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Correct bolt diameter for a 2.5" class 300 RFWN Flange

Correct bolt diameter for a 2.5" class 300 RFWN Flange

Correct bolt diameter for a 2.5" class 300 RFWN Flange

Hi guys.

I'd just like to run something by you if you don't mind. I feel like I know what the answer would be but I just want some expert confirmation before I make someone spend some money.....

We've got a 2.5" class 300 RFWN flange. Bolt hole in these flanges is 7/8" as per ASME B16.5. In my piping specs, I called off a 3/4" bolt. My spec then disappeared into the drawing office and I never saw it or heard about the job again.

Now months down the line, the drawings have been done (I wasn't given them to check), materials have been purchased and site works have begun.

I've been asked why did I request M16 bolts for these flanges - they don't fit, they're too small, the nuts are almost going through the holes, and why did I do this?

I'm a bit confused - I requested 3/4" and that's what should have appeared on the drawing. After checking the drawing, the draughtsman has ignored my spec and calls off M16's and now I'm being asked to approve this change. They are wanting to use washers both sides of the flange hole to provide a surface for the bolt and the nut to hit, but, I'm not happy with this. My preference would be to buy new hardware because having smaller than specified hardware on a flange connection of this pressure isn't safe. And using washers in this way, in my opinion, isn't recommended.

Do you agree?

Any input would be appreciated.

Thank you


RE: Correct bolt diameter for a 2.5" class 300 RFWN Flange

According to the tables I normally use, that flange should have 7/8" holes and use eight 3/4" studs 3-3/4" long. In any process service, I would not allow this flange to be placed into service with smaller studs. The smaller studs greatly increase the chance of the gasket dropping down out of position allowing a leak. We have seen this when undersized studs were used. In one instance, the flange leak could not be isolated and the unit had to shut down costing millions of dollars in lost production.

Johnny Pellin

RE: Correct bolt diameter for a 2.5" class 300 RFWN Flange

I agree 100% with your action.

There are some oil companies who have banned use of washers precisely because of this reason, where undersized bolts and nuts were used with washers which then took all the shear load and failed in service, but no one could see that the hole was much bigger than the stud.

JJ highlights an important practical construction consideration

What they are proposing is just plain wrong and you need to stand your ground and refuse to change or sign. This one is worth lying down in front of the bulldozer. (IMHO)

why are they haggling over 8 3/4" studs and nuts??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Correct bolt diameter for a 2.5" class 300 RFWN Flange

So because a drafter/designer, who is probably not qualified to make the decision, decides to deviate from the spec without prior receiving approval to do so, and then a checker, who probably should not be checking if he does not understand the importance of the spec and decides not to uphold the piping material to the spec, allows the deviation without questioning it, that means that you should just approve the change??

This is preposterous to me, and I agree that you should stand your ground. Just because others screwed up does not mean that you should follow suit and go against your better judgement IMO.

RE: Correct bolt diameter for a 2.5" class 300 RFWN Flange

@JJPELLIN Thank you for your input totally agree with what you're saying. And the point about the gasket is a very good one. Adds even more confidence to my argument against it!

@Littleinch thank you for the back up. I knew I was right to stick to my guns but some expert sounding board always helps! And it's multiple flanges, in the region of 80, so it would be a significant amount of hardware to reorder. So obviously - they're looking for an alternative to cover their mistake which costs the least

@DGrayPPD Yes that's exactly what's happened! Do you work at my place?? Thank you for the comment. All adds weight to my argument and my belief to just buy the correct hardware and do the job properly.

Why take unnecessary risks?!

Thanks again all


RE: Correct bolt diameter for a 2.5" class 300 RFWN Flange

Perhaps the pull-down menu in the software didn't offer the correct option and it was cheaper to specify the wrong bolts than it was to modify the CAD program.

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