1. What is cavitation
Cavitation is a hydraulic phenomenon in which vapor bubbles spontaneously form within the pumped liquid when the local pressure drops below the fluid vapor pressure at operating temperature. When these bubbles migrate to higher-pressure regions inside the pump (the discharge zone), they collapse violently and asymmetrically, generating shock waves and liquid microjets at velocities exceeding 100 m/s that impact metal surfaces.
The result is progressive metal erosion (called pitting or cratering), mechanical vibration, characteristic noise similar to rocks inside the pump, flow and pressure drop, and dramatic reduction in the service life of internal components — especially impeller, mechanical seal and bearings. A pump operating under cavitation can have its service life reduced from years to weeks.
In field vocabulary, "to cavitate" is the verb that describes this regime: a pump starts to cavitate when the suction pressure drops below the liquid vapor pressure, and stops cavitating when that condition is corrected — by reducing suction losses, lowering fluid temperature or reducing speed.
In continuous industrial pump operation, cavitating is an undesirable condition that demands investigation: in the vast majority of cases it points to installation, selection or operation outside design intent — it should never be treated as an acceptable system characteristic.
The cavitation bubble is not an air bubble: it is vapor of the pumped liquid itself, nucleated on micro-impurities and dissolved gases the instant local pressure crosses the vapor pressure. It is born microscopic, grows while crossing the low-pressure zone and implodes within microseconds upon reaching higher pressure.
When the collapse happens next to a metal wall, the implosion is asymmetric: liquid rushes through the bubble center and forms the microjet that pierces the boundary layer and hammers the metal — this mechanism, repeated continuously at high frequency, is what carves the characteristic pitting.
Two practical points follow from this physics. First, in pumps cavitation is always a suction-side phenomenon: bubbles form where pressure is lowest (the impeller eye in centrifugal pumps, the gear-tooth space in gear pumps) and collapse just downstream — which is why damage concentrates in those regions.
Second, air ingress is not cavitation: entrained air forms bubbles that compress and cushion without imploding, producing similar symptoms (noise, flow drop) with a different cause and a different fix — telling the two apart is the first step of any diagnosis.
2. How to identify cavitation in the field
Cavitation manifests through five symptoms identifiable without sophisticated instrumentation: (1) Noise — gravel-like or metallic hammering sound, audible from a few meters away; (2) Vibration — increasing amplitude, detectable by hand on the bearing housing; (3) Performance drop — flow and/or pressure below specification, intermittent or progressive; (4) Visible erosion — pitting marks on the impeller suction face (centrifugal) or on the gear suction face (gear pumps); (5) Premature mechanical seal failure — the seal loses service life due to vibration and instability of the liquid film on the sealing faces.
| Symptom | Hypothesis to verify | Immediate action |
|---|---|---|
| "Gravel" or metallic hammering noise | Vapor bubble collapse — insufficient NPSHa at suction | Check suction strainers and valves; recalculate installation NPSHa |
| Increasing vibration at the bearing housing | Asymmetric bubble collapse on the impeller or gears | Reduce speed (if possible) and inspect the suction line |
| Flow and/or pressure drop | Partial fluid vaporization at the pump inlet | Check fluid temperature and suction reservoir level |
| Pitting on the impeller or gears | Prolonged cavitation — microjet erosion | Fix the root cause (NPSH, viscosity × speed) before replacing parts |
| Premature mechanical seal failure | Vibration and liquid film instability at the sealing faces | Check for air ingress at suction connections and seals |
3. The 7 real causes of cavitation
In FB Bombas application engineering experience, cavitation causes in industrial installations concentrate on seven recurring factors: (1) Excessive geometric suction height — the pump tries to draw liquid from a height greater than atmospheric pressure allows; (2) Suction piping friction losses — undersized piping, excess bends, partially closed valves or dirty filters; (3) Fluid temperature above design — vapor pressure increases exponentially with temperature, reducing NPSHa; (4) Fluid more viscous than specified — in gear pumps, excessive viscosity prevents complete filling of the gear tooth chamber at operating speed; (5) Speed above recommended — especially critical in FBE gear pumps with viscous fluids (see viscosity × RPM table in manual MTEC-01/01); (6) Air ingress at suction — joints, packing or connections with compromised sealing allow air entry; (7) Partial vaporization — volatile fluids or fluids near boiling point at operating temperature.
4. NPSH: the parameter that prevents cavitation
NPSH (Net Positive Suction Head) is the quantitative tool for preventing cavitation. The concept splits into two values: NPSHa (available) is the effective pressure energy that the installation provides at the pump inlet — it depends on atmospheric pressure, geometric height, friction losses and vapor pressure. NPSHr (required) is the minimum pressure the pump needs at its inlet to operate without cavitation — this value is provided by the manufacturer for each model and operating condition.
The NPSH margin must be selected for the liquid, pump and installation conditions. A fixed margin in meters cannot approve every FBCN centrifugal or FBE gear pump. Where the curve states NPSH3, it represents a 3% head-drop test criterion, not the onset of all cavitation.
