Cavitation in Industrial Pumps: What It Is, How to Identify and How to Prevent

1. What is cavitation

Cavitation is the formation of vapour bubbles in a liquid where local pressure falls below the vapour pressure at the operating temperature, followed by the collapse of those bubbles when they reach a region of higher pressure. In a centrifugal pump, pressure drops as the liquid accelerates into the impeller; the Hydraulic Institute describes the collapse eroding impeller passages and the vibration damaging seals and bearings.

The damage comes from collapse near the wall. According to Brennen (Cavitation and Bubble Dynamics, §3.6), each collapse close to a solid surface produces shock waves and microjets; the local, transient stresses repeat, fatigue the surface and detach pieces of material. That is why a cavitated area looks jagged and crystalline, unlike the smoother wear caused by solid particles. In iron and steel, corrosion adds to the damage.

In the field, "to cavitate" is the verb for this regime: a pump cavitates when pressure somewhere at its inlet falls below vapour pressure, and stops cavitating when the cause is corrected, whether by reducing suction losses, raising the level, lowering the temperature or bringing flow back into the curve range. In continuous service, cavitating is not an acceptable system feature: it points to installation, selection or operation outside the design.

A cavitation bubble is not air: it is vapour of the liquid itself, born on microscopic nuclei such as gas microbubbles or particles when local pressure crosses the vapour pressure (Brennen, ch. 1). It grows in the low-pressure zone and collapses when it meets higher pressure again. Near a wall, the collapse is asymmetric and drives a microjet against it (§3.5); the book cites shocks of about 100 bar at the wall (§3.2).

Two practical points follow. First, in a pump cavitation starts on the suction side, where pressure is lowest, and the damage concentrates just downstream, at the impeller inlet of a centrifugal pump. Second, entrained air is not cavitation: when gas bubbles grow in the low-pressure zone without it reaching vapour pressure, Brennen calls the phenomenon pseudo-cavitation (§1.15). The symptoms look alike, but the cause and the correction differ.

2. How to identify cavitation in the field

Five signs point to cavitation: (1) crackling that the US DOE and Hydraulic Institute sourcebook compares to marbles passing through the pump; (2) rising vibration, measured with an instrument at the bearing housing, never by hand; (3) a drop in head, flow or pressure; (4) jagged, crystalline erosion at the impeller inlet, unlike smooth particle wear; (5) accelerated seal and bearing wear (DOE, pp. 14 and 20).

None of these signs is exclusive to cavitation: air ingress, misalignment, a worn bearing or recirculation produce similar symptoms. Compare noise, measured vibration, flow and pressures with the recorded normal condition of the pump. Inspect internal parts only with the equipment stopped, isolated and released under the site procedure, and do not use hand contact with hot or moving parts as a diagnostic method.

3. The 7 real causes of cavitation

Seven conditions cover the causes to investigate in an industrial installation. The list organises the diagnosis and is not FB Bombas failure statistics: each item reduces available NPSH, raises the required value or produces symptoms similar to cavitation.

• Excessive suction lift: with the pump above the liquid level, the elevation difference uses up part of the atmospheric pressure, and the minimum reservoir level is what counts in the calculation.
• Suction losses: a pipe smaller than the nozzle, too many fittings, a partly closed valve, air pockets or fouled pipe (DOE, p. 20); on the FBEI, the diameter must be at least equal to the nozzle, with as few fittings as possible (MAN001-10).
• Temperature above design: the vapour pressure of water rises from 2.34 kPa at 20 °C to 47.41 kPa at 80 °C (NIST); section 4 shows the effect on available NPSH.
• Operation far from best efficiency: in a centrifugal pump, cavitation usually appears at high flow, on the right side of the curve, where NPSHr rises; at low flow, vortices can cause similar damage (DOE, p. 14).
• Viscosity and speed in a gear pump: suction must fill the chambers between the teeth on every revolution, and Table 2 (MTEC-01/01) lowers the speed as viscosity rises (section 5).
• Air ingress: poorly sealed joints, packing and connections on the suction side let air in. It is not cavitation, but it produces similar noise and flow loss, and it should be ruled out first.
• Liquid near boiling: at 100 °C, the vapour pressure of water is 101.42 kPa (NIST), above the standard atmosphere of 101.325 kPa; in an open tank, only the liquid column above the pump sustains available NPSH.

4. NPSH: the parameter that prevents cavitation

Available NPSH (NPSHa) is the absolute suction head above vapour pressure, in metres of liquid: it is a property of the installation at one operating condition. Required NPSH (NPSHr) comes from the manufacturer for each model, impeller, speed and flow. Under the Hydraulic Institute definitions, NPSH3 is the value at which first-stage head drops 3%; it does not mark the onset of cavitation, which at that point already reduces head.

The margin comes from the application. ANSI/HI 9.6.1 measures the margin over the published NPSHr, equal to or greater than NPSH3, with recommendations by type of service, and the US DOE and HI sourcebook cites the common rule of NPSHa 25% above NPSHr (p. 38). No fixed margin in metres approves every FBCN centrifugal or FBE gear pump.

For suction from a reservoir, NPSHa = Pa ± Hz − Hf − Pv, with every term in metres of liquid: Pa is the absolute pressure on the reservoir; Hz, the level difference, positive with flooded suction; Hf, suction losses; Pv, the vapour pressure at the operating temperature. The article on NPSH in centrifugal pumps details each term and how to read NPSHr on the curve.

