Time to failure 3

[engrom]

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?

 

[SJones]

API RP 579 and either some empirical corrosion rate data or an assumed rate based on published literature.

Steve Jones
Materials & Corrosion Engineer

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

 

[engrom]

Thanks Jones for that info. I am looking at a sort of built-in software or calculation sheet for such estimation. Appreciate if have any info on certain websites which give more knowledge on this.

 

[robsalv]

Er... no time would be my answer.

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.

 

[engrom]

I thank you for that....appreciate your reply, and be live on forum

 

[rorschach]

is this thickness at the bottom of a pit? if pitting is occurring, then there is no such animal as an estimate because there is no way to predict how fast a pit will propagate. If pitting is possible/occurring, then the theory of generallized corrosion allowances will fool you every time.

 

[stanier]

Apart from the interruption to the process I would have thought the cost of replacing the DN25 line would so small compared to the risk that you would do it!

 

[NickelMet]

If that SBP were in my refinery, we would recommend replacement due to the thin wall. IMHO, you really need to weigh the risk of the continued use. What is the cost to you should it fail versus the cost to replace?

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

 

[PAN]

rmsaj,
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.

 

[engrom]

Guys, Thanks for a live discussion on this.....appreciated. Let me clarify some more points, yes, we are planning for a permanent repair. The concern for not going for immediate repair option is due to high deferment on production, it could be planned during a shutdown which is supposedly in two months time. Time-to-failure need to be estimated for an overview, how long we can run it on risk. The defect is on a Process vapour line from power gas scrubber of a riser platform. Definitely, as PAN said I'm aware of the minimum allowable, but it's a matter of playing on razor's edge. Consequence of failure, apart of production loss, is low. Pressure retaining wall thickness for this is calculated to be 0.1mm. Advises are well taken, but I'm really looking at some sort of in-built software/calculation sheet to estimate such cases, by inputting factors in line with API RP579.

 

[stanier]

The problem with software or calculations is that you are on a razor's edge. The assumptions made or boundary conditions in any analytical software will impact the results.

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!

 

[robsalv]

Here's a suggestion for you Rmsaj, put a line rated clamp over the thin point so that you have secondary containment. If the thin point leaks then so what, it's clamped and contained. In Australia a mob called Furmanite design PV code compliant bolt on line clamps.

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...

 

[engrom]

Well said, robsalv.... No, not at all. I've made my recommendations and it is the management to decide. I have no risk or threat for sure. (Options like Plidco clamp and DML repair to be decided by Management). But, it is out my interest to know things, are there any such in-built software to predict such a risky condition? I am aware of TTF as per API, but has someone come across any such thing like a prediction table?

Ooops, dont want to carry more on my head....Risk to be taken by someone else!