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NORA Oilheat Technicians Manual

Chapter 4 Fuel Units and Oil Valves Chapter 4—Fuel Units and Oil Valves 4-5 vacuum with an inch (") mark. Example: 10" of vacuum. Vacuum brings oil to the pump. We need about .75" to 1" Hg of vacuum for each foot we lift the oil, 1" of vacuum for each 10 feet of horizontal run, and ½" for a clean oil filter. For example, if we have 4 feet of lift from an underground tank, plus 10 feet of oil line run to the burner, and add the oil filter, the calculated vacuum reading should be 5.5". A vacuum gauge reading of from 5" to 6" is acceptable. When the motor turns the pump shaft, oil enters the strainer chamber through the intake port, either by gravity or by the vacuum developed on the intake side of the gear pump. As the gears rotate, the teeth squeeze the oil and discharge it on the pressure side to the pressure regulating valve. The pressure adjusting screw on the regulating valve controls spring tension, which determines the pressure at which the oil will force the piston open and be discharged through the nozzle port. This pressure is about 80% to 95% of the operating pressure. The minimum factory set operating pressure is 100 PSI. The pump can deliver 5 to 20 times the amount of oil required by the nozzle. This excess oil is bypassed by the pressure regulating system and returned to the strainer chamber. The total oil capacity of a gearset is referred to as TGSC or Total Gear Set Capacity. The bypassed oil returns through internal porting in the pressure regulating valve and pump body. As the excess return oil is no longer at pressure, some of this oil is used for the lubrication of the shaft pump seal. In order for the excess oil to return to the strainer chamber, the bypass plug located between the pump and the strainer chamber must not be installed. If this plug were in place, the excess oil could not return to the strainer chamber and would require a return line from the pump back to the tank. If there were no return line, the high-pressure oil would be forced into the front seal chamber, which would rupture this seal. In most pressure fuel units, this seal can only withstand 10 PSI of pressure. You should always check to be sure that the bypass plug has not been installed when using the unit on a one pipe installation. The bypass plug is only installed when using a two pipe system. Pressure regulating valve operation The discharge oil pressure of the fuel unit can be adjusted between 100 and 200 PSI. Normal pressure setting on a highpressure burner is 100 PSI, but some burners are designed for higher oil pressure. For variations in this pressure setting and its resulting effect on nozzle performance, see the Nozzle Chapter # 5, of this manual. Also refer to manufacturers’ specifications for recommended pump pressure on flame retention burners. On burner shutdown, spring tension against the pressure regulating piston will cause the piston to close, shutting off oil discharge to the nozzle at a pressure approximately 20 percent below operating pressure. Therefore, if the pump pressure is adjusted to its normal operating 100 PSI, the shutoff pressure will be about 80 PSI. For pumps with high-speed cut off, the cut off pressure may be different than 20 percent. What is important is that the pressure should drop and hold. For acceptable vacuum, figure 1" Hg per foot of lift plus 1" per ten feet horizontal run and add 1/2" for the oil filter


NORA Oilheat Technicians Manual
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