What is a check valve and what is the use of a check valve
A check valve is an automatic shut off valve. Under pressure and flow, the closure mechanism opens that would allow the media to flow freely. These valves can prevent backflow in a piping system, they are also known as non-return valves. Check valves are installed to protect the piping system and equipment, if the check valves are not installed properly then it could end up in costly repairs and shutdown. Check valves would only allow the flow of fluid in one direction. There are many designs of the check valve. Some rely on the weight of the closing element and fluid flow only to close them. Others have a spring to help close them. Pressure surges could be caused by sudden closure and it would damage the pumping system and piping, so the check valves must protect them from these surges. Check valve slam could happen because of the sudden stoppage of reverse flow through a closing check valve. In order to prevent the slam, the check valve must close very quickly or a pump control valve must close very slowly, anything in the middle would cause havoc in the pumping system.
How does a check valve work
The working of the check valve is automatic. Check valves are two-port valves, which means that they have two ports in the body one is for the fluid entrance and the other one is for the fluid to leave. The flowing material in the pipeline would activate the check valve. The pressure of the fluid passing through the valve in the correct direction would open it automatically and if the flow tries to reverse, then the valve closes automatically. The pressure of the fluid passing through the system opens the valve, while any reversal of the flow will close the valve. The closure is done by the weight of check mechanism, by back pressure, by a spring, or by a combination of these means. These valves are operated entirely by the reaction to the line fluid and they don’t need any external actuation. These valves only operate at minimum pressure and it is called cracking pressure.
What are the parts of the check valve
How to select a check valve
Mostly check valves are selected by comparing the required closing speed, with the closing characteristics of the valve. This selection would lead to good results in the majority of the applications. Another important thing in valve selection is sizing.
What are the types of check valves
Swing check valve
A swing check valve is a valve that uses a disc to block the flow, it is the movable part that swings on a hinge or trunnion, either onto the seat to block reverse flow or off the seat to allow forward flow. Mostly large check valves are swing check valves. The swing check valve has a flap or disk that hangs down in the flow path. When the flow is in the forward direction the pressure of the fluid forces the disc to hinge upwards and thus allows flow through the valve. During the reverse flow, the disc would shut against the seat and stops the flow.
What are the advantages of a swing check valve
- Suitable for non-pulsating flow
- Not good for vertical upward flow
- They are available in wafer design for mounting between flanges
- Swing check valves can be used for vertical and horizontal applications
Lift check valve
Lift check valves are mostly used for high-pressure service, where the velocity of flow is high. They are suitable for horizontal and vertical lines with upward flow. Lift check valves can be used with steam, air, gas, water, and vapor lines with high flow velocities. Lift check valves are available in three body patterns horizontal, angle, and vertical. The configuration of the lift valve is similar to globe valves and the only difference is that the disc or plug is automatically operated. A cone-shaped plug separates the inlet and outlet ports. When the flow into the valve is in the forward direction, the pressure of the fluid lifts the cone of its seat, opening the valve. The cone will return to its seat, due to reverse flow and it will be held there by reverse flow pressure.
What are the advantages of a lift check valve
- They are good for pulsating flow
- It can be used in the vertical upward flow
- They are economical and reliable in operation
- It only needs less maintenance
- Seat wear is limited because of the use of metal seat
Piston check valve
Piston check valves are used to protect the pump or similar equipment. These valves allow the flow in one direction and it prevents the flow reversal due to backpressure. These valves provide a better pressure drop in pipelines. The design of the piston valve will provide a tight seal and a fast reaction to the closure impulse. Piston check valves are used in conjunction with globe and angle valves in the piping system experiencing very frequent changes in the flow direction. These types of valves are used in water, steam, and air systems.
What are the advantages of a piston check valve
- It can be used for pulsating flow
- Subject to fouling with paraffin and debris
Ball check valve
This valve consists of a rubber-coated ball that is normally seated on the inlet to the valve sealing off the inlet. When pressure is exerted on to the ball it will move off its seat along a guide rail, allowing fluid to pass through the inlet. When the fluid pressure drops then the ball slides back into its position on the inlet seat. These valves are mostly used in liquid systems because they won’t provide a tight seal using a ball.
What are the advantages of a ball check valve
- It can be used in vertical lines
- Usually for sizes 1-in. and smaller
- It doesn’t have a tendency for slam shut or flow reversal
Diaphragm check valve
A flexible rubber is placed in a mesh or perforated cone with the point in the direction of flow in the pipeline. Flow in the forward direction deflects the diaphragm inwards. Which would allow the free passage of the fluid? When there is no flow or backpressure exists, the diaphragm would return to its original position, closing the valve.
What are the applications of diaphragm check valves
- These valves are widely used in the chemical industry and for slurries
- It can be used in purge gas lines, which feed into lines, handling slurry or gluey substances. Under these conditions, the diaphragm valve would operate at great reliability, while other valves will hang up quickly
Stop type check valve
A stop check valve is a non-return globe valve, these check valves override the control to stop flow regardless of flow direction or pressure. In addition to closing in response to backflow or insufficient forward pressure, it can also deliberately shut by an external mechanism, thereby preventing any flow regardless of forwarding pressure. With the valve stem raised it acts as a lift check valve that would allow flow only from below the disc. If there is no flow, or if flow reverses, the disc drops into the seats. When the stem is lowered to the closed position, the disc cannot lift and flow is stopped in both directions.
What is the difference between the check valve and the non-return valve
The check valve and non-return valve allows the fluid flow only in one direction, while the check valve can also prevent the backflow, these valves close automatically if the fluid flow is in a reverse direction.
Why do we need a check valve
- It can protect any item of equipment that can be affected by reverse flow, such as flow meters, control valves, and strainers
- It can prevent flooding
- Prevention of reverse flow on system shut down
- Prevention of flow under gravity
- Relief of vacuum conditions
- It can check pressure surges associated with hydraulic forces
What are the general characteristics of check valves
- Low-pressure change
- Quick opening
- Low maintenance costs
- It can be used for high and low pressure
- Reliable and long service life
- They are used for both horizontal and vertical flow
What are the advantages of check valves
- It prevents backflow
- Maintains pressure
- They also serve as a backup system
- They are self-actuated and doesn’t need any external means to actuate the valve either to open or close
What are the disadvantages of check valves
- It can’t be used with the pulsating system
- The closing element will slam close and it would cause damage and excessive wear
- In the open position, the valve disc may stick
What are the applications of check valves
- Check valves are used in many industries such as refining, petrochemical, chemical…
- These valves are also used in wastewater management systems and in manufacturing
- Check valves are used in domestic heating systems to prevent vertical convection, especially in combination with solar thermal installations
- Check valves are often used when multiple gases are mixed in one gas stream
- Check valves are used with some types of pump, piston-driven and diaphragm pumps such as metering pumps and pumps for chromatography commonly use inlet and outlet ball check valves
To get more info about the control valve check the following links