Relief valves are the most common type of valve found in a hydraulic system. Their purpose is to prevent high pressure in the hydraulic circuit from damaging the system components. Relief valves are normally closed; whenever one is open, it is converting the pressure energy of the hydraulic system directly to heat. High oil temperatures can also cause damage to hydraulic components, so any system designed to have the relief valve open a significant amount of the time needs to plan for heat management.
To better understand why we need a relief valve let’s look at a typical open loop hydraulic system. With the pump running weare creating flow from the tank out to the circuit and back to the tank. When we try to do some work with this flow, the pressure rises because the work creates a resistance to flow. The pump continues to pump so the pressure rises until it can overcome the resistance. Always remember pumps create flow and tolerate pressure. Pumps do not create pressure; the pressure comes from the resistance to flow. When the flow is restricted, pressure intensifies at the speed of sound so restrictions in the system result in pressure rising very quickly. Normally the pressure rises to what is needed to do the work, such as lifting the cylinder or turning the motor. If the flow is blocked or cannot overcome the load, pressure will rise until it breaks the system components. The relief valve is there to open a path for oil to flow back to the tank, preventing the system pressure from damaging the components.
There are two basic types of relief valves commonly found in hydraulic systems: direct acting and pilot operated. Selecting the correct one can prevent issues and make your system run better. Before we get into the different types of relief valves there are some terms we need to review so we can better understand how relief valves work.
- Cracking Pressure: This is the pressure where the first drop of oil starts to pass through the relief valve back to the tank.
- Full Flow Pressure: This is the pressure required to allow all the pump flow across the relief valve back to the tank.
- Reseat Pressure: This is the pressure where the relief fully closes after being open. This pressure is typically less than cracking pressure.
Direct Acting Relief Valves
A direct acting relief valve is most simply understood as a poppet held on a seat by a spring. The pressure setting is adjusted by pre-tensioning the spring to the desired setting. The other side of the poppet is exposed directly to the high-pressure oil. When the pressure is high enough to start pushing the poppet off the seal, we have reached cracking pressure. As the pressure continues to rise the poppet is pushed farther and farther off the seat until all the pump flow is going to the tank. This is full flow pressure.
The difference between cracking and full flow pressure is dependent on the K factor of the spring and the flow rate. K factor defines the stiffness of the spring. A higher K factor spring requires more force to collapse the spring a set distance. The higher the flow rate, the farther the spring needs to collapse to reach full flow. As always, you need to select a valve appropriate for the flow rate your system requires. If your relief is too small for the flow rate, you could see extremely high full flow pressures that can cause premature failure of system components.
Direct acting relief valves tend to be very fast to respond to changes in system pressure. Relief speed is typically found in the literature provided by the valve manufacturer. It is measured in milliseconds (ms). There are 1000 ms in 1 second so it is a very short amount of time. Direct acting reliefs can be as fast as 2ms, but typically they are 5-10ms. The only way to know how fast your valve responds is to look it up in the manufacturer’s literature. Typically, the part number is marked on the body of the valve. The other thing to know about direct acting reliefs is that they tend to be much noisier than a pilot operated relief. When you hear a relief valve squealing it is most likely a direct acting relief.
Pilot Operated Relief Valves
Pilot operated relief valves are also referred to as a balanced relief. These relief valves use a pressure balance to control a larger internal spool. Adjacent to the spool is a very small direct acting relief sensing the system pressure. When the internal direct acting relief cracks open, it upsets the balance holding the spool in position, which causes the spool to move, opening an oil path to the tank. Pilot operated relief valves tend to be much slower than direct acting reliefs because it takes time for the spool to move once the balance is lost. Typically, this time to react is around 100ms, 10X longer than a direct acting relief. The advantage is once the balance is lost the valve opens to full flow at a very small pressure differential. Crack to full flow pressure with a pilot operated relief valve is typically 50-100 PSI.
Direct Acting vs. Pilot Operated
When selecting a relief valve there are many tradeoffs to consider. Direct acting reliefs tend to be very responsive to changes in system pressure; in other words, they open very fast. The drawback is they tend to have a high-pressure delta between crack and full flow pressure. Depending on the size of the valve, pressure can rise 500 PSI higher than crack pressure to reach full flow. This can make it difficult to protect sensitive components from being damaged by excess pressure. Direct acting reliefs also tend to be noisy. They can make a high-pitched squeal when they are passing oil to the tank which can be annoying to people who work around the equipment.
Pilot operated valves are typically much slower to react to changes in system pressure because of the time it takes to get the valve open. The advantage is that, once unbalanced, they have a very small delta from crack to full flow. Pilot operated relief valves make noise as well, but it is a much duller sound that is less distracting.
The reason this matter is that hydraulic cylinders are a commonly used hydraulic actuator. Inherent to their design is that when you reach the end of stroke, you have effectively blocked pump flow. When this happens, the pressure in the system starts intensifying at an incredible rate, putting a lot of stress on the pump, conductors and valves in the system. The faster the relief opens, the lower the pressure spike will be. When hooked up to data recording systems, we have seen short pressure spikes beyond 6000 PSI on systems with functioning reliefs setting at 3000 PSI crack. These spikes occur during the 100 ms it takes to get a pilot operated relief valve open. In many cases, the solution is to add a small direct acting relief valve to the system. It opens very quickly, limiting the pressure spike during the time it takes the slower pilot operated relief valve to open. Using both types of relief in the system gives us the benefits of both. The pump takes less damage from spikes and has a much longer service life, while maintaining the benefits of a pilot operated relief valve.
Choosing the Right Relief Valve
At HSI, we have helped customers in industrial settings where a noisy relief valve can be a real nuisance for the staff that have to work around it. Switching to a pilot operated relief can eliminate the high-pitched squeal. For the reasons explained above, you need to understand the hydraulic circuit to determine if the pilot operated relief will be fast enough to protect the system. If you’re uncertain that your pilot operated relief valve is up to the task, your best option may be to pair it with a small direct acting relief to mitigate the damage from pressure spikes.