NRV Valve

What is NRV valve?

NRV valve in full means non-return valve. NRV valve is also known as a one-way valve or check valve. This is a type of valve that allows fluid to flow in one direction only. These valves work by opening when fluid flows forward but it closes when the fluid flows backward or when the fluid tries to reverse the direction of flow. NRV valves are different from other types of valves which are used to throttle, open, or close fluid flow using handwheel or actuators. Most of the NRV valves do not have handwheel or actuators. NRV valve manufacturers make these valves automatic based on the principle of pressure difference between the inlet and outlet to open or close the fluid flow. 

How does the NRV valve work? 

NRV valve operates by applying the principle of pressure difference. The difference between the outlet pressure and the inlet pressure in an non return check valve is known as cracking pressure. Cracking pressure depends on the design and size of the NRV valve. When the cracking pressure is of the same level as the upstream fluid pressure, the NRV valve opens and fluid flows through the valve. When the cracking pressure becomes more than the flow upstream pressure, the fluid attempts to flow from the outlet to the inlet side. In this instance, the NRV valve closes to prevent the fluid from flowing backward. NRV valve manufacturers design the valve with a sign showing fluid flow direction on the valve body. NRV valve helps to prevent fluid from getting back which may cause contamination to the rest of the fluid for example, in applications such as sewage pipelines, foods, and beverages among others. 

Figure: Working of a NRV valve.

Types of NRV valves 

Swing NRV valve 

This is one of the most widely used NRV valves. In this type of valve, a disc is used to close and open fluid flow. The disc swings on a shaft or hinge. To open the NRV valve, the disc swings from the seat which allows space for fluid to flow through. To close fluid flow, the disc swings back onto the seat to block fluid from reversing. Swing NRV valve manufacturers design these valves such that their flow resistance is very low which helps to improve pumping efficiency. To enhance better performance swing check valve is designed with a spring and lever or weight and lever. The weight of the disc and return flow helps to improve the shut-off characteristics of the swing NRV valve. Swing NRV valves are unobstructed and automatically close when the pressure decreases. 

Figure: Swing NRV valve.

Spring-loaded NRV valves 

These are NRV valves that use a spring mechanism to help in opening and closing fluid flow. Two types of these valves are spring-loaded in-line and spring-loaded Y-shaped NRV valves. In-line spring-loaded NRV valves use a guided disc and compression sprig in their operation. In an in-line spring-loaded NRV valve, the fluid flow is opened when the pressure is more than the cracking pressure and spring force. As such, the fluid pushes the disc allowing it to proceed through the NRV valve to the piping system. When the inlet fluid pressure reduces, the disc is pushed against the orifice by the spring which then helps to close the valve preventing backward fluid flow. The spring-loaded Y-shaped NRV valve is the same as the in-line valve. NRV valve manufacturers differentiate the Y-shaped and the in-line valve by having the disc and the spring of a Y-shaped valve positioned at an angle that makes a Y-shape. The Y-shaped NRV valve can be serviced and inspected while it is still connected to the pipeline. 

Figure: Spring loaded Y shaped NRV valve.

Figure: In-line NRV valve.

Ball NRV valve 

This is a type of NRV valve which uses a ball-shaped element for closing and opening fluid flow. When a ball NRV valve closes, the element shaped like a ball rolls to the valve seat to align with the seating surface. Some ball NRV valve manufacturers use spring-loaded mechanisms to help the valve in shutting-off. For the ball NRV valves that do not have a spring mechanism, reverse flow is used to move the ball to the seat to help create a sealing surface. Ball NRV valve manufacturers recommend these valves for use in applications that have viscous fluids as well as where there are sediments and scales since they are capable of withstanding such high viscosity and sediments. Ball NRV valves have a long service life as the ball and seats can withstand wear for a long duration. These valves operate quietly and are suitable for use where fluid rapidly changes flows. Ball NRV valves can have the ball and seat removed for repair without opening the piping system. 

Figure: Ball NRV valve.

Duckbill NRV valve 

This is a unique type of NRV valve which is designed as a one-piece elastomeric valve. This valve is basically meant to prevent fluid flow from reversing. Duckbill NRV valve manufacturers design these valves such that fluid flows through a soft tube feeding into the downstream section of the valve where backpressure closes fluid flow. These valves are made of good fabric reinforced polymer materials to enhance corrosion and wear resistance which may be caused by continuous valve operation working on abrasive slurries, effluent, or sludge. Characteristics of duckbill NRV valve are: 

• Low head loss. 
• Zero mechanical parts. 
• Zero corrosion.
• Low cracking pressure. 
• Self-draining to eliminate standing water.
• Zero slamming silent operation. 

Figure: Duckbill NRV valve.

