Positive Displacement Pump
Why are you limiting yourself to either a gear pump or a vane pump?Centrifugal pumps are widely used for pumping petrol in petroleum refineries and you would be making a mistake not to consider the use of a centrifugal pump.
As for pumping out of underground storage tanks, there are many pump manufacturers that sell vertical submerged pumps where the drive motor is installed dry and above-ground with a vertical shaft leading to the centrifugal pump submerged within the storage tank. I think this would be much preferable to either a gear pump or a vane pump.
Why do you consider petrol a low viscosity, easily vaporized fluid? Low viscosity and easily vaporized as compared to what? Yes, it has a lower viscosity and a lower initial boiling point than heavy fuel oil or asphalt. But it has a higher viscosity and higher initial boiling point than liquified propane or butane. You seem to think that petrol is a difficult fluid to pump ... but it is not.
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