what is called nominal diameter in pipes

what is called nominal diameter in pipes? explain in detail relating nominal diameter,inside diameter,& outside diameter

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Dear saleh:

Nominal diameter in *pipe* ends up with a relationship from the stated size, to the actual OD. From there, expressed modifiers alter the wall thickness, but the OD is unaffected.

If you are in the USA, "iron pipe size" would be given as "1 inch" for example. The actual OD is 1.315 (if I recall correctly). The standard wall thicknesses (under IPS) were STD, XS, and XXS. Now we have "Schedules" like 5, 10, 40, 80, and


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In general in the USA, the pipe size, with either STD or Schedule

40 wall, the ID for sizes 1 inch to 12 inches is really close to 1 inch to 12 inches.

Nominal is important, because pipe fittings use this surface for defining a connecting seal.

David A. Smith

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N:dlzc D:aol T:com (dlzc)


Didn't seem too much like a homework question?

esp with the operative phrase

"explain in detail"

cheers Bob


If this is a homework problem, the intent is for you to do some work other than posting to two ng's.

You're using gmail, learn to use their search engine.

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Dear BobK207:

As it happened, I was doing some research recently (like Friday) on the "iron pipe size" standard. We have a "historical artifact" that was installed in 1936, and were wondering what surprises would be obtained in trying to connect to it.

If the OP relies on others for his/her understanding, then he/she won't survive the exams. Hopefully he/she will learn that early in a "summer" session.

English is slippery for a non-native speaker.

David A. Smith

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N:dlzc D:aol T:com (dlzc)

Hi Saleh:

If you're talking about the American Standard for pipe, it's confusing as hell. It's based on old standard OD's that were chosen years ago so that the nominal pipe size was the ID of the pipe. This used a standard wall thickness that we don't use anymore. For some reason the new standard adopted the OD sizes from the old standard and chose new, different wall thicknesses. So now the nominal size indicates neither the OD nor the ID of the pipe...not even close. A 1/8" pipe, for example has an OD of .405 and an ID of .269 (for standard grade), neither of which is anything close to 1/8" (.125"). There is no convenient formula to convert nominal sizes to actual sizes...you have to look them up.

The OD of pipes is a given (basically arbitrary) number that always remains the same, no matter the wall thickness of the pipe. Pipe wall thickness is designated by a "schedule" number. Standard (STD) weight pipe is called "schedule-40" (why the number 40 I don't know). Extra strong (XS) pipe is "schedule-80". There are also other bizarre sizes like "schedule-120" and "schedule-160" and "double extra strong" (XXS). Large pipe can also have sizes like "schedule-20", or even "schedule-10". Note that the OD is the same for all these sizes; only the ID changes for different schedule numbers.

The upshot is, you need to see what is available from your pipe supplier and work from that.

Don Kansas City

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Get an Education and read the specification "ASTM A53" section "Scope" nominal is defined for pipes there.


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Some pipes are based on the inner diameter or were at one time. Looks like the smaller schedule 80 pipes follow this.

In any event, pipe wall thickness were decreased as material properties improved over the years. The outer diameter sizes were maintained to allow different pipes to fit together. That's why the ID are not round numbers.

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Jeff Finlayson

technically speaking - as I understand it and I had checked it years ago:

the nominal diameter is a hydraulic dimension, i.e., it was originally the effective hydraulic diameter at a flow of steam (I think - although it may have been water) in iron pipe at a given temperature and pressure. You can calc out the effective hydraulic diameter of iron pipe dimensions and it comes up nominal to roughly 5-10%.

That is why the actual material diameter is all over the place relative to nominal diameter, as the pipe size goes up. (Unlike hose and tube). With higher flows and other materials, that relationship is no longer used.

As to the geometrical dimension - unless it has changed in the past 10 years:

The OD tolerance for pipe is held quite tight, so it is a fair bit more expensive than round tube. On the other hand, pipe ID is pretty sloppy.

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I might just add that the OD of the pipe is always held constant for a nominal size because it gets a standard pipe thread on the ends. This way, any pipe of a given nominal size will thread into any fitting with the same nominal designation, regardless of the strength of the pipe. So a 1" pipe nipple always fits into a 1" elbow no matter what its schedule number is.

Don Kansas City

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