In our previous article (Pipes and Bends –Part 1) we discussed about the difference between a pipe and a tube, meaning of nominal diameter and schedule of a pipe. In this article we will discuss bends, elbows and miter bends.
Bend or Elbow
There is always a doubt about the terms bends and elbows on ships. They are frequently used as synonyms. The difference between them is as follows:
- Bend is a generic term for any offset or change of direction in the piping. It is a vague term that also includes elbows.
- An elbow is an engineering term and they are classified as 90 deg or 45 deg, short or long radius.
- Elbows have industrial standards and have limitations to size, bend radius and angle. The angles are usually 45 deg or 90 degrees. All others offsets are classified as pipe bends.
- Bends are generally made or fabricated as per the need of the piping; however elbows are pre fabricated and standard, and are available off the shelf.
- Bends are never sharp corners but elbows are. Pipe bending techniques have constraint as to how much material thinning can be allowed to safely contain the pressure of the fluid to be contained. As elbows are pre fabricated, cast or butt welded, they can be sharp like right angles and return elbows which are 180 degrees.
- Elbow is a standard fitting but bends are custom fabricated.
- In bends as the pipe is bent and there is no welding involved, there is less pipe friction and flow is smoother. In elbows, the welding can create some friction.
- All elbows are bends but all bends are not elbows.
- Bend has a larger radius then elbows.
- Generally the most basic difference is the radius of curvature. Elbows generally have radius of curvature between one to twice the diameter of the pipe. Bends have a radius of curvature more than twice the diameter.
Short Radius and Long Radius
Elbows are again classified as long radius or short radius elbows. The difference between them is the length and curvature. A short radius elbow will be giving the piping a sharper turn than a long radius elbow.
- In a long radius elbow the radius of curvature is 1.5 times the nominal diameter. In a standard elbow the radius of curvature is 1.0 times the nominal diameter of the pipe.
- Long radius elbows give less frictional resistance to the fluid than the short elbows.
- Long radius elbows create lesser pressure drop than short radius elbows.
- Short radius is less costly than long radius elbows.
- The short radius elbows are used where there is scarcity of space.
In addition to this classification the elbows are 45 degrees, 90 degrees and 180 degrees also called as a return elbow.
The 45 degrees elbow turns the fluid /piping at 45 degrees and so on.
Another type of bend is a Miter bend. A Miter bend is a bend which is made by cutting pipe ends at an angle and joining the pipe ends. A true miter bend is a 90 degree bend made by cutting two pipes at 45 degrees and joining them by welding. Similarly three pipes cut at 22.5 degrees will give a 90 degree miter bend.
In the next article we will discuss about various pipe fittings.