For propulsion machinery spaces intended for centralized or unattended operation (ACC or ACCU or ABCU notation), low level conditions of fuel oil settling and service tanks are to be alarmed at the centralized control station. Where tanks are automatically filled, high level alarms are also to be provided. For ACCU notation, these tanks are to be sized for at least 24-hour operation without refilling, except that for automatically filled tanks, 8-hour operation will suffice, see also 4-9-3/15.1.1, 4-9-4/3.3, and 4-9-4/Tables 3, 5B, 6.
Use of a settling tank with or without purifiers or use of purifiers alone is not acceptable as an equivalent arrangement to providing a service tank. 4-6-4/Table 2 shows examples of acceptable arrangements.