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 cook top (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.
How are valves made?
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).
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.
Types of valves:
In piping following types of valves are used depending on the requirements. The cost of Valve in the piping system is up to 20 to 30% of the overall piping cost. And the cost of a given type and size of the valve can vary 100%. It means that if you choose ball valve over butterfly valve for the same function. It can cost you more. So, the selection of valves is essential to the economics, as well as operation, of the process plants.
1. Gate valve:
Gate valve is the most common type of valve in any process plant. It is a linear motion valve used to start or stop fluid flow. In service, these valves are either in fully open or fully closed position. Gate valves are used in almost all fluid services such as air, fuel gas, feed water, steam, lube oil, hydrocarbon, and all most any services. Gate valve provides good shutoff.
2. Globe Valve:
Globe valve is used to stop, start, and regulate the fluid flow. Globe Valves are used in the systems where flow control is required and leak tightness is also necessary. Globe valve provides better shut off as compared to gate valve and it is costlier than gate valve.
3. Check Valve:
The check valve prevents back flow in the piping system. The pressure of the fluid passing through a pipeline opens the valve, while any reversal of flow will close the valve.
4. Plug valve:
Plug valve is Quarter-turn rotary motion Valve that uses a tapered or cylindrical plug to stop or start the flow. The disk is in plug shape, which has a passage to pass the flow. Plug valve used as on-off stop valves and capable of providing bubble tight shutoff. Plug valve can be used in vacuum to high-pressure & temperature applications.
5. Ball Valve:
A Ball valve is a quarter-turn rotary motion valve that uses a ball-shaped disk to stop or start the flow. Most ball valves are of the quick-acting type, which requires a 90° turn of the valve handle to operate the valve. The ball valve is Smaller and lighter than a gate valve of same size and rating.
6. Butterfly Valve:
A Butterfly valve is a quarter-turn rotary motion valve, that is used to stop, regulate, and start the flow. Butterfly valve has a short circular body. Butterfly Valve is suitable for large valve applications due to Compact, lightweight design that requires considerably less space, as compared to other valves.
7. Needle Valve:
Needle valves are similar to a globe valve in design with the biggest difference is the sharp needle like a disk. Needle valves are designed to give very accurate control of flow in small diameter piping systems. They get their name from their sharp-pointed conical disc and matching seat.
8. Pinch Valve:
The pinch valve is also known as clamp valve. It is a linear motion valve. Used to start, regulate, and stop fluid flow. It uses a rubber tube, also known as a pinch tube and a pinch mechanism to control the fluid. Pinch Valve is ideally suited for the handling of scurries, liquids with large amounts of suspended solids, and systems that convey solid material pneumatically.
9. Pressure Relief Valve:
A pressure Relief valve or pressure safety valve are used to protect equipment or piping system during an over pressure event or in the event of vacuum. This valve releases the pressure or vacuum at pre-defined set pressure.
How do safety valves work?
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).