Pre-wash Procedures for Tanks

General Requirements:

• A prewash procedure is required to meet certain MARPOL Annex II Requirements.

• This blog post explains how these prewash procedures shall be performed and how the minimum volumes of washing media to be used shall be determined.

• Smaller volumes of washing media may be used based on the actual verification testing to the satisfaction of the administration.

• If the reduced volumes are approved then an entry to that effect must be recorded in the manual.

• If a medium other than water is used for the prewash, then the provision in Regulation -13 (Control of discharges of residues of NLS) of this Annex shall apply.

For Ships built before 1 July 1984:

• The pre-wash procedures shall be performed to the requirements of Annex II.

A number of Cleaning machine cycles to be used in each location:

Category of Substance	Number of Cleaning machine Cycle
	Non-Solidifying Substances	Solidifying Substance
Category X	1	2
Category Y	1/2	1

Prewash Procedures for Non-Solidifying Substances:

1. Tanks shall be washed using a rotary water jet, operated at sufficient high water pressure. In the case of Category X substances cleaning machines shall be operated in such locations that all tank surfaces are washed. In the case of Category Y substances, only one location needs to be used.
2. During washing the amount of water in the tank shall be minimized by continuously pumping out slops and promoting flow to the suction point (positive list and trim). If this condition cannot be met then the washing procedure shall be repeated three times, through striping of the tanks between washings.
3. Those substances which have a viscosity equal to or greater than 50 mPa.s at 20 degrees celsius shall be washed with hot water (temperature at least 60 deg. celsius), unless the properties of such substances make the washing less effective. 
4. The number of cycles of the cleaning machines used shall not be less than that specified in the table. The cleaning machine cycle is defined as the period between the two consecutive identical orientations of the tank cleaning machine (rotation through 360 degrees).
5. After washing, the tank cleaning machine(s) shall be kept operating long enough to flush the pipeline, pump and filter, and discharge to shore reception facilities shall be continued until the tank is empty.

Prewash Procedures for Solidifying Substances:

1. Tanks shall be washed as soon as possible after unloading. If possible tanks shall be heated prior to washing.
2. Residues in hatches and manholes shall be preferably be removed prior to prewash.
3. Tanks shall be washed by means of a rotary water jet operated at sufficiently high water pressure and in locations to ensure that all tank surfaces are washed.
4. During washing, the amount of water in the tank shall be minimized by pumping out slops continuously and promoting flow to the suction point (positive list and trim). If this condition cannot be met, the washing procedures shall be repeated three times with thorough stripping of the tank between washings.
5. Tanks shall be washed with hot water (the temperature at least 60 degrees Celcius) unless the properties of such substances make the washing less effective.
6. The number of cycles of the cleaning machine used shall not be less than that specified in the table. A cleaning machine cycle is defined as the period between two consecutive identical orientations of the machine (rotation through 360 degrees).
7. After washing, the cleaning machine(s) shall be kept operating long enough to flush the pipelines, pump and filter, and discharge to shore reception facilities shall be continued until the tank is empty.

For Ships built on or after 1 July 1994 and recommendatory for ships built before 1 July 1994.

• The pre-wash procedures shall be performed to the requirements of Annex II.

Prewash procedures for non-solidifying Substances without recycling:

1 Tank shall be washed using a rotary water jet, operated at sufficient high water pressure. In the case of Category X substances cleaning machines shall be operated in such locations that all tank surfaces are washed. In the case of Category Y substances, only one location needs to be used.

2 During washing the amount of water in the tank shall be minimized by continuously pumping out slops and promoting flow to the suction point (positive list and trim). If this condition cannot be met then the washing procedure shall be repeated three times, through striping of the tanks between washings.

3 Those substances which have a viscosity equal to or greater than 50 mPa.s at 20 degrees celsius shall be washed with hot water (temperature at least 60 deg. celsius), unless the properties of such substances make the washing less effective. 

4 The quantities of wash water used shall not be less than those specified in the Paragraph (Minimum quantity of water to be used in a Prewash). 

