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NPT/BSP thread compatability

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jonesey

Mechanical
Oct 6, 2002
60
NO
How compatible are NPT threads with BSP threads ? I've heard varying comments where it's not a problem to screw one type into the other, and other sources that say it is not possible. We are only using threaded connection after block valves typically for connecting to instruments/vendor supplied items.
 
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You can screw about two or three turns if you try hard, rest is left to your desicion. Personally I never attempt it.

 
We've done work at a fairly new tobacco processing plant that has a lot of pre-fab skid assemblies from the UK that were piped together here (Canada). Mismatched pipe threads and flanges cause a LOT of problems. I have known people who put the smaller sizes (1/2" and 3/4") together with no apparent problems. Above that, they don't match properly. I've seen a number of back-welded threads, and flanges with the bolt holes drilled out to oversize in that facility. If your assemblies have to hit any kind of piping code requirements, I'd recommend staying away from trying to marry-up even the smaller sizes.
 
Buy adapter fittings (one side BSP, the other NPT). You should be able to find what you need at good hardware suppliers, try
BSP thread sizes typically are 1 pitch number higher than the NPT threads. I.e. in 1/4" pipe size, the NPT thread is 18 threads/inch; BSP is 19 threads/inch. There are other differences (in thread form for instance) as well. Bottom line is the two don't mate up.
 
I have experience with one size only, 3" nominal.
Can be screwed in, but will not work at all, the threads as I recall, are much finer on the BSP vs. NPT, so there is no way to obtain a seal. Taper appeared similar, just thread count per inch was finer on the BSP.

PUMPDESIGNER
 
As I understand it, BSP threads are Whitworth form threads with a 55° angle. NPTs are 60° threads. Also, the pitches are differnent for any given nominal size. These threads are definitely not compatable.
 
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