Check valves are crucial devices in many systems. The main applications of check valves are in pump outlets, pipelines, and compressors to protect equipment by preventing the backflow of fluid.
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They prevent backflow by using the pressure differential between the inlet and outlet areas. With backflow prevention, fast-acting check valves also prevent water hammering caused by pressure surges and vibrations.
Available in non-slam, ball, dual plate and other types, check valves have various types and varying parts, and some check valves operate with unique mechanisms.
This post explains what a check valve is, discusses its functions, parts, types, and working principle and highlights other essential information.
A check valve, also called a non-return valve, retention valve, or one-way valve is a device through which fluid flows in only one direction. A check valve has two openings or ports, one of which is called an inlet because fluid enters the valve through it, and the other is called an outlet because fluid leaves the valve through it.
Check valves function to prevent the reversal of fluid flow (backflow) in the system to which they are applied. They are used in applications where the backflow of fluid may contaminate the fluid moving into the system.
The check valve works with changes in pressure in the system. The valve opens to allow flow through the inlet when the inlet pressure is higher than the outlet pressure and closes to restrict flow when the outlet pressure is higher than the inlet pressure.
The valves may also close due to the check valve weight, a spring, or other means depending on the valve type. These valves are automatically activated and are hardly operated on-site or remotely as most of them do not have handles or stems.
The check valve symbol is composed of a triangle with two horizontal lines extending from two opposite sides of the triangle. A vertical line is inserted at the meeting point of the pointed end and the horizontal line.
The horizontal lines indicate that the check valve has two ports-inlet and outlet. The pointed triangle and vertical line show that flow is allowed upstream, but backflow is restricted in the check valve.
Check valves are differentiated into types based on the movement of the valve closure element or member and the process with which it shuts off to prevent backflow. The types of check valves include:
The swing check valve is the most commonly used. It operates with a disc which swings on a hinge or a shaft. As inlet pressure increases, the disc swings off the valve seat and fluid is allowed to flow through the valve, and as inlet pressure decreases or there is backflow, the disc swings back to the seat to restrict the fluid flow.
This check valve offers minimal restriction to flow when open. When springs are not used to assist the valve in closing, the valves should be mounted to allow the disc to shut with gravity.
The lift check valve can be installed in many ways, including horizontally, vertically and angularly. It uses a disc which lifts to allow the flow of fluid upon an increase in inlet pressure.
This check valve requires the inlet pressure to be higher than the cracking pressure to allow the fluid flow. The inlet pressure pushes the disc against gravity or a spring force. The valve closes when the inlet pressure decreases below the cracking pressure or there is backflow.
Dual plate check valves are commonly used in industrial operations and refineries. They are compactly built and can handle huge fluid backflow that other weaker types of check valves will not handle.
These check valves have double metal plates and are lightweight, making them easy for installation, transport and storage. They are mostly designed as non-slam types and usually result in low fluid pressure losses.
The purpose of these check valves is primarily to handle water hammer that results from accumulated pressure in the valve. Also called nozzle check valves, the disc in this valve has an internal spring which compresses when the valve is open.
When the inlet pressure decreases, the spring pushes the disc forward to the valve seat, closing softly to prevent water hammering. The mechanism of the non-slam check valves prevents pressure spikes and vibrations in the valve.
The wafer check valves consist of wafer-style discs that swing to allow or restrict fluid flow. The valve disc is attached to a hinge or spring, which controls the opening and closing.
These valves offer minimal obstruction to flow and, because of their slim and lightweight construction, are applied in many operations. They are very similar to the butterfly check valves and may be used instead of the butterfly check valves.
The diaphragm check valves allow fluid flow by flexing its rubber diaphragm open when inlet pressure is increased. When inlet pressure decreases, or there is a backflow of fluid, the diaphragm closes to seal the passage.
The two diaphragm check valves are the free-floating normally open valve and the fixed-flexing normally closed valve.
