Cavitation Damage Repair: How to repair pitted impeller and casing after cavitation

Cavitation is one of the most destructive failure modes in centrifugal pumps, causing pitting, erosion, and material loss on impeller vanes, volute cutwaters, and casing walls. When cavitation has occurred — often due to insufficient NPSH, low-flow operation, or improper system design — the question facing maintenance engineers is whether the damaged pump can be repaired or must be replaced. With proper assessment and repair techniques, moderately cavitated impellers and casings can be restored to service, extending pump life by 3–5 years at a fraction of replacement cost. However, not all cavitation damage is repairable — severe damage compromises structural integrity and hydraulic performance, making replacement the only safe option.

This guide covers cavitation damage assessment, repair methods, material selection, acceptance criteria, and preventive measures for centrifugal pump impellers and casings.

Understanding Cavitation Damage

How Cavitation Damages Pump Components

Cavitation occurs when the local pressure at the pump impeller eye drops below the vapor pressure of the liquid, causing vapor bubbles to form. These bubbles are carried to higher-pressure regions where they collapse (implode), generating microscopic shock waves (up to 10,000 bar) and micro-jets of liquid that impact the metal surface at velocities up to 500 m/s. Over time, this repeated impact causes:

• Pitting: Small, deep holes (0.5–5 mm diameter) on impeller vane surfaces, especially on the suction side (low-pressure side) of the vanes near the inlet edge.
• Erosion: General material loss and roughening of surfaces, particularly at the volute cutwater and impeller outlet.
• Material fatigue: Repeated shock loading causes work hardening and eventual crack initiation at pit edges.
• Structural degradation: In severe cases, impeller vanes become thin, develop cracks, or break off entirely.
• Performance loss: Pitted and eroded surfaces increase hydraulic losses, reducing flow, head, and efficiency by 5–30%.

Typical Damage Locations

Types of Cavitation

• Classic (NPSH) cavitation: Caused by insufficient available NPSH. Damage on impeller vane suction side near inlet. Most common type.
• Recirculation cavitation: Caused by operation at low flow (<50% BEP). Internal recirculation at impeller inlet and outlet creates low-pressure zones. Damage on both sides of vanes and at cutwater.
• Vane-passing (rotating) cavitation: Caused by interaction between impeller vane and volute cutwater. Damage at cutwater and vane outlet. Often accompanied by high-frequency noise.
• Gas cavitation (aeration): Caused by entrained gas in the liquid. Gas bubbles collapse less violently than vapor bubbles, causing more generalized erosion. Common in condensate and hot water service.

Damage Assessment: Repair or Replace?

Inspection Methods

1. Visual inspection: Remove pump, clean all wetted surfaces, examine with bright light and magnifying glass. Document all damage with photographs and measurements.
2. Dimensional inspection: Measure impeller vane thickness at multiple points (compare to original drawing or new impeller). Measure casing wall thickness with ultrasonic gauge. Check wear ring clearance.
3. Dye penetrant inspection (DPI): Apply penetrant to detect surface cracks, especially at pit edges and vane roots. Critical for structural integrity assessment.
4. Ultrasonic testing (UT): Measure remaining wall thickness in pitted areas. Detect subsurface cracks and voids.
5. Magnetic particle inspection (MPI): For ferromagnetic materials (carbon steel, cast iron), detect surface and near-surface cracks.
6. Performance testing: If pump can be tested, compare current performance (flow, head, efficiency) to original curve. >10% head drop or >5 point efficiency drop indicates significant hydraulic damage.

