Non Return Valve

What is a non-return valve?

A non-return valve is also known as a check valve. This is a valve that is used to prevent fluid from flowing backward in a pipeline. A non-return valve is also known as a one-way valve. This is because non-return valve manufacturers design these valves such that fluid flows in one direction only. Non-return valves work on the principle of differential pressure. From this principle, and in terms of a non-return valve, the valve opens when downstream pressure is lower than the upstream pressure. The valve closes when the downstream pressure becomes more than the upstream pressure. Non-return valve manufacturers design these valves to be automatic. As such, these valves work independently unlike other valves like gate valves which may need a valve operator. As such, most of the non-return valves do not have a handle or stem. Non-return valves are made of either metal or plastic materials. These valves are meant for use in different applications such as turbines, furnaces, boilers, and compressors among others. The choice of the non-return valve depends on certain factors such as media type and levels of pressure and temperature. Non-return valves are simple in design, inexpensive, and small in size.

Figure: Non-return valve.

Terms used in non-return valves 

Cracking pressure 

Cracking pressure is also known as opening pressure or head pressure. In a non return check valve cracking pressure is the minimum amount of differential upstream pressure between the valve inlet and outlet for which the non-return valve will open. All non-return valves are designed for specific applications with specific cracking pressure. 

Backpressure 

This is the pressure difference between the inlet and outlet of the non-return valve.

Reseal pressure 

This is the difference in pressure between the inlet and outlet when the non-return valve is closing and at which there is no fluid leakage. Reseal pressure is also known as closing pressure. 

How does a non-return valve work? 

The non-return valve works by using the principle of pressure difference. Cracking pressure is very important in opening and closing fluid flow. The non-return valve opens when the cracking pressure is equal to the upstream pressure and at this point, fluid starts entering the valve. When the upstream pressure becomes less than the cracking pressure the fluid tries to flow from the outlet side to the inlet side of the non-return valve. In this situation, the valve closes preventing backward fluid flow. The closing process of the non-return valve depends on the valve design and size. Preventing the backward flow of fluid is very important, especially in cases where fluid flowing backward has substances that may contaminate upstream fluid or cause other technical problems. 

Figure: Working of a non-return valve.

Types of non-return valves

Spring-loaded in-line non-return valve 

This is the most common type of non-return valve. As the name suggests, this type of valve uses an in-line spring when closing fluid flow. For the fluid to get into the valve it has to have enough pressure to overcome the cracking pressure and spring force. Once the fluid overcomes the cracking pressure, it forces the disk to open the valve orifice. When the orifice opens, fluid starts moving into the non-return valve. When the outlet pressure becomes less than the cracking pressure, the cracking pressure and spring force push the disk to the opposite direction of the orifice and close the non-return valve. The spring has a short travel distance which accelerates the closing time. Spring-loaded in-line non-return valve manufacturers employ a design that makes these valves able to stop fluctuations of pressure in the piping system and also stop water hammers from occurring. Water hammer is a phenomenon that occurs when there is a sudden fluid stoppage in the piping system. 

Figure: Spring-loaded in-line non-return valve.

Spring-loaded Y shaped non-return valve 

This is another type of non-return valve which uses a spring but it is loaded at an oblique. The non-return valve manufacturers design this valve such that the spring and the valve disk are installed at a certain angle as shown in the figure below. It is through this angle that the valve has a Y shape and hence the name spring-loaded Y-shaped non-return valve. This type of nrv valve works the same way as the spring-loaded in-line non-return valve. However, when cleaning and repairing the Y-shaped non-return valve there is no need to remove it from the piping system. This type of valve is heavy and its installation occupies more space relative to other valves. 

Figure: Spring loaded Y shaped non-return valve.

 Ball non-return valve 

This is a non-return valve that uses a ball-shaped element to open and close the fluid flow. To close the valve, the ball element travels to the seat to attain a tight ball alignment. In other cases, the ball non-return valve manufacturers design the valve to use a spring-loaded mechanism to help shut the valve. The other valves that do not have the spring mechanism use reverse flow to move the ball seat to create a fluid seal. Ball non-return valve manufacturers design these valves so that they can be used in applications that have sediments and scales as well as in viscous fluids. This type of non-return valve operates quietly and is suitable for use in applications where fluid rapidly changes flow. The ball non-return valve uses replaceable seats and both ball and seats can be removed without the need to open the pipe. 

Figure: Ball non-return valve.

Swing non-return valve 

This is a non-return valve also known as a tilting disk non-return valve. This type of non-return valve has inlet and outlet ports and a disk. The disk opens once the inlet pressure becomes more than the cracking pressure. When the inlet pressure becomes less than the backflow pressure, the disk swings and closes the swing check valve. The angle between the valve seat and the vertical plane is known as the seating angle. The seating angle ranges between 0^o and 45^o degrees. When the seating angle of a swing non-return valve is increased, the disk travels a shorter distance. As such, the disk closes the valve more quickly and reduces water hammer problems. 

