A semi-welded plate heat exchanger supplied as a technical replacement equivalent to ALFA LAVAL MK15BW was installed for steam heating of vegetable oil.
Hot Side – Steam
• Inlet: 140°C
• Outlet: 137°C
Cold Side – Vegetable Oil (90%)
• Inlet: 40°C
• Outlet: 80°C
Heat Duty: 1,093 kW
Plate Material: AISI 316
Design Pressure: 16 bar
Design Temperature: 140°C
The semi-welded plate heat exchanger was selected to improve sealing reliability in oil service.
After a period of operation, the unit developed internal leakage.
Inspection revealed:
• Multiple deep pitting cavities
• Localized corrosion spots
• No weld seam cracking
• No mechanical damage
• No manufacturing defect
The corrosion morphology is consistent with chloride-induced pitting corrosion of stainless steel 316.
Semi-welded plate heat exchangers provide:
• Improved safety
• Reduced cross-contamination risk
• Higher pressure stability
However:
Semi-welded construction does not increase corrosion resistance.
Corrosion resistance depends solely on alloy composition and chemical environment.
Under standard conditions, refined vegetable oil:
• Contains negligible chloride
• Has low water content
• Does not typically cause pitting corrosion
Pitting corrosion in 316 requires:
• Chloride presence
• Elevated temperature
• Moisture or water phase
• Deposit formation
Therefore, the corrosion indicates that the actual process medium contained aggressive components beyond normal vegetable oil characteristics.
Cold side outlet temperature: 80°C
Chloride pitting susceptibility increases significantly above 60°C.
At elevated temperatures:
• Passive film stability decreases
• Pitting initiation potential drops
• Corrosion propagation accelerates
Temperature acts as a corrosion multiplier.
The failure mechanism is:
Chloride-induced localized pitting corrosion of AISI 316 plates under elevated temperature conditions.
The aggressiveness of the actual operating medium exceeded the corrosion resistance limit of 316 stainless steel.
When selecting materials for semi-welded plate heat exchangers in oil heating applications:
✔ Verify chloride concentration
✔ Confirm water contamination risk
✔ Consider worst-case chemical scenarios
✔ Evaluate operating temperature impact
If chloride uncertainty exists, consider:
• Duplex 2205
• 254SMO
• Titanium
Material upgrade during design is significantly less costly than premature failure.
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