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Show constraints that are typical in a retrofit situation. In most cases it is desirable to minimize the following: • Heater downtime (shutdown duration); • Plot space requirements; • Alteration of heater combustion characteristics; • Addition of equipment requiring high maintenance; • Constraints on future heater modifications. Heater Downtime The consideration of downtime is of critical importance from the user's standpoint. Depending on the technology applicable, the time required for the system installation and commissioning can differ substantially. Table 1 shows the typical shutdown times needed for various technologies along with the amount of construction that can be done prior to the actual heater shutdown. These shutdown ~riods are of the utmost importance due to the loss of revenue resulting from heater shutdown. Revenue loss can exceed $500,000 per day for some critical heaters such as crude, vacuum, and coker heaters. A higher level of pre-shutdown work clearly implies smaller downtimes and therefore smaller revenue losses. In many cases, retrofit projects can be implemented during~ periods of scheduled maintenance shutdowns. However, careful planning is needed to avoid conflicts between maintenance and retrofit projects. For example ' burner removal and tube inspection might have to be scheduled in series since the latter may need scaffolding and the former may need floor replacement - clearly, incompatible tasks for the same time period. Another concern related to minimizing downtime is the -availability of adequate labor and fabrication facilities. Costs are significantly impacted if labor has to be imported from out-of-state, as was the case in a recent Rule 1146 shutdown at a local refmery. Plot Space No less important than downtime is the aVailability of unencumbered plot space. This is particularly true of certain technologies such as Selective Catalytic Reduction (SCR), discussed later. Alternatives such as stacking of equipment and creative piping and ductwork arrangements may help mitigate the problem. However, sharper turns in the ductwork often lead to higher system pressure drop and larger fan sizes. Space required for safety, code requirements, maintenance lay-down, and unit access cannot be used. It should also be noted that the potential plot space should be free of contamination since cleanup costs can be significant. Combustion Characteristics . -The next major constraint is maintaining the original heater combustion characteristics. This refers to the tube side heat flux since, ideall y, the retrofit should not result in altered process conditions. Typically, the substitution of standard burners by low NOx burners having longer flames may result in significant changes in the tube side flux, including the possibility of flame impingement and coke formation. This is mainly a concern in critical heaters (e.g., crude and vacuum preheaters, reformers, crackers, and coker heaters). There are cases in which altered combustion characteristics can be beneficial to heater operation. In cases where coke formation is presently a problem the replacement of natural-draft burners by forced-draft burners may diminish the 4 |