Corrosion Allowance - Very Basic Questions 4

[ChEMatt]

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

 

[vesselfab]

depends on how much corrosion is anticipated, depending on service, during the service life of the vessel.

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.

 

[SJones]

http://www.eng-tips.com/gsearch.cfm...&ie=ISO-8859-1&q=corrosion+allowance&siteurl=

http://www.eng-tips.com/viewthread.cfm?qid=279142&page=1#1170

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[moltenmetal]

Corrosion service conditions, anticipated lifetime etc. all go into this decision.

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.

 

[MJCronin]

My experience has been a bit different.

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

 

[roca]

Internal Corrosion allowance is normally set by the client (end user) requirements/specs or the engineering contractors process/metallurgy dept based on corrosion calcs/reports and obviously depends on the process involved and vessel lifetime required.
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.

 

[stanier]

You also need to consider the ongoing NDE and maintenance. Is the corrosion inhibitor system fail safe? Are corrosion coupons used to monitor corrosion and thus effectiveness of any chemical dosing. What level of integrity is there in the control system to monitor process upsets? Is the feed stock going to change such as low to high sulphur crude or coal?

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.

 

[ChEMatt]

This particular application is a shell and tube exchanger, with a potentially corrosive liquid on the tube side (so the tube side is SS), and boiler feed water on the shell side.

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

 

[muld0020]

HX's with boiler feedwater typically uses 1/16" CA on CS.

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.

 

[DSB123]

I've known where the Client /User would not provide the expected corrosion rate for his product and there were no figures available. The original pipework was thick walled cast iron but was to be changed to stainless steel during bidding. Subsequently the product was found to be carbonic acid which was also high in chlorides at around 100 Deg C. The EPC company was asked for corrosion rates as was the Client but no-one could provide. This being the case we opted for 9mm corrosion allowance and went for Carbon Steel since the Company would not go for Duplex due to costs.

As regards nozzle sizes - I personally would not use anything less than 2"nb for mechanical strength purposes alone.

 

[sheiko]

Material selection is done on the basis of the calculated corrosion rate, i.e loss of thickness.
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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