Interchangeability of Threads 1

[manian]

Are Line Pipe Threads and NPT Threads interchangeable? If not, please explain what are the differences between these threads. If NPT threads would suffice the need, why Line Pipe Threads are required ? What are the torque requirements for Line Pipe Threads and NPT Threads ? How to calculate the torque values ?

 

[MikeVV]

Please clarify what you mean by "line threads." I'm not aware of this term. Several different tapered threads are made under the National Pipe Thread specification(s). NPT threads are tapered and obtain their ability to seal by forcing the threads together with this taper. Torque requirements are not often found because the threads are effective over a range of torques, that allows fittings to be positioned properly. (The toughest decision a plumber makes is, "should I go one more turn...") &lt;grin&gt;. <br>
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The torque values would be difficult to calculate. The tapered thread complicates the approach. An estimate of the pressures between thread faces can be made as a function of torque but I believe that most specifications of torque are based on acceptable past performance. I have found torque specifications published by manufacturers for internal company use. MIL standards also contain torque requirements as part of the qualification testing process. Qualified fittings must be sampled. A performance test (static hydroproof) is performed on fittings that have been assembled to proper torques. This is a good reference for operational data.<br>
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[manian]

"Line Pipe Threads" are taper threads similar to NPT threads and are used extensively in the flow line pipes. Flow lines are the piping that run between the Oilfield Wellhead and the Group Gathering Station. In some cases may be even upto the refinery. These lines are to be capable of containing crude oil pressures of e.g 5000 psi and hence safety is of utmost importance. These threads are covered in American Petroleum Institute's Specification 5B. Since high pressures are involved, the torque should be high enough to avoid leakages and at the same time to be low enough so that the metallic threads are not damaged. i would be thankful if some one could provide torque data for these threads.

 

[MikeVV]

Aeroquip Corp has information for "NPTF" threads - National Pipe Thread - Fuel. This may be the same specification that you are working with. Also, thread torque information is provided by Del Aerospace Inc at: <br>
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http://home.earthlink.net/~delaero/<br>

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- Hope this helps...

 

[manian]

Thank you very much.<br>
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but NPTF threads are just variants of NPT threads i.e with fine pitches & some other dimensional differences. But LP threads are different. Thanks for the web page address. But it has only torque values of conical seal fittings and not for taper threads.<br>
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Thanks a lot any way.

 

[MikeVV]

Just tryin' to help &lt;grin&gt;. You are working an area that appears to be very specialized - I'm glad I was able to meet you because maybe you can help me sometime!

 

[ricey]

This may be a bit late for your requirements, but here goes: in my experience many engineers equate API (line pipe threads) & NPT as one and the same. They are not. The API standard (from memory) highlights a few variations on the taper length, which would not be compatible with a standard NPT dimension. Secondly, the torque values for the NPT taper thread would also be affected by the type of lubriction used to effect the seal on the taper eg PTFE tape or liquid sealant. As one of your replies states; it really is suck it and see with taper threads. Ovetorquing can have a catastrophic, explosive effect on the female port of any NPT installation.<br>
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Why not junk NPT in favour of HP UNF coned fittings? Up to 150,000 psi and with no dubiety over the torques or performance of the thread. Just a thought.