A Short Article on Elbow as Pipe Fitting
Steel pipe elbow (sometimes also refereed as bends) is a key part in a pressure piping system used to change the fluid flow direction. It is used to connect two pipes with same or different nominal diameters, and to make the pipe and thus the fluid direction turn to a certain direction of 45 degree or 90 degree. This change in fluid flow direction adds pressure losses to the system due to impact, friction and re-acceleration.
Classification of Steel Pipe Elbows
Steel pipe elbows can be classified through following various parameters,
- Direction Angle
- Length and Radius
- Connection with Pipe
- Material of Construction
Types of Elbows based on Direction Angle
According to fluid flow direction of the pipes, elbows can be divided into different degrees, such as 45 degree, 90 degree, 180 degree, which are most common elbows. Also there are 60 degree and 120 degree elbows for some special pipelines. This degree is just an representation of the angle by which the fluid flow is going to change after flowing through the said elbow.
Types of Elbows based on Length and Radius
Elbows are split into two groups which define the distance over which the flowing fluid change direction; the center line of one end to the opposite face. This is known as the “Center to Face” distance and is equivalent to the radius through which the elbow is bent.
If the radius is the same as pipe diameter (Center-to-Face dimension of 1.0 X diameter ), it called Short Radius Elbow (SR elbow) used normally for low pressure and low speed pipelines or in tight areas where clearance is the main issue. If the radius is larger than pipe diameter (Center-to-Face dimension of 1.5 X diameter) then we call it a Long Radius Elbow (LR Elbow) used for high pressure and high flow rate pipelines.
Types of Elbows based on Connection with Pipe
As per the connection with pipe, elbows could be classified as Butt Welded Elbow, Socket Welded Elbow and Threaded Elbow.
Butt Welded Elbow
Butt weld elbow is a steel elbow formed by hot pressing or forging. Its connection form is to directly weld the elbow and steel pipe. Butt welded elbows have beveled ends to allow for ease of welding. This bevel allows for full penetration weld in most of the cases. Butt welded elbows are mainly used for elbow with higher pressure and temperature.
Socket Welded Elbow
Socket weld elbow is also welded to the pipe and fittings end. Unlike the butt weld elbow, the socket weld elbow has a trapezoidal area at the end. We can insert pipe end into this area (the diameter of SW elbow is matched with the outer diameter of pipe), then do welding work to connect them together. Therefore, the actual diameter of SW elbow end is greater than the elbow diameter.
The shape of threaded elbow is similar to the SW elbow, except that the inner surface of the trapezoidal area has been machined into thread. It is easier to install and remove, good for pipeline repair and maintenance.
Types of Elbows based on Material of Construction
Stainless Steel Elbow : ASTM A403 WP Gr. 304, 304H, 309, 310, 316, 316L, 317L, 321, 347, 904L
Carbon Steel Elbow : ASTM A 234 WPB , WPBW, WPHY 42, WPHY 46, WPHY 52, WPH 60, WPHY 65 & WPHY 70.
Low Temperature Carbon Steel Elbow : ASTM A420 WPL3, A420 WPL6
Alloy Steel Elbow : ASTM / ASME A/SA 234 Gr. WP 1, WP 5, WP 9, WP 11, WP 12, WP 22, WP 91
Duplex Steel Elbow : ASTM A 815, ASME SA 815 UNS NO S31803, S32205. Werkstoff No. 1.4462
Nickel Alloy Elbow : ASTM / ASME SB 336 UNS 2200 ( NICKEL 200 ), UNS 2201 (NICKEL 201 ), UNS 4400 (MONEL 400 ), UNS 8020 ( ALLOY 20 / 20 CB 3, UNS 8825 INCONEL (825), UNS 6600 (INCONEL 600 ), UNS 6601 ( INCONEL 601 ), UNS 6625 (INCONEL 625), UNS 10276 ( HASTELLOY C 276 )
Some Other Types of Elbows
In addition to the above defined elbows there are few more types of elbows which are not used on regular basis. Few of them has been defined below.
A reducing elbow is a type of fitting which is used to join two pipes of different sizes. The reducing elbow is so called because it looks like a reducing piece and elbow combined into one. The reducing elbow eliminates one pipe fitting (reducer) and reduces the welding by more than one-third. Also, the gradual reduction in diameter throughout the arc of the reducing elbow provides lower resistance to flow and reduces the effect of stream turbulence and potential internal erosion. These features prevent sizable pressure drops in the line.
For many suppliers it is a non standard item resulting into high price with a long delivery time. The use of a “normal” elbow with a separate reducer is an option if the situation allows.
Male Pipe Elbows & Female Pipe Elbows
Male pipe elbows and female pipe elbows are popular tube fittings which provide an fluid flow directional change of a tubing run. While a male pipe elbow is used to connect fractional tube to female tapered pipe thread, a female elbow is used to connect fractional tube to male NPT thread. These types of tube fittings like male elbows and female elbows have been specifically designed for use on instrumentation, process and control systems and equipment employed in chemical, petroleum, fluid power, electronic and pulp and paper plants.
Wall Thickness of Elbows
The weakest point on an elbow is the inside radius. ASME B16.9 only standardizes the center to face dimensions and some “squareness” dimensional tolerances. The wall thickness at the weld line location even is standardized, but not through the rest of an elbow. The standard states that the minimum tolerance will be within 12.5% of the minimum ordered wall thickness of the pipe. A maximum tolerance is specified only at the ends of the fitting. Many providers of butt weld elbows provide one schedule greater thickness so that sufficient wall thickness, after forming, remains.
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