What is Cavitation?

Within the fluid flowing inside a pump,if the local pressure at any point drops below the saturation vapor pressure corresponding to the fluid's temperature,vapor bubbles will form within the liquid,and any dissolved gases will also be released.These bubbles travel with the fluid into high-pressure regions,where they collapse extremely rapidly(typically within T≤0.001 s).The instant of collapse generates high-frequency shock waves and micro-jets,with pressures reaching up to tens of megapascals.These repeatedly impact the metal surface,causing material fatigue and erosion.This entire process is known as cavitation.

(Bonve Pumps Cam Rotor Pumps)

Locations Where Cavitation Occurs

The location of cavitation is closely related to the pump's operating flow rate:

-When the flow rate is greater than the design flow,the liquid impacts the backside of the blades,and the low-pressure zone is located on the front side of the blade inlet near the front shroud.

-When the flow rate is less than the design flow,the liquid impacts the front side of the blades,and the low-pressure zone is located on the backside of the blade inlet near the front shroud.

NPSH and Suction Lift

Net Positive Suction Head(NPSH)refers to the excess energy per unit weight of liquid at the pump suction port,over and above its vaporization pressure,measured in meters of water column.

Required Net Positive Suction Head(NPSHr)is an inherent characteristic of the pump itself,reflecting the degree of internal pressure drop.A smaller value indicates better cavitation resistance.

Available Net Positive Suction Head(NPSHa)is determined by the suction system and represents the actual energy available to prevent cavitation.

Suction Lift(Δh)is the maximum allowable installation height for the pump,calculated by the formula:

Δh=10.33 meters-NPSHr-0.5 meters

For example,if a pump's NPSHr=4.0 meters,then the suction liftΔh=10.33-4.0-0.5=5.83 meters.

Classification and Meaning of NPSH

-NPSHa:System/available NPSH.The larger it is,the safer the operation.

-NPSHr:Pump required NPSH.The smaller it is,the better the pump's anti-cavitation performance.

-NPSHc:Critical NPSH,corresponding to the point where pump performance begins to deteriorate.

-[NPSH]:Allowable NPSH,typically taken as 1.1 to 1.5 times the NPSHc.

Difference Between Required and Available NPSH

-NPSHr is determined by the pump's own structure,speed,and flow rate,and is essentially independent of the medium's properties(neglecting thermodynamic effects).It essentially represents the dynamic pressure drop in the pump's inlet region.

-NPSHa depends on the suction system layout,liquid surface pressure,pipeline losses,and liquid temperature.It must be ensured that NPSHa>NPSHr,and a safety margin of 0.5 to 1.0 meters is usually added.

Main Hazards of Cavitation

1.Erosion of Wetted Parts:Bubble collapse generates high-frequency mechanical shock and localized high temperatures,leading to combined mechanical erosion and chemical corrosion.

2.Pump Performance Degradation:Cavitation disrupts energy transfer within the impeller,causing the head and efficiency curves to drop,and may lead to flow interruption in severe cases.Pumps with low specific speeds may experience a sudden,steep drop in performance,while those with medium to high specific speeds often show a gradual then sharp decline trend.

How to Improve Cavitation Resistance?

I.Enhancing the Pump's Own Cavitation Resistance

1.Optimize the inlet structure:Enlarge the flow area,increase the inlet curvature radius,round the blade inlet edges,improve surface finish,and extend the blade inlet edge.

2.Employ a front-mounted inducer to pre-pressurize the liquid.

3.Use a double-suction impeller,which halves the inlet flow velocity.

4.Adopt a slightly larger positive incidence angle design to improve performance at high flow rates,while balancing efficiency.

5.Select materials with high strength,toughness,and corrosion resistance,such as stainless steel or hard alloys.

6.For specific conditions,such as handling high-viscosity or particle-laden media,consider positive displacement pump types like rotor pumps,whose cavitation characteristics differ from centrifugal pumps.

II.Increasing the Available NPSH of the Suction System

1.Increase the pressure at the inlet liquid surface.

2.Lower the pump's installation height or change to a flooded suction arrangement.

3.Shorten the suction pipeline,reduce elbows and valves,and increase pipe diameter to lower flow velocity and resistance.

4.Reduce the temperature of the conveyed medium(especially when it's near saturation).

Preventing and controlling cavitation requires a comprehensive approach involving pump selection,system design,and operational maintenance.Understanding the true meaning of NPSH and properly matching NPSHa and NPSHr is key to avoiding cavitation and ensuring long-term,stable pump operation.In practical applications,appropriate protection strategies should be selected based on process conditions and pump characteristics to achieve a balance between economy and reliability.

Bonve pump always adheres to the philosophy of "Wholeheartedly Making Good Rotor Pumps," committed to providing global customers with customized conveying and mixing solutions. If you have technical inquiries, selection needs, case references, or other cooperation intentions related to Bonve rotor pumps, please feel free to contact us.  

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