Whenever we are dealing with piping and vessels that use nuts and bolts as their joining method, there was always this “rule of thumb” about “short-threading” and “3-threads showing.” But now these “rules of thumb” may be no more, based on ASME PCC-1–2019, 8.2 Bolt Lengths. I was raised/trained in this as a fundamental in process safety, and we have cited short-threading many times during audits and investigations. But now, there is language stating neither “short-threading” nor “3-threads showing” may no longer be “fundamental”. I almost believe this is another case of “dumbing down” our RAGAGEPs. It seems when OSHA and EPA begin to understand process safety matters better and issue citations on some critical, albeit small in size ONLY matters that the RAGAGEP used in the citation gets a revision to make it more difficult for OSHA/EPA to use in their citations. For example, take these two statements from PCC-1:
- However, it has been shown that the full strength in a threaded fastener can be developed with one thread less than complete thread engagement (see para. 16.15 references), a potential consideration in certain post-construction situations.
- Excess thread protrusion beyond the nut should be minimized.
Let’s discuss what these revisions mean…
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8.2 Bolt Lengths Check bolts for adequate length. This length should consider the presence of washers, nut height, and required thread protrusion. The requirements in the NEW CONSTRUCTION CODES are based on complete thread engagement for the full depth of the nut. However, it has been shown that the full strength in a threaded fastener can be developed with one thread less than complete thread engagement (see para. 16.15 references), a potential consideration in certain POST-CONSTRUCTION SITUATIONS. The use of bolt tensioners requires that the threaded portion of the bolt extend at least one bolt diameter beyond the outside nut face on the tensioner side of the joint. Galvanized or coated bolts may require special tensioner puller sleeves. |
I am puzzled at the different requirements for “new construction” and “post-construction” requirements. I am guessing the pipe inspector will establish the status of each flanged joint so they can properly assess the integrity of the flange. This seems insane to me, but I can hear some of the arguments from the businesses now… “that flange passes because it is not new construction.”
| 8.2.1 Excess thread protrusion can hinder joint disassembly due to corrosion, paint, or damage. A practice that facilitates joint disassembly (see section 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. Excess thread protrusion beyond the nut should be minimized. |
We have discussed the concerns with having a lot of threads showing beyond the bolt; as corrosion can indeed cause issues with maintenance. But this is why it is “3-threads showing” and not 5 or more threads showing.
The old ways were easy to MEASURE and VERIFY, but now, good luck on your annual visual inspections done by in-house personnel.