Corrosion Allowances for Equipment and Pipes

it depends on many factors.. it would be necessary to know also the type of fluid and chemical composition.. one possible reason could be that youi're injecting corrosion inhibitor and usually the inhibitor efficiency if vessel is lower than in piping so this can justify the difference in corrosion allowance or it could be that in the vessel there coulb be a free water phase that separate from the fluid..

The calculation of corrosion allowance in piping and equipment is the same, you've to multiply the corrosion rate ( mm/y) times the design life (y) and you've the corrosion allowance.

http://www.mesteel.com/cgi-bin/w3-msql/goto.htm?url=http://www.mesteel.com/info/metal_calculators/Corrosion_Allowance_Calculator.htm

S

corrosion control & prevention

2 thoughts:

1. it is a lot easier to change out piping when the corrosion allowance is gone, so maybe that is why the amount is lower.
2. Maybe someone had some forsight and saw that a 6mm CA on the piping would take it into the next higher schedule, ie. from sch 40 to sch 80 and they didn't want to to that.

2.5 you can also have flow accelerated corrosion which could possibly cause greater corrosion in the piping, in which case you have an interesting situation.

When we start design of any system such as an equipment or a pipeline, we have to decide from the 1^st stage of design how much the depreciable/design life of that system. May be the designer decides that the life of such a vessel is 40 years, and the life of the pipeline is only 20 years only. So depending on type of material, type of fluid, its temperature, and its concentration we can define exactly the corrosion rate, and by determining/designing the depreciable/design life we can define the corrosion allowance to be used.

Corrosion Allowance (mm) = Corrosion Rate (mm/year) x Depreciable or Design Life (year)

And as we know theoretically that the corrosion rate of such material like CS must be the same, but this is not correct at all. This will depend on a lot of parameters like heat treatment and carbon & alloy contents. For example, we have a pressure vessel constructed from CS ASME 516 Grade 70, and we fabricated its internal from CS Steel 37, after 5 years in operation we found that the Steel 37 have bee rusted and CS ASME 516 Grade 70 still OK.

See the following CR4 Threads: Corrosion Allowance & Re: Pipe schedule calculation

That is a GA I think, you do explain this stuff well my friend.

I sometimes add to the corrosion rate on certain tanks, vessels and piping that I know will have a high corrosion/erosion rate. I have 15 large pressure vessels that are used to digest wood chips into pulp that have a high corrosion and erosion rate. The vessels start out with a 2" wall thickness but nearly 1-1/4" of the thickness is corrosion allowance. On average these vessels loose around .100" thickness per year due to corrosion from the high caustic mixture that is used to break down the wood chips. The erosion from the wood fibers and dirt that I was in the chips being blown through a 12" nozzle when the cook is done and the pulp sent to a holding tank also adds to the wear rates.. I usually add another 1/8-3/16" extra corrosion allowance to the lower 2 shell sections because this is a higher wear area than the top sections.

I have had good service from cladding these vessels with E312 S/S corrosion resistant weld overlay and this might be an option for anyone who wants to add some type of corrosion protection to an existing vessel. This is in many ways better than adding a corrosion allowance in the design of a new vessel. I say that because by just adding a corrosion allowance to the vessel will not give you the benefits of overlaying the vessel with a material selected because it has better resistance to the corrosion caused by the particular service of the vessel. Also by adding a corrosion resistant overlay after the vessel has run for years may be easier on the finances because it will lower the cost of the vessel when it is built and add years to the life of the vessel.

Thank you my friend pipewelder,

It is a good effort to protect your tanks and vessels by corrosion resistant weld overlay which used for vessels already constructed.

Also, in your case you can use lining (not cladding) by welding thin sheets 2-3 mm into base metal. Cladding is a metal joining process which used for producing a plates like as carbon steel as a base metal to be cladded with corrosion resistant clad metal, and this process shall be carried out at the mill. Then we used these plates to fabricate the vessel. The ASME code covers that branch of cladding by specifications ASME SA-263, SA-264 and SA-265.

One additional point to consider is the operational service. For example, a storage tank with valve and piping outlet. Storage tank may hold corrosive material for a longer period of time and the piping only sees the corrosive material when the tank is drained. Exposure time in the vessel may be longer, hence greater CA.