Sep. 09, 2024
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Here is the most complete guide on the internet to check valves.
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A check valve is an automatic safety device designed to permit fluids and gases to flow in one direction while preventing reverse flow. When pressure is applied, fluids enter the valve through the inlet, pushing the disc off the valve seat and allowing the fluid to exit through the outlet.
Flow remains steady as long as the pressure is sufficient to keep the disc open. When the pressure drops, the disc automatically returns to its seat, shutting off the flow.
The pressure required to open the valve is known as the cracking pressure, which can range from 3 psi to 350 psi depending on the size, purpose, and function of the check valve.
Check valves ensure that gases or liquids flow in the correct direction and create tight seals to prevent leaks. By regulating the flow of liquids, check valves help protect pumps and compressors from significant damage.
Check valves are known by various names depending on their use and design. Common names include clack, non-return (NRV), reflux, retention, and one-way valves. Despite their simple design and function, these valves are crucial for protecting pumps, piping systems, and fluid flow.
The swing check valve, shown in the image below, is the most common type. It features a full port design, allowing the disc to be completely out of the way during fluid flow. The disc opens with the direction of the flow and remains open while the flow continues. When the flow slows or stops, the disc moves back into the closed position due to gravity.
The primary function of a check valve is to prevent reverse flow, which can damage pumps and other mechanisms. When a check valve closes, it can cause water hammer, where the fluid in the system slams against the closed valve. This necessitates that check valves be resilient, durable, and constructed from high-quality materials to withstand such impacts.
Check valves can generate noise and may require noise suppression. To address this, various control mechanisms are incorporated into their design. Springs, levers, or weights are used to manage surges and prevent the valve from slamming shut.
Check valves come in numerous varieties to suit various applications. They are used in both industrial and commercial settings. In gardening, check valves regulate the flow of fertilizers and water in irrigation systems. In the aerospace and aircraft industries, they control corrosive fluids, hydraulic systems, and fuel flow.
While the primary function of check valves remains consistent across applications, the types of check valves vary based on factors such as flow rate, media gravity and temperature, line size, pressure, and flow velocity, as illustrated in the chart below.
Application of Check Valves Type Flow Media Type Velocity Range FPS(m/s) Recommended Check Valve Uniform With Insignificant Reversal Water Or Oil 1 to 6 (0.3 to 2) Swing Check w/ Lever and ctrl wt. Steam, Water, Gas 7 to 100 (2 to 30) Simple Swing Uniform Water Or Oil 5 to 10 Max (a.5 to 3) In-line Guided Disc Pulsating Air Or Gas 5 to 10 Max (a.5 to 3) In-line Guided Disc w/Cushion Chamber Uniform With Normal Reversal Water Or Oil 7 to 10 (2 to 3) Swing w/ Spring Assist To Close Uniform With Severe Reversal Water Or Oil 7 to 10 (2 to 3) Swing w/ Dashpot Uniform Or Pulsating Steam, Water, Or Gas 8 to 160 (2.5 to 50) Tee- Or Inclined- Pattern Lift Uniform Or Pulsating (Severe Revesal) Steam, Water, Or Gas 10 to 160 (3 to 50) Tee- Pattern Lift w/ Dashpot Uniform Steam, Water, Or Gas 12 to 250 (4 to 75) Tilting Disc Uniform Or Pulsating Steam, Water, Or Gas Or Oil 20 to 250 (6 to 75) Wye-Pattern Lift Uniform Or Pulsating (Severe Revesal) Wye-Pattern Lift w/ dashpotSelecting the appropriate check valve requires careful consideration of the application's specific needs. Engineers must choose a valve that matches the type of media, pipe size, pressure gradient, fluid velocity, and pumping mechanism. Evaluating these factors is essential to ensure safe operation and to protect equipment.
Check valves come in a broad range, from standard models available at home improvement stores to custom designs tailored for specific applications. Given the constant development of new check valves, it's challenging to provide a comprehensive list. However, despite their variety, all check valves share certain common descriptive factors.
A swing check valve is a self-activating valve designed to prevent backflow or allow flow in only one direction. It features a disc that pivots on a hinge away from the valve seat to permit forward flow. When the flow ceases, the disc swings back into position to block reverse flow. The disc must be heavy enough to resist the impact of the return flow. Swing check valves are known for minimizing turbulence and having a low pressure drop.
Lever and weight swing check valves are ideal for situations where water hammer might occur, while those with lever and spring mechanisms are suited for high-pressure and high-flow velocity applications. Regardless of the type, it is essential to regularly inspect and clean the disc of swing check valves.
Wafer check valves feature a slim disc that swings to regulate flow. When the media flows in the intended direction, it lifts the disc off its seat. If the flow reverses, the disc moves back to seal the valve and prevent backflow. Often, a spring or lever is incorporated to ensure quicker closing and to help minimize water hammer.
The thin, short, and compact design of wafer check valves makes them ideal for small pipe systems and systems that move solid or semi solid media.
