Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association
St. Louis, MO
In his paper presented at the EASA Convention 2016, Gene Vogel explores the phenomenon of cavitation in centrifugal pumps, detailing its causes, effects, and methods for diagnosis. Cavitation occurs when liquid is subjected to pressure changes, leading to the formation and implosion of vapor bubbles. This process releases energy that can cause significant damage to pump components, including impellers, shafts, and bearings.
Vogel begins by explaining the basic principles of cavitation. As liquid enters the suction eye of the impeller, the pressure drops, causing the liquid to vaporize if the pressure falls below its vapor pressure. The implosion of vapor bubbles releases energy that damages the impeller surface and generates vibratory energy. This damage is often described as similar to sand, gravel, or marbles passing through the pump.
Impeller damage from cavitation manifests as pitting and erosion marks on the working side of the vanes. Over time, these pits can develop into holes that erode completely through the impeller. Stainless steel impellers show cavitation damage as a mottled surface due to their tough nature. Hard coatings can reduce cavitation damage, but the effects on bearings and seals remain.
To prevent cavitation, the pressure on the liquid must remain above its vapor pressure. This requires a comparison between the Net Positive Suction Head Required (NPSHR) and the Net Positive Suction Head Available (NPSHA). The NPSHR is provided by the pump manufacturer, while the NPSHA is calculated based on atmospheric head, suction static head, suction friction head, and vapor pressure. The NPSH margin should be 1.15 or greater to prevent cavitation.
Vogel discusses the vapor pressure of fluids, noting that it varies with temperature and pressure. For example, water vaporizes at 212°F at sea level but at 201°F in Denver due to lower atmospheric pressure. Cryogenic liquids like hydrogen and nitrogen require extremely low temperatures or high pressures to remain liquid.
The case study presented involves two test loops designed to induce cavitation and monitor its effects. Test loop 1 used a 25 hp split case pump with a pressure-vacuum pump to control suction head. Despite significant cavitation, vibration data did not show expected increases, likely due to the massive pump casing damping the vibration energy. Test loop 2 used a ¾ hp end suction pump with a variable frequency drive. This test confirmed classic vibration characteristics of cavitation, with increased random vibration frequencies in the 400 to 1000 Hz range as suction head dropped.
Vogel concludes that while vibration analysis can be effective for diagnosing cavitation in some pumps, it may not be reliable for pumps with massive impeller housings. Other methods of detecting cavitation should be considered for such pumps.
Key Points Covered:
- Causes and effects of cavitation in centrifugal pumps
- Impeller damage from cavitation
- Comparison of NPSHR and NPSHA to prevent cavitation
- Vapor pressure of fluids and its impact on cavitation
- Case study involving two test loops to induce and monitor cavitation
- Vibration characteristics associated with cavitation
Key Takeaways:
- Cavitation causes significant damage to pump components
- Maintaining pressure above vapor pressure is crucial to prevent cavitation
- NPSH margin should be 1.15 or greater
- Vibration analysis may not be reliable for all pumps
- Other methods of detecting cavitation should be considered for pumps with massive impeller housings
- Understanding cavitation and its effects can help improve pump reliability and performance
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