Fuel Oil Firing – Pressure Control
If a fuel oil system feeds more than one firing control valve, then load changes on one firebox should have an acceptably small effect on the other firing valves. For very small systems this may need only a supply pump with a flat head-versus-flow curve and a generously sized line.
Large systems typically circulate substantial oil in addition to that which is burned. The circulating oil provides two benefits:
1. The increased oil flow reduces the effect of heat losses and heat tracing on temperature control.
2. The oil supply pressure is stabilized by the increased flow as load changes of individual firebox firing control valves become a smaller fraction of the total oil flow.
Referring to Figure 600-11, a large system is typified by the following:
1. Fuel oil tanks have tank heaters
2. A fuel oil pump passes at least 300% of the combined normal firing rate of all the plants; this 300% is distributed as follows:
a. 100% circulates through the supply header pressure control valve
b. 100% circulates through restriction orifices
c. 100% flows through the burners
3. Supply and return lines are sized for negligible line loss (typically one size bigger than so-called “economic line size”).
4. A pump discharge pressure controller returns the pump’s excess capacity to the pump suction. The control valve passes 100% of the combined normal firing rate when all fireboxes are running, 200% when all fireboxes are extinguished. (Fuel is usually returned to the pump suction via a stilling chamber, which is vented to the fuel tank suction to remove any gas present in the returning fuel. In this way fuel heating is minimized and tank temperature is kept below the boil-over point. This stilling chamber may be either internal or external to the tank as shown on Figure 600-11).
5. Each plant has fuel oil header block valves and blinds about 100 feet from the
6. Just outside the 100-ft. blocks a flow orifice interconnects the plant’s oil supply and return headers. The orifice passes 100% of that individual plant’s normal firing rate.
7. A flow orifice interconnects the main oil supply and return headers just outside the plot limit block valves for the plant at the end of the headers. The orifice is needed to maintain header circulation when that plant is blocked in. The orifice is blocked in when the plant is running. It is sized to pass 100% of the normal firing rate of the plant.
8. A burner header pressure controller for each firebox adjusts the firing control valve to counter any remaining pressure bumps in the oil pressure. The plant operator or the demand controller firebox adjusts the pressure controller’s set point to vary the firing rate. If the demand controller is board mounted, then the pressure controller should be too.
9. Finally, each furnace has a manual valve connecting the burner manifold to the plant’s return header for startup. This valve is normally closed when the furnace is running. It is opened on startup to heat the system and to establish control of the oil header pressure for stable burning. Preheating the system gives the correct oil viscosity at the burners to minimize smoking on initial light-off.