February 15, 2007

Finding and fixing cracks in high-temperature headers

Peter Kennefick, Thielsch Engineering Inc.

Major failures of superheater and reheater outlet headers are relatively infrequent. But when headers do fail, repairing them may take weeks, and replacing them may take up to six months. Forced outages are expensive, so it is imperative to develop and follow a routine inspection program for high-pressure/temperature steam headers. Early detection, monitoring, and repair performed on your schedule is always preferable to that phone call in the middle of the night.

 

Ideally, the inspection of a high-temperature header should begin with a magnetic particle examination of all of the welds on it—including circumferential butt welds, seam welds, header-to-tube stub welds, and any penetration and attachment welds. The inspection should be followed by a volumetric examination of any butt welds and a remote visual (borescopic) examination of the interior of the header, and then by in-situ metallographic examination (replication) and hardness testing.

If your plant is new or doesn't have a history of steam header problems, or if your budget or outage schedule wouldn't allow a comprehensive header inspection, consider a limited inspection. Restrict it to headers prone to service-related deterioration and to areas on them where a row of tube holes substantially overlaps or bisects a circumferential butt weld (Figure 1). Recent reports from the field indicate that these areas may be susceptible to severe ligament cracking. The level of susceptibility has three factors: the designs of the header and the weld, the composition of the weld's final filler material, and the stresses associated with normal operation.

 

 

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