Q & A > Question Details
When a furnace is intended to operate at more than design capacity, tube skin temperatures are expected to reach beyond tube design temperatures. It is because the increase in heat flux is higher than design value. Let us say that tube skin temperatures are 50 deg.c above the tube design temperatures. How can we make a decision whether the heater can be run at more capacity or not based on tube skin temperatures ?
11/09/2019 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
A furnace is not normally designed to operate at SOR at its maximum skin temperature. This approach would not allow for any coking of the furnace tubes, which always happens to some extent over time. The runlength that can be achieved between furnace decokes is a function of the rate at which the skin temperature is increasing and the difference between SOR and maximum skin temperatures. When you increase the feed to the unit you increase both the SOR skin temperature and the rate at which the skin temperature will increase (i.e. coking rate increases). The result will be a shorter interval between shut downs to decoke. If that duration is acceptable, there is no problem and you can just make the move. If the duration between decokes becomes unacceptably short, then you need to try and rerate the tubes for higher EOR skin temperatures. Depending on the service degradation mechanisms like creep, decarburization or hydrogen dusting can come into play, so you need to get a corrosion engineer and a qualified mechanical engineer involved to assess both corrosion risks as well as mechanical integrity of the tubes and the more severe conditions.