Design Considerations for Water Pressure Reducing Valve (PRV) Stations in Commercial Buildings

Pressure Reducing Valve (PRV) Stations are an important component of a water-distribution system in a commercial building. The 2015 Uniform Plumbing Code Section 608.2 states that PRVs are required at any point where the system static pressure exceeds 80 PSI. Typically, this applies to mid- and high-rise buildings when the pressure boost required at the ground floor to serve the upper floors in the building is over 80 PSI. When you need to design a PRV Station, you must consider the station pressure drop, water flow, and safety devices.

To calculate the Pressure Drop across the PRV Station, we have to determine the inlet and outlet pressures. The inlet pressure is determined by the PRV location in the building. The lower the PRV is in the building, the higher the static inlet pressure will be. Typically, the PRVs are fed by a Pressure Boosting System that feeds the entire building, so the inlet pressure may also fluctuate a little, depending on the demand in the rest of the building.

The outlet pressure is determined by two factors. First is the number of floors the PRV Station is serving, and the second factor is whether the station is feeding the floors above or below the station. A good rule of thumb is that each floor will result in a pressure change of 5 PSI. If the floors fed by the PRV Station are the floors above, then you would need a higher outlet pressure at the PRV Station (around 65 to 75 PSI) because the pressure will drop about 5 PSI each floor higher in the piping. If the PRV Station is feeding the floors below, the outlet pressure would need to be lower (around 40 to 50 PSI) because the pressure will increase 5 PSI for each floor lower in the piping.

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Water PRV Sequence of Operation

A High-Performance Water Pressure Reducing (PRV) Station requires valves that are properly sized for the flow rate and pressure drop of the zone they serve.  Also, the High Flow and Low Flow valves must have the proper pressure setpoints to balance the flow rates across them.  Cougar USA can help in the design, installation, and start-up of PRV’s to ensure constant pressure to the fixtures downstream of the PRV Station.

Water PRV Sequence of Operation Tech Talk Transcript:

Hi, I’m Tim Zacharias with Cougar USA on this Tech Talk, we’re going to be covering the sequence of operation for a Water Pressure Reducing Valve station or PRV station.

For this example, we are looking at a PRV station here that’s designed for a high-rise building. We’re generating a lot of pressure with our booster system, down the lower floor to be able to have usable pressure up at the top of the building, and in the lower floors, we’re having to knock that pressure back down to get it below 80 PSI.

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Avoiding Cavitation in Water Pressure Reducing Valves

High-rise buildings present multiple challenges for water distribution due to the high pressures required to reach the top of the building. The high pressures in the lower levels of the building cause high-pressure drops across Pressure Reducing Valves (PRVs), over 100 PSI or more, creating the potential for cavitation within the valves.

Cla-Val explains cavitation in this white paper, saying “Cavitation occurs when the velocity of the fluid at the valve seating area becomes excessive, creating a sudden, severe reduction in pressure that transforms the fluid into a vapor state, resulting in the formation of literally thousands of minute bubbles. The subsequent decrease in velocity and pressure rise that occurs after the valve seating area, when the pressurized condition resumes, causes these vapor bubbles to collapse at the rate of many times per second. Should this occur in close proximity to any metal surface, damage can take place. Over time, this can lead to valve failure.”

The damaging effects of cavitation include excessive noise, erosion of the valve and eventual valve failure. When designing a system with pressure drops greater than 100 PSI, there are two ways to avoid cavitation.

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