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”.
The fuel is sent to the nozzle at the pressure value set by “RG”, only the exceding fuel is sent on the return line “R”. In the one-pipe system the by-pass screw “B” is removed and the return “R” is pluged; 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, instantly the pressure comes down and the spring of the pressure adjustment screw “RG” moves the piston which stops the oil flow to the line and allows to the fluid to go through the return line “R”.