Use of inert gas to stop acid corrosion in cargo tank vapour space 1

[xltsport]

I'd like some feedback and experience from all the marine/materials/corrosion engineers on the positive effects of the use of inert gas - either from boiler flue gas systems or from independent inert gas generators using MGO - to prevent or significantly slow down the rate of corrosion of the steel in the upper vapour space of uncoated cargo tanks carrying heated fuel oil cargoes with a sulphur content of ~3.5%.
This is specifically related to vessels operating permanently in the Arabian Gulf where peak daytime deck steel temperatures can reach 70 degrees+ and in night time can drop to 30 degrees, with air humidity ranging from 40 ~ 95%(day/night)

 

[navarch1]

Unfortunately, some types of inert gas systems actually increase the rate of corrosion in vapor spaces. The by-products of combustion in flue-gas type inert gas systems combined with higher temperatures in the area, can create a problem with acids, chemicals, etc. that can also harm the steel. Nitrogen systems are the best bet if you want to minimize corrosion in this area, however, in most of the barges we do, when we do not coat the entire cargo tank we at least use tank coatings on the upper 10% of the cargo tank and the deck overhead. (coating the vapor space)

 

[MASE81]

Navarch1 is exactly correct.
Corrosion in the main deck in the cargo block is amplified by the corrosive by-products of combustion from the boiler flue gas. Additionally, when the gas is "scubbed" it gains some water content as well. Add a bit of heat and you have an excellent environment for corrosion. You need only examine the inside of an Inert Gas line, or Deck seal with damaged coating to find out quickly it can eat through steel. I've seen Stainless steel nonreturn valves "disappear" .
Most modern tankers have tops coated for a distance of 1-2 meters down...i.e the typical vapor space, at the new construction phase. For an existing tanker to "recoat" the deck involves extensive time out of service in the yard, scaffolding the entire cargo block, etc.. so its expensive and not practical.
Similarly a nitrogen generator system large enough to service an operational crude carrier would be cost prohibitive as well. Unless you are doing some sort of life extension, conversion, or you bought the vessel for a song, Tanker economics typically cant justify it.
Better to wait until the steel needs replacement.
Suggest you conduct some UT surveys to determine exactly how extensive your particular problem is. Your class society surveyor will be glad to assist.

Rgds

 

[mbridger]

The only exception to tha above two comments is by use of an inert gas generator which utilises the reverse osmosis principle to produce pure nitrogen (in stages from 5%O2 downwards) This may would displace a lot of the H2s however further boil off from cargo could mix with the N2 but I cannot recall what the reaction would be without oxygen present if any???

These systems are used on offshore storage and production vessels.

 

[ssreese1500]

Good point about the osmosis type systems, but i do know they are expensive and i have no idea what the maintenance would be like.

If i were you, I would price out a reverse osmosis system and forget a burner type. Then do a cost benefit versus estimates from your paint rep to coat the upper 10%. Its not going to be cheap i assure you. I recently did the upper 10% of a very large tank, and after staging, blasting, grit removal, etc., the cost was close to US $200,000

From the paint side, figure you can get 8-10 years if the coating is applied correctly. As far as just waiting for the steel to diminuate, my opinion is that is a bad idea. Coatings and Prevention are almost without exception ALWAYS cheaper than steel replacement if you are planning on keeping this vessel for a long time. Besides, you dont have your friendly surveyor taking you out of service when you least expect it, or God forbid have a spill.