Due to high speeds of the medium in the narrowest section of the valve, local underpressure occurs (p2). If this pressure drops below the medium’s boiling pressure, cavitation occurs (steam bubbles), possibly leading to material removal (abrasion). Also, when cavitation sets in, the noise level increases abruptly. Cavitation can be avoided by limiting the pressure differential across the valve as a function of the medium temperature and the prepressure.
Progression of speed
Progression of pressure p
Δpmax = differential pressure with valve almost fully closed at which cavitation can largely be avoided
p1 = static pressure at valve inlet
p3 = static pressure at valve outlet
M = pump
ϑ = water temperature
Example for low-temperature hot water
Pressure p1 at valve inlet: 500 kPa (5 bar)
Water temperature: 120 °C
From the chart above it can be seen that with the valve almost fully closed, the maximum permissible differential pressure Δpmax is 200 kPa (2 bar).
Example for cold water
Spring water cooling as an example for avoiding cavitation:
Cold water = 12 °C
p1 = 500 kPa (5 bar)
p4 = 100 kPa (1 bar)
Δpmax = 300 kPa (3 bar)
Δp3-3’ = 20 kPa (0.2 bar)
ΔpD (throttle) = 80 kPa (0.8 bar)
p3’ = pressure downstream from the consumer in kPa
To avoid cavitation in the case of cold water circuits, it must also be made certain that there is sufficient static counter-pressure at the valve’s outlet. This can be ensured by installing a throttling valve downstream from the heat exchanger, for example. In that case, the maximum pressure drop across the valve should be selected according to the 80 °C curve in the flow chart above on page 56.