Bolting threads for flanges 1

[STALguy]

Hi All,
What type threads should I select for flange bolting (ASME B16.5)? I understand that for smaller size bolts (up to 1") UNC is the normal way to go. But what about the larger sizes? Do I stay with UNC or do I switch to constant pitch threads like 8UN or similar? Which is more common? Are there advantages/drawbacks with one or the other?

Thanks
Gunnar Breimer

PS
Being European I am not that familiar with American engineering practises.

 

[pennpiper]

I don't think you should try to customize or reinvent something that is already done.

Here is a link to the standard bolt size chart for ASME B16.5 Flanges

http://www.texasflange.com/dims/Bolting Chart.pdf

Here is a link to the details for the tpi (threads per inch) for UNC bolt threads:

http://www.efunda.com/designstandards/screws/screwunc.cfm

Stick with the standard.

 

[JohnBreen]

Get a copy of ASME PCC-1 (Post Construction Code, Guidelines for pressure Boundary Bolted Flange Joint Assembly. Also:

BP Amaco's Handbook is at

http://www.scribd.com/doc/7784298/Apiping-Joint-Handbook

........and you may find these useful as well:

http://www.awhem.org/PDFs/TR9501A.pdf

http://www.aztecbolting.com/pdf/pipeflangebolting.pdf

http://www.scribd.com/doc/13162417/Gasket-Handbook

 

[STALguy]

The problem is that niether ASME nor Texasflange give the thread, only the thread diameter.

If we take the example of a 2" bolt. A 2" UNC has 4,5 threads per inch, while an 8-UN has 8 threads per inch. That to me would suggest that it would be easier to achive the required clamping force in the flange with less torque if you use an 8-UN thread rather then a UNC.

In ASME B1.1 sect. 3.6.1 it says: "8-Thread Series. The 8-thread series (8-UN) is a uniform-pitch series for large diameters <snip>. Although originally intended for high-pressure-joint bolts and nuts, <snip> for diameters larger than 1 in."

 

[STALguy]

Thanks John Breen,

I found my answer in your first link, section 5.2. It clearly says that for flanged joints the bolting should be UNC up to and including 1" bolts and 8-UN for bolts larger than 1".

Again thanks John, I have made bookmarks of your links. Brilliant!