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How do flow control valves work?

What are flow control valves and why do you need them in your hydraulic system? In this week’s blog we examine how flow control valves help to regulate system speed and adjust to variances in pressure and temperature.

The speed of a system actuator is regulated by its flow rate, with the rate of flow also determining energy transfer rates at various pressures. Hydraulic actuators respond to changes in pressure by opening or closing the valve. Automatic flow valves do not require an external power source to operate, so they use fluid pressure to enable them to work effectively.

Types of flow control

There are a number of different types of valves which control flow rate using various different methods with the most common examples described below:

Fixed and variable orifice flow control – The traditional method of flow control and the most basic. Orifice based flow control can vary from a simple fixed drill hole to a calibrated needle valve which controls the variable flow rate. When teaching the basics of flow control, this is a good method for students to get their heads around the concept due to its simplicity.

Flow regulators – Moving on from the orifice method are flow regulators, which use an orifice but sense fluid rate as a pressure drop with adjustments to inlet and outlet pressures made by the use of a compensating piston. This all gives greater control of flow rates under a range of different operating conditions.

Demand compensated flow controls – This process basically separates flow into primary and secondary circuits so priority functions in the primary circuits can be catered for, leaving secondary circuit flow for other lower priority tasks.

Pressure compensated flow valves – Equipped with an adjustable, variable rate orifice, inlet and load pressures are both controlled using a compensator. The compensator keeps the pressure constant to a degree of accuracy of up to approximately 5%.

In addition to the pressure compensator, valves are available with a temperature compensator as the flow can vary dependant on system temperatures, so the combination of both compensators keep things as steady as possible.

Priority valves – These valves adjust the flow rate specifically for the primary circuit and reduce flow pressure for use in the secondary circuit hence the ‘priority’ name in the title of the valve.

If any discrepancies should occur with flow rate fluctuations, the primary circuit will always have priority over the secondary circuit.

Other methods of flow control – Flow dividers are another popular technique for dividing flow between circuits and include examples such as rotary dividers and proportional flow logic valves which all essentially perform similar functions but in different ways.

How flow rates are measured

To correctly measure the flow rate of hydraulic fluid, three principles need to be taken into account - Mass flow, weight flow and volumetric flow:

Mass flow – refers to the levels of inertia regarding accelerative and decelerate forces and is measured in kg/sec or min

Weight flow – Commonly measured in Ib/sec or min, the weight flow calculates the power created

Volumetric flow – This looks at linear speeds and rotational speeds of pistons and motor shafts and is normally calculated using in3/sec or min

At Hydraproducts we stock a large range of hydraulic valves as part of our New Components Division so feel free to browse our full range here.


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