Designing Equipment to Avoid Pump Cavitation

Pump Impeller

Pump cavitation is a destructive phenomenon observed mainly in centrifugal pumps. It is characterized by the formation of vapor bubbles at or near the pump’s impellers. These bubbles implode or pop, causing a release of high-pressure shockwaves and concentrated water jets that can damage even the highest-grade pump and piping materials.

Early symptoms of pump cavitation include excess noise, often described as the sound of marbles, rocks, or gravel, near the pump location. The noise is distinct and highly audible, usually allowing for a swift diagnosis. However, if left untreated, pump cavitation can lead to a range of issues, including:

  • Deterioration and failure of the pump enclosure

  • Deterioration of the pump impeller

  • Decreased output flows and pressures

  • Inefficient pump operation and increased power consumption

  • Excessive vibration in the piping system, and subsequent seal and bearing failure

Cavitation can occur in any industry in which pumps are routinely used for normal operations. Chemical processing, food processing, and petroleum/oil and gas production are just a few of the many industrial applications frequently affected by pump cavitation.

Causes of Pump Cavitation

Cavitation is caused by the rapid formation and eventual collapse of vapor bubbles within a liquid. When the impeller spins quickly, an area of low pressure is created near the blades. If this pressure is less than the vapor pressure of the liquid, the liquid boils and forms vapor/steam bubbles. When the bubbles reach an area where the pressure is above the vapor pressure, the bubble collapse, resulting in the release of destructive energy in the form of shockwaves and high-pressure jets.

There are two types of pump cavitation: suction cavitation and discharge cavitation.

Suction Cavitation

Suction cavitation occurs when excessive suction pressures near the pump inlet cause the fluid to become heated to the point of vaporization. Since vapor bubbles occupy thousands of times more volumetric space than liquids, the pump becomes “starved” due to lack of sufficient inlet flow. As the bubbles circulate the pump and encounter areas of higher pressure, they eventually implode against the face of the impeller. Continuous exposure to suction cavitation can result in characteristic Swiss cheese- or sponge-like wear on the impeller.

Suction cavitation may be caused by:

  • Obstructed filter

  • Excessively high pump rpm and pressure

  • Pipe blockages

  • Poor design of the piping system

  • Poor pump suction

Discharge Cavitation

Discharge cavitation, on the other hand, occurs when the pressure at the discharge end of the pump is excessively high. These high discharge pressures limit the amount of fluid flowing out of the pump, resulting in recirculation of high-velocity fluid between the pump impeller and the housing. The ensuing vacuum effect forms bubbles, which eventually implode, causing damage to the impeller and the shaft.

Common causes of discharge cavitation include:

  • Clogged filters

  • Pipe blockages

  • Poor design of the piping system

Avoiding Pump Cavitation

The key to preventing pump cavitation is understanding the net positive suction head (NPSH) of the piping system. Essentially, the pressure of the fluid at all points within the pump must remain above the vapor pressure to prevent boiling of the liquid. The NPSH is the main parameter used to determine whether the pressure of the fluid being pumped is enough to avoid cavitation.

The net positive suction head available (NPHSa) is defined as the difference between the actual pressure at the pump inlet flange and the vapor pressure of the fluid being pumped. Conversely, the net positive suction head required (NPHSr) is the minimum net positive suction head required to avoid cavitation; this value is typically provided by the pump manufacturer. To ensure that pump cavitation is avoided, net positive suction head available must be greater than the net positive suction head required plus a predetermined safety margin.

This relationship is expressed as the formula below:

NPHSa ≥ NPHSr + safety margin

Design engineers often specify the safety margin, but it may also be determined by the pump’s manufacturer.

Design Consideration by Industry

Various industries — such as the chemical, food and beverage, and petroleum sectors, among many others — deal with a range of different liquids, each with unique physical and chemical properties, including:

  • Density

  • Viscosity

  • Boiling point

  • Specific volume

  • Specific gravity

  • Temperature

Careful consideration must be given to these properties, as well as operating altitudes, which can affect the pump’s best efficiency point (BEP).

Preventing Pump Cavitation

Pump cavitation, although easily identified by its excessive noise and vibration, is a highly destructive force that can cause extensive damage to even the highest-quality pumps and equipment.

However, deterioration can be easily avoided by selecting the appropriate pump for the given application and having a full understanding of the pump’s best efficiency point for the specific fluid under consideration.

 

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Image Credit: Surasak_Photo/Shutterstock.com

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