A Guide to Flow Control Valves Types and Use Cases

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:

Volumetric flow rate, Qv, expressed in units of in.3/sec. or  in.3/min. – or as cc/sec. or cc/min. in SI metric measure – is used to calculate the linear speeds of piston rods or rotational speeds of motor shafts.

Weight flow rate, Qw, expressed in units of lb./sec. or lb./min., is used to calculate power using English units of measure.Mass flow rate, Qg, expressed in units of slugs/sec. or slugs/min. for English measure

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or kg/sec. or kg/min. in SI metric measure

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is used to calculate inertia forces during periods of acceleration and deceleration.

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.

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