Corrosion allowance vs. Mill tolerance (whom to add first in calculati 3

[Meck91]

Folks,

I tried to calculate pipe wall thickness as per ASME B31.1 Para. 304.1.2 & the “Pressure design thickness (t)” comes as 0.38106” for 24” SA-106 Gr. B pipe. Design pressure & temperature is 550 Psig & 600 Degree F respectively. Now,

1) When I added corrosion allowance of 1/16” in to the pressure design thickness (t), the value comes as [0.8106” + 1/16” = 0.44356”]
And later I added 12.5% wall thickness tolerance to 0.44356”, the adjusted thickness comes as 0.5069”. For 24” pipe the next greater commercial wall thickness available is 0.562” that corresponds to 24” schedule 30 pipe.

Now,

2) If I reverse the process and add 12.5% wall thickness first into the pressure design thickness (t), and later add corrosion allowance the final pipe thickness comes as 0.49799”, which means that I can use 24” XS pipe having a wall thickness of 0.500”.

Which method should I use (1) & (2). I know that the code formulas are for worst case scenario or in corroded condition, in that case I have to add corrosion allowance at the very first step and later to add mill tolerance. Some on-line calculation available also used method (1), however, some colleagues think to use method (2).

Would appreciate your feedback and/or comments.

Regards,

Meck91

 

[boilerone]

I have gone through this before. To me the first one is correct because need to follow the order of how the thinning naturally happens. Without looking at what schedule is available, say, the nominal thickness to be ordered is T and because manufacturing tolerance happens first and corrosion happens later, then it is required that,

T*0.875 - 1/16 >= 0.38106

Therefore,

T >= (0.38106 + 1/16) / 0.875 = 0.50693

 

[prex]

The correct procedure is obviously:
-take the nominal pipe thickness less undertolerance to obtain the minimum wall thickness in the new condition
-withdraw the corrosion allowance to get the actual minimum available pipe thickness
-compare the latter to the required wall thickness.
If you reverse this procedure, you'll see that it corresponds to your procedure number 1, so I confirm what boilerone says.

prex

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[racookpe1978]

Are you sure - and I mean really, really sure - that you know everything about every assumption and every future accident, surge, and problem that will happen to your pipe that you want to take advantage of a 0.002" (0.5000 - 0.49799") change in your calculated pipe wall?

The corrosion allowance of 12.5% is itself simply an assumption, based on average conditions and average corrosion over the average expected life of the pipe. All you're doing is playing lawyer ("But I followed the formula") to (literally) justify shaving less than a hair's width in your pipe wall.

And, in fact, 24 inch Sch 30 pipe is often more expensive than regular Sch 40. (More readily available might also reduce your cost - more people will be able to immediately ship after an RFP for Sch 40 than Sch 30. What will that do to your schedule and pre-fab? Can that decrease reduce even more cost?)

How long is your pipe? How many fittings? What schedule fittings?

 

[rneill]

For the B31 Code, method 1 is correct. However, this is somewhat code specific other codes (e.g., CSA Z662 in Canada) may specify method 2.

 

[DSB123]

racookepe1978,
You quote 12.5% corrosion allowance. This is "Mill Tolerance" not corrosion allowance.

rNeill,
You are not correct. The "Mill Tolerance" is applicable to reducing the nominal thickness of the selected pipe schedule thickness. You cannot add 12.5% of a calculated thickness. The procedure is calculate the pressure thickness then add the corrosion allowance to this to give the minimum required thickness (tm). Select a pipe schedule/nominal thickness and reduce by the applicable mill tolerance to give the minimum actual thickness (tmin).

The check that tmin > tm . If it is then the selected thickness/schedule is acceptable. If it ain't then select a thicker schedule.

Simple Really.

 

[eliebl]

Specify a different tolerance. If I remember correctly this is permitted, although not commonly recommended. This adds a bit to the QC and material recieving side of things but may allow for a lesser wall thickness.

EJL

 

[DSB123]

eliebl,
I think what you mean to say is order the pipework as "minimum thickness" pipe such that the supplier has to meet the purchase requisition. You cannot really specify a different tolerance as it is what it is in the manufacturing Code - nothing more nothing less - but you can as I say order the piping with a minimum thickness from the supplier.

 

[pennpiper]

The original question addressed calculating the wall thickness of "Pipe". Other posters have given good advice about "rounding up to the next schedule" or just asking the vendor for pipe with "a minimum bore".

These recomendations are all valid.
But I would like to offer "Meck91" the following.

You are NOT just designing a "Pipe". You are designing a "whole piping system". A whole piping system will have fittings and flanges in addition to the pipe. You may need to price-out the whole system based on your "Sch 30" calculated wall and the next higher schedule but more commercially available therefore cheaper pipe, fittings and flanges.

If you buy schedule 30 pipe and cannot get schedule 30 fittings or schedule 30 bore flanges then you also open yourself to additional extra costs in fabrication for the Taper Boring of the thicker wall fitting or flange.

My point: Consider all the material of the system and the complete fabrication process.

 

[boilerone]

Pennpipe has a very good point. Even though it is still the initial calculation which picks the pipe schedule, many times support design and stress analysis comes back to force to use a higher schedule.

 

[SnTMan]

I recently designed a small S&T exchanger to use 12" Sch 30 SA-106B for the shell. Guess what? There was not even a 10 ft. piece available from stock anywhere (practically). Had to go to Std. Definitely recommend checking availiblity.

Regards,

Mike

 

[EngAddict]

I always design to sizes I know is readily available, like STD, XS, 40, 80, 160, and pretty much give the rest the flick. Stainless is a bit different though. Quite often the fabricator will request a size for flange and pipe that he has left over or a heap in stock, then it will change but if you work on standard sizes you can't go wrong.

It doesn't matter whether you add the corrosion allowance to the calculated min thickness, or you subtract it from the pipe thickness, as long as you do it after you have considered the under tolerance of the pipe.

If you really can't afford to reduce the pipe size, the 12.5% reduction can be omitted if the pipe meets dimensional tolerance. However I would be extremely cautious using this approach.

 

[Mboundi]

DNV OS-F101 is an offshore standard, but gives criterium that is valid for all application, i think:

Sect. 13 E300:

"Calculation of nominal thickness

T=(T1+Tcorr)/(1-%Tfab)

T1=pressure design thickness
Tcorr=corrosion allowance
%Tfab=thickness tolerance "

That corresponds to method 1.

 

[rneill]

Mboundi, generally that method is the conservative option so you should be safe to use it for all applications but different design codes do specify the means of dealing with this issue so to dot the i's and cross the t's you should double check that you have meet the specific requirements of the code that applies to your project.