When I was a kid, my dad explained how tapered BSP worked (with hemp/boss white or teflon tape).
But in all my recent wibbings with plumbing, I don't think I have ever come across a tapered BSP thread.
Where they ever a thing? Seems to be all parallel now and dump shedloads of teflon on until it feels convincing.
Strictly speaking they are BSPT [T aper]
Most fittings do seem to be parallel now and they can be used with PTFE by building a little "knee" up. This is easer if the build up can be butted up against a union or similar, but with care a small bumb can be formed on the male thread. One has to avoid the temptation of just whacking a few metres of tape around the pipe as it simply gets displaced as you screw the fittings up.
Oddly enough I saw a colleague use loctite for the purpose this week. It was certainly a new one for me and it was for high pressure potable water. Not sure of its WRAS rating, so on the odd occasion when I still dabble with pipework, I'll stick to PTFE.
AB
There are certainly specific Loctite products for pipe threads. We used to use one on instrument fittings on industrial compressors at one company that I used to work at. That was at 210 Bar.
SteveW
Tapered threads seal n the thread. Parallel thread have a washer to seal on the body of whatever it's attached to.
Taper threads are mostly used on steel pipe in large/commercial/industrial heating systems.
No they dont, they "seal" the thread no more than parallel, probably less actually. Thread tape or similar seals the thread. No tape and your taper thread passes as effectively as any other.
IMHO the most advantageous thing about taper was the ease at which taps, pressure guages and the like could be positioned.
AB
The tape is a lubricant, Why do you think it's made of PTFE?
Yes they do. There is a clearance between parallel threads, and it is still there when they are done up, just all at one side. But with tapered threads you compress your hemp/boss white or PTFE up until all the air has been excluded. And even if you still have a very small leakage path, it blocks up with solids until it does give a true seal.
Even with a male taper in female parallel and no sealant you can get the same sort of seal that you get from olives in a compression fitting, where there is sufficient plastic deformation of the metal to produce a very good seal indeed. Though (unlike in a compression fitting) there is a chance of a helical leak path at the tips and roots.
Tapered threads dont seal. Certainly not enough to stop any fluid I have ever pumped from leaking.
If you dont use thread tape you need a fitting with something akin to an olive, although I know that some plumbers actually back up the olive and compression fitting theads with PTFE tape ?????
AB
And very good they are too. Screwfix also has two "cheap" versions which are also fine for domestic temperatures. They have different strengths, one is intended for permanent joints, the other for ones which you might have to undo at some time. Very handy if you get asked to look at connections which have been bodged in the past.
Loctite also do a modern replacement for hemp, it's a very strong fibre (possibly Kevlar?) about the thickness of thread that you might use on leather items. This is meant for taper fittings of course.
poundland polypropylene rope does that job fine. £1 for what was it, 2
0 or 30m. Ordinary products marketed for specialised purposes always seem t o attract silly prices.NT
The beauty of hemp is the fibres swell in the presence of moisture so any minor imperfections/drips heal quickly in use
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