Dual Check Valve 

What is a dual check valve? 

A dual check valve is a special type of check valve which is used to prevent reverse flow of fluid. This type of valve consists of two automatic, spring-loaded check valves that work together to prevent reverse fluid flow and thus prevent fluid contamination and damage to piping equipment like pumps. The dual check valve is suitable for use against back pressure and back siphonage. In water supply applications, dual check valve manufacturers aim to separate fresh water from non-health hazards. These valves have been used in applications such as fire protection, household water supply, medical, chemical and irrigation systems. The use of two automatic check valves instead of using one is very safe because one valve will work even when the other valve has been stuck open. Dual check valve manufacturer recommend checking and testing these valves regularly to ensure that they are functioning properly to help for example, save human lives from health hazards. A dual check valve is similar to another check valve known as a double check valve which has two independent check valves for preventing reverse fluid flow. However, a double check valve has shut-off valves and test cocks or ports while a dual check valve does not have such.

Figure: Dual check valve.

How does a dual check valve work? 

A dual check valve works based on the principle of pressure difference. The pressure energy of the flowing energy is used to open the valve thus allowing forward fluid flow. As such, a dual check valve opens when the inlet pressure is more than the outlet pressure. When the outlet pressure becomes more than the inlet pressure the fluid attempts to flow backward which is the reverse direction of flow. Fluid reversing may cause contamination of other fluids. To prevent the reverse fluid flow, dual check valve manufacturers design this valve to be sensitive to pressure changes so that in the instance the fluid attempts to flow back the valve disc or ball closes. These valves are arranged in series to ensure that if one valve is stuck open the other valve will prevent backward fluid flow. Also, having the two valves functional enhances efficiency and thus ensures there is zero contamination of the fluid or mixing of fluid.

Figure: Working of a dual check valve.

Types of dual check valves

Wafer dual check valve

This is a dual plate check valve which is made of wafer type disk that is installed on the hinge and spring. This type of dual check valve works on the same principle of pressure difference. When the upstream pressure is more than the downstream pressure the valve disc opens to allow fluid flow. Immediately the suction pressure drops fluid attempts to reverse flow direction and thus the spring forces the disk to shut the valve. Both check valves have a disc that opens to around 90^o when the valve is fully opened to allow fluid flow. This design of the dual plate check valve is good due to its high operating speed, small size, and lightweight. 

Figure: Wafer dual check valve.

Sanitary dual check valve 

This is a special dual check valve that is meant for use in applications that need sterile or clean processing. This type of valve is made of materials and design that enhance hygiene such as ease of cleaning and which are not harmful to human consumption. Dual check valve manufacturers design these valves for use in applications such as foods, dairy, pharmaceuticals, chemicals, and medical applications. Good defining features of these dual check valves are easy cleaning, polished contact surfaces, and crevice-free. Most dual check valve manufacturers produce the valve such that the component of the valve that comes in contact with the fluid or medium is made of 316 or 304SS stainless steel. This material is resistant to corrosion and is also non-toxic which is a critical property when making sanitary valves.

Forged dual check valve 

This is a dual check valve that is made through the forging method. This method of manufacturing valves involves using thermal energy to heat the material used to make the valve which makes it easier to shape. The hot material is then processed using mechanical devices to shape it to the required final product. Forged dual check valves are known to be very strong and tough for use in high pressure and high-temperature applications such as pharmaceuticals, water supply, and agriculture among others. However, forged dual check valves are expensive to produce compared to the casted types. Also, not all materials can be forged to make valves so there are limited forged dual check valves.

Casted dual check valve 

This is a dual check valve that is made through the casting method. The casting method involves heating the material used to make the valve to high temperatures until it is in molten form. The molten material is then poured into a mold and given time to cool and solidify. The valve is then tested through non-destructive tests to ensure that it does not have defects that may cause leakage while the valve is in use. Through the casting method, it is possible to produce dual check valves of different and complex shapes, sizes, and patterns. Also, dual check valve manufacturers use this method because it is cheaper to make valves compared to making valves using the forging method. The casting process involves less labor relative to the forging method thus casted dual check valves are cheaper. 

