What is a check valve?
Check valves, also known as non-return valves (NRV), are used to prevent backflow automatically in a piping system. Generally, check valves are kept open by the pressure of the flow and closed by the backpressure or by the weight of the checking mechanism.
Types of check valve
Some common type of check valves is mentioned below
Swing check valve
The swing check valve consists of a disk that revolves around a pin. When the flow pressure becomes higher than the resistance provided by the disk, it swings open to allow the flow. When the flow drops or reverses, the valve disk swings back to its original position, closing the flow. In these types, the flow moves approximately in a straight line.
Broadly, there are two types of swing check valves.
conventional type swing check valve
Here, a disk swings open on a hinge pin or shaft, with the forward flow pressure and swing-close when the flow stops or reverses. Below is a working animation of a conventional swing check valve.
Conventional swing check valve advantages and where recommended to use
- Conventional swing check valves shall be used for sizes ≥ 2”.
- Swing check valves are inexpensive and can be utilized in both horizontal and vertical pipelines (flow direction up) without any alteration, provided they do not incorporate any counterbalance or dashpot.
- A counter counterbalance weight or dashpot can be used where required to prevent slamming or opening with a minimum pressure differential.
- Although widely used, swing check valves are generally best suited to gravity flow and pump systems where velocities are relatively low and where low pressure drop is required.
- It is very efficient for constant-flow fluids.
Conventional swing check valve disadvantages and where not recommended to use
- They are unsuitable for frequent flow reversal as it tends to chatter. The chatter can be avoided by using a swing check valve with outside weight and lever.
- Unsuitable for wide velocity variations.
- Unsuitable for application with pulsating flow, such as a reciprocating compressor.
- Closer response is generally slower than lift check valves.
- Swing check valve should not be installed wth flow in the down position. The weight of the fluid upstream of the disk would tend to open the disk when the line pressure was less upstream, thereby permitting the flow to reverse, destroying the check feature of the valve.
Swing check valve tilting disk type
It has a disk that rotates eccentrically on a shaft mounted above the horizontal axis of the pipe. The tilted disc check valve is designed to stay fully open and stable even at low flow rates, and to close quickly when the flow stops. Its disc has a dome shape, allowing fluid to flow smoothly on both the top and bottom surfaces. The disc is spring-loaded, so when the forward flow pressure drops, the spring pushes the disc to close the valve quickly. These valves are available in wafer and lug-type designs.
Swing check valve tilting disk type advantages and where recommended to use
- The center of gravity of the tilted disc check valve is close to its rotation axis.
This allows the valve to open and close quickly without damaging the seat or disc. - Because it reacts fast when the flow reverses, very little reverse flow occurs, which helps reduce water hammer.
- Because of its design, the tilted disc check valve works better than a swing check valve when the fluid flow is low or pulsating.
- The pressure drop in a tilted disc check valve is much lower than in a lift check valve but slightly higher than in a straight-flow swing check valve. Its design allows fluid to flow from both the top and bottom of the disc. It has only a small pressure drop at the center.
- Tilted disc check valves provide better sealing than swing check valves, especially when there is a higher-pressure difference across the valve.
- Tilted disc check valves can be installed in both horizontal and vertical pipelines. For vertical installation, the flow should move upwards to help the disc close properly with gravity.
Dual disk wafer check valve
A dual disk wafer check valve (also called a double door or butterfly-type check valve) has two spring-loaded half disks mounted on a hinge that open when fluid flows in the forward direction. When the flow stops or reverses, the springs and reverse pressure push the disks closed to prevent backflow. Its slim wafer design allows easy installation between flanges, making it compact and efficient for both horizontal and vertical pipelines.
These provide fast response due to the short path of travel of the half disk center of gravity and low inertia resulting from the light weight, short moment arm of the half disk.
Valve’s installation design should ensure that the disk does not foul with adjacent piping components during open and closure motion.
Dual disk check valve, where recommended to use
- Dual disk check valves are recommended in high flow velocity services, e.g, Gas services.
- Dual disk check valves depend upon internal spring loading for closure and are therefore the most suitable type of check valve for location in a vertical pipe with upward flow.
- These kinds of check valves accommodate various fluids like gases, liquids, and steam etc.
