Common working conditions such as sudden load changes during excavator operation and pressure fluctuations during machine tool processing, may cause a sudden increase in hydraulic system pressure. The hydraulic relief valve activates rapidly to release pressure when the pressure exceeds the limit.
Now, let’s delve into the hydraulic relief valve and see how it safeguards stable production in various industries.
What is a Hydraulic Relief Valve?
The hydraulic relief valve is one of the most widely used pressure control valves. It is an automatic safety protection device installed in the hydraulic system. The hydraulic relief valve can monitor and control the internal pressure of the system, preventing it from exceeding the preset safety limit.
How does a Hydraulic Relief Valve Work?
Under normal operating conditions, the hydraulic relief valve is in the closed position. At this moment, the hydraulic pressure acting on the valve core is less than the spring force at the other end. The valve core is tightly pressed against the valve seat by the spring force, which prevents the hydraulic oil from flowing from the inlet to the outlet.
When the system pressure rises and exceeds the preset value, the hydraulic pressure overcomes the spring force. The hydraulic pressure pushes the valve core upwards and opens the overflow channel.
Excess oil that exceeds the pressure limit will flow through the overflow channel of the hydraulic relief valve and be returned to the hydraulic tank or the low-pressure circuit.
As the oil leaks out, the system pressure drops. When it drops below the set value, the spring will then push the valve core back to the valve seat and close the oil discharge channel. The system will return to its normal working condition.
Hydraulic Relief Valve Types
Hydraulic Direct Acting Relief Valve
The hydraulic direct acting relief valve has a simple structure. The system pressure acts on one end of the valve core, while the other end is held in place by an adjustable spring.
When the system pressure rises and exceeds the preset force of the spring, the liquid pressure will push the valve core open. This allows the oil to flow back to the oil tank and limits the further increase in pressure.
Hydraulic direct acting relief valves have fewer parts and lower manufacturing costs. And its response speed is extremely fast, usually less than 10 ms.
Due to the limited spring stiffness, the pressure stability of the hydraulic direct acting relief valve is poor. The pressure fluctuates significantly during overflow.
In high pressure conditions, it is necessary to equip large stiffness springs. This may lead to unstable and inaccurate pressure regulation, and the valve core will vibrate.
Therefore, the hydraulic direct acting relief valve is suitable for low pressure and low flow scenarios. For example, you can install it on overload protection branches of small hydraulic fixtures, machine tool auxiliary hydraulic circuits, or construction machinery.
Hydraulic Pilot Operated Relief Valve
The hydraulic pilot operated relief valve adopts a two-stage control structure. It consists of a pilot valve and a main valve. The pilot valve is usually a small direct-acting relief valve.
The system pressure acts on both the main valve core and the pilot valve simultaneously. When the pressure reaches the set value, the pilot valve opens first, allowing a small amount of control oil to flow out.
This causes a pressure imbalance between the top and bottom of the main valve core. Then the main valve is opened to carry out a large flow of pressure relief.
The biggest advantage of this design is that it only requires a small pilot valve spring force to control high-pressure, high-flow oil accurately and stably.
The structure of the hydraulic pilot operated relief valve is relatively complex, and its response speed is about 50-100 ms. It is suitable for systems with medium to high pressure and high flow rate.
For example, you can use it in the main hydraulic circuit of injection molding machines, large construction machinery, or precision hydraulic equipment.
According to the Classification of Valve Core Structure
There are two common types of valve cores in hydraulic relief valves: the poppet valve and the spool valve.
- The Poppet Valve is fitted with a conical valve core and valve seat. This structure determines that its sealing performance is extremely excellent. The hydraulic relief valve with the poppet valve structure responds very quickly to pressure changes and is capable of achieving fast and precise pressure control.
- The Spool Valve consists of a cylindrical spool that slides axially inside the valve body. The hydraulic relief valve with a spool valve structure can achieve more stable and precise pressure regulation. When the flow rate changes significantly, the pressure fluctuation of the system is smaller. Meanwhile, the cylindrical structure is also more sturdy and durable. It can handle a larger flow rate.
How to Adjust the Hydraulic Relief Valve?
Determine Position
After finding the hydraulic relief valve in the system, you need to confirm the position of its adjustment screw or knob and lock nut. Most hydraulic relief valves set the opening pressure through an adjusting screw and locking structure.
Loosen and Start Adjusting
You need to loosen the locking nut first, and then turn the adjusting screw slowly. The direction of pressure regulation is usually:
- Clockwise: Increase the opening pressure of the valve. The system allows for a higher pressure to be reached before the release pressure.
- Counter-Clockwise: Reduce the opening pressure. This means that the pressure will be released earlier.
