Corrosion in Alloy Pipe

The corrosion allowance is whatever the present wall thickness is minus the minimum calculated wall thickness.

Making certain I'm reading this right: The OP is not finding a calculation for corrosion allowance in the code because there is no calculation for corrosion allowance in the code. The code only provides a calculation for minimum wall thickness. Any corrosion allowance is provided solely at the discretion of the designing party, and only through the employment of their judgment, or plant standards, or whatever else.

Is that a correct understanding?

Any corrosion allowance is provided solely at the discretion of the designing part.

Nope! it is given by the calculation of the minimum wall and the current wall thickness. ASME gives either the calculation or/and the minimum wall. I ran many crews in refinery's doing corrosion analysis to forecast the replacement of the pipe or vessel and in every case the unit was replaced WAY before the unit hit minimum wall. You have to live with insurance companies and lawyers.

OP was requesting corrosion allowance for steam pipe, and even cited the standard, please read before you react. I quoted an excerpt to indicate what should be done in allowing for some corrosion over the projected life of the project.

Please note also, that one cannot arbitrarily add thickness to the pipe schedule in highly stressed areas, as the extra thickness will result in problems with differential stresses on the pipe wall.

Never had a failure in over 40 years. Everything done was to ASME. Shell, Monsanto , Gulf and many others. They designed the piping systems as well as the vessel systems all we did was to make/inspect it to specifications. Corrosion survey was typically carried out over a 2 year period where the rate was established by depreciation over several inspections based on usage of the pipe or vessel. This established a rate so the suggested replacement time was based on that rate.

Ron: Sure that works in the oil patch, and it is a great rule of thumb when initial corrosion rates (and perhaps the latter rates also) are unknown, but guessed at.

The system designer engineers (if worth their salt corrosion) would have already designed the corrosion thickness into the pipe.

What you are doing is measuring the existing pipe in situ as compared to spec min wall, then extrapolating time to failure based upon thickness loss in X years. That is a perfectly fine way, since no one can really predict failure point, as corrosion rates will increase or decrease over time due to changing factors (including retention/deposition of corrosion products that may inhibit further loss of metal).

But thanks for filling me in on O&G practices with respect to offshore or even onshore operations.

I think the term you were shooting for rather than "usage" of the pipe or vessel is termed "wastage", since the material is wasted away.

I am talking about a335 p91 pipe, in my company are telling i can use 0 mm but i an worry because i think that in 25 years can affect the corrosión.The problem is the asme b31.1 only say The designer choose this valor and i have not argumentación for use 1 mm that always have used.

What alloy are we talking about? I think most of the alloys used for steam piping would have about the same corrosion rate as carbon steel in the same environment. Therefore, they would probably have about the same corrosion allowance.