Pump Cavitation and How to Prevent It?
In the field of pump applications (especially in household appliances, medical equipment, industrial circulation systems, etc.), the phenomenon of cavitation is known as the "silent killer." It silently erodes the pump body, reduces efficiency, shortens service life, and can even cause sudden failures. As a professional manufacturer of micro pumps, TOPSFLO today takes you deep into understanding cavitation and shares our methods for preventing it.
What is Pump Cavitation?
Cavitation is a physical disturbance that occurs when the pressure of a liquid drops suddenly to its vaporization pressure at a specific temperature. Its destructive process can be divided into three stages:
① Bubble Formation
When the pump impeller rotates at high speed, a local low-pressure area forms at the inlet of the blades (as shown in the figure). If the pressure here is ≤ the saturated vapor pressure of the liquid at its current temperature, the liquid will instantaneously boil and vaporize, producing dense vapor bubbles.
② Bubble Migration
The bubbles are carried by the liquid flow towards high-pressure areas such as the impeller outlet, where the external liquid pressure increases sharply.
③ Bubble Collapse
The high-pressure liquid intensely compresses the bubbles, causing them to implode within microseconds, instantly releasing:
• Ultra-high-pressure shock waves (up to 3000-4000 atmospheres, equivalent to the pressure at 10,000 meters deep in the ocean)
• High-temperature micro-jets (local temperature exceeding 1000°C)
• High-frequency impacts (tens of thousands of times per second)
Serious Hazards of Cavitation
1. Physical Damage to Components
• The shock waves from collapsing bubbles erode the surfaces of metal or plastic components, forming honeycomb-like pits (e.g., in the pump cavity, impeller).
• Dople Case Study: Although our PPE+30%GF reinforced plastic impellers possess high rigidity, long-term cavitation can still cause surface fatigue cracking. While ceramic center shafts/spacers are wear-resistant, severe vibration may cause micro-cracks.
2. Severe Performance Degradation
• Bubbles occupy flow passage space, blocking water flow, causing a head drop of 5%~15%, and efficiency losses of up to 30%. The pump "appears to be running but cannot transport the medium efficiently."
3. Vibration and Noise Pollution
• Cavitation causes high-frequency impacts, producing a sharp "cracking sound" accompanied by severe vibration of the unit, affecting the user experience (e.g., in silent medical equipment, household appliances).
4. Accelerated Seal Failure
• Continuous vibration accelerates the wear of EPDM seals, and the precise clearance matched with ceramic shaft sleeves may be compromised, leading to an increased risk of leakage.
Conditions for Pump Cavitation and Identification
Whether cavitation occurs depends on the combined characteristics of the pump itself and the suction system. Therefore, analyzing the conditions for cavitation requires considering both aspects. The core criterion for pump cavitation is expressed by the following relationship:
NPSHc ≤ NPSHr ≤ [NPSH] ≤ NPSHa
When NPSHa ≤ NPSHr (NPSHc): Cavitation begins.
When NPSHa > NPSHr (NPSH c): The pump operates without cavitation.
In the formula:
• NPSHa (Net Positive Suction Head Available): Characterizes the margin of energy provided by the suction system at the pump inlet over the vapor pressure. The larger this value, the less likely cavitation is to occur.
• NPSHr (Net Positive Suction Head Required): Represents the minimum NPSHa required by the pump itself to maintain operation without cavitation. The smaller this value, the better the pump's cavitation resistance. Its physical essence reflects the dynamic pressure drop from the pump inlet to the lowest pressure point on the impeller.
• NPSHc (Critical NPSH): Refers to the NPSH corresponding to a specified drop (typically 3%) in pump performance (e.g., head).
• [NPSH] (Allowable NPSH): A design margin set to ensure safe pump operation, usually taken as [NPSH] = (1.1 ~ 1.5) × NPSHc.
Calculation of NPSHa: NPSHa=Ps/ρg+Vs/2g-Pc/ρg=Pc/ρg±hg-hc-Ps/ρg
Be Alert to These Signals of Cavitation:
• Abnormal Noise: A continuous crackling or popping sound (like gravel flowing through the pump), distinct from normal operating sounds.
• Severe Vibration: Caused by hydraulic imbalance due to bubble formation and collapse.
• Sudden Performance Drop: Flow rate, head, and efficiency are significantly lower than expected.
• Material Damage: Characteristic honeycomb-like or sponge-like erosion pits appear on the impeller or pump cavity.
Cavitation Prevention Measures
The key to preventing pump cavitation lies in increasing the NPSHa to ensure it is greater than the NPSHr (NPSHa > NPSHr). Specific measures include:
• Reduce Installation Height: Decrease the geometric suction lift (hg), or use a flooded suction installation (increase the flood height) whenever possible.
• Reduce Suction Line Losses: Increase the suction pipe diameter, shorten the pipeline length, and reduce local resistance components such as elbows and valves.
• Avoid Overflow Operation: Prevent the pump from operating for extended periods at flow rates significantly beyond its rated capacity.
• Adjust Operating Conditions: If signs of cavitation appear during operation, reduce the pump's flow rate or speed.
• Optimize Suction Source Conditions: Ensure the suction tank has smooth flow patterns, is free of vortices, and maintains a stable liquid level that meets requirements.
• Increase Impeller Inlet Vane Width: This increases the inlet flow area, reduces the inlet absolute and relative velocities (Vo and Wo from the inlet velocity triangle), thereby reducing NPSHr and improving cavitation resistance.
• Optimize Inlet Vane Thickness: Thinner inlet vane edges, more streamlined profiles, and positioning the maximum vane thickness farther from the inlet lead to a smaller pressure drop at the vane inlet, improving the pump's cavitation performance.
