Is a check valve a double valve?

　　A check valve is a two-way valve, meaning it has two openings on the valve body, one for fluid to enter and the other for fluid to exit. There are various types of check valves used for a wide range of applications. Check valves are often part of common household items. Although they come in various sizes and costs, check valves are typically very small, simple, or inexpensive. Check valves operate automatically and are mostly not controlled by a person or any external control; therefore, most do not have any valve handles or stems. The body (housing) of most check valves is made of plastic or metal. An important concept of check valves is the opening pressure, which is the small differential pressure upstream between the inlet and outlet when the valve is operating. Typically, check valves are designed to and can therefore specify a specific opening pressure.

　　Types of check valves
　　Ball check valve
　　A ball check valve is a one-way valve where the closing element, the movable part that blocks the flow, is a ball. In some ball check valves, the ball is spring-loaded to help keep it closed. For those designs without a spring, reverse flow is needed to move the ball toward the seat and form a seal. The inner surface of the main seat of a ball check valve is more or less conical to guide the ball into the seat and form a positive seal when reverse flow stops. Ball check valves are usually very small, simple, and inexpensive. They are commonly used in liquid or gel micro-pump dispensers, spray devices, for pumping air in some rubber balls, manual air pumps, and some other pumps, as well as refillable dispensing syringes. While the ball is typically made of metal, it can be made from other materials; in some special cases, it is made from highly durable or inert materials, such as sapphire. High-pressure HPLC pumps and similar applications often use small inlet and outlet ball check valves with (synthetic) ruby balls and sapphire-made seats or balls and ruby seats, which are hard and chemically resistant. After prolonged use, these check valves may wear out or the seats may crack, requiring replacement. Therefore, these valves can be replaced and are sometimes placed in a small plastic body that fits tightly within a metal fitting that can withstand high pressure and screw into the pump head. There are similar check valves where the disc is not a ball but some other shape, such as a spring-activated lift valve. Ball check valves should not be confused with ball valves, which are a different type of valve where the ball acts as a controllable rotor to stop or direct flow.
　　Swing check valve
　　A swing check valve or tilting disc check valve is a one-way valve where the disc, the movable part that blocks the flow, swings on a hinge or pivot to either block reverse flow at the seat or move away from the seat to allow forward flow. The seat opening cross-section can be perpendicular to the centerline between the two ports or at a certain angle. While swing check valves can come in various sizes, large check valves are typically swing check valves. A common problem caused by swing check valves is known as water hammer. This occurs when the swinging check closes and the flow suddenly stops, causing pressure fluctuations that exert high-speed shock waves on the pipes and valves, putting significant stress on the metals and vibrations in the system. Undetected water hammer can damage pumps, valves, and pipes within the system. The flap valve in a toilet flushing mechanism is an example of this type of valve. The tank pressure that keeps it closed is overcome by manually lifting the flap. It then remains open until the tank drains and the flap falls due to gravity. Another variation of this mechanism is the valve flap, used in applications such as fire and life safety systems. The hinged door remains open only in the direction of flow. The valve flap typically also has a spring that keeps the gate closed when there is no forward pressure. Another example is the backwater valve (for sanitary drainage systems), which prevents flooding caused by sewage backflow. This risk often occurs in sanitary drainage systems connected to combined sewage systems and stormwater drainage systems. It can be caused by heavy rainfall, melting, or flooding.
　　Lift check valve
　　A lift check valve is a one-way valve where the disc, sometimes referred to as a lift, is lifted away from its seat by the higher pressure of the inlet or upstream fluid to allow flow to the outlet or downstream side. Guides keep the disc moving in a vertical line, so the valve can be properly reseated later. When the pressure is no longer high, gravity or higher downstream pressure will cause the disc to fall back onto its seat, closing the valve to stop reverse flow. An inline check valve is similar to a lift check valve. However, this valve typically has a spring that 'lifts' when there is pressure on the upstream side of the valve. The pressure required on the upstream side of the valve to overcome the spring tension is referred to as the 'opening pressure.' When the pressure through the valve falls below the opening pressure, the spring will close the valve to prevent backflow during the process.
　　Applications
　　Check valves are used in many fluid systems, such as in chemical and power plants and many other industrial processes. A typical application in the nuclear industry is in water control systems, discharge pipelines, make-up water, miscellaneous processing systems, N2 systems, and monitoring and sampling systems. In the aerospace and aviation fields, check valves are used in places where there are high vibrations, extreme temperatures, and corrosive fluids. For example, they are used in spacecraft and rocket propellant control for reaction control systems (RCS) and attitude control systems (ACS) as well as aircraft hydraulic systems. Check valves are also commonly used when multiple gases are mixed into a single gas flow. Check valves are installed on each individual gas flow to prevent the gases from mixing in the original gas. For instance, if fuel and oxidizer are to be mixed, check valves are typically used on the fuel and oxidizer sources to ensure that the original gas cylinders remain pure and thus non-flammable. In 2010, NASA's Jet Propulsion Laboratory slightly modified a simple check valve design intended to store liquid samples indicating Martian life in different containers of the equipment without worrying about cross-contamination.
　　Domestic use
　　1. Certain types of irrigation sprinklers and drip irrigation devices have small check valves built in to prevent the system from draining when shut off.
　　2. Check valves used in home heating systems prevent vertical convection, especially when used in conjunction with solar thermal devices, also known as gravity brakes.
　　3. Rainwater collection systems connected to the main water supply from utility providers may require one or more check valves to prevent rainwater from contaminating the main water supply.
　　4. Hydraulic jacks use ball check valves to apply pressure on the lifting side of the jack.
　　5. Check valves are commonly used in inflatable toys, such as toys, mattresses, and boats. This allows the objects to expand without continuous or uninterrupted air pressure.