The free-floating diaphragm does not require a cracking pressure, while the fixed flexing diaphragm requires an amount of inlet pressure to contract and allow fluid flow.
Spring-loaded inline valves can also be called nozzle or silent check valves. These valves require an inlet pressure high enough to overcome the cracking pressure and spring force for the valve&#;s orifice to open and allow fluid flow.
The valve closes by spring force when there is a decrease in the inlet pressure or back pressure pushing the disc to shut the valve. These valves also prevent pressure variations; hence they are good for handling water hammer.
Installation of the in-line check valves can occur vertically or horizontally.
The spring-loaded y-check valves have a &#;y&#; shape, as their name implies. They operate like the spring-loaded inline check valves, where high inlet pressure is required to overcome the cracking pressure and spring force.
However, unlike the in-line valves, the spring of the y-check valve and the valve disc is at an angle which enables them to be serviced while still connected to the system. These valves require more space than the in-line valves.
The stop check valve function is a combination of two valve functions. These valves function as blocking and throttling valves and as check valves to restrict fluid reversal in the system.
The stop check valves have an actuator on the top of the valves used to control the opening and closing of the valves. These industrial check valves find applications in steam operations such as boiler circulation, steam generation, or power plants.
The ball check valves use a spherically shaped ball to seal the valve. The ball is moved by a spring or acted upon by gravity. When the inlet pressure increases, the ball is pushed by the pressure away from the curved seat to allow the flow of fluid through the valve.
As the inlet pressure decreases or there is backflow, the disc shuts off the valve due to spring force or gravity.
These check valves find application in operations where the suction line is under the pump. The foot check valve comprises a check valve and a strainer in the inlet area.
The strainer restricts pipe fragments or substances from entering the valve and clogging downstream instruments.
The duckbill valve, as the name implies, has an end resembling a duck beak. The flat end, with increased inlet pressure, opens to allow the fluid to flow through it, and when there is a decrease in inlet pressure, the valve closes and returns to its original state.
Tilting disc check valves are a modification of the swing check valves. In this valve, the hinge at the top of the disc in the swing check valve is replaced by a pivot point with a double offset from the center of the disc. This valve can open at lower pressures than the swing check valve.
The purpose of check valves is to prevent flow reversal in the system. To restrict backflow, every part of the check valve plays a role. Check valves have different types and forms, but they all use the same principle.
Check valves have varying parts as the components that make up a particular check valve may not be the same in another. However, these are three parts that can be found in most check valves:
This part of the check valve allows the fluid to enter. This part is usually shut or open with the valve closure element.
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This is the vessel through which the fluid passes.
The fluid leaves the check valve through this part. It usually serves as the first port for a backflow.
Different types of check valves have unique parts. Some parts may look the same but do not have the same function in different types of check valves. Below are some check valves and their parts.
Other check valves include unique parts such as stoppers, spacers, springs, cages, etc.
Check valves are designed to allow fluid flow in one direction only. They operate automatically using the pressure in the fluid to either open and allow fluid to move through or close to prevent fluid from moving out.
The main functions of the check valves are:
Backflow of fluid can cause contamination, water hammer and other system damage. Some check valves, such as the stop check valve and the foot valve, have other functions asides from preventing backflow.
For example, the stop check valve may also be used as a throttle or isolation check valve, and the foot valve also functions to block debris from moving further into the pipeline.
The check valve may also be used as an auxiliary system in cases where the pressure can increase above the pressure of the main system. Check valves such as the non-slam check valve primarily prevent water hammer in the system.
If a backflow of fluid occurs and the system does not have a check valve installed, a water hammer may occur and destroy the pipeline parts.
The check valve working principle involves two major areas: Opening and shutting the valve.
For the valve to be open, it requires a minimum upstream pressure provided by the fluid. The minimum pressure is the differential between the pressures at the inlet and outlet areas. It is called the check valve &#;cracking pressure&#;.
When the cracking pressure is overcome, the valve opens and allows fluid flow. Cracking pressures are specific to different valve types. Hence, proper research should be done to ensure the system can generate the cracking pressure of the check valve.
