Wafer Type NRV

What is wafer type NRV? 

Wafer type NRV means wafer type non-return valve. This is a type of valve used to prevent reverse fluid flow in different industrial applications conveying fluids such as liquids, slurries, gases, vapors, and steam. Wafer-type NRV manufacturers design this type of valve without a valve handwheel or actuators. The valve operates automatically based on pressure difference that is they open when suction pressure exceeds the outlet pressure. A spring is used to help the valve close immediately suction pressure reduces or when there is backflow. Having the spring in a wafer-type NRV helps to make the valve close fluid flow when installed in either vertical or horizontal orientation. The disc of a wafer-type NRV consists of either single plate or two half-plates pivoted on a hinge that flap allowing fluid flow through the valve. These valves have short takeout lengths which allow them to close fast. Wafer-type NRV manufacturers design these valves with a slim profile which makes them unlikely to clog and thus they are ideal for applications with solids or debris. Wafer-type NRV manufacturers design these valves to allow fluid flow in one direction only. Any attempt by the fluid to flow in the reverse direction causes the valve disc to close. Also, some of these valves come with a weight and lever mechanism to help in closing the valve with aim of reducing water hammer. 

Figure: Wafer type NRV.

Components of a wafer type NRV

Valve body 

The body is the main component of a wafer-type non-return valve. The body serves as the main pressure casing and holds the valve ports. The body of a wafer-type NRV valve is always designed using strong metallic materials to ensure the valve can withstand fluid pressure and protect interior parts from impact damage when the valve falls down or from falling materials. 

Disc/plates 

This is the part used to close the valve when fluid attempts to reverse flow direction. The disc consists of single or two half plates pivoted about a pin and flap when there is high fluid pressure. 

Seats 

Seats are components mounted between the valve body and the disc/plates to ensure the valve is free from fluid leakage when the disc closes fluid flow. Also, the seat provides the surface for the plates to the seat when they revert to the seating position. 

Spring 

Spring is a component used to help the disc return to its seating position. The spring gets stretched when the suction pressure forces the disc to open. When the suction pressure reduces or fluid attempts to reverse flow direction the spring reverts together with the valve disc to the seating position. Wafer-type NRV manufacturers design the spring with a strength relative to suction pressure. A spring with very high strength may not be able to open when suction pressure is low.

Gasket 

This is a component of a wafer-type non-return valve that serves to prevent fluid leakage between the mating faces of the valve and the pipe. The gasket material determines its operating temperature. Very high temperature may damage the gasket if it is made of rubber or plastic. 

Figure: Components of a wafer type non-return valve.

How does a wafer-type NRV work? 

A wafer-type non-return valve operates based on pressure differential. This is a difference in pressure between the inlet and outlet ports of the wafer-type NRV. The valve disc opens when the inlet pressure is more than the outlet pressure. This pressure provides the force needed to flap the valve discs. When the discs are flapped they leave space for the fluid to flow through the valve. The discs remain open provided there is high suction pressure than the outlet side. When the suction pressure reduces or fluid attempts to reverse flow, the spring gains energy to revert the valve plates to their seating position thus helping in preventing reverse fluid flow. 

Figure: Working of a wafer type non return check valve.

Types of wafer type non-return valves 

Reduced port wafer type NRV 

This is a wafer-type NRV designed with the diameter of the valve port less than the diameter of the pipe. This design of a wafer-type NRV causes friction losses in the piping system which can reduce pumping efficiency. Wafer-type NRV manufacturers recommend these valves for use where pressure loss, material characteristics, and turbulence are not major concerns. However, despite the pressure loss, these valves are of low cost and small size. As such, if the flow rate is not in strict demand, a reduced port wafer type non-return valve will be more economical to use. Also, the weight of this wafer-type NRV will be relatively lower. 

Full port wafer type NRV 

This is a wafer-type NRV that is designed with a valve port diameter equal to the pipe diameter. This type of valve does not have friction losses and also it is easier to clean unlike the reduced port type. As such, this type of valve helps to enhance piping efficiency. However, full port wafer type non-return valves are mostly bigger relative to the reduced port type. As such, the cost of this valve is higher. Wafer type NRV manufacturers recommend the use of this valve where fluid flow resistance should be as minimal as possible to avoid interfering with piping system performance. Also, this type of valve is suitable for use where the media consists of solids and liquids to help reduce the chances of material buildup which may cause blockage. 

