What is cavitation in industrial pumps?

Cavitation is the formation of vapor bubbles within a liquid when the local absolute pressure drops below its vapor pressure at a specific temperature. In centrifugal industrial pumps, it often occurs near the impeller inlet, where flow acceleration can create low-pressure areas. It is not necessary for the entire liquid to be heated to its usual boiling point.

As bubbles are transported to higher-pressure areas, the vapor condenses and the bubbles collapse abruptly. This repetitive collapse generates shock loads and, near solid surfaces, liquid micro-jets. These mechanisms can erode the metal of the impeller, even when the pumped liquid does not contain abrasive particles.

Cavitation is not synonymous with air suction from a leak or a vortex in the tank. The two phenomena may have similar noise and unstable operation, but they require different handling. Proper diagnosis must precede the replacement of components.

How cavitation destroys the impeller

Wear usually begins as surface roughness and microscopic pits. With continued operation, these points evolve into craters, material loss, and alteration of the vane profile. The surface may acquire a characteristic appearance as if it has been subjected to repeated small impacts. The location of the damage depends on the impeller geometry and the mechanism causing the cavitation.

The destruction is not limited to the appearance of the metal. The change in geometry affects the flow, reduces hydraulic efficiency, and can exacerbate the conditions that created the problem in the first place. At the same time, pressure pulses and vibrations put strain on the assembly's mechanical operation.

  • Drop in flow and head: the pump struggles to reach the required operating point.
  • Increase in vibrations: unstable flow and uneven wear can increase mechanical stresses.
  • Strain on seals and bearings: vibrations can contribute to leaks and premature failures.
  • Unplanned production downtime: advanced wear may require impeller replacement and additional repairs.

A more resistant metallurgical choice might delay cavitation erosion, but it does not eliminate the hydraulic cause. Simply replacing the impeller without checking the installation often leads to the recurrence of the damage.

Causes of cavitation and the role of NPSH

A key factor is insufficient suction pressure. Possible causes include low tank level, high elevation distance when the pump is located above the liquid, a clogged filter, a partially closed valve, or a small-diameter pipe. Long pipe lengths and many bends also increase pressure losses.

Higher temperatures increase the liquid's vapor pressure, limiting the available margin before vapor creation. In vacuum tanks, the reduced absolute pressure above the liquid has a similarly adverse effect. Increasing flow or speed can also strain suction conditions.

For evaluation, NPSH (Net Positive Suction Head) is used. NPSHa is the head available from the installation, while NPSHr is the head required by the pump for a specific flow and rotation speed, according to manufacturer data. Both are usually expressed in meters of the pumped liquid column.

For supply from a large tank, with negligible velocity at the free surface, NPSHa is calculated from the absolute pressure head above the liquid, plus the geodetic level difference to the pump's reference level, minus suction losses and the vapor pressure head. The level difference is positive when the surface is higher than the reference level.

A marginal exceedance of NPSHr is not enough. Curves often use the NPSH3 criterion, which is a condition with a 3% drop in head due to cavitation, not a guarantee of its absence. The appropriate margin is determined by the manufacturer and the application. Furthermore, operating far from the Best Efficiency Point (BEP) can cause local recirculation and cavitation that are not adequately captured by a simple NPSH check.

Symptoms and proper diagnostic procedure

A common symptom is a noise like gravel inside the casing. It may be accompanied by increased vibrations, pressure fluctuations, and reduced flow. However, sound alone does not confirm cavitation: worn bearings, foreign bodies, and trapped air can produce a similar picture.

  • Record flow, speed, liquid temperature, and suction and discharge pressures.
  • Check tank level, filter cleanliness, and the actual position of valves.
  • Examine potential air ingress, insufficient suction submergence, or vortex formation.
  • Compare the operating point with the pump curves and the permissible operating range.
  • Calculate NPSHa under the worst-expected conditions, not just in a single momentary measurement.

Gauge readings must be correctly converted to absolute pressures where required. Correction for measurement position and velocity head is also necessary when the calculation is based on pipe measurements. A single vacuum reading is not enough for a safe conclusion.

If an internal inspection is required, it must be preceded by shutdown, isolation, lockout/tagout, depressurization, and safe draining. For hot, flammable, or hazardous liquids, the installation's specific procedures must be followed.

How cavitation is prevented and treated

Treatment aims at increasing the available NPSH, reducing the pump's requirements, or correcting the operating point. First, obvious limitations are restored, such as clogged filters and incorrect valve positions. Then, the diameter, path, and configuration of the suction piping are evaluated.

Depending on the application, increasing the level in the tank, lowering the pump's position, or reducing the temperature may help, provided the process allows it. Pressure changes in vessels require specific study and verification of their design limits.

Reducing speed with a frequency converter can improve conditions, but the required flow, head, and operating limits must be checked. At excessive flow, discharge regulation may help, but only after technical assessment. Do not throttle the suction to reduce flow, because this further reduces the available pressure.

At very low flow, a different approach is needed, such as a properly designed minimum flow line. If the pump is unsuitable for the process, a different hydraulic size or model should be considered. Monitoring vibrations, pressures, and filter differential pressure helps in the early detection of deviations.

Frequently Asked Questions

Can there be cavitation without loud noise?

Yes. Early or local cavitation is not always distinct amidst the noise of an industrial facility. Evaluation requires measurements and correlation with operating conditions.

Does a stainless steel impeller eliminate the problem?

No. Resistance depends on the material and conditions, but no material fixes insufficient suction pressure or improper operation.

Does cavitation mean the pump is running without liquid?

No. It can occur while the pump is full of liquid. Dry running is a different problem, which is also dangerous for the seal and other components.

When is an immediate shutdown necessary?

Intense new vibrations, a serious drop in performance, overheating, or leaks require immediate assessment and a controlled shutdown according to safety procedures. Continuing operation until failure can multiply the damage.