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NORA Oilheat Technicians Manual

Chapter 4—Fuel Units and Oil Valves 4-7 uninterrupted flow of oil to the second stage that pressurizes the oil to the nozzle. Figure 4-5 shows the oil flow in a two stage pump. The first gear set provides the vacuum to fill the strainer chamber as well as the low pressure oil supply to lubricate the shaft seal. From the shaft seal chamber, the oil flows to the low-pressure side of the pressure regulating valve and then back to the tank. The second set of gears provides the pressure for the oil taken from the strainer chamber, with the surplus oil being bypassed through the porting in the pressure regulating valve and back to the strainer chamber. Note that the bypass plug is installed in this unit because two stage pumps must always be two pipe. If a two-stage pump is connected to one pipe, it becomes a single stage pump. The first stage will only take oil from the cover and return it to the cover. Installing two stage pumps on oneline systems is a waste of money. Not only does the pump cost more, it also uses more electricity to turn the second set of gears. Even though single stage pumps are capable of creating 20" of vacuum, we need two stage pumps because fuel oil starts to break up or “vaporize” at vacuum levels as low as 6.7" (Figure 4-6). When this happens, foamy oil collects in the pump and the pump begins to cavitate. The pump sends this foam directly to the nozzle, causing unstable atomization, smoke, and soot. Also when the burner shuts down, the air bubbles in the nozzle expand, pushing oil out of the orifice, creating after drip. The two stage pump may correct the foaming oil problem. The first set of gears brings the oil into the pump, and returns any foam back to the tank via the return line. It is important to use the lower intake port so even at 17" of vacuum, only foam free oil is picked up and delivered to the nozzle. It is recommended that all two-stage pumps be mounted right side up so the air will collect in the top of the pump and can be sent back to the tank. In a two pipe system with a two stage fuel unit, it is not advisable to exceed 17" of intake vacuum. Figure 4-7 shows the effect that 20" of vacuum has on a pump. With long oil line runs where excess vacuum is required, or for overhead Figure 4-6 Figure 4-7 Chapter 4 Fuel Units and Oil Valves


NORA Oilheat Technicians Manual
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