Pipe and tube

[mklotz]

What is the difference between pipe and tube? At first glance, the naive
might say "none". After all, they're both just hollow cylinders. That's
wrong, of course. There is a difference (in the metalworking arena) and I
hope to clear up a bit of the confusion here.

For any hollow cylinder, there are three important dimensions - the outside
diameter (od), the inside diameter (id) and the wall thickness (wt). Since
these three are related by a simple equation:

od = id + 2 * wt

one can completely specify a piece of pipe/tube by supplying any two of these
numbers.

Tubing is more frequently used in structures so the od is the important
number. Strength depends on the wall thickness. So tubing is specified by
the od and the wt. Very logical and simple to measure. The id is simply
whatever falls out of the equation above.

Pipe is normally used to convey gases or fluids so the internal
cross-sectional area (defined by the id) is important. It's therefore not
surprising that pipe is specified by the id. Although anyone who's ever done
any plumbing knows that the id on the pipe label is only a *nominal* id. As
an example, a (nominal) 1/8 wrought steel pipe will typically have a
*measured* id of 0.269 (schedule 40) or 0.215 (schedule 80). (More below
about those schedule numbers.)

While the designation for tubing is straightforward, that for piping is
obscure for some perverse reason unclear to me. All pipe of a given nominal
size has the *same od*. An abbreviated list:

nominal
size OD

1/8 0.405
1/4 0.540
3/8 0.675
1/2 0.840
3/4 1.050
1 1.315
1-1/4 1.660
1-1/2 1.900
2 2.375

Now, the folks (ASME?) who codify this stuff, in an effort to make things
difficult for us, instead of specifying the wall thickness directly, decided
to use (seemingly arbitrary) schedule numbers to specify the wall thickness.

For instance, a (nominal) 1/8 schedule 40 pipe will have a wall thickness of
0.068 (id=0.269) while a 1/8 schedule 80 pipe will have a wall thickness of
0.095 (id=0.215).

And, no, these schedule numbers do not reflect a constant wall thickness. For
instance, a (nominal) 1/4 schedule 40 pipe has a wt = 0.088 while the same
pipe in schedule 80 has wt = 0.119.

Schedule numbers range from as small as 5 up through 40, 80 (common) to as
high as 100, 120 and 160. There may be others. This is not my area of
expertise. Larger schedule numbers correlate one-for-one with thicker walls,
which seems to be the only predictable thing about schedule numbers.

To the best of my knowledge, there is no mathematical relationship that can be
used to translate schedule number into equivalent wall thickness. You're
forced to consult a table. Machinery's Handbook has such a table for wrought
steel pipe (pg. 2378 in the 23rd edition). Do these tables also apply to pipe
made of other materials (e.g., plastic)? I don't know, but I doubt it. That
would be too simple. Since I don't want to make a career out of plumbing
minutiae, I'll let you research it for your application.

I can only guess that the schedule number relates to some burst pressure and
thus the relationship to wall thickness is non-linear. But that's only a
guess - anyone who knows the real story please correct me.

Why do you care? Well, beyond the satisfaction of simply knowing some obscure
metalworking stuff, this should help you in selecting and specifying hollow
cylindrical elements for that project you have planned. It should help you to
understand why you won't have much success trying to bend tubing with a pipe
bender. On the latter, the bending dies are sized to the (constant) pipe ods
mentioned above. It's unlikely they'll fit any tubing you buy since tubing od
generally comes in straightforward sizes like 1/4, 3/8, 1/2, etc. If you want
to bend tube in a pipe bender, count on making some purpose-built dies - a
tricky lathe job. Or buy a tubing bender.