Stop NRV valve 

This is a type of NRV valve which is designed special relative to other NRV valves. The special feature of the stop NRV valve is the control mechanism in form of a handwheel, lever, or actuator. This control mechanism is used to help in closing the valve regardless of the level of pressure. Stop NRV valve is a modification of the normal globe stop valve with the valve stem fixed to the globe disc. Stop NRV valve manufacturers design these valves such that the stem head is floating in the disc. The disc moves up and down when the stem is rotated to help in opening and closing fluid flow respectively. When fluid attempts to flow backward, the disc quickly closes to prevent reverse fluid flow. As such, stop NRV valve manufacturers design these valves for use in starting and stopping fluid flow as well as for automatically preventing backward fluid flow to prevent damage to equipment like pumps and boilers. 

Figure: Stop NRV valve.

Diaphragm NRV valve 

This is an NRV valve that is designed with a rubber flexing diaphragm to help prevent backward fluid flow. The diaphragm is self-centering and is responsible for stopping and allowing fluid flow. The diaphragm NRV valve body is used to center the sealing surface or seating area. The sealing surface is placed on the suction side of the valve body. This type of NRV valve has a smooth and fine surface that is curved or concave back to the inlet side. The center has an opening. The reverse flow of fluid makes the rubber disc lie across the sealing surface which helps to close the inlet port opening preventing upstream fluid flow back into the inlet port. 

Figure: Diaphragm NRV valve.

What should you consider when purchasing/selecting an NRV valve? 

NRV valve material

The material used to make the NRV valve should be compatible with the fluid to be transported through the valve. Also, the material should be compatible with the pipe such that when the two are connected they can provide a tight seal free from fluid leakage. 

NRV valve performance 

NRV valve manufacturers recommend the use of an NRV valve that can withstand the application fluid pressure and temperature. NRV valves made of plastic materials have low strength and low melting temperatures and thus should not be used for high temperatures starting at 60 ^oC. Metallic seated NRV valves have high strength for high pressure and high temperature and thus can be used even at high temperatures without distorting the valve. 

NRV valve type and size

The NRV valve purchased should be able to fit in the intended pipeline to ensure no fluid leakage and serve for a long time. The type of valve should be suitable for use in that specific applications as recommended by the NRV valve manufacturer. Some valves will corrode if used in acidic applications while others will work perfectly even in corrosive media. 

Maintenance 

The NRV valve selected for use should be reliable in its service so that it does not need repair every day which would be expensive. While considering repair, it is necessary to purchase a valve that will resist clogging and jamming relative to the application to help reduce the chances of halting work, repair and maintenance. 

Special requirements 

The NRV valve to be selected should be considered for factors such as response time, normally open versus normally closed, cracking pressure, horizontal verse vertical installation, fluid density, and fluid flow direction. 

Applications of NRV valves 

• Food and beverage applications. 
• Pharmaceutical industries. 
• They are used in the production of different chemicals. 
• Oil and gas applications. 
• Wastewater transportation. 
• Prevent the backflow of hot or cold fluids in boiler systems. 
• Irrigation systems. 
• Steam power plants. 

Advantages of NRV valves 

• Simple in design. 
• These valves are energy efficient. 
• Automatic thus saving on the cost of hiring a valve operator or including actuator. 
• Versatile as they are used in different industrial applications. 
• Low-pressure drop. 
• NRV valves can maintain pressure. 
• These valves can be repaired and cleaned. 
• They are durable. 
• They do not allow reverse fluid flow. 

Disadvantages of NRV valves 

• Not recommended for pulsation systems. 
• They are prone to water hammering problems. 
• Cannot be inspected in open or closed conditions. 
• These valves allow fluid flow in one direction only. 

Troubleshooting NRV valves 

Leakage through valve connection 

• Loose bolts or screws. Tighten the bolts or screws to the torque recommended by the NRV valve manufacturer. 
• Worn out gasket. Replace the gasket. 

No fluid flow through the NRV valve 

• The pipe connected to the NRV valve is blocked by foreign materials. Check the pipeline connected to the NRV valve and remove any material blocking fluid flow. 
• Low inlet pressure. Increase inlet pressure above the outlet pressure. 

NRV valve cannot stop reverse fluid flow

• The NRV valve closes slowly. Replace the NRV valve with another one that can close faster. 

Leakage through the NRV valve body 

• Impact damage on the body. Repair the valve body but in case the valve is meant for hazardous fluids contact the NRV valve manufacturer for further details.

Summary 

An NRV valve is a type of valve used to prevent fluid from flowing in the reverse direction. These valves are meant to help prevent backward fluid flow which may cause damage to equipment like pumps as well as prevent fluid contamination, unnecessary fluid mixing, and losses in different applications. Most NRV valves are automatic thus reducing the cost of valve actuators or valve operators. Some of the factors recommended for consideration by NRV valve manufacturers before purchasing are valve material, valve performance, valve type and size, maintenance, and special requirements like cracking pressure and orientation of installation.

Different types of NRV valves are swing NRV valves, spring-loaded NRV valves, ball NRV valves, duckbill NRV valves, stop NRV valves, and diaphragm NRV valves. These valves are used in different applications such as irrigation, wastewater, foods and beverages, pharmaceuticals, boiler applications, steam power plants, chemicals, and oil and gas applications among others. Advantages of using NRV valves are low-pressure loss, simple in design, versatile, durable, no reverse fluid flow, automatic, maintaining pressure and they are energy efficient.