Fuel Oil Firing – Viscosity Control
High pour point fuel oils need viscosity control, more so for some types of burners than for others. Viscosity can be controlled by regulating the oil temperature close to the pump discharge. Or a continuous viscometer can be used to control the oil viscosity, once again, by adjusting the oil temperature close to the pump discharge.
Generally, heat tracing of the flowing portions of the supply and return headers should be turned off because it interferes with the viscosity control. All hot fuel oil piping should be insulated to minimize heat loss. The flow through the headers will always be at least 100% of the normal firing rate, due to the restriction orifices. This flow will keep the headers hot, except perhaps in cold climates. Heat tracing is a precautionary measure against unforeseen adverse circumstances, such as long time loss of flow, etc.
With the recirculation arrangement described under “Pressure Control,” the only lines that need continuous heating are the in-plant oil lines, plus a few dead-ended stubs. Unheated lines would be flushed with wash oil for startup and shutdown.
Oil heater turndown is a problem on a large system. One solution varies the steam condensate level in the vertical shell of a heat exchanger while the oil flows through the tubes. The condensate can blank off any desired fraction of the heating surface area. The steam pressure should be lower than the oil pressure. If the steam pressure is higher than the oil pressure, then exchanger leaks will cause problems due to condensate in the oil.