corrosion allowance

By MBSims

Date 01-28-2012 01:25

NIL = zero, per Webster's dictionary.

By G.S.Crisi (****)

Date 01-31-2012 18:06

Curious question: what Code or Standard has the vessel been made to? ASME VIII? AD Merkblatter? Someother one?
Giovanni S. Crisi
Sao Paulo - Brazil

By Joey

Date 02-06-2012 05:58

Prof Crisi

Curious questions : In your experience as an Engineer, have you encountered a vessel head /or shell without the corrosion allowance?
Meaning, the actual thk of material provided is equivalent to the calculated minimum required thickness.
Imagine, without corrosion allowance, when your actual thickness corroded below the min required thickness, your vessel remaining life is zero.
Derating below original design conditions must be done to continue to operate the vessel.

~Joey~

By ozniek

Date 02-07-2012 10:46

Hi Joey

In the case of corosion resistant alloys (Nickel; Stainless Steel etc) the corrosion allowance is often zero. In the case of carbon steel, this would be very unusual.

Regards
Niekie

By G.S.Crisi (****)

Date 02-07-2012 18:56

Joey,
In my 35 years of project engineering experience, more than once I've seen a carbon steel vessel with zero corrosion allowance specified. This, as Niekie says, is unusual, but I've seen it more than a couple of times.
When is this so? When the fluid contained in the vessel isn't corrosive to carbon steel. Hydrocarbons are an example. Attention though! The non corrosive property must be not only stated on engineering handbooks and corrosion manuals, but confirmed by actual experience.
Corrosion tables that are commonly found on books are often based on laboratory tests and don't reflect what happens actually in industry. Example: a laboratory test consists in dipping a specimen into a corrosive medium and leaving it there for a number of days, i.e., the conditions are static. Conditions in industry are very often dynamics, i.e., the fluid moves into a pipe or an equipment. Corrosive properties of a fluid are different when it's static than when it's moving. In the latter case, of course, corrosion is stronger.
Giovanni S. Crisi

By Joey

Date 02-08-2012 12:43

Prof Crisi

I still need to eat more rice before I can level your long experience :roll:

.

I have reviewed hundreds of PV design calculations when I'm doing auditing of statutory equipment in Smokey. I've seen CA=0 ; CA=1.5mm ; CA=3.0mm. But I have not encountered a newly fabricated PV having an actual thickness (nominal thk) equivalent to the calculated minimimum required thk. Even during the time that I'm Technician doing ultrasonic thickness gauging.

As an Engineer, when your calculation shows that you only need a min thickness of 0.50mm to operate certain pressure of PV, will you not allow using a 2mm thk material...when that is the closest thickness available in the market?

Yes, in paper it was specified CA =0... but the excess thk on actual material used can be considered as CA also. Please correct me if I'm wrong.

~Joey~

By G.S.Crisi (****)

Date 02-08-2012 19:27

Joey,
First comment. As an American, you think in inches, not millimeters. 0.5 millimeters is just a little bit more than 1/64 inches, so there are no pressure vessels 0.5 mm thick.

Second comment. Brazil is a metric country, but for reasons too long to explain now pipes, shapes and plates are made, calculated and used in inches, like in the USA. Regular plate sizes rolled here by steel mills are 1/8, 3/16, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, 1, 1.1/4 and 1.1/2.

Third comment. Let's suppose that I'm calculating a pressure vessel. Corrosion allowance is zero. The calculations end to a thickness of 0.69 inches. 0.69 inches is not a regular size. The next upper thickness is 0.75 or 3/4 inches. Of course, I'll chose a plate 3/4 inches thick. The extra, not needed thickness is due to the fact that the calculated thickness fell between two regular sizes, and I must select the upper one.
Now, let's suppose that the vessel has a corrosion allowance of 1/8 or 0.125 inches. 0.69 (uncorroded) + 0.125 (corrosion allowance) = 0.815 inches. The 3/4 inches plate is not suitable any more, and I must go to a 7/8, or 0.875 plate.

Giovanni S. Crisi

By Joey

Date 02-09-2012 12:29

Prof Crisi

First comment : I'm Pinoy and not American, (mm) is now included in ASME codes. The dimensions given in my earlier reply are examples only. However, if you wish to argue that there are no such dimensions in PV, well I can give you some clues on where you will encounter such dimensions.... :lol:

equipment tag starting with "E"....you normally do some expansion at the end to have 100% bonding....so is this not part of pressure vessel?

3rd comment : You are correct to conclude that the nominal thickness is not equal to the calculated minimum required thickness.

Have a nice day.

~Joey~: :lol:

By G.S.Crisi (****)

Date 02-10-2012 16:56 Edited 02-10-2012 19:17

I don't wish to argue anything with anybody. I'll clear up what I wanted to say.
On your former posting you said that " ...... when your calculation shows that you only need a minimum thickness of 0.50 mm to operate certain pressure of a pressure vessel ......". At reading this phrase, I thought that you were speaking of the wall thickness of the vessel.

I though also that, being an American, you were not familiar with metric units. Far from arguing, I wished to explain you that it is impossible that a pressure vessel has 0.5 mm (or 1/64 inches) wall thickness. My intention was not to be offensive. When one's not familiar with a certain unit, he can incur in such kind of mistakes. Example: Britishmen use (or at least used) to measure the weight of a person in "stones". I have no idea of how heavy a stone is. If someone says me that I'm one stone heavy, and another one says that I'm 100 stones heavy, I won't know who's right.
I hope I've cleared up this misunderstanding.

I've learned now that you're not American. I confess my ignorance: what country are Pinoys native from?

Giovanni S. Crisi

By CWI555

Date 02-13-2012 16:31

Pinoy = Phillipines.

By ozniek

Date 02-09-2012 12:15

Hi Joey

I think this is a matter of definitions. The general definition for a corrosion allowance is the thickness that the designer has allowed for degradation during the life of the vessel. So, typically for carbon steel vessels this may be 3mm. As you mentioned, due to real world economics, different components may have more "thickness above minimum design thickness required" than the corrosion allowance, but you would not say that the corrosion allowance is 3.6mm for the shell, 4.2mm for nozzle A, 6.6mm for nozzle B, 3.1mm for the knuckle of the heads, 7.2mm for the flange of the heads etc.. The CA is given as 3mm, because that is what the designer has alloted for corrosion during the design life of the vessel. That is the "definition" of corrosion allowance.

If the corrosion at some stage exceeds the 3mm allowed, then the designers will again look at the minimum thickness required for pressure retention (and other stresses) and specify this thickness as the minimum thickness for the particular component (e.g. a small bore nozzle) thus allowing further use of the vessel without repair.

Getting back to the original issue regarding the zero CA. This is what the designer has allowed for corrosion, (Nothing) because s/he does not expect any general corrosion mechanism to operate on the vessel. Obviously some of the individual vessel components will be thicker than the minimum design thickness required, but that does not automatically change the corrosion allowance.

Regards
Niekie

By dlmann

Date 02-07-2012 04:29

Manufactured to ASME VIII, Div. I - 2007 edition

By Joey

Date 02-06-2012 06:06

Donnie Mann

You may conduct UT measurement and compare the readings against the manufacture's design data.
The extra thickness = manufacturer's allowance = corrosion allowance.

~Joey~