Check valves are self-activating safety valves that permit gases and liquids to flow in only one direction. The purpose of a check valve is to prevent process flow from reversing in the system which could damage equipment or upset the process. They are classified as one-way directional valves. Check valves are commonly found protecting pumps in liquid applications or compressors in gas systems where backflow could cause the pump or compressor to shut down. They are also applied in process systems that have varying pressures, which must be kept separate. They do not need an outside power supply to operate since they use the pressure drop created by the media flow.
|Prevents backflow||Can’t be used with pulsating systems|
|Maintains pressure||Closing element may slam close causing damage and excessive wear|
|Serves as backup system|
|Most can be used horizontally as well as vertically|
Check valves are used to prevent backflow and maintain pressure. They are commonly used as: a backup in case a pump fails, to help compressors and reciprocating pumps function, and when the system pressure exceeds the primary equipment capabilities. Check valves can be seen in industries such as refining, petrochemical, chemical, etc., oilfield production, water, steam, refining petro, and viscous fluids. Check valves are also commonly found in waste-water management systems and in manufacturing.
Check valves use a variety of technologies to allow and stop the flow of liquids and gases.
Swing check valves allow full, unobstructed flow, and automatically close as pressure decreases. Full closure occurs when flow reaches zero. Swing check valves are recommended for use in systems that use gate valves because of the low pressure drop across the valve. The valve is composed of a disk and hinge which are suspended from the body by a hinge pin. The seating in this valve is either metal-to-metal or metal seat to composition disk. A composition disk is recommended for services where particles may be present in the media, where noise is objectionable or where positive shutoff is required. The body of the swing check valve is available in a Y-pattern or straight body design. The straight body design has the disc hinged at the top so that it seals against the seat, which is integral with the body. A single-disc swing check valve is designed with the closure element attached to the top of the cap.
Advantages: Turbulence and pressure drop are very low, replaceable seat rings, angled seating surface permits easier opening at lower pressures, more positive sealing, and less shock when closing under higher pressures.
Disadvantages: Cannot be used in systems with pulsating flows. The continual flapping would be destructive to the seating elements.
Lift-check valves feature a guided disc and a similar seating arrangement as globe valves. They are best used in systems with globe valves and plastic check valves. There are three body patterns (horizontal, angle, and vertical) which can be installed in horizontal or vertical lines with upward flow. For proper function of lift check valves, the flow must enter below the seat. The pressure of the upward flow raises the seat within guides from the seat. The disk falls back down to close the pipe when the flow stops or reverses. When installed horizontally, the disk is suspended by a system of guide ribs.
Silent or center guide valves are similar to lift check valves, with a center guide extending from inlet to outlet ports. The valve stopper is a spring and bushing actuated to keep the movement “quiet.” Lift check valves are composed of a metallic body with seats either integrated with the body or contained in renewable seat rings. The disk construction is similar to the construction of globe valves, with either metal or composition disks. Piston check valves are very similar to lift check valves as well. They provide the best seal tightness and are the most practical for small sizes.
A ball check valve uses a free-floating or spring loaded ball resting in a seat ring as the closure element. It is similar in construction to a ball valve. The mechanics of check valve operation are relatively simple. Most check valves contain a ball that sits freely above the seat, which has only one through-hole. The ball has a slightly larger diameter than that of the through-hole. When the pressure behind the seat exceeds that above the ball, liquid is allowed to flow through the valve; however, once the pressure above the ball exceeds the pressure below the seat, the ball returns to rest in the seat, forming a seal that prevents backflow. They are best used for services with viscous fluids or when there is sediment in the system because the ball remains clean with smooth operation.
Advantages: Can be used with steam, air, gas, water, and vapor lines with high flow velocities, can be installed in lines with vertical flow, available with renewable seat rings.
A double-disc or wafer check valve consists of two half-circle disks hinged together. The two disks fold together upon positive flow and retract to a full-circle to close against reverse flow. Wafer check valves were designed as a smaller, less expensive option from lift and swing check valves. Wafer check valves are designed to fit between a set of flanges. The disc in disc check valves moves in a plane at right angles to the flow of the fluid, resisted by the spring that is held in place by the retainer. As the upstream pressure increases and overcomes the force of the spring, and the weight of the disc, the disc lifts off its seat allowing flow through the valve. The spring allows the valve to be installed in any direction. These should not be confused with a double check valve, which is an assembly that contains two distinct check valves.
Disc check valve. Image Credit: Spiraxsarco.com
Duckbill check valves are flow-sensitive, variable-area, valves. They get their name from their shape, which consists of two flaps shaped like a duck’s bill. In zero flow conditions, the valve remains closed. As the flow increases, the pressure on the flaps increases and the valve opens.
