Machining Limits for Heat Exchanger Flanges

[SZM94]

Hi all,

First time here, hoping to be around for a long time.
My question is in regards to machining of heat exchanger flanges, specifically, how to go about determining when to stop machining and either weld build up or replace the flange altogether.

I want to, without having to re-design the flange, determine a minimum flange (bolt face) thickness to which the flange could be machined down to.
Generally when machining the raised face/gasket seating surface, you would machine the adjacent bolt face by the same amount in order to maintain uniformity.
Initially I was using the nominal-CA to determine when to stop machining and although I feel this is OK when considering the actual gasket seating surface which could corrode... Would the bolt hole face also have this Corrosion Allowance designed into it? Since it wouldn't ever be exposed to the corrosive environment?

I have gone through ASME Sec VIII Appendix 2 but to me it seems that the thickness is selected by the designer and then altered based on the rigidity factor.

Thank you in advance.

Syed

 

[SnTMan]

SZM94, I going to speak only of heat exchanger body or girth flanges, typically designed to Apx 2. B16.5 and B16.47 flanges are not addressed by the following.

Flange thickness will be governed by either applied stresses or by rigidity, and by either the operating case or the gasket seating case. The greatest of these thicknesses is selected, typically rounded up to, say the next 1/16". Any facing is then added. Per TEMA RCB-1.515, corrosion allowance is not generally added to the gasket surface.

For a recessed flange such as at a tubesheet, the facing, typically 3/16", is added to the portion outside the gasket OD. For a raised face flange such as at a shell cover the facing, typically 1/4", is added inside the gasket OD.

So, to find the amount of thickness that may be removed by machining, the difference between the governing design thickness and the thickness through the thinnest portion of the flange, exclusive of the facing, is what is available. The facing may be removed entirely, but this has drawbacks, such as that the gasket may now be unconfined.

You may have guessed the excess thickness is liable to be less than 1/16".

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Christine74]

I wouldn't be too concerned about calculating the minimum required thickness of the flange. Generally speaking flanges designed per Appendix 2 are quite robust. For example most Appendix 2 flanges are designed based on a maximum bolt stress of 25,000 psi, yet many owner/users will routinely torque stud bolts to 2 or 3 times this amount without damaging the flanges.

Another issue to consider when remachining the shell-to-tubesheet or channel-to-tubesheet body flanges is that machining off too much can cause issues with piping alignment.

-Christine