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, email@example.com.
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.
What else needs to be considered when implementing or adding relief valves in a hydraulic application?