Corrosion allowance (again!!) 1

[carletes]

Dear all,

As far as know, corrosion allowance when using ASME BPVC must be defined by designer/purchaser according to their experience. But, is there any publication (more or less "official" or a recognized book) that recommends corrosion allowance depending on shell material, fluid contained etc?? Any good web site discussing this topic?

I am a young designer and perhaps don't have enough experience that code requires.

Thanks for any help

 

[deanc]

Try:

http://www.fitness4service.com

http://www.nace.org

Both good sites.

 

[jproj]

We design vessels to ASME Section VIII, Div. 1 and also Section 1. Minimum corrosion allowance we use for carbon steel is 1/16" and 0" for stainless steel (in water/steam service). Often our clients specify a higher corrosion allowance for safety factors. Highest I've ever designed for is 1/4" (not very common). Generally, if a client specifies added corrosion allowance, it's 1/8". As far as standards go, I don't have any references. HEI recommends 1/8" CA, but it is not a code, just a set of recommendations. The only NACE specifications I know about revolve around weld grinding.

Hope this helps

jproj

 

[MJCronin]

carletes,

My two cents worth....

There is nothing like an overly conservative corrosion allowance to drive a piping cost estimate through the roof. Any corrosion allowance determination must consider 1)the process use of the system 2) the intended system lifetime and 3) the exterior coating of the system.

For example, a corrosion allowance of "zero" is reasonable for a carbon steel nitrogen piping system, used indoors with a good exterior paint.

Similarly, a "zero" or minimum corrosion allowance would also be reasonable for a 98% sulfuric acid system, operating for only a few months with a design velocity below 3 feet per second.

My opinion only......

MJC

"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields

 

[workermonkey]

ooh i like that quote.

carletes,

i find myself in this same situation and the company i work for uses the same guidelines that jproj lays out. i design everything with a 1/16 corosion allowance but most of what i do is steam, water and oil.

 

[Sharik]

It used to be in the older ASME Section VIII, Div. 1 (<1992), there was a stipulation for corrosion allowance for carbon steel vessels (UCS-25) in air, steam or water service, less than 1/4&quot; thick. These had to have a corrosion allowance of 1/16&quot;. This 1/16&quot; would be added to the base metal and the total base metal could still be less then 1/4&quot;.

As far as I know, this is the only reference to a required corrosion allowance in ASME Section VIII, Div. 1 and this paragraph has even been removed from the later editions of the Code.

So since the Codes are telling you what you can't do, if there is no mention of a corrosion allowance then one isn't required (unless stipulated by the Owner).

I think the corrosion allowance has confused a lot of people in the past. I've seen where a vessel had corroded beyond it's given corrosion allowance and the Owner has initiated extensive and expensive weld buildup repairs.

But if the API 510/570 definition of corrosion allowance is used (nominial thickness - minimum required thickness) there is usually still a lot of metal to be corroded after the 'given' corrosion allowance is used up, before the minimum required thickness is reached.

It is unlikely a vessel is built to the design minimum thickness. Usually it is something like: minimum calculated required thickness = 0.803&quot;, nominal thickness used = 1&quot;. So already there is a built in corrosion allowance of 1&quot; - 0.803&quot; = 0.197&quot;.

For errosive or corrosive services, extra material (corrosion allowance) is designed into the vessel (especially where it would be much to expensive to use a more exotic material) or the use of clad or lined material is considered.

When we are designing mew equipment, we take the metal loss histories of similar existing equipment in similar service conditions and try to balance the cost of the extra material for the required metal loss rates for an acceptable run-life time to the initial cost of the added extra material or other exotic material.

 

[LSThill]

CORROSION ALLOWANCE
Corrosion allowance listed below shall be applied in pipe wall thickness calculations for all cases (full flange rated specifications or on the basis of line conditions [Limited by Pipe]):

Material Services NPS NPS
<1-1/2 >2

All Line Pipe 0 0
High Alloy – Class 150 or Less 0.031 0.031
General Services 0.050 0.063
Corrosive/Abrasive 0.125 0.187


lst
THE ABOVE SHALLL BE SUBJECT TO THE pURCHASER REVIEW.