Riello 40 F20 Installation, Use And Maintenance Instructions page 31

H = Difference of level.
L = Max. length of the 
suction line.
I.D.= Internal diameter of the
oil pipes.
D5220
A filter must be installed on the suction fuel line.
2401
L meters
H
I. D. I.D.
meters
8 mm 10 mm
0 35 100
0.5 30 100
1 25 100
1.5 20 90
2 15 70
3 8 30
3.5 6 20
5
GB
The pump vacuum should not exceed a
maximum of 0.4 bar (30 cm Hg).
Beyond this limit gas is released from
the oil.
Oil lines must be completely airtight.
The return line should terminate in
the oil tank at the same level as the
suction line; in this case a non-return
valve is not required.
When the return line arrives over the
fuel level, a non-return valve must be
used.
This solution however is less safe than
previous one, due to the possibility of
leakage of the valve.
PRIMING THE PUMP
Start the burner and wait for the priming.
Should lock-out occur prior to the arrival
of the fuel, await at least 20 seconds
before repeating the operation.