What is the difference between elbow and bend in piping?

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.

How to distinguish between pipe elbow and pipe bending?

Pipe elbows and pipe bends are both very common pipe fittings products which are used to change the flowing direction in a piping systems.Some times they are interchangeable,but not same.

What is a Piping Bend?

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 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 fittings.

What is a Piping Elbow?

A PIPING 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 degrees or 90 degrees. There can also be custom-designed elbows, although most are categorized as either “short radius” or long radius”.

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:

  1. Bend is a generic term for any offset or change of direction in the piping. It is a vague term that also includes elbows.
  2. An elbow is an engineering term and they are classified as 90 deg or 45 deg, short or long radius.
  3. 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.
  4. Bends are generally made or fabricated as per the need of the piping; however, elbows are prefabricated and standard and are available off the shelf.
  5. Bends are never sharp corners but elbows are. Pipe bending techniques have constraints as to how much material thinning can be allowed to safely contain the pressure of the fluid to be contained.  As elbows are prefabricated, cast or butt welded, they can be sharp like right angles and return elbows which are 180 degrees.
  6. Elbow is a standard fitting but the bends are custom fabricated.
  7. 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.
  8. All elbows are bends but all bends are not elbows.
  9. Bend has a larger radius than elbows.
  10. Generally, the most basic difference is the radius of curvature. Elbows generally have a radius of curvature between one to twice the diameter of the pipe.  Bends have a radius of curvature more than twice the diameter.

A Typical Piping Elbow with Elbow Angle

Elbow Radius

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. 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 b. Short radius elbows (Radius = 1.0D): to be used only in locations where space does not permit the 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 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 change in a direction causing the 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 an equation for calculating minimum thickness required (t) in finished form for a given internal design pressure (P) as shown below:

Code Equation for Minimum Elbow Thickness Calculation

Here,

R1 = bend radius of welding elbow or pipe bend
D = outside diameter of the pipe
W = weld joint strength reduction factor
Y = coefficient from Code Table 304.1.1
S = stress value for material from Table A-1 at the maximum temperature
E = quality factor from Table A-1A or A-1B Add any corrosion, erosion, mechanical allowances with this calculated value to get the thickness required.

End Connections

For connecting elbow/bend to pipe, the following type of end connections are available

  • Butt-welded: Used along with large bore (>=2 inch) piping
  • Socket welded: Used along with pipe size
  • Grooved end: A grooved end fitting has a groove or shoulder along the edge. This fittings groove allows for a seal without the need for welding.
  • Flanged: Flange connections are very important in the petroleum and chemical industries, and bolts and nuts are an essential part of them.

Butt-welded Elbows

The pipe is connected to butt welded elbow as shown in Fig. 4 by having a butt-welding joint.

Butt-welded fittings are supplied with bevel ends suitable for welding to the pipe. It is important to indicate the connected pipe thickness /schedule while ordering. All edge preparations for butt welding should conform to ASME B16.25.

Dimensions of butt welded elbows are as per ASME B16.9. This standard is applicable for carbon steel & alloy steel butt weld fittings of NPS 1/2” through 48”.

A typical butt welded elbow

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 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 dia. and layout is congested, butt welded elbows are preferred.

Butt-welded joints can be radiographed and hence preferred for all critical services.

Material standards as applicable to butt welded fittings are as follows:

ASTM A234

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 upon chemical composition. Selection would depend upon pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specification are listed below:

ASTM A403

This specification covers two general classes, WP & CR, of wrought austenitic stainless steel fittings of seamless and welded construction. Class WP fittings are manufactured to the requirements of ASME B16.9 & ASME B16.28 and are subdivided into three subclasses as follows:

  • WP-S-Manufactured from a seamless product by a seamless method of manufacture.
  • WP – W These fittings contain welds and all welds made by the fitting manufacturer including starting pipe weld if the pipe was welded with the addition of filler material are radiographed. However, no radiography is done for the starting pipe weld if the pipe was welded without the addition of filler material.
  • WP-WX These fittings contain welds and all welds whether made by the fitting manufacturer or by the starting material manufacturer are radiographed.

Class CR fittings are manufactured to the requirements of MSS-SP-43 and do not require non-destructive examination. Under ASTM A403 several grades are available depending upon chemical composition. Selection would depend upon pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specification are listed below:

ASTM A420

This specification covers wrought carbon steel and alloy steel fittings of seamless & welded construction intended for use at low temperatures. It covers four grades WPL6, WPL9, WPL3 & WPL8 depending upon chemical composition. Fittings WPL6 are impact tested at temp – 50° C, WPL9 at -75° C, WPL3 at -100° C and WPL8 at -195° C temperature.

The allowable pressure ratings for fittings may be calculated as for straight seamless pipe in accordance with the rules established in the applicable section of ASME B31.3.

The pipe wall thickness and material type shall be that with which the fittings have been ordered to be used, their identity on the fittings is in lieu of pressure rating markings.

 

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