Flow control is an important aspect of hydraulic system designs as it is used to regulate speed. Various types of flow control valves and other devices can be used which enables the speed of an actuator to be controlled by regulating the hydraulic system's flow rate.

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Flow rate also determines rate of energy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (stroke) equals the work done on the load. The energy transferred must also equal the work done. Actuator speed determines the rate of energy transfer (i.e., horsepower), and speed is thus a function of flow rate.

Directional control, on the other hand, does not deal primarily with energy control, but rather with directing the energy transfer system to the proper place in the system at the proper time. Directional control valves can be thought of as fluid switches that make the desired "contacts." That is, they direct the high-energy input stream to the actuator inlet and provide a return path for the lower-energy oil.

It is of little consequence to control the energy transfer of the system through pressure and flow controls if the flow stream does not arrive at the right place at the right time. Thus, a secondary function of directional control devices might be defined as the timing of cycle events. Because fluid flow often can be throttled in directional-control valves, some measure of flow rate or pressure control can also be achieved with them.

Different Types of Flow Measurement

Controlling flow of a hydraulic system does not necessarily mean regulating volume per unit of time from a valve. Flow rate can be specified three different ways, so it is important to be aware of how flow is to be specified or measured:

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Weight flow rate, Q, expressed in units of lb./sec. or lb./min., is used to calculate power using English units of measure.Mass flow rate, Q, expressed in units of slugs/sec. or slugs/min. for English measureor kg/sec. or kg/min. in SI metric measureis used to calculate inertia forces during periods of acceleration and deceleration.

Because they control the quantity of fluid that flows through the valve per unit of time, the same control valves are used for all three types of flow rates.

Control of Flow Rate with Valves

There are eight types of flow control valves which are used most often in hydraulic circuits. 

Orifices 

A simple orifice in the line, shown in Figure 1 (a) below, is the most elementary method for controlling flow. (Note that this is also a basic pressure control device.) When used to control flow, the orifice is placed in series with the pump. An orifice can be a drilled hole in a fitting, in which case it is fixed; or it may be a calibrated needle valve, in which case it functions as a variable orifice, Figure 1 (b). Both types are non-compensated flow control devices.

Understanding the Difference Between Pressure and Flow Control

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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.

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