Corrosion Allowance - Very Basic Questions
Corrosion Allowance - Very Basic Questions
(OP)
When designing a new pressure vessel, what determines whether or not one should specify a corrosion allowance? And how much should that allowance be?
Onwards,
Matt





RE: Corrosion Allowance - Very Basic Questions
seen very clean service specifiy 0.125" CA.
seen medium service specify 0.25" CA
It pretty much depends on the owner/user's process, process engineer, corrosion engineer viewpoint and as always, the users specifications.
RE: Corrosion Allowance - Very Basic Questions
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: Corrosion Allowance - Very Basic Questions
Watch out for small nozzles- even a very small corrosion allowance on these can result in nozzles of impossible or ridiculous wall thickness. This is one reason amongst many that 2" is a minimum nozzle size for CS vessels in many owner specs.
RE: Corrosion Allowance - Very Basic Questions
With stainless steels and other corrosion resistant alloys, chosen to perform because of the corrosive service....I suggest 0.0"
All carbon steels in almost all other srevices 0.0625"
Sulfuric acid, several other acids - 0.125"
Condensate return piping -- back to the boiler - always schedule 80.
Obviously, a lot of judgement and subjectivity comes into play
RE: Corrosion Allowance - Very Basic Questions
Corrosion allowance is added in most cases to fixed and removable carbon steel vessel internals.
External vessel corrosion allowance is also sometimes specified / required by the end user.
Corrosion allowance is also sometimes specified / required by the end user on support legs / skirts / saddles.
RE: Corrosion Allowance - Very Basic Questions
In designing a plant or pipeline you do not start out with the premise that it is going to corrode X amount in Y years unless you are very certain of your process conditions.
RE: Corrosion Allowance - Very Basic Questions
I specified that the tube side have zero corrosion allowance, and the shell side 1/8" as it is carbon steel.
Thanks for the responses guys, very helpful!
Onwards,
Matt
RE: Corrosion Allowance - Very Basic Questions
Since this is a heat exchanger you should specify what standard should be used such as TEMA C. TEMA R is for process exchangers and has the 1/8" CA on Carbon Steel and 0" on alloy. Typically steam/boiler feedwater exchangers are TEMA C which is 1/16".
If you specify the standard you would have recieved the correct CA. However, it is safe side to increase the CA it just costs more.
RE: Corrosion Allowance - Very Basic Questions
As regards nozzle sizes - I personally would not use anything less than 2"nb for mechanical strength purposes alone.
RE: Corrosion Allowance - Very Basic Questions
If such loss is only a few mm, then ordinary carbon steel is selected, with an increased thickness, called a corrosion allowance, typically up to 6 mm only.
If the corrosion rate is high, a corrosion resistant alloy steel must be selected instead, such as stainless steel.
In some cases, it is possible to inhibit corrosion by injecting a chemical, called corrosion inhibitor, to decrease the corrosion rate. In such cases the pipes can remain in carbon steel but adequate corrosion monitoring, usually by means of weight loss coupons and corrosion probes, must be put in place to ensure inhibition is effective.
Carbon steel is not suitable where clean service is required and must be galvanized to ensure cleanliness.
Material selection is specified by the Material and Corrosion Engineer and shown on the Material Selection Diagrams.
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