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A man's hand turns the red wheel of an isolation valve.

Valves

by Chris Woodford. Last updated: February 28, 2016.

What's the world's favorite form of transportation? The car? The bicycle? The jet airplane? If I had to hazard a guess, I'd pick none of these things. Instead, I'd opt for the humble pipeline. You might not notice pipes, but they're transporting vast amounts of fluid (liquid and gas) around the world quietly and efficiently, day in and day out. To work efficiently, pipes need a way of regulating how much fluid can pass through them; they also need a way of switching the flow off completely. That's the job that valves do: valves are like mechanical switches that can turn pipes on and off or raise or lower the amount of fluid flowing through them. Let's take a closer look at how they work!

Photo: Opening an isolation valve by turning a wheel. Typically, an isolation valve cuts off the supply of fluid to a particular place, such as the water flowing into your home, either during an emergency or for routine maintenance. Once the valve is closed, you can safely carry out repairs without the fluid all flooding out. Photo by Brian Sloan courtesy of US Navy.

What are valves?

A giant wind tunnel air valve with a man standing beside it.

A small blue and white plastic poppet valve on top of a sports drink bottle.

A valve is a mechanical device that blocks a pipe either partially or completely to change the amount of fluid that passes through it. When you turn on a faucet (tap) to brush your teeth, you're opening a valve that allows pressurized water to escape from a pipe. Similarly, when you flush the toilet, you open two valves: one that allows water to escape to empty the pan and another (called a ball valve or ballcock) that admits more water into the tank ready for the next flush.

Valves regulate gases as well as liquids. If you have a gas cooktop (hob) on your stove, the controls that turn the gas up or down are valves. When you turn up the heat, you're opening a valve that allows more gas to flow in through the pipe. More gas burns with a bigger flame so you get more heat.

Valves are pretty much guaranteed to be in any machine that use liquids or gases. There's a valve in your clothes washer that turns the water supply on or off each time the drum rinses out. There are also valves in the cylinders of your car engine, opening and closing several times a second to admit air and fuel and to allow burned exhaust gases to escape.

It's not just machines that use valves. Your body has some pretty important valves inside your heart that allow it to pump blood to your lungs (where it picks up oxygen) and then around your body.

Photo: Valves come in all sizes. Left: Some are tiny, like this poppet valve, which slides up and down on top of a drinks bottle to let the water in and out when you pull or push it with your teeth. Right: Other valves are gigantic. This 7.3-m (24-ft) diameter butterfly valve from a wind tunnel dwarfs the man standing next to it! Photo by courtesy NASA on the Commons.

How are valves made?

A stop valve being operated by hand.

Valves are usually made of metal or plastic and they have several different parts. The outer part is called the seat and it often has a solid metal outer casing and a soft inner rubber or plastic seal so the valve makes a closure that's absolutely tight. The inner part of the valve, which opens and closes, is called the body and fits into the seat when the valve is closed. There's also some form of mechanism for opening and closing the valve—either a manual lever or wheel (as in a faucet or a stop cock) or an automated mechanism (as in a car engine or steam engine).

An open butterfly valve.

It's often critically important for valves that are switched off to allow absolutely no escape of liquid or gas through a pipe to avoid accidents, explosions, pollution, or the loss of valuable chemicals (even a dripping faucet can be expensive if your water is metered). That's why the seal on a valve needs to be perfectly secure and a valve that's turned off must be tightly closed. Turning off a high-pressure flow of liquid or gas by obstructing it with a valve is physically hard work: in other words, you need to use a lot of force to do it. That's why some valves are operated by long levers (as in our top photo) or large wheels (as in the photo shown here). If really big valves require too much force for a human to supply, they're operated by hydraulic rams.

Photo (left): This stop valve is manually operated: you open and close it by turning the wheel. A wheel like this makes a valve easier to open because it multiplies the force you apply at the rim to produce a bigger and more useful force at the center. If you're not sure why, take a look at our article on tools and machines. Photo by Conor Minto courtesy of US Navy.

Photo (right): This butterfly valve swivels open in the center to let air through a pipe. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

Types of valves

Artwork comparing eight common types of valves and how they open and close to let fluid flow.

The many different types of valves all have different names. The most common ones are the butterfly, cock or plug, gate, globe, needle, poppet, and spool:

Artwork: Eight common types of valves, greatly simplified! Color key: the grey part is the pipe through which fluid flows; the red part is the valve and its handle or control; the blue arrows show how the valve moves or swivels; and the yellow line shows which way the fluid moves when the valve is open.

How do safety valves work?

Wesson safety faucet patent drawing from 1923.

Valves are often used to contain dangerous liquids or gases—maybe toxic chemicals, flammable petroleum, high-pressure steam, or compressed air—that mustn't be allowed to escape under any circumstances. In theory, a valve must be perfectly secure and, once closed, must never allow liquid or gas to get past it. In practice, that's not quite true. Sometimes it's better for a valve to fail, intentionally, to protect some other part of a system or machine. For example, if you have a steam engine powered by a water boiler in which steam is building up, but the pressure suddenly gets too high, you need a valve to blow open, let the steam escape, and release the pressure safely before the entire boiler explodes catastrophically. Valves that work in this way are called safety valves. They're designed to open automatically when the liquid or gas they contain reaches a certain pressure (though many systems and machines have safety valves that can be opened manually for the same purpose).

Artwork: An example of a safety valve fitted into an ordinary hot-water faucet (tap). In a conventional faucet, you turn the orange handle at the top clockwise or counterclockwise to make the valve screw up or down. That allows water to flow from left to right through the horizontal pipe, around the bend (through the gap where the valve was), and out through the vertical pipe on the right. You can turn the handle by different amounts to screw the valve open to a different height, letting different amounts of water through. In this design by Paul Wesson, patented in 1923, there's an extra, safety valve at the bottom, colored green. It has a conical shape and is normally held tightly in place by the yellow spring coiled around it. However, if the water pressure builds up too much, it pushes against the cone, opens the valve, and the water escapes downward, releasing the pressure. Artwork from US Patent: 1,449,472: Safety Faucet by Paul B. Wesson and Hampden Brass Company, courtesy of US Patent and Trademark Office.

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Woodford, Chris. (2008) Valves. Retrieved from http://www.explainthatstuff.com/valves.html. [Accessed (Insert date here)]

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