Damage Classification

Repair Decision Criteria

Repair is justified when:

• Damage is Class 1–2 (light to moderate)
• No cracks detected by DPI/MPI/UT
• Remaining wall thickness ≥ 75% of original
• Impeller vane thickness ≥ 80% of original at root
• Performance loss <10% (head) and <5 points (efficiency)
• Repair cost <50% of replacement cost
• Pump is not in critical safety service (high pressure, toxic fluid)

Replace when:

• Damage is Class 3–4 (heavy to severe)
• Cracks detected, especially at vane root or hub
• Through-wall pitting or breakthrough
• Remaining wall thickness <75% of original
• Impeller is significantly out of balance (cannot be corrected)
• Pump is in critical service (high pressure, toxic, flammable fluid)
• Replacement cost is comparable to repair (small pumps)

Repair Methods

Method 1: Grinding and Smoothing (Light Damage, Class 1)

Procedure:

1. Clean damaged area thoroughly (solvent degrease, then grit blast if needed)
2. Use a die grinder or rotary tool with carbide burr to smooth pits and blend edges
3. Grind in the direction of flow (to avoid creating turbulence-inducing ridges)
4. Blend repaired area into surrounding surface with gradual transition (no sharp edges)
5. Polish with progressively finer grit (80 → 120 → 240 grit) to achieve Ra ≤ 3.2 μm
6. Inspect with DPI to ensure no cracks remain after grinding
7. Re-balance impeller (grinding removes material, changing balance)

Limitations: Only removes material — cannot restore lost thickness. Suitable for light pitting where structural integrity is not compromised. Do not grind below minimum wall thickness.

Method 2: Weld Repair (Moderate Damage, Class 2–3)

Procedure:

1. Pre-clean: degrease, grit blast to white metal finish in repair area (extend 25 mm beyond damage)
2. Pre-heat: for carbon steel and low-alloy steel, pre-heat to 150–250°C (depending on material and section thickness). For stainless steel, pre-heat to 50–100°C (if needed)
3. Grind out pits: remove all damaged material until sound metal is exposed. U-groove the area for proper weld penetration
4. DPI inspection after grinding to confirm no cracks remain
5. Weld: use matching or overmatching filler material. TIG (GTAW) for thin sections and stainless steel; MIG/MAG (GMAW) for thicker carbon steel. Use stringer beads (not weave) to minimize heat input
6. Interpass temperature: maintain between 150–250°C for carbon steel; <150°C for stainless (to avoid sensitization)
7. Post-weld: grind weld flush to original contour. Do not leave weld reinforcement (causes turbulence)
8. Post-weld heat treatment (PWHT): for carbon steel >25 mm thick or high-strength steel, stress-relieve at 600–650°C. For 304/316 stainless, solution anneal at 1050–1100°C followed by water quench (if service is corrosive)
9. DPI/MPI inspection after repair
10. Re-balance impeller to G2.5 or G6.3 (per API 610)

Weld Filler Material Selection:

Important: Welding cast iron impellers is generally not recommended — cast iron is prone to cracking during welding and the repair may fail in service. For cast iron pumps, consider epoxy repair (Class 1–2) or replacement.

Method 3: Epoxy / Polymer Repair (Moderate Damage, Class 2, Low-Temperature Service)

Procedure:

1. Clean: degrease, grit blast to SA 2.5 (near-white) in repair area (extend 25 mm beyond damage)
2. Grind out pits to sound metal, create rough profile for mechanical adhesion
3. Apply primer (if specified by epoxy manufacturer) to improve adhesion
4. Mix epoxy according to manufacturer's instructions (two-part, precise ratio)
5. Apply epoxy to repair area, building up to original contour. Use a template or gauge to match original impeller profile
6. Shape epoxy while still workable (before gel time — typically 30–60 minutes)
7. Cure at room temperature for 24–48 hours, or accelerate with heat (60–80°C for 2–4 hours)
8. Grind and polish to final contour and surface finish (Ra ≤ 3.2 μm)
9. Inspect for voids, cracks, or delamination (tap test — solid sound = good, hollow = void)
10. Re-balance impeller

Epoxy Material Types:

Limitations of epoxy repair:

• Temperature limit: typically 80–200°C (depending on type)
• Chemical resistance: not suitable for strong solvents, ketones, or strong oxidizers
• Adhesion: can delaminate if surface preparation is poor or if thermal cycling is severe
• Not for high-pressure or critical safety service
• Epoxy is softer than metal — may erode faster in abrasive service
• Must verify compatibility with process fluid (immersion test recommended)

Method 4: Laser Cladding / Hardfacing (Severe Service, High Value Pumps)

Laser cladding uses a high-power laser to melt a metallic powder (Stellite, Inconel, or stainless steel) onto the damaged surface, creating a dense, metallurgically bonded coating with minimal heat input. Suitable for high-value impellers in critical service.

