Fuel oil settling and service tanks
The tank arrangement and drain routine that determines how much water and cat fines reach the separator, and the overflow pipe detail that decides it.
The settling tank and the service tank are the two stages between the bunker tank and the separator, and how they are arranged and run determines how much water and how many catalyst fines the treatment plant has to deal with. The settling tank gives water and the heaviest solids time to fall out under gravity; the service tank holds the cleaned fuel the engine draws from.
MAN Energy Solutions Service Letter SL2017-638 sets out the practice that matters. Tank bottoms should be sloped. Settling and service tanks should be drained at regular intervals. The service tank overflow pipe should be led all the way to the bottom of the tank and returned to the top of the settling tank, or a separate recirculating line and pump should be fitted, so that overflowing fuel does not simply carry the settled layer forward. Where a service tank has been out of use for a week or two, its whole contents should be cleaned before use.
In heavy weather the arrangement matters more, not less: rolling stirs settled cat fines back into suspension, so the service letter recommends running the standby separator simultaneously with reduced flow and taking suction from a high suction line rather than the tank bottom. The full article will cover tank sizing and residence time, the drain and sounding routine, heating coils and the pour point plus 10 degrees C storage rule, and how tank practice shows up in an engine-inlet sample.