Non Return Flap Valve

What is a non-return flap valve? 

A non-return flap valve is a type of valve used to allow fluid flow in one direction. The fluid flow is allowed to proceed in the forward direction only and whenever fluid attempts to flow back the valve closes. The opening or closing of the valve depends on the pressure difference between the inlet and outlet ports of the valve. Non-return flap valve manufacturers design this type of valve without an actuator or handwheel since it operates automatically by allowing fluid to flow forward and blocking fluid flowing backward. Preventing reverse fluid flow prevents fluid contamination such as in foods and water applications. This valve also prevents damage to pumps and boilers which could happen if there is reverse fluid. Non-return flap valves can be used in both low and high temperatures as well as corrosive and non-corrosive media. The area of application is highly dependent on the material used to construct the valve mostly internal parts. 

Figure: Non-return flap valve.  

Components of a non-return flap valve 

Body 

This is an external part of a non-return flap valve that is used to house internal parts of the valve. Non-return flap valve manufacturers design the body with strong materials like ductile iron, stainless steel, and ductile iron among others to make the valve body strong so that it can withstand fluid pressure. The body’s high strength is also meant to make the valve safe against impacts from falling objects as well as high temperatures. The body contains the ports of a flapper check valve. 

Bonnet/cover 

This is another part of a non-return flap valve that is placed on top of the body to protect internal parts. The body and the cover are connected using different elements such as bolts, threads, welding, or pressure seal. This helps to ensure no leakage which can cause pressure loss or damage to the environment. Both the body and cover are made of the same metallic material to enhance high strength. 

Disc 

This is the component of a non-return flap valve that is used to prevent reverse fluid flow. The disc works in support of the spring. However, some non-return flap valve manufacturers do not use spring and the valve depends on backflow to prevent reverse fluid flow.

Seat

This is a component placed inside the valve body to prevent leakage between the body and disc when the check valve closes. The seat provides the seating surface where the disc lies to prevent reverse fluid flow. 

Spring 

This part is used to support the disc in closing the valve immediately after the suction pressure reduces or when the pump is turned off. Non-return valve manufacturer’s design the spring with strength proportional to the suction to ensure the fluid will be able to open the valve. If the spring is made of very high strength the fluid may not be able to open the disc. 

Gasket 

This is one of the parts of a non return flap valve used to prevent fluid leakage. It is placed between mating surfaces of the valve body and bonnet and body and the pipe. Gaskets are made of different materials that determine the area of application of the non return valve. 

Figure: Components of a non-return flap valve.

How does a non-return flap valve work? 

The working principle of a non-return flap valve depends on pressure differential. The valve opens when the upstream fluid pressure exceeds the downstream fluid pressure. This pressure provides the force that acts on the valve disc to force the disc flap forward. As such, a free space is created allowing the fluid to flow through the non-return flap valve. Once the disc is opened, fluid keeps flowing forward until when the pump is turned off or when there is reverse fluid flow. The disc is forced to close the valve by a valve spring when the suction pressure reduces or when there is backflow. This prevents back fluid flow thus preventing upstream fluid contamination as well as damage to piping system equipment like boilers or pumps. 

Figure: Working of a non-return flap valve.

Types of non-return flap valves 

Straight non-return flap valve

As the name suggests this non-return flap valve is designed such that the fluid flows straight through the valve. This is valve designed with a circular disc. The disc flaps on a hinge pin to open and close the fluid flow. Non-return valve manufacturers design this valve such that when the fluid flows upstream it forces the disc up leaving space for fluid flow. This valve without a spring closes when the pressure reduces and the disc is returned to its seating position by backflow of fluid. On the body is a seat ring on which the disc fits when it closes preventing reverse fluid flow. Non-return flap valve manufacturers design the valve with the seating surface oriented at a slight angle to make the disc open even at low-pressure levels. This also helps to provide a tight seal and reduce shock when the disc closes. 

Figure: Straight non-return flap valve.

Wafer non-return flap valve 

This is a non-return flap valve designed with plates that serve to open and close the fluid flow. There are two types of this valve that is the single plate and the dual plate. The common characteristic of these two types is that they have slim bodies. The single plate wafer non-return flap valve uses one plate. The plate which serves as the valve disc is hinged on one side and it is free on the other side. The fluid flowing through the valve acts on the free end of the plate forcing it to flap upwards and thus opening fluid flow. The dual plate wafer non-return flap valve uses two plates known as half-discs. The discs/plates are pivoted on a post mounted at the center of the valve. The center post is known as the hinge pin. Springs are used to hold the plates/discs in a closed position against the valve seats and the body. This prevents the plates from turning freely unless they are forced by fluid flowing forward. Once the suction pressure reduces or there is backflow the plates are forced back to the seating position by the valve springs. 

