The suction vacuum generated by the gears sucks up the fuel through the suction line; it crosses the filter and it is sent, under pressure, to the hydraulic valve which has cut-off function.
The hydraulic valve opens when oil pressure gets over spring strength settled by pressure adjustment screw and the oil reaches nozzle line. In two pipe systems the exceeding oil flows into the tank through the return line; in one pipe system, after the removing the by-pass screw, it goes back to the gears.
When the burner stops, the oil pressure immediately comes down and the spring of the pressure adjustment screw, moves the piston which stops the fluid flow to the line and, at the same time, allows to the fluid to go through the return line.

Series NR pump is manufactured with a cavity for the insertion of an heater cartridge to maintain fluid the oil without direct contact and viton components.