When you are designing a hydraulic system or making a purchase, you may confuse the hydraulic pressure relief valve and the pressure reducing valve. Incorrect valve selection can lead to overpressure risks or unstable downstream pressure. This article will help you understand the differences between these two types of valves.
What is the Pressure Relief Valve?
The pressure relief valve is a pressure control device. It is used to prevent the hydraulic or fluid system from exceeding the preset maximum pressure. When the system pressure is higher than the set value, the valve automatically opens and releases the excess fluid. It can protect pumps, pipes, actuators and other components from overload damage.
What is the Pressure Reducing Valve?
The pressure reducing valve is a type of pressure regulating device. It automatically regulates the flow of fluid, reducing and stabilizing the high, fluctuating input pressure to a specific low output value.
Pressure Relief Valve vs Pressure Reducing Valve
Function
Pressure Relief Valve
The main function of the pressure relief valve is to limit the maximum pressure in the hydraulic system. It remains in a closed state under normal conditions. When the system pressure exceeds the set value, the valve will automatically open and redirect the excess hydraulic oil back to the hydraulic tank or the return pipeline.
In some hydraulic systems, when the actuator reaches the end of its stroke, the load suddenly increases, or the oil passage is blocked, the system pressure may rise suddenly. The relief valve is designed to provide overload protection for your entire fluid system.
It can help you avoid potential hazards such as pipeline leakage and seal damage caused by system overpressure. At the same time, it also reduces unplanned downtime due to equipment failures and lowers your maintenance costs and downtime losses.
Pressure Reducing Valve
The main function of the pressure reducing valve is to reduce the high pressure of the main system and maintain the downstream circuit at a stable set pressure level continuously. When the outlet pressure changes, the valve will automatically adjust the size of the internal opening to maintain a stable working pressure.
Unlike the pressure relief valve that only opens when there is overpressure, the pressure reducing valve remains open in normal operation. At the same time, the pressure reducing valve is more focused on maintaining continuous pressure control for local branches.
In your hydraulic system, not all actuators require the same pressure. For example, clamping devices, lubrication circuits, or precision hydraulic actuators usually require lower working pressure than the main system.
If you do not use a suitable pressure reducing valve, excessive pressure may lead to a decrease in positioning accuracy, pipeline leakage, and even shorten the service life of seals and actuators.
A stable pressure output helps you improve production consistency, especially in high-pressure precision requirements conditions such as hydraulic machines and automated production lines. And reasonable pressure reduction can also reduce unnecessary energy loss and lower your long-term operating costs.
Working Principle
Pressure Relief Valve
The pressure relief valve is usually composed of a valve body, a valve spool, a spring, a pressure regulating mechanism, and internal oil passage channels.
When the system pressure continues to rise and exceeds the set value, the liquid pressure exerted on the valve spool will be greater than the spring’s preload force. The valve spool begins to move, opening the return flow path, allowing some hydraulic oil to flow back to the oil tank or the oil return circuit, thereby limiting the further increase of the system pressure.
Once the system pressure returns to the safe range, the valve will automatically close. There are two types of pressure relief valves: direct-acting and pilot-operated pressure relief valves.
Pressure Reducing Valve
A pressure reducing valve is usually composed of a valve body, a valve spool, a pressure regulating mechanism and a pressure feedback device. When hydraulic oil enters the valve body and flows downstream, the pressure will be continuously fed back to the control chamber of the valve spool and compared with the spring setting force.
When the downstream pressure gradually increases and approaches the set value, the valve spool begins to move and reduces the flow cross-section, stabilizing the pressure output through throttling. When the pressure exceeds the set value, the valve further adjusts the opening degree to maintain the downstream pressure within the target range you have set.
Installation Location
Pressure Relief Valve
The pressure relief valve is usually installed at the outlet of the hydraulic pump or at the beginning of the main pressure oil circuit, where the system pressure is first formed. It is usually installed in parallel with the main oil circuit.
This enables the pressure relief valve to sense the highest pressure of the entire system at the earliest time and release it when the pressure exceeds the set value, thereby protecting all the hydraulic components at the rear end.
Alternatively, you can install secondary relief valves near the actuators such as hydraulic cylinders and hydraulic motors. These valves are specifically designed to handle the risk of pressure accumulation caused by load inertia or external forces.
