QUESTION
I’m looking into a resolution for machining down a girth flange face on a TEMA/Section VIII S&T exchanger where we’ve found some pitting on the shell flange face where it bolts to the tubesheet. We will be replacing the tubesheet & tube bundle with a new one but have concerns about the existing shell flange face due some existing pitting that we know exists. It’s been a problematic flange joint for our team to seal historically.
Can anyone direct me to where I can find how much code allowable thickness I can machine off the shell flange face before I have to weld buildup to restore the flange face?
The problem with the weld buildup path is that the metallurgy will require us to PWHT & will severely impact our schedule if we have to do this. So the hope is to only have to machine down the flange face to remove the pitting & then bolt this exchanger back up with the new bundle.
The shell was originally designed with 1/8" corrosion allowance I know. There was no machining allowance ever designed into this shell flange face though.
REPLIES
Christine74
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TEMA does not require that the corrosion allowance be applied to the gasket surface of the flange, and in my experience it it’s extremely rare to see a machining allowance applied to a flange designed during that era.
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TEMA is really intended for new construction; as far as I know it doesn’t cover anything with regard to remachining flanges.
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Girth flanges for heat exchangers are typically made from forged rings designed in accordance with Appendix 2 of ASME Section VIII Div. 1. Custom forgings are usually ordered to size, so there’s very little excess thickness. Manufacturers typically round up to the nearest 1/16 inch or so.
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Appendix 2 calculations are quite conservative. In the 1968 Code the allowable stress for B7 studs was 20,000 psi, but these same studs can usually be torqued all the way to their yield point (105,000 psi) without damaging the flange.
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Heat exchanger flanges get remachined all the time in the field without first applying weld overlay, and I’ve never heard of anyone requesting calculations to be submitted to confirm that the flanges still meet Code.
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A more common issue with repeated or excessive machining of flange gasket surfaces at the shell-to-tubesheet or channel-to-tubesheet flanges is that it can affect the piping alignment by reducing the center-to-center distance between the channel nozzles and the shell nozzles.
SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=460331
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