What is a relief valve in a hydraulic system?

Tank pressure control safety device

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Like other pressure relief valves (PRV), pilot-operated relief valves (PORV) are used for emergency relief during overpressure events (e.g., a tank gets too hot and the expanding fluid increases the pressure to dangerous levels). PORV are also called pilot-operated safety valve (POSV), pilot-operated pressure relief valve (POPRV), or pilot-operated safety relief valve (POSRV), depending on the manufacturer and the application. Technically POPRV is the most generic term, but PORV is often used generically (as in this article) even though it should refer to valves in liquid service.

In conventional PRVs, the valve is normally held closed by a spring or similar mechanism that presses a disk or piston on a seat, which is forced open if the pressure is greater than the mechanical value of the spring. In the PORV, the valve is held shut by piping a small amount of the fluid to the rear of the sealing disk, with the pressure balanced on either side. A separate actuator on the piping releases pressure in the line if it crosses a threshold. This releases the pressure on the back of the seal, causing the valve to open.

The essential parts of a PORV are a pilot valve (or control pilot), a main valve, a pilot tube, the dome, a disc or piston, and a seat. The volume above the piston is called the dome.[1]

Mode of functioning

The pressure is supplied from the upstream side (the system being protected) to the dome often by a small pilot tube. The downstream side is the pipe or open air where the PORV directs its exhaust. The outlet pipe is typically larger than the inlet. 2 in × 3 in (51 mm × 76 mm), 3 in × 4 in (76 mm × 102 mm), 4 in × 6 in (100 mm × 150 mm), 6 in × 8 in (150 mm × 200 mm), 8 in × 10 in (200 mm × 250 mm) are some common sizes.

The upstream pressure tries to push the piston open but it is opposed by that same pressure because the pressure is routed around to the dome above the piston. The area of the piston on which fluid force is acting is larger in the dome than it is on the upstream side; the result is a larger force on the dome side than the upstream side. This produces a net sealing force.

The pressure from the pilot tube to the dome is routed through the actual control pilot valve. There are many designs but the control pilot is essentially a conventional PRV with the special job of controlling pressure to the main valve dome. The pressure at which the control pilot relieves is the functional set pressure of the PORV. When the pilot valve reaches set pressure it opens and releases the pressure from the dome. The piston is then free to open and the main valve exhausts the system fluid. The control pilot opens either to the main valve exhaust pipe or to atmosphere.

Snap acting

At set pressure the valve snaps to full lift. This can be quite violent on large pipes with significant pressure. The pressure has to drop below the set pressure in order for the piston to reseat (see blowdown in relief valve article).

Modulating

The pilot is designed to open gradually, so that less of the system fluid is lost during each relief event. The piston lifts in proportion to the overpressure. Blowdown is typically short.

Comparison to conventional pressure relief valves

Advantages

• Smaller package on the larger pipe sizes.
• More options for control.
• Seals more tightly as the system pressure approaches but does not reach set pressure.
• Control pilot can be mounted remotely.
• Some designs allow for changes in orifice size within the main valve.
• Can be used in engines.
• Higher Operating-to-Set Pressure ratio.

Disadvantages

• More complex, resulting in various fail-open failure modes.
• More expensive at smaller sizes (starts to even out as pipe size increases).
• Small parts in pilot valve are sensitive to contaminant particles.
• Soft goods typically installed in pilot valves cannot withstand higher temperature fluids.

See also

References

Hydraulic pressure relief valves are the most common type of valve found in a hydraulic system. There are two basic types that can help prevent high pressure in the hydraulic circuit from damaging the system components. 

Relief valves are normally closed. Whenever one is open, it converts 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. In this guide, we will delve into what direct-acting and pilot-operated relief valves are and which one may be right for your hydraulics system.

Hydraulic System Inc. has the hydraulic system components and parts you need to make your hydraulics run efficiently.

How Does A Pressure Relief Valve Work?

In open-loop hydraulic systems, pumps create flow and tolerate pressure, but they do not generate pressure themselves; pressure arises from resistance to flow. When work is applied, pressure rises due to this resistance. If the flow is restricted, the pressure intensifies rapidly, risking damage to system components. The primary purpose of a relief valve is to act as a safety mechanism, diverting oil flow back to the tank to prevent pressure-induced damage.

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. There are three types of pressure that hydraulic pressure release valves help maintain:

• 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 hydraulic 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.

You need to select a valve appropriate for the flow rate your hydraulic 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.

What Is A Direct Acting Relief Valve?

A direct acting relief valve is a poppet held on a seat by a spring. The other side of the poppet is exposed directly to the high-pressure oil. Cracking pressure occurs when the pressure is high enough to start pushing the poppet off the seal. As the pressure increases, the poppet is pushed farther and farther off the seat until all the pump flow goes to the tank. This is full-flow pressure.

Direct acting 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 ms in 1 second; Direct acting reliefs can be as fast as 2 ms but typically are 5-10 ms. 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.

What Is A Pilot Valve?

Pilot pressure relief valves, also referred to as balanced 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, it reacts in around 100ms, ten times longer than a direct acting relief. Once the balance is lost, the valve opens to full flow at a very small pressure differential. Crack pressure to full flow pressure with a pilot operated relief valve is typically 50-100 PSI.

Do you own a heavy-duty truck? Learn more about fluid powers for heavy-duty truck hydraulic systems.

The Main Differences Between Direct Acting Vs. Pilot Operated

Direct-acting reliefs tend to be very responsive to changes in system pressure, opening very fast. The main 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.

Pilot operated valves are typically much slower to react to changes in system pressure because of the time it takes to open the valve. The key advantage is that once unbalanced, they have a very small delta from crack pressure to full flow. Pilot operated pressure relief valves make noise as well, but it is a much duller sound that is less distracting.

Why Does Relief Speed Matter? 

Having a small delta from crack pressure to full flow is very important. Hydraulic cylinders are a commonly used hydraulic actuator. When you reach the end of a stroke, the pump flow has been blocked. 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 PSI on systems with functioning reliefs setting at 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 pressure 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.

Hydraulic Specialty Has The Right Relief Valve For Your Hydraulics

At Hydraulic Specialty Inc., we help customers in industrial settings where noisy relief valves can be a real nuisance for the staff working around. Switching to a pilot-operated relief can eliminate the high-pitched squeal. Before going with a pilot operated relief valve, you need 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.

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