Understanding the Difference Between Pressure and Flow Control

Pneumatic systems are a commonly used and cost-effective means of delivering power and energy to tools, instrumentation and industrial processes. All pneumatic systems rely on both pressure and flow to operate. While there is a difference between pressure control and flow control, they are closely related and adjusting one will impact the other. Here, we aim to explain the difference between pressure and flow control, simplify the relationship between the two and discuss the types of pressure control devices and flow control valves that are commonly found in pneumatic applications.

Defining Pressure and Flow in Pneumatic Systems

Pressure is a force applied across a given area and controlling it relates to how pressure is routed and contained within a pneumatic system to provide sufficient and reliable energy. Flow is the speed and volume at which the pressurized compressed air moves and controlling flow relates to how fast it is permitted to move within a given time period through a pneumatic system.

You cannot have a functional pneumatic system if you do not have both pressure and flow. Without pressure, the air within the pneumatic system would not have enough force to provide power to the application. Without flow, the pressurized air would remain contained and unable to reach its destination.

Pressure Control vs. Flow Control

Simply put, pressure is related to the force and strength of the air. In pressure control, the generated force is equal to pressure multiplied by the area in which it is contained, so a high input of pressure in a small area will create the same force as a low input of pressure in a large area. Pressure control regulates both input and output forces to maintain a constant, balanced pressure at the level required for the application. Typically pressure control is achieved via a pressure-regulating device.

Flow is related to air volume and speed, which means flow control either opens or restricts the area through which the air can flow to control how much and how fast pressurized air will move through the system. A smaller open area will result in less air flow at a given pressure within a given period of time. Flow control is usually achieved via a flow control valve that opens and closes to precisely allow or prevent the flow of air.

Pressure and flow control are different but equally important parameters in a pneumatic system and they are dependent upon each other in order for the pneumatic system to function. As such, changing one variable will inevitably affect the other, impacting the overall system.

In an ideal pneumatic system, controlling one variable in order to control the other might work, but real-world applications are seldom ideal representations. For example, using pressure to control flow will not provide the required precision and will likely lead to increased energy costs as more air flow than necessary will be provided. Using pressure to control flow may also cause over-pressurization in the system, leading to damaged components or product.

Conversely, attempting to control pressure by controlling flow will likely lead to pressure drops when the airflow is increased, resulting in an unstable supply of pressure that may not offer enough energy to supply the application, while increasing airflow may supply too much pressure and waste energy.

For these reasons, it is often recommended to manage flow control and pressure control separately in a pneumatic system.

Pressure and Flow Control Devices

Flow control valves are used for controlling or adjusting air flow (speed) through the pneumatic system. There are different types available to suit a variety of applications, including:

• Proportional control valves: These pneumatic flow control valves are used for controlling or adjusting air flow through the system. As amperage is applied or varied to the solenoid of the valve, the output flow of the valve varies accordingly.
• Ball valves: These valves contain an inner ball attached to a handle that, when turned, permits or prevents the flow.
• Butterfly valves: These employ a metal plate attached to the handle to open (allow) or close (block) the flow.
• Needle valves: Needle valves provide flow control via a needle that opens or closes to allow or block the flow of air through the system.

To control pressure (or force/strength), a pressure control valve or pressure regulator will be the weapon of choice. Typically, pressure control valves are closed valves, except for pressure reducing valves, which are usually open. There are several types available, including:

• Pressure relief valves: Pneumatic systems are often required to operate within a defined pressure range and, without controlling the force of air, equipment or product could be damaged, so pressure relief valves serve to limit maximum pressure by diverting excess pressure.
• Pressure reducing valves: Used to maintain lower pressure in a pneumatic system, these two-way valves close after receiving sufficient pressure to prevent over-pressurization.
• Sequencing valves: Normally closed, two-way sequencing valves, these valves regulate the sequence of actuator movement in a system that has more than one actuator, allowing pressure to pass from one actuator to the next in the system.
• Counterbalance valves: These valves are usually closed and help maintain a set pressure in a portion of the pneumatic system. They are mainly used to counterbalance an external force.

For more information on controlling pressure and flow in your pneumatic system, please reach out to an expert at JHFOSTER.