Prewash quantity calculation

The minimum quantity of water to be used in a prewash is determined by the residual quantity of noxious liquid substance in the tank, the tank size, the cargo properties, the permitted concentration in any subsequent wash water effluent, and the area of operation. The minimum quantity is given by the following formula:

Q = k * ( 15 * r exp 0.8 + 5 * r exp 0.7 * V / 1000 )

where

Q = the required minimum quantity in m3

r = the residual quantity per tank in m3. The value of r shall be the value demonstrated in the actual stripping efficiency test, but shall not be taken lower than 0.100 m3 for a tank volume of 500 m3 and above and 0.040 m3 for a tank volume of 100 m3 and below. For tank sizes between 100 m3 and 500 m3 the minimum value of r allowed to be used in the calculations is obtained by linear interpolation.

For Category X substances the value of r shall either be determined based on stripping tests according to the Manual, observing the lower limits as given above, or be taken to be 0.9 m
.
V = tank volume in m3

k = a factor having values as follows:

Category X, non-Solidifying, Low-Viscosity Substance, k = 1.2

Category X, Solidifying or High-Viscosity Substance, k = 2.4

Category Y, non-Solidifying, Low-Viscosity Substance k = 0.5

Category Y, Solidifying or High-Viscosity Substance k = 1.0

The table below is calculated using the formula with a k factor of 1 and may be used as an easy reference.