10 replies to this topic

[SAprocessGD]

Hey,

With regards to pipe fittings,

It is my understanding that an elbow consists of a sharp "bend" usually at 45 or 90 deg angles. What is the difference between an elbow and a bend - or are they considered the same type of fitting?

Feel free to add any additional information. Thank-you!

[Art Montemayor]

An "elbow" and a "bend" are two different animals - as the terms are generally employed in the process industry.

A BEND is the generic term for what is called in piping as an "offset" - a change in direction of the piping. A bend is usually meant to mean nothing more than that there is a "bend" - a change in direction of the piping (usually for some specific reason) - but it lacks specific, engineering definition as to direction and degree. Bends are usually custom-made (using a bending machine) on site and suited for a specific need.

An ELBOW, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45^o or 90^o. There can also be custom-designed elbows, although most are catagorized as either "short radius" or long radius".

A bend can be an elbow; an elbow does not mean a bend. If you use the term elbow, it should also carry the qualifiers of type (45 or 90^o) and radius (short or long) - besides the nominal size.

Additionally, if you are using tube and not pipe, there is a marked difference. As you can bend tube for pressure applications you will get less interuption and turbulence in your flow; this translates to less pressure drop than with fitted 90 or 45 elbows.

[Vimalesh Agnihotri]

Art Montemayor:

Good Explanation. Thanks.

[rowanlim]

Thanks for the explanation, now I learned something new!

[ogpprocessing]

If you use the term elbow, it should also carry the qualifiers of type (45 or 90^o) and radius (short or long) - besides the nominal size.

Dear Art,

Could you please explain us about the difference between short radius and long radius elbows? And for which specific applications long and short radius elbows should be used?

[Modesty Blaise]

Long radius elbow is used to change the direction of flow gradually, 90 degrees.
Short radius elbow is used to change the direction of flow rapidly, 90 degrees.
45° elbow is used to change the flow only 45°.
Return bends are used to change the direction of flow 180°.

Regards,

Attached Files

•  Elbows and bends.pdf   154.85KB   316 downloads

Edited by Modesty Blaise, 22 April 2011 - 06:36 PM.

[Art Montemayor]

ogpprocessing:

The physical difference between Long Radius and Short Radius Elbows is graphically illustrated in the attached Workbook that I have put together for this thread.

The following is the criteria I use when deciding which to use:

Long Radius Elbows are used when:

• there is a need to keep the frictional fluid pressure loss down to a minimum;
• there is ample space and volume to allow for a wider turn and generate less pressure drop;
• the fluid being transported is abrasive or has solids in it.

Short Radius Elbows are used when:

• there is a need to reduce the cost of elbows;
• there is a scarcity of space and volume to allow a Long Radius type.

I hope this helps.

Attached Files

•  Chem Engineer Piping Guide.xls   1.66MB   405 downloads

[nmh]

Thanks for info

[Madhan Kumar]

Dear Art Montemayor Sir,

Thanks for your useful spreadsheet.



Cheer's
MK_Chem

[Elizabeth_I]

Thanks for the information, it's amazing to find the answers here!