Submersible Solutions
Cavitation & Air Entrainment in Centrifugal Pumps
Understanding symptoms, causes and key differences to ensure pump longevity.
Bob Domkowski | SWPA
Cavitation and air entrainment are phenomena that can adversely affect centrifugal pump operation, performance and working life. These two issues can present similar symptoms, but reducing and correcting them requires different resolution pathways. The most common presentation to the owner is a continuous noise emanating from the pump unit as though small rocks are being pumped through the hydraulic end.
Typically, each centrifugal pump is factory tested by the manufacturer prior to shipment of the unit, confirming that its hydraulic performance is within published Hydraulic Institute (HI) standards. Quite often, a centrifugal pump unit can be unfairly blamed for operational issues once installed within a system where unfavorable hydraulic conditions already exist.
Pump Unit Cavitation
Cavitation can occur when the suction pressure within the pump unit falls below the vapor pressure of the liquid, causing vaporization to begin. This will cause vapor bubbles to form within the pumped media.
The formed bubbles collapse violently as they reach the higher-pressure areas within the pump housing. The violent collapse of these bubbles causes damage to the pump impeller and pump housing castings, resulting in pitting of these components.
The most common causes of centrifugal pump unit cavitation include:
- Inadequate net positive suction head available (NPSHa)
- High pumped media temperature
- Excessive pump unit flow rates
Typical overt signs of centrifugal pump unit cavitation include:
- Pump unit noise: Grinding or tumbling rock sounds and/or hissing sounds during operation are caused by the formation and collapse of the vapor bubbles (Image 1).
- Pump unit physical damage: Pump unit pitting or erosion of internal components can occur due to the collapsing bubbles impinging on internal surfaces (Image 2).
- Pump unit performance issues: Fluctuating discharge pressure or reduction in pump unit flow rates indicate the pump is not operating properly.
- Pump unit vibration: Pump unit monitoring systems detecting an increase in vibration levels indicates likely internal unit damage.
| IMAGE 1: Cavitation graphic representation (Image courtesy of Wilo)
| IMAGE 2: Typical impeller cavitation damage (Image courtesy of ABB)
Pump Unit Air Entrainment
Air entrainment, as the name suggests, is the introduction of air into the pumped media. This creates an aerated liquid. While air entrainment typically does not present as significant physical damage to internal pump components, over time, internal damage will lead to reduced pump unit output and possibly seal and bearing damage.
The most common causes of air entrainment include:
- In submersible pump wet-pit installations, a high-side elevation of the invert of the conveying line provides a “waterfall” effect as the liquid drops into the wet-pit, aerating the pumped media.
- In dry-pit installations, air can leak into the suction.
Preventing air entrainment in centrifugal pump units is important for maintaining efficient pumping operation. Several key activities include:
- Pump unit priming: Fully prime the pump unit to ensure there is no entrained air at startup (conventional pump units).
- Suction piping design: Always employ a downward slope in the piping arrangement to facilitate fluid flow. Eliminate any suction piping high points where air can accumulate (conventional pump units).
- Submersible pump station design: Ensure that the invert of the influent pipe is not located at a significantly higher elevation than the wet-pit fluid level and thus cascading downward, causing highly aerated liquid. For very deep wet-pits, influent drop pipes could be a consideration (Image 3).
- Piping inspection: Inspect suction piping on a regular basis and immediately repair any leaks to prevent air from entering the system.
- Baffles and vanning: The installation of baffles, inlet flow conditioning devices and internal vanes (Image 4) in suction piping to prevent channeling of fluid and promote quiescent approach flow to the pump unit suction inlet can be quite beneficial to quiet pump operation in support of good pump performance. Ensure the wet-pit is adequately ventilated so fluid flow is not diminished.
- Regular maintenance: Conduct routine maintenance checks to ensure all components are functioning correctly and to identify potential issues.
| IMAGE 3: Influent drop pipe (Image courtesy of Xylem, Inc.)
Cavitation vs. Air Entrainment
Cavitation occurs as vapor bubbles are formed within the pumped media. Air entrainment is simply as stated, involving actual air entering the pumped media.
Cavitation generally results in significant to severe damage of the pump unit, while air entrainment affects pump unit performance and efficiency. Understanding these differences is important for effective maintenance and troubleshooting of centrifugal pump units.
In most cases, the operator can conduct a simple test where the pump unit discharge is temporarily throttled by means of the discharge valve to help determine whether the noise emanating from the pump unit is cavitation or air entrainment.
When throttling the discharge, there are two likely results:
- The pump unit noise and vibration diminish or end completely. If this occurs, cavitation is most likely the pump malady.
- The pump unit noise and vibration remain fairly constant. In this case, the pump unit flow rate does not minimize the problem, and thus the likely causation is air entrainment of the pump unit.
| IMAGE 4: Inlet flow conditioning devices (Image courtesy of FlowOptimizers, LLC)
Pump unit cavitation issues have the potential to reduce pump life from 10 to 15 years to as few as two years in very extreme situations. Despite continuing advancements in pump hydraulic design, ever-evolving and improving technical measurement and predictive maintenance programs, and additional choices regarding pump unit materials of construction, cavitation continues to be a harmful and economically damaging phenomenon.
Consulting with pump manufacturers, following best practices in pump system design, incorporating the most current standards developed and published by HI and employing an established preventative maintenance program should result in properly operating pump units delivering long operational life for pump owners.
Bob Domkowski is the principal/engineering consultant at Brashley Technical Services, LLC in Milford, Connecticut. He has more than 40 years of pump industry experience, is a past president of the Submersible Wastewater Pump Association (SWPA) and is an editorial board member of Pumps & Systems magazine. For more information, visit brashleytech.com.
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