How much water passes a pressure reducing valve
How much a valve can pass is set by its flow coefficient. Cv is the number of gallons per minute of water that flow with 1 psi of differential across the valve, and the working equation is Q = Cv × √(ΔP/SG), where SG is specific gravity. A valve with Cv 25 fed at 100 psi and set to 40 psi sees 60 psi of differential and passes roughly 194 GPM.
The most common sizing mistake is choosing a reducing valve by pipe size alone. Two 1 in valves from different makers can differ in Cv by more than a factor of two, so always read the published Cv (or Kv, where Cv = 1.156 × Kv). An oversized valve hunts and chatters at low demand, while an undersized one cannot hold the set pressure when everything opens at once.
Once the differential exceeds about half the absolute inlet pressure, pressure at the valve throat can drop below the vapor pressure of the liquid and cavitation or flashing begins. Left alone it erodes the seat quickly and makes the valve loud. The ratio shown here is only a screening check; a proper assessment uses the FL liquid pressure recovery factor from IEC 60534 together with the fluid's vapor pressure. Have a mechanical engineer and the valve manufacturer confirm the final selection.
Frequently Asked Questions
Cv is GPM of water at 1 psi differential, Kv is cubic meters per hour at 1 bar. They convert as Cv = 1.156 × Kv, so check which one the catalog lists before sizing.
Split it across two valves in series so each takes part of the drop. Forcing one valve to do it all brings cavitation damage, high noise and an unstable downstream setting.