Dagny
Well-known member
I saw a can of expando at united refrigeration so I bought it but I haven't used any yet.
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Tape, dope or both for black pipe for compressed air
- Thread starter SouthernIllinois
- Start date
andyvh1959
Well-known member
The "perfect" threads are not at the end of the thread form on the minor OD, those threads are at least four thread pitch up from the minor OD. Proper pipe thread forms cause the crest of the male thread to crush into the root of the female thread, assuming quality made threads. Yet, thread sealant or teflon tape is required to fill any gaps and effect the seal. But still, the first two threads at the minor diameter do not seal, because the crest and root never meet to deform into each other. That metal deformation is acheived around the fourth thread up from the minor diameter. See page four from the Parker Tube FItting catalog showing the pipe thread to port interaction, note the gaps in the male to female thread engagement at the minor diameter:yes, they are the perfect threads and do the sealing...
rlitman
Well-known member
That's kind of an idealized view, and properly all true only for NPTF, but we're not dealing with NPTF, nor is NTPF a standard thread you would encounter in a hardware or plumbing store. Also, NTPF requires deformation of the thread in order to seal, so it is a one-time use product, and really only exists in brass to my knowledge.The "perfect" threads are not at the end of the thread form on the minor OD, those threads are at least four thread pitch up from the minor OD. Proper pipe thread forms cause the crest of the male thread to crush into the root of the female thread, assuming quality made threads. Yet, thread sealant or teflon tape is required to fill any gaps and effect the seal. But still, the first two threads at the minor diameter do not seal, because the crest and root never meet to deform into each other. That metal deformation is acheived around the fourth thread up from the minor diameter. See page four from the Parker Tube FItting catalog showing the pipe thread to port interaction, note the gaps in the male to female thread engagement at the minor diameter:
For NPT and BSP, there is no crushing or deformation (well, there shouldn't be, though over-tightening can lead to this), so the helical leak path always runs the entire length of the threaded connection. Of course, the first thread or two still shouldn't contribute much to the seal, but where exactly along the taper everything mates up tightest can really vary when differing materials meet each other (steel, stainless, brass, various irons, PVC, acetyl, nylon, PP, copper and aluminum all come to my mind, each with their own sets of issues).
I don't know if any of this matters to the OP, who would be fine with dope, or properly applied tape, or both.
andyvh1959
Well-known member
The company I work for has been producing both brass and steel NPTF for decades. and have used both in hydraulic systems many times. So perhaps my viewpoint is different. Also, NPTF can be reused in a limited sinse, IF, proper thread sealant is appied and the connection has to be turned in the same amount of turns from hand tight to again effect the seal. But yes, NPTF does have a limited reuse capability.
When I worked in the hydraulics testing lab at Dayco-Eastman we regularly used steel NPTF connections in limited sizes and shapes to power test circuits in excess of 5,000 psi. But again a limited application. So again my viewpoint is different. The jaws of life equipment used by EMTs and crash rescue also commonly use 1/4 NTPF steel (only) connections for the hoses, up to 10,000psi.
When I worked in the hydraulics testing lab at Dayco-Eastman we regularly used steel NPTF connections in limited sizes and shapes to power test circuits in excess of 5,000 psi. But again a limited application. So again my viewpoint is different. The jaws of life equipment used by EMTs and crash rescue also commonly use 1/4 NTPF steel (only) connections for the hoses, up to 10,000psi.