An example shows the weight of temperature: water in an open tank at sea level, the pump 2.5 m above the level and 0.8 m of losses. At 20 °C, NPSHa is 6.81 m, with 3.81 m to spare over a hypothetical NPSHr of 3.0 m. At 80 °C, the same installation falls to 2.36 m, 0.64 m below that NPSHr: the pump starts to cavitate with nothing changed in the piping.

That is why temperature is the most treacherous variable in the diagnosis. The example converts each term with the NIST density at the liquid temperature and neither selects a pump nor sets a margin: the real comparison uses the curve NPSHr at the duty point and the application margin.

5. Cavitation in gear pumps vs centrifugal pumps

In a centrifugal pump, the liquid accelerates into the impeller and pressure drops at the inlet (DOE, p. 20); NPSHr comes from the curve and grows with flow. On the published FBCN 50-160 curve at 3,500 rpm (174 mm impeller, clean water at 20 °C), it stays near 1.7 m at 40 m³/h and reaches about 4.7 m at 100 m³/h. FB Bombas Engineering limits FBCN flow to 1.1 Qot (MTEC-03/00, Table 1).

A gear pump is a positive displacement pump: on every revolution the teeth carry liquid from suction to discharge, and suction must fill the chambers between them. On the FBE, selection starts from Table 2 (MTEC-01/01), which pairs each viscosity range with a speed, a drive and the models. Each row applies to the models it lists, not to every size.

FB does not publish an NPSH value for the FBE: Table 1 (MTEC-01/01) lists flow and power by discharge pressure and speed. For the FBEI, suction is tied to cavitation: the strainer is 40 to 80 mesh, depending on the model, with the largest possible open area "to avoid pump cavitation" (MAN001-10, p. 4). There is no universal ranking of which family resists more; each is checked against its own series data.

6. Hydraulic cavitation: valves, turbines and hydropower plants

Hydraulic cavitation is the generic name for the phenomenon in any liquid system. Wherever liquid accelerates and local pressure falls below vapour pressure, bubbles form and collapse: at the vena contracta of partly closed valves, at the throat of venturis and orifice plates, on ship propellers and on hydraulic turbine runners. The physics is the one described in section 1; only the geometry where pressure drops changes.

In hydropower plants, cavitation wears the runners of reaction turbines. Engineering Monograph EM-20 of the U.S. Bureau of Reclamation records large runners in cast stainless steel, or carbon steel with stainless overlay, to withstand cavitation (p. 13), and links operation at small gate openings to low efficiency and accelerated cavitation damage (p. 8).

Control starts with the machine elevation. EM-20 uses the Thoma cavitation factor, σ, and sets the static draft head by Hs = Hb − σ·h, where Hb is atmospheric minus vapour pressure, in metres of water, and h is the maximum head at full gate. The experience curve places the turbine 0.3 m below the elevation at which damage and loss of performance approached unacceptable values (p. 22).

The parallel with a pump is direct: Hb − Hs plays the role of available NPSH, and σ·h that of required NPSH. In industrial plants, the common blind spot is a throttled valve ahead of the pump, which drops suction pressure and creates installation cavitation that no pump replacement solves. On the FBEI, the suction valve must be fully open before start-up (MAN001-10, p. 13).

In a centrifugal pump, flow is controlled by a discharge valve, by speed or with several pumps (DOE, p. 8), never by throttling the suction. In a positive displacement pump, a closed discharge makes pressure rise until the motor overloads or the piping ruptures, which is why it usually has a relief valve (DOE, p. 41). FB Bombas Engineering forbids using that valve on the FBEI to control pressure or flow and requires protection on pumps without one (MAN001-10, p.

4).

7. Cavitation prevention checklist — FB Bombas

The points below gather what FB Bombas Engineering and the cited sources require to prevent cavitation. Check each one at the worst operating condition, not the average.

• Calculate NPSHa at the minimum level, maximum temperature and highest expected flow, and compare it with the curve NPSHr plus the application margin.
• Install the pump as close as possible to the suction point (MAN001-10, p. 4); with flooded suction, the liquid column adds to NPSHa.
• Use a suction pipe at least as large as the nozzle, with as few fittings as possible (FBEI, p. 4).
• On the FBEI, fit a 40 to 80 mesh strainer, depending on the model, with the largest possible open area (p. 4), and keep it clean.
• Control liquid temperature: above design, vapour pressure rises and NPSHa falls.
• On the FBE, select speed from Table 2 (MTEC-01/01) with the actual viscosity, at start-up and in operation.
• On the FBCN, operate between 0.1 and 1.1 Qot on bearing frames 30 and 40, and between 0.15 and 1.1 Qot from frame 50 upwards (MTEC-03/00, Table 1).
• Seal suction joints, packing and connections: air ingress mimics cavitation and is hard to find.
• When starting the FBEI, prime the pump and fully open the suction valve; in operation, check that suction pressure matches the design (MAN001-10, p. 13).
• Record noise, measured vibration, flow and pressures under normal conditions, to have a baseline when a symptom appears.

8. When to consult the application engineer

If symptoms persist, send the model, serial number, curve, liquid, temperature, flow, speed and measured pressures, with the suction drawing. With these data, FB Bombas application engineering recalculates the installation NPSH and may recommend a suction piping change, a different model or impeller diameter, another speed or a booster pump on the suction side. Contact: comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837.