What are check valves? Types, Operation and Symbols of Check Valves (with PDF) | What is piping (2023)

Check valves are mechanical devices used in plumbing and piping systems to prevent backflow. A check valve only allows liquid to flow in one direction, which is why it is called a one-way valve or check valve. The check valve works according to the differential pressure principle. This means that the check valve only opens when the inlet pressure is higher than the outlet pressure.In situations where the outlet pressure is higher, the valve closes and prevents backflow. Closing can also be achieved by the weight of the locking mechanism, by a spring, or by a combination of all of these means. One of the main uses of check valves is at the pump outlet to protect equipment from reverse flow.

Check valves are automatic valves and unlike other valves do not require human intervention or external control to open or close. The sole purpose of the check valve is to prevent flow reversal or backflow and they come in a variety of sizes, styles and materials.

How check valves work

As already explained, a non-return valve works according to the differential pressure principle. For a check valve to open, it must reach a minimum pre-pressure known as the cracking pressure. The opening pressure changes depending on the design and size of the check valve. When the upstream pressure reaches the cracking pressure, the valve opens and allows the liquid to enter. When the upstream pressure falls below the cracking pressure, a back pressure is created and the flow tries to move from the outlet to the inlet. At this point the check valve closes and flow stops. The closing mechanism of a check valve varies depending on the design and type of valve. Spring or gravity pressure usually assists in the closing process.

Since the check valve only works in one direction, the manufacturers provide an arrow on the valve body that indicates the direction of flow.

Types of check valves

Different types of check valves are available depending on the movement of the shutter element.

• check valve
• wafer check valve
• Spring loaded check valves
  • Spring loaded inline valves
  • Spring loaded Y
• ball check valves
• Diaphragm check valves
• Raise check valve
• Stop check valve
• foot valve
• Entenschnabelventil
• Dual plate check valve

check valve

A check valve is the most common check valve. The shutter or disc swings on a hinge or shaft. To allow flow, the disc swings off the seat and swings back onto the seat to block backflow. In the open position, a check valve offers very little resistance to flow. To achieve optimal performance, a lever and weight or lever and spring are often assembled. The disc weight and the backflow influence the shut-off behavior of the valve. Swing Check Valve is also known as a tilting disc check valve.

The check valve allows full, unobstructed flow and automatically closes when the pressure drops. The animation video below explains the main parts and how a typical check valve works

wafer check valve

Wafer type check valves are very slim and compact in design and use an oscillating disc to allow or block flow. They are lightweight and suitable for various applications. They are economical and come in a variety of sizes. The figure below shows the typical operation of a wafer check valve.

Wafer style check valves are ideal for applications that require low pressure drop as the valve operates at a very low pressure differential.

Spring loaded check valves

There are two types of spring loaded check valves; Spring loaded in-line valves and spring loaded Y valves.

Inline valves are also known as nozzle check valves or silent check valves. These valves use a centrally guided stem and disc arrangement together with a compression spring. To open the valve, the flow pressure must be greater than the spring force and the opening pressure. In this case, the flow pushes on the disk that allows the flow. When the inlet pressure drops, the spring pushes the disc against the orifice and closes the valve.

The principle of operation of spring-loaded Y-type check valves is similar to that of in-line check valves. The only difference is that the spring and moving disc are placed at an angle to form a Y-shape. The main advantage of Y check valves is that they can be inspected and serviced while the valve is still connected to the system.

ball check valves

Ball check valves are easy to use and are commonly used on small pumps and in low head systems. Ball check valves incorporate a spring-loaded or free-floating ball flap that closes at pressures below the cracking pressure. In order to guide the ball into the seat and create a positive seal, the sealing seat is tapered. However, these valves can easily wear out with prolonged use and require frequent maintenance.

Diaphragm check valves

Diaphragm check valves consist of flexible rubber diaphragms or self-centering discs to prevent backflow. When the inlet pressure is increased, the diaphragm opens and flow begins. There are two types of diaphragm check valves;

• Free floating normally open valve and
• Normally closed valve with a fixed bend.

No opening pressure is required for normally open diaphragm valves, since the self-centering elastomeric diaphragm is free-floating. However, you need back pressure to close the valve. On the other hand, normally closed valves require a certain inlet pressure in order to overcome the elasticity of the fixed diaphragm.

Due to the very low opening pressure, diaphragm check valves are used in low-pressure and vacuum applications.

Raise check valve

A lift check valve is also referred to as a piston check valve. It consists of a guided disc that lifts (lifts) from the valve seat, creating space for media flow. The inlet pressure must be greater than the cracking pressure to overcome gravity and/or a spring force. The valve closes when the inlet pressure falls below the cracking pressure or when back pressure is present.

Stop check valve

A check valve is basically two valves built into one body. It can act as a shut-off valve for isolation or regulation purposes. Again, it can act as a check valve to prevent backflow. Unlike other check valves, check valve has additional external control mechanism in perpendicular or angular direction. Check valves are popular in steam systems such as power plants, boiler circuits, steam generators, turbine cooling and safety systems.

foot valve

A foot valve is a check valve that has a strainer installed on the inlet side to prevent debris from entering the valve.

Entenschnabelventil

Duckbill check valves are unique, one-piece, elastomeric components that allow flow through a soft tube that empties into the downstream side of the valve where back pressure collapses the tube and shuts off flow.

