The Centrifugal Pump Cavitation is one of the major factor to be considered to avoid pump failure. In this article, we will see the Centrifugal Pump Cavitation Effects and Prevention methods.
- 1 What is Cavitation?
- 2 CAVITATION EFFECTS
- 3 Two Main Reason for Cavitation
- 4 How to prevent Centrifugal Pump Cavitation
- 5 Results of Cavitation in Pumps
- 6 Cavitation Prevention Methods
What is Cavitation?
Cavitation is the formation and collapse of vapor bubbles in a liquid.
The cavitation process occur in two phases
First Phase – Bubble formation occurs at a point where the pumping liquid pressure is less than the vapor pressure
Second Phase – The bubble collapse or implosion occurs at a point where the pressure will increase above the vapor pressure.
Now we will see the effect of cavitation in pump during the first and second phase.
Cavitation Effect On Bubble Formation Phase
In this phase, the pumping liquid flow is reduced as the liquid is displaced by vapor. Due to this, a mechanical imbalance occurs as the impeller passages are fill with lighter vapors. This results in vibration and shaft deflection, eventually resulting in bearing failures, packing or seal leakage, and shaft breakage. In the case of multi-stage pumps, this can cause loss of thrust balance and thrust bearing failures.
Cavitation Effect On Bubble Collapse Phase
- Mechanical damage occurs as the imploding bubbles remove segments of impeller material.
- Noise and vibration result from the implosion. The noise that sounds like gravel is the user’s first warning of cavitation.
Two Main Reason for Cavitation
NPSH (R) EXCEEDS NPSH (A)
Due to low pressure, the water vaporizes (boils) and higher pressure implodes into the vapor bubbles as they pass through the pump causing reduced performance and potentially major damage.
Suction or discharge recirculation
The pump is designed for a certain flow range if there is not enough or too much flow going through the pump. The resulting is turbulence and vortexes can reduce performance and damage the pump.
How to prevent Centrifugal Pump Cavitation
While designing a pumping system or selecting a pump, one must thoroughly evaluate Net Positive Suction Head (NPSH) margin to prevent cavitation.
Net Positive Suction Head (NPSH) margin = NPSHA – NPSHR
Proper analysis of both the net positive suction heads available in the system (NPSHA) and the net positive suction head required by the pump (NPSHR) will reduce the formation of cavitation.
It is a measure of the pressure drop as the liquid travels from the pump suction flange along the inlet to the pump impeller. This loss is due primarily to friction and turbulence.
Turbulence loss is extremely high at the low flow of the pump and then decreases with flow towards the best efficiency point of the pump. Friction loss usually increases with the increase in pump flow rate. As a result, the internal pump losses will be high at low flow, dropping at generally 20–30% of the best efficiency flow, then increasing with the flow.
The NPSHr of the pump is obtain from the actual pump curve. (Click here to read about the How to read a centrifugal pump curve?). The pump manufacturer determines the actual NPSHR for each pump over its complete operating range by a series of tests. (Click here to read about the Centrifugal pump performance test). The detail test procedure is describe in the Hydraulic Institute Test Standard 1988, Centrifugal Pumps 1.6.
The industry has agreed on a 3% head reduction at constant flow as the standard value to establish NPSHR. (Refer below figure).
The NPSH margin required will vary with pump design and other factors, and the exact margin cannot be precisely predicted. For most applications, the NPSHA will exceed the NPSHR. Normally the NPSH margin will be of 1m is consider for pump selection to avoid cavitation.
Results of Cavitation in Pumps
The following are the damages in pump due to cavitation.
- Erosion of impeller due to cavitation will cause unbalance of the impeller.
- Cavitation produces high vibration and damage bearings.
- Due to cavitation, pump life will reduce and also cause premature failure of the pump.
Cavitation Prevention Methods
- NPSH available shall be higher than then the NPSH required by the pump.
- Provide a straight run of three to five times the length of the pipe diameter (D) to the pump suction flange to prevent added turbulence at the impeller eye.
- Avoid boil-off vapor mixing with pumping liquid.
- The absolute pressure must not be allowed to drop below the vapor pressure of the fluid. This will prevent boiling which will cause cavitation.