AN extensive study of oil foaming has brought Mr. Pigott to the following conclusions:
All suction-side resistance of either pressure or sump pumps must be reduced as much as possible.
Velocities in suction piping should be kept below 5 fps at all points. Bends that cannot be eliminated should be long radius. Valves and other restrictions must be avoided.
Spur-gear pumps can be improved by optimum exposure of teeth on the suction side, use of side pockets, and reduction of clearance. Without help, these pumps still won't be good enough for the highest altitudes.
Oil tank pressurizing will provide boost for the pressure pump.
Centrifugal boosters are useful, except with cold oil or with much aeration. For the pressure pump, the booster should be located directly at the oil tank, and for the scavenge pump, at the sump. Both must be drowned, no lift.
One form of positive-displacement rotary is available for fixes or new designs that is almost as good as a first-class centrifugal in low suction-side losses, and satisfactory in thick oil or high aeration. Boosting is not needed.
Oil tank efficiency as air separators varies with oil level and with scavenge pump pressure. Bad tanks vary from 16% air to 3%; good tanks from 6% to 2%; as varied by tank level and pump pressure - 185 F oil.