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





RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
T*0.875 - 1/16 >= 0.38106
Therefore,
T >= (0.38106 + 1/16) / 0.875 = 0.50693
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
-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.
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RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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?
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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.
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
EJL
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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.
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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.
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
Regards,
Mike
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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.
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati
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.
RE: Corrosion allowance vs. Mill tolerance (whom to add first in calculati