For suction from a reservoir, NPSHa = Pa ± Hz − Hf − Pv, with all terms in meters of liquid head: Pa is the absolute pressure head over the reservoir; Hz, the elevation difference; Hf, suction losses; and Pv, the head corresponding to vapor pressure at operating temperature. Confirm the curve and applicable margin with engineering.
A worked example shows the weight of each term. Typical installation: water at 20 °C, open tank at sea level (Pa = 10.33 mwc), pump 2.5 m above liquid level (Hz = −2.5 m), suction losses of 0.8 m and vapor pressure of 0.24 mwc from the saturated steam table. NPSHa comes to 6.79 m — comfortable against a catalog NPSHr of 3.0 to 3.5 m.
Now the same installation with water at 80 °C: vapor pressure jumps to 4.83 mwc and NPSHa plummets to 2.20 m — the pump that ran with margin starts cavitating with nothing changed in the piping. That is why temperature is the most treacherous variable in diagnosis: NPSHa is not a property of the pump, it is a property of the installation at a specific operating condition.
NPSHa(20 °C) = 10,33 − 2,5 − 0,8 − 0,24 = 6,79 m
NPSHa(80 °C) = 10,33 − 2,5 − 0,8 − 4,83 = 2,20 mExample: water at 20 °C, open tank at sea level, 2.5 m suction lift, 0.8 m losses — vs the same installation at 80 °C. Values in standard water column (ρ = 1,000 kg/m³).
5. Cavitation in gear pumps vs centrifugal pumps
FBCN centrifugal pumps are more sensitive to cavitation than FBE gear pumps. This happens because in centrifugal pumps, the liquid enters the impeller center at high velocity, and the pressure drop at the impeller eye inlet is abrupt — any NPSH insufficiency generates cavitation immediately. In gear pumps, the mechanism is different: cavitation occurs when the viscous fluid cannot fill the space between gear teeth at the rotation speed.
Therefore, in FBE pumps, cavitation is directly related to viscosity: the more viscous the fluid, the lower the speed must be to allow complete chamber filling.
Practical data from the FBE manual (MTEC-01/01): for fluids at 30 to 250 SSU, maximum speed is 1,750 rpm (direct drive). For 2,500 to 7,500 SSU, it drops to 850 rpm. For 10,000 to 50,000 SSU, it stays between 500 and 300 rpm (with gearbox). Above 50,000 SSU, speed must be between 300 and 150 rpm. Operating above these limits causes immediate cavitation, regardless of available NPSH.
6. Hydraulic cavitation: the same phenomenon beyond pumps
Hydraulic cavitation is the generic name for the phenomenon in any hydraulic system — it is not exclusive to pumps. Wherever liquid accelerates and local pressure falls below vapor pressure, bubbles form and collapse: at the vena contracta of partially closed valves, at the throat of venturis and orifice plates, on ship propellers and on hydraulic turbine runners. The physics is identical to what "What is cavitation" describes; only the geometry where the pressure drop occurs changes.
In the hydroelectric industry, cavitation is one of the main wear mechanisms of Francis and Kaplan turbines: bubbles collapse on the runner blades and in the draft tube, and control happens at plant design — the turbine setting relative to tailwater level (submergence) is chosen to keep pressure above the cavitation limit, a criterion expressed by the Thoma coefficient (σ). The parallel with pumps is direct: turbine submergence plays the same role as the installation NPSHa.
In industrial plants, the most common blind spot is the control valve: a throttled valve just before the pump collapses suction pressure and manufactures an "installation" cavitation that no pump replacement will fix. Valves in the suction line must operate fully open; flow control belongs on the discharge side.
7. Cavitation prevention checklist — FB Bombas
Based on FB Bombas field experience with industrial operation since 1944, this checklist covers the verification points to prevent cavitation: (1) Calculate installation NPSHa and compare with catalog NPSHr — maintain margin ≥ 0.5 m (centrifugal) or ≥ 1.0 m (gear with viscous fluids); (2) Size suction piping with velocity ≤ 1.5 m/s for centrifugal and ≤ 0.5 m/s for gear with viscous fluids; (3) Minimize length and fittings in suction line — each 90° elbow equals ~30 pipe diameters of straight pipe in friction loss; (4) Install pump as close as possible to suction reservoir, preferably below liquid level (flooded suction); (5) Verify suction filters are clean with passage area ≥ 3× pipe area; (6) Control fluid temperature — cool before pump if possible; (7) For FBE gear pumps: strictly follow maximum speed table by viscosity range; (8) For FBCN centrifugal pumps: operate between 0.15×Qopt and 1.1×Qopt (optimum flow) — operation outside this range increases cavitation and internal recirculation risk; (9) Check sealing of all suction line connections — air ingress is a frequent and difficult-to-diagnose cause; (10) Monitor bearing temperature during startup — temperature above 90°C indicates problems that may be associated with cavitation (reference: FBEI manual, MAN001-10, startup procedure).
8. When to consult the application engineer
If your pump already shows cavitation symptoms, the first step is to reduce speed (if possible) and check the suction line. If symptoms persist, FB Bombas application engineering can recalculate the installation NPSH and recommend: suction piping resizing, pump model or size change, operating speed change, or suction booster pump installation. Contact: comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837.