Here is the most complete guide on the internet to check valves.
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A check valve is a safety device designed to allow fluids and gases to flow in one direction. Under pressure, fluids enter the valve at the inlet and exit through the outlet. Uninterrupted flow continues until the pressure drops, or the pump is shut down.
An important aspect of check valves is their cracking pressure, the pressure at which the disk to the valve opens, which can vary from 3 psi to 350 psi depending on the size, purpose, and function of the check valve.
Check valves are an automatic safety device that prevents backflow, ensures gases or liquids are flowing in the right direction, and creates a tight seal to prevent leaks. By controlling the flow of liquids, check valves protect pumps and compressors from serious damage.
The names of check valves change according to their use and how they are manufactured. Their many names include clack, non-return (NRV), reflux, retention, and one-way valves.
The most common type of check valve, swing valve, can be seen in the image below. It is a full port design where the disk is completely out of the way when the fluid is flowing through. The directional flow opens the disk. As the flow continues, the disk remains open. As the flow slows or stops, the disk moves, by the force of gravity, into the closed position.
To control surge and prevent the valve from slamming closed, springs, levers, or weights are added to the assembly to assist in the closing function.
There are endless varieties of check valves to fit multiple applications. Check valves have industrial and commercial use. In gardens, they control the flow of fertilizers and water in irrigation systems. The aerospace and aircraft industries use check valves to control corrosive fluids, hydraulic systems, and the flow of fuel.
Though the function of check valves is the same regardless of where they are being used, the types of check valves vary according to the flow rate, media gravity and temperature, line size, pressure, and velocity of the flow, which can be seen in the chart below.
Each application for a check valve uses a different type. In some cases, a specially designed check valve is used to fit a specific application. The descriptions below are of some of the more common check valves.
The swing check valve is a full-port design, where the disk is out of the way of the flow stream. It is used in applications with a high percentage of solids and a low on/off cycle.
The swing distance of the disk causes the valve to close slowly, which causes the reverse flow to slam the disk closed producing a water hammer.
A tilting disk check valve has its pivot point at the center of the disk so that the fluid flows over the top and bottom of the disk. They are often used where there is frequent flow reversal. When the disk is open, the flow keeps it open, which is different from a swing check valve where the velocity of the flow keeps it open. The disk is small and light with a center of gravity that is close to its pivot point. It has a low pressure drop with low flow rates and a higher pressure drop with high flow rates.
Ball check valves use a ball to block fluid flow and can be spring loaded. Reverse flow moves the ball back to seal off the flow. When there is enough pressure in the flow, the ball is lifted away from the flow. When the pressure decreases, it comes down to close the opening.
A butterfly check valve is also known as a dual, folding disk, double disk, or splits check valve. The halves of the disk 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 disks. 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 disk or ball off the seat. When the flow weakens, slows, reverses, or stops, gravity forces the disk or ball downward onto the seat. The design is similar to a normal Globe valve. They are ideal for systems with a high flow and velocity rates.
Silent check valves, also known as poppet check valves, avoid water hammer or shock by closing before fluid flow reversal. The benefit of a silent check valve is that they close quickly and smoothly, which prevents damage to surrounding materials.
A stop check valve can be any variety of check valve. What makes it different from a normal check valve is a mechanism such as a lever, handwheel, stem, or other device that can permanently close the valve to stop liquid flow. When closed, the disk is prevented from moving off the seat, as can be seen in the diagram of a lift check valve and globe valve below.
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 have a rubber diaphragm or disk. The valve is centered in the seating area with the sealing surface on the inlet side of the valve and has an opening in the center. Backflow causes the rubber disk to cover the sealing opening, which closes the opening to the inlet.
A foot check valve is at the bottom of a pipe connected to a pump. They act like a ball check valve with a screen to block debris. The opening of the valve is larger than the opening of the line. Foot valves keep the pump primed. When the pump is operating, suction pulls water up through the pipe and foot valve. When the pump is turned off, the water in the pipe is pulled back by the force of gravity. The foot check valve blocks the path of the water as it falls and is closed by the weight of the water.
Pneumatic check valves, or air check valves, control air flow from a compressor by letting air in and preventing it from going out. They are placed in pneumatic circuits that need air flow in one direction.
