33 CFR §157.15

1. (a)Number. A tank vessel must have the following number of slop tanks that comply with the requirements of this section:
   1. (1)A new vessel of less than 70,000 tons DWT and an existing vessel must have at least one slop tank.
   2. (2)A new vessel of 70,000 tons DWT or more must have at least two slop tanks.
2. (b)Capacity. Slop tanks must have the total capacity to retain oily mixtures from cargo tank washings, oil residue, and ballast water containing an oily mixture of 3 percent or more of the oil carrying capacity. Two percent capacity is allowed if there are—
   1. (1)Segregated ballast tanks that meet the requirements in § 157.09, § 157.10, § 157.10a, or § 157.10b; or
   2. (2)No eductors arrangements that use water in addition to the washing water.
3. (c)Design. A slop tank required in this section:
   1. (1)Must minimize turbulence, entrainment of oil, and the creation of an emulsion by the use of separate inlet and outlet connections; and
   2. (2)May carry bulk oil when not being used as a slop tank.