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) 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.1 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 used while making piping layouts for such situations.
Elbow angle can be easily calculated using simple geometrical technique of mathematics. Lets give an example for you. Refer to Fig.2. Pipe direction is changing at point A with the help of an elbow and again the direction is changing at the point G using another elbow.
For connecting elbow/bend to pipe the following type of end connections are available
- Butt welded: Used alongwith large bore (>=2 inch) piping
- Socket welded: Used alongwith pipe size
- 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 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”.
- 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 use of socket welded joints is prohibited, pipe bends are normally used. These bends are usually of 5D radius and made at site by cold bending of 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 initial stage of the project for procurement purpose. So pipe bends are preferred. However, pipe bends do occupy more space and particularly in pharmaceutical plants where 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:
- 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.