Pipe ID and Pipe OD

Pipe is defined by ID, tubing is defined by OD.

Pipe is defined by ID, tubing is defined by OD.

Just to elaborate slightly "Pipe is defined by ID" because it's original purpose was as a vessel to carry fluids hence the inside was the important dimension AND "tubing is defined by OD" because it's original purpose was structural hence the outside was the important dimension!

However, engineers like tubing for stuffing fluid in cos' it's handy and some joker made a whole load of fittings for tubing to facilitate it's use as a pipe. Now pipe and tube is indelibly mixed!

Practically it was of importance to me for fits.

Interior Dimension is 3/4" for instance and outside Dimension is 1" for instance.

I had no idea at all philosophy was involved.

I thought this sort of thing was simply about physics.

I thought I needed to know both interior dimension and exterior, outside dimensions when putting stuff together in places.

Big ID to small ID over distance maintains pressure when doing plumbing.

Big wires come in the house, and then small wires spread throughout.

Similar "Philosophy".

The reason for this "philosophy" seems to be buried in history. I'm not sure this is a correct explanation, which I heard long ago: Once upon a time, pipe was made from wrought iron according to inside diameter, with a wall thickness to suit typical pressures. This established an O.D. for each pipe size, and pipe is actually manufactured by O.D. With the advent of steel, the wall thickness could be less. The O.D. was maintained and the I.D. enlarged. Thus what started out as 1/2-inch pipe (0.840 O.D.) is now still 0.840 O.D., but 0.622 I.D. (Schedule 40). The old 1/2-inch designation was retained, but only as a nominal, rather than actual, I.D.

I don't think there are any formulas for this; instead it is semi-arbitrary, and you have to look things up in a table. Engineering handbooks and pipe catalogs have this information, as do many online resources. It takes a bit of getting used to, but there actually is some underlying sense to it all.

GA. Unfortunately, this thread and all others referenced herein are riddled with errors. Correct information is present in each of these threads, too, but an unfamiliar reader won't be able to tell which is which.

Among the more unusual items are 1) spiral-welded pipe formed from helically wound skelp, and 2) elliptical tube, which is used in some high-end bicycles and in at least one brand of evaporative condensers (resulting in an advantageous air flow path over the staggered tubes).

Playing into all of this are 1) history and 2) the manufacturing processes involved. I wish I could find a video from an evap condenser manufacturer--it would be quite something to watch. If there is enough interest in this, I will try to describe it later.

There are non standard pipes available and they are called pipe.

There are standard tubes available (ASTM/DIN/JIS)

The differences are not very clear except the usage and fitting.

The tubes (circular) typically has a slightly closer tolerance on OD to accommodate the fittings - the compression fittings do not act well to seal in case the tolerances are as wide as pipes.

The pipes do not have to accommodate the compression fittings and hence do not impose this restriction.

Eg Tube 1" will have a tolerance ±0.1mm on OD, for pipe it may be about 0.5mm

For 4" it is 0.38mm on tube , 0.8mm for pipe

How often are we going to hear this same question and the multitude of incorrect answers and wild attempts at justifying the fuzzy logic of those incorrect answers.

The original poster asked (16 months age):

"Some pipes are denoted /described by ID & some by OD.

What is the idea behind this philosophy?"

Pipe is not denoted/described by the ID. Pipe is denoted by it's "Nominal" size. This goes for all sizes and in both imperial and metric designations. It just so happens that the pipes larger than 12" (imperial) have an Outside Diameter the same as the "Nominal" pipe size (NPS). All the other NPS sizes and all the metric sizes are identified by a metris Diameter Nominal (DN) size that do not actually correspond to the actual measurment. Please, as all of us older long time pipers have done, learn to accept it, you will not change it

Here are two questions (and my answers) related to this same question

Pipe vs. Tube

The question:

What is the difference between Pipe and Tubing?

My answer:

Tubing is normally "cold drawn" seamless with the O.D. the same as the size designation (i.e.: 1" inch tubing has an O. D. of 1 inch). You can get different pressure ratings of 1" tubing by changing the wall thickness. Thus the I. D. is smaller. Tubing normally comes in twenty foot lengths or in coils. Tubing is normally joined with compression type tubing fittings.

Pipe is made seamless or rolled and welded (sometimes referred to as ERW pipe). The pipe sizes up to 12' has a nominal size which is close but not the same as the actual O. D. Pipe in sizes 14" and larger have an O. D. the same as the nominal size. Both the seamless pipe and the ERW pipe come in different wall thicknesses depending on the pressure/temp criteria. Pipe normally comes in lengths called "single random" (20'+/-) or "double random" (40'+/-). ERW pipe is also furnished in very long lengths, spooled on large coils for pipeline needs. Pipe is normally joined using butt weld pipe fittings.

Pipe Sizes - NPS vs. metric

The question:

"How does nominal size relate to actual diameter?"

My answer:

Pipe sizes, along with the related pipe fittings, pipe flanges and valves are designated in both inches (NPS - Nominal Pipe Size) and metric units (DN - "Diameter Nominel").

Pipe is identified by "nominal" or "trade" names that are loosely related to actual dimensions. For instance, a 2-inch galvanized steel pipe has an inside diameter of about 2 1/8 inches and an outside diameter of about 2 5/8 inches. It is called "2-inch pipe" only for the sake of convenience.

There are the inch names for pipe products called NPS or "nominal pipe size" and their metric equivalents called DN or "diametre nominel". The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings.

Where I come from, a simple question deserves a simple answer.

Thanks for giving us the anal retentive version.