Duckbill check valve. Image Credit: tideflex.com
Butterfly check valves have a similar construction and flow characteristics as butterfly valves. They are often used together in flow management systems. Butterfly check valves are designed so the body provides enough room for easy movement of the butterfly disk. The design allows for flexible sealing against the bore of the valve body at 45° and reduces the slamming action. The valve body is a length of pipe with connections on both ends and a finely bored finished. The simple body design permits large diameter valves and the installation of seat lines made of many materials. When installing a butterfly check valve, it is important to ensure the flow entering the valve comes from the hinge post end of the valve to protect the valve from stopping all flow.
Advantages: Quiet operation, can be used in the same applications as butterfly valves, can be installed horizontally or vertically and can handle vertical flow, and design reduces slamming action.
Disadvantages: Cannot be used with clean or sterile applications since the valve cannot be easily cleaned. Dirt may cause excessive wear on the disk.
Umbrella check valves are elastomeric self-actuating devices. These valves simply press into a hole and can be designed to function within a specified pressure range. The name umbrella comes from the general shape of the device.
Image Credit: DFT Inc.
Backflow preventers are assemblies of two check valves and two shut-off valves. They may also include test cocks for each chamber within the assembly. For more information, visit Engineering360’s Backflow Preventers Specification Guide.
Cartridge/manifold valves are directional control valves that are inserted into manifolds to provide cost-effective, compact system design.
Foot valves are a type of check valve with a built-in strainer. They are used at the point of liquid intake to retain liquid in the system.
Poppet check valves open and close ports with a sealing device composed of a disk, cone, or sphere that is pressed against the seating surface using a spring.
Tilting disk check valves use a pivoted disk as the closure element. They are similar to swing check valves but the pivot axis is close to the center of the flow stream. The disk remains closed against the seat under no- flow conditions and is forced open by the flow. Tilting disk check valves are best used in applications which require rapid response and when freedom from sticking is essential. Rapid response is possible because the center of mass of the disk is close to the pivot axis. When fully open, the valve exhibits a low pressure drop.
Advantages: very low pressure drop, fast closing, will not “stick” in dirty systems, minimizes disk slamming.
Disadvantages: Can’t be used in rapidly fluctuating flow, seat tightness may deteriorate at low differential pressure.
Performance specifications include valve size, pressure rating, media temperature, and valve flow coefficient.
Pressure drop – When selecting a check valve it is important to understand the pressure drop that will occur in the flow management system. The changes in pressure in the system are responsible for controlling the flow and therefore function of the check valve. The shutoff tightness depends on the reverse pressure drop, where the downstream pressure exceeds the upstream pressure. This means that the larger the reverse pressure drop, the greater the downstream pressure will push against the closure element providing better shutoff.
Additional information on valve flow can be found on Engineering360’s Valve Flow and Sizing page.
Fluid flow in the desired direction opens the valve, while backflow forces the valve closed. Backflow is prevented by the weight of the check mechanism, back pressure, by a spring, or by a combination of these means. The mechanism is commonly referred to as a disk, but ball and poppet are other interchangeable terms used. Ideally, the check valve would provide zero resistance to normal flow and full resistance to flow (leakage) in the reverse direction.
The media plays an important role when selecting the material the valve body and disc will be made of as well as the type and speed of the actuator. There are a wide variety of materials that could be in the valve system.
Check valves are made of a wide variety of materials including metallic and nonmetallic options. When selecting a material, the operating environment (i.e. ambient heat), lifespan (i.e. maintenance), and media (i.e. gas or corrosive liquid) should be considered.
Connection types for check valves should be selected based on the connections already available in the system. For proper function, it is critical that the valve is able to make a strong and dependable connection with the piping in the system.
|Threaded||The valve has internal or external threads for inlet or outlet connection(s).|
|Compression fitting||A sealed pipe connection without soldering or threading. As the nut on one fitting is tightened, it compresses a washer around the second pipe, forming a watertight closure.|
|Bolt flange||The valve has a bolt flange(s) for inlet or outlet connection.|
|Clamp flange||The valve has a clamp flange(s) for inlet or outlet connection.|
|Union||The valve has a union connection for inlet or outlet connection(s).|
|Tube fitting||The valve has a connection for directly joining tubing at the inlet and/or outlet connections.|
|Butt weld||The valve has a butt weld sized connection for inlet or outlet connection.|
|Sock weld/solder||The valve has a socket weld connection for inlet or outlet connection.|
FlowServe- Edward Valves (pdf)
PDHEngineering- Valve Fundamentals
Skousen, Philip L. Valve Handbook. New York: McGraw-Hill, 1998. Print.
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