Cavitation

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Cavitation

A phenomenon which occurs within a moving liquid when the static pressure at some location falls to or below the vapor

pressure of the liquid.

To prevent cavitation:NPSHA > NPSHR

Quadna – Engineering Innovative Solutions for Industrial Applications

Cavitation

Quadna – Engineering Innovative Solutions for Industrial Applications

Cavitation - Impeller

Cavitation – Impeller

Cavitation can occur at the inlet of the impeller if the suction pressurefalls below the liquid vapor pressure.

Quadna – Engineering Innovative Solutions for Industrial Applications

Cavitation - Impeller

Cavitation – Impeller

Large vapor volumes cancause reduction in head generation, or loss of prime.

Large vapor volumes can causesurging and unstable flows.

• Bubbles collapsing create intense pressures and

(10,000 psi) shock waves.

• Intense pressures near metal surfaces exceed material strength and fatigue metals.

• Creates pitted surfaces.

Cavitation

Cavitation is caused by liquid changing into aCavitation is caused by liquid changing into avapor and then back into a liquid. The vapor and then back into a liquid. The Formation of partial vacuums in a flowing liquidFormation of partial vacuums in a flowing liquidas a result of the separation its parts. When as a result of the separation its parts. When These collapse, pitting or other damage is These collapse, pitting or other damage is Caused on metal surface in contact.Caused on metal surface in contact.

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There are two forms of cavitation:There are two forms of cavitation:• Suction Cavitation, which occurs at the eye of the impeller, (also referred to as Suction Cavitation, which occurs at the eye of the impeller, (also referred to as “ “true cavitation”.true cavitation”.

• Discharge Cavitation, which occurs at the tip of the impeller, (also referred to as Discharge Cavitation, which occurs at the tip of the impeller, (also referred to as recirculation cavitation).recirculation cavitation).

Cavitation is NOT AIR in the PUMP!

What Damage Does Discharge Cavitation Cause?

• Break ShaftsBreak Shafts

• Shortens Bearing LifeShortens Bearing Life

• Destroys Impellers and VolutesDestroys Impellers and Volutes

• Ruins Mechanical SealsRuins Mechanical Seals

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Total Dynamic Head Too “HIGH”

What Damage Does Suction Cavitation Cause?

• Shortens Bearing LifeShortens Bearing Life (Increased radial & thrust loads)(Increased radial & thrust loads)

• Destroys ImpellersDestroys Impellers

• Destroys Wear PlatesDestroys Wear Plates

• Ruins Mechanical SealsRuins Mechanical Seals (Vibration & thrust loads)(Vibration & thrust loads)

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Total Dynamic Suction Lift Too “HIGH”Discharge Head Too “LOW”

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Chemical Attack Abrasive Wear

Discharge Cavitation Suction Cavitation

Shaft Breakage ProblemsRadial Fatigue BreakRadial Fatigue Break

• Total Dynamic Head Too “High”Total Dynamic Head Too “High”

• Air (Hydraulic Imbalance)Air (Hydraulic Imbalance)

• Mechanical ImbalanceMechanical Imbalance

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Torsional Break (Twisting Break)Torsional Break (Twisting Break)

• Pump Lock-UPPump Lock-UP

• Reverse Rotation (Open Check Valve)Reverse Rotation (Open Check Valve)

• Short CyclingShort Cycling