46 CFR §153.1108

1. (a)When a high viscosity or solidifying cargo is unloaded from a cargo tank, the cargo tank must be prewashed unless § 153.1114 or paragraph (c) of this section allows the prewash to be omitted.
2. (b)When a prewash is required for a tank that has unloaded a solidifying cargo or a cargo having a viscosity exceeding 25 mPa sec at 20 °C, the wash water used in the prewash must leave the tank washing machine at a temperature of at least 60 °C (140 °F).
3. (c)The prewash required under paragraph (a) of this section may be omitted if the approved Procedures and Arrangements Manual contains a procedure for measuring the temperature of all interior cargo tank surfaces throughout unloading and under the measuring procedure the temperature of these surfaces remains above—
   1. (1)The temperature of the cargo's melting point if the cargo is a Category B or C solidifying NLS; or
   2. (2)The temperature at which the cargo's viscosity exceeds—
      1. (i)25 mPa.s, if the cargo is a high viscosity Category B NLS; or
      2. (ii)60 mPa.s, if the cargo is a high viscosity Category C NLS.