Nominal Pipe Size and Diametre Nominal

[TSN79]

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 :)

 

[Topher925]

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.

 

[mgb_phys]

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.

 

[FredGarvin]

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

 

[Redbelly98]

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.

 

[mgb_phys]

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.

 

[Redbelly98]

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!

 

[Topher925]

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.

 

[FredGarvin]

How long ago was this? I thought all cars had been metric for quite some time now.

 

[Topher925]

How long ago was this? I thought all cars had been metric for quite some time now.

Way to rub it in Fred.

 

[gmax137]

Hey... I'm still working on machines with Whitworth fasteners.

 

[dingpud]

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.

 

[FredGarvin]

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.

 

[Redbelly98]

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.)