The Value of Hydraulic Pressure Relief Valves

Hydraulic Valves, which are used to control hydraulic power by regulating the flow of fluid, are as complex as they are vital to a hydraulic system.

For more information, please visit Huade Hydraulic.

As control managers of the system&#;s flow, valves are capable of redirecting pressurized fluid, controlling flow to specific areas, or completely closing a line.

Valves have the ability to manipulate flow, the direction of flow, and perhaps most importantly, the amount of pressure coursing through a hydraulic system.

Hydraulic pressure relief valves not only limit the maximum pressure within a hydraulic circuit; they also create a new path for flow when that pressure exceeds a limit.

Let&#;s examine further how pressure relief valves operate to dispense built-up pressure, what can happen when they fail, and how to problem-solve any common issues.

What Do Pressure Relief Valves Do?

Pressure Relief Valves limit maximum pressure in a hydraulic circuit by opening an alternate path that relieves pressure that exceeds a preset level.

All fixed-volume circuits require a pressure relief valve to avoid excess pressure that can cause system failure. They can be direct-acting or pilot-operated to perform this function.

Pressure relief valves are the most widely used type of pressure control valve in nearly every hydraulic system.
How they work is pretty simple:

1. The relief valve opens when fluid pressure exceeds the set limit
2. Excess pressure is relieved
3. When the pressure falls, the valve closes

For more understanding, let&#;s examine how pressure relief valves appear on a schematic and in a simple circuit.

credit: learnmech.com/pressure-relief-valve-diagram-working/

In this example, a poppet is held inside the valve by a heavy spring. The poppet is forced off its seat when the pressure exceeds a set value, and flow is released through an outlet into a tank.

Why Are Hydraulic Relief Valves Important?

A pressure relief valve has the critical task of protecting a system from any overloads of actuators that can compromise the system. 

When a pump unloads or actuators are in motion, fluid movement is not a problem. A relief valve is essential when the actuators stall or to balance hydraulic force with spring force, opening in response to pressure and releasing heat. 

Without a pressure relief valve, when pump flow is stalled or stopped, the system would sustain damage, power wastage, overheating, or even failure due to excess pressure. 

Sticking, Leaking, and Common Pressure Issues

Relief valves can last up to 30 years when maintained properly. However, problems can arise that may need some troubleshooting. 

In the meantime, look for signs of common pressure issues, such as sticking or leakage, that can lead to pressure relief valve failure.

If the system is not reaching the designed pressure or is exceeding the maximum pressure, it can be a sign of valve issues. Checking the relief valve for common problems can determine the problem.

Here are some reasons these common issues may occur:

• Wrong calibration &#; the relief valve is calibrated to the wrong pressure
• Not closing fully &#; the valve can be worn or damaged after years of service, or debris may be preventing closure
• Sticking &#; Contaminants like dirt, dust, and corrosion can cause the valve to remain stuck in the closed position
• Leakage &#; If the valve is not the proper size, is not closing, or is damaged from debris or extreme temperatures, leakage can occur

Wear and corrosion, excess debris, and leakage may lead to pressure relief valve replacement. 

However, to avoid these issues, calibrate and install pressure relief valves properly and plan on performing routine checks and adjustments.

Here are 6 steps to use while setting, checking, and adjusting a hydraulic relief valve:

The company is the world’s best Hydraulic pressure valve supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

1. Consult schematics and locate the relief valve, taking note of which circuit may need adjusting 
2. Locate and remove the hydraulic hose on the system side of the relief valve
3. Connect a pressure gauge between the valve and pump (use an adapter if there is not a port installed)
4. Loosen the pressure relief valve as far as it will go until the pressure reading is as close to zero as it can get
5.  Adjust the relief valve
   &#; Turn the adjuster clockwise until the gauge pressure matches the schematic to &#;crack&#; pressure (the pressure at which the relief valve opens)
   &#; Tighten the adjuster lock nut securely
6. Shut down machinery and remove JIC plugs and caps, reconnecting any hoses removed in step 2, then start machinery and test the relief valve by starting the circuit and checking that pressure matches the set pressure from step 5

Knowing More About Pressure Relief Valves

With their ability to relieve the pressure coursing through the system, hydraulic pressure relief valves are vital preventative instruments that function to make sure damage is not done if pumps and actuators are halted.