A tilting disc check valve features a disc that pivots at its center, allowing fluid to flow over both the top and bottom of the disc. This design is particularly useful in applications with frequent flow reversals. Unlike a swing check valve, where the flow velocity keeps the disc open, a tilting disc valve relies on the flow to maintain the open position. The disc is small and lightweight, with its center of gravity near the pivot point. This valve exhibits a low pressure drop at low flow rates but experiences a higher pressure drop at high flow rates.
Ball check valves use a ball that moves up and down in the valve to block the flow. The seat of the valve is designed to fit the ball and has a conically shaped chamber that guides the ball to the seat. The weight of the ball can vary depending on the pump capacity and the possibility of water hammer.
When reverse flow occurs, the ball returns to its seat, sealing off the flow. With sufficient pressure, the ball is lifted away from the seat. As the pressure decreases, the ball drops back down and is guided to its seat to block the flow.
Link to Yaxing Valve
Featured content:A butterfly check valve is also known as a dual, folding disc, double disc, or splits check valve. The halves of the disc open toward the centerline, as seen in the diagram, during the forward flow of the fluid. With reverse flow, the halves open and seal the pipe. The short distance that the halves have to travel lessens the slamming effect on the discs. Butterfly check valves operate very quietly making them ideal for heating, ventilation, and air conditioning systems.
The flow for lift check valves enters below the seat of the valve. The pressure of the flow lifts the disc or ball off the seat. When the flow weakens, slows, reverses, or stops, gravity forces the disc or ball downward onto the seat. They are ideal for systems with high flow and velocity rates.
Silent check valves, or poppet check valves, prevent water hammer or shock by closing before fluid flow can reverse. They close quickly and smoothly, reducing the risk of damage to surrounding materials.
Duckbill check valves are made of rubber or synthetic elastomer and have the shape of a duck&#;s beak. The open end of the valve is stretched over the outlet to the supply line. The other end retains its normal shape. Upstream pressure forces the duckbill lips open allowing the flow. As the pressure increases, the lips open wider. As the pressure gets lower, the lips return to a flatten and close.
Diaphragm check valves feature a rubber diaphragm or disc that seals the valve. The valve is centered in the seating area, with the sealing surface on the inlet side and an opening in the center. When backflow occurs, the rubber disc covers the sealing opening, effectively closing it to prevent reverse flow.
A foot check valve is installed at the bottom of a pipe connected to a pump. It functions similarly to a ball check valve but includes a screen to block debris. The valve's opening is larger than that of the pipe. Foot valves help maintain the pump&#;s prime. During operation, suction from the pump pulls water up through the pipe and foot valve. When the pump is turned off, gravity causes the water in the pipe to flow back. The foot check valve prevents this by blocking the water's path and is held closed by the weight of the water.
Pneumatic check valves, also known as air check valves, regulate the flow of air from a compressor by allowing air to enter but preventing it from escaping. They are used in pneumatic circuits where airflow needs to be controlled in a single direction.
Since the invention of the self-sealing valve in the early 20th century, check valves have become essential for addressing backflow issues across various industries. Any sector involved in transporting products through pipelines relies on check valves within their systems.
Check valves are cost-effective devices that help prevent waste and protect equipment. They ensure that fluids traveling through piping networks are not lost due to poor handling. By automatically stopping liquid flow when pressure drops or flow decreases, check valves block backflow and retain the fluid within the system.
Check valves in pumps is one of their more common uses.
Parallel Pump System &#; In a parallel pump system, a check valve prevents the duty pump from pumping into the standby pump.
Pressure Relief &#; Spring loaded check valves are used in gas applications for low pressure conditions. If the pressure in the system rises above the cracking pressure, the check valve safely releases the gas.
In the pharmaceutical industry, check valves are essential for fluid movement, as they regulate pressure to ensure product quality, efficiency, and production. Only sanitary check valves are suitable for this application, as they must be easily cleanable without disassembly and feature silent closing with a tight seal.
Orifice check valves are employed in the hydraulic systems of landing gear actuators. During gear retraction, fluid flows to raise the gear. When extending the gear, the check valve regulates the flow out of the actuator to prevent uncontrolled dropping. In addition to landing gear systems, check valves are also used in aircraft hydraulic, fuel, and pneumatic systems.
In an irrigation system, check valves are positioned near the source to prevent backflow and siphoning of irrigation water, ensuring that water does not flow back into the source.
Check valves are found in vehicles that were made before the invention of the electronic injection system. Old car fuel pumps have a check valve located at the pump&#;s inlet and one at the outlet, They are designed to force the fuel to flow in the correct direction. When the check valve goes bad, the fuel pressure goes down.
Water in a home should only move from the supply line and out through a fixture or from a drain to the sewer. Check valves are located in homes to prevent cross connection or backflow. They come in several varieties including ball and swing check valves. Domestic check valves can be seen below on a diagram of a hot water system for a home.
