Piping Elbows and Bends are very important pipe fitting which is used very frequently for changing direction in the piping system. Piping Elbow and Piping bend are not the same, even though sometimes these two terms are interchangeably used.
A PIPING 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 a 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. The use of bends is economic as it reduces the number of expensive pipe fittings.
A PIPING ELBOW, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece (based on ASME B16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45 degrees or 90 degrees. There can also be custom-designed elbows, although most are categorized as either “short radius” or long radius”.
In short “All bends are elbows but all elbows are not bend”
Depending on various piping parameters, elbows can be classified as follows:
Refer to Fig. 1 shows various types of piping elbows
Fig. 1: Types of piping elbowsWhenever the term elbow is used, it must also carry the qualifiers of type (45 or 90 degrees) and 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 (See Fig. 2 below). Even though An elbow angle can be anything greater than 0 but less or equal to 90° But still a change in direction greater than 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly are used while making piping layouts for such situations.
Fig. 2: A Typical Piping Elbow with Elbow Angle (phi)Elbow angle can be easily calculated using a simple geometrical technique of mathematics. Let’s give an example to you.
Refer to Fig. 3. Pipe direction changes at point A with the help of an elbow and again the direction is changing at point G using another elbow.
<div class="wp-block-image">Fig. 3 Example figure for elbow angle calculationdiv class="wp-block-image">
In order to find out the elbow angle at A, it is necessary to consider a plane that 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 the pipe is moving along line AG. Plane AFGD contains lines AD and AG and the 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) = √( x2 + z2)/y Similarly elbow angle at G is given by: tan (phi1)=√ (y2 +z2)/x
Elbows or bends are available in various radii for a smooth change in direction which is 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 are ample space and volume to allow for a wider turn and generate less pressure drop.
ss='code-block code-block-7' style='margin: 8px 0; clear: both;'>b. Long radius elbows (Radius > 1.5D): Used sometimes for specific applications for transporting high viscous fluids like 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 the most commonly used. In the pipeline industry, a piping bend of up to 60D is quite common.
c. Short radius elbows (Radius = 1.0D): to be used only in locations where space does not permit the use of long radius elbows 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 the nominal pipe size in inches.
T
here are three major parameters that dictate the radius selection for the elbow. Space availability, cost, and pressure drop Pipe bends are preferred where pressure drop is of major consideration. The use of short-radius elbows should be avoided as far as possible due to abrupt changes in a direction causing the high-pressure drop.Whether an elbow or bend is used the minimum thickness requirement from the code must be met. Code ASME B31.3 provides an equation for calculating the minimum thickness required (t) in finished form for a given internal design pressure (P) as shown below:
Fig. 4: Code Equation for Minimum Elbow Thickness CalculationHere,
For connecting the elbow/bend to the pipe, the following type of end connections are available
Dimensions of stainless steel butt welded fittings are as per MSS-SP-43. Physical dimensions for fittings are identical under ASME B16.9 and MSS-SP-43. It is implied that the scope of ASME B16.9 deals primarily with the wall thicknesses which are common to carbon and low alloy steel piping, whereas MSS-SP-43 deals specifically with schedule 5S & 10S in stainless steel piping.
Dimensions for short radius elbows are as per ASME B16.28 in the case of carbon steel & low alloy steel and MSS-SP-59 for stainless steel.
Butt-
welded fittings are usually used for sizes 2” & above. However, for smaller sizes up to 1-1/2” on critical lines where the use of socket welded joints is prohibited, pipe bends are normally used. These bends are usually of a 5D radius and made at the site by cold bending of the pipe.Alternatively, butt welded elbows can be used in lieu of pipe bends but usually smaller dia lines are field routed and it is not possible to have the requirement known at the initial stage of the project for procurement purposes. So pipe bends are preferred. However, pipe bends do occupy more space and particularly in pharmaceutical plants where a major portion of piping is of small diameter and the layout is congested, butt welded elbows are preferred.
Butt-welded joints can be radiographed and hence preferred for all critical services.
ASME B31.
3 allows the application of miter bends subject to meeting its pressure requirements.Material standards as applicable to butt welded fittings are as follows:
This specification covers wrought carbon steel & alloy steel fittings of seamless and welded construction. Unless seamless or welded construction is specified in the order, either may be furnished at the option of the supplier. All welded construction fittings as per this standard are supplied with 100% radiography. Under ASTM A234, several grades are available depending on chemical composition. Selection would depend upon the pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specifications are listed below:
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