The Difference Between Flow Control Valve and Pressure Control Valve
In hydraulic and pneumatic systems, flow control valves and pressure control valves are two crucial components. They are used to control the flow rate and pressure of fluids to ensure efficient system operation. Although these two types of valves differ in function, they play equally important roles in many industrial applications. This article will introduce different aspects of flow control valve and pressure control valve.
What is a Flow Control Valve?
A flow control valve is a device used to control the flow rate of fluids. It controls the flow rate of fluids whether liquids or gases by adjusting the size of the valve opening. Flow control valves are typically used to ensure the fluid flow rate in a system remains within the required range that prevent damage to the system from excessively high or low flow rates.
Types of Flow Control Valves
There are various types of flow control valves and the specific type chosen depends on the application requirements and fluid characteristics. Common types of flow control valves include:
- Hydraulic flow control valves: Used in hydraulic systems to help regulate oil flow.
- Pneumatic flow control valves: Used in pneumatic systems to control the flow rate of gases.
- Pressure-compensated flow control valve: Automatically adjusts the flow rate according to pressure changes that ensure a constant flow.
- Automatic flow control valve: Typically used in systems requiring automatic flow regulation.
- Sprinkler flow control valve: Used in sprinkler systems to ensure stable liquid flow.
Classification by working medium/application area
This is the most direct classification method. The valve design must be adapted to the fluid it handles.
- Hydraulic flow control valve: Used in hydraulic systems, the medium is usually hydraulic oil that features high pressure resistance and robust construction.
- Pneumatic flow control valve: Used in pneumatic systems, the medium is compressed air.
- Flow control valve for water system: Used in various water-medium pipelines.
- Water hose nozzle with high flow control valve: A water gun/water hose nozzle with flow control function.
- Shower flow control valve: Specifically designed for bathrooms that enhance the experience or saving water.
- Coolant flow control valve: Commonly used in the cooling circulation systems of engines, machine tools and other equipment.
- Boiler flow control valve: Used in boiler heating or water supply systems to regulate the circulating water volume.
Classification by Technical Principle and Function
These valves incorporate more complex technology on top of their basic functions to achieve more precise or specialized purposes.
Automatic flow control valve: Automatically regulates flow without manual intervention. It can be driven by changes in the fluid’s own pressure or flow rate or it can automatically adjust based on signals from a flow meter or control system.Pressure-compensated flow control valves and overflow control valves can both be considered types of automatic flow control valves.
Pressure-compensated flow control valve: An advanced flow control valve in hydraulic and pneumatic systems. It is a high-performance type of hydraulic or pneumatic flow control valve that maintains a constant set flow rate regardless of fluctuations in the valve’s inlet or outlet pressure.
Overflow control valve: Is a safety valve. Under normal flow conditions, the valve is fully open; when the flow rate exceeds a preset safety value, the valve will automatically and quickly close to prevent large-scale leakage of the medium.
What is a Pressure Control Valve?
A pressure control valve controls pressure in a system. Different from flow control valves, pressure control valves do not directly control the flow rate of fluid, but rather protect the system from overpressure damage by limiting pressure. They are commonly used in hydraulic and pneumatic systems to ensure that pressure is maintained within a safe range.
How do flow control valves and pressure control valves work?
The core principle of a flow control valve is throttling. It creates resistance by generating a variable-sized narrow orifice in the fluid passage, thereby precisely controlling the flow rate through the valve. Its ultimate purpose is to regulate the movement speed of actuators.
Force balance is the core principle of a pressure control valve. It compares the fluid pressure with a preset spring force: When the system pressure exceeds the spring setpoint, the valve core actuates, either returning excess fluid to the tank or limiting further pressure increases. Its fundamental function is to limit the maximum system pressure for safety or to provide a stable and lower pressure for branch circuits.
Advantages and Limitations of Flow Control Valve and Pressure Control Valve
Pros of Flow Control Valves
- Precise control of fluid flow rate.
- Suitable for regulating the flow rate of liquids and gases.
- Suitable for various industrial applications such as hydraulic systems, pneumatic systems and sprinkler systems.
Cons of Flow Control Valves
- Cannot directly control pressure; may require use with a pressure control valve.
- May not provide precise flow control in systems with large flow variations.
Pros of Pressure Control Valves
- Effectively prevents system overpressure that protects equipment.
- Suitable for systems requiring stable pressure.
Cons of Pressure Control Valves
- Cannot directly control flow; may require use with a flow control valve.
- Not suitable for applications with large flow variations.
Application
Applications of Flow Control Valves
1. Hydraulic Cylinder Speed Control
- Inlet Throttling: Controls the flow rate of hydraulic fluid entering the hydraulic cylinder that provides smooth movement, but back pressure will be generated inside the cylinder.
- Return Throttling: Controls the flow rate of hydraulic fluid leaving the hydraulic cylinder that provides better control over the load, resist “rushing” phenomenon and ensure smooth operation.
