Perforated pump casing on a brackish-water cooling line
A seafood processing plant on the Tamil Nadu coast runs a 37 kW, 1450 rpm centrifugal pump delivering 90 m³/h of brackish cooling water at about 18,000 ppm chloride, with sand drawn in from an open sump. The casing is grey cast iron FG 200. After seven months of service the volute has perforated near the cutwater, the impeller vane leading edges have lost about 3 mm of metal in a smooth, scoured pattern, and a white powdery deposit fills the low spots. There is no red rust anywhere on the wetted surfaces. The plant engineer must restore the pump during the next annual shutdown on a budget of about ₹85,000 and cannot alter the pump hydraulics or the pipeline layout. The store holds a spare FG 200 casing and a used SS 316 casing taken off a decommissioned line; the existing shaft is 410 stainless steel running in a packed stuffing box.
1. Name the two damage mechanisms acting on the casing and impeller, and cite the specific evidence from the description that supports each. Which commonly blamed mechanism can you largely rule out here, and why?
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Mechanism 1 - erosion by suspended sand. The impeller leading edges have lost 3 mm in a smooth, scoured pattern. Metal loss concentrated where the relative velocity and impact angle are highest, with no corrosion product in the pits, is the signature of abrasive erosion. The open sump means grit is continuously supplied.
Mechanism 2 - graphitic corrosion (selective leaching) of cast iron in chloride water. Grey cast iron in high-chloride, low-velocity pockets loses iron selectively, leaving a soft graphite network that holds the original shape until it is mechanically weak. Evidence: perforation at the cutwater, a white deposit (hardness scale and corrosion salts) sitting in low spots, and no red rust - a rust-coloured surface would be expected if general oxidation dominated.
The two act together: erosion scours away the protective scale so fresh iron is exposed to chloride attack, while the graphite residue is easily washed out.
Largely ruled out - cavitation. Cavitation damage is a dense, sponge-like pitting over a pressure-recovery zone, and it would normally be reported together with noise and vibration. The pattern here is directional and smooth, so cavitation is at most a minor contributor, not the primary cause. Note also that the cutwater region is where local velocity is highest, which fits flow-accelerated erosion-corrosion rather than vapour cavitation.
2. Choose between the spare FG 200 casing and the used SS 316 casing, and justify the choice on corrosion, erosion and budget grounds within ₹85,000. If procurement were allowed, which option would you put forward instead?
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Refitting FG 200 is not acceptable as a straight swap. The failed casing lasted only 7 months with the same water and the same sand. FG 200 has no useful resistance to chlorides and its graphite structure makes selective leaching inevitable. A repeat failure would occur within a similar interval, and the ₹85,000 budget would be consumed again next year, with the added cost of a shutdown.
SS 316 is better but not a clean solution. The molybdenum addition raises resistance to chloride pitting, and the alloy avoids graphitic corrosion, so the perforation mechanism is eliminated. However, 316 is not a seawater alloy: at 18,000 ppm chloride, with warm water and stagnant crevices behind the wear rings and at the stuffing box, pitting and crevice attack are likely. Erosion by sand will also continue because 316 is soft (about 150–200 HV), though it will erode more evenly than cast iron.
Judgement: take the used SS 316 casing as the interim repair because it is already in store, costs only fitting labour and machining of the fits, and removes the dominant mechanism at far below ₹85,000 - but treat it as a monitored interim, with wall-thickness checks at the cutwater every quarter.
Better engineering option: an FG 200 casing with a bonded rubber or polyurethane lining (about 3–6 mm) and a polyurethane-coated or bronze impeller. This addresses erosion and chloride corrosion simultaneously, keeps the existing bolt pattern and hydraulics, and typically costs less than a new 316 or duplex casing.
3. The SS 316 casing is fitted onto the existing 410 stainless shaft with its packed stuffing box. Identify the new risk this creates, and give two workshop-level countermeasures.
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The risk is galvanic (dissimilar-metal) corrosion of the shaft. SS 316 is noble relative to 410 martensitic stainless, and the potential difference in brackish water is of the order of 100–200 mV. The critical point is the area ratio: the casing is a large cathode and the exposed shaft under the packing is a small anode, so all the galvanic current concentrates on a small area. The 410 shaft will pit and groove just under the packing rings, and the packings will then leak and cut a wear groove, leading to shaft breakage rather than a slow leak. The graphite packing itself, if graphitised or graphite-impregnated, adds another cathodic couple.
Countermeasure 1 - break the metallic contact. Fit a bronze or SS 316 shaft sleeve under the packing so the running surface is close to the casing in potential, insulate the sleeve/gland where practical, and use a non-graphitic packing (PTFE or aramid-based) so the couple is not reinforced.
Countermeasure 2 - protect the anodic area. Fit two or three zinc or aluminium sacrificial anodes (or a small impressed-current unit) inside the casing, sized on wetted area, and inspect them at every packing change. Complement this with planned condition monitoring: measure shaft diameter under the packing at each overhaul, check anode consumption, and record casing wall thickness at the cutwater. Also keep the pump running rather than idling in brackish water, since stagnation promotes crevice attack.