If you are designing a hydraulic system, you should be aware that if the pressure relief valve is installed too far away, the pipeline between the pump and the valve may still be at risk of overpressure. If the installation position is incorrect, the system may experience delayed pressure response, resulting in shock or fluctuations.
Pressure Reducing Valve
The pressure reducing valve is usually installed in series at the inlet of the branch that requires lower working pressure, usually located after the control valve and before the actuator.
For example, you can install the pressure reducing valve before the control branch of the clamping cylinder, the lubrication system oil supply circuit, the low-pressure branch in the multi-actuator, and the inlet position of the precision hydraulic actuator.
This installation method makes sure that even if the main system pressure is high, the pressure of the downstream components remains stable within the appropriate range.
Proper installation of the pressure reducing valve can help you enhance the pressure independence of the multi-loop system, reduce the long-term impact of high pressure on precision components, improve the stability of the actuator operation, and minimize the wear of seals.
Application
Pressure Relief Valve
- Engineering machinery
In engineering machinery such as excavators, cranes and loaders, the load changes dramatically. The pressure relief valve can quickly open to release the excess pressure, preventing the hydraulic pump and cylinder from constantly bearing excessive loads.
If your equipment needs to operate with frequent heavy loads, the pressure relief valve helps reduce the fatigue damage to the system caused by hydraulic shock, and also minimizes unplanned shutdowns due to overpressure.
- Hydraulic press and molding equipment
Hydraulic presses, injection molding machines, and die-casting equipment usually need to operate continuously under high-pressure conditions. The pressure relief valve can limit the maximum pressure of the system and protect the mold, hydraulic cylinder, and main circuit in case of abnormal pressure.
A stable and reliable pressure relief valve can help you improve the stability of the equipment and reduce maintenance costs under high-pressure conditions.
- Hydraulic system with multiple actuators
In systems that contain multiple hydraulic actuators, different branches may cause pressure fluctuations or flow variations. Pressure relief valves are typically installed as main system protection devices in the main pressure circuit.
Pressure Reducing Valve
- Clamping and positioning system
In hydraulic fixtures, workpiece positioning devices, and automated clamping circuits, the actuators usually do not require the full pressure of the main system. The pressure reducing valves are typically installed at the front end of the clamping branch to maintain a stable low-pressure output.
If your equipment has high requirements for repetitive positioning accuracy, the pressure reducing valve can help you improve the consistency of the actions while reducing the long-term wear on the fixtures and actuators caused by high-pressure impacts.
Lubrication and low-pressure circuit
In the lubrication system, cooling circuit, and some auxiliary hydraulic circuits, excessively high pressure may cause unstable oil flow or component damage. The pressure reducing valve can maintain a stable outlet pressure, enabling downstream components to operate under more suitable conditions.
- Hydraulic system with multiple actuators
In a multi-actuator hydraulic system, the pressure requirements of different actuators are different. You can install the pressure reducing valve separately in a certain branch to control the pressure of that branch.
What Happens If You Use the Wrong Valve?
Using a Pressure Relief Valve Instead of a Reducing Valve
If you use a pressure relief valve instead of a pressure reducing valve, it cannot stably control the working pressure of a certain branch.
For hydraulic branches that require stable low-pressure operation, if only relying on the Pressure Relief Valve to control pressure, downstream pressure usually fluctuates with changes in system load. After long-term operation, there may be unstable actuator movement, inconsistent clamping force, increased pressure impact, and accelerated wear of seals.
If your device involves high repetitive movements or precise control, this pressure fluctuation may also affect machining accuracy and equipment consistency.
Using a Pressure Reducing Valve Instead of a Relief Valve
The pressure reducing valve does not have the functions of quickly releasing high pressure and protecting against system overload. If you use the reducing valve instead of the relief valve, your entire system will lose overpressure protection.
When facing sudden load changes, pipe blockages, or abnormal pump conditions that cause instantaneous high pressure, the opening and closing response speed of the pressure reducing valve is slow, and the flow diameter cannot meet the standard for rapid pressure relief.
The pressure in the main pipeline will continue to rise abnormally, which can easily cause pipe cracking. Meanwhile, the excessive pressure will continuously impact downstream components along the pipeline. This can easily lead to production line interruption and shutdown, causing you production delays and economic losses.
