Check valves are mechanical devices used in piping and pipeline systems to prevent back-flow. A check valve allows fluid flow only in one direction that’s why it is called a one-way valve or non-return valve. Check Valve works on the principle of differential pressure. It means the check valve will only open if the upstream pressure is more than the downstream pressure. In situations, if downstream pressure is more, the valve will close preventing back-flow. Closure can also be accomplished by the weight of the check mechanism, by a spring, or by a combination of all these means. One of the main applications of check valves is at the pump outlet to protect the equipment from flow-reversal.
Check valves are automatic valves and unlike other valves, human intervention or external control is not required for their opening or closing. The only purpose of the check valve is to prevent flow reversal or back-flow and they are available in various sizes, designs, and materials.
Working of Check Valves
As already stated that a check valve operates on the principle of differential pressure. For a check valve to open, it must attain a minimum upstream pressure known as cracking pressure. Depending on the check valve design and size, the cracking pressure changes. When the upstream pressure reaches the cracking pressure, the valve opens allowing the fluid to enter. When the upstream pressure falls below the cracking pressure, back pressure is generated and the flow attempt to move from outlet to inlet. At this point, the check valve closes, and the flow halts. The closing mechanism of a check valve varies depending on the design and type of the valve. Spring or Gravity pressure normally assist the closing process.
As the check valve works only in one direction, manufacturers provide an arrow on the valve body indicating the flow direction.
Types of Check Valves
Depending on the movement of the closure member, various types of check valves are available.
- Swing Check Valve
- Wafer Check Valve
- Spring Loaded Check Valves
- Spring Loaded Inline Valves
- Spring Loaded Y
- Ball Check Valves
- Diaphragm check valves
- Lift check valve
- Stop check valve
- Foot Valve
- Duckbill Valve
- Dual Plate Check Valve
Swing Check Valve
A swing check valve is the most widely used check valve. The closing member or the disc swings on a hinge or shaft. To allow the flow, the disc swings off the seat and it swings back onto the seat to block the reverse flow. In an open position, a swing check valve offers very little resistance to the flow. To achieve optimum performance, often a lever and weight or a lever and spring are mounted. The disc weight and the return flow has an impact on the shut-off characteristics of the valve. Swing Check Valve is also known as the Tilting Disc Check valve.
The swing check valve allows full, unobstructed flow and automatically closes once pressure decreases. The following animation video explains the main parts and working methodology of a typical Swing Check Valve
Wafer Check Valve
Wafer check valves are very slim and compact in design and use a swinging disc to allow or block flow. They are lightweight and suitable for various applications. They are economical and are available in various sizes. The following figure shows the typical working of a Wafer Check Valve.
Wafer check valves are ideal for services requiring low pressure loss as the valve operation takes place at very low pressure difference.
Spring Loaded Check Valves
There are two types of Spring Loaded Check Valves; Spring Loaded in-line valves and Spring loaded Y-valves.
In-line valves are also known as Nozzle Check Valves or Silent Check Valves. These valves employ a centrally guided stem-disc assembly along with a compression spring. To open the valve, the flow pressure must be more than the spring force and cracking pressure. In that case, the flow pushes the disc allowing the flow. When the inlet pressure reduces, the spring pushes the disc against the orifice and shut the valve.
The operating principle of Spring-loaded y-check valves are similar to in-line check valves. The only difference is that the spring and movable disc are located at an angle to form a ‘y’ shape. The main advantage of Y type check valves is that it can be inspected and serviced while the valve is still connected to the system.
Ball Check Valves
Ball check valves are simple in operation and commonly used on small pumps and in low head systems. Ball check valves involve a spring-loaded or free-floating spherical ball clapper to shut at pressures below the cracking pressure. In order to guide the ball into the seat and create a positive seal, the sealing seat is conically tapered. However, these valves can easily wear due to prolonged use and require frequent maintenance.
Diaphragm check valves
Diaphragm check valves consist of rubber flexing diaphragms or self-centering discs for preventing backflow. When the inlet pressure is increased, the diaphragm flexes open and flow start. There are two types of the diaphragm check valves;
- Free-floating Normally Open Valve and
- Fixed Flexing Normally closed Valve.
In the case of normally open diaphragm valves, no cracking pressure is required as the self-centering elastomeric diaphragm is free-floating. However, they need back pressure to close the valve. On the other hand, normally closed valves need a certain inlet pressure to overcome the elasticity of the fixed diaphragm.
Due to very low cracking pressure, Diaphragm check valves finds its use in low pressure and vacuum applications.