- In fire water or process systems where there is a fire risk, wafer check valves should be used in a drilled-lug style. This design keeps the valve securely held between the flanges, even if a fire damages or weakens the gasket and flange area. With drilled lugs through the valve body, the bolting passes through the valve body itself, so the joint remains supported and does not separate during high heat. This added mechanical strength protects the flange bolts and ensures the pipeline stays intact during a fire emergency.
Dual disk check valve disadvantages and where not recommended to use
Not recommended for size ≤2”.
Should not be used in the following services-
- Dual disk wafer check valves should not be used in SVC lines containing ≥25% hydrogen because their thin wafer body and internal components may not provide the necessary leak-tightness and mechanical integrity required for high-diffusion hydrogen service. Liquids above auto-ignition temperature.
- Dual disk wafer check valves should not be used in steam service because their thin body and spring-loaded internals cannot reliably withstand high temperatures, leading to loss of sealing and premature failure.
- Dual disk wafer check valves should not be used above 260 °C because their spring and hinge components cannot maintain mechanical integrity at such high temperatures.
- Dual disk wafer check valves should not be used in reciprocating compressor or pump services because their disks and springs cannot handle the frequent pulsations and vibrations, leading to rapid wear and failure.
Lift check valve
There are mainly three types of lift check valves.
Lift check valve ball type
A lift check valve ball type uses a spherical ball that lifts off its seat when fluid flows in the forward direction, allowing flow. When the flow stops or reverses, gravity or backpressure pushes the ball back onto the seat, blocking reverse flow.
Lift check valve ball type advantages and where recommended to use
- The valve has low head loss.
- Lift check valve ball type gives a tight closure with minimum back-pressure.
- It has an option to select different ball weights according to need. Selecting a heavier ball can give faster closure and nullify the water hammer effect.
- Ball check valves are commonly used in wastewater systems because their smooth, full-bore design allows solids to pass without building up. The rotating ball also has a self-cleaning action that prevents impurities from sticking and keeps the valve working smoothly.
Lift check valve ball type disadvantages and where not recommended to use
- Ball check valves with a metal ball should not be used in gas or vapor services because of the potential for ball or seat damage due to irregular flow.
- Lift check valves, ball type, should not be used for sizes ≥ 2”.
- Should not be used with wide varying pressure, pulsating flow, or for frequent flow reversal.
Lift check valve disk type
A lift check valve disk type has a disk. When fluid flows in the forward direction, the pressure lifts the disk upward, allowing the fluid to pass through. When the flow stops or reverses, gravity and the reverse pressure push the disk back down onto the seat, closing the valve and preventing backflow.
Due to its narrow globe valve-like flow path, the pressure drop through a lift check valve disk type is higher.
Lift check valve piston type.
A lift check valve piston type has a guided piston that lifts upward when forward flow pressure pushes against it, allowing fluid to pass. When the flow stops or reverses, the piston drops back onto the seat due to gravity and reverse pressure, preventing backflow.
Lift check valve piston type advantages and where recommended to use
- The lift check valve piston type incorporates a piston and cylinder for a dampening effect.
- The lift check valve piston type is ideal for high-pressure applications due to its higher resistance and sealing properties.
Lift check valve disadvantages and where not recommended to use
- All lift check valves cause a relatively high pressure drop, and these types are selected for applications that could result in surge problems.
- Lift check valves are suitable for reciprocating pump outlets and comparable applications.
- Piston and disk types are avoided in dirty and viscous services and reserved for clean liquids and non-condensing gases. , The reason is free movement of the obstructor may be difficult to achieve in non-clean services, and slow response and jamming may occur afterwards.
Axial flow check valve
An axial flow check valve uses a streamlined spring-loaded disc that moves back and forth along the flow direction. When fluid flows forward, the pressure pushes the disc straight back, opening the valve with very low resistance. When the flow stops or reverses, the spring pushes the disc forward onto the seat, closing the valve quickly and preventing backflow.
Axial check valve advantages and where recommended to use
- An axial flow check valve is recommended in systems with wide pressure variations, frequent flow reversals, because its spring-loaded, smoothly guided disc moves in a straight line, with its short disk travel, allowing it to react very quickly and be extremely responsive to changes in flow. This fast, controlled movement minimizes slamming and water hammer, which reduces pressure surges and keeps the system stable.
