Minimization of Tank Washings on Chemical Tanker
Koichi UEDA* and Hiroshi YAMANOUCHI*
A tank washing on a chemical tanker is considered in this investigation. Tanks should be washed by means of rotary water jet operated at sufficiency high water pressure and in location to ensure that all tanks surfaces are washed. The discharge into the sea of chemical substances or tank washings containing such substances should be prohibited to protect marine environment from pollution produced by ships(1). If tank containing such substances or mixtures are to be washed, the resulting residues shall be discharged to a reception facility until the concentration of the substance in the effluent to such facility is at or below the prescribed concentration until the tank is empty. Using much water to wash the tank is a burden to a shipping service and it needs heavy cost and much fuel for the incineration. Therefore the methods of the reduction of the amount of the wash water are considered. The washing with water jet is studied experimentally. As the results, by making the nozzle diameter of the washrag machine small, the quantity of washing water can be reduced. In the case of most of Japanese domestic chemical tankers, the distance between washing nozzle and tank wall surface is about 5 m, so the water jet flow from the small diameter nozzle becomes water droplet flow. Then washing with droplet flow is investigated. The plate surface is washed well with droplet flow and with the falling film flow. The water quantity for washing can be reduced by using washing machine with small diameter nozzles.
Key Words: Marine Environment, Chemical Tanker, Tank Washing, Washing Machine, Water Jet
After unloading the liquid chemical substances on the chemical tanker, when the next cargo is different substance, tanks are washed. The discharge into the sea of noxious liquid substance, ballast water, washings, or the other residues or mixtures containing such substances shall be prohibited for prevention of sea pollution. For tank washing of chemical tanker, water jet is usually used. It is necessary to reduce the wash water quantity because it needs heavy burden for the treatment of tank washings. The prewash procedure for chemical tanker(2) and more effective tank cleaning methods(3) were studied, but in these study it needs much water to wash until the concentration of the substance in the effluent becomes 0.1%.
An example of the tank washing test result of chemical tankers is shown in Table 1. In the tests, the washing was done until the concentration of the substance in the effluent is below 0.1%. The average quantity of water to be used in the washing is 1.03 m3. Thc average concentration of residue is 2.3%. As the 98% of washrags is water, so the possibility of reduction of wash water quantity is investigated in this paper. The small diameter nozzle washing machine is effective to decrease the quantity of wash water(4)(5). Using small diameter nozzle decrease the remaining water quantity in the tank during washing, then it is effective to reduce wash water quantity. In the case of using small diameter nozzle, it needs to understand the washing state with water droplet flow and with the falling water film flow. In the case of most of domestic chemical tankers, the tank size is generally about 150 m3 and the distance between washing nozzle and tank wall surface is about 5 m, so the water jet becomes droplet flow. So the washing with the droplet flow is studied experimentally. In the case of the distance between the nozzle and the test wall is 5 m, the washing state of water-soluble liquid clinging to the wall is investigated by photographing and the distribution of droplet of water jet is investigated. The washing with water droplet is ascertained.
Generally the tank is washed with water jet, but at the vertical tank wall surface can be washed with the falling water film flow. So the washing; effect by falling water film flow is experimented with clinging liquids of the different viscosity. When the liquid of clinging to the wall is water-soluble, it is washed with film flow easily. To design washing machine, it is necessary to understand the washing degree with falling water film flow. To reduce the wash water quantity, it is useful to optimize the scanning of water jet. In order to estimate the washing effect, using Φ60 mm diameter, 2 m length cylinder, the washing effect with falling film flow is investigated. Colored water, colored glycerin and colored ethylene glycol and the water solution of them are used as test liquid.
The velocity in the falling film flow is as followings. Here, u is velocity, y is the distance from the cylinder surface.