Lift check valve
A lift check valve is also known as piston check valve. It consists of a guided disc that raises (lifts) up from the valve seat and creating space for media to flow. The inlet pressure must be more than the cracking pressure to overcome gravity and/or a spring force. The valve will close when the inlet pressure decreases below the cracking pressure or there is a back-pressure.
Stop check valve
A stop-check valve is basically, two valves built into one body. It can act as a globe valve for isolation or regulation purpose. Again, It can acts as a check valve to prevent back flow. Contrary to other check valves, Stop check valve has an additional external control mechanism in perpendicular or angular direction. Stop Check valves are popular in steam services like power plants, boiler circulation, steam generators, turbine cooling and safety systems.
A foot valve is a check valve which has a strainer installed on the inlet side to prevent the debris entering into the valve.
Duckbill Check Valves are unique, one-piece, elastomeric components that enable flow to proceed through a soft tube that feeds into the downstream side of the valve wherein back pressure collapses the tube and cuts off the flow.
What is Check Valve Slam?
The check valve slamming phenomenon can be described as follows:
- The ideal check valve is one that closes at the moment the fluid being transported reaches “zero” velocity prior to flow reversal.
- But, No check valve is closed at the point of “zero” flow.
- Closure of the valve occurs after flow reversal has taken place.
- The mean velocity of the fluid is backward at the instant of closure.
- The magnitude of the reverse velocity, Vr, causes the phenomenon of “check valve slam”
- Check valves have “dynamic behavior”, ie “speed of response”. Different check valves behave differently.
- Check valves have “Dynamic Characteristics”, ie different speeds of response, which determines the maximum reverse velocity, Vr max, and, hence, the degree of “check valve slam”.
- The quick slamming creates a pressure spike that is a probable cause of water hammer.
Non-Slam Check Valve
Non-slam check valves are specifically designed valves where the closing member close without
slamming preventing excess pressure spikes. The disc of a non-slam check valve includes an internal spring opposing the opening fluid flow pressure. When the flow media is strong enough, the spring compresses and the valve opens. Again when the flow decreases, the disc is smoothly pushed back toward the valve seating surface by the spring force and stops. For vertical piping runs or complex applications requiring constant and controllable pressure levels, Non-slam check valves are an ideal solution.
The main advantage of non-slam check valves is their ability to effectively prevent water hammer. Hence, pressure swings, vibrations, and component damages are ideally eliminated. As Non-slam valves have a short
stroke, they facilitate quick soft closing of the disc to prevent water hammer. As they consist of only one moving part, the disc itself, non-slam check valves experience minimal wear over time. However, the non-slam check valves are not piggable.
Dual Plate Check Valve
Dual-plate check valves are wafer-type compact valves with a small overall length. They provide excellent hydrodynamic properties that result in very low-pressure losses and they are technically efficient. Their low weight provides advantages during installation, transport, and storage. Dual-Plate check valves are suitable for liquid, gas, steam, condensate, water supply, and in oil and natural gas services. They are designed as non-slam type. With suitable springs, they can be installed in any position.
Desirable Design Characteristics of a check valve
For a check valve to work smoothly the following design characteristics are desirable:
- Moving parts of the valve are of low inertia
- Distance/angle through which the moving element(s) have to travel is minimal and
- Mechanical assistance of closure motion of moving element(s), e.g. spring.
- Tight seal leakage
- Lower pressure loss
Check Valve Symbols
The symbol of check valve can vary slightly from company to company. Normally any one of the following check valve symbols are prevalent in industries.
Materials of Check Valve
Industrial check valves are available in various materials like
- Carbon Steels
- Stainless Steels
- Duplex Stainless Steel
- High Nickel Alloys
Check valve Standards
The following are the various check valve standards followed during piping design
- API Standards: API Spec 6D, API Std 594
- ASME Standards: ASME B16.34
- AWWA Standards: AWWA C508, AWWA C510
- BSI Standards: BS 1868, BS 1873, BS 2080, BS 5152, BS 5153, BS 5154, BS 5160, BS 5352
- MSS Standards: MSS SP-42, MSS SP-61, MSS SP-71, MSS SP-80, MSS SP-84
Applications of Check valves
Check valves are used in various industries like Offshore Oil & Gas Production, Onshore Oil & Gas Production, Gas Plant, LNG, Liquid Gases, Refinery, Petrochemical, Chemical, Fertilizer, Terminals, Pipeline, Power, Desalination, Water, etc. They are widely used in Pump and Compressor discharge, Heat Exchangers, Reactors, Vessels, and Separators to prevent flow reversal. Few typical applications are shown below: