Bolting Requirements in Code

[RNicastro]

For years I have been doing Pressure Vessel Inspections and have gone by what was taught when i learned. I was taught to assure that all bolted joints had at least three exposed threads beyond the nut. Where does this come from. I have been looking but not found it yet. Is there a place in any code applicable to API that requires this? If so what and where. I am writing a proceedure and want to put it down by chapter and verse.

Thanks in advance for any help

 

[metengr]

How about good engineering practice.

 

[TBP]

The 2001 ASME B31.1 says:

"108.5.1 General

(A) Bolts, bolt studs, nuts, and washers shall comply with applicable standards and specifications listed in Table 126.1 and Table 112. Bolts and bolt studs shall extend completely through the nuts."

The 2004 ASME B31.3 states:

"335.2.3 Bolt Length. Bolts should extend completely through their nuts. Any which fail to do so are considered acceptably engaged if the lack of complete engagement is not more than one thread."

Every decent pipefitters book I've ever seen shows the number, diameter and length of studs required for any given class of flange, and these studs are commercially available in these lengths. The published standard lengths also leave enough "extra" length to allow for blanks to be installed. Why would anybody use shorter? They sell these by the pound - the money saved by having them a little short is the square root of nothing.

 

[RNicastro]

Thanks for the assistance. That is all that I need to put some teeth into my proceedure. They guys in the field see that it is in the spec but they want something concrete from code. I am creating a whole lot of work for them.

 

[TGS4]

ASME PCC-1 also offers some additional wisdom (from 8.2.1):

Corrosion of excess threads can hinder joint
disassembly. A practice that facilitates joint disassembly
(see para. 15) is to fully engage the nut on one
end (no bolt projection beyond the nut) so that all
excess threads are located on the opposite end; the
excess threads should not project more than 13 mm
(½ in.) beyond the nut, unless required for the use
of hydraulic bolt tensioners [see para. lO.l(c)]

 

[codeeng]

Go to Annex F in B16.5. This gives you a method for calculating bolt/stud lengths. I don't think you'll find anything specific on thread projections past the nut but 2 threads is the general rule of thumb. Certainly if the bolt/stud is chamfered then the chamfer needs to be outside the nut at the very least.

 

[proinwv2]

Well certainly, any threads beyond the last full thread in the nut do nothing EXCEPT show an inspector that the nut is fully engaged.

Excessive extension beyond the nut can be a corrosion issue as mentioned earlier and can also be a safety issue.

I certainly hope no one is out there counting exposed threads. Gee, if they are are they counting from the vanishing point of the threads, from the nut face, from the bottom of the chamfer in the nut...?

Paul Ostand

http://www.ostand.com

 

[TBP]

So, leaving some "extra" threads so flanged joints can accept a blank & two gaskets, instead of the normal operating single gasket is wrong? The commercially available stud lengths will allow for blanking lines, and well, they're commercially available. What purpose is to be served by using studs too short to allow for blanking? Why would anyone want to cut their own studs on a regular, on-going basis when standard lengths are on the shelf - by the box - at wholesalers?

 

[proinwv2]

No one said that extra threads were wrong, and in fact if they are needed, they are correct.

Who is cutting the threads off?

Paul Ostand

http://www.ostand.com

 

[TBP]

I wasn't thinking about trimming studs after they're installed, but rather that B7 rod can be purchased in lengths, similar to pipe. Some outfits or maintenance shops will have a length or two of different diameters on hand for emergency use (we do). They could conceivably routinely cut their own studs to any length they desire, but I can't imagine it being cost effective - it would be equivalent of making your own pipe nipples. The stud lengths shown in pipefitters handbooks for various classes and sizes of flanges will leave 2 or 3 threads exposed on each end, when used with 2H nuts. IPT's "Pipe Trades Handbook" has an excellent set of tables for this.

 

[proinwv2]

You can purchase alloy studs to various standard lengths, which can be economical. I guess that it would make sense to stock a few lengths and use an (gulp) extra long one for convenience/economics.





Paul Ostand

http://www.ostand.com

 

[unclesyd]

I would have to say that 90% of Studs are cut from bar stock upon order.
The nominal length unless modified by the order is from tables using ANSI flange dimensions with a 1/8" gasket. The most common modification for the length is if the stud is to be installed with washers, either one or two.
The standard cut length is nominally ±1/16" but with automatic saws it will be around ±1/32".
About 25% of the orders will call out a non-standard length with can vary up to 3/4" from the nominal length.
I've seen several orders that narrow the limits to ±0.010" on length and a very specific call out on chamfer. This particular customer also uses ground washers and torques every fastener on site.

In the fastener industry there are a lot of nuts.

 

[TBP]

Normally, shops or contractors will have the lengths they need, on hand. The vast majority of the time when we cut studs, it's because of a mistake - such as having studs for 3" Class 300 flanges show up, instead of for 4" Class 300, or some surpise maintenance situation.

When I see studs that looking a "little short", it's a situation like the crew didn't have the 4-1/2" long studs they needed for the 4" Cl 300, but they had some 4-1/4" (for 3" Cl 300) in the shop or truck - "We'll use them up." Or somebody just grabbed the wrong ones on the way out the door - it's pretty easy to do.

The supplyhouse we normally use will routinely ship us flanges "dressed" - whatever gasket we call for, B7 studs & 2H nuts - when we ask for it. We don't have to tell them the number, diamteter or length of studs. They don't cut them either - the various standard diameter & length combo's are on the shelf.

There are always exceptions to any given rule, but under normal conditions, why would anybody spend the time to spec studs shorter (or longer) than an industry standard length? And using standard lengths eliminates the "how many threads past the nut" discussions like this. You'll be left with 2 or 3 threads showing, so it'll meet B31.1 or B31.3 and the inspector can see that from a distance.