In heating boilers, check valves prevent hot water from circulating when the thermostat is not active. They stop water from returning to the boiler, which helps avoid excessive pressure buildup. Additionally, in a home heating system, a check valve ensures that boiler water does not flow into the domestic water supply, protecting it from contamination used for drinking, bathing, or cooking.
Liquid fuel check valves are engineered to work with various fuels, including jet fuel, hydraulic oil, synthetic oil, and air. They are designed to ensure stable operation from the valve's initial crack to full flow. The valve features an inverted spool with an integrated pressure sensor to maintain balance and stability, keeping a consistent delta pressure within the valve. This design allows for smooth and uniform valve opening.
To understand how a check valve functions, it's crucial to first examine its construction. A basic check valve consists of four key components: the body, seat, disc, and cover, as illustrated in the diagram below.
Depending on the design and function of the check valve, additional components such as springs, balls, stems, and hinge pins may be incorporated into the basic elements.
Swing check valves feature an unguided disc that swings open fully when flow pressure is applied. They are available in various sizes and designs to suit different applications.
Lift check valves use a guided piston that rises when flow enters the valve. The seat has a barrel design and is secured with screws and seals. The valve's opening matches the size of both the inlet and outlet.
Tilting check valves are designed to address some issues associated with swing check valves, which have been the most common type. They can stay open at low flow velocities and close swiftly when the flow stops. They are particularly effective in high-velocity applications.
The folding disc check valve features a split disc design and a wafer body pattern with a soft seat. The discs are held in place by a pivot rod and open in response to the flow pressure. Springs on the discs assist in closing the valve.
Vertical check valves are also known as spring check valves A spring keeps the disc pressed against the seat in a vertical or spring check valve. The valve remains closed due to the spring until sufficient pressure is applied to compress the spring and open the valve. While the pressure is maintained, the valve stays open and the spring remains compressed. When the pressure drops, the spring pushes the disc back onto the seat, shutting off the flow. Vertical and spring check valves are known for their reliable backflow protection.
Check valves are typically constructed from durable materials to endure high-pressure conditions. Common materials used in their manufacture include PVC, CPVC, bronze, brass, iron, and stainless steel.
Polyvinyl Chloride (PVC): PVC is corrosion resistant and flexible. The smooth surface of PVC allows the check valve parts to move easily.
Chlorinated Polyvinyl Chloride (CPVC): CPVC has the same qualities as PVC but is able to endure high temperature applications.
Bronze: Bronze can be used for low and medium pressure applications, be cast in complex configurations, and is corrosion resistant.
Brass: Brass has the same abilities as bronze as well as the same machinability and is less expensive than bronze.
Cast Iron:Cast iron check valves are used for hot and cold water, HVAC, steam, gas, and utility services due to its excellent corrosion resistance.
Ductile Iron:Ductile iron has more than 3% carbon so it can be bent and shaped easily. It is stronger than cast iron and is easier to form into check valves.
Iron: Iron is used for steam, water, oil, and gas applications. It can endure a wide range of temperatures and pressures. Its excellent performance balances its high cost.
Stainless Steel: Stainless steel is corrosion resistant, durable, and can be used in harsh conditions, including chemical applications.
Polypropylene (PP):PP is used to make check valves due to its exceptional resistance to corrosion, which is superior to CPVC and PVC.
Polyvinylidene Difluoride (PVDF):PVDF plastic is used in applications where exceptional purity and resistance to acids, solvents, and hydrocarbons are a necessity.
Cast Steel:Cast steel is used to produce check valves due to its sudden impact resistance without deforming, breaking, or bending. It can also be easily shaped to any type of check valve.
The major benefit of check valves is their ability to perform without having to be monitored or controlled. Their basic design allows them to be inserted into a pipe&#;s flow and be able to control the flow without being managed.
Regardless of their many benefits, check valves are like any other type of mechanism and have drawbacks.
Water hammer is caused by a pressure surge when there is a sudden stop in the flow of a gas or fluid and the valve suddenly closes, which causes noise and vibrations. Water hammer can damage the system and lead to costly repairs
Water hammer can be mitigated by using check valves that close quickly, thereby preventing pressure surges and shock waves. Silent check valves are one effective solution to address this issue.
Reverse flow can be damaging and costly, as it may cause a pump to spin in the wrong direction. This issue can be addressed with tightly fitting, fast-closing check valves. Spring-assisted check valves, in particular, are beneficial because they react quickly to prevent reverse flow.
Some check valve systems experience chattering due to frequent opening and closing of the valve, often caused by an oversized valve. Proper sizing is crucial when installing a check valve to ensure it fits the application. The disc must remain stable when open and create a complete seal when closed to prevent issues like fluttering, repeated cycling, and potential valve failure.
Improper installation and assembly of a check valve can lead to future issues. The initial step is to select the appropriate check valve for the application, considering factors such as flow capacity, positioning, and orientation. Installing a check valve too close to a pump, for example, can result in turbulence and potential damage to the valve.
Check valves should be installed at least several straight pipe diameters upstream from circulators, elbows, tees, and strainers to avoid turbulence and prevent the disc from rattling against the seat.
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