BSPP is the better system in terms of sealing. If the valve requires the ability to clock, BSPT allows for it.

Hi TugboatEng,

To my knowledge, "BSPP to BSPP" fitting will have chance of leakage more as compared to "BSPT (Male) - BSPP (Female)" fitting.

Please correct me if I'm wrong.

If you're using straight threads only (BSPP) you need a seal, typically at the base of the male threads. Depending on the fitting this is an o-ring or rubber washer typically, but there are likely solutions without a rubber seal. Since you're mating to a shoulder engagement depth is a known quantity and the piping can typically go together closer to the 3D model designs - this is why tugboat mentions BSPP can't be "clocked" as it has no engagement range but a hard stop.

If you're using tapered threads (BSPT) the threads themselves are designed to create a seal. You typically need tape and dope to assist the sealing as engagement depth and fit are critical to the seal, and why using tapered threads on both sides is a bit better fit, but tapered male will still seal in a parallel female thread. Since the engagement depth can vary based on manufacturing tolerances and other factors the fit of the system can sometimes vary enough that assembly prints can't be followed exactly - but again as tugboat mentioned, since the thread engagement has an acceptable range you can work within you will have some ability to choose the final rotational position and "clock" the fitting during assembly.

I've never chosen a BSPT male x BSPP female arrangement to know how well it works, but it's shown as acceptable.

It is general practice to use taper threads on valves. The wedging action of the tapered threads has higher friction that straight or parallel threads. Thus, the tapered threads are less likely to loosen with repetitive operation of the valve.

Both types of threads will initially seal reliably if put together correctly. Any leaks are primarily due to workmanship rather than the thread form.

Coming from USA we have NPT and NPSM. Fluid connections are always NPT to NPT (tapered). NPSM (straight mechanical) is not meant for sealing applications without a gasket. It's typically used for electrical conduit and not fluid.

We do not combine tapered and parallel threads.