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.

[C-Bag]

Now THAT is a useful post.

Do you know how many times I've asked plumbers and sales people what IS the difference between pipe and tube? And all I've ever gotten was a blank stare that seemed to convey either "are you stupid?" or "shaddup and buy it". This points out that pipe is a murky quagmire of nonsensical specs and it seems purposely done to only make sense to plumbers. And it doesn't really make sense, they just got used to it.

[mklotz]

I hear you C-bag. Don't get me started on idiotic nomenclature systems.

Labeling drills with fractions is bad enough but numbering them and then making the biggest numbers correspond to the smallest sizes. Plus, if you make a drill bigger than #1 now you're into the double idiocy of things like #0, followed by #00, #000, etcan. We already had that with machine screws, the infamous #0-80, #00-90, #000-120 and #0000-160, so what do they do? They use letters of the alphabet A-Z, thus creating a closed-at-both-ends nomenclature that won't easily permit the addition of larger/smaller sizes or intermediate sizes.

It may be logical in the wire-drawing factory to label wire with a number denoting how often it's been pulled through the dies, but why allow that system out into a world where most people care about the diameter of the wire and not the details of how it was made? Plus this leads to a counter-intuitive system where higher numbers mean smaller wire.

But let's not risk any consistency, they chortle. Let's label music wire with thicker wire having the larger number just to keep them on their toes. I wonder that they didn't label it with the note it would sound when plucked on an eighteenth century zither.

The standards agencies in most metricized nations have had the good sense to label things with the size; a 6 x 1 metric screw tells you right off everything you want to know. Note that nomenclature systems are not an element of the metric system (one of my pet peeves is that everyone seems to think so) but are the product of folks who gave some thought to how they would name things.

Sorry for the rant; this stuff sets my teeth on edge.

[Frank S]

Now THAT is a useful post.

Do you know how many times I've asked plumbers and sales people what IS the difference between pipe and tube? And all I've ever gotten was a blank stare that seemed to convey either "are you stupid?" or "shaddup and buy it". This points out that pipe is a murky quagmire of nonsensical specs and it seems purposely done to only make sense to plumbers. And it doesn't really make sense, they just got used to it.

C-bag it probably started out as a secrete handshake type of thing. But just suffice to say that tubes are made to a tighter tolerance. If you really want to get blank stares ask them why some pipes are welded seams and others are seamless.
If you do some searching around pipe yards you will find that many of the smaller sizes 8" and udder in sch 40 will be welded seam while many of the sch 80 pipes 3/4 and above will be seamless and all sch 100 and above will be seamless, as will most pipes labled structural or high pressure grade.
Round tubing has some strange designations as well the ornamental or decorative grades will all be welded seam there is also internally flattened welded seam then seamless and at the top of the list will be DOM or drawn over mandrel tubing.
IF you are really in the mood for some seeing some head scratching ask them if their pipes are API grade.
A lot of folks think of Oilfield tubing as being pipe and it is often called pipe such as 2 3/8 upset tubing or 2 7/8" or 3 1/2" But even after the ends are cut off and the OD's are the same as 2", 2 1/2" & 3" pipes it is still and always will be tubing.
Have you ever given any thought as to how they know when to replace a down hole string without even pulling it out of the ground? Well they have scientific methods now and log records of the age and amount of fluids which have been pumped through it but one tried and true way is to weigh the entire string with a column of what ever fluid that has been flowing in it. They know the weight of the tubing (pipe) when it was new and the weight of the fluid. by looking at a chart or graph they can tell exactly what the difference in the wright of the tubing is now. Or they can send a bore Gage to the bottom let it expand then read off the ID of every inch of the string on the way up. For pipelines buried or not they have what is called pigs and robots they send through the pipeline all they have to do is follow it and read out the internal size as it travels the pig cleans as it travels while the robot tells the operator everything else even with pictures and videos

[Toolmaker51]

Probably a tremendous over simplification, but here it is. As pipe and tube were utilized differently, a strict uniformity wouldn't have appeared sensible. Pipe schedules relate to pressure, so regardless size they'll contain the same force. Wall thickness and internal surface area and inextricably connected. The connections are the weak link whether slip or threaded. Pipe threads are not intended to create a mechanical connection in the way nuts and bolts do. Piping requires support to protect the joints, the taper of pipe threads is less about leak proof than trying to align cylindrical threads; attaining a seal benefits from the taper secondarily. Having assembled fire sprinklers, full lengths, overhead, countless fittings, etc makes one appreciate a system intimately. Scheduled pipe does reduce field efforts, at time of engineering and permit inspections, and when replacement occurs, instead of potential miscalculations.
Tubing structure designs consider torsion, compression, etc; mechanical forces controlled by wall thickness and external surface area whereas connected to other forms, such as plate where machining and welding form the joints. Most structures require heavier material foundation/ bases that could reduce proportionally with height. Wall thickness and/ or taper accomplish that. It's easy to purchase cylindrically telescopic sizes of round, square, and rectangular tube. Very long 'tubes' are now formed in specific faceted tapers, as used in large wind generators, high tension electrical towers. Round tapers have been around longer, such as flag poles. Size limits in length are imposed by what can be transported over the road, diameters by bridges en-route to construction sites.
Another basis is older. The societies bent on colonization imposed measurement on subject colonial territories, which influenced development of tools and materials from there on. The more isolationist groups concentrated on logical standardization, whether adopted as simply convenient or wholly anew. The empires drove both, as trade would dictate, being more or less in control of production.
WWII began the first real documentation of incompatible components, especially mechanical fasteners which occurred first. The differences were retained; with perfectly accurate representation of dimensions so a producer could complete parts intended for a distant user. There have been successive meetings, notably in the 60's and 70's. Can't imagine the volume of paper and pencils consumed before pocket calculators...
Marv's observation on drill sizes is correct, with an additional condition in colonial business. Not to say much logic or foresight is evident. That was to meet demand of increasingly diverse products and processes; in physical size, complexity, and fits desired for functional purposes; multiplying in both directions.