Single stage and double stage fuel oil pumps and their suction capacities.
The limitation of a fuel oil pump feeding a burner may often be the suction side. With higher suction lift the "vacuum" in the suction line will evaporate the light fractions of the fuel oil - and the pump starts to choke.
In general there are two types of fuel feeding pumps
single stage pumps
double stage pumps
A single stage pump consist of only one set of pumping gears and the lift capacity is limited compared to the two stage fuel pump with double set of pumping gears.
a single-stage pump is suited for low-level lifts up to 5 - 10 inches of vacuum
a two-stage pump is suited for medium-level lifts up to 10 - 15 inches of vacuum
Higher "vacuum" levels requires use of separate lifting pumps and dedicated feeding reservoirs to the fuel pumps.
The total "vacuum" in a fuel oil suction line can roughly be estimated to
1 inch of vacuum for each foot of lift
1 inch of vacuum for each 90o elbow
1 inch for each 10 feet of 3/8 inch horizontal suction pipe
1 inch for each 20 feet of 3/8 inch horizontal suction pipe