Max torque that can be applied to a threaded body

  • Thread starter aander3120
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  • #1
What is the max amount of torque that could be applied to 1 in male threaded NPT pipe when it is threaded into a 1 in female threaded body that is made from ASTM 126 cast iron. The dimensions of the threaded female body are shown below.

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  • #4
JBA
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Are you looking for the estimated torque to fitting tensile failure or a recommended assembly torque?
 
  • #5
Both would be nice, but mainly looking to what amount of torque would cause failure.
 
  • #6
JBA
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This is something best determined by a series of tests on a number of pipe/fitting samples. Since the general dimensions and accepted application pressures for most pipe fittings are set by industry standards; and, the method of "hand tight plus (n) number of turns" for a pressure tight npt joint is well established, there is little incentive for such testing by either fitting manufacturers or users. If you have a special fitting where the fitting wall less than or greater or you want to exceed the pressure limit of the accepted standards, you are probably going to be required to do those tests yourself.

The failure point of given wall thickness and yield strength pipe and a given wall thickness and tensile strength fitting due to the taper wedging effect can be determined by FEA analysis; but, relating that to a reasonably accurate resulting joining torque is still going to be very difficult; and, still must be verified by a series of tests on a number of pipe/fitting samples.
 
  • #7
CWatters
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Google found some recommended torques for NPT pipe threads. Example...

https://www.surpluscenter.com/images/techhelp/NPT.pdf

but note the comment...

Torque installation of pipe fittings is not a recommended practice. Thread taper and quality, different port and fitting materials, plating thickness and types, varying thread sealants, orientation, and other factors reduce the reliability of a torqued connection.
That begs the question what's the alternative? Tighten until it stops leaking?
 
  • #8
JBA
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The generally accepted procedure is to always use a pipe thread sealant and to use the finger tight plus the number of turns, generally the higher value, given in the third column of the table in your referenced website document. This based upon an accumulated 4 years of overseeing the construction of petroleum storage facilities including the installation of utility service piping systems using npt connections. During that time, I have never seen any plumber use a torque value on the npt fitting and piping joints. To be honest, they always use thread sealant and generally tighten joints until "it feels right" based upon their prior experience.
 
  • #9
CWatters
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I have never seen any plumber use a torque value on the npt fitting and piping joints. To be honest, they always use thread sealant and generally tighten joints until "it feels right" based upon their prior experience.
and I do the same when reassembling anything mechanical like a lawn mower.

But this "feels right" method feels really unscientific. Essentially we are relying on our own internal uncalibrated torque meter. I wonder if anyone has done a study of the "hand tight and an extra quarter turn" or the "just right" methods?
 
  • #10
JBA
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At some point, there must have been some analysis or testing done to develop the generally recommended number of turns past finger tight range for npt joint assembly.
Parker Hannifin Corporation Tube Fittings Division Columbus, Ohio is one of the world's premier manufacturers of npt and tubing fittings for high pressure service; and, if you go to page S7 of the below of the attached document on applications by Parker you will find, in the second column, their discussion and recommendations for npt threaded connection assembly that terminates with the same table of "number of turns past finger tight" values found is in many other documents.

http://www.parker.com/literature/Tube Fittings Division/Assembly_Installation.pdf

While possibly "not scientific" some methodologies are simply the result of long periods of successful application. Anyone, including tradesmen, that uses npt connection fittings quickly establishes their own "until it feels right" standard, in addition to the recommended number of turns, that may or may not exceed the number of recommended assembly turns, but rarely, if ever, is less than those values because such assemblies are almost assured to leak under pressure.
 
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