(Video) Control Valves Types,Operation and Troubleshooting

Was ist Check Valve Slam?

The phenomenon of check valve slamming can be described as follows:

• The ideal check valve is one that closes the moment the transported liquid reaches “zero” velocity before the flow reverses.
• However, at the point of “zero” flow, no check valve is closed.
• The valve is closed after the flow reversal has taken place.
• The mean velocity of the fluid is backwards at the moment of closing.
• The magnitude of the reverse speed Vr causes the “check valve slap” phenomenon.
• Check valves have "dynamic behavior", ie "speed of response". Different check valves behave differently.
• Check valves have "dynamic properties", ie different response speeds, which determine the maximum reverse speed Vr max and therefore the degree of "check valve slamming".
• The rapid slamming creates a pressure spike that is a likely cause of water hammer.

Non-slam check valve

Non-slam check valves are specially designed valves in which the closing element closes without
Slamming prevents excess pressure spikes. The disc of a non-return check valve contains an internal spring that opposes the opening fluid flow pressure. When the flow medium is strong enough, the spring is compressed and the valve opens. When the flow decreases again, the spring force gently pushes the disc back towards the valve seat surface and stops. For vertical pipe runs or complex applications that require constant and controllable pressure levels, non-slam check valves are an ideal solution.

The primary benefit of non-slam check valves is their ability to effectively prevent water hammer. This optimally eliminates pressure fluctuations, vibrations and component damage. Because non-slam valves have a short circuit
Hub, they facilitate a quick, gentle closing of the pane to avoid water hammer. Because they consist of only one moving part, the disc itself, non-slam check valves experience minimal wear over time. However, the non-slam check valves are not piggable.

Dual plate check valve

Double plate check valves are compact valves in wafer design with a small overall length. They offer excellent hydrodynamic properties resulting in very low pressure drops and they are technically efficient. Their low weight offers advantages during installation, transport and storage. Dual Plate Check Valves are suitable for liquid, gas, steam, condensate, water service, and oil and natural gas service. They are designed as a non-slam type. With suitable springs, they can be installed in any position.

Desirable design features of a check valve

For a check valve to function smoothly, the following design features are desirable:

• Moving parts of the valve have low inertia
• The distance/angle that the moving element(s) must travel is minimal and
• Mechanical support for the closing movement of moving elements, e.g. Feather.
• Density leakage
• Less pressure drop

Check valve symbols

The check valve symbol may vary slightly from company to company. Usually one of the following check valve symbols is widely used in the industry.

Check valve materials

Industrial check valves are available in different materials such as:

• carbon steels
• Stainless steel
• Duplex stainless steel
• Alloys with a high nickel content
• Titan

check valve standards

Below are the various check valve standards that are followed in the design of pipelines

• API-Standards:API Specification 6D, API Std. 594
• ASME standards:ASME B16.34
• AWWA-Standards:AWWA-C508, AWWA-C510
• BSI-Standards:BS 1868, BS 1873, BS 2080, BS 5152, BS 5153, BS 5154, BS 5160, BS 5352
• MSS-Standards:MSS SP-42, MSS SP-61, MSS SP-71, MSS SP-80, MSS SP-84

Check valve applications

Check valves are used in various industries such as offshore oil and gas production, onshore oil and gas production, gas plants, LNG, liquefied petroleum gases, refineries, petrochemical, chemical, fertilizer, terminals, pipelines, power, desalination, water, etc. Widely used in pumps - and compressor discharges, heat exchangers, reactors, vessels and separators to prevent flow reversal. Some typical applications are listed below:

Spring check valves vs. check flaps

Check valves are highly effective, low cost check valves in industry. However, there are some basic differences between check valves and spring check valves.

Spring Check Valves vs. Check Valves: Works

With a check valve, the flapper “swings” away from the seat to allow forward flow. It swings back onto the seat to stop the flow. On the other hand, a spring loaded check valve uses a spring to help close the valve.

Check Valves vs. Spring Check Valves: Installation Limitation

Check valves are only suitable for applications with horizontal flow, which severely limits the mounting position. Although swing check valves offer greater flow capacity, they sometimes do not fit into existing piping configurations.

In contrast, spring-loaded check valves are suitable for any flow direction. Spring check valves therefore offer more flexibility compared to check valves in difficult space conditions with demanding dimensions.

Spring check valves vs. check flaps: minimizing water hammer

A spring loaded check valve must be able to minimize the damaging effects of water hammer. On the other hand, a check valve can increase the problem.

Let's understand the concept of water hammer with an example: Suppose there is a check valve in a water pipe. Downstream of this check valve is a quarter turn ball valve. Now, during operation, if someone abruptly closes the quarter-turn ball valve, a pressure wave is generated that flows through the pipeline. This phenomenon is known as water hammer. If the check valve is a check valve, the flapper will remain open until that pressure wave returns to the check valve. The pressure wave causes the valve to slam shut, creating an audible noise and causing excessive wear in the check valve and other piping system components. However, if a spring-loaded check valve is installed, the spring will close the valve before the pressure wave gets there. So, a spring check valve can effectively reduce the impact of water hammer. Spring check valves are also known as "silent check valves". In the spring check valve, a spring is used to assist the poppet in closing the check valve silently before reversing fluid flow.

Check valves vs. spring check valves: costs

As mentioned earlier, check valves are cheaper compared to spring check valves.

Click here to learn more about different types of valves