Advantages:

• Minimal heat input (low dilution, no distortion)
• Dense, non-porous coating (better than epoxy or welding)
• Can apply corrosion-resistant or wear-resistant overlay
• Precise control of coating thickness (0.5–3 mm)
• Suitable for complex geometries (impeller vanes)

Limitations:

• High cost (2–5x conventional welding)
• Requires specialized equipment and skilled operators
• Not widely available (may need to ship pump to specialist)
• Best for large, high-value pumps (not economical for small pumps)

Method 5: Casing Repair

Casing (volute) repair follows similar principles but has additional considerations:

• Pressure boundary: The casing is a pressure vessel — any weld repair must be qualified to pressure vessel code (ASME BPVC Section VIII or equivalent). Hydrotest after repair.
• Cutwater repair: The cutwater (tongue) is a critical hydraulic feature. After welding/grinding, the cutwater profile (angle, radius, gap to impeller) must match original design. Use a template or coordinate measuring machine (CMM).
• Casing wall thickness: After grinding out pits, verify remaining wall thickness meets pressure design requirements. If too thin, weld build-up or replace casing.
• Gasket faces: If casing joint face is damaged, machine flat and verify flatness (≤0.05 mm per 100 mm). Do not use epoxy on gasket faces.
• Bearing fits: If bearing housing or seal chamber is damaged, machining may be required. Verify concentricity and alignment after repair.

Post-Repair Verification

Nondestructive Testing (NDT)

• DPI/MPI: All weld-repaired areas must pass dye penetrant or magnetic particle inspection (no cracks, no linear indications)
• UT thickness: Verify wall thickness meets minimum design requirement
• Pressure test: Hydrostatic test casing at 1.5x design pressure for 30 minutes (no leakage, no deformation)
• Visual: Surface finish Ra ≤ 3.2 μm on hydraulic surfaces; no sharp edges or pits remaining

Dimensional and Hydraulic Verification

• Impeller dimensions: Verify outlet diameter, vane exit angle, and eye diameter match original (within ±1%)
• Wear ring clearance: Set to manufacturer's specification (typically 0.2–0.4 mm radial)
• Balance: Impeller must be dynamically balanced to G2.5 (API 610) or G6.3 (general purpose) at operating speed
• Performance test: If possible, test pump after repair. Head and efficiency should be within 5% of original (new) performance. If not, repair was inadequate or damage was too severe.

Documentation

Maintain a repair record including:

• Damage assessment (photos, measurements, NDT results)
• Repair method and materials (filler metal, epoxy type, welding parameters)
• Welder qualification and procedure (WPS/PQR)
• Post-repair NDT results
• Balance report
• Hydrotest report (casing)
• Performance test results (if available)
• Repair date, technician name, and approvals

Preventing Future Cavitation

Repairing cavitation damage without addressing the root cause is a waste of money — the damage will recur. Common root causes and fixes:

Our stainless steel chemical pump range and API 610 OH2 process pump range are designed with low-NPSHr impellers and robust construction to resist cavitation. For cavitation damage assessment, repair procedures, and root cause analysis, talk to our engineers with your pump nameplate data, damage photos, and operating conditions.

Common Problems and Solutions

Problem 1: Weld repair cracks during or after welding. Improper pre-heat, high heat input, or wrong filler material. Solution: use correct pre-heat temperature (150–250°C for carbon steel). Use low-hydrogen electrodes (E7018) or TIG with low heat input. Maintain interpass temperature. PWHT after welding for thick sections. For cast iron, do not weld — use epoxy or replace. DPI after each weld layer to catch cracks early.

