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Air line fitting threads - calibrate me please

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  • Air line fitting threads - calibrate me please

    Just started getting into the new money pit that is compressed air. Before anyone says it, yes it's mainly due to jealousy....I'm sure you understand!
    I could do with a sanity check on fitting threads please - to know how tight it's supposed to be or if I just have naff fittings.

    I'm mainly using BSPP and BSPT threads - knowing which it's going to be before you receive them is just guesswork. I do have one NTP thread and have ordered a separate fitting for that.
    I know the basics of the thread form and the idea of tapers and teflon tape but I've not had to seal anything without o-rings previously.....and then only up to about 7 or 8 whole psi!
    Fogbuster kit went together very nicely and no leaks on anything - but that only needs to go as high as 30psi.
    I made up a main line to go from my (teeny) compressor to where I want to use things. Barbed end with a single-ear clamp was perfect straight away. The other end was a male 1/4 BSPT which I taped and screwed on a female 1/4" BSPT fitting. To get it sealed up to about 100psi (max pressure) I needed to re-tape it from 3 turns (as recommended on t'interweb) to 5 turns and also tighten it about as tight as I could get it with a pair of spanners. 3 turns of tape and tight weren't enough - neither were 4.
    So, question is simply: does it normally need to be bust-a-nut tight with that many turns of tape or do I just have the quality of fittings I deserve for the money I paid?

    I know it doesn't need to be pushing the max pressure and I expect I'll be running it at more like 60 for the main line and then drop it down from a wall block to each tool. That said, I wanted to know that my connections were good for the max pressure they could see.

  • #2
    both styles of thread are tapered. Thread sealant or tape and a moderate amount of torque on the fittings should suffice to effect a seal on your equipment, if it leaks, tighten it some until it doesn't.Jim

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    • #3
      Iā€™m finding the liquid sealer with ptfe works really well
      mark

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      • #4
        BSPP threads are parallel, BSPT threads are tapered.
        In the US NPT threads are tapered and NPS threads are straight.

        The question that begs for an answer is why is anyone using a standard developed in the 19 century in the year 2021?

        I dislike tapered pipe threads in every possible way.

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        • #5
          IF both of those 1/4 fittings are, in fact, tapered threads then they should seal with 2 wraps of tape at most and no more than two turns with spanner after tightening by hand. One full turn is often sufficient. Anything more exciting than that and you've got some quality issues, or you are trying to mash a straight thread in to a tapered one. Tapered into straight can work, but I wouldn't rely on it. I was always taught that straight pipe threads are not to be used on pressure tight joints.

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          • #6
            Originally posted by Bented View Post
            The question that begs for an answer is why is anyone using a standard developed in the 19 century in the year 2021?
            I know. I get "If it ain't broke, don't fix it" but it does seem odd when (almost) everything else has gone metric and the nominal dimension doesn't even refer to something that still exists!

            Originally posted by Bented View Post
            I dislike tapered pipe threads in every possible way.
            I'm beginning to come round to your way of thinking! Right now this is just a one-off. I'll know more when more things turn up for me to swear at.

            boslab I've sent an email to Henkel (Loctite) to ask which of their products are most suitable - there seem to be at least 4 in this area! I'm liking the idea of painting it on, aligning it (for a gauge) and letting it gap-fill.

            tom_d Thanks, that's a helpful benchmark. I can't say I counted the number of rotations after hand-tight but it was definitely about as tight as I could get it with two spanners without getting more exotic.
            I do wonder if brass fittings might seal easier since they're more deformable.

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            • #7
              Originally posted by Cenedd View Post

              .... and the nominal dimension doesn't even refer to something that still exists!

