What is Check Valve?
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications.
Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, and inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.
An important concept in check valves is the cracking pressure which is the minimum differential upstream pressure between the inlet and outlet at which the valve will operate. Typically, the check valve is designed for and can therefore be specified for a specific cracking pressure.
How does a check valve work?
A check valve is a device that only allows the flow of fluids in one direction. They have two ports, one as an inlet for the media and one as the output for the media. Since they only allow media flow in one direction, they are commonly referred to as ‘one-way valves or ‘nonreturn valves.’ The main purpose of a check valve is to prevent backflow in the system.
A check valve relies on a pressure differential to work. They require a higher pressure on the input side of the valve than the output side to open the valve. When the pressure is higher on the outlet side (or the input side pressure is not high enough), the valve will close. Depending on the valve type, the closure mechanism is different. Unlike other valves, they do not need a handle, lever, actuator, or human to correctly work.
They are commonly installed in applications that backflow would cause an issue. However, since they are non-return valves, they are a cheap, effective, and easy solution to solve a potential issue. Backflow can cause an issue if the backflow is contaminated, and therefore, contaminates the media upstream.
For example, a sewer line will have a non-return valve to ensure that waste can leave but not re-enter the system. They are also used if backflow will cause damage to equipment upstream that can only allow media to flow in one direction.
For example, a reverse osmosis filter can only have water pass through it in one direction, so a one-way valve is installed downstream to prevent this. There are various sizes, designs, and materials to ensure there is a check valve for every application.
Types of Check Valves
Check valves are available in the following designs:
- Swing Check Valves
- Tilting Disk Check Valves
- Wafer Check Valves
- Disk Check Valves
- Piston Check Valves
- Ball Check Valves
- Duo-Check Valves
- Non-Slam Check Valves
1. Swing Check Valve
Swing Check Valves are available in straight body design and y-pattern design. The disk is suspended from the body by means of a hinge pin and seals against the seat which is integral with the body.
These valves are typically used in sizes 2 inches and over. Swing check valves can be installed in both horizontal and vertical positions. They are not suitable for pulsating flow. The closure response of the swing check valve is slower compared to the lift check valve due to longer disc travel and inertia of the disk.
2. Tilting Disk Check Valve
Tilting Disk Check Valves fit between two flanges and provide a compact installation for large diameter applications. These valves can be installed in horizontal and vertical lines.
They provide a quick closing response and are particularly suited for pulsating flows with compressible fluids.
3. Wafer Type Check Valve
Wafer Type Check Valve has short face-to-face dimensions and low weight allows a simple space-saving installation between the companion flanges.
The valves are suitable for mounting between weld neck or slip-on type companion flanges of different standards. They are specially developed for applications where a low-pressure loss is essential. Opening and closing of the valve will take place at an extremely low-pressure difference over the valve disc.
The eccentric disc shaft combination with the disc seat guarantees a positive shut-off returning media. Wafer check valves are becoming the preferred type of check valve for most applications, due to their compact design and relatively low cost.
4. Disc Check Valve
The Single Disc Check Valve (Wafer Type Disc Check Valve) consists of four main components: the body, a disc, a star guide, and spring. The Disc check valves are opened by the pressure of the fluid and closed through compression spring as soon as flow stops thus preventing reverse flow.
The design of Wafer Design, Sandwich Type Single Disc Spring Loaded Check Valves allows them to be installed between any flanges of different standards and in any position; including vertical pipelines where the fluid flows downwards.
5. Piston Check Valve
Piston Check valves are generally used to protect pumps or similar equipment, allowing the flow only in one direction and preventing flow reversal due to backpressure. The piston check valves are designed with globe valve bodies, producing an increased drop pressure in the pipeline. This design provides a tight seal as well as a fast reaction to the closure impulse.
Metal seated check valves may not provide drop tight sealing when used in gas systems or fluid systems with low backflow pressure or fluids containing particles.
6. Ball Check Valve
The ball check valve is one of the few check valves that work well in both water and wastewater applications. Ball check valves are simple in operation and commonly used on small pumps and in low head systems.
