11 Suction Pumps
If a suction pumping system is not working correctly, the problem could be with the underground
installation, the product, or with the Wayne™ Compact Pumping Unit (CPU).
11.1 Compact Pumping Unit
The CPU consists of the pump, air separator chamber, and a by-pass (relief) valve. See Fig
11.1.
The compact pumping unit is a positive displacement rotary type gear pump. These pumps are
called "self-priming" because they do not require an auxiliary device to remove air or vapor from
the suction line in order to function. Rotary pumps operate with extremely close clearances be-
tween the rotating member and the pump casing. This feature enables them to be good "self-
primers" and ideal pumps for gasoline dispensing systems.
When first put into operation, the interior of the compact pumping unit is wetted with oil to provide
a liquid seal that makes priming quick and easy. The oil used for priming also serves to lubricate
the internal parts of the pumping unit to prevent wear caused by running the pump dry. A check
valve is installed in the suction line in order to hold the pumping unit full of product between
sales. This eliminates the need for the pump to re-prime itself at the beginning of each sale.
The actual pumping by the CPU is accomplished by the rotor and shaft assembly, the idler, and
the relief valve assembly. As the rotor and shaft turn, they turn the idler. Because the idler is lo-
cated slightly off center in the rotor, this rotation causes gaps to be opened between the idler
and the rotor. This opening of gaps is what causes the lowering of pressure on the suction side
of the pumping unit. On the pressure side of the pumping unit, these gaps are closed. The
product, which is displaced by the closing of the gap, is forced toward the outlet of the pumping
unit, causing an increased pressure.
184 WM048523 Rev 08 01/2019
11 Suction Pumps
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