Direct acting Pressure reducing valves (PRV)

In continuation to my previous article on the basics of Pressure reducing valves, I thought of sharing my knowledge on the assembly, working principle, selection and installation of a direct acting pressure reducing valves. I believe this article will help you to understand more technical details about a pressure reducing valves and clear your general doubts on the working & installation principles.

Based on the construction, Direct acting PRV are classified in to piston type and Diaphragm type. But mostly, Diaphragm types are preferred in the current scenario as it has the advantage of tight sealing (leak proof) and long life. So, let’s focus more on the Diaphragm type direct acting PRV.

So, let me start with the detailed explanation on the construction & assembly of a direct acting pressure reducing valve. Usually, these PRVs are with very simple mechanism with major components categorized in to –

• A complete valve: Inlet flow of water, water flow through the PRV and outlet flow of water. The second one is the key mechanism where the pressure is getting reduced to the desired pressure and that’s what we are going to see how it happens!
• Adjusting spring & Diaphragm: This is considered to be a very critical components in a PRV. The spring is being adjusted through a provision like screw, in order to set the desire outlet pressure. This happens through the compression & extension of the spring.
• Next is the protection phase i.e., a Strainer – It is a recommended component for any type of PRV in order to protect the PRV from damage / reduce life span due to debris deposition in it. There are many varieties that you can see in the market but the most common one are strainer installed externally before the inlet of the PRV and the integral one.

Let’s see how a direct acting PRV works with these three functional part to reduce the water pressure in the pipeline.

The incoming water from the main line flows through the strainer first, in order to filter out any debris / unwanted particles that are present in the water before entering in to the valve. After filtering out the debris, the incoming water pressure pushes on a small area of the seating disc which will tends to open the valve. The valve operates by means of a force equilibrium system i.e. force of the diaphragm operates the force of the adjusting spring. The pressure at the downstream pushes against a larger area of the diaphragm trying to close the valve. The point where the valve closes in a static flow is the set point and it can be adjusted by changing the tension on the spring. More tension assists in opening the valve and raising the set point. Less tension decreases the force trying to open the valve so it closes at a lower set point. Increased demand drops the downstream pressure and thus the pressure trying to close the valve. The valve then opens, restoring the pressure to the desired level.

I often get few questions from my customers on the PRV working;

1. What happens when PRV installed in opposite direction? Does it increase the pressure? The name of the valve is self-explanatory that it only reduces the pressure and cannot increases the pressure.
2. What happens when the inlet pressure falls below the set pressure? If the inlet pressure falls below the set outlet pressure, the outlet pressure will also go down. PRV maintains a set pressure when supply pressure does not drop below the valve’s pre-set level

SIZING / SELECTION OF A DIRECT ACTING PRV:

In order to select a PRV, the first thing you should keep in mind is “DO NOT SELECT A PRV BASED ON THE LINE SIZE”. This is the main problem that we often faces in the buildings after installing the PRV or after certain point of time where we get complaints – noise occurrence, leak through diaphragms, reduced life span of the valve, etc.

Please keep in mind, PRV selection (regardless of any type) has to be done based on the flow & pressure and not with line size. Every PRV will technical data sheet will have a performance curve which will clearly shows the flow vs reduced pressure drop. The selection & sizing has to be done based on the same.

I will take you back for a minute to explain you about the reduced pressure drop or fall of pressure again. Reduced pressure dropt is the reduction of pressure at the downstream side of the PRV when the downstream fixtures are open to allow flow through the valve.

(Sorry for the pencil-drawing) :)

Let is consider a PRV is set at 50 psi, which is static pressure setting and represents the PRV downstream pressure in a no-flow condition. Once the fixtures start to open, water begins to flow through the PRV and the downstream pressure will fall. When more fixture are open means more flow through the PRV, resulting in more reduced or fall-off pressure. The amount of fall-off pressure depends on the inlet pressure of the PRV, the size of the PRV and the flow through the valve.

I will show you one instance on how to use the performance curve chart for selecting a PRV. By considering;

• Upstream pressure : 100 PSI
• Downstream pressure : 50PSI (Set pressure)
• Flow : 20 GPM

The x-axis reads the flow capacity in gallons per minute and y-axis reads the reduced pressure drop in pound square inch. As shown in the performance curve (Drawn by me for an example). To determine which size to be used, first locate the flow rate on the x axis and follow the line until it intersects a curve near the allowable reduced pressure drop on the y axis. This will allow us to select the 1” PRV for our requirement.

Please note that the over-sizing of PRV can lead to wire draw which is caused by erosion of the seat due to wearing caused by high velocity water over a metal edges and Under-sizing can cause hammering or noisy operation of the valve. The recommended best practice is that the minimum flow through the valve should be 10% to 15% of the maximum flow rate specified for the system.

INSTALLATION CONSIDERATIONS:

In order to ensure protecting the fixture and appliances from high water pressure, it is recommended to install a PRV immediately after the shut off valve. Installation of PRVs are categorized in to three configurations;

(Sorry for the pencil-drawing) :)

1. Single stage installations are most commonly seen in one-family homes and also it is preferred to use when the pressure reduction ratio is 3:1 (recommended) or less i.e., ratio between the incoming pressure to the desired outgoing pressure.
2. Two stage series installation are used when there are considerable difference between the incoming supply pressure and unstable system pressure. When the pressure reduction ratio is greater than 3:1 ratio, two stage series installation is recommended.
3. Parallel / low flow bypass installations are used when continuous water supply is required without any interruption of flow even during the maintenance. More consistent pressure can be maintained using two PRV piped in parallel. As shown in the figure, the smaller valve shall be sized to handle 1/3 of the flow rate at an acceptable fall-off pressure and the larger valve would then sized for 2/3 of the flow rate at the same fall-off pressure. If the smaller valve is set for 75 psi and the larger one is set for 70 psi, then under low-flow conditions, the larger valve would be closed and the smaller valve would regulate the pressure.

I believe my articles on the pressure reducing valves would have helped you to enhance your knowledge on the PRV and at this juncture, I would like to thank the trainers of WATTS Works Learning center, NA for providing me a great training on water specialty products.

Please feel free to contact me if you have any questions or clarification.