For a decade or extra, warmth restoration steam generator (HRSG) customers have heard that Grade-91 tubing (T-91) is prone to microstructure injury from improper warmth remedy. Now, a brand new injury mechanism in T-91 tubing has been recognized, which HRSG customers heard about on the February 2018 HRSG User’s Group Conference.
During the dialogue on warmth switch elements, the brand new injury mechanism, dubbed “exfoliation,” was mentioned to have first proven up as deposits on the steam turbine cease valves at a 500-MW mixed cycle plant within the Northeast U.S. When initially found, the deposits have been attributed to flow-accelerated corrosion (FAC). But lab evaluation confirmed they weren’t the magnetite sometimes related to FAC, so Intertek specialists have been known as in to analyze. Further research recognized the deposits as spinel, an oxide scale recognized to happen in fired boilers, the place reheater tubes could function as sizzling as 1,300F, however not recognized to happen in HRSGs, the place reheater tubes sometimes function at just one,050F.
The reheater tubes on this HRSG, nevertheless, operated hotter than that due to supplementary firing. Commonly known as duct burners (Figure 1), the supplementary-firing system was elevating this HRSG’s reheater tube temps to 1,150F—excessive sufficient, doubtlessly, to clarify the formation of spinel oxides. As a consequence, Intertek specialists monitored that risk because the plant cycled by means of its regular operations. The staff noticed a really clear course of occurring on this HRSG. Spinel oxides would type and cling tightly to the T-91 tubes till enough thickness constructed as much as fracture from the thermal gradients of the plant’s biking operations.
|1. Duct burners can elevate tube temps into the spinel formation zone. Courtesy: John Zink Hamworthy|
As fractured layers broke off, or “exfoliated,” they’d journey alongside the steam path and type deposits on the downstream elements seeing excessive steam velocities, such because the steam turbine cease valves. This course of occurred again and again: Spinel layers would type, exfoliate, after which reform. Any move abnormalities—comparable to water hammer, a leaking attemperator, or a slug of condensate throughout begin up—drove the method extra rapidly.
To handle the T-91 exfoliation downside, HRSG customers ought to periodically examine their duct-burner flame patterns, as a result of elongated flames can impinge on, or “lick” the tubes, and lift steel temperatures larger into the spinel-formation zone. An even higher instrument for monitoring flame patterns is a steady, on-line monitoring system, aiming infrared thermography cameras on the aircraft simply upstream of the duct burners (Figure 2). An identical, superior monitoring instrument is passive acoustic mapping, which is an imaging methodology based mostly on the acoustic emissions obtained by an array of ultrasonic sensors.
|2. Infrared thermography is a good way to watch duct-burner flames. Courtesy: Scientific Engineering Inc.|
—Doug Hilleman, PE is principal engineer with Intertek Group PLC and Rob Swanekamp is government director of HRSG User’s Group Inc.
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