A polyethylene unit experienced an ethylene gas leak within a high pressure tube and shell heat exchanger resulting from water side corrosion of UNS N04400 (Monel Alloy) tubes. To address historical water side exchanger fouling issues, the cooling tower was transitioned to a polymeric dispersant program capable of handling higher stressed chemistry. A failure analysis revealed that tube wall loss was localized to areas beneath residual deposits that had accumulated at baffle plates. This paper discusses the conditions resulting in an oxidizing salt driven under-deposit corrosion mechanism of UNS N04400 tubes in cooling tower water service. Preventative measures and improvements in inspection and cleaning procedures are discussed.
Polyethylene units polymerize ethylene gas under pressure with organic peroxides as an initiator; however a percentage of ethylene does not convert to polymer. Unreacted ethylene gas is diverted through a high pressure recycle loop that separates residual waxes from the gas before reintroducing to the main process stream. A DuPont Polyethylene Unit uses a series of vertical high pressure tube and shell heat exchangers (2,700 psig) with cooling tower water (shell side) to condense waxes from the recirculated process feed. Process enters the tubeside of the heat exchangers at 450°F, resulting in high tube metal temperatures. Nickel Alloy 400 (UNS N04400) tubes were specified to minimize the potential for water side corrosion in these high heat [flux] exchangers. However, high tube metal temperatures could still lead to scale formation and fouling from solids in the cooling tower water.
Historically, during the first eight years of operation, the plant had experienced fouling issues leading to loss of heat transfer which was addressed through routine acid chemical cleaning of the water side. To improve heat transfer efficiency and eliminate the need for chemical cleaning, a Stress Tolerant Polymer (STP) dispersant was added to the water treatment plan to prevent deposition. Although heat transfer issues and the need for chemical cleaning were eliminated, routine eddy current inspections of the heat exchanger tubes began to reveal localized wall loss on the outside diameter (water side). In one case, a through-wall leak developed resulting in ethylene gas entering the cooling water system. The total time in service at that point was 14 years.