Center to End Dimensions for Elbows Other Than 45, 90 and 100.

[Pavan Kumar]

Hi All,

I need the method to calculate the center to end dimensions for long radius elbows with elbow angles other than 45, 90 and 180 Deg. Information provided in some sources I found on the internet( link copied below) is bit confusing. Kindly also provide the basis for your answers.

https://www.pipingengineer.org/pipe-elbow-center-calculator/

For example if I want to calculate the center to end distance "B" for a 45 Deg elbow. If I use the formula mentioned then

B = Tan (45/2) * 1.5* D

for a 2" 45 Deg LR elbow

B = Tan(45/2)*1.5*2 = 1.2426 inches = 31.56 mm

The value from the standard dimensions chart, link copied below, for 2" elbow says

B= 35 mm.

http://www.wermac.org/fittings/dim_elbows_a.html

I want to know how.

If I understand this, I want to calculate the center to end dimension "B" for an elbow angle of 24.76 Deg for my project.


Thanks and Regards,

Pavan Kumar.

 

[1503-44]

The standard fittings have straight tangent lengths on the ends.
If you cut a 45 in the middle to make 2 x 22.5°, you won't have a tangent at the cut location.

 

[3DDave]

The "2" in that table is the pipe size, not the radius. The radius is given for the A value of the 90 degree elbow.

I don't see where 1.5 comes from.

Tan(theta/2) = the straight end distance / radius of the bend at the center of the elbow.

The formula given was because the elbows are not in the table by radius; only by setback. So if all you know is setback, then you need to divide by the tangent(theta/2) to get the original radius.

 

[LittleInch]

ASME B16.9 section 6.2.4 implies that dimension B is different if you cut the elbow.

SO if you get one made use that formula, but if you cut a 45 elbow you won't get the same dimension.

At least that's my reading of it...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Pavan Kumar]

Hi3DDave,

I have given an example of a Long radius 45 Deg elbow. For a LR elbow r=1.5 D. for a 2" 45 Deg LR elbow

As per ASME B16.9 Section 6.2.4, the "B" dimension for segmented elbow is given by

Bs= A x Tan(Θ/2)

where is A is the dimension A for the 90 Deg elbow from which the segmenting is being done

A) With this lets says we are cutting a 45 Deg C elbow from a 2" 90 Deg LR elbow

A for 2" LR 90 Deg elbow = 76 mm

Theta = 45 Deg

Therefore Bs = 76 * Tan(45/2) = 76 * 0.4142 = 31.48 mm.

Per the table below the B dimension for 45 Deg elbow is 35mm. That was my question.

http://www.wermac.org/fittings/dim_elbows_a.html

B) Lets say I want to cut a 2" elbow with 24.76 Deg C angle what would be the "B" dimension?.

is it Bs = 76* Tan(24.76/2) = 16.68 mm ?

Thanks and Regards,
Pavan Kumar

 

[3DDave]

Probably best to not post copyrighted material.

 

[LittleInch]

6.2 4 is not easy to interpret but at the start it says "factory made segments.."

Now I read that to means that the B dimension is somehow an odd dimension.

B16.9 provides 45 and 90 factory made elbow dimensions. It doesn't say that you can cut either and get the same thing. So in your B this would be the dimension if you forged a 25 elbow , NOT if you cut it out of a 45 elbow. At least that's my interpretation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TBP]

I would talk to a shop that does pipe bending, and get them to make the custom bend, cut the ends to whatever it is they can cut them to, and weld that section of pipe in. Alternatively do a double 90 or 90 & 45 offset if you can take the elevation/dimensional change in your assembly.

 

[KevinNZ]

This may a bit of subject, but cutting, say, a 45 bend from a 90 may give problems since the 90 may not be round in the middle. The butt weld to the cut 45 would then be misaligned.

 

[Pavan Kumar]

Hi 3DDave , LittleInch ,TBP ,KevinNZ ,

I am modeling an existing pipe from a distillation tower overhead to the overhead condenser in CAE Pipe software to do the Pipe Stress Analysis. I have to enter the dimensions in this model. I need the "B" dimension for a bend that has an elbow angle of 22.8 deg to input the value in the model. I need the correct way to calculate this "B' dimension. I was using a 45 deg elbow as an example to verify the formula given in B16.9. An accurate input would help me model correctly. This is an existing bend. I am NOT fabricating new one now.

I downloaded a free copy of B16.9 from internet. I will avoid posting it next time.

Thanks and Regards,
Pavan Kumar

 

[3DDave]

The standard radius for that 45 degree fitting is 84.5 mm; for the 90 degree fitting the radius is 76 mm.

It seems like the off-standard fitting could be either one of those or some other radius. If it is a highly stressed area you may have to either make an accurate measurement from the installation or to try likely limits on different runs of the analysis.

When I apply that given:B, find:radius, formula to the 3D version of the 45 degree 2 inch fitting, I get the same radius as the 90 fitting (152 mm); I don't know why the standard for smaller radii don't have consistent radii.

 

[racookpe1978]

Do you want (need) a uniform bent elbow, or one with a variable inlet and outlet tangent (straight) length?
Are you placing the specialized pipe bend as a macro (assembly) with an original at the workpoint on the bend's centerline arc, at the inlet CL, or on the point of rotation?

 

[LittleInch]

For the analysis I would just use whatever figure ASME B 16.9 calculates.

If this is going to impact your results by 3mm on a 2" elbow I think you've got other problems.

Sometimes there is no perfect answer, so you need to state your assumption (following the calculation in B16.9), stick the dimension in and then run the analysis.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Pavan Kumar]

Hi All,

I got the problem solved by measuring the "B' dimension of the bend and inputting in the model.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi All,

I found the 45 Deg and 90 Deg elbows in my piping model are actually Mitre Bends. Now I need to get the standard dimensions for 45 and 90 Deg Mitre bends. I mainly want the values of the standard radius of bend by pipe size. It would be helpful if anyone can point me to a reliable source.

Thanks and Regards,
Pavan Kumar