due to the unavailability of a processing unit, the worst-affected components are the radiant tube hangers or shock intermediate tube support (ITS). These shock ITS are often a nuisance for refinery inspection teams, as cracks are quite common (see Figure 4 ). In one such occurrence, the refinery inspection team reached out with
Figure 4 A wide crack is observed across the entire width of the ITS
The burner reconfiguration had a very positive impact on heater operation, with the flame height being reduced to half its existing height. The maximum flame height was now restricted to approximately 55% of the tube length, with crisp and stable flames that did not overlap with each other. This optimised burner configuration led to a reduction in the BWT by more than 90-100°C, as evident from the operating data before and after the revamp. The relaxation in BWT also allowed the operator to exercise greater flexibility in operation over the entire range of operation, including the end-of-run (EOR) operating scenario. Nonetheless, a detailed assessment by a furnace designer is a must to identify the best possible solution for the problem at hand. Repeated failure of shock section tube supports Tube supports are one of the most critical yet overworked components of the fired heater system. With an increasing tendency to process opportunity crudes, more severe operating conditions, and a compulsion to fire heavy oil
a recurring complaint: cracks were detected in the shock ITS, although the heater duty was well within the range, and the BWT was also within the design temperature of the shock ITS. Usually, the shock ITS is designed for anticipated BWT plus a margin of 100°C. During a detailed investigation of this issue, it was found that although the heater was running well within its design duty, the BWT at times had very high excursions from its expected value, which could be due to multiple reasons, including those explained in the earlier section. The problem was further investigated, and the flue gas temperature profile across the convection bank was worked out (see Table 1 ). An important observation during the investigation was that the flue gas temperature gradient for the current operating condition exceeded the API recommended limit of 222° across a single ITS for bottom rows, which is certainly not advised on account of the unusual stresses. At a higher BWT temperature of 900°C, as observed quite frequently, the temperature gradient across the bottom ITS
further increases, reaching ~320°C in both convection sections. Thus, to avoid a high temperature gradient exceeding the API limit of a single ITS, a different ITS design was adopted, which could comply with the API requirement, thus preventing gradient
Flue gas temperature profile
Parameter
Convection section
Convection section
of Heater 1
of Heater 2
Flue gas entry temperature Flue gas temp exiting Row 1 Flue gas temp exiting Row 2 Flue gas temp exiting Row 3 Flue gas temp exiting Row 4
805 745 692 646 538
824 761 708 662 552
Table 1
Refining India
50
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