When adjusting the screws, you should pay attention to making small adjustments each time to avoid turning them too much at once. We suggest that the adjustment range should not exceed 1/4 turn each time.
Monitoring Pressure while Working
After restoring the system to working condition, you need to observe the pressure gauge reading. Then slowly increase the pressure until it reaches the relief pressure you have set. At this point, you should record the pressure and confirm that the valve initiates the pressure relief action at that pressure value.
Tighten and Verify
After reaching the appropriate set pressure, you can tighten the locking nut to fix the setting. Then release the system pressure and conduct 2-3 tests to confirm that the relief valve opens and closes accurately at the set pressure. At the same time, you need to observe the stability and noise of the system in the overflow state.
Final Confirmation and Recording
After completing the adjustment, you should record the set pressure value for subsequent maintenance and inspection.
Hydraulic Relief Valve Troubleshooting
Abnormal Pressure
During the usage process, you may encounter situations where the system pressure fails to reach the set value, the pressure fluctuates sharply, or the pressure exceeds the set value but cannot be relieved.
Common Reasons
Hydraulic oil contamination is one of the main causes. Impurities in the oil can block the valve core or damping hole and lead to pressure transmission failure.
The pressure regulating spring may experience fatigue, deformation, and even damage due to long-term pressure impact. This will lead to fluctuations in the set pressure.
In addition, if the inlet and outlet ports are installed in the wrong position or the locking nut is loose, it will also cause abnormal system pressure.
Troubleshooting Steps
You can first check and replace the contaminated oil and filter element. Then tighten the locking nut and recalibrate the pressure. If the problem persists, you may need to disassemble the valve. Check and clean or replace the deformed springs, clogged valve seats, and other damaged parts.
Abnormal Noise and Vibration
If you hear harsh knocking, buzzing, or cavitation sounds accompanied by valve body vibration, it will affect the operating environment and exacerbate component wear.
Common Reasons
This type of problem is often caused by cavitation due to the mixing of air or high oil flow rate in the system.
Resonance of the pressure regulating spring or loose pipeline connections during installation can also cause valve body vibration accompanied by abnormal sounds.
Troubleshooting Steps
You can expel the air by using the exhaust valve or by having the actuating element move back and forth. If cavitation occurs, you can try to increase the diameter of the return oil pipe to reduce the flow resistance. At the same time, tighten the loose pipes and valve bodies, and replace the bent and worn-out springs.
Leak Problem
Leakage is divided into external leakage and internal leakage. The external leakage is manifested as obvious oil leakage at the joint, while internal leakage will cause unstable system pressure and weak performance of the actuating elements.
Common Reasons
External leakage is usually caused by aging and wear of seals or O-rings. Internal leakage may be caused by deformation of the valve body or damage to the sealing surface between the valve core and valve seat.
Troubleshooting Steps
When encountering leakage, you can replace the aging seal. At the same time, tighten the valve body bolts evenly to avoid further leakage caused by uneven stress.
If you suspect an internal leak, you need to disassemble the valve and grind the worn sealing surface. If the clearance exceeds the tolerance, it is recommended that you replace the valve core component.
Valve Core Stuck
You may encounter situations where the hydraulic relief valve does not open and close smoothly, the pressure response lags behind, or there is a sudden increase in pressure. This is usually caused by the valve core getting stuck.
Common Reasons
If you don’t replace the oil and clean the filter element regularly, the impurities in the oil will get stuck in the valve core and hinder its normal movement.
If the system oil temperature is too high or too low, it will affect the viscosity of the oil and cause the valve core to operate slowly.
Furthermore, if you tighten the bolts too much during assembly, it may cause the valve hole to become eccentric, which could also lead to the valve core getting stuck.
Troubleshooting Steps
You can use specialized cleaning agents to clean the valve core and chamber thoroughly, and ensure that the system oil temperature is maintained within the normal range.
When reassembling, you must tighten the bolts according to the specified torque. If the valve core has severe scratches, it should be replaced immediately.
The Application of Hydraulic Relief Valves
Installed near the Outlet of the Hydraulic Pump
The hydraulic relief valve installed near the hydraulic pump or at the outlet is usually referred to as the system’s main safety valve or the main relief valve. Its function is to safeguard the safety of the entire hydraulic power system and main pipeline. Usually, the set pressure is the maximum working pressure of the system.
When the system pressure rises abnormally and reaches its set value, the hydraulic relief valve will open immediately. It will guide the oil discharged by the pump to the oil tank and prevent the key components such as the pump, motor and core pipelines from being damaged due to overpressure.
Installed on the Downstream Branch
Hydraulic pressure relief valves can also be installed downstream of specific branches or assembled on actuators such as hydraulic cylinders and motors. In complex hydraulic systems, relying only on the main hydraulic relief valve is difficult to avoid all risks.
