HFO settling tank

An HFO settling tank is a heated tank between the bunker storage tanks and the purifiers where heavy fuel oil is held and warmed so gross water and heavy sediment fall out by gravity before centrifuging. Steam heating coils hold the fuel at about 50 to 70 degrees C to reduce viscosity and speed settling, and a self-closing test cock and drain at the bottom let the engineer draw off and check for water. Settling pre-treats the fuel so the downstream separator handles only the fine emulsion, and most ships fit two settling tanks for continuous bunkering.