485. Originally steam ship f/o service and settling tanks were designed with only a high suction and a low suction and no other valves. What was the purpose of the low suction?

The low suction, positioned at the bottom of the fuel oil service/settling tank, facilitated water and sludge removal. Water and sludge, being denser than fuel, settle at the bottom of the tank; the low suction allows for periodic removal of this layer without emptying the entire tank.