Consider adding an optional roll-bar to the piping system for safety; ball check valves have the highest tendency to slam due to the ball’s high inertia long stroke. When ball check valves encounter high pressures and dynamics, severe slamming may result.
7. Dual Plate Wafer Check Valve
The Dual Plate, Wafer Check Valve employs two-spring-loaded plates hinged on a central hinge pin. When the flow decreases, the plates close by the action of torsion spring before flow reversal takes place.
All features put together to make the Dual Plate Check Valve the most efficient & versatile design. It is also referred to as a silent check valve. It is much easier to install between standard gaskets and line flanges and therefore is more cost-effective to install and to maintain. Its design complies with API 594 and API 6D, testing with API 598.
It is also called Butterfly Check Valve.
8. Non-Slam Check Valve
The swing check valve closes abruptly because of gravity and causes pressure surge resulting in shock waves. These high-pressure waves cause severe stress on the piping system.
This problem can be minimized by installing a non-Slam type check valve. Non-Slam check valves do not rely on gravity. As the upstream velocity of the fluid slows the spring assist on the valve start to close the disc.
By the time the upstream velocity comes to 0, the disc is completely closed. With the reverse flow eliminated, the force necessary to produce a water hammer on either side of the valve is substantially reduced.
How do you choose the right check valve?
There are several factors to consider when deciding on a check valve for your needs:
- Material Selection: Material used for valve construction must stand up to piping system and media demands associated with the specific type of gas or liquid flowing through the piping system.
- Performance Requirements: Performance considerations include the capability of the valve to handle various temperature and pressure instances.
- Type and Size: Selection of the right check valve includes control considerations, typically related to increased safety in hazardous applications as well as sizing the valve to properly accommodate the piping system’s flow conditions.
- Maintenance: In addition to considering the frequency of maintenance or replacement, operators must also think about the valve’s resistance to clogging, jamming, or otherwise malfunctioning.
- Special Requirements: Check valve selection also includes several special requirements unique to their function, including response time, cracking pressure, normally closed vs. normally open, vertical vs. horizontal mounting, flow direction (upward or downward) and fluid density.
How do you prevent check valves from failing?
One of the disadvantages of check valves is the difficulty of determining the condition of the enclosed valve. Consequently, there are certain measures necessary for recognizing their failure and preventing it.
Several indicators that signal failure or potential failure include valve vibration (chattering), measured reverse flow, excessive component wear, audible indications of leakage (water hammer), and leaking. You can prevent failure by:
- Proper sizing
- Routine inspections for cleanliness
- Proper installation
- Replacing valves as needed
Check valves play an important role in controlling flow within high-performance piping systems. Selecting the right check valve provides greater piping system efficiency and improves operational safety.
Applications of Check Valve
On the basis of the work function of the check valves, it can be used for the following different applications;
- To prevent the backflow of fluid creating damage to the system.
- To ensure the prevention of contamination due to backflow of the fluid.
- To avoid the siphoning action in a pipeline.
- To maintain the vacuum seal.
Advantages of Check Valves
- Prevent backflow
- Able to endure high- and low-pressure conditions
- Perform as a backup and safety system
- Able to be used horizontally and vertically
- Fast acting.
- Protect pumps and compressors from damage caused by backflow
- Reduce downtime and loss of production
- Prevent water hammer
- Reduce sudden valve failure
- Lower maintenance costs
- Few moving parts
- Smaller footprint
- Able to cope with variable flow conditions
- Do not require power to operate
- Horizontal Check Valve
Disadvantages of Check Valves
- Do not operate in pulsating systems
- Closing element can slam closed causing damage and wear
- Operate in a completely enclosed system
- Impossible to check if they are open or closed
- Internal parts cannot be checked
- Disks can get stuck in the open position.
- Noise from slamming disks
- Water hammer
- Reverse flow problems
Common Problems with Check Valves
- Water Hammer. 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. 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.
- Oversizing. 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.