief valves for the system&#;s new normal operating pressure. Your valve technicians can go in and adjust the valve&#;s set pressure to address this issue.
If that doesn&#;t solve the problem, and the rest of your system is working properly, then your pressure relief valve likely needs to be changed. After years of service, the valve could be damaged or eroded from environmental dirt and debris, which block the valve from fully closing. This can cause chattering, where the valve isn&#;t all the way open, but is opening and closing rapidly, stopping it from properly doing its job. When your system cannot reach pressure, you will likely experience production slowdown or downtime.
Pressure relief valves and safety relief valves are what keep a facility safe. If your system builds up more than safe maximum pressure, safety relief valves open up to let off additional pressure, keeping your facility, your employees, and your equipment safe.If your system is above pressure and your pressure relief valves have not released, this could be a relief valve failure. You will need to ensure that the valves are set to the correct set pressure.Contaminants, like dirt, lint, rust, sludge, or even the misalignment of the valve can cause the pressure relief valve to stick. At this point, you might see that your system is above pressure, or you will notice other pressure relief valves in the system releasing to make up for this valve&#;s malfunction.While the other two signs appear obvious, this last sign is more noticeable when you&#;re directly inspecting your pressure relief valves. Leaking valves are a problem, and can contribute to slower, less efficient production, but they can be more difficult to notice, as their effect on the entire system can be much smaller. This is part of the reason that valve testing and maintenance is so important.
If your pressure relief valve has no pressure, it is likely that the balance hole is plugged, the spring is broken, or the valve has a loose fit. In the case of a loose fit or broken spring, replacement is a must. The valve does not function properly within your system, which means it&#;s not protecting your facility, your employees, or your equipment in the event that there is a larger problem.
Pressure valve leakage is a little more complicated to troubleshoot, as there are many possible causes. It could be the valve, it could be that misalignment is causing a failure to reseat after a correct opening, or it could be that there is more pressure in your system than the valve&#;s set pressure. A quality inline safety relief valve testing system can help you here.
Properly setting, inspecting, and maintaining pressure relief valves in a hydraulic system is key to operating a safe facility.
A failing pressure relief valve is most often a symptom of a greater problem. In most cases, It is important to look at the &#;why&#; behind pressure relief valve failure, rather than just replacing the valve in question.
This article from the Journal of Emerging Technology and Advanced Engineering provides a helpful flowchart outlining the troubleshooting procedure to take in the event that you experience pressure relief valve failure.
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