Long radius vs short radius elbows: which one should you specify?

Open any piping material take-off and you will find elbows listed as 90 degree or 45 degree, long radius or short radius. The difference looks small on a drawing, but it changes pressure drop, the space the line needs and the price of the fitting. This guide explains the two types so you can specify the right one the first time.

What the radius actually means

The radius of an elbow is measured from the centre of the bend to the centreline of the pipe. For a long radius elbow it equals 1.5 times the nominal pipe size (1.5D). For a short radius elbow it equals 1.0 times the nominal pipe size (1D). A 6 inch long radius elbow therefore turns the flow through a radius of 9 inches, while a 6 inch short radius elbow turns it through 6 inches.

Which standard covers which

Long radius elbows, 45 degree elbows, tees, reducers and caps are covered by ASME B16.9 for factory-made wrought buttweld fittings. Short radius elbows and returns are covered by ASME B16.28. For stainless steel, MSS SP-43 covers lightweight fittings made for corrosion-resistant service at lower pressure. Always state the standard on the enquiry, because dimensions and tolerances differ between them.

Long radius elbows: the default choice

The gentler curve of an LR elbow keeps the flow smoother. That means lower pressure drop, less turbulence and less erosion at the outer wall of the bend, which matters in slurries, steam condensate and any line carrying solids. Lower pressure drop also means lower pumping energy over the life of the plant. For these reasons most process, utility and fire-water piping specifications call for long radius elbows unless the drawing says otherwise.

Short radius elbows: when space rules

An SR elbow turns the line in a tighter footprint. It is useful on crowded pipe racks, on skid-mounted units, and anywhere a long radius elbow would clash with steelwork or equipment. The price you pay is higher pressure drop and more wear at the bend. Short radius elbows are normally kept for low-velocity, clean fluids.

Side by side

How to specify an elbow completely

An elbow enquiry that gets a fast and accurate quote includes every item on this list.

• Angle: 90 degree, 45 degree or 180 degree return
• Radius: long radius or short radius
• Nominal pipe size and wall thickness or schedule, matching the pipe it welds to
• Material grade and standard, for example ASTM A234 WPB or A403 WP316L
• Seamless or welded construction
• Dimensional standard, such as ASME B16.9, B16.28 or MSS SP-43
• Certificates required, such as EN 10204 3.1

Match the wall to the pipe

The elbow bevel is cut to ASME B16.25, normally 37.5 degrees, and it must match the pipe wall for a clean, full-penetration weld. Specify the elbow in the same schedule as the connecting pipe, or the welder will have to deal with a thickness mismatch at every joint.

A simple rule of thumb

Specify long radius unless layout forces you otherwise, and record the exception on the drawing. That keeps the line hydraulics predictable and avoids a mixed stock of elbows on site where the wrong one can get installed.

Need long or short radius elbows in carbon, stainless or alloy steel? Send us your size, schedule and grade list and we will come back with a quote.