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.
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 fundamental rule is: NPSHa must ALWAYS be higher than NPSHr, with a safety margin. For FBCN centrifugal pumps, FB Bombas recommends a minimum margin of 0.5 to 1.0 meters. For FBE gear pumps with viscous fluids, the margin should be larger (1.0 to 1.5 meters) because suction friction losses increase significantly with viscosity.
The NPSHa formula is: NPSHa = Pa ± Hz - Hf - Pv, where Pa = atmospheric pressure (or tank pressure), Hz = geometric height (positive if flooded, negative if suction lift), Hf = friction losses in suction piping, Pv = fluid vapor pressure at operating temperature.
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. Cavitation prevention checklist — FB Bombas
Based on FB Bombas field experience with over 82 years of industrial operation, 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).
7. 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.