Check valves are double-pass valves, meaning they have two openings in the body, one for fluid entry and one for fluid exit. Various types of check valves are available for a wide variety of applications. Check valves are usually part of common household items. Although they come in a variety of sizes and costs, check valves are usually very small, simple, or inexpensive. Check valves operate automatically, most of which are not controlled by human or any external control; As a result, most do not have any valve handle or stem. The main body (housing) of most check valves is made of plastic or metal. An important concept of check valves is the open pressure, which is the minimum differential pressure upstream between the inlet and outlet when the valve is in operation. Typically, check valves are designed for and can therefore specify specific opening pressures.
Type of check valve
Ball check valve
A ball check valve is a one-way valve wherein the shutoff member, the movable part, to block the flow, is a ball. In some ball check valves, the ball is spring-loaded to help keep it closed. For those without springs, reverse flow is required to move the ball toward the seat and seal. The inner surface of the main seat of the ball check valve is more or less conical to guide the ball into the seat and form a positive seal when reverse flow is stopped. Ball check valves are usually very small, simple and inexpensive. They are commonly used in liquid or gel micro pump dispensers, spray devices, some rubber balls for pumping air, manual air pumps and some other pumps, and refillable dispensing syringes. Although balls are usually made of metal, they can be made of other materials; In some special cases, it is made of a highly durable or inert material, such as sapphire. High-pressure HPLC pumps and similar applications typically use small inlet and outlet ball check valves with (artificial) ruby ball and sapphire seats [6] or ball and Ruby seat [7], hardness and chemical resistance. Over time, the check valve will eventually wear out or the seat will crack and need to be replaced. The valve is therefore replaceable, sometimes in a small plastic body fitted tightly into a metal fitting that can withstand high pressure and be screwed into the pump head. There are similar check valves in which the disc is not a ball but some other shape, such as a spring-actuated lift valve. Ball check valves should not be confused with ball valves, which are a different type of valve in which the ball ACTS as a controllable rotor to stop or guide the flow.
Swing check valve
Swing check valves or tilting disk check valves are one-way valves in which the disc, when movable, blocks flow and the hinge or pivot on the swing, either blocks reverse flow in the seat or leaves the seat to allow forward flow. The seat opening cross-section may be perpendicular to the center line between the two ports or at an Angle. Although swing check valves may be available in a variety of sizes, large check valves are usually swing check valves. Common problems caused by swing check valves are known as water hammers. This occurs when the swing check is closed and the flow stops abruptly, causing pressure fluctuations that cause high-speed shock waves to act on pipes and valves, putting a lot of pressure on the metal and vibration in the system. Undetected water hammer can damage pumps, valves and piping in the system. The baffle valve in flushing toilet mechanism is an example of this type of valve. The tank pressure to keep it closed was overcome by manually lifting the baffle. Then it stays open until the tank drains and the baffle falls due to gravity. Another variant of this mechanism is the disc for use in applications such as fire protection and fire life safety systems. Hinged doors remain open only in the direction of inflow. The disc usually also has a spring that keeps the gate closed when there is no forward pressure. Another example is the backwater valve (used in sanitary drainage systems), which prevents flooding caused by backflow of sewage. This risk most often occurs in sanitary and drainage systems connected to the combined sewerage and stormwater drainage systems. This can be caused by heavy rainfall, melting or flooding.
Lift check valve
A lift check valve is a one-way valve in which the disc, sometimes referred to as an elevator, can be lifted off its seat by a higher pressure of the inlet or upstream fluid to permit flow to the outlet or downstream side. The guide keeps the disc moving in a vertical line so that the valve can be reinstalled correctly at a later time. When the pressure is no longer high, gravity or higher downstream pressure will cause the disc to drop onto its seat, closing the valve to stop reverse flow. An in-line check valve is a check valve similar to a lift check valve. However, the valve usually has a spring that "rises" when there is pressure on the upstream side of the valve. The pressure required to overcome spring tension on the upstream side of the valve is called "open pressure". When the pressure through the valve is lower than the opening pressure, the spring will close the valve to prevent backflow during the process.
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