What Is A Valve?
Definition of a Valve
Valve (noun) — any device for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of a fluid media.
In its simplest form, by squeezing a garden hose to stop flow, your hand and that section of hose become a valve. In its most complex form, a valve has built in electronics or other sensing devices that react to real-time conditions, and the valve will control flow with extreme precision according to how it is programmed.
Practically speaking, most valves have an inlet, an orifice or seat, a disk (or plug, seal etc.) that seals against the orifice, and an outlet. The inlet(s) and outlet(s) are also known as “ports.”
The orifice seat and seal principle can be accomplished a number of ways, in fact, it seems that the valve industry is constantly inventing new ones. Perhaps the most common is the globe style valve, in which the seal moves to press against a “volcano” style orifice. Another common type is the ball valve, in which a ball with a hole through it is rotated within two seals. When the hole is aligned with the inlet and outlet, the valve is open. When the ball is turned, and the solid sides of the ball align with the inlet and outlet, the valve is closed. A plug valve is similar; it has a through hole in a cylindrical or conical shape instead of a ball.
As stated above, the orifice seat and seal appear in many forms. In a typical pinch valve, an all-rubber sleeve is “pinched” closed — very much like the garden hose — in this case, the sleeve functions both as seat and seal. In a swing-type check valve, the seal is a flapper that swings to seal against the orifice. It is held closed by pressure from the valve outlet, and opens under pressure from the inlet.
Beyond these most basic principles, a number of other factors come into play, most notably, actuation. In other words, the force or mechanism that makes the valve open, close, or do whatever its function is.
The simplest form of actuation is manual. A manual valve requires the operator to open, close, or otherwise control the valve “by hand.” Your kitchen faucet is a manual valve. Common industrial manual valves include hand-operated shutoff valves and manual ball valves.
Automatic valves, also known as self-actuating, perform their specific function without external assistance. A safety relief valve on a home water heater is an example of an automatic valve. When pressure in the tank is greater than the spring force built into the valve, the safety valve automatically pops open. Common automatic industrial valves include pressure regulators, check valves, vacuum breakers, and by-pass relief valves.
Mechanically actuated valves require an external device, motor, or other force to operate. These are referred to simply as actuated valves. An example is the solenoid valve in your automatic dishwasher. An electric signal acts upon a coil, which electromagnetically pulls a metallic stem that is attached to the seat; the valve opens and allows flow. At the instant the external force (electricity) is removed, the magnetic field vanishes and a spring closes the valve. Common “actuated” industrial valves include air-actuated ball valves, motorized ball valves, and solenoid valves. A well-designed actuator is modular; it can be mounted on different valves and can be service/replaced without disturbing the liquid handling components.
Some valves use a combination of manual and automatic, automatic and actuated, or manual and actuated. The simplest example is found in the everyday toilet tank; the valve requires manual opening, but then has automatic shutoff via a float. An example of an industrial valve is an air-actuated ball valve with a limit stop; it requires an external force (compressed air to the actuator) to open, but then stops automatically depending on where the limit stop is set.
Other considerations center on what the valve actually does. Most valves are “normally closed.” They remain closed until acted upon by some force. If the valve then closes again when the force is removed, it is a “fail-safe” valve. The solenoid valve in your automatic dishwasher is normally closed and — hopefully — fail safe.
Another type of valve is “normally open.” They are open until acted upon, and often are described as “fail-safe/open.” Normally-open valves are frequently found in cooling systems, where maximum flow is desired at all times, and the valve is closed only when system maintenance is required.
“Throttling” valves are valves that are opened or closed incrementally, restricting flow. The spigot you attach your garden hose to is regularly used as a throttling valve — you open it a little to gently water a flower bed, or wide open for washing a car.
“Diverter” or sampling valves are used to re-direct flow. These have three ports — two inlets or two outlets — and are commonly referred to as 3-way valves. The small adapter you attach to your spigot that enables you to switch between two garden hoses is a diverter valve. In industrial applications, diverter valves are used for blending two inlets, isolating output, sampling, and similar applications.
“Multi-port” valves theoretically includes diverter valves, but more often refers to valves with four or more ports.
Multi-port valves tend to be more complex, and are often designed as a “manifold” instead. Manifold port configurations are limited only by the designer’s imagination and the constraints of the material used for the manifold body. Manifolds can not only have a variety of inlet/outlet combinations and flowpaths, they can also be used to combine a number of different types of valves into one functioning system.
NOTE: Most of the plastic valves people come in contact with in everyday life are radically different from the rugged industrial valves described on these pages. The average plastic valve used on a home aquarium or swimming pool tends to be molded of low-grade white plastics, and is designed for low-cost. For a more detailed explanation of the differences between consumer-oriented plastic valves and industrial plastic valves, please click here.
The basic principles described above could apply to virtually any type of valve material, whether it is a large cast metal valve at an oil refinery, a rigid polyethylene valve for an irrigation system, or an exotic alloy valve for high pressure steam cleaning at a pharmaceutical firm.
More specifically, the industrial plastic valves found on these pages have many elements in common that enable you to better understand their design, construction, and ultimately, selection for a given application.
view of a relief valve with its most basic parts indicated by callouts
The basic components common to most industrial plastic valves are:
- The body. This is usually an all-plastic material. Because this is the primary part in contact with liquid, the plastic material is selected based on its compatibility with the process liquid.
- The bonnet (spring housing, seal housing, air chamber, etc.) also built of plastics. This looks like, and is often confused with, the valve body. As a rule of thumb, if the valve appears to have separate top and bottom sections, the top is generally the bonnet. The bonnet is frequently constructed of the same material as the body, but in cases where it does not contact liquid, it is often built of a different material for cost considerations.
- The main seal (also known as the disk), built of elastomers or fluoroplastics.
- The stem, shaft or other sealing mechanism. The stem is frequently a combination of plastic and steel, sometimes working in combination with a spring. Wetted parts are built of plastics. This is generally housed in the bonnet.
- The body and stem seals, built of elastomers. These are necessary to create a completely sealed unit.
- Fasteners (if any), available in a variety of materials, but usually stainless steel. Because plastic valves tend to be used in either high purity or highly corrosive applications, it is important that the fasteners not be exposed to any process liquid, and have minimal atmospheric exposure.
–Rick Bolger, Marketing & Advertising Manager, Plast-O-Matic Valves, Inc.
–Rick Bolger, Marketing & Advertising Manager, Plast-O-Matic Valves, Inc.