6/16/2016 How valves work | Types of valves ­ Explain that Stuff

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

hat's the world's favorite form of transportation? The car? The bicycle? Thejetairplane? 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'retransporting 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 muchfluid 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 onand off or raise or lower the amount of fluid flowing through them. Let's take a closer look athow 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 valve isa

mechanical device that blocks a pipe eitherpartially or completely to change the amount offluid that passes through it. When you turn on a

faucet (tap) to brush your teeth, you're opening a valve that allows pressurized watertoescape from a pipe. Similarly, when you flush the toilet, you open two valves: one thatallows water to escape to empty the pan and another (called a ball valve or ballcock) thatadmits more water into the tank ready for the next flush.

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

Valves are pretty much guaranteed to be in any machine that use liquids or gases. There'sa valve in your clothes washer that turns the water supply on or off each time the drumrinses out. There are also valves in the cylinders of your car engine, opening and closingseveral 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 insideyour heart that allow it to pump blood to your lungs (where it picks up oxygen) and thenaround 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 7.3­m (24­ft) 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?Valves are usually made of metal or plastic and theyhave several different parts. The outer part is calledtheseat and it often has a solid metal outer casing anda soft inner rubber or plastic seal so the valve makes aclosure that's absolutely tight. The inner part of thevalve, which opens and closes, is called the bodyandfits into the seat when the valve is closed. There's alsosome form of mechanism for opening and closing thevalve—either a manual lever or wheel (as in a faucetor a stop cock) or an automated mechanism (as ina car engineor steam engine).

It'soften

critically important for valves that areswitched off to allow absolutely no escape ofliquid or gas through a pipe to avoidaccidents, explosions,pollution, or the lossof valuable chemicals (even a drippingfaucet can be expensive if your water ismetered). That's why the seal on a valveneeds to be perfectly secure and a valvethat's turned off must be tightly closed.Turning off a high­pressure flow of liquid orgas by obstructing it with a valve is physically hard work: in other words, you need to use a

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lot of force to do it. That's why some valves are operated by long levers (as in our topphoto) or large wheels (as in the photo shown here). If really big valves require too muchforce for a human to supply, they're operated byhydraulic 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 (NASA­GRC).

Types of valvesThe many different types of valves allhave different names. The mostcommon ones are the butterfly, cockor plug, gate, globe, needle, poppet,and spool:

Ball: In a ball valve, ahollowed­out sphere (the ball)sits tightly inside a pipe,completely blocking the fluidflow. When you turn thehandle, it makes the ballswivel through ninety degrees,allowing the fluid to flowthrough the middle of it.

Butterfly: A butterfly valve is adisk that sits in the middle of apipe and swivels sideways (toadmit fluid) or upright (to blockthe flow completely).

Cock or plug: In a cock orplug valve, the flow is blockedby a cone­shaped plug thatmoves aside when you turn awheel or handle.

Gate or sluice: Gate valvesopen and close pipes by lowering metal gates across them. Most valves of this kindare designed to be either fully open or fully closed and may not function properlywhen they are only part­way open. Water supply pipes use valves like this.

Globe: Water faucets (taps) are examples of globe valves. When you turn thehandle, you screw a valve upward and this allows pressurized water to flow up

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through a pipe and out through the spout below. Unlike a gate or sluice, a valve likethis can be set to allow more or less fluid through it.

Needle: A needle valve uses a long, sliding needle to regulate fluid flow precisely inmachines like car enginecarburetors and central­heating systems.

Poppet: The valves in car engine cylinders are poppets. This type of valve is like alid sitting on top of a pipe. Every so often, the lid lifts up to release or admit liquid orgas.

Spool: Spool valves regulate the flow of fluid in hydraulicsystems. Valves like thisslide back and forward to make fluid flow in either one direction or another around acircuit of pipes.

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?Valves are often used tocontain dangerousliquids or gases—maybetoxic chemicals,flammable petroleum,high­pressure steam, orcompressed air—thatmustn't be allowed toescape under anycircumstances. In theory,a valve must be perfectlysecure and, once closed,must never allow liquid orgas to get past it. Inpractice, that's not quitetrue. Sometimes it'sbetter for a valve to fail, intentionally, to protect some other part of asystem or machine. For example, if you have a steam engine poweredby a water boiler in which steam is building up, but the pressuresuddenly gets too high, you need a valve to blow open, let the steamescape, and release the pressure safely before the entirely boilerexplodes catastrophically. Valves that work in this way are called safetyvalves. They're designed to open automatically when the liquid or gasthey contain reaches a certain pressure (though many systems andmachines have safety valves that can be opened manually for the same

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purpose).

Artwork: An example of a safety valve fitted into an ordinary hot­water 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.