Corrosion allowance 4

[0707]

Piping designer, based on service corrosivity, adds corrosion allowances to the calculated pipe thickness. In a new oil refinery construction project, which would be typical corrosion allowances?

Is there any rule to be followed?

Thanks

Luis

 

[EdStainless]

In service where there is experience of uniform general corrosion the corrosion allowance will be based on the corrosion rate, expected service life and reasonable saftey factors.
In other words, it all depends. I am sure that some will come along and give you some specific examples.
The most important step is the first one, proof that you will see uniform general corrosion. Without that a corrosion allowance does not improve life or saftey.

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[0707]

For Carbon Steel piping in refinery service, a typical C.A. assumed and given will be = 1.5 mm (1/16"). In some extreme cases, a C.A. of 1/8" or 3.2 mm is given (but not as a general rule). For high temperature services on bottom cuts of corrosive hydrocarbon services should we use 6.35mm corrosion allowances?
From my search on this subject I suppose that there is no rule or recommended practice and the things are a little bit established ad hoc empirical based on licensors experience.
I think that actually in a new refinery project construction a RBI approach should be made to define expectable corrosion rates then based on those expectable corrosion rates designer should add corrosion allowances with the following criteria: high corrosion rates high corrosion allowance, medium corrosion rates medium corrosion allowance, low corrosion rates low corrosion allowances.
Thanks for you sharing
Luis

 

[unclesyd]

In the chemical business several other factors also enter into the CA equation.
One in particular is will the corrosion products, say iron contaminate the process. If iron will contaminate your process you have to select a material that will not corrode and contaminate the process.

Where you have highly corrosive materials you may have to set corrosion rate limits on different materials based on measured corrosion. In one service we used max rate of 0.006 IPY on SS piping with a 0.006 IPY CA. We slayed in the process of continual evaluation of the piping systems and at same time evaluating other materials. Other services we had 0.018 IPY on CS with 0.125" CA.

With the cost of material and cost of product these days designing a system and giving a CA requires a lot more study and evaluation than just a few years ago.

 

[SJones]

http://www.eng-tips.com/viewthread.cfm?qid=130588

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[ggrindle]

Interesting question.

Because for something with only an expected life of say 1 day....all you have to worry about is the immediate stresses...i.e pressures & mechanical stresses.

If something is designed with a 30 year life then the corrosion allowance needs to be for 30 years of corrosion wall loss.

If something was designed for 30 years of life and you are in year 29 then the remaining corrosion allow can be minimal.

This is something that I struggle with all the time. We have engineers at our plant who look at something that has been in service for 25 years and do a new installation calculation and determine that it does not have sufficient wall left. But when I look at their calculations it is because they are including 30 years of corrosion allowance. When their calculations are re-done to have only 5 years of corrosion allowance then there is still suffient wall left to operate.

There is an ASME course on "fitness for continuted operation" that sort of covers this.

 

[SJones]

Assuming linear wall loss rate and no change in operating conditions that have had a positive (or negative) effect on the corrosion process. Generally, it doesn't happen in a straight line which is why corrosion monitoring is performed.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[stanweld]

0707,
We often specified 1/4" corrosion allowance for specific piping in the Sulfur Recovery Systems based on Owner desires to mitigate initial capital expenses. We always knew that these systems would probably not last 10-years and would actually be lucky to last 5-years. As a result of high maintenance costs incurred, many of those sytems have been replaced with more corrosion resistant alloys.
In essence, whenever we applied a 1/4-inch corrosion allowance, expected service life was always short.

 

[rorschach]

There is one thing you need to remember about corrosion allowance. it is based on the general corrosion mechanism ONLY. If erosion corrosion or pitting is possible, then all bets are off when it comes to corrosion allowances. ESPECIALLY when pitting is a possibility. It is VERY important to keep that firmly in mind.

 

[rconner]

I believe a gentleman named Allen Hazen, reportedly a brilliant chemistry student at MIT in 1888 and I think also later of some not insignificant, worldwide engineering repute, is credited with the following statement nearly a century ago (reportedly commenting specifically with regard to pitting of steel not protected with a cement or cement mortar coating/lining):

“It is my feeling that it will not generally pay to increase the thickness of steel plates very greatly because of this consideration (viz., that thickening the plates will not cure the trouble but will merely prolong the life of the metal), but that the money will be better spent in better coating and in more careful inspection of the steel plates, or, in other words, by preventing the pitting instead of trying to make the plate thick enough so that the pitting will not go through it.”

While it is certainly possible Mr. Hazen did not have intimate experience with the quite specific applications discussed thus far in this thread, I think his sort of dual-pronged comments are perhaps some interesting nonetheless.

 

[zdas04]

It has been my experience with a number of clients in Oil & Gas that whenever there is a CA established by policy that is to be used in all design calculations it is based on fear and superstition. Companies get a warm and fuzzy feeling that if they specify a 1/16 inch CA (per policy) then the pipe will last forever. I've rarely seen anyone even think to question annual corrosion rates to see what the projected life of the set-in-stone CA will be in a particular projet.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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