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Show More often than not, this first step is the most troublesome part o~ the. entire proj~t, mainly since it is often ignored. Clearly, a meaningful evaluation IS not posslble without original design data (along with documented prior modifications) and accurate current operating data. For older heaters, the original design data either may not exist or are often not adequate. Furthermore, earlier modifications are sometimes not well documented in the drawings or data sheets. Similarly, obtaining consistent and accurate operating data is not a trivial task. Sometimes modifications are sought to be made in heaters that are currently not operational. In other instances, where the heater operates under cyclic conditions, the entire range of operation may not be adeq~tely . covered without a major data gathering effort. The latter cannot be achieved Wlthout significant prior planning since current heater operation cannot be readily altered to provide all of the data, e.g., those due to change in feedstock. It is important that every effort be made to fully understand the present status of the heater, including a survey of its physical condition and operating performance test(s). A survey of the existing heater and its associated equipmenLshould note the following: physical conditions including structural steel integrity, panel warpage, duct corrosion, wear of the burner and associated equipment, integrity of the heat transfer surfaces, refractory damage, and damper deterioration. General heater maintenance records and the condition of key components of the control system should also be evaluated. Performance tests consist of past operational data records and "spot checks." These should contain such necessary information as process fluids characterization, flowrates, inlet and outlet temperatures and pressures, fuel composition and its fluctuations, fluegas and tubeskin temperatures proflles, and draft profiles. Other conditions such as firing patterns and flame characteristics (length, width, color) should also be noted. Once the original design basis data and the performance data are gathered, they can be compared using computer simulations. Simulations serve to calibrate the model itself and help establish data consistency checks. Model calibration may help point out deficiencies in heater operation. For example, if it so happens that heat balances for various runs are achievable only by assuming unreasonable stack temperatures, the model simulation is an excellent tool to determine its possible causes. Further, some causes may indicate equipment deficiencies. Such information is useful in determining not only the future design basis but also in evaluating the retrofit cost. 4. RETROFIT CONSTRAINTS Once a computer model has been developed and design parameters have been agreed upon by both the user and the design engineer, the latter must evaluate the NOx reduction technology to be used for each heater. However, this cannot be done without considering some of the major constraints inherent in any retrofit situation. These constraints, some of which are contributed by the technologies themselves, are discussed below. Of course, other constraints such as new and emerging technology developments, the impact on overall costs, and the refinery-wide compliance strategy will affect the technology selection for any given heater as well. Figure 1 shows the overall decision diagram that is typically followed in arriving at the optimum heatertechnology combination to meet specified regulatory standards. In contrast to new designs, heater retrofit projects have significant constraints imposed on them that may limit the applicability of some otherwise promising technologies to particular heaters. The following discussion pertains to some of the more important 3 |