BSP and NPT threads - compatible?

How interchangeable/mateable are these thread types? I’ve got a client in India who’s part has ½” male BSP, and my parts have ½” FNPT on them. The *system * (we’re sampling combustion gas from a power plant) has no pressure, but will have to pass a leak check at about 15-20 in Hg vac.

Some of what I’ve read says, “no”. Then some of it leads me to believe they can be connected but their performance will be compromised. Is 15” Hg within a safe window… away from the zone of compromise?

I can buy a FBSP half-coupling* and weld on my part. I can get an adapter bushing, too. We can solve the problem. But dude is in a hurry and I’m wondering what our options are.

*I think is what it’s called.

The safe answer is don’t do it, they will leak.

Taking a closer look, at the 1/2" size, they both have the same pitch (14 TPI) but the threadforms are different. The NPT has a 60 degree thread and the BSP has a 55 degree angle. Since these threads are designed to seal on the threads, the difference in thread angle is subject to leak. You can try to really torque them to try to deform the threads to make up the difference, but there is no really good way to make sure that happens.

You can use thread sealant (not Teflon tape, but actual sealant), which will work for the short term. But, sealant will dry and crack, which means it may pass the leak test at first, but fail if it needs to pass the test at a later date.

Thanks for the input, e(fam). I agree… we should play it safe. Welding the half-coupling will be easy enough, and McMaster has lightening-quick response/ship times.

One of my cow-orkers tells me that “some sizes will fit/seal, some won’t”. He went on to say that the 1/2" size is compatible, but I’m not sold.

I’m still hoping that someone comes along with real-world anecdotes of doing this. Or maybe it’s such a stupid idea that no one *has *done it.

Well, your coworker is somewhat right. Some sizes have a different pitch (threads per inch), so they won’t even go together. They also have different major diameters, but since they are tapered threads, for the 1/2" size, they are close enough. It’s the different thread profiles. Like I said, if you torque them hard enough, the threads may deform enough to seal, but there is no way to tell and you would definitely have to torque them more than matching connections.

I’m sure it’s been done and there is a very good chance it would work. But, like I said, the safe answer is don’t do it.