For example, if a downstream valve accidentally gets stuck, it may cause a sudden increase in local pipeline pressure. The cylinder bears the sudden loads or the inertial forces generated when the motor brakes. These factors could also cause local pressures to exceed the overall safety limits of the system.
You can install the hydraulic relief valve at these key points to enable targeted monitoring of local pressure. When the pressure exceeds the limit, it will activate the pressure relief function to prevent damage to a single actuating element or a branch circuit. At the same time, this will not affect the normal operation of other parts of the system.
Common Application Scenarios
In hydraulic circuits of excavators, loaders, cranes, and other equipment, hydraulic relief valves are often installed at the outlet of hydraulic pumps and next to actuating components such as oil cylinders and hydraulic motors.
When there is a sudden change in load or mechanical jamming during heavy excavation or heavy lifting, the system pressure will increase instantly. The hydraulic relief valve will open promptly to guide the excess oil back to the oil tank. This will prevent damage to the pump body or the oil cylinder.
If the pressure is too high during the clamping process of the injection molding machine, it may cause damage to the mold or defects in the product. The hydraulic relief valve can limit the maximum pressure.
In agricultural machinery, hydraulic circuits for tractors, harvesters, and irrigation systems all rely on hydraulic pressure relief valves. It can work stably in harsh outdoor environments such as dust, vibration, and temperature fluctuations.
The Difference between a Hydraulic Relief Valve and a Pressure Reducing Valve
Function
- The main function of a hydraulic relief valve is to prevent excessive system pressure. It protects pumps, pipelines, and actuators from being damaged by high pressure.
- The function of the pressure reducing valve is to lower the high pressure fluid from the upstream to the required low pressure at the downstream end. And it maintains this pressure stability continuously. When the main system pressure is high, but some components can only withstand lower pressure, the pressure reducing valve comes into play.
Working Principle
- The hydraulic relief valve is usually in a normally closed state. It does not operate when the system is functioning normally and the pressure does not exceed the set value.
When the system pressure exceeds the set value, the pressure relief valve opens to release pressure. After the pressure returns to a safe level, the valve closes and the system returns to normal.
- The pressure reducing valve is usually in a normally open state, and hydraulic oil can flow to the downstream normally.
When the downstream circuit pressure reaches the set value, the pressure reducing valve closes partially to maintain a constant downstream pressure. If the downstream pressure drops, the valve opens to replenish the pressure.
That is to say, the pressure reducing valve monitors the downstream pressure continuously and adjusts the flow area dynamically to stabilize the output pressure.
Application Scenarios
- Hydraulic pressure relief valves are mainly used as safety protection devices. Whether it is large industrial equipment, construction machinery or ordinary hydraulic circuits. When your system needs to prevent damage caused by overpressure, the hydraulic relief valve should be selected.
- When your system requires different pressure levels, a pressure reducing valve should be selected. For example, your main system is under high pressure, but some components cannot withstand such high pressure or need to stabilize at a lower pressure. You will need a pressure reducing valve.
FAQs
Is there any Requirement for the Installation Direction of the Hydraulic Relief Valve?
In most cases, hydraulic relief valves can be installed horizontally, vertically, or diagonally. You should install it according to the arrow indications on the valve body to avoid reverse installation of the inlet and outlet ports.
Can a High-Flow Hydraulic Relief Valve be Used in a Low-Flow System?
We do not recommend using high-flow valves in low-flow systems. This will affect the control accuracy and lead to waste.
The valve core structure and spring stiffness of the high-flow hydraulic relief valve are both designed to meet the requirements of high-flow relief. When used in low flow systems, a small amount of oil cannot push the valve core to achieve stable opening and closing.
In addition, the volume and cost of high flow hydraulic relief valves are higher than those of low flow models.
How to Establish the Frequency of Maintenance and Inspection for Hydraulic Relief Valves?
Based on the usage intensity of the equipment, you can observe the valve body weekly or monthly for oil leakage, corrosion, or damage.
You can conduct a functional test once every quarter or once every half year. The specific method is to let the system reach a certain pressure and observe whether the hydraulic relief valve can start according to the set value. And check whether the hydraulic relief valve can be fully closed after the pressure returns to normal.
If your hydraulic system operates in harsh environments, the frequency of inspections should be increased.
Target Hydraulics, the Professional Manufacturer of Hydraulic Relief Valves
The hydraulic pressure relief valve is the key to maintaining equipment safety. Its response is quick and it can control the pressure precisely. The sturdy design ensures long-lasting durability and reduces maintenance costs.
We have a wide range of standard models of hydraulic relief valves. According to the specific operational requirements, we can also offer customized services tailored to your needs. Please feel free to contact us！