Plastic dual check valves 

This is a dual check valve that is made of plastic material. Dual check valve manufacturers design these valves for use in low-temperature and low-pressure applications. These valves should not be used for temperatures above 60 ^oC. This is because the materials used to make these valves are very weak above this temperature and thus they can get damaged if used above such temperatures. Some of the materials used to make these valves include polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC). These materials are excellent in corrosion resistance. Plastic dual check valves have long services because of the long life span of the plastic materials. It is easy to manufacture a plastic dual check valve as it needs less energy relative to metallic valves which makes its cost cheaper relative to metallic ones. However, they have weak thermal strength. 

Figure: Plastic dual check valve.

Factors to consider when selecting a dual check valve

Type of media 

Before purchasing a dual check valve it is important to know the properties of the media the valve will work on. Some of the dual check valves are meant for use in clean water applications while others are meant for use in slurry media. Other dual check valve manufacturers produce these valves for use in applications that are corrosive such as the transfer of acids. So the properties of the fluid must be well defined to help purchase the best valve which will last longer and which will not cause unnecessary costs through repair and maintenance. 

Pressure and temperature levels 

Pressure and temperature are very important factors to consider when purchasing a dual check valve. Dual check valve manufacturers design these valves for use in different temperatures and pressure levels. For example, valves made of plastic materials are meant for use in low-temperature applications and if used at a temperature above 60 ^oC they get damaged since they cannot withstand such levels of thermal energy. Also, plastic dual check valves cannot withstand high temperatures relative to metallic ones. In addition, metallic dual check valves are designed for specific temperatures and pressure. As such, it is very important to note the operating pressure and temperature where the dual check valve will be used. 

Applications of dual check valves 

• Used in the freshwater supply. 
• They are used in wastewater treatment plants. 
• Irrigation systems. 
• Foods and beverages industries. 
• Medical industries. 
• Chemical manufacturing industries. 
• Fire protection systems.

Advantages of dual check valves 

• Small in size. 
• Light in weight relative to other valves like ball valve and gate valve.
• These valves do not allow reverse fluid flow. 
• Dual check valves maintain pressure levels. 
• These valves have a low-pressure loss. 
• Durable provided they are used according to the manufacturer’s instructions. 
• Can be used in different industrial applications. 
• Are automatic so there is no need for actuators or valve operators which may raise the cost of operating the valve.
• Free from fluid leakage. 

Disadvantages of dual check valves 

• Allow fluid flow in one direction only. 
• Can cause a lot of damage if not well maintained.
• Expensive relative to single check valves. 
• Cannot be inspected while in operation. 

Troubleshooting dual check valves

External leakage 

• Loose connection between the pipe and dual check valve. Tighten the connection between the valve and the pipe. 
• O-rings are worn out. Replace the O-rings. 

Dual check valve leaking when fully closed 

• O-rings are worn out. Replace the O-rings. 
• There are solid particles in the valve. Remove the solid particles. 
• The valve seat is damaged. Replace the valve seat or the dual check valve. 

The dual check valve is not working 

• The valve was not installed correctly. Uninstall the valve and install it following the dual check valve manufacturer’s instructions.
• There is low upstream pressure. Check the suction pressure is high enough to allow fluid flow through the dual check valve. 

Summary 

A dual check valve is a type of valve that is used to prevent reverse fluid flow. It consists of two check valves that are arranged in series. These two check valves work together automatically to prevent backflow and siphonage. Having two check valves enhances efficiency in restricting reverse fluid flow. This type of valve is better relative to other types of check valves because in case one check valve is stuck open, the other valve will work to ensure no fluid reverses direction of flow. Dual check valve manufacturers also design these valves to prevent fluid contamination as well as damage to other piping elements such as pumps.

Some of the factors to consider when selecting a dual check valve are the type of media, pressure, and temperature rating. The applications include freshwater supply, wastewater treatment, medical applications, foods and beverages, and pharmaceutical and chemicals applications among others.