"Corrosion under deposit in external side of exchanger CS tubes"

Question:

"I´m now working in Static Equiment Preventive Maintenance. One of the most important problem we have is pitting corrosion in the external face of CS tubes in tower water coolers ( normally when tower water goes in shell side ). I´m very interetes in get information about this tipe of corrosion caused by diferential aireation under the diposits in the external side of tubes if you have problems of fouling. Which is the most important factor in severity of the problem ( Tª, tower water components as Clorides, Oxigen concentration, deposit composition etc.?. What is the estimated life for this type of coolers ( with tower water in shell side a Tª above 60 ºC )?. Where I can information about this problem? We are trying to avoid or reduce this problem by chemical periodical ( every four or two year )cleanning in shellside, but it´s dangerous because if you have a damaged tube, with this acid treatment after cleanning it could leak in the start up of the equipment. What do you think about chemical cleannings as a preventive method to avoid corrosion under deposit? The solution we are determined in the worse cases ( vertical coolers we can´t cleanning ) is to change material tubes an use Cooper-Nicker Alloy."

Answer:

Shellside corrosion of carbon steel in cooling water service is a serious problem and one not easily solved. The problem is particularly severe in vertical exchangers where corrosion of carbon steel at the bottom tubesheet usually governs tube life. Exchanger tube life can be as short as 1-2 years. The two parameters most affecting tube life are deposits and high heat flux through the tube wall from elevated temperature tubeside fluids. Oxygen is secondary as all recirculating cooling tower water contains dissolved oxygen. Some design/operating modifications that have been tried to increase tube bundle life are to: (1) re-pipe the exchanger to place the cooling water on the tube side, (2) to provide flush ports at the tubesheet to try and keep the tubesheet free of deposits and, (3) retube with duplex stainless steels. Retubing with 304SS/316SS is not recommended due to the risk of stress corrosion cracking (scc) from chlorides in the cooling water. You don't say what type of acid you are using. It's my opinion that chemical cleaning only one or two times a year may not be enough, depending on the quality of the cooling water. Acids may eliminate calcarous deposits and iron oxides, but they will not remove sludge, organics, trash, etc. You may want to consider chemical cleaning combined with backflushing. As for the copper nickel tubes, they should definitely be an improvement compared with carbon steel. I still like duplex stainless steels for this application also, i.e., alloy 2205SS.

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