The suction vacuum generated by the gears sucks up the fuel through the suction line "A"; it crosses the filter and it is sent under pressure to the pressure adjustment screw "RG". During the prepurge cycle the "NC" solenoid valve prevent the exit of the fuel from the nozzle outlet "U". When the voltage is applied to the "NC" solenoid valve, the fuel is sent to the nozzle at the pressure value set by pressure adjustment screw "RG", only the exceeding fuel is sent on the return line "R". In the one-pipe system the by-pass screw "B" is removed and the return line "R" is plugged; the whole fuel is sucked up by the gears without crossing another time through the filter. During the operation it is possible to measure the suction vacuum by the vacuum gauge port "V" and the pressure by the pressure gauge port "P"; it is also available on the pump an auxiliary delivery port "P1". When the burner stops the voltage to the "NC" solenoid valve is cut-off and immediately the oil flows to the return line "R".
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