# Bend geometry

I'm quite confused on how bends are categorised. Some would refer it to 1.5D or 3D bends for instance, while some will use the ratio of the bend diameter (D) to pipe bore (d).

I somewhat understand the two separately, but couldn't go about how to convert between the two. Can someone help me understand them better?

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SteamKing
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I'm quite confused on how bends are categorised. Some would refer it to 1.5D or 3D bends for instance, while some will use the ratio of the bend diameter (D) to pipe bore (d).

I somewhat understand the two separately, but couldn't go about how to convert between the two. Can someone help me understand them better?
The nomenclature "1.5D" and "3D" refers to the radius of the bend of the fitting to the nominal diameter D of the fitting. AFAIK, D is not the bend diameter.

Below, find a sample table of ANSI fittings:

The nomenclature "1.5D" and "3D" refers to the radius of the bend of the fitting to the nominal diameter D of the fitting. AFAIK, D is not the bend diameter.

Below, find a sample table of ANSI fittings:

Alright, I understand about the 1.5D and 3D definition. How about the D/d ratio?

SteamKing
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Alright, I understand about the 1.5D and 3D definition. How about the D/d ratio?
Well, what about it? Your original post wasn't too clear on that.

I think that D is used for the outside or nominal diameter, while d is used for the inside diameter in some cases. Most piping catalogs include a diagram which lays out the key dimensions of the fitting. OD and ID are used for the respective diameters so that their meaning is unambiguous.

Well, what about it? Your original post wasn't too clear on that.

I think that D is used for the outside or nominal diameter, while d is used for the inside diameter in some cases. Most piping catalogs include a diagram which lays out the key dimensions of the fitting. OD and ID are used for the respective diameters so that their meaning is unambiguous.

Please refer to the link below, section 1.3.4.2. I'm quite confused about the bend geometry here as compared to the regular 1.5D and 3D ratio.

https://books.google.com.my/books?i...considered in terms of the bend angle&f=false

SteamKing
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I'm sorry, but for some reason, this link isn't working for me. Can you provide an alternate means of viewing this page?

EDIT: Nevermind. Got it working.

SteamKing
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It appears this reference is discussing design rules for designing things like pneumatic conveying systems, which don't use regular steel pipe designed for pumping liquids, for example. That's one reason bend radii are so large, for example D/d = 24:1. In the latter case, it appears that the bend radius D = 24 × d, pipe diameter.

The bend radii are very large so that the pneumatic carriers:

don't get stuck in the piping when they are being pushed thru the system by compressed air.

It appears this reference is discussing design rules for designing things like pneumatic conveying systems, which don't use regular steel pipe designed for pumping liquids, for example. That's one reason bend radii are so large, for example D/d = 24:1. In the latter case, it appears that the bend radius D = 24 × d, pipe diameter.

The bend radii are very large so that the pneumatic carriers:

don't get stuck in the piping when they are being pushed thru the system by compressed air.

Okay, I get what you meant. For instance, I'm using 3D elbows (in the general context), so how do I interpret it in terms of the D/d ratio?

SteamKing
Staff Emeritus
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Okay, I get what you meant. For instance, I'm using 3D elbows (in the general context), so how do I interpret it in terms of the D/d ratio?
Then the bend radius of the pipe or conduit is going to be so many times the bore of the pipe or conduit.

For example, D/d = 24:1 means the bend radius D = 24 × Pipe diameter d.

It's not clear how pneumatic conduit is made: If a piece of straight conduit is bent to the required radius or what.

For fluid piping, there are standard sizes of elbows and other fittings made to match the nominal diameter and wall thickness schedules of straight piping. The bend radii of these elbows is either 1.5 × nominal diameter, the so-called "short radius" elbow, or 3.0 × nominal diameter, the so-called :long radius" elbow. Anything else must be specially manufactured for liquid piping systems or fabricated by bending a length of straight pipe.

I think that D is used for the outside or nominal diameter.

SteamKing
Staff Emeritus