Combining different bolt sizes on a flange 2

[jtravs]

Hi All,

I'm designing a custom flange to seal a small high pressure gas vessel. Due to various design constraints (in terms of flange shape and other aspects of the vessel design) it is challenging to reach the required bolt stress area using a single bolt cross section. However, it is easy to achieve by combining 4 M8 bolts and 4 M6 bolts. Are there any code rules or design guidelines that prohibit such a combination? I cannot find anything about it in either ASME VIII or in EN13445. In both, the design rules are simply based on the total bolt stress area. Are there any practical reasons why this should be avoided?

Thanks,
John

 

[LittleInch]

Yes.

This is in normal circumstances forbidden.

Unless you have, for some reason, different sized holes, the bearing surface for the nuts is going to be different. The risk is usually that someone uses the wrong sized bolt with a washer which then fails or the nut pulls through or breaks the flange. The stud will then also not be centred in the hole.

There isn't much difference between M8 and M6 and hence the risk that someone will force an M8 bolt into an M6 hole and vice versa is usually too large to make this an acceptable solution. I think you would need M10 and M6 if you're going to do this.

If this is a "custom flange" then it will need to be designed to first principles and probably using FEA or similar. AFAIK, ASME VIII Div 1 Appendix 2 I think is the only way to do this to some sort of recognised code, but usually ends up with bigger flanges than a standard ASME B 16.5 flange.

It's probably not prohibited as I don't ever recall seeing such a design so the risk is that on one else will realise what you're doing and then just use M6 bolts in all the holes and then complain that some of the holes are a bit big....

It's certainly a "novel" design... Maybe it's been done before, but is not mainstream. IMHO.

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

 

[jtravs]

Hi LittleInch,

Thanks for your response. For clarity both the flange holes and the receiving part will be designed such that there is no way to put an M8 bolt into the M6 hole. Of course, the opposite is always possible, but the person assembling it would have to be rather stupid. I suppose we should account for such a case though.

Thanks,
John

 

[r6155]

Be careful
See ASME VIII Div 1 Appendix 2
Bolts, studs, nuts, and washers shall comply with
the requirements in this Division. It is recommended that
bolts and studs have a nominal diameter of not less than
1/2 in. (13 mm). If bolts or studs smaller than 1/2 in.
(13 mm) are used, ferrous bolting material shall be of alloy steel.
Precautions shall be taken to avoid over‐stressing small‐diameter bolts.

Regards

 

[LittleInch]

"but the person assembling it would have to be rather stupid a pipe fitter who may not know better [ fixed it for you]. I suppose we should account for such a case though. [Not sure how, but definitely yes].

Doing anything which is not "normal practice" has a high potential for someone to get it wrong or "make it fit"....


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

 

[r6155]

small high pressure gas vessel

If you continue with this design, it may be necessary to retain an lawyer after the pressure test.

 

[jtravs]

Thanks everyone for your feedback - I will work harder to revise the design to avoid this.

 

[LittleInch]

I think that's a very good plan.

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

 

[Christine74]

I agree that this practice could lead to confusion in the field but there are no restrictions against this is Appendix 2 of ASME Section VIII Division 1. Per the code you can meet the required bolting are however you wish.

-Christine

 

[SnTMan]

Agree w/ Christine here. You are not prohibited by Apx 2 from using different sized bolts.

Don't do it. The mechanics will hate you, and rightly so

Regards,

Mike

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

 

[r6155]

How can you ensure equal loading of different bolt diameters during the installation sequence?
I would like to see them in the workshop.
This is not a good engineering practice.

Regards

 

[XL83NL]

GEP (Good Engineering Practice) is (sometimes) something different than meeting (minimum) Code requirements, which is the question being addressed here.

Huub
- You never get what you expect, you only get what you inspect.

 

[dik]

I don't like mixing bolt sizes... generally all one size, else I might use 1/2"dia with 3/4"dia (significant size difference) so it is unlikely that they will be mixed up... or maybe use 5/8"dia for all.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GD2]

This is one thing why Maintenance guys hate Designers.
How a fitter is supposed to torque the bolts? Will he give a damn to if it is M4 or M8 bolts?
I will not do this design.

GDD
Canada

 

[dik]

Just to clarify... never different size bolts in one connection... but maybe having two different size fasteners in the same project.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EdStainless]

If you put load sensors in the flange gap you will find that you cannot achieve uniform load on the gasket.
Even if the preloads match the relaxation under load will differ.
Go to 12 M6 and be happy.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[pperlich]

Interesting question. The only time I've seen mixed bolt sizes in a connection is on my farm tractor, rim to hub connection. I'm not sure why they did it, and it is annoying. Best to avoid that situation as you seem to have decided to do.

Phil

http://www.quicktanks.com