PRV Valve

What is PRV valve? 

PRV means pressure reducing valve. A PRV valve is a control valve used to reduce high unregulated inlet pressure to constant outlet pressure despite the fluctuations in the upstream pressure. A PRV valve reduces excess pressure in a piping system. This valve is also known as pressure reducing regulator. PRV valves are used in applications such as steam, water, gas, and oil industries. PRV valve helps to reduce pressure to safe working levels before the fluid reaches any plumbing apparatus within the area of application. Very high pressure can result in plumping problems and thus PRV valves are very necessary to prevent such. A good example of a PRV valve application is often found in a municipal water supply system where it is used to reduce water flow pressure to ensure no damage to plumping fixtures when it flows to the home piping system. 

Figure: PRV valve.

Components of a PRV valve

Valve body 

This is the component used to house the interior parts of the PRV valve. PRV valve manufacturers design the body using strong materials such as stainless steel and carbon steel to make sure it has high strength enough to withstand fluid pressure as well as weight loads of other components. 

Handle

This is component on which the valve operator can apply force to rotate a screw that moves down or upwards to vary fluid pressure.

Diaphragm 

This is the component of a PRV valve that is pressed upwards to allow the valve to open to allow fluid flow into the low-pressure pipeline.

Adjusting screw 

This is a screw with a lock nut and is used to change spring pressure. The screw is rotated from the handle and it moves up or downwards to change pressure.

Strainer 

This is a component used to filter dirt or debris from the main line to prevent the PRV valve from blockage. 

Inlet port 

This is the port through which the fluid at high pressure enters the PRV valve. 

Outlet port 

This is the port through which the fluid leaving the valve passes. This is the fluid whose pressure has been regulated so as not to damage plumping fixtures downstream. 

Spring 

Spring is used to change the pressure at which the PRV valve opens.

Figure: Components of a PRV valve.

How does a PRV work? 

A PRV valve manufacturer designs this valve with a dome shape. This valve is installed ahead of the shutoff valve where fluid pressure needs to be regulated to prevent damage on other piping fixtures. PRV valve manufacturers design this valve with an adjusting screw on the top. There is a variable spring-loaded diaphragm in the valve that is used to automatically widen and narrow based on the amount of fluid pressure entering the PRV valve. When the fluid enters the PRV valve at high pressure, the internal mechanism compresses the diaphragm to reduce fluid flow. This can help reduce fluid pressure which then reduces stresses on the fixtures and pipe installed past the PRV valve. On the other hand, when the incoming fluid pressure reduces, the diaphragm widens allowing more fluid to flow through the valve. An adjustment screw mounted on the top can be tightened to enhance the tension on the spring inside (this lowers the fluid pressure at the exit) or loosened to allow fluid to flow freely through the valve (which increases the exit fluid pressure). When the piping system is protected by the PRV valve, inner valves of appliances, shutoff valves, and faucets are less likely to have fluid leakage and fluid pressure fluctuations are evened out. 

Types of PRV valves 

Direct acting PRV valve 

This is a PRV valve that is ideal for small pressure loads where precise fluid pressure control is not necessary. PRV valve manufacturers design this valve to have compact size, easy installation and low cost. However, this valve has more set pressure variation relative to other PRV valves. PRV valve manufacturers design this valve for point-of-use services. Direct-acting PRV valve is the simplest type of PRV valve. This valve operates with convoluted bellows or a flat diaphragm.. This valve has +/- 10% accuracy of the downstream set point. This valve is designed such that adjustment spring movement helps to open the valve directly. Compression of the spring produces an opening force that helps increase fluid flow. When the pressure rises downstream, leveling occurs when the downstream pressure is fed to the underside of the adjusting spring to counter-balance against spring compression. The compressive force of the spring is restricted to enhance enough spring sensitivity to level up with changes in downstream pressure. This results in a simple fluid pressure control via the valve orifice. However, fluid with high flow rate causes pressure variation. 

Figure: Direct-acting PRV valve.

Pilot operated PRV valve

This is a PRV valve used for large loads that need close pressure control. These valves provide a quick response to changes in load and they are suitable for a wide range of fluid flow rates relative to the direct-acting types. Pilot-operated PRV valves are larger and they are more expensive. PRV valve manufacturers design this valve to use a pilot valve to load a diaphragm or piston that increases the downward force needed to open the larger valve. This enhances large flow capacity with low variation in pressure. The closing and opening of the pilot valve can be controlled by balancing the force between the secondary pressure and the adjustment spring. The pilot valve helps to deliver pressure to the diaphragm or main valve piston. The downward force resulting from the pilot flow pressure is multiplied by the diaphragm or piston areas. This enhances the opening of the large or main valve which enhances very high flow rates. 

