Valve also refers to vacuum tubes used as amplifiers or rectifiers in HiFi, guitar, and recording equipment. Many vacuum tubes work by using a signal to control a grid that restricts the flow of electrons from one end of the tube to the other. The electrons are flying from one polarity to the other, and the grid controls the degree of this polarity difference. This is how most amplification stages work...using a small signal to control a larger signal.

In the process industries, such as refining, pulp and paper and water purification, valves are named for one of three things: The type of valve, such as a butterfly valve; The operator of the valve, such as motor operated valve; The function of the valve such as isolation valve. Needless to say, a combined description such as a motor operated butterfly isolation valve is more complete yet to fully and unambigiously describe a valve, you really need a specification sheet like ISA 20.50.

The common feature amongst valves is that they open an orifice to allow some kind of media to flow through it. The orifice can be revealed in either a rotary motion such as in a ball valve or a linear motion as in a globe or spool valve. The interaction between the orifice and what blocks it gives the valve certain qualities which make it more or less suitable for particular applications. For example, when a quarter turn ball valve is opened (these are commonly used as gas line isolation valves in homes), the hole increases in size at an increased rate when turned at an even rate throughout the whole of the opening stroke. This property is know as the "equal percentage" characteristic and can be summarized as: When you turn the valve through say 5 degrees, the ratio of the opening size between the start and end positions is the same for any starting position.

Other valve characteristics are possible such as linear or quick opening. These are best shown on graphs. If you plot percentage open across the bottom and area of opening (or Cv, valve co-efficient) upwards, all the curves start at the origin and end in the upper right corner. The linear curve is a straight line from one to the other, equal percentage is like a parabola, hugging the bottom until all of a sudden rocketing up to the upper right corner. The quick open is a mirror image of the equal percentage, hugging the vertical axis until it makes a dash across the common end point. The use of these valve characteristic is applied in the area of control valves which modulate in a middle position nudging and bobbing in response to some varying signal from the control system.

For non-control valves, the principal criteria is tightness. Many moons ago, the only sealing technology was a metal on metal seal. With the development of elastomers such as PTFE (poly tetra flouro ethylene), sold under the trade name Teflon, valves have been expected to not let a bubble of leakage through (hence bubble-tight).

Piping systems and applications demand a wide variety of valve solutions from exotic alloys and plastics for handling corrosive acids and bases to expanded outlet segment control valves for optimizing the control of the flow of wood pulp in solution. Special end connections, materials of construction, operators, painting treatments and non-destructive testing: these are all possible special requirements that the valve manufacturer must be able to supply.

A list of different kinds of valves:

Valves are vital to the internal workings of any four stroke internal combustion engine. They allow air/fuel into, and exhaust gas out of, an engine cylinder. An internal combustion engine has one or more cylinders, where a fuel/air mixture is detonated by a spark plug to create motion. The fuel/air mixture is sucked in through a valve, and exhaust gases are pushed out through another valve. Each cylinder has two or more valves.

I'll ignore how the valves are opened and closed for this writeup (partly because some of it's already done for me); elves are pushing them around or something.

Some understanding of how valves relate to the inner workings of engines would probably be useful here. I'll not go into too much detail of how a four-stroke engine works, suffice it to say that it contains one or more cylinders (most commonly there are four), each of which contains a piston. The pistons produce power by going through what's called the ottoman cycle (assuming some basic knowledge of engines here):

  1. The piston starts at the top of the cylinder bore and descends. As it does, an inlet valve at the top of the cylinder (the cylinder head) opens to allow the air/fuel mixture into the cylinder. The descent of the piston creates a vaccuum effect that sucks the mixture in. This is the intake stroke.
  2. The piston reaches the bottom of the bore. The inlet valve closes and the piston travels back up the bore, compressing the mixture trapped inside the cylinder. This is the compression stroke.
  3. The piston reaches the highest point of its travel and the air/fuel mixture is at maximum compression. At this point a device on the roof of the cylinder creates a spark, which detonates the air/fuel mixture. The explosion pushes the piston back down the bore and ultimately drives the crankshaft (which the piston is connected to). This is the power stroke.
  4. The piston reaches the bottom of the bore again. The cylinder is now full of waste gas left over from the explosion. This has to go. An exhaust valve at the top of the cylinder now opens and as the piston rises again it pushes the waste gas out through it. This is the exhaust stroke.
That's it. Lather, rinse, repeat. Several squillion times.

