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How hydraulic valves work

Valves are tools that direct, control or regulate the flow of a fluid.

They can allow or interrupt the flow, regulate the flow rate or pressure and their body has inlet and outlet ports, inside of which there is a moving element for flow regulation or interruption.

Since the dawn of civilization human beings used primitive valves, as they settled down for long periods in a place instead of living as hunters-gatherers. Water supply was essential for nutrition and irrigation permitting small cities to survive. 7000 years ago in Jericho, Israel, water used to be stored in wells, to be successively distributed thanks to simple primitive systems of supply. Ancient civilizations became prosperous thanks to their ability to manage water resources till the ancient Greeks invented first true valves.

First bronze valves, both cylinder plug and gate-valve type, were made with metallurgical procedures and equipped with mechanical devices.

Vitruvius mentioned in his writings some Greek valves similar to the modern butterfly valves. Even Hero in his writing “Milliarion” described a horse head valve, which had the same function as a modern ball valve.

In the Roman age the use of valves and taps in water network increased to reduce water waste following the raise of new aqueducts and amazing development in metallurgy. Romans used cylinder plug valves, or rarely conical, which body and male were made through casting.

With the decline of the Roman Empire and with the barbarians devastating cities, during the Middle Age monasteries were the places where this kind of technology was kept alive.

In this video we’re going to explain the most famous types of valves and how they work.

For over ten years Jaes Company has been providing industrial spare parts and offers every type of valve from leading producers.

There are many types of valve and several classification criteria:

The first way to classify a valve is by its shape; as a matter of fact, the valve usually takes the name of the external shape of the plug or from how it moves.
The plug is the valve’s main part and it’s used to start, stop or regulate the flow. For example, the ball valve gets the name from the sphere-shaped plug. The globe valve, instead, gets its name from the external shape, whereas the gate valve from movement of the plug up and down, like a proper gate.

The second method is by drive type. In order to activate a valve you need a force that can be manual using a lever or a handwheel, or derive from actuators, for example:
- electromagnetic (with a solenoid);
- electromechanical (with an electric motor);
- pneumatic (by a pneumatic cylinder);
- hydraulic (by an hydraulic cylinder)
- thermostatic (thermal expansion makes the plug move).
In this classification method one must also consider whether they are rotary or linear motion valves. Rotary valves require a 1/4 of a turn of the plug to completely start or stop the flow, thus they are called fast-acting valves; some examples are ball valves and butterfly valves.
In linear motion valves the plug gets lifted and lowered to start, regulate or stop the flow; some examples are gate valves and globe valves.

The third classification method is by type of application. Valves can act as:
- shut off valve (to stop and start the flow);
- control valve (for intermediate adjustments of flow or pressure of a fluid);
- air relief valve (to eliminate excess air);
- safety valve (to allow the leak of fluid in overpressure condition);
- check valve (to permit the flow just in one direction).

Let’s now see various types of valves.

If you’re interested in the type of valve you’re watching and want to know more, click the upper right link to watch a video about it.

Ball valve.
It’s a fast-acting rotary motion valve with a sphere-shaped plug with a through-hole.
The hole turns till it’s in line with the inlet and outlet of the body to start the flow, and rotates again into perpendicular position to stop it.
Ball valves are suitable just to start and stop the flow.

Plug valve.
It is a rotary motion valve, operating the same way as the ball valve, although the plug is cylindrical or conical. Thanks to the shape, plug valves can have more than two ports, splitting the flow into as many ports as the valve has.
Butterfly valve.
It’s a rotary motion valve with a disc as a plug, which rotates a 1/4 of a turn. By rotating, the plug starts and stops the flow, but it is also efficient in regulating it.
Butterfly valve are suitable for big flows of fluid or gas at low pressure.

Gate valve.
It’s a linear motion valve with a plug that can be even rectangular. It gets the name from the plug that moves up and down, starting and stopping the flow, like a proper garage door.
This kind of valve is not suitable for regulating the flow because it’s subject to vibrations when it’s open.

Globe valve.
It is a linear motion valve with a plug that presses vertically on the seat, exerting a great clamping force. It is used to stop, start and regulate the flow of a fluid.
The stem can either be at right angles, also called “straight-flow” or “T-pattern”, or tilted, which is called “Y-pattern” where the flow is less deviated.

Needle valve.
It is designed to make fine adjustments on the flow.
It operates the same way as the globe valve, but it has a needle-like plug pressing vertically on the seat, thus the amount of flow is more accurate.

Diaphragm valve.
It’s a linear motion valve, with a diaphragm that can be raised or lowered to either start or stop the flow.
Since just the inside of the valve and the diaphragm are exposed to the flow, it is suitable for corrosive fluids, dangerous fluids or that cannot be contaminated.

Pinch valve.
It consists of a rubber sleeve and a pinching mechanism to stop the flow. It is used to stop, start and regulate the flow of a fluid and is especially suited for slurries or liquids with residues.
The rubber sleeve is the only part exposed to the fluid, so pinch valve is also suitable for corrosive fluids, dangerous fluids or that cannot be contaminated.

Relief and safety valves.
These valves are designed to prevent damages to devices, eliminating the possible overpressure.
Relief valve opens gradually when pressure is too high, decreasing to the ideal level, whereas safety valve pops fully open and closes tightly as soon as normal pressure is set again.

Check valve also called “non-return valve”.
This type of valve is designed to prevent the reversal of the flow. The valve is activated directly by the flow pressing, whereas any reversal of the flow will close the valve. There are several model of this type, like the swing check valve, the disc check valve and the ball check valve.
Among all these models of check valves there is a special type called Tesla valve. It is a fixed geometry valve, which means it has no moving parts. As a matter of fact, thanks to several changes in direction it creates motions of the fluid, that make it flow in only one direction.

One of the most important data to identify a valve is its flow coefficient.

The flow coefficient (with symbol Kv) describes the water flow between 5° C and 40° C. It is expressed in “cubic metres per hour” (m3/h) and measures the volume passing through a valve with a differential pressure (i. e. the difference between inlet and outlet pressure) of 1 bar.
The American system expresses the coefficient Cv, which is described as the water flow at 60° F in “US gallons per minute” (gpm) with a differential pressure of 1 psi.
Kv flow coefficient is equal to 0.865 Cv (1 Kv = 0.865 Cv).

Using this coefficient you can:

- measure the valve from the flowrate and the pressure drop
Kv = Q / √ΔP

- calculate the rate of flow of the valve from the pressure difference
Q = Kv x √ΔP

- calculate the pressure drop across the valve from the flowrate and flow coefficient Kv
ΔP = (Q / Kv)2

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