Take account corrosión allowance in flange thickness? in a bolted semiconfined gasket conection.

[seamantgn]

I'm designig a aes tema heat exchanger.
I use a std pv soft for do it.
I've done a design without error messages in the design soft calculation repport.
One colleague has asked me if i take account corrosión allowance in the thickness of the body flanges.
The body flanges are calculated according to appendix 2 of div 1, sección VIII, BPVC.
To check this i calculate the flange with corrosion allowance and without.
But if the corrosión allowance was 3mm the flange thickness without corrosión allowance was 4 mm higher.
I also saw if i try to design with 3 mm of corrosion allowance and 5 mm machining thickness. In the repport appear a total thickness of for example 32 mm but the value wich take account to make the calculation - verification it's 27. Always take account the machinning value but not the corrosión.
I don't know if the corrosión allowance must be take account in the flange thickness (holes circle).

I think may be are a mistake:
I've read about the design procedure of "bolted flange connections with ring type gaskets" for the case of a welding neck flanges. First it's must calculate loadings and bolt area - boltings next operating and seating loads and moments then the shape and stress formula factors. Then it will must propose a flange thickness and check with the stress formulas if the stresses are bellow to the allowable stress.
If we want save material the max stress will must be near to the allowable stress.


Now if corrosión allowance must be considered: if i have a 3 mm corrosión allowance i must increase the thickness value obtained in those 3 mm?

Thanks for any help.
Regards

 

[SJEC]

Considering CA on Flange thickness typically is not required.

Reason:
1. your flange both sides are not directly contact with the liquid. so there is no corrosion.
2. Flange design is typically rigid design. a local corrosion (such as inter corner of flange) will not affect integrity of flange.

 

[seamantgn]

Thanks SJEC!!
The flange wich i refer it's one of a main flanges placed at each side of the tubesheet in a TEMA AES heat exchanger.
The type of flange it's a welding neck flange with a semiconfined gasket (jacketed). Then the flange it's female with 5 mm machining thickness with a diameter ligthly bigger than the gasket diameter. The gasket inside diameter it's bigger than the flange inside diameter and this is lightly bigger than the raised male diameter of the tubesheet. To make a semiconfined arrange. Then we see what the portion of face of flange bellos to the diameter of the gasket will be un contacto with the fluid. Then we think in corrosión only un this part of face.
Then I think you probably are right, it's local corrosion
But the book what i was reading (Farr guide book for pressure vessels.. Edited by ASME), have one example with corrosión equal to 0 . And include a spreadsheets to calculate the flange thickness wich include the corrosión allowance as input.
The book it's 1997 Edition.

I asked my self if the code was modified after , and then the program who must apply the last edition don't consider the corrosión or have a mistake.

Regards

 

[TomBarsh]

ASME Code currently requires consideration of corrosion allowance for bolted flanges designed to ASME Section VIII Division 1, Appendix 2. Note that ASME B16.5/B16.47 do not address the effect of corrosion on the published pressure-temperature ratings for these types of flanges.

My own software provides for consideration of corrosion allowance on the Appendix 2 flanges, and I am certain (without doubt) that all commercial software for design/analysis of Appendix 2 flanges also considers corrosion allowance on these flanges.

 

[SnTMan]

Per TEMA RCB 1.515, not required to add corrosion allowance to flange surfaces other than the ID. Also see preceeding sections for language about gasket surfaces, etc.

Regards,

Mike















regards,

 

[seamantgn]

Thanks SJEC, Tom Barsh and SnTMan !!

The construction STD for the exchager it's TEMA.
Then will not apply corrosion allowance to the "face" of the girth flange, following the parr.
RCB 1.515. The part of the semiconfinated gasket face inside de gasket it's exposed to the fluid bit, not al the face. I agree this part will be exposed to a local corrosion but not al the face.

The issue it's that the soft calculate if apply a general corrosion a value diferent than 0 a thickness higher than wich one calculated for a corrosion equal to 0. And the difference in thickness it's major than the value of corrosión applied generally un 1 mm. All this when we selected a TEMA C exchanger.
May be with general corrosión it's put in the side of the security considering a local corrosión ignoring the par. of TEMA.