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Question

  • One of our Unit has four furnaces viz. F-1, F-2, F-3 & F-4. There are two fire boxes: one for F-1/F-2 and one for F-3/F-4. Both fire boxes have two parallel convention banks and a common stack. The furnaces have the issue of low steam generation from convection banks, high stack temperature, BFW flow mal-distribution in convection banks and hence a lower efficiency than design. Sketch of the furnaces with current & design flows are provided below for reference. What could be the possible reasons and remedial solutions.

    Jan-2024

Answers


  • Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

    The question does not specify what kind of processing unit provides the flue gases (Catalytic Cracking, Distillation, etc.), but I believe that it's possible to consider some verifications generally.

    It's important to consider if  the furnaces have soot blowers, if the response is yes, how is the availability of these devices? The deposition of soot or ash in the furnace wall and specially in the convection tubes (which are generally finned) can significantly reduce the thermal exchange in the furnaces, leading to a poor thermal efficiency and the main side effect is a high stack temperature like mentioned in the question. This is specially true in furnaces using FCC flue gases due to the carryover of catalyst fines in the flue gas which will be deposited in the furnaces wall and convection tubes.

    Another key point is the distribution of BFW (Boiler Feed Water), which seems to suffer with a poor distribution. This is a key factor to the performance of the furnace and the flow and symmetry criteria needs to be accomplished in order to reach the expected design. A good strategy to identify poor flow performance of gases and BFW is a CFD (Computational Fluid Dynamics) study using a simulated model of the furnace.

    A general review of the design with verifications of fin distribution on the convection tubes, their area, and the diameter of the convection tubes facing the eventual operating changes of the process conditions of the furnace over the years (capacity raising) should help to understand the poor performance of your furnace system.

    Jan-2024