              .
              It most certainly does refer to something that exists.
              Nominal pipe size refers to the ID of the pipe for "Heavy" wall thickness specification.
              "Heavy" has been refined to be called Schedule 80 wall thickness. They are not
              exactly the same, but very close. Schedule 80 being just slightly thinner.
              So 1/2" IPS (Iron Pipe Size) refers to 1/2" Heavy wall pipe Inside Diameter measurement.
              The designation of nominal actually does match up to an exact physical size.
              Because you don't know the answer does not mean there actually is an answer.

              -Doozer
              DZER

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              • #8
                I knew the answer in that I was aware it referred to vintage pipe OD's but by your own admission those no longer exist. Schedule 80 is "very close" but many things are very close and not considered the same. Beyond the dangerous waters I'm trying to avoid, how many 1/4" BSP threads are on a 1/4ā€ schedule 80 pipe that would render the measurement meaningful? At least when you get a 1/4" screw thread, it refers to something that is consistently measurable no matter what (if anything) you screw it into.
                I understand where it came from, I'm just not sure it's in line with current standards - whether those are metric or (UN) imperial.

                1/4" NPT ball swivel fitting turned up and the 1/4" hose it fits on nicely with a compression fitting - now those are MUCH better. Still haven't got the fitting for the other end of the hose though. Getting closer to a functional air gun....but irritatingly far away still!

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                • #9
                  If your doing pressure pipe, tapered threads is the standard. Pipe thread or dope is just for lubrication of the threads while tightening. Two or three at the most turns of Teflon tape, more will flake off inside the pipe and plug up strainers or air valves. Same with the pipe dope, you can tell an amateur job of piping by how much slopped on pipe dope or tape you can see!! Here in US NPT is the standard and its tapered, you will find Metric on imported.

                  Nominal pipe size is suppose to be the ID of the pipe, sure but not really close. Schedule 80 pipe is heavy wall but its smaller on the inside so standard pipe dies can still be used on the OD. Schedule 40 is standard pipe used most often.

                  PS As an electrician threaded rigid conduit was threaded with non tapered threads.
                  Last edited by wmgeorge; 05-06-2021, 09:36 AM.
                  Retired - Journeyman Refrigeration Pipefitter - Master Electrician

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                  • #10
                    Originally posted by Bented View Post
                    BSPP threads are parallel, BSPT threads are tapered.
                    In the US NPT threads are tapered and NPS threads are straight.

                    The question that begs for an answer is why is anyone using a standard developed in the 19 century in the year 2021?

                    I dislike tapered pipe threads in every possible way.
                    Someday when you get in the "real world" and need to pipe something you will know.
                    Retired - Journeyman Refrigeration Pipefitter - Master Electrician

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                    • #11
                      At my last job I eliminated pipe threads in every way possible. Flanged connections of all kinds, plastic and metal welds, O-ring sealed, hose clamped etc. Leak issues almost completely beaten. Only threads left were 1/4 npt on instruments and if metal to metal we used a Loctite sealant.

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                      • #12
                        If you know what your doing you won't have leaks in pipe fittings. Larger pipe over 2 inches or so is usually welded or joined by means other than threads. It just depends on the application. Here is just one > https://www.victaulic.com/
                        Retired - Journeyman Refrigeration Pipefitter - Master Electrician

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                        • #13
                          Originally posted by wmgeorge View Post

                          Someday when you get in the "real world" and need to pipe something you will know.
                          True enough, have only worked in machine shops for the last 40 tears (-:

                          I make these fittings regularly, various sizes from 1" to 3" NPT.

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                          • #14
                            Originally posted by Cenedd View Post
                            I knew the answer in that I was aware it referred to vintage pipe OD's ....
                            ID is the size reference.

                            -Doozer

                            DZER

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                            • #15
                              Originally posted by Cenedd View Post
                              ... how many 1/4" BSP threads are on a 1/4ā€ schedule 80 pipe that would render the measurement meaningful?
                              1/4" size ID is more about flow restriction calculation than anything else.
                              Plumbers don't care about pipe size, just what flow fulfills the need.
                              Not everyone thinks like a machinist.

                              -Doozer
                              DZER

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