If the pressure at the inlet (upstream pressure) decreases, falling below the cracking pressure, or there is a backflow of fluid, the valve shuts. The closing operation and closing element depend on the valve type and design.
For proper operation of the check valve, the valves should be installed correctly. The valves should be examined for correct installation because installing wrongly could damage the system.
Various factors need to be considered before the selection of a check valve for a system. Some of these factors are:
The fluid makes contact with the valve while passing through it. Hence, the material of the valve must be compatible with the fluid to prevent contamination.
The check valve must be able to handle the fluids for the intended operation. Most check valves can handle normal fluids without suspended solids, but specific check valves can handle fluids with suspended solids, such as raw wastewater or sewage water.
The fluid must be able to generate enough pressure to overcome the cracking pressure of the selected valve. Also, in some cases, it should be confirmed that the fluid can generate enough back pressure to seal the valve.
The fitting of the valve with pipe should be considered. The size of the check valve and its parts should correspond with the required size of the pipe.
The fluid and external temperature conditions should be compatible with the valve type. Also, the installation of the check valve type should give enough space for repairs and inspections.
For check valves to work effectively and with optimal efficiency, they have to be installed properly. Mistakes in the installation of the valves may result in damage to the valve and the pipe system. Here are some installation best practices that apply to most check valves:
Most check valves from industrial valve manufacturers are of different materials and parts. Therefore, for proper check valve maintenance, following the maintenance guide from the manufacturer is always best.
However, below are some effective maintenance guidelines for most check valves.
The material, standard, sizes and designs of the check valve should be compatible with the application. Below, some of these specifications are discussed.
These are the types of check valve materials to consider.
Check valves made from PVC material are insusceptible to corrosion. They can be used with corrosive fluids such as acids and other caustic chemicals. However, they may be damaged by aromatic and chlorinated hydrocarbons.
Check valves made from polypropylene are also insusceptible to corrosive fluids such as inorganic acids and bases. They have a maximum temperature of around 80 degrees and find applications in water systems and food products.
These check valves find applications in systems that use air, water, oil and fuel. They are more susceptible to corrosive fluids than the other material types. They are mostly used in low-pressure applications and small systems.
These valves are resistant to corrosive fluids and low-temperature conditions. They also offer good structure and have solid construction. However, they are a less economical option than the PVC and brass material types.
The check valve has several types and designs, and some types are more suitable for certain applications than other types. Some check valve designs are listed below:
Other check valve designs have characteristics that make them suitable for specific operations.
The check valve standards should be complied with when buying a check valve. These are some standards followed during piping design:
Check valves depend on fluid pressure at the inlet to open or close. Hence, knowing how much the valves open to allow fluid flow through the pipe is essential to effectively operating the valves. Check valve sizes inform on the spatial capabilities of the valve and if it is compatible with the pipes.
The most important consideration in selecting a check valve size is its application. The operation the check valve is intended for determines the size used. Calculating the valve size requires the material viscosity, pressure, temperature, and flow rate; with these flow characteristics, a valve size can be customized for the application.
An operation in which a wrong check valve size is used may lead to higher pressure losses, unstable flow and a shorter valve lifespan.
The pressure ratings of the check valve vary depending on the manufacturer. Some common ratings are:
This post discussed all you need to know about the check valve, including its function, parts, installation and maintenance requirements, and what you need to consider before buying an industrial check valve.
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The check valve allows fluid to flow in one direction only, blocking its passage in the opposite direction. Fluid backflow can damage circuits, especially hydraulic circuits. These valves can also prevent liquid or steam leaks if there are faults in the pipe structure. They are also installed in sewage pipes to prevent backflow and the passage of bad smells.
Check valves can be found in hydraulic pumps and motors, ventilation systems, pump pipes, hydraulic and pneumatic connections, heating systems, as well as in the evacuation systems of kitchen sinks, washbasins, bathtubs, toilets, washing machines, etc.