S&T Exchanger - Machining Down Flange Faces 2

[Krausen]

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.

Thank you in advance.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[TGS4]

You'll have to run the calcs to see the Code minimum thickness.

There are weld build up techniques that don't necessarily require PWHT.

 

[Krausen]

Being that this exchanger was designed in 1970, should I be looking at the TEMA code from this time? Or should I go off current Section VIII-1 now for the code minimum thickness on this shell flange?

Due to the metallurgy of this specific exchanger shell (1.25Cr-0.50Mo alloy), our standards will require us to PWHT any weld buildup we do on these flange faces.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[SnTMan]

Krausen, I don't suupose you have the original calculations available, but I have to ask, how do you know no machining allowance on the flange face? Do you know whether CA is applied to the face?

With these old jobs, practices were more varied than recent times.

Regards,

Mike

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

 

[Krausen]

SnTMan - I do not have the original calcs yet. I'm still trying to find these. I also do not know with 100% certainty that no machining allowance was designed into these flanges. That is just a guess. I'm also assuming the 1/8" C.A. that I have confirmed can be applied to the flange face.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[SnTMan]

Krausen, kind of a difficult situation. I suppose you could basseline your flanges using original (~1970) Code rules and allowables. I don't know if TEMA had its' own flange design rules at the time but I doubt it, it does not more recently. It did in the past have a published series of body flange designs that were listed in TEMA standards, older Ladish catalogs and perhaps elesewhere. Possible you could have these.

Anyway you could then compare calculated thickness to nominal and see if you have enough fat to face it.

I also don't see why, with approval of the involved parties, you could not perform the flange calculation using current rules and allowables to justify removing a small amount of thickness. After all this is a post construction activity.

Couple of flys in that ointment are consideration of pass-rib area and flange rigidity, which may not have been accounted for originally.

Be nice if you could find the old calcs. Best of luck,

Mike



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

 

[Christine74]

1) 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.

2) TEMA is really intended for new construction; as far as I know it doesn't cover anything with regard to remachining flanges.

3) 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.

4) 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.

5) 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.

6) 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.


-Christine

 

[SnTMan]

Great stuff from Christine74 as usual

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

 

[Krausen]

Thank you Christine74. This is the first I've heard of calc requests to confirm the girth flange still meets Code after machining down too. It's good to hear the Section VIII-1, App. 2 calcs are conservative for these, but there must be a point where machining off flange face material becomes a problem. So I will dive into this.

Very good point on potential for piping alignment issues after excessive machining down of the girth flange faces as well.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[MJCronin]

This seems like a very common repair for refurbished vessels and Heat Exchangers ...

Is there a Code case on this issue ???


MJCronin
Sr. Process Engineer

 

[Christine74]

I'm not aware of any Code Cases but paragraph 2.3 of ASME PCC-2 covers remachining flanges. They include an option for installing a split ring on the back side to add rigidity back to the flange when machined below the required thickness.

I assume that modifying the flange in this way would be considered an NBIC Alteration.


-Christine

 

[r6155]

How deep is the pitting?
Can you send us a photo?

Regards

 

[SnTMan]

Krausen, since you are replacing the bundle: If you can estimate the required flange machining reasonably closely, you can compensate with increased tubesheet thickness, or perhaps by adjusting the "turn" dimensions.

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

 

[r6155]

Krausen
“our standards will require us to PWHT any weld buildup we do on these flange faces”.
You have two alternatives:
1)meet your standards
2)change your standards

Regards