Time to failure
Time to failure
(OP)
Folks, could anyone advise any software or calculation sheet to identify time to failure. One of the small bore piping (DN25) with nominal wall thickness 4.55mm now at one point has min. remaining wall thickness as 0.5mm (API 5L GRADE B, piping class 1141) and the vessel to which piping is attached has a working pressure 860kPa, Design pressure 1034 kPa. External corrosion pitting on this piping is active. How can I calculate what is the minimum time period I can operate the vessel before piping failure?





RE: Time to failure
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Time to failure
RE: Time to failure
I wouldn't rely on 0.5mm of wall?? With active external corrosion how could it be accurately measured? RT is not accurate enough.
What's the corrosion rate? At best it's (4.55 - 0.5) / years of service = X mm/yr. Too many factors could influence this though.
Remaining thickness divided by corrosion rate/yr = time to failure. However it's likely to be less than this given that at some minimum thickness the metal will no longer resist pressure.
API RP579 will allow you to work out the remaining equivalent strength based on the loss area, material, corrosion allowance etc etc.
RE: Time to failure
RE: Time to failure
RE: Time to failure
RE: Time to failure
Since you state it is an API line class, are you using API 570 (Piping Inspection) to evaluate the line? There are calculations in that standard that can give you minimum allowable pressure, minimum wall thickness, corrosion rates, and remaining life.
~NiM
RE: Time to failure
The retiring thickness of your pressure piping system should be controlled. For an example, the measured thickness of pipe size 1" should not be less than 2.0 mm. Otherwise, I will replace that section. This may be conservative idea for you. However, it works in my company.
I do not understand about the meaning of "piping class 1141". Is this your inhouse piping class? Please clarify.
RE: Time to failure
RE: Time to failure
If you are honest with yourself this is all about money. If you shutdown the process it will cost $X/day. A temporary repair will cost $Y. A permanent repair will cost $Z. DO the numbers.
In the meantime have the permanent replacement of pipeline installed ready for tie in when the shutdown comes. And pray a lot!
RE: Time to failure
I'm assuming your in a technical plant support role. If yes, it's not your job to accept the risk of the line popping, it is your management's job. You are a technical person whose role is to maintain pressure containment integrity. There is no way you can reliably predict containment performance of a dangerously thin pit - and if you were to do so, you'd be going out on a limb that would not be supported by any code, best practice, or regulatory jurisdiction.
Recommend that your organisation fit the secondary containment, and if they dont and it leaks, then at least you can say hand on heart you made the right recommendation but were not supported by the organisation.
Geez, I think I woke up on the wrong side of the bed this morning...
RE: Time to failure
Ooops, dont want to carry more on my head....Risk to be taken by someone else!