A single valve is a trumpet-shaped piece of solid or hollow metal which sits in a small bore in the cylinder head. The exhaust valves in some cars are filled with sodium to help them cool.

xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|   ||   |xxxxx
xxxxx|__/  \__|xxxxx

(combustion chamber)

When the valve opens it descends into the cylinder, allowing gas to flow past it:

xxxxx|   ||   |xxxxx
xxxxx| G || G |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| A || A |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| S || S |xxxxx
xxxxx|   ||   |xxxxx
xxxxx| F || F |xxxxx
      L  ||  L
  W O __/  \__ O W

If any readers have ever experienced something like a cambelt failure it might be clearer now why such damage can occur (repair bills of thousands are not unheard of). See, an open valve is directly in the path of the piston. It is only the exquisitely precise and ingeniously self-regulating timing of an engine that prevents the pistons and valves from colliding.

In older engines, the fuel/air mixture was not compressed as much (the bores were longer or the pistons were shorter), so piston travel did not overlap with the valve travel. It was impossible for the piston to hit any valves on the upstroke, even if they were open. If the timing belt failed in an engine like this, the engine would just stop running.

More recent and current engines use much greater compression. The piston travels further up the bore (which may also be shorter) and does overlap with the valve travel. So, if the valves stop operating for any reason (because of a cambelt failure, for example) and any are left protruding into the cylinder head, they're going to get smacked by the piston as it rises. This is why cambelt failure is so bad. Pistons move fast. Although very small parts, and frequently quite cheap themselves, valves are critical and expensive to reach if damaged. They can also take other parts with them like valve guides, the cylinder head, the piston and in severe cases maybe even the crankshaft, camshaft(s) and hydraulic lifters.

There is at least one valve each on inlet and exhaust duties in a four-stroke engine. The majority of modern four-cylinder engines of two litres and above have two inlet and two exhaust valves per cylinder. This allows more air/fuel into the cylinder at once and allows exhaust gases to escape more efficiently. There are also some 12-valve engines that have one inlet and two exhaust valves per cylinder, presumably in an attempt to compromise between the performance of a 16-valve engine and the economy of 8-valve engines (it is commonly small hatchback cars that have this sort of engine).

There's a bit more to how valves work - they are opened by force but have to close themselves. I won't get into how they're opened; suffice it to say they are pushed downwards from above at precise intervals. The diagram below is an exploded view of the valve assembly from my car's engine, which I imagine is similar to that of most engine designs:

xxxx|     ___     |xxxx
xxxx|    |___|<--------- F
xxxx| ___________ |xxxx
xxxx||___________|<----- E
xxxx|         ___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----___ |xxxx
xxxx| ___-----___|<----- D
xxxx||___-----___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----___ |xxxx
xxxx| ___-----___||xxxx
xxxx||___-----    |xxxx
xxxx|     ___     |xxxx
xxxx|    |___|<--------- C
xxxx| ___________ |xxxx
xxxx||___________|<----- B
xxxx|_____   _____|xxxx
xxxxxxxxxx| |xxxxxxxxxx
xxxxxx           xxxxxx
          |-| <--------- A

This whole assembly sits in a small bore in the cylinder head (x). There is a circular hole at the bottom of this that leads to the combustion chamber. The valve stem goes through there, and the valve seat is a short distance below.

