NPT Threads

  1. Hi Guys,
    My friend is asking me to do the calculation for NPT thread. He is asking if 1 1/4 NPT thread is able to hold 10 000Psi pressure acting on it.
    In my opinion there is no need to do the calculation for the thread. Of course it will hold it.
    Don't quote me on this but I think that there is no difference in the way how low and high pressure npt threads are cut. High pressure fittings are more bulky due to pressure.
    What are your thoughts?
  2. jcsd
  3. Q_Goest

    Q_Goest 2,989
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    NPT generally refers to the US style tapered thread. There is a European counterpart with both tapered and straight thread design. I presume you mean the US type.

    In general, the fitting isn't limited by the thread, it is limited by the overall dimensions of the fitting. Pipe fittings can be made to almost any standard including one that the supplier comes up with, so for any pipe thread fitting you need to verify the pressure rating with the supplier or to a recognized standard. The US standard that would generally cover these is ASME B16.11. That standard states that a class 6000 threaded fitting is equal to a Sched. XXS pipe in the size given. So for a 1-1/4", Class 6000 fitting built to ASME B16.11, the fitting pressure rating is equal to a 1-1/4" pipe, schedule XXS. Depending on material, it could have a pressure rating of 10,000 psi, but you would have to check. Note it is also a function of the temperature the fitting will be operated at.
  4. Hi Thanks,
    That makes sense. Pressure rating of the pipe = pressure rating of the thread when it comes to NPT threads.
  5. Q_Goest

    Q_Goest 2,989
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    Some people/companies/industries will derate a pipe with a threaded end because the threads have removed material which would otherwise be used to resist hoop and axial stress. I don't know how common that is throughout industry, but our company does that. In fact, our company doesn't allow pipe less than sched 80 to be threaded. Thinner walled pipe must be welded. Note also that the piping codes require that you take wall tolerance into account when determining pressure rating.
  6. Yeah those scheduled pipes are not accurate but we always apply 1.5 safety factor to anything that is under pressure.
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