3.6 Pump cavitation

Pump cavitation

Pump cavitation occurs when the static pressure somewhere locally within the pump falls to or below the vapor pressure of the fluid. Some of the fluid then vaporizes giving rise to the formation of vapor bubbles. These vapor bubbles are entrained in the fluid and transported until they reach an area, further down stream or in the pump, where the pressure is equal to or greater than the vapor pressure. At this point the vapor bubbles implode since they can no longer exist as vapor . Each implosion causes a severe pressure pulse. The continuous repetition of this sequence of events occurring many times and at high frequency can cause mechanical damage to the pump materials.

Furthermore, the hydraulic performance of the pump is reduced by the onset of cavitation. Cavitation is therefore an undesirable phenomenon in connection with pump design and should be avoided wherever possible.

The risk of cavitation is greatest where the pressure within the pump is lowest. The lowest pressure in a centrifugal pump occurs on the suction side of the impeller blades, slightly downstream of the front edges, figure 3.6a. At the pump inlet connection at the same level as the impeller shaft the pressure is somewhat greater – Δ_P. At this point, which lies at the static suction head h_s above the lower fluid surface, the flow velocity is v_s. By applying Bernoulli’s equation see equation 8.2a >>> — the flow through the suction pipe becomes:

or

where

p_min = lowest pressure within the pump (Pa or N/m²)
Δ_p = local pressure reduction (Pa )
v_s = flow velocity at inlet connection (m/s)
h_s = static suction head (m)
h_Ls = head loss in suction pipe (m)
p_a = pressure on liquid surface (usually atmospheric pressure) (Pa)

Displacement pumps require a corresponding pressure differential Δ_p in order to avoid cavitation within the pump. The lowest pressure in a pump is determined partly by external factors such as the pressure on the lower liquid surface (usually atmospheric) p_a, the static suction head h_s and the flow losses in the suction line h_Ls and partly by factors concerned with construction. The latter consists  of the local pressure drop Δ_p and the flow velocity at the inlet connection v_s . In order to avoid cavitation it is necessary to ensure that p_min is greater than the vapor pressure of the fluid p_v.