Figure: Wafer non-return flap valve (single plate type on the left and dual plate type on the right).

Split body non-return flap valve 

This is a non-return flap valve that is designed with two pieces of the body. The two pieces of the body are connected using bolts and nuts. The bolting method creates a tight seal which makes these valves suitable for use in high-pressure applications. Non-return flap valve manufacturers design this valve such that the disc is oriented at a certain angle. This design helps to reduce pressure loss and make the disc close without slamming. 

Figure: Split body non-return flap valve.

Factors to consider when selecting/purchasing non-return flap valves 

Type of media 

Non-return flap valve manufacturers design valves for use in different applications. However, each valve is meant to be used for a certain fluid(s). Some of the valves are meant for low viscosity fluids while others are meant for highly viscous fluids. Other non-return flap valves are used for corrosive media such as acids while others are meant for use in non-corrosive fluids. Use of the valve according to the non-return flap valve manufacturer’s instructions enhances long service life. 

Flow velocity and pressure 

Non-return flap valve manufacturers design each valve for use in a certain range of fluid velocity. Using the valve for very high velocity causes the valve to chatter or make a lot of noise. Also, non-return flap valve manufacturers design these valves for use in certain pressure ranges. Using the valve for very high pressure than it is recommended may cause the valve to leak fluid which may cause environmental pollution or other losses. Also, if the fluid pressure is low it may not be possible to open the valve disc. In conclusion, it is very important to use the valve for the velocity and pressure given by the non-return flap valve manufacturer. 

Temperature 

Non-return flap valve manufacturers design different valves with different values of operating temperatures. Some valves are made of soft seat that is seats made of plastic or rubber materials. These materials are recommended for use at temperatures below 60 ^oC. Other seats are made of metallic materials which are very strong against high temperatures and recommended for use above 60 ^oC of temperatures. Using the valve against the temperature range given by the non-return flap valve manufacturer may damage the valve or cause the fluid to chemically react with the valve material.

Applications of non-return flap valves 

• Freshwater supply.
• They are used in wastewater treatment. 
• Used in the manufacturing of different chemicals.
• Paper and pulp manufacturing plants. 
• Gas and oil refinery systems. 
• Steam power plants.
• Food and beverage applications. 

Advantages of non-return flap valves 

• Work on the principle of pressure difference where they open and close fluid flow automatically.
• These valves are free from fluid leakage. 
• They do not allow reverse fluid flow.
• High efficiency because the valve disc causes a low-pressure drop.  
• Fast acting.

Disadvantages of non-return flap valves

• Prone to slamming and water hammer problems. 
• They are not suitable for use in pulsating fluids. 

Troubleshooting non-return flap valves 

External fluid leakage 

• Loose connection. Tighten the connecting elements to the strength values recommended by the non-return flap valve manufacturer. 
• Worn out gasket. Replace the gasket. 

Internal fluid leakage 

• Worn out seat. Replace the seat. 
• Worn out O-rings. Replace the O-rings. 
• Solid particles in the valve. Open the valve according to the non-return flap valve manufacturer’s instructions and clear the foreign materials. 

The valve does not open 

• Low suction pressure. Ensure the suction pressure is up to the level recommended by the non-return flap valve manufacturer. 
• Downstream valve is not open. Ensure the downstream valve. 
• The non-return flap valve is installed wrongly. Install the valve such that the direction of fluid flow from the source matches the flow arrow indicated on the body of the non-return flap valve. 

Valve makes a lot of noise or vibrations 

• Fluid flow velocity is very high. Ensure the fluid velocity matches the fluid velocity range given by the non-return flap valve manufacturer.

Summary 

A non-return flap valve is a type of valve used to prevent reverse fluid flow. These valves do not have control mechanisms such as actuators or handwheel as they operate automatically by using the difference in pressure between the inlet and outlet. The valve opens when the fluid pressure is high to provide a force that acts on the disc to make it open and allow fluid flow. The disc closes when there is backflow or when the pump is turned off.

Non-return flap valve manufacturers design these valves with strong metallic materials that make them suitable for use in low and high pressure as well as high and low temperature applications.