inside the furnace box is a safety hazard, with many end users, legislations, and licensors recommending a furnace trip as soon as the box pressure reaches a certain positive value. Given the safety interlock's utmost importance, special care is taken to operate within the integrity operating window (IOW) limit. Often, this becomes a limitation, especially for older furnaces, when production demands are high. In one such occurrence, the end user was facing severe limitations due to furnace box pressurisation in their crude unit. As crude unit healthy operation defines the overall profitability of the refinery, compromised operation means curtailed gross refining margin (GRM), and the operation team has to be vigilant. The problem was referred to the author’s company, and a fired heater audit was conducted over two shifts through EIL’s EngRT-Htr digital platform for operational data analysis. Through the digital platform, it was revealed that the heater operation was actually hampered due to severe leakage in the downstream air preheater. Assuming the fuel flowmeter readings are correct, the air quantity measured by the venturi flowmeters was at least 1.8 times higher than that required for the fuel being fired. The oxygen analyser located at the arch was cross-checked and found to be reading reasonably correctly. On the other side, the airflow rate measured at the forced draft fan outlet was far higher than the oxygen reading. This indicated appreciable leakage in the air- side circuit. Subsequently, a manual survey was recommended to the client, utilising a portable oxygen analyser. As anticipated, the oxygen content in the flue gas at the air preheater (APH) outlet was observed to be around 14 vol%. This high percentage of oxygen in the flue gas at the APH outlet indicated that almost half of the air entering the APH was leaking into the flue gas side, overloading the induced draft fan beyond its capacity. Effectively, the induced draft fan was handling around 100,000 kg/hr of flue gas instead of the 54,000 kg/hr being generated in the heater system. Accordingly, action points were identified, and the APH was taken up for replacement in the next available turnaround opportunity. A simple digital tool could help identify the ID fan’s strained operation, which can lead to firebox
300
Complete heater Radiant section only
250
200
150
100
50
0
At 8” tube
Convection inlet
At crossover
50% crossover + 50% at 8” tube
Steam injection location
a detailed investigation regarding the steam injection location was carried out (see Figure 1 ) to determine the residence time for different possibilities of operation. As can be seen from Figure 1, the residence time inside the heater can be very high and exceed acceptable limits if the velocity steam bypasses the crossover location injection point and is fully redirected to the downstream larger coil size (in this case 8in). This effectively meant that for almost 80-85% of the travel length inside the radiant section, the RCO was dry and no vaporisation could be induced, which otherwise would have occurred with the aid of steam injection at the crossover. Although the end user may not be aware of its effect until the designer of the furnace conducts a simulation study, the residence time of the fluid increased almost seven times that of the normal case scenario of steam injection at the furnace inlet. Thus, a very high residence time, coupled with a high film temperature, led to rapid coking in this furnace, which resolved upon reinstatement of the crossover steam injection point. It is worth noting that when multiple steam injection provisions are present in vacuum furnace coils, it may be wiser to take advantage of this flexibility by evaluating the best possible location for overcoming the chances of coking, if any. High firebox pressurisation leading to curtailment of throughput Fired heaters are designed to operate at negative draft conditions. Positive pressure Figure 1 Residence time of hydrocarbon inside the heater coil
Refining India
48
Powered by FlippingBook