3
$\begingroup$

I am confused about the difference between NPT, outer diameter of a pipe, and nominal pipe size.

Let's say I have a 1/8" NPT pipe. Does this mean the outer diameter of my pipe is 1/8"? Is the nominal pipe size equal to the outer diameter of the pipe?

$\endgroup$
2
  • $\begingroup$ Welcome to Engineering SE. Could you clarify what you are confused about. Nation Pipe Standard is a thread standard, pipes are measured by the inside diameter, and Nominal Pipe Size is a pipe standard. An 1/8" pipe can have different ODs because of different wall thicknesses. None of these are the same thing but they are related. $\endgroup$ – Drew_J Sep 8 '17 at 23:49
  • 2
    $\begingroup$ Can't have different OD because the threads always match. You can connect 2" sch 20 to 2" sch 120, etc. $\endgroup$ – blacksmith37 Sep 9 '17 at 17:06
2
$\begingroup$

Pipe comes in different wall thicknesses, designated schedule 40, Sch. 80, Sch. 10, etc. Higher numbers are thicker walls. The outer diameter is the same for all schedules, so the inner diameter varies. The nominal size is close to - but not exactly - the actual inner diameter for schedule 40 pipe.

Here is a chart summarizing the dimensions

$\endgroup$
3
  • $\begingroup$ I have seen this chart before, and I still think I do not understand it. For instance, in the first row of the chart it says, "Pipe Size (inches): 1/8; External Diameter .405, Internal Diameter .27;" 1/8 = 0.125. So, why is pipe that has an external, and internal diameter greater than 1/8" called 1/8" pipe? I suppose this is what ASTM A53 states? $\endgroup$ – Thomas Wilk Sep 11 '17 at 16:48
  • $\begingroup$ wikipedia also has the same note about the nominal size roughly corresponding to schedule 40 inner diameter. en.wikipedia.org/wiki/National_pipe_thread. It is however clearly not true for the smallest sizes. They are effectively (inexplicably) a whole size off with 1/8 Sch 40 -> 1/4 Id, 1/4->3/8 ID , etc. $\endgroup$ – agentp Sep 11 '17 at 17:15
  • $\begingroup$ @ThomasWilk - The chart defines the dimensions, but as you say the name is deceiving for smaller sizes. Probably would have been better to call rhem "Red", "Yellow", "Green", and "Blue". But whether iy makes sense or not, those are the names and dimensions. $\endgroup$ – Mark Sep 11 '17 at 22:37
0
$\begingroup$

The NPS designation is a name that indicates a size but it does not define that size. Like "two by four" is the name of a certain size of lumber, it not the actual size of the lumber. There is a particular size of pipe that is defined by the name AND schedule ; such as 2 inch, schedule 80.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.