Nominal diameter

[SeanSweeney]

Very daft question but this is driving me nuts.

The nominal diameter as I was aware is the estimate internal diameter of a pipe. But I seem to be seeing references to DN being the outside diameter.

What is correct. (european standard)

(materials - cast iron and PVC (normally given as internal and external diameters respectively)



Cheers. Sean.

 

[BigInch]

If its "NOMINAL" diameter it is a nominal diameter which is NOT an inside diameter or an outside diameter... Its "nominal".

Examples,

12" "Nominal" pipe is 12.75 OD, but 18" NOMINAL pipe is 18" OD.

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[RWF7437]

"Nominal" means "in name only". Thus it could be the inside diameter, the outside diameter, or Neither !

If you know either both or one of the actual diameters why would you care what the nominal diameter is ? Whatever the practice is in your part of the world you can learn it but it won't help you a great deal to know that.

good luck

 

[SeanSweeney]

Thanks for your answers.

It seems that the DN isn't so useful. I want to specify a drainage pipe of something like 300mm. There are two major pipe producers in Norway, Wavin and Pipelife. Wavin does 300mm and Pipelife 315mm with IDs 255 and 276. How does one specify for a pipe using DN. The only way for me would be based on a minimum ID of 255 - bore, making DN redundant.

Is this a local problem in Norway (i.e. do your markets have better conformity regs.)

Cheers. Sean.

 

[RWF7437]

Sean, In the U.S. the situation is the same. Nominal diameter doesn't tell you anything useful. The way to get around this is to specify the pipe material or materials which are acceptable, the "sizes" (nominal sizes), and refer to industry standards for other details. It is rare in practice that a difference between 300 mm and 315 mm ( 5%)is significant in the design of anything we Civil Engineers do. Perhaps in some applications in other fields this difference is significant but unless the performance of the product in the finished project is affected ot is simply not worth worrying about. Generally, we try to produce "performance specifications" rather than "prescriptive specifications".

good luck

 

[BigInch]

The answer is ... NEVER specify a pipe only by its diameter.

ALWAYS specify the OD and the Wall Thickness (WT).

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[katmar]

It is very difficult to make any general statement for European standards because there are so many. I would regard the Nominal Diameter in a European context as the "approximate" size. For example you may talk of a 300 mm pipe or a 500 mm pipe, where the actual dimensions would depend on the standard being used.

One thing that you can keep in mind is that for pipes (as opposed to tubes) the outside diameter is constant for a given standard and the wall thickness, and therefore the ID, varies with the pressure rating.

In the US context it makes a bit more sense. The ANSI sizes are based on the old IPS (Iron Pipe Size) standard and the Nominal Diameter is roughly the ID for sizes up to 12" for the "Extra Strong" wall thickness. For sizes 14" and above the Nominal Diameter is the OD in the ANSI system.

Katmar Software
Engineering & Risk Analysis Software

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[BigInch]

I'm not exactly sure why you think approximately = ID for nominal sizes to 12" and the OD for pipes >12 makes sense. Personally I don't see the logic, but if you think so, I guess that's a good thing.

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[katmar]

Hey BigInch, you are right. The only reason it makes sense is because we have grown up with it. But at least there is some systematic element to it, which I have not discovered in the European standards. It may just be my limited experience - the industries I have worked in tend to use ANSI sizes.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

Had to laugh, because I've struggled with it for my entire life, always muttering under my breath, "Why? Why? Why?". IF.... IF I could rework the standards, I'd use Nominal D = OD, but I do believe that in this case, what's done... is done. Kinda like the 4'-8[½]"

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[Artisi]

BigInch
4'8 1/2" - guess you are referring to British standard railway gauge - at least you don't forget it, well I haven't - probably first heard it a good 50 years back in primary school.

 

[BigInch]

Nope. Its the wheel base of a Roman Chariot, which was later adopted by European roadway carriges and ,since it was a regular roadway carrige that was adapted for steam power to run on rails, it needed the same width between the rails. Then they just kept building them that way. Its so institutional its even had an effect on the Space Shuttle design.

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[Artisi]

Thanks for the reminder on the Roman chariot wheel base - it seems you don't remember everything although I have seem the wheel track in the UK left by the Romans.

 

[BigInch]

I was being cute.

As for the shuttle, it seems that the booster rockets travel by rail to assembly point and consequently their diameter is (somehow) limited by the width of the US railway gage, which was handed down through the Roman-Brits heritage. I'll have to measure the Spanish rail gage. I don't know what it is.

Sean. Appologies for the hijack. I'm finished.

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[LittleWheels]

Booster rocket diameter was actually limited by a tunnel as I understand it - but I've been wrong before.

 

[Artisi]

Not the first time a rail system has determined the design of a piece of equipment - many years back the Allis-Chalmers pump company had to design the pump volute of a pump unit that could be carried on an open framed rail car capable of passing thru a rail tunnel.

 

[thomas17]

According to the European standards in metal pipes the nominal diameter is equal to the internal diameter. In plastic and cement pipes the nominal diameter is the external diameter. That is how it is categorized. Hope I was useful.

 

[BigInch]

thomas17,
These web pages seem to disagree with your ID theory.

http://www.worldwidemetric.com/PDFs/Piping_Comparison.pdf

http://www.spiraxsarco.com/resource...distribution/pipes-and-pipe-sizing.asp#head12

This Tutorial considers Schedule 40 pipework as covered in BS 1600.

Tables of schedule numbers can be obtained from BS 1600 which are used as a reference for the nominal pipe size and wall thickness in millimetres. Table 10.2.1 compares the actual bore sizes of different sized pipes, for different schedule numbers.

In mainland Europe, pipe is manufactured to DIN standards, and DIN 2448 pipe is included in Table 10.2.1.

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