Suction cavitation, or classic cavitation, occurs when a pump is under low pressure or high vacuum conditions. When the liquid being pumped enters the eye of a centrifugal pump, the pressure is significantly reduced. In some cases, the pressure drop is great enough to cause the liquid to flash to steam when the local pressure falls below the saturation pressure for the fluid that is being pumped. Bubbles or cavities will form at the eye of the impeller, and subsequently, the formed vapor bubbles move into regions of higher pressure as they travel towards the pump discharge. In the higher pressure region, the vapor bubbles collapse suddenly on the outer portions of the impeller. This can cause significant damage to all moving parts of a centrifugal pump.
Typical causes of suction cavitation:
- The pump is running too far right on the pump curve
- Poor suction conditions (NPSH requirements)
- Blockage in the pipe on the suction side
- Inappropriate piping design
- Clogged filters or strainers
To prevent this type of cavitation, the Net Positive Suction Head Available (NPSHa) in the system must be higher than the required NPSH of the pump. This problem is typical for suction cavitation, and therefore this type of cavitation is also called inadequate NPSHa cavitation.
Besides the change of the pump, problems with suction cavitation can also be solved by:
- Lowering the temperature
- Reduction of motor RPM if possible
- Increase in the diameter of the eye of the impeller
- Use of an impeller inducer.
- Use of two parallel pumps with lower capacity.
- Use of a booster pump to feed the principal pump.
Special case of cavitation occurs at the suction side as a result of inappropriate piping in suction line. Use of restrictions, sharp elbows and other hydraulic equipment can turbulize the flow this can contribute to cavitation formation.
Net Positive Suction Head or NPSH for pumps can be defined as the difference between the suction pressure and the saturation pressure of the fluid, expressed in terms of the height of the liquid column. NPSH is used to measure how close a fluid is to saturated conditions. Lowering the pressure at the suction side can induce cavitation. If cavitation occurs, the violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as “pumping gravel”. Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.
In general, there are two suction heads defined in hydraulics:
- NPSH Available (NPSHa): The absolute pressure at the suction port of the pump. NPSHa is a function of water temperature. As the inlet temperature increases, NPSHa decreases because the saturation pressure decreases.
- NPSH Required (NPSHR): The minimum pressure required at the pump’s suction port to keep the pump from cavitating. NPSHa is not a function of water temperature.
NPSHA is a function of your system and must be calculated, whereas NPSHR is a pump function and must be provided by the pump manufacturer. During operation, the available NPSH must be maintained at a level greater than the NPSH required by the pump manufacturer. It has been found that cavitation rates increase rapidly with the increase in the volume flow rate. This can be seen from the picture. As the volume flow rate increases, NPSH required increases, but the available NPSH decreases.
How to increase NPSH available?
To avoid suction cavitation, NPSH available must be increased as much as possible. The only way to increase NPSH available is to increase the pressure at the pump inlet:
- Lower the pump level
- Raise the reservoir level
- Reduction of motor RPM if possible
- Reduce minor losses upstream of the pump
- Reduce major losses upstream of the pump
- Shorten the length of the pipe
- Use a smoother pipe
- Increase in the diameter of the eye of the impeller
- Use of a booster pump to feed the principal pump.