What thread types are standard for chemical plants?

[mjcook]

The chemical plant I work in uses the constant-pitch 8-thread series for fasteners over 1" in diameter.

Many items we get that have tapped holes in them like valves and expansion joints, though, are tapped for coarse thread fasteners, or 7 TPI for 1-1/8" and 1-1/4" diameters.

What do most of you use in your plants? Is there a standard in the chemical industry for these larger fasteners? It causes problems having a mix of 7 TPI and 8 TPI fasteners floating around when specifying torques.(not to mention the headache for mechanics when they grab a bucket of bolts and there are two different thread types) Thanks for your input.

 

[rconner]

It sounds like you are probably talking about UNC (Unified Coarse-thread series). Many references involving threading could be pulled up with these key words/acronyms (and your knowledge of what you have), including perhaps ASME/ANSI B1.1.

 

[mjcook]

That's the question. UNC fasteners would have 7 TPI for 1-1/8" through 1-1/4", 6 TPI for 1-3/8" through 1-1/2", etc.

There are also constant pitch series (UN series), like the 8 TPI series my pllant uses. In John Bickford's book, " An Introduction to the Design and Behavior of Bolted Joints", he says, "The 8-thread series is used on large-diameter fasteners and was originally intended for bolts used in gasketed joints containing high pressure. It's also widely used as a substitute for coarse series fasteners when the basic fastener diameter exceeds 1 in."

So, based on what I've read, we're not the only plant doing using the 8-thread series, but I'd like to understand how prevalent it is. I don't know what pressure is considered high pressure in this context - everything in my plant is contained by 150# or 300# piping. As I mentioned before, many tapped components you buy for CPI service are for the UNC series. Also, go to any gasket manufacturer's catalogs and look at their table for converting torque to seating stress - they all assume UNC.

My concern is based on torque values. If I assume 6 TPI and the fastener has 8, there's a huge discrepancy in desired versus actual torque. I can't figure out how my plant standardized on the UN series, and I'd like to determine which is more typical in industry.