A introduction to check valve
2008-08-07

Check valves, also referred to as “non-return” or “one-way directional” valves, are very simple devices that allow fluid, air or gas to flow in only one direction. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. They are operated entirely by reaction to the line fluid and therefore do not require any external actuation. Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. When the fluid moves in the pre-determined direction, the check valves open. Any backflow is prevented by the moveable portion of the valves. A swinging disc, ball, plunger or poppet moves out of the way of the original flow. Since these devices are slightly larger than the through hole, the pressure of backflow will cause them to tightly seal, preventing reversal of flow. So an important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Domestically, check valves are found in many devices such as faucets, toilets and dishwashers. Industries use check valves to control flows of all types—from the thinnest gas to radio active materials, from molten metal to highly corrosive materials. Check valves are available in a wide range of sizes and costs, many check valves are very small, simple, and/or cheap. The simplest check valves can be purchased at the local hardware store, but check valves may also be precision-designed for highly sophisticated systems. Check valve is often made of a wide variety of materials. Some are made of exclusively manufacture plastic. Others may be brass check valves, electro less nickel plated brass check valves and stainless steel check valves. The components of check valves can be made of different materials, such as cast iron check valves or bronze check valves, depending upon their application.

There are a number of reasons for using check valves, which include: • Protection of any item of equipment that can be affected by reverse flow, such as flow meters, strainers and control valves. • To check the pressure surges associated with hydraulic forces, for example, water hammer. These hydraulic forces can cause a wave of pressure to run up and down pipe work until the energy is dissipated. • Prevention of flooding. • Prevention of reverse flow on system shutdown. • Prevention of flow under gravity. • Relief of vacuum conditions. As with isolation valves, there are a number of different check valve designs, each suited to specific applications. The different types of check valve have different applications. Stop-check valves A stop-check valve is a check valve with override control to stop flow regardless of flow direction or pressure. When the valve is open, it acts as a check valve, but the valve can be deliberately shut to stop flow.

Ball check valves A Ball check valve is a check valve in which the disc, the movable part to block the flow, is a spherical ball. In many ball check valves, the ball is spring-loaded to stay shut, but also many do not have a spring inside. For those designs without a spring, reverse flow is required to move the ball toward the seat and create a seal. The interior surface of the seats of ball check valves are more or less conically-tapered to guide the ball into the seat and/or form a positive seal when stopping reverse flow.

Lift check valves The lift check valve is a globe valve without a stem, handle or other opening/closing parts and with an added cover. Because of its large fluid resistance, this valve is used primarily for small-bore applications.

Wafer check valves  Both lift and swing check valves tend to be bulky which limits their size and makes them costly. To overcome this, wafer check valves have been developed. By definition wafer check valves are those that are designed to fit between a set of flanges. This broad definition covers a variety of different designs, including disc check valves and wafer versions of swing or split disc check valves.

Diaphragm check valves A diaphragm check valve uses a flexing rubber diaphragm positioned to create a normally-closed valve. Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount, the pressure differential, for the check valve to open allowing flow. Once positive pressure stops, the diaphragm automatically flexes back to its original closed position.

Swing Check Valves A hinged disc assembly is suspended from the body to allow it to move freely. This configuration minimizes pressure loss and eases fluid flow. Swing check valves can be used for either horizontal or vertical (fluid flowing upward) pipe layouts.

Disc check valves The disc check valve consists of four main components: the body, a disc, a spring and a spring retainer. The disc moves in a plane at right angles to the flow of the fluid, resisted by the spring that is held in place by the retainer. The body is designed to act as an integral centering collar that facilitates installation. Where a 'zero leakage' seal is required, a soft seat can be included. When the force exerted on the disc by the upstream pressure is greater than the force exerted by the spring, the weight of the disc and any downstream pressure, the disc is forced to lift off its seat, allowing flow through the valve. When the differential pressure across the valve is reduced, the spring forces the disc back onto its seat, closing the valve just before reverse flow occurs. This is shown in Figure 12.3.4. The presence of the spring enables the disc check vale to be installed in any direction. Clapper valves A clapper valve is a type of check valve used in or with firefighting, and has a hinged gate (often with a spring urging it shut) that will only remain open in the out flowing direction. Diaphragm check valves A diaphragm check valve uses a flexing rubber diaphragm positioned to create a normally-closed valve. Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount, the pressure differential, for the check valve to open allowing flow. Once positive pressure stops, the diaphragm automatically flexes back to its original closed position.