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What is a piston check valve?
A piston check valve is a type of valve that is used to prevent reverse fluid flow. This type of valve is mechanically automatic and thus it does not need an actuator or valve operator. Piston check valve manufacturers design this valve to allow fluid flow in one direction only. A piston check valve is designed with a dash-pot that consists of a cylinder and a piston for providing cushioning effect when the valve is operating. There are guides in the piston type check valve that help to enhance the proper alignment of the disc and seat configuration when the piston closes. The fluid flows through the valve from below the valve disc. This design helps the fluid flow rate and pressure to force the valve disc to float on the seat which then allows fluid to pass through the valve. The piston check valve closes when the fluid tries to reverse the direction of flow in which the disc is assisted by its weight and gravity. Piston check valve manufacturers produce different varieties of these valves for use in different applications such as in high and low temperature. Piston type check valves used in low temperatures have the seats made of soft materials like rubber and plastics. Those used for high temperature applications have the seats made of metallic materials like brass, bronze, and steel.
Figure: Piston-type check valve.
Components of a piston type check valve
Valve body
This is the part of the piston check valve that contains the internal parts of the valve. The valve body is made of strong materials such as ductile iron, cast iron, stainless steel, and carbon steel among others. The body also serves as the main pressure barrier to ensure fluid pressure is not lost. The body contains the inlet and outlet valve ports where fluid passes through the valve to the piping system.
Bonnet
This is top part of a piston check valve. The bonnet is placed on top of the valve body and the two are connected using various methods such as bolts and nuts, threads, or welds. The weld and bolt connections are suitable for use in high-pressure applications as they offer a tight seal. The screwed or threaded connection involves connecting the body and the bonnet through threads. The welding connection guarantees a very tight seal despite high pressure. However, the welding connection makes it hard to open the valve while in the field for inspection, cleaning or repair.
Disc
This is the part of a piston check valve that is used to close and open the valve.
Spring
The spring is used to help in closing the valve immediately suction pressure starts reducing.
Seat
This part acts as a seal between the valve body and the disc to prevent fluid leakage.
Figure: Components of a piston type check valve.
How does a piston check valve work?
A piston check valve works by employing the principle of pressure difference. To open a piston check valve, the fluid pressure must be greater than the outlet pressure. In this case, the fluid pressure energy becomes high enough to lift the valve disc off the valve seat. When the disc moves upwards, it leaves space for the fluid to pass through the valve. The fluid keeps flowing through the piston check valve until a time when the suction pressure drops. At that instance, the fluid starts reversing the direction of flow from the outlet side to the suction side due to low inlet pressure. However, piston check valve manufacturers design these valves with high sensitivity to pressure so that the valve closes immediately before significant fluid starts flowing backward. Fluid reversing backward causes damage to equipment like boilers or pumps. It can also bring losses due to unnecessary contamination. So the use of piston check valves is inevitable in certain industrial applications.
Figure: Working of a piston check valve in open state (left side) and closed state (right side).
Types of piston check valves
Z-shaped piston check valve
This is a piston check valve that has a z-shaped partition inside the valve. The Z-shape is where the valve seat is mounted. This is one of the most famous and simplest piston check valves. Piston check valve manufacturers make the seat with a horizontal arrangement. This type of design allows the disc to move perpendicular to the axis of the pipeline. Z-shaped piston type check valve is easier to do maintenance, installation, and cleaning.
Figure: Z-shaped piston check valve.
Y-loaded piston check valve
This is a piston check valve in which the disc and the horizontal axis are at an angle of around 45o degrees. This design makes it possible to inspect and clean the valve without disconnecting it from the piping system. The working principle of this piston type check valve is the same as the Z-shaped piston check valve. The only difference is that in the Y-loaded type the piston moves up and down at an angle of around 45o. The y-loaded piston check valve is heavy and it occupies more space for installation.