Because of this multiplication, a small change on the pilot valve opening results in a significant change in the fluid flow and downstream pressure via the main valve. As such, when the adjustment spring force makes a small change on the pilot, it results in a fast response and a wide range of flow rates. Pilot-operate PRV valves have the advantage of tight downstream pressure control and fast response. PRV valve manufacturers produce two types of pilot-operated valves which are:

• Internally piloted PRV valve. This valve has two valves that is main valve and the pilot valve in one unit. The accuracy of this valve is around +/- 5%. 
• Externally piloted PRV valve. This type uses double diaphragms instead of piston operators in the internally piloted type. This valve has +/- 1% accuracy. 

Figure: Pilot-operated PRV valve.

Factors to consider when purchasing a PRV valve 

Operating pressure 

PRV valve manufacturers design different PRV valves for use at different pressure levels. Each valve has the output and input pressure levels. The inlet and outlet pressure ratings of the application need to be known to select the best PRV valve. This will also help determine the needed accuracy.

Fluid flow requirements 

Selection of the right PRV valve will need one to know the fluid flow rate and pipe size to ensure that the pipe and valve will withstand such flow loads. A valve operating on very high loads may experience leakages and thus losses.

Type of media 

Some PRV valves are meant for use in non-corrosive fluids while others are meant for use in corrosive, viscous, and or containing solid particles. As such, knowing the specific type of fluid will help the PRV valve manufacturer or vendor to provide the best valve for that job. 

Operating temperature 

PRV valve manufacturers use different materials to make different components of the PRV valve. Each material has its thermal properties different from other materials. As such, it is important to know the range of operating temperature to make the PRV valve manufacturer or vendor select the best valve that cannot be damaged by low or very high temperatures. 

Applications of a pressure-reducing valve 

• They are used in the domestic and commercial water supply. PRV valves are used to regulate water pressure from the municipal pipeline to the pipeline feeding home apparatus. This helps to prevent water leakage and damage to faucets. 
• PRV valves are used to reduce pressure in humidifiers, unit heaters, sterilizers, and other process equipment. 
• These valves are used in compressed air systems such as actuated tools, inflating tires, and cleaning among others. 
• They are also used in steam power plants to regulate the flow of steam and water. 
• PRV valves are used in mining operations. 
• They are used in oxyfuel cutting and welding. 
• They are used in hospitals to control the flow of oxygen. 

Advantages of PRV valves 

• PRV valves can be automated or manual. 
• These valves are fast in operation. 
• PRV valves are of simple and compact design. 
• They are free from fluid leakage. 
• These valves do not need separate feedback controllers or measuring apparatus.
• PRV valves are easy to maintain. 
• They are reliable in their operations. 
• They can be used in low and high-temperature applications. 
• They are versatile as they are used to serve in various industries. 
• These valves help to prevent damage to plumbing fixtures due to excess pressure from the main supply. 
• PRV valves help to enhance comfortable and constant fluid flow rates. 

Disadvantages of PRV valves 

• PRV valves are expensive. 
• They occupy a lot of installation space relative to other valves. 

Troubleshooting PRV valves 

Pressure overshoots during normal load conditions 

• Foreign materials or dirt is clogged between the head and pilot valve seat. Open the valve according to the PRV valve manufacturer’s instructions and clean off the dirt or foreign materials. 
• Foreign materials between the seat and main valve head. Clean the seat and the head. 
• Pressure sensing lines or orifices are blocked. Remove blocking materials and clean the lines and orifices. 

Pressure overshoots during light loads 

• Dirt between the seat and main valve head. Open the PRV valve and clean off the dirt. 
• The bypass valve is leaking or is not shut off tightly. Tighten the bypass valve and repair it as necessary. 

PRV valve does not open 

• Defective diaphragm. Replace the diaphragm. 
• The orifice is blocked. Remove blocking dirt. 
• The pilot valve seat is clogged with dirt. Clean the pilot valve seat or replace it as necessary. 
• Blockage in the pipeline strainer. Clean the strainer. 

Low delivery pressure 

• The adjustment of the pilot valve has not been properly adjusted. Adjust it to the pressure recommended by the PRV valve manufacturer. 
• Undersize valve. Check the load against the rating given by the PRV valve manufacturer’s instructions. 

Valve does not close 

• The bypass valve is leaking or it is open. Repair the valve. 
• Blocked pilot sensing line. Remove the blocking materials. 
• Pilot ruptured. Replace assembly of the pilot diaphragm. 

Summary 

PRV means pressure reducing valve. This is a control valve used to regulate high inlet pressure to constant outlet pressure. This pressure reduction is necessary as it helps prevent damage to plumping fixtures upstream. Such fixtures are designed to withstand low fluid pressure and thus the need for a PRV valve. PRV valve manufacturers design this valve with a handle used to adjust the required fluid pressure. The handle has a threaded stem and spring that helps to either raise or lower the level of pressure downstream of the PRV valve. There are various types of these valves and the most common two are direct-acting PRV valves and pilot-operated PRV valves. These valves are used in applications like steam power plants, municipal water supply, hospitals, air compressors, and mining operations among others. Advantages of this valve are versatile, fast operation, simple and compact design, zero leakage, high strength, and reliable operations. However, PRV valves are expensive compared to other valves. Also, these valves are of large size and thus they occupy large installation space.