How to Choose the Right Pressure Reducing Valve and Pressure Relief Valve?
Clarify the Requirements of Your System
If you need to prevent system overpressure, protect the hydraulic pump and pipelines, and avoid pressure shock damage to components, what you need is a pressure relief valve. It is usually installed in the main pressure circuit.
If you need to provide a lower pressure for a certain branch, maintain stable downstream pressure, and improve the consistency of actuator operation, this is more suitable for a pressure reducing valve. It is usually installed in a local branch and regulates the outlet pressure by continuously sensing the downstream pressure.
Select Based on System Structure
If your hydraulic system only has a single pressure requirement, usually a pressure relief valve can provide you with basic system protection.
For systems with multiple actuators or multiple pressure levels, you usually need to use pressure relief valves in combination with pressure reducing valves.
Parameters to Consider When Making a Choice
Working Pressure
You need to confirm in advance the maximum working pressure of the system, the allowable pressure of the branch lines, and the pressure fluctuation range.
The set value of the pressure relief valve is usually higher than the normal working pressure and does not exceed the safety range of the system. The set value of the pressure reducing valve needs to be determined according to the requirements of the downstream actuator.
Flow
For the pressure relief valve, you need to determine the maximum output flow of the hydraulic pump, as well as the maximum discharge flow that the system may generate when the actuator stops or the oil circuit is blocked. Then, leave a certain safety margin when making the selection.
For the pressure reducing valve, you need to select the appropriate valve diameter and flow range based on the maximum and minimum flow requirements of the downstream actuator. The valve should be capable of handling peak flow conditions and also make sure stable pressure control even at low flow rates.
Response Time
If your equipment needs to operate continuously, perform frequent actions, and has high requirements for pressure stability, you also need to consider the response time of the valve.
FAQ
Why Does a Pressure Reducing Valve Sometimes Cause Pressure Fluctuation?
Excessive valve diameter, frequent flow changes, oil contamination, or rapid downstream load fluctuations can cause pressure instability. For example, if the size of the pressure reducing valve is too large to adapt to the actual operating flow range, the valve spool may continue to adjust within a very small range, resulting in unstable pressure control.
If the Oil Viscosity Changes, Do the Set Values of These Two Valves Need to Be Recalibrated?
Yes. The change in oil viscosity will affect the resistance of the valve spool movement and the internal leakage volume, thereby causing deviations in the opening pressure, response speed or downstream pressure stability.
For low-temperature startup, continuous high-temperature operation, or hydraulic fluid replacement, it is recommended that you recheck the system pressure parameters. And recalibrate the valve settings based on actual operating conditions to make sure pressure control stability and equipment operation safety.
What Causes a Pressure Relief Valve to Open Frequently?
The frequent opening of the pressure relief valve might be a problem with the valve itself, or it could be that your hydraulic system has been in a high-pressure relief state for a long time. Overload of the actuator, blockage of the oil circuit, too low set pressure of the valve, or excessive flow rate of the hydraulic pump can all cause the system to overflow.
If your pressure relief valve frequently opens for a long time, you can check the load, the circuit design, and the pump flow configuration.
How Do I Know If a Pressure Relief Valve Is Set Too High?
If the set value of the pressure relief valve is too high, the hydraulic system may continue to operate beyond the safe pressure level. This will cause the hydraulic pump, seals, and pipelines to bear additional loads for an extended period.
During actual operation, you may notice abnormal increases in oil temperature, increased pump noise, bulging of hoses, or leakage at the seals.
Why Does a Pressure Reducing Valve Generate Heat?
The Pressure Reducing Valve needs to reduce the downstream pressure through throttling during operation. During this process, some hydraulic energy will be converted into heat, so there will be a temperature rise in the valve and the nearby oil circuit.
When the system has a large pressure difference for a long time, continuous high flow operation, or frequent pressure regulation, the heating will be more obvious.
Final Thoughts
Based on your actual working conditions, appropriately matching the pressure relief valve and pressure reducing valve can help your equipment achieve more stable pressure control, higher operational reliability, and lower long-term maintenance costs. If you still have any further questions about these two types of valves, please feel free to contact us.
