Is the check valve a two-way valve?

Check valves are two-way valves, meaning that they have two openings in the valve body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they come in various sizes and costs, check valves are usually very small, simple or cheap. Check valves work automatically and most are not controlled by a person or any external control; therefore, most do not have any valve handle or stem. The body (housing) of most check valves is made of plastic or metal. An important concept of a check valve is the opening pressure, which is the small differential upstream pressure between the inlet and outlet when the valve is running. Typically, check valves are designed for, and can therefore be specified for, a specific cracking pressure.

Types of check valves
Ball check valve
A ball check valve is a one-way valve in which the closing 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 designs without springs, reverse flow is required to move the ball toward the valve seat and form a 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 the reverse flow is stopped. Ball check valves are usually very small, simple and cheap. They are commonly used in liquid or gel micropump dispensers, spray devices, some rubber balls for pumping air, etc., manual air pumps and some other pumps, and refillable dispensing syringes. Although the balls are usually made of metal, they can be made of other materials; in some special cases, highly durable or inert materials, such as sapphire. High pressure HPLC pumps and similar applications commonly use small inlet and outlet ball check valves with seats [6] or ball and ruby seats [7] made of (artificial) ruby balls and sapphires, which are hard and chemical resistant. After long-term use, this check valve will wear out or the seat will crack and need to be replaced. Therefore, this valve can be replaced, sometimes in a small plastic body that fits 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 poppet valve energized by a spring. A ball check valve should not be confused with a ball valve, which is a different type of valve in which the ball acts as a controllable rotor to stop or direct the flow.
Swing check valve
Swing check valves or tilting disc check valves are one-way valves in which the movable part of the disc blocks flow, and the hinge or pivot on the swing either blocks reverse flow on the seat or leaves the seat to allow forward flow. The seat opening cross-section may be perpendicular to the centerline between the two ports or at an angle. While swing check valves can come in a variety of sizes, large check valves are typically swing check valves. A common problem caused by swing check valves is known as water hammer. This happens when the swing check is closed and the flow suddenly stops, causing pressure fluctuations that cause high-speed shock waves to act on the 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 within the system. The flapper valve is an example of this type of valve in a flush toilet mechanism. The tank pressure holding it closed was overcome by manually lifting the flap. It then remains open until the tank drains and the flap falls due to gravity. Another variation of the mechanism is a valve flap, used in applications such as fire fighting and fire life safety systems. The hinged door remains open only in the inflow direction. The valve flap usually also has a spring, which keeps the gate closed when there is no forward pressure. Another example is a backwater valve (used in sanitary drainage systems) to prevent flooding caused by backflow of sewage. This risk often occurs in sanitary drainage systems connected to combined sewage and storm water drainage systems. This could be due to 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 the higher pressure of the inlet or upstream fluid to allow flow to the outlet or downstream side. The guide keeps the disc moving in a vertical line, so the valve can be properly reinstalled later. 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. One in-line check valve is a check valve similar to a lift check valve. However, the valve typically has a spring that will "lift" 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 called the "opening pressure". When the pressure through the valve is lower than the opening pressure, the spring will close the valve to prevent backflow in the process.
Application areas
Check valves are used in many fluid systems, such as chemical and power plants and many other industrial processes. Typical applications in the nuclear industry are feedwater control systems, dump lines, make-up water, miscellaneous treatment systems, N2 systems, and monitoring and sampling systems. In the field of aircraft and aerospace, check valves are used where there are high vibrations, large temperature extremes and corrosive fluids. For example, spacecraft and launch vehicle propellant propellant control for reaction control systems (RCS) and attitude control systems (ACS), as well as aircraft hydraulics. Check valves are also commonly used when multiple gases are mixed into one gas stream. A check valve is installed on each individual gas flow to prevent gas mixing in the original gas. For example, if the fuel and oxidant are to be mixed, check valves are often used on the fuel and oxidant sources to ensure that the original cylinder remains pure and therefore non-flammable. In 2010, NASA's Jet Propulsion Laboratory slightly modified a simple check valve design designed to store liquid samples indicative of Martian life in different reservoirs of the device without worrying about cross-contamination.
Domestic use
1. Some types of irrigation sprinklers and drip emitters have small check valves built in to prevent the system from draining when closed.
2. Check valves for domestic heating systems to prevent vertical convection, especially when used in combination with solar thermal devices, also known as gravity brakes.
3. Rainwater collection systems connected to the utility provider's primary water supply may require the installation of one or more check valves to prevent rainwater contamination of the primary water supply.
4. The hydraulic jack uses a ball check valve to apply pressure on the lifting side of the jack.
5. Check valves are usually used for inflatable toys, such as toys, mattresses and boats. This allows the object to expand without continuous or uninterrupted air pressure.