Piping elbows and bends are very important pipe fitting which are used very frequently for changing direction in piping system. Piping elbow and piping bend are not the same, even though sometimes these two terms are interchangeably used. A bend is simply a generic term in piping for an “offset” – a change in direction of the piping. It signifies that there is a “bend” i.e, 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 made by using a bending machine (hot bending and cold bending) on site and suited for a specific need. Use of bends are economic as it reduces number of expensive fittings. An elbow, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece (based on ASME B 16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45 degree or 90 degree. There can also be custom-designed elbows, although most are catagorized as either “short radius” or “long radius”.
In short “All bends are elbows but all elbows are not bend”
Whenever the term elbow is used, it must also carry the qualifiers of type (45 or 90 degree) are radius (short or long) – besides the nominal size.
Elbows can change direction to any angle as per requirement. An elbow angle can be defined as the angle by which the flow direction deviates from its original flowing direction. Even though an elbow angle can be anything greater than 0 but less or equal to 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly used while making piping layouts for such situations.
In order to find out the elbow angle at A, it is necessary to consider a plane which contains the arms of the elbow. If there had been no change in direction at point A, the pipe would have moved along line AD but pipe is moving along line AG. Plane AFGD contains lines AD and AG snd elbow angle (phi) is marked which denotes the angle by which the flow is deviating from its original direction.
Considering right angle triangle AGD, tan (phi) = v( x2 + z2)/y
Similarly elbow angle at G is given by : tan (phi1) = v (y2 +z2)/x
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Elbows or bends are available in various radii for a smooth change in direction which are expressed in terms of pipe nominal size expressed in inches. Elbows or bends are available in three radii,
a. Long radius elbows (Radius = 1.5D) : Used most frequently where there is a need to keep the frictional fluid pressure loss down to a minimum, there is sample space and volume to allow for a wider turn and generate less pressure drop.
b. Long radius elbows (Radius > 1.5D) : Used sometimes for specific applications for transporting high viscous fluids likes slurry, low polymer etc. For radius more than 1.5D pipe bends are usually used and these can be made to any radius. However, 3D & 5D pipe bends are most commonly used.
c. Short radius elbows (Radius – 1.0D) : to be used only in locations where space does not permit use of long radius elbow and there is a need to reduce the cost of elbows. In jacketed piping the short radius elbow is used for the core pipe.
Here D is nominal pipe size in inches.
There are three major parameters which dictates the radius selection for elbow. Space availability, cost and pressure drop.
Pipe bends are preferred where pressure drop is of a major consideration. Use of short radius elbows should be avoided as far as possible due to abrupt change in direction causing high pressure drop.
Minimum thickness requirement:
Whether an elbow or bend is used the minimum thickness requirement from code must be met. Code ASME B 31.3 provides equation for calculating minimum thickness required in finished form for a given internal design pressure
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