Fuel to be used is first transferred from storage tanks to a settling tank in which it is heated to allow some water and sludge to settle out by gravity and be drained off. The fuel is then passed through the purification system and discharged to a daily tank. There are usually two daily tanks, used alternately,one in use while the other is being recharged. Settling and service tanks are lagged to conserve heat. A recommended standard of treatment for residual fuel to be used in a large engine requires two centrifuges of adequate capacity, operating in series. The first acts as a purifier to remove water, solubles, sludge etc while the second acts as a clarifier to remove solids. The purifier must be fitted with the correct disc or dam to match the oil density. The oil is heated before purification (max.temp.98°C) and the rate of throughput is limited to assist efficient separation. Both centrifuges must be cleaned frequently. Such systems can operate effectively on oils of densities up to 0.99. From the service tanks the treated oil is pumped through a pressurized fuel system to the engine. With the oil temperatures necessary for high viscosity fuel, and the possibility that a trace of water may still be present, it is necessary to maintain the engine pump suctions and circulating connections under pressure to inhibit boiling, gasification and capitation.