- Bypass Throttling: Directs a portion of the pumped oil back to the oil tank with lower efficiency but simpler control.
2. Hydraulic Motor Speed Control: Controls the flow rate entering the hydraulic motor to adjust its speed.
3. Pneumatic Cylinder Speed Control: In the pneumatic circuit, the extension and retraction speed of the cylinder is controlled by adjusting the air flow rate entering and exiting the cylinder that achieve “slow entry and slow exit” to avoid impact.
4. Cooling System: Precisely controls the flow rate of coolant entering the cooling parts of the equipment.
Applications of Pressure Control Valves
1. Relief Valve
- Safety Protection: Normally closed.
- Safety Valve: Opens when system pressure exceeds the set value that releases excess flow back to the oil tank to prevent system overpressure damage.
- Constant Pressure Relief: Normally open. Used in conjunction with a throttle valve in a fixed displacement pump system, it continuously overflows excess oil to maintain a constant system pressure.
2. Pressure Reducing Valve: Provides a stable low pressure to a branch circuit within the same hydraulic system that offers a stable secondary pressure lower than the main system pressure.
3. Sequence Valve – Controls the sequence of actions: When the pressure reaches the set value, the valve opens. It allows oil to flow to another actuator that achieves the logical sequence of “Cylinder A completes its action, then Cylinder B follows.”
4. Unloading Valve: Saves energy: When the system pressure reaches the set value in a variable displacement pump system, a signal is sent to reduce the pump’s displacement to almost zero that achieves unloading, saves energy and reduces heat generation.
Precautions in Practical Operation
| Item | Flow control valve | Pressure control valve |
| What you care about is…? | “How fast does it move?” | “How much force can it apply?” or “Is the system safe?” |
| Operating core | For speed regulation, pay attention to speed stability and prioritize pressure-compensated speed types. | For pressure limiting/regulation, pay attention to the accuracy of the pressure setpoint and the unobstructed flow of the oil drain circuit. |
| Maintain critical | Keep the fluid clean to prevent the throttle port from becoming clogged. | Connect the drain port correctly and check the set pressure regularly. |
Precautions for Flow Control Valves
1. Importance of Pressure Compensation
The flow rate of ordinary throttle valves fluctuates with changes in the pressure difference between their inlet and outlet that lead to unstable actuator speed. Therefore flow control valves with pressure compensation must be used in applications requiring high speed stability. They automatically counteract the effects of pressure fluctuations that ensure constant flow.
2. Oil Cleanliness
Throttle orifices are typically very small and easily clogged by contaminants such as sludge and metal shavings that cause flow changes or complete jamming. Maintaining oil cleanliness is a prerequisite for normal operation.
3. Installation Direction and Type It needs to be confirmed whether the one-way flow control valve or a two-way flow control valve. One-way flow control valves typically throttle only in one direction that allow free flow in the opposite direction. They must not be installed backwards. Clearly define whether it is inlet throttling, return throttling or bypass throttling; their performance differs and they cannot be interchanged arbitrarily.
4. Adjustment Operation
When adjusting handles or screws, the movements should be smooth and slow to avoid impact. If the valve has a lock nut after adjustment, be sure to tighten it to prevent setpoint drift due to vibration.
5. Energy Loss and Heat Generation:
Throttling essentially converts excess energy into heat that leads to increased oil temperature. The system’s heat dissipation capacity must be considered during design and use.
Precautions for Pressure Control Valves
1. Correct Setting Pressure
The set pressure of a safety valve must never exceed the rated operating pressure of system components that typically set at 1.1 to 1.2 times the system operating pressure.
The set pressure of a pressure reducing valve must be lower than its inlet pressure; otherwise it will not function.
2. Drain Port Connection
For pilot-operated pressure valves, especially pressure reducing valves and externally controlled sequence valves, the pilot control oil drain port usually marked “L” or “Y” must be directly and unobstructed back to the oil tank. Back pressure at this port will severely affect valve performance that causes inaccurate pressure adjustment or valve malfunction.
3. Avoid Misuse
Never use a pressure reducing valve as a relief valve. Pressure reducing valves are normally open, while relief valves are normally closed; their functions are completely different. Sequence valves and relief valves have similar structures, but the oil drain of a sequence valve returns to the oil tank independently, while the oil drain of a relief valve is internal; they should not be confused.
4. Adjustment Operation
When adjusting the pressure, observe the pressure gauge while slowly rotating the adjusting handle. It is recommended to gradually adjust from low pressure to high pressure. Before starting the system, you should check that the pressure valve’s set value is within the safe range.
5. Stability and Vibration:
Pressure control valves may oscillate near the critical pressure point. This may be due to air mixed in the oil, valve core wear or system resonance. The cause needs to be investigated, rather than simply increasing or decreasing the pressure.
Flow Control Valve – Recommended by SNS Pneumatics
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