Single plate wafer type NRV 

This is a wafer-type non-return valve designed with a single plate. This means that the valve disc is made up of only one plate. The single plate is responsible for opening and closing fluid flow. One advantage of this type of wafer type NRV is that it is cheaper compared to the valve using two plates as the disc. The disc is pivoted at one end and the other end is free. The disc opens when fluid acts at the free end. Any attempt of the fluid to flow backward forces the disc to close. However, wafer type NRV valve is prone to a slamming effect when it closes which makes it an unpopular choice compared to other types. 

Figure: Single plate wafer type NRV.

Dual plate wafer type NRV 

This is a wafer-type NRV that uses two plates that work together to prevent reverse fluid flow. Wafer-type NRV manufacturers design this valve such that the two plates can work automatically based on forward and reverse fluid flow. These valves are more expensive compared to the single plate type. Also, dual plate check valve has a slimming effect. This type of valve has good hydrodynamic features that result in a low-pressure drop.

Figure: Dual plate wafer type check valve.

Wafer type NRV with a counterweight 

This is a wafer-type non-return valve designed with a mechanism that helps to prevent water hammer. Water hammer happens due to the valve closing quickly. This effect is very dangerous as it can lower the structural integrity of the piping system. The counterweight mechanism helps to enhance reliable time taken to close the valve. Wafer-type NRV manufacturers design this valve with an anti-hammer system that uses an oil cylinder to help slow down closure movement and thus reduces the slamming effect and noise. This type of valve opens when fluid flows in the forward direction and closes when fluid attempts to flow in the reverse direction. However, this type of wafer type non-return valve is more expensive relative to the other types. The high cost emanates from the counterweight system. 

Figure: Wafer type NRV with counter weights.

Applications of wafer type NRV

• Protect against steam boiler flooding. 
• They are used in vacuum breaker systems. 
• Prevent reverse fluid flow in compressed gas and air systems. 
• Water and wastewater systems to prevent reverse fluid flow and water hammer effects. 
• Oil and gas production industries. 
• Petroleum refining applications. 
• Petrochemical and chemical manufacturing industries. 
• Paper and pulp industries for bleaching lines and chemical recovery. 
• Marine applications for example, in shipboard services, tanker loading terminals, and unloading lines. 
• Foods and beverages industries. 

Advantages of wafer type NRV 

• Operate on their own without the use of a handwheel or valve actuator. 
• These valves do not allow reverse fluid flow. The fluid flows in the forward direction and any attempt to reverse is restricted as the valve closes immediately. 
• High efficiency relative to other valves because their pressure drop is minimal.
• Versatile as they are used in various applications from household to industrial applications. 
• Easy to install, repair, and clean. 
• Alleviate water hammer and related effects. 
• Light in weight relative to other non return valve. 
• Can be installed in either horizontal or vertical positions since they do not depend only on gravity to prevent reverse fluid flow. 
• Fasting acting relative to other valves such as globe valve.

Disadvantages of wafer type NRV 

• Cannot allow fluid in both directions. 
• These valves are not suitable for use in pulsating fluid flow. 
• Cannot be inspected while in operation. 
• They can be trapped wide open and then cause reverse fluid flow leading to losses or damage to the piping system. 

Troubleshooting wafer type non-return valves 

The valve does not open 

• Wrong direction of installation. Install the valve according to the instructions given by the wafer-type NRV manufacturer. Verify the fluid flow direction matches the flow arrow shown on the valve body. 
• There is no enough suction pressure. Increase the suction pressure to the level recommended by the wafer-type NRV manufacturer. 
• Fluid flow blockage on the pipe connecting the valve. Check the pipe for any blockage and remove any blocking material. 
• The downstream valve is not open. Verify the downstream valve is open and if not open it. 

The valve does close 

• The valve disc has been stuck wide open. Open the wafer type NRV and remove any material restricting disc or spring closure. 

Internal fluid leakage 

• Damaged or worn-out O-rings. Replace the rings. 
• Damaged seats. Replace the seat. 
• Debris is in the seat. Open the wafer-type NRV and clean all the debris. 

External fluid leakage 

• Loose connection between pipe and wafer type NRV. Tighten the connecting elements to the torque range recommended by the wafer-type NRV manufacturer. 
• Worn out O-rings. Replace the O-rings. 
• Worn out gasket. Replace the gasket. 

Summary 

Wafer-type non-return valves are valves used to prevent fluid from reversing direction. These valves operate automatically without using a handwheel or actuator. The working principle of these valves is based on the pressure difference.

Wafer-type NRV manufacturers design these valves to open when the inlet pressure is more than the discharge pressure. The valve closes when there is backflow or when the pump is stopped. The valve disc is aided by spring mechanism to close the valve. This is where the spring is stretched by fluid pressure but immediately after pressure reduces it starts reverting taking the disc back to the seating position.