Figure: Swing-non-return valve.

Lift non-return valve 

This is a non-return valve that uses a disk to block and allow fluid flow. Lift non-return valve manufacturers design these valves for use in high velocity and high-pressure fluid flow applications. The fluid turns at 90^o degrees for discharge through the outlet. When the outlet pressure becomes less than the cracking pressure the fluid attempts to flow back to the inlet section and thus gravity force or spring force is used to close the non-return valve. A lift non-return valve can be installed in either a horizontal or vertical direction. However, for a lift non-return valve that relies on gravity force it needs to be installed such that the disk travels in the vertical direction. 

Figure: Lift non-return valve.

Stop non-return valve 

This is a non-return valve that is designed similar to other non-return valves but it has external control mechanisms such as a handwheel, lever, or actuator. This type of non-return valve is similar to a non-return globe valve as shown in the figure below. Stop non-return valve manufacturers design this valve with the control mechanism so that it can be used to close the valve despite the fluid flow pressure. The stop non-return valve has its stem head floating in the disc. The disc is not connected to the stem and is used to allow and block fluid flow. This helps to control the flow but in case fluid tries to flow back the disc quickly closes to prevent fluid flow in the reverse direction. The stem can also be lowered manually to stop fluid flow. In short, a stop non-return valve is used as another version of a globe valve to open and close fluid flow as well as automatically prevent backward fluid flow which prevents damage to equipment like pumps and or boilers. 

Figure: Stop non-return valve.

Applications of a stop non-return valve 

• Used in pumping systems to help prevent fluid from flowing backward which can cause damage to the pumping system. 
• Chemical applications to prevent fluid contaminations.
• They are used in the foods and beverages industries. 
• Control flow of fluids used in power plants. 
• Oil and gas industries. 
• Wastewater, potable water, and irrigation systems. 
• Boiler applications to prevent hot water from flowing backward. Also, the non-return valve is used to prevent water flow from the pump into the boiler. 
• Prevent the backward flow of compressed gas or air in compressors. 

Advantages of non-return valves 

• Ensures no backward flow of fluid. 
• It prevents pumps from damage due to backward fluid flow. 
• Help to prevent production losses which may occur when fluid mixes such as in foods and chemical industries. 
• Very low-pressure drop. 
• Cheap to repair and do maintenance. 
• Maintain the required pressure. 
• These valves can be used in the either vertical or horizontal direction. 
• Versatile as they can be used in different industrial applications. 
• These valves are durable. 
• They do not need a valve operator. 

Disadvantages of non-return valves 

• Fluid flow occurs in one direction only. 
• Not suitable for use in pulsation systems. 
• These valves cannot be inspected while in open or closed state. 
• Create water hammer problems in the system. 

Troubleshooting a non-return valve 

Non-return valve not allowing fluid flow from suction to the outlet side

• The suction pressure is lower than the outlet pressure. Increase the suction pressure level according to the manufacturer’s instructions. 
• Pipeline is blocked. Check the valve for blockage and remove any material blocking the fluid flow. 

The non-return valve does not stop fluid flowing in the reverse direction 

• The non-return valve is obstructed by foreign materials. Shut fluid flow and depressurize the piping system. Remove the valve from the piping system and disassemble the valve to access the foreign materials and clean them. Reassemble the valve according to the non-return valve manufacturer’s instructions. 
• The pipe thread seal has failed and blocked the orifice. Replace the thread seal. 

Fluid leakage through valve connection 

• Loose connecting elements like bolts, nuts, and screws. Tighten the screws according to the torque recommended by the non-return valve manufacturer. 

Leakage through the valve body 

• The non-return valve body has been damaged by a falling object. Repair the valve body or contact the non-return valve manufacturer for further instructions. 

Summary 

A non-return valve is a type of valve which is used to prevent fluid flow from reversing in the piping system. It is also known as a one-way valve since the fluid flows only in one direction. Non-return valve manufacturers design these valves such that there is no need for a valve operator as the valve shuts and opens automatically. The valve operates on the principle of differential pressure. In this principle, the valves opens when the upstream fluid pressure is more than the downstream pressure. This valve closes when the downstream pressure is more than the upstream pressure. Non-return valves work based on pressure to open and close automatically which helps to prevent backward fluid flow which can cause damage to other systems like pumps and or contaminate fluid in the pipeline.

There are various types of non-return valves which include spring-loaded in-line non-return valve, spring-loaded Y-shaped non-return valve, ball non-return valve, swing non-return valve, lift non-return valve, and stop non-return valve. Applications of non-return valves include chemical industries, foods and beverages, power plants, irrigation, wastewater, boiler and pump applications among others. Advantages of using a non-return valve are low-pressure drop, versatility, durability, no reverse flow, cheap to repair, can be installed horizontally or vertically, and they are automatic.