David E. Oakley, Senior Project Engineer, Software Development
HTRI recently launched Xfh® Ultra 1.0, its new fired heater simulation and rating tool. Ease of setup and fast execution time make Xfh Ultra ideal for rapid investigation of a wide range of operating scenarios, including increasing throughput or improving equipment efficiency.
This software is typically used in evaluating ways to increase the capacity or duty of a heater, as illustrated by the case study below.
Problem
A fired heater in a refinery crude distillation unit (CDU) has been identified as a bottleneck that limits throughput. The heater, consisting of a cabin firebox with attached shock bank and finned convection banks (shown schematically in Figure 1), has been operating reliably within safe limits. Preliminary investigation of the burner design data, combustion air register, and equipment on the fuel line indicates that it is possible to increase the firing rate by 20%. The plant manager wants to know the maximum safe operating capacity that could be achieved by increasing the firing rate, while maintaining the same process outlet conditions.
Evaluation
In our investigation, we first want to establish a baseline calculation that matches the available plant data. It can be a significant effort to reconcile all the plant data with predictions from fired heater software. Unless carefully calibrated or verified, limited plant instrumentation can be unreliable and misleading. In addition, heaters operating for a long time may not be clean, with fouling buildup and coking layers on internal/external tube surfaces.
Using Xfh Ultra, we can simulate separate tube internal/external fouling factors and tubeside fouling layer thickness for each coil to match the actual performance of the heater. Figure 2a displays a summary view from the Xfh Ultra report for the established base conditions (mass flow = 140 kg/s) of the heater.
As Figure 2a indicates, plenty of draft (i.e., negative pressure below atmospheric) is available at the firebox roof to ensure safe operation. The stack damper is partially closed for control to an optimum level just below 3-mm wc (water column). The software predicts a process peak film temperature of 428 °C, which is high for normal CDU operation. However, there is no evidence from the plant to indicate coking problems. Tube temperatures remain well within limits for the given tube metallurgy (5Cr-1/2Mo). In addition to the summary shown in Figures 2 (a), (b), and (c), Xfh Ultra provides more information for each coil (see Figure 3 as an example). Where necessary, it allows users to drill down to investigate the film, tube (average and peak), and fin (average and peak) temperatures of individual tubes for more detailed analysis.
After analyzing the base case (current operation), we increase the throughput 20% (168 kg/s) by fixing the required process terminal conditions. The software calculates the fuel flow and heat input, as shown in the summary report for the new condition (Figure 2b).
As expected, the higher firing rate makes it necessary to open the stack damper to ensure a desirable level of draft at the firebox roof. API 560 recommends an additional margin of safety (a minimum draft of 2.5-mm wc ), which could be achieved at 120% of the design heat release. A check run is performed, at increased throughput of 20%, at 202 kg/s (Figure 2c) with the stack damper fully open, to verify that the draft is sufficient with the new design conditions.
A second area of concern is the peak film temperature. Figure 2b indicates that, at 168 kg/s, the peak film temperature does not change significantly. Drilling down into the detailed results reveals that the tubeside heat transfer coefficient increases sufficiently to counteract the impact of a higher heat flux due to the increased tubeside velocity. Although peak tube temperatures increase slightly, they are acceptable for the 5Cr-1/2Mo tube material.
If we change the throughput again to 202 kg/s, the program displays a warning indicating a fin temperature limit will be violated, as shown in the example below:
Results
Using Xfh Ultra, we can confirm that the operator can safely increase throughput by 20%, but that increasing by a larger percentage is not advisable due to material temperature limits. For new operating conditions, we should consider performing more detailed performance checks for each coil (Figure 3), for example, to ensure that fin temperatures and heat fluxes remain within acceptable limits.
This short case study illustrates how Xfh Ultra can be used to rapidly evaluate new fired heater operating scenarios to increase throughput. In practice, the promising conditions that the Xfh Ultra calculation runs indicate is just a starting point for more thorough evaluation before a higher throughput is initiated. This evaluation should include the impact on all components of the process, and the plant metallurgist should review the predicted peak tube metal temperatures to ensure minimal impact on the remaining life of the tubes.
