No they are not needed. They meassuerd regulary the amount of water in the fuel befor it goes in the gravity tank, wich was at the same time also a seperation tank and they meassured it again from the gravity tank to the diesels.
Also the flow meter not only shows how much fuel passes at a given moment, it also counts how much fuel has passed, like the speedometer in a car, it not only shows the speed at any given moment, it also counts the driven distance.
Also also, small leakages are pretty much irrelevant, they didnt need to know on the liter exact how much fuel they have and bigger leaks would be noticed within a day and would be stopped when possible, by feeding the diesel from this tank and/or by pumping the fuel into another tank