Nominal Pipe Size and Diametre Nominal

  • Thread starter TSN79
  • Start date
  • Tags
    Pipe
In summary, the term 'Diameter Nominal' or 'NPS' is used in the US to identify pipes, but these names are only loosely related to the actual dimensions. This is due to historical reasons and the difficulty of changing infrastructure. This issue is further complicated by the use of different sizing systems in other countries. Engineers and physicists may also have different perspectives on this issue. Some resources, such as www.eng-tips.com and www.mcmastercarr.com, can provide more information on mechanical fasteners and pipe threads. In conclusion, using tubing instead of pipes may be a simpler solution when precise measurements are needed.
  • #1
TSN79
424
0
I'm wondering about the the term 'Diametre Nominal', or 'NPS' as it's called in the US. I've realized that pipes that are identified by "nominal" names are only loosely related to the 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. I find this "inaccuracy" very strange. And another thing, most DN measures are relatively even numbers like 15, 20, 25, 40, 50 and so on. But then there is also a 32 in there which totally breaks the "even-ness"! That puzzle's me the most! If someone has a good explanation to my wonderings I would be grateful :)
 
Engineering news on Phys.org
  • #2
Unfortunately, we American's are insane and we like making things hard for ourselves. I'm sorry but that's the best answer I can give. I have found that things like this are governed by the "olden days" and were set due to ease of manufacturing and once a standard is set, its hard to change. If you don't like my answer I suggest you try www.eng-tips.com. Its a great resource for questions like this.
 
  • #3
Pipe sizes are based on the equivalent internal diameter, because you normally care about the flow rate. Lead pipes had thicker walls because lead isn't as strong, so when we switched to copper the external diameter reduced. because the wall thickness was reduced.
 
  • #4
Admittedly, there is a lot of old ways that are still in use today because of the cost or difficulty in changing the infrastructure. One of the issues that clouds things for American pipe sizes is that the early days didn't use all of the schedule sizes we have today. The old ways of stating wall thickness was based on a word description versus the current method which relates to the maximum stress. The new pipe schedule numbering was an effort to make some sense of things.

I put this topic up there with why a lumber 2x4 is really 1.5 x 3.5
 
  • #5
What mgb_phys said jives with what I remember reading a year or so ago, that things changed when they switched to stronger materials and could get by with thinner walls. But they needed to keep the threading the same so that for example all 2" pipes would still mate together ... that meant changing the I.D. rather than the O.D.
 
  • #6
Redbelly98 - that makes more sense, I must have had it the wrong way round.
It's even worse in the UK, there's metric, BA and Whitworth!
In France+Germany an adjustable wrench is known as an 'English', because if you are working on any british equipement you never know what the bolts are.
 
Last edited:
  • #7
Reminds me of the time a coworker and I needed to loosen a nut on his car. We went into the lab, grabbed a variety of wrenches (in 1/16" increments) thinking one of them just had to fit. Walked out to the parking lot, determined that the correct size was in between 3/8" and 7/16". Back we go into the lab, hunting for a 13/32" wrench. (Not sure why we didn't just grab an adjustable, probably we had tried it and the space was too small to fit one in there.)

We couldn't find a 13/32, so we went to one of the technicians who was also a whiz with cars, to ask if he had a 13/32 wrench. He look at us blankly. "A 3/8 is too small, and a 7/16 is too big", we told him. "Oh," he said, "sounds like you need a 10 mm".

He was right. We grabbed a 10mm from the set of metric wrenches we kept in the lab, and that did the trick. The weird thing was, we were 2 Ph.D. physicists who needed to be reminded about thinking in metric!
 
  • #8
The weird thing was, we were 2 Ph.D. physicists who needed to be reminded about thinking in metric!

Classic demonstration of the difference between engineers and physicists.:smile:
 
  • #9
How long ago was this? I thought all cars had been metric for quite some time now.
 
  • #10
How long ago was this? I thought all cars had been metric for quite some time now.

Way to rub it in Fred. :rofl:
 
  • #11
Hey... I'm still working on machines with Whitworth fasteners.
 
  • #12
www.mcmastercarr.com

These guys usually have a decent description of what is what when it comes to mechanical fasteners, pipe threads, etc.

One of the ones that gets me is the difference in diameter between PVC and CPVC pipe. Just ridiculous.
 
  • #13
I guess the moral of the story is that, if you don't want to look up wall thicknesses and ODs in a table (like you have to with pipes), try to work with tubing whenever possible.
 
  • #14
FredGarvin said:
How long ago was this? I thought all cars had been metric for quite some time now.

About 5 years ago. Not sure how old the car was at the time, but it was a Honda or Toyota which should have been a dead giveaway to us.

(If we had some kind of "Duh..." smiley/emoticon, I'd put one here.)
 

1. What is Nominal Pipe Size (NPS)?

Nominal Pipe Size (NPS) is a standard used to refer to the size of a pipe. It is a numerical value that does not correspond to the actual dimensions of the pipe, but rather serves as a designation for different pipe sizes.

2. How is NPS determined?

NPS is determined by the inside diameter (ID) of the pipe and its wall thickness. The wall thickness is standardized based on the pipe's schedule, which is a ratio of the pipe's wall thickness to its outside diameter (OD).

3. What is Diametre Nominal (DN)?

Diametre Nominal (DN) is a metric equivalent to NPS. It is also used to refer to the size of a pipe, but is based on the outside diameter (OD) of the pipe rather than the inside diameter (ID).

4. What is the difference between NPS and DN?

The main difference between NPS and DN is the measurement used. NPS is based on the inside diameter of the pipe, while DN is based on the outside diameter. NPS is more commonly used in the United States and Canada, while DN is used in most other countries.

5. Are NPS and DN interchangeable?

No, NPS and DN are not interchangeable. While they may refer to the same pipe size, the actual dimensions and wall thickness may differ. It is important to use the correct designation when selecting pipes and fittings to ensure proper compatibility and functionality.

Similar threads

  • Mechanical Engineering
Replies
15
Views
654
  • General Engineering
Replies
2
Views
8K
Replies
3
Views
1K
  • General Engineering
Replies
1
Views
3K
Replies
6
Views
1K
  • General Engineering
Replies
4
Views
9K
  • General Engineering
Replies
5
Views
10K
Replies
11
Views
8K
  • Mechanical Engineering
Replies
4
Views
8K
Replies
2
Views
12K
Back
Top