CHAPTER 9: Relief and Unloading Pressure Controls

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Pressure-control valves

Several types of pressure-control valves are found in fluid power circuits. Some keep the whole system from excess pressure while others only protect a portion of the system. Others allow flow to an isolated circuit after reaching a preset pressure. Some bypass fluid at low or no pressure when activated.

This chapter only covers relief valves and unloading valves because they are closely associated with hydraulic pumps. The other pressure-control valves are part of the control circuit and will be dealt with after directional control valves.

Why relief valves?

All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. When a pump is unloading through an open-center circuit or actuators are in motion, fluid movement is not a problem. It is when the actuators stall with the directional valve still shifted that a relief valve is essential.

Pressure compensated pump circuits could run successfully without relief valves because they only move fluid when pressure drops below their compensator setting. (Most designers still use a relief valve in these circuits for reasons explained later.)

In either case, a relief valve is similar to a fuse in an electrical system. When circuit amperage stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse amperage, the fuse blows and disables the circuit. Both devices protect the system from excess pressure by keeping it below a preset level.

The difference is that when an electrical fuse blows it must be reset or replaced by maintenance personnel before the machine can cycle again. This requirement alerts the electricians to a possible problem and usually causes them to look for the reason before restarting the machine. Without the protection of a fuse, the electrical circuit would finally overheat and start a fire.

In a hydraulic circuit, a relief valve opens and bypasses fluid when pressure exceeds its setting. The valve then closes again when pressure falls. This means a relief valve can bypass fluid anytime . . . or all the time . . . without intervention by maintenance. (It also means the system can run hot even with a heat exchanger installed.)

Many fixed-volume pump circuits depend on this bypassing capability during the cycle, and some even bypass fluid during idle time. A well-designed circuit never bypasses fluid unless there is a malfunction, such as a control switch not closing or an operator overriding the controls. This eliminates most overheating problems and saves energy.

Relief valve operation

There are two different designs of relief valves in use: direct acting and pilot operated. Both types have advantages and work better in certain applications.
Some terms relating to relief valves and their function are:
&#; Overshoot: The actual pressure reading when a relief valve first opens to bypass fluid. (It can be up to twice the actual pressure setting.)
&#; Hysteresis: The difference in pressure between when a relief valve starts letting some flow pass (cracking pressure) and when full flow is passing.
&#; Stability: The fluctuation of pressure as a relief valve is bypassing at set pressure.
&#; Reseat pressure: The pressure at which a relief valve closes after it has been bypassing.
&#; Pressure override: The difference in the pressure reading from the time a relief valve first opens (cracking pressure) until it is passing all pump flow to tank.

Direct-acting relief valves

Figure 9-1 shows a cutaway view and the symbol for a direct-acting relief valve. The valve has a poppet that is pressed against its seat by an adjustable spring. An adjusting knob can be change the force on the spring to raise or lower maximum pressure. The poppet remains seated while pump flow goes to the circuit and pressure is lower than the relief valve setting. If pressure tries to go above spring setting, the poppet is forced off the seat just enough to pass excess pump flow to tank.

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.

Steps to Setting a Pressure Relief Valve

• Step #1: Consult and review the machine drawings to figure out which circuit needs adjustment. Locate the relief valve for the circuit. Relief valves are always piped in parallel to the pump and are normally as close to the pump as possible.
• Step #2: Locate and remove the hydraulic hose on the system side of the relief valve. Cap off the hose and valve with the correct JIC caps or plugs. Do not cap off the return or tank side of the relief valve. Capping off or plugging unused hoses and fittings prevents the loss of hydraulic fluid and system contamination. Using any method of capping or plugging a hose other than with the proper JIC plug or cap is unsafe and should never be attempted. This deadheads the hydraulic circuit in order to isolate the system to just the pump and relief valve.
• Step #3: Connect a pressure gauge between the relief valve and the pump. Most equipment will have a port already installed for this purpose. If there is no port, use an adapter to install the gauge.
• Step #4: Loosen the pressure relief valve adjustment as far as it will go. The relief valve will normally have a locking hex nut and an Allen head adjuster or hand wheel adjuster. Start the equipment and activate the hydraulic circuit. The pressure reading on the gauge should be close to zero.
• Step #5: Adjust the relief valve by turning the adjuster clockwise until the reading on the gauge builds to the pressure called for on the machine drawings. This is what is known as the valve "cracking" pressure, which is the pressure at which the relief valve starts to open. Tighten the adjuster lock nut securely, being careful not to disturb the valve setting.
• Step #6: Shut down the machinery and allow the pressure to bleed off. Remove the JIC plugs and caps, and reconnect any hoses removed in Step 2. Start the machinery, and test the relief valve by starting the circuit. The pressure reading on the circuit should not rise above the pressure that was set on the relief valve.

3 Signs of Pressure Relief Valve Failure

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.

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.

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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.

Here are 3 signs of pressure relief valve failure to watch out for.

Sign #1: Cannot Reach Pressure

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 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.

Sign #2: Over Maximum Pressure

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.

Sign #3: Relief Valve is Leaking

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.

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.

Conclusion

Properly setting, inspecting, and maintaining pressure relief valves in a hydraulic system is key to operating a safe facility.

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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