Drain stub bore hole cracking - Case study

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Courtesy of Babcock & Wilcox MEGTEC LLC

Purpose
This plant service bulletin alerts customers to inspect drain line stub bore holes for radial cracking.

Problem
Numerous incidents have been reported of header cracking caused by water quenching the drain line stub bore hole and header ID, especially reheater headers. This problem occurs when drain lines from boiler, superheater and reheater headers of a single unit or of multiple units are connected to a common blowdown tank. The quenching may result in cracks in welds, bore holes or other sections of the headers.

Background
During start-up and normal operation, the blowdown tank experiences unit operating pressure. Since the reheater operates at a much lower pressure than the boiler or superheater, fault)* or unclosed drain valves could allow water from the blowdown tank to flow back up to the reheater header through the drain line and header drain line bore hole. This backflow of water from the drain line could be at a much lower temperature than the header itself and could subject the header, the drain line, and the drain line bore hole to thermal shocks, which may cause cracks. Craze cracks may form directly downstream of the drain and can be recognized by an 'elephant-hide' appearance on the inside diameter of the component (see Figure 1). The cracks emanating from the drain bore hole typically appear as radial cracks (see Figure 2) which could extend through the header wall and result in drain stub leaks.

Recommendations
All units having drain piping from boiler, superheater or reheater headers connected to a common blowdown tank should be inspected. Those units that already have experienced drain line stub leaks should be inspected first, with other units having a lower priority for scheduling inspections.

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