Problem 2: Epoxy repair delaminates after a few months. Poor surface preparation, wrong epoxy for fluid/temperature, or thermal cycling. Solution: grit blast to SA 2.5 (near-white) before epoxy — no shortcuts. Verify epoxy chemical compatibility with process fluid (immersion test). Check temperature limit (epoxy may soften above rating). For thermal cycling service, use flexible epoxy or consider welding. Ensure surface is dry and above dew point during application.

Problem 3: Pump performance still poor after repair. Repair did not restore original hydraulic contour, or damage was more severe than assessed. Solution: compare repaired impeller dimensions to original drawing (outlet diameter, vane angle, eye area). Use CMM or template to verify contour. If performance is >10% below original, the impeller likely needs replacement. Consider that casing may also be damaged (cutwater erosion) — inspect and repair casing too.

Problem 4: Impeller cannot be balanced after repair. Asymmetric material addition (weld/epoxy on one side only) or material removal (grinding) changes balance. Solution: add material symmetrically where possible. After repair, dynamically balance on a balancing machine. If balance cannot be achieved within G6.3, the impeller is too distorted — replace. Keep a record of original impeller weight for comparison.

Problem 5: Casing weld repair leaks under pressure. Weld defect (porosity, incomplete fusion) or casing wall too thin after grinding. Solution: UT thickness check before welding — minimum wall must meet pressure design. Use qualified weld procedure (WPS/PQR). Hydrotest after repair at 1.5x design pressure. If leak persists, excavate defect and re-weld, or replace casing. Do not use epoxy on pressure-boundary casing repairs.

Problem 6: Cavitation recurs after repair (same damage location). Root cause not addressed — repair only treats the symptom. Solution: perform NPSH analysis (calculate NPSHa, compare to NPSHr curve). Check operating point (is pump running at BEP?). Install pressure gauges on suction and discharge. Check for suction strainer clogging, air leaks, or high fluid temperature. Fix the root cause before repairing again — otherwise the repair will fail repeatedly.

Problem 7: Stainless steel weld area corrodes faster than base metal. Weld heat-affected zone (HAZ) sensitized (chromium carbide precipitation), or wrong filler metal. Solution: use low-carbon filler (ER316L) for 316SS base metal. Keep interpass temperature <150°C. Solution anneal after welding (1050–1100°C, water quench) if service is corrosive. Use electrolytic pickling paste to remove heat tint and restore passive layer. Passivate after repair per ASTM A967.

Selection Summary

1. Assess damage: visual, dimensional, DPI/MPI/UT, performance test
2. Classify damage: Class 1 (light) → grind; Class 2 (moderate) → weld or epoxy; Class 3 (heavy) → weld with engineering approval; Class 4 (severe) → replace
3. Verify repair is justified: cost <50% of replacement, no cracks, ≥75% wall remaining, performance loss <10%
4. Select repair method based on material, damage level, temperature, and service criticality
5. For weld repair: use qualified procedure, correct filler, pre-heat, PWHT if needed
6. For epoxy repair: verify chemical/temp compatibility, proper surface prep (SA 2.5), post-cure inspection
7. For casing repair: pressure-boundary qualification, hydrotest, cutwater profile verification
8. Post-repair NDT: DPI/MPI, UT thickness, hydrotest, balance check
9. Performance test after repair (if possible): verify head/efficiency within 5% of original
10. Address root cause of cavitation (NPSH, low flow, strainer, temperature) — repair will fail otherwise
11. Document all repairs with photos, measurements, NDT, and procedure records
12. For critical or high-value pumps, consider laser cladding for superior repair quality
13. Our stainless steel chemical pumps and API 610 OH2 process pumps feature low-NPSHr impellers and repairable designs