Expanded Pipe

The Best Transition Design for Expansion Joints With Oversize Bellows

Many expansion joints with internal sleeves are designed with oversize bellows to eliminate any reduction in diameter. This is important to consider if the media contains abrasive particles that could erode surfaces that redirect the media. A full bore sleeve is also important to use for process conditions that could be affected by turbulence or pressure drop. In addition, a full bore sleeve minimizes the possibility of flow induced vibration in high fl ow rate applications like safety relief valve piping.

The transition for producing the oversize bellows attachment can be achieved in a variety of ways. A simple fi llet welded ring transition can be used. However, this design presents sharp angular surfaces to the fl ow stream. These sharp surfaces are subject to erosion and the shape is not the best profi le to minimize turbulence. This design is not recommended for high temperature service (over 800°F) or for any service where the pipe is subject to radiographic examination since the circular fi llet welds cannot be evaluated by radiography.

For high temperature applications such as FCC process and flue gas lines and applications that require low turbulence like ethylene transfer lines, a tapered transition is best. This can be achieved with a welded cone or by expending the process pipe to form a smooth “S” shaped transition. Senior Flexonics Pathway’s standard transition is expanded pipe.

The expanded pipe transition is superior to the welded conic transition in many ways. The use of an expanded “toriconical” comer rather than a sharp welded corner will greatly reduce the stress concentrations at the junction. This is the area of highest pressure and thermal stress. Experimental results have shown that the measured maximum longitudinal stress due to pressure in a welded comer is more than twice the stress in an expanded corner.’ In addition, welds are “metallurgical notches” that have historically been the starting point for catastrophic failures. For this reason, welds usually require special examination to identify inherent defects. These concerns are eliminated by the use of expanded pipe transitions.