- For the reason stated above, it can also handle Process pulsations from compressors, turbines, or variable flow, which are smoother, lower frequency, and within the valve’s controlled spring response range. That’s why it’s recommended in compressor discharge lines subject to pulsating or low flow conditions in which tilting disk-type valves may chatter.
Axial check valve disadvantages and where not recommended to use
- These are not recommended for extreme pulsations of reciprocating pump applications, which create very sharp, high-frequency, high-amplitude pulsations, causing the disc to slam open and closed rapidly.
Diaphragm check valve
A diaphragm check valve uses a flexible rubber or elastomer diaphragm that lifts or flexes when fluid flows forward, allowing the fluid to pass. When the flow stops or reverses, the diaphragm returns to its original position and seals against the seat, preventing backflow. Because the diaphragm moves smoothly without metal-to-metal contact, it provides quiet, low-maintenance operation.
Diaphragm check valve advantages and where recommended to use
- It provided stable operations with pressure variation, pulsating flows, and frequent flow reversal systems.
- It is used to handle viscous or abrasive fluids and slurries.
- Diaphragm check valves are important in water treatment because they prevent backflow and help keep the water clean. They are also corrosion-resistant and can handle many types of fluids, even those that are chemically aggressive.
- In chemical processing plants, their flexible diaphragm and durable materials like PTFE let them work reliably even in harsh chemical environments.
- In pharmaceutical manufacturing, where hygiene is extremely important, diaphragm check valves are used as they are easy to clean and have very low leakage, making them ideal for sterile and sensitive production processes.
- The location can be either a horizontal or vertical line.
Diaphragm check valve disadvantages and where not recommended to use
- This is less commonly used than conventional lift and swing check valves.
Selecting check valves on dynamic response and pressure drop
Swing check valves are used for pigging. If pigging is not required and high-pressure drop can not be tolerated, the following alternatives, listed in order of their anti-slamming properties, are used.
- Axial flow check valves
- Dual plate check valves
- Tilting disk check valves
If pressure drop can be tolerated, a piston-type check valve may be used.
Selecting check valves for unstable and pulsating flow
- Standard Swing check valves are to be avoided.
- Tilting disk and dual disk valves are better.
- Axial flow check valves are even better.
- Axial flow check valves should always be used at the compressor outlet other than a reciprocating compressor.
- For a reciprocating compressor and pulsating flow piston or ball-type check valve, a spring-loaded check valve may be used.
Check valves installation recommendations
- If there is a flow-changing fitting like an elbow or a tee, the check valve should be installed at least three pipe diameters downstream from it to ensure proper operation.
- Lift-type and swing-type check valves should not be used on vertical pipes because gravity is needed for their disks to fall back onto the seat properly. In vertical lines, the flow direction can interfere with this movement, causing the valve to not close fully or to chatter, leading to poor sealing and faster wear.
- However, if a swing or tilting-disk check valve must be installed vertically (with upward flow), the disk should not reach a fully stalled (90°) open position because it may get stuck and fail to close when the flow reverses. Keeping the disk slightly short of full open ensures it can always swing back freely and seal properly.
- Any lift check valve mounted on a vertical pipe shall be provided with a spring. This is because gravity alone cannot reliably push the disk or piston back onto the seat. The spring helps the valve close quickly and properly when the flow stops or reverses, preventing backflow.
- Dual disk valves are suited for vertical (with upward flow) applications. This is because their two spring-loaded disks do not rely on gravity to close. The springs help the disks open easily with forward flow and close quickly when the flow stops or reverses, making them reliable in vertical installations.
- No check valves other than axial flow nozzles and dual disk type check valves shall be used at vertical pipes with flow downwards. This is because they have spring-assisted mechanisms that do not depend on gravity to close. Other types, like swing or lift checks, rely on gravity or disk weight, so in downward flow, they may stay open, chatter, or fail to seal properly.
- Typical flow resistance coefficient for a standard 6” valve is as follows-Swing type – 1.0; Axial flow type – 1.4; Lift type (oblique) – 2.0; Piston type -7.0
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