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Thread: NPT vs ORB Fitting

  1. #1
    Associate Engineer
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    NPT vs ORB Fitting

    Hi All,

    Can one tell me when to use a NPT fitting and when to use a ORB fitting?

    Are both equivalent?
    Any preference for PTFE connection?

    Thank you

    -F. Laporte

  2. #2
    Technical Fellow
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    Quote Originally Posted by flaporte View Post
    Can one tell me when to use a NPT fitting and when to use a ORB fitting?
    Yes.
    Quote Originally Posted by flaporte View Post
    Are both equivalent?
    No.
    Quote Originally Posted by flaporte View Post
    Any preference for PTFE connection?
    Depends on the installation.

    If you can't take the time to describe THIS application, then how on earth do you expect to get an answer that is useful and appropriate for THIS application?

  3. #3
    Associate Engineer
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    Thank you for your kind answer Mr. PinkertonD.

    I was wondering if there was a general rule of thumb about NPT vs ORB.
    But It guess the answer is pretty straightforward from your reponse.

    There is this pvdf 1/16 ID barb - 1/8-27 NPT fitting that threads into a PTFE valve. 50% peroxide is flowing through it. The assembly is sealed with epoxy and the fitting is screwed until 1 thread remains visible. But in some occurences, it leaks. The opportunity to replace it with a O-Ring boss fitting was brought up and led to the question: Are NPT and ORB fittings equivalent?

    -F. Laporte

  4. #4
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    Ignoring the sarcasm, they can be equal in some applications, so research for each...
    Pressure rating tables
    Material performance tables <-- fluid reaction to base material, o-ring material and environment
    Temperature performance Tables <-- o-rings are limited in range

    It seems a little weird that you are installing down to one thread visible and hoping for the epoxy to handle the rest. The "T" in "NPT" is for "Taper" and that means the fitting is tightened down to a leak-proof torque, not a pretty-location.

    It seems a little weird that you are using epoxy as I suspect it is too brittle when set and vibration or knocks and bumps are allowing avenues of egress to form between the thread and the units. Why not tape and/or sealant?

    So, yes and no they may or may not be equivalent. Only your research can tell what is best for your application. As to PTFE same again. If the fluid is eating the o-ring then PTFE may be the next option to try.

  5. #5
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    Thanks PinkertonD.
    I find it weird too. I'm new in this company and I was put into this leak issue. I need to understand why it was designed this way and how to improve it. Thus your inputs are important.

  6. #6
    Technical Fellow jboggs's Avatar
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    The only thing that seals NPT connections is the "tightness" of the fit. Any installation spec that is not based on that fact will eventually lead to failure. I had one employer that tried to use an existing NPT fitting to also anchor a machine label in place. So, the installers tightened the fitting until they bottomed out on the label. It worked - sometimes. A fitting cannot be tight in the thread and tight on something else at the same time. It also may, or may not, have a consistent amount of thread showing. Tighten an NPT connection to a torque value, regardless of the thread left showing.

    SAE type ports, or ORB (O-Ring Boss) threads as you call them, are a very different animal. They are typically seen in more precise, higher pressure applications than NPT fittings. I like them, but you won't find replacement parts at the local Lowe's hardware if you know what I mean. In this case, the O-ring itself is the seal, so the application must meet all the factors normally associated with O-rings (pressure, temperature, fluid compatibility, surface cleanliness, etc.) They must be handled and installed very carefully.

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