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.
The first is to use the Cla-Val Anti-Cavitation Trim option on the 90-01 Pilot Operated PRVs. The Anti-Cavitation trim controls the water flow through the disc and seat of the valve in a way that dissipates cavitation and its effect and allows the pressure drop to be taken across a single valve.
The second option is to use multiple valves in series to reduce the pressure in stages so that the pressure drop across each individual valve is less than 100 PSI. In the example above, the station pressure drop is 150 PSI (200 PSI to 50 PSI) with the first PRV reducing the pressure from 200 to 110 PSI, and the second from 110 to 50 PSI. Both the High Flow (Blue 90-01 Valves pictured) and the Low Flow (Bronze CRD-L’s) are configured in series for the staged pressure drop.
The choice to use the Anti-Cavitation Trim or Valves in Series will depend on each application. Cla-Val has a Factory Sizing Program that provides PRV selections that handle the pressure drop across the entire flow range of the station without excessive velocity or noise. At Cougar, we take the valve selections from Cla-Val and combine them with our control system to provide a complete solution for Water PRV Stations.