5 After prewashing, the tanks and lines shall be thoroughly stripped.

Prewash Procedures for Solidifying Substances without recycling:

6 Tanks shall be washed as soon as possible after unloading. If possible tanks shall be heated prior to washing.

7 Residues in hatches and manholes shall be preferably be removed prior to prewash.

8 Tanks shall be washed by means of a rotary water jet operated at sufficiently high water pressure and in locations to ensure that all tank surfaces are washed.

9 During washing, the amount of water in the tank shall be minimized by pumping out slops continuously and promoting flow to the suction point (positive list and trim). If this condition cannot be met, the washing procedures shall be repeated three times with thorough stripping of the tank between washings.

10 Tanks shall be washed with hot water (the temperature at least 60 degrees Celcius) unless the properties of such substances make the washing less effective.

11 The quantities of wash water used shall not be less than those specified in the Paragraph (Minimum quantity of water to be used in a Prewash). 

12 After prewashing, the tanks and lines shall be thoroughly stripped.

Prewash Procedures with recycling of washing medium:

13 Washing with a recycled washing medium may be adopted for the purpose of washing more than one cargo tank. In determining the quantity, due regard must be given to the expected amount of residues in the tanks and the properties of the washing medium and whether any initial rinse or flushing is employed. Unless sufficient data are provided, the calculated end concentration of cargo residues in the washing medium shall not exceed 5% based on nominal stripping quantities.

14 The recycled washing medium shall only be used for washing tanks having contained the same or similar substances

15 A quantity of washing medium sufficient to allow continuous washing shall be added to the tank or tanks to be washed.

16 All tank surfaces shall be washed by means of a rotary jet(s) operated at sufficiently high pressure. The recycling of the washing machine may either be within the tank to be washed or via another tank, e.g. a slop tank.

17 The washing shall be continued until the accumulated throughput is not less than that corresponding to the relevant quantities given in the Paragraph (Minimum quantity of water to be used in a Prewash).

18 Solidifying substances and substances with a viscosity equal to or greater than 50 mPa.s at 20 degrees celsius shall be washed with hot water (temperature at least 60 deg. celsius), unless the properties of such substances make the washing less effective.      

19 After completing the tank washing with recycling to the extent, the washing medium shall be discharged and the tank thoroughly stripped. Thereafter, the tank shall be subjected to a rinse, using a clean washing medium, with continuous drainage and discharged to a reception facility. The rinse shall as a  minimum cove the tank bottom and be sufficient to flush the pipelines, pump and filter.

Minimum quantity of water to be used in a prewash:

20 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(15r0.8 + 5r0.7 x V/1000)

where

Q = the required minimum quantity in cubic meters.

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

V = tank volume in cubic meters.

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 factor of 1 and may be used as an easy reference.

Stripping quantity (Cubic meters)	Tank Volume (Cubic meters)
	100	500	3000
<=0.04	1.2	2.9	5.4
0.10	2.5	2.9	5.4
0.30	5.9	6.8	12.2
0.90	14.3	16.1	27.7

21 Verification testing for approval of prewash volumes lower than those given in Paragraph (Minimum quantity of water to be used in a Prewash) may be carried out to the satisfaction of the Administration to prove the requirements of the Regulation 13 (Control of Discharge of Residues of NLS) are met, taking into account the substances the ship is certified to carry. The prewash volumes so verified shall be adjusted for other prewash conditions by application of the factor k as defined in the Paragraph (Minimum quantity of water to be used in a Prewash)

For all Ships

Prewash Procedures for persistent floaters to which Regulation 13 of Annex II of MARPOl shall apply:

• Persistent floaters with a viscosity equal to or greater than 50 mPa.s at 20 degrees celsius and/or a melting point equal to or greater than 0 degrees celsius, shall be treated as solidifying or high-viscosity substances for the purpose of the prewash.

• Where it is determined that the use of small amounts of cleaning additives would improve and maximize the removal of cargo residues during a prewash, then this should be done in consultation and with prior agreement from the reception facility.

 

Reference: IMO Publications & Documents - International Conventions - MARPOL - Annex II - Appendix 6