Cavitation is the only common hydraulic failure that destroys a pump while every pressure gauge in the system reads normal. The damage happens on the inlet side, where oil pressure falls below vapour pressure, bubbles form, and then collapse violently as the oil reaches the pump's pressurised side.
The energy released in that collapse erodes metal, generates noise and heat, and removes material from exactly the surfaces that control flow. This article explains how to recognise it, how to calculate whether the suction side is adequate, and what to change so it does not recur.
Liquid does not boil only at high temperature; it also boils when pressure falls far enough. In a pump inlet, oil accelerates into the pump and the pressure at the piston inlet port drops. If it reaches vapour pressure, bubbles form. Those bubbles are carried into the bore, where pressure rises rapidly, and they collapse in microseconds, producing microjets and shock waves that pit the metal.
Two consequences follow, and both are commercially important. First, the damage is not caused by dirt, so filtration changes will not stop it. Second, the damage is progressive: each collapse removes a little material and leaves a slightly rougher surface, which encourages the next collapse. A pump can run for months with mild cavitation and then fail suddenly when a pit becomes a crack.
Cavitation has a distinct fingerprint. Compare it against the other causes of similar symptoms before ordering parts.
| Evidence | Cavitation | Air ingress | Abrasive wear |
|---|---|---|---|
| Damage location | Inlet side of bores, inlet kidney of the plate | Little metal damage | Both kidneys, evenly distributed |
| Damage appearance | Pitting, sponge-like erosion, rough craters | None, but oil is foamy | Fine concentric scratches |
| Noise character | Crackling, rattling, worse under load | Whistling, gurgling at the suction | Steady rumble |
| Oil appearance | Often clear, may darken with heat | Foam that collapses slowly | Metal particles, dark colour |
| Response to throttle | Noise increases with speed | Varies with suction line movement | No relation to speed |
The speed relationship is the most useful field test. Cavitation worsens sharply as pump speed rises, because inlet velocity and pressure drop rise together. Abrasive wear does not care about speed in the same way.
Suction performance is not a matter of opinion. Three quantities determine whether the inlet is adequate, and all can be estimated before any part is changed.
| Parameter | Practical target | Consequence if missed |
|---|---|---|
| Inlet flow velocity | 1–2 m/s in the suction line, up to about 1.5 m/s for long runs | Excessive pressure drop at the inlet |
| Inlet absolute pressure | Comfortably above vapour pressure with margin for cold oil | Bubbles form at the piston inlet |
| Suction line length and bends | Short, straight, few fittings | Adds losses that are invisible on a gauge |
| Strainer pressure drop | Monitored, and replaced not cleaned indefinitely | Progressive restriction as debris accumulates |
| Oil viscosity at cold start | Within the pump's published limit for the ambient temperature | Cold-start cavitation, the most damaging kind |
| Reservoir level and geometry | Above the inlet with adequate dwell time for de-aeration | Air entrainment and vortex formation |
Inlet velocity is the calculation most worth doing: flow in litres per minute divided by 600 times the square of the pipe bore in centimetres gives velocity in metres per second. A 150 L/min pump drawing through a 30 mm line runs at about 3.5 m/s, which is roughly double the recommended figure and a reliable cause of cavitation at speed.
Field cavitation almost always traces to one of six causes, and each has a different remedy.
This list is written to be used during commissioning and at each major service. Each item is cheap; each failure it prevents is not.
| Item | Check | Frequency |
|---|---|---|
| Suction velocity | Calculate from flow and bore; keep at or below 1.5–2 m/s | On installation and after any pump change |
| Strainer condition | Differential pressure indicator, or inspect and replace | Every service |
| Hose integrity | Check for soft spots, delamination, collapsed liner | Annually and after any overheating event |
| Flange tightness | Torque check; look for oil weep and air draw | Every service |
| Reservoir level | Above minimum with machine on slope, not just level ground | Daily |
| Cold start procedure | Idle to warm, no load until oil reaches operating band | Every cold start |
| Oil viscosity grade | Matched to ambient temperature range, not to habit | Seasonally |
| Breather | Clean and correctly sized; a blocked breather pressurises or vacuums the tank | Every service |
Once pitting has occurred, replacing the damaged parts without correcting the suction side guarantees a repeat. The correct sequence is to fix the inlet first, then assess the pump, and to record the inlet velocity achieved so the repair can be verified rather than assumed.
On the bench, cavitation damage concentrates on the valve plate inlet kidney and the bore inlet region, so plate and block are usually replaced as a pair. Where the pump is a Kawasaki K7V installation, the K7V63 and K7V125 repair parts cover the plate and block assemblies affected by inlet erosion. For excavator main pumps in the K5V212 frame, this K5V212DTP diesel-driven unit is specified for 500–3,000 rpm operation, a speed band where inlet conditions must be verified rather than assumed. Linde HMF motor installations use HMF28 to HMF135 motor spare parts, where inlet-side erosion is a frequent finding on units run at low reservoir levels.
Before ordering, confirm the suction fix and send the pump model and nameplate data, the part numbers required, the machine model and the quantity. We will also confirm the recommended inlet velocity for that frame size, so the replacement part is not the second failure of the same root cause.
Yes. A blocked strainer, a collapsed hose liner or an undersized line will cause it with the tank full. Reservoir level is only one of six common causes.
Usually the opposite. A finer strainer increases inlet pressure drop and makes cavitation more likely. Filtration belongs on the pressure or return side, with the suction protected only against gross debris.
Light pitting on the plate can be addressed by replacing plate and block as a pair. Erosion of the bore inlet or housing means the pump is beyond economic repair.
Because inlet velocity, and therefore pressure drop, rises with speed. Noise that appears only at high rpm is a strong indication of a marginal suction design rather than a worn pump.
