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.
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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.
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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:
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.
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.
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.
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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.
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.
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.
A hydraulic control valve regulates the flow of fluid to enable various types of functions within a hydraulic system. The valve is designed to adjust flow rate by adapting the rate of energy transfer. By doing so, it can reduce or increase the speed of a motor, actuator, cylinder, or other device. Similar valves are used in faucets and showers.
Another benefit is a hydraulic valve can depressurise part of a hydraulic circuit to, for example, allow hose fittings or other components to be changed. Changing the flow rate is the primary means of managing a control valve&#;s performance. There are different ways of measuring this, including volumetric flow rate (units of volume per unit time), weight flow rate (units of weight per unit time), or mass flow rate (mass per unit time).
Different valves serve different purposes. Therefore, one can understand the need for specific types of valves depending on the application.
A directional control valve is designed to start, pause, stop, and change the direction of flow of fluids in a hydraulic system. Also called a switching valve, it is identified based on the number of working ports and spool positions (a 2/2 valve has two ports and two positions and a 4/3 has four ports and three positions). Spools can change positions within the valve body to manage fluid flow. While a simple binary valve either blocks or allows fluid flow, a three-position valve can block all ports to stop fluid, extend the cylinder, or retract it.Pressure control valves release excess pressure from the hydraulic system. They have relief, reduction, sequencing, counterbalancing, and unloading functions, depending on the valve. By regulating pressure, the valve can prevent leakage. It can also help you avoid a burst pipe or tube.Flow control is a function that regulates flow rate to change actuator speed. The flow rate also influences the rate of energy transfer at the achieved pressure level. Both fluids and gases can be controlled/adjusted while backflow towards components is prevented. Flow control valves come in fixed flow, adjustable flow, throttling flow, and pressure-compensated flow control models.Flow control valves help run automated processes in factories, operate warehouse equipment, and manage various systems in food processing and materials handling facilities. Numerous types have been developed to accommodate specific applications and equipment. Some common hydraulic control valve types include:The two subtypes include parallel gate valves, with a flat gate placed in-between two parallel seats, and wedge-shaped valves with two inclined seats that are slightly mismatched with an inclined gate.Choosing a hydraulic control valve requires determining the best one for your application. In addition to type, factors to consider include the valve configuration, media type, port size/type, operating voltage, flow rate, operating pressure, and temperature range. From plumbing to aerospace, hydraulic valves are found in many applications.At White House Products, Ltd. , we supply a wide range of hydraulic valves from leading manufacturers. Browse our online catalog or call +44 (0) for help finding the product with your required specifications. Back to blog posts
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