A double check valve, also known as a dual check valve, has two check valves assembled in a series with two different forms of operation. Though the two valves are part of the same assembly, they operate independently. If one of the check valves fails, the other will remain operational. A benefit of the dual check valve configuration is the reduction of pressure along the line as one of the check valves operates.
Since the invention of the self-sealing valve, at the beginning of the 20th Century, they have become the answer to backflow problems in a wide range of industries. Any industry that transports products through pipes has a check valve somewhere in their system.
Check valves are cost saving devices that prevent waste and protect equipment. Fluids that travel through a piping network need to be prevented from being lost due to poor handling. Check valves automatically stop liquid flow when pressure drops or the flow decreases by blocking backflow and containing the fluid.
Check valves in pumps is one of their more common uses.
Single pump system – The check valve controls a flow from a suction tank and blocks it from reversing when the pump stops.
Parallel pump system – In a parallel pump system, a check valve prevents the duty pump from pumping into the standby pump.
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Series pump system – A series of pumps is used when pumping materials in a pipe line over long distances. If any of the pumps fail in the series, the check valve activates to prevent losses.
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.
Foot valve – The foot valve blocks debris and prevents water from returning to the well when the pump is turned off.
The pharmaceutical industry requires check valves for the movement of fluids. Check valves regulate the pressure in the fluid flow for product quality, efficiency, and production. Only sanitary check valves can be used for this process. They require easy cleaning, without the need for disassembly, and silent closing with a tight shut off.
Orifice check valves are used in the hydraulics of the landing gear actuator system. When the gear is raised, fluid flows to lift the gear. When the gear is lowered, the check valve stops the gear from dropping by controlling the flow out of the actuator. Aside from the landing gear, check valves on aircraft are used in other hydraulic systems as well as fuel and pneumatic systems.
In an irrigation system, check valves are located near the source of the system to prevent backflow, siphonage, of irrigation water back to 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.
Check valves on heating boilers prevent hot water from circulating when the thermostat is not calling for heat. They stop water from going back into the boiler, which would build up excessive pressure. In a home heating boiler, a check valve keeps boiler water from entering the domestic supply to keep it from being used for drinking, bathing, or cooking.
When discussing how a check valve works, it is important to first understand how they are constructed. A basic check valve has four individual parts: body, seat, disk, and cover, which can be seen in the diagram below.
Springs, balls, stems, hinge pins, and other features can be added to the basic elements according to the purpose and design of the check valve.
Swing check valves have an unguided disk that moves into a fully open position when the pressure from the flow is applied. They come in several sizes and designs to fit different applications.
Lift check valves have a guided piston that lifts when the flow enters the check valve. The seat has a barrel design and is screwed on and sealed. The opening to the check valve is the same as the same size as the inlet and outlet.
The design of tilting check valves is to overcome some of the problems of swing check valves, which have been the most common type. They are capable of remaining open when the flow has a low velocity and close quickly when the flow stops. They perform well in applications with high velocity.
The folding disk check valve is a split disk design with a wafer body pattern that has a soft seat. The disks are secured by a pivot rod and open with the pressure of the flow. Springs on the disks help close the valve.
Vertical check valves can come in an inline ball or fully guided disk design. The ball in line version can be horizontal or vertical. Most vertical check valves have a spring assistance feature to quickly close the valve to minimize water hammer.
Most check valves are made from durable materials to be able to withstand high pressure conditions. Common materials used to make check valves include PVC, CPVC, bronze, brass, iron, and stainless steel.
PVC: PVC is corrosion resistant and flexible. The smooth surface of PVC allows the parts of a check valve to move easily.
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.
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.
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 prevented by having faster closing check valves, which stops pressure surges and shock waves. Silent check valves are one possible solution.
Reverse flow is costly and can damage a pump by causing it to spin backwards. This problem can be corrected with tight fitting fast closing check valves. One of the benefits of spring assisted check valves is their ability to react quickly and prevent reverse flow.
Some check valve systems have chattering caused by repeated opening and closing of the valve. This is caused by oversizing of the check valve. When installing a check valve, it has to be sized to fit the application. The disk has to be stable in the open position and make a complete seal when closed, which can prevent repeated opening and closing, fluttering, and failure of the check valve.
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