Without a pressure relief valve present, the system can sustain damage, power wastage, overheating, or even failure due to excess pressure.

Hydraulic oil can do an incredible amount of work very efficiently. However, it also can cause extreme damage to pumps, motors, hose, and fittings when there is no safety relief valve. Every hydraulic system should have at least one, if not two, relief valves. One functions as the primary and the other is a backup. The main relief ensures the system does not work above a certain pressure. The backup relief valve kicks in should the main relief fail. Also, the backup valve usually is set 200-300 psi higher than the main relief.

Common relief valve failure scenario

In a mobile system, the control valves have pressure relief built-in to either the end stations or each individual operating station. This lets the machine designer customize the operation to the function rather than having one pressure set for all functions. In this example, we&#;ll consider a track-driven excavator. There may be 10 valves together in a manifold that are operating cylinders and motor functions. The system pump may deliver up to 5,000 psi, but the relief valve will limit it to 3,500 psi.

The cylinders operate the boom arms and the motors turn the turntable to get the bucket into the correct position. Motors also run the tracks, which propel the device. If a seal in one of the cylinders tears up internally, it can send debris into the hydraulic valve. This will clog the valve&#;s internal relief passage and cause the relief valve to not open.

When the system relief valve does not open, the pump will go to full pressure and a hose connection, which is only rated for 4,000 psi, will burst. Hydraulic fluid will spray onto the concrete around the machine and may inflict serious injury to those working nearby.

Damage and downtime costs

It will take several hours to clean up the hydraulic mess around the machine and replace the blown connection. It will take even longer for humans to heal from any injury due to hydraulic failure. There are many questions that need to be addressed, such as:

• How long would it take in your machine&#;s environment before techs discover the cause?
• Will the relief valve or the cylinder seal be replaced?
• Is the replacement hose connection at risk to blow again?
• How much does this downtime cost?
• How much do the oil, hose, and parts cost?

All of these questions prevent companies from moving forward with their operations.

Preventing common relief valve failure

A secondary safety relief valve, partnered with a pressure sensor or over-pressure warning device, would have safeguarded this system. When the primary relief fails, the secondary relief would only allow the system to go slightly above the working pressure and not high enough to fail components. Instead of going to 5,000 psi, the system might only have gone to 3,700 or 3,800 psi.

In this situation, the operator likely would have noticed a change in function efficiency from the equipment and have notified management or maintenance. Further investigation by maintenance would have solved the problems and the system would be fixed without any critical system failures, oil leakage, or injury.

To install a secondary relief in most systems, all that is required is to tee into the pressure line with the relief valve and add a line from the relief valve back to the reservoir. You should also add a gauge, but that&#;s optional. A few hoses and fittings can save hundreds or thousands of dollars.
Any time you are running over the relief valve, you are adding heat to the hydraulic system. The relief valve should be large enough to handle the flow of the system pump(s). The secondary relief valve can be smaller since it only will be used in an emergency. However, it still needs the ability to divert the complete amount of the pump flow.

Selecting the correct relief valve

Relief valves come in many shapes and sizes. Many designs start with a ball covering a hole called a spring relief valve. The ball is held in place by a spring and varying the amount of spring pressure will vary the amount of pressure that the ball will hold back.

Some valves are direct-acting, that is, with the media pressure acting directly against the ball or spool. There are pilot-actuated relief valves that use a much smaller operating surface and a spring to open and close a flow path, which operates the mechanism of a secondary flow path.

This secondary flow path can have a high flow or require precise operation. Direct-acting valves may experience system shock and bounce as they are &#;directly&#; in the flow path and the ball may bounce up and down off the seat as the pressure changes during the cycle. The pilot-operated valves act as a buffer against the bounce.

Relief valves can come with multiple settings. These are selectable by either solenoid valves such as on/off or proportional solenoid valves, or unlimited settings driven by programmable logic controller (PLC) input.

Paul Badowski, Cross Company. This article originally appeared on Cross Company&#;s blog. Cross Company is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, .

MORE ANSWERS

Keywords: process valves, actuators, hydraulics

Users should have at least two relief valves in a hydraulics application: one as a primary and one as a backup.

Relief valves can help prevent major safety incidents from occurring.

Knowing what kind of relief valve is best for a particular application is also part of the selection process.

Consider this

Contact us to discuss your requirements of hydraulic flow control valve. Our experienced sales team can help you identify the options that best suit your needs.