Cavitation is the only common hydraulic failure that destroys a pump while every pressure gauge in the system reads normal. The damage happens on the inlet side, where oil pressure falls below vapour pressure, bubbles form, and then collapse violently as the oil reaches the pump's pressurised side.
The energy released in that collapse erodes metal, generates noise and heat, and removes material from exactly the surfaces that control flow. This article explains how to recognise it, how to calculate whether the suction side is adequate, and what to change so it does not recur.
Liquid does not boil only at high temperature; it also boils when pressure falls far enough. In a pump inlet, oil accelerates into the pump and the pressure at the piston inlet port drops. If it reaches vapour pressure, bubbles form. Those bubbles are carried into the bore, where pressure rises rapidly, and they collapse in microseconds, producing microjets and shock waves that pit the metal.
Two consequences follow, and both are commercially important. First, the damage is not caused by dirt, so filtration changes will not stop it. Second, the damage is progressive: each collapse removes a little material and leaves a slightly rougher surface, which encourages the next collapse. A pump can run for months with mild cavitation and then fail suddenly when a pit becomes a crack.
Cavitation has a distinct fingerprint. Compare it against the other causes of similar symptoms before ordering parts.
| Evidence | Cavitation | Air ingress | Abrasive wear |
|---|---|---|---|
| Damage location | Inlet side of bores, inlet kidney of the plate | Little metal damage | Both kidneys, evenly distributed |
| Damage appearance | Pitting, sponge-like erosion, rough craters | None, but oil is foamy | Fine concentric scratches |
| Noise character | Crackling, rattling, worse under load | Whistling, gurgling at the suction | Steady rumble |
| Oil appearance | Often clear, may darken with heat | Foam that collapses slowly | Metal particles, dark colour |
| Response to throttle | Noise increases with speed | Varies with suction line movement | No relation to speed |
The speed relationship is the most useful field test. Cavitation worsens sharply as pump speed rises, because inlet velocity and pressure drop rise together. Abrasive wear does not care about speed in the same way.
Suction performance is not a matter of opinion. Three quantities determine whether the inlet is adequate, and all can be estimated before any part is changed.
| Parameter | Practical target | Consequence if missed |
|---|---|---|
| Inlet flow velocity | 1–2 m/s in the suction line, up to about 1.5 m/s for long runs | Excessive pressure drop at the inlet |
| Inlet absolute pressure | Comfortably above vapour pressure with margin for cold oil | Bubbles form at the piston inlet |
| Suction line length and bends | Short, straight, few fittings | Adds losses that are invisible on a gauge |
| Strainer pressure drop | Monitored, and replaced not cleaned indefinitely | Progressive restriction as debris accumulates |
| Oil viscosity at cold start | Within the pump's published limit for the ambient temperature | Cold-start cavitation, the most damaging kind |
| Reservoir level and geometry | Above the inlet with adequate dwell time for de-aeration | Air entrainment and vortex formation |
Inlet velocity is the calculation most worth doing: flow in litres per minute divided by 600 times the square of the pipe bore in centimetres gives velocity in metres per second. A 150 L/min pump drawing through a 30 mm line runs at about 3.5 m/s, which is roughly double the recommended figure and a reliable cause of cavitation at speed.
Field cavitation almost always traces to one of six causes, and each has a different remedy.
This list is written to be used during commissioning and at each major service. Each item is cheap; each failure it prevents is not.
| Item | Check | Frequency |
|---|---|---|
| Suction velocity | Calculate from flow and bore; keep at or below 1.5–2 m/s | On installation and after any pump change |
| Strainer condition | Differential pressure indicator, or inspect and replace | Every service |
| Hose integrity | Check for soft spots, delamination, collapsed liner | Annually and after any overheating event |
| Flange tightness | Torque check; look for oil weep and air draw | Every service |
| Reservoir level | Above minimum with machine on slope, not just level ground | Daily |
| Cold start procedure | Idle to warm, no load until oil reaches operating band | Every cold start |
| Oil viscosity grade | Matched to ambient temperature range, not to habit | Seasonally |
| Breather | Clean and correctly sized; a blocked breather pressurises or vacuums the tank | Every service |
Once pitting has occurred, replacing the damaged parts without correcting the suction side guarantees a repeat. The correct sequence is to fix the inlet first, then assess the pump, and to record the inlet velocity achieved so the repair can be verified rather than assumed.
On the bench, cavitation damage concentrates on the valve plate inlet kidney and the bore inlet region, so plate and block are usually replaced as a pair. Where the pump is a Kawasaki K7V installation, the K7V63 and K7V125 repair parts cover the plate and block assemblies affected by inlet erosion. For excavator main pumps in the K5V212 frame, this K5V212DTP diesel-driven unit is specified for 500–3,000 rpm operation, a speed band where inlet conditions must be verified rather than assumed. Linde HMF motor installations use HMF28 to HMF135 motor spare parts, where inlet-side erosion is a frequent finding on units run at low reservoir levels.
Before ordering, confirm the suction fix and send the pump model and nameplate data, the part numbers required, the machine model and the quantity. We will also confirm the recommended inlet velocity for that frame size, so the replacement part is not the second failure of the same root cause.
Yes. A blocked strainer, a collapsed hose liner or an undersized line will cause it with the tank full. Reservoir level is only one of six common causes.
Usually the opposite. A finer strainer increases inlet pressure drop and makes cavitation more likely. Filtration belongs on the pressure or return side, with the suction protected only against gross debris.
Light pitting on the plate can be addressed by replacing plate and block as a pair. Erosion of the bore inlet or housing means the pump is beyond economic repair.
Because inlet velocity, and therefore pressure drop, rises with speed. Noise that appears only at high rpm is a strong indication of a marginal suction design rather than a worn pump.
