The heater tube vibration usually occurs in the feed vaporizing service (reboiling, atm, vacuum fractionation). Feed vaporizes as it flows inside radiant tubes driven by outside combustion heat to make the liquid to vapor phase change. The heater is designed for hydraulic and thermal symmetry of all coils (passes) and around target firebox heat-flux profile, but in reality is significant variation. Ideally the oil after convection section is routed to radiant elevation having highest heat-flux (middle of box) to minimize oil-film temperature at initial vaporization. Above the forth tube (from floor) flue gas temperature sharply increases and forms high-flux zone. Oil vaporization is not linear along tube length and increases near outlets, where the high-heat flux should be avoided. Correcting and balancing firing to equalise coils heat load should be done.
Intensive vaporization and resulted unstable two-phase flow regime. Two-phase flow with changing the volume fraction of two phases and their properties form a particular flow regime that cause tube vibration. Slug flow and large bubbles cause rough vibration and transient annular flow, bubble /mist fine vibration. Earlier the tube flow regime was predicted by using flow pattern charts but today by CFD (comput. flow dynamics).
This should be avoided by balancing hydraulic and thermal parameters i.e. throughput (coil flows) and burner firing . So, good tube observation and understanding is needed to set operating strategy to avoid tube vibration and achieve desired outcome related to throughput, fractionation and specified product yield and quality.
Apparently, here an unbalanced heater firing creates more heat in some of the coils, higher oil vaporization and unstable two-phase flow. Recommendation is to first check tube crossover supports, for free tube movement, then fixed points for zero outside load on radian. tubes, Then, balance firing as best as possible, so that coils are heated near uniformly. Observe and adjust burner firing for an uniform coil heating. Try to increase mass-flow through problematic coils, while keeping coils flow balance within 5 %. If tubes are fouled could go up to 10 %. Try to reduce the vibrating coil vaporization rate to 40 %, if in reboiling service even though the design may be 50 %. Last, but undesired step can be reducing the throughput and heat load to prevent risk of tube damage .