Change in Mechanical Integrity of a flange after adding holes

[Feltonn]

Hello all,

I have a set of temperature measuring instruments that are bolted into a pressure vessel using welding neck flanges, with a raised face. Specs: ASME B16.5 300 Pound 1.5" Carbon Steel.

I have a variety of these temperature probes, in order to avoid inserting the wrong one in the wrong vessel I want to add some guiding pins in the flanges. The idea is to have two small holes in every flange and add pins in one of the flanges, every instrument will have its distinct angle between the two pins. Therefore each is keyed in a unique way, making it impossible to make a wrong connection.
The new holes will be in parallel to the existing bolt holes, at the same distance from the centre of the flange. New hole diameter will be 8mm.

Drilling these holes will compromise the integrity of the flange. I therefore compared the moment of inertia according to the following formulas:

I' = (Pi D^4)/64 + (Pi D^2 R^2)/4

I = I'flange - I'centre_hole - 4x I'bolt_holes - 2x I'pin_holes

D = Diameter
R = Distance centre flange to centre of the hole

I found that adding the two holes will in this case decrease the moment of inertia by 1.4%, which is acceptable for me. I am wondering what your opinion is on this, is this result to be expected?

Furthermore:

1. Will it suffice to calculate the moment of inertia, or am I missing another important phenomenon besides bending resistance?
2. What does ASME B16.5 recommend, do they approve of drilling holes in the flange up to a certain diametre?

As a reference, I've attached my calculations in an Excel sheet.

 

[TGS4]

I understand what you are doing and the rationale for doing so. There is a certain logic here, but remember that Codes and Standards are not necessarily based on a type of logic that we would normally assume.

In your situation, you have modified an ASME B16.5 flange. Therefore, it is NOT an ASME B16.5 flange, and you may not apply the temperature-pressure ratings. If this is going to be part of an ASME Section VIII, Division 1 vessel, then you would need to qualify the flange to Appendix 2. Depending on how close your vessel design pressure is to the (otherwise standard B16.5) rated pressure of the flange, it may work, or it may not.

That said, there may be sufficient grey space here to find a middle ground with the AI. Talk with them.

 

[LittleInch]

Can't you just paint them different colours??

Drilling holes like this is an interesting idea, but leads to issues of potential corrosion, freezing and mechanical integrity.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TWP]

Extra on holes 8 mm is no problem at all; relatively small and influence is only local. Locally (near 8 mm holes) somewhat higher stresses. Not very important and OK if local. In general, all flange ring stresses at bolt circle dia. are relatively low. All in all more a theoretical problem than a real one. I wouldn't bother too much. Th. Pl.

 

[Feltonn]

Thank you all for your advice.
The flanges have been drilled and the sensors are in place, each with a unique key.
Colorcoding wasn't an option, due to company regulations. It is a hot vessel, thus freezin is not very likely to occur. Corrosion can be, so inspections will be needed.

 

[pennpiper]

This sounds like a big waste of time and money. It also sounds like an insult to the intelligence of the maintenance people.

I suggest you meet with the maintenance department leadership and look at the installation instructions and improve them if required, otherwise make sure there is proper Quality Control on such installations.

prognosis: Lead or Lag

 

[MartinLe]

May I supply an anecdote that does nothing so solve your problem? A guy I know works with a major chemical cooperation. Apparently their installation guys sometimes ran into trouble matching different pressure rating flanges, and drilled extra holes into a loose flange ... ending up with a DN100 flange with 64 holes.

 

[Feltonn]

Nice anecdote ;-)

I understand your point of view pennpiper, and it was considered. However, this solution was finally chosen and applied to a special case as a safety precausion.

 

[ricklts]

why not just drill your holes outboard of the bolt circle? Orifice flanges do this for jackscrews everyday and are not derated. you could add two small dowels at any combination that makes sense to you in an area of the flange that is outboard of any pressure/load/design requirements. this would be similar to alignment pins used on many HX's.