  • A: Valve stem - the thin end of the valve. It has a thread on the end of it. It slots through the rest of the components and F is screwed onto it.
  • B: Valve spring washer - this sits on the bottom of the valve bore and provides something for D to bear on.
  • C: Valve stem oil seal - this stops oil from the top of the engine (camshafts, tappets/hydraulic lifters etc) from getting into the combustion chamber. You don't want oil down there.
  • D: Valve spring - this pulls the valve back up into the cylinder head when the pressure from above has ceased. It is very strong.
  • E: Valve spring seat - this is sort of a 'bowl' that the top of the valve spring sits in. It is held in place by:
  • F: Valve stem key - this screws onto the thread on the top of the valve stem. It holds the assembly below it in place.

So, when the valve assembly is pushed downwards from above, it compresses the valve spring, which is ultimately attached to the top of the valve stem. When the pressure is released, the valve spring immediately expands (they are very strong springs, remember?) and pulls the valve back into its seat in the cylinder head.

Just as an aside, the ultimately limited speed at which valve springs are able to close an open valve is one of the factors that limits the maximum revolutions per minute of overhead valve engines. Formula 1 engines use a method of electromagnetic valve actuation, allowing potentially insane engine speeds.

See also: Internal combustion engine valve actuation methods

  • My car. Bless.
  • Vauxhall Motors Ltd & Bell and Howell PSC; "EPC Electronic Parts Catalogue"
  • Haynes Publishing; "Vauxhall/Opel Omega"; "General engine overhaul procedures"

Valve (?), n. [L. valva the leaf, fold, or valve of a door: cf. F. valve.]


A door; especially, one of a pair of folding doors, or one of the leaves of such a door.

Swift through the valves the visionary fair Repassed. Pope.

Heavily closed, . . . the valves of the barn doors. Longfellow.


A lid, plug, or cover, applied to an aperture so that by its movement, as by swinging, lifting and falling, sliding, turning, or the like, it will open or close the aperture to permit or prevent passage, as of a fluid.

A valve may act automatically so as to be opened by the effort of a fluid to pass in one direction, and closed by the effort to pass in the other direction, as a clack valve; or it may be opened or closed by hand or by mechanism, as a screw valve, or a slide valve.

3. Anat.

One or more membranous partitions, flaps, or folds, which permit the passage of the contents of a vessel or cavity in one direction, but stop or retard the flow in the opposite direction; as, the ileocolic, mitral, and semilunar valves.

4. Bot. (a)

One of the pieces into which a capsule naturally separates when it bursts.


One of the two similar portions of the shell of a diatom.


A small portion of certain anthers, which opens like a trapdoor to allow the pollen to escape, as in the barberry.

5. Zool.

One of the pieces or divisions of bivalve or multivalve shells.

Air valve, Ball valve, Check valve, etc. See under Air. Ball, Check, etc. -- Double-beat valve, a kind of balance valve usually consisting of a movable, open-ended, turban-shaped shell provided with two faces of nearly equal diameters, one above another, which rest upon two corresponding seats when the valve is closed. -- Equilibrium valve. (a) A balance valve. See under Balance. (b) A valve for permitting air, steam, water, etc., to pass into or out of a chamber so as to establish or maintain equal pressure within and without. -- Valve chest Mach., a chamber in which a valve works; especially Steam Engine, the steam chest; -- called in England valve box, and valve casing. See Steam chest, under Steam. -- Valve face Mach., that part of the surface of a valve which comes in contact with the valve seat. -- Valve gear, or Valve motion Steam Engine, the system of parts by which motion is given to the valve or valves for the distribution of steam in the cylinder. For an illustration of one form of valve gear, see Link motion. -- Valve seat. Mach. (a) The fixed surface on which a valve rests or against which it presses. (b) A part or piece on which such a surface is formed. -- Valve stem Mach., a rod attached to a valve, for moving it. -- Valve yoke Mach., a strap embracing a slide valve and connecting it to the valve stem.


© Webster 1913.

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