Know your fittings... For safety sake! BSPP vs NPT vs BSPT

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Tominco

Tominco

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Apr 1, 2015
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Colorado, United States
Knowing what fittings we're dealing with will go a long way in keeping us all safe! Bookmark this thread so, you can refer to it when you need to. And, please toss me some accuracy if you think this is helpful. ;)

I got this article from Ralston Instruments. It is the best representation and explanation I have found.
Air gunners and Paintballers generally use ~1/8" sized fittings. We may upsize to 1/4" and back down again, but our primary fittings are ~1/8". I found out that there are actually 5 types of fittings in this size category! The vast majority of us will use either NPT, BSPP, or BSPT. Even BSPT is pretty rare. 
Connecting a NPT to a BSPP is not safe. You might be able to get it to work and think everything is fine. But, you are leaving yourself open to failure and possible injury. The best case scenario is that you'll only lose air at higher pressures. But, why leave yourself open to any of that? Generally, these fittings are pretty inexpensive and the shipping ends up costing more than the part (unless you're talking stainless steel). Another hazard to using the wrong fitting is when you're forcing this and one of your pieces is made out of aluminum. This can put a tremendous amount of strain on the tiny threads and possibly strip them right out. Then you'll have an expensive repair on your hands. 
I found some NPT to BSPP and vice versa on eBay. I also found some steel as well as stainless steel fittings at a hydraulic supply company. My favorite place to shop for high pressure fittings is Northshorecompressor.com. However, they don't seem to carry any BSPP stuff. 
I would also suggest spending the extra $ for stainless steel vs steel fittings. They are made much cleaner and more precise and generally look better as well. 
-BTW, "BSPP" is sometimes abbreviated as just "BSP"-
Always remember to double check your fittings pressure ratings before ordering! 
My suggestion, get converter fittings from this to that, and that to this. Have them labeled and stashed in a drawer so that when you run into that situation where you need this to go into that, you can make it happen and get on with your project. 
I hope you find this info useful and educational. Stay safe and, Happy Shooting!
Tom
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There is one other important difference in the National Taper, and the British Taper threads in the 1/8" size, and if I recall correctly in the 1/4" size as well. That is, the thread pitch is also slightly different. This means that not all the threads are engaging and holding back the forces of the pressure on the fittings. You wind up with a significant strength loss, and thus any factors of safety that were built into the design.

There is one other place that the BSPP will have a seal, it is at the bottom. This is often just a standard o-ring. The o-ring gland is shown in the diagram in the last picture. It requires that the male fitting seats all the way down to the small shoulder. The thread relief step at the bottom is not present in the drawing of the male fitting which mates with the shoulder in the female fitting.
 
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Good post Tominco. I'm surprised this is not discussed more. I work in the natural gas industry. I make LNG (liquid natural gas). I am much more afraid of of pressure release than fires. We generally are working with less than 1200 psi. I know of several deaths from well under 1200 psi. Even though I am dealing with more volume generally....... It doesn't take much to launch a piece of shrapnel from a low grade stainless or brass for that matter. We airgunners have tanks that can fill/overfill to 4500 psi. Just because it fits together does not mean it is rated for high pressure. I see all kinds of fittings being sold from questionable overseas sellers and it makes me shudder to think of what the standards are. If guys are cobbling together a pcp setup please think twice. Buy from a reputable seller that has a name behind them. Sure you are going to pay more but is your life worth the extra $80.00 you may spend in proper fittings?
 
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Also being that there is so much importation/exportation of products, many may not be too keen as to what mates up to what. When I got my Prod and my Hill MK4 pump, it was almost impossible to find mating hardware as the AirVenturi brand most use was out of stock nationwide in the US. I could have rigged something up but I know better. Many do not. I too am surprised this isn't talked about as much or a sticky provided on various web forums to address this for newbies.
 
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