Marine Engineering Study Materials

Quality of Cooling Water

The cooling water of the engine should be only demineralized (distilled) water with proper treatment, which is necessary for keeping effective cooling and preventing corrosion of the system. Though the distilled water matches best to the requirements for cooling water, it is necessary to add corrosion inhibitor. Because untreated distilled water absorbs carbon dioxide from the air and then becomes corrosive.

Sea water or fresh water contaminated by sea water even in small amount is not allowed to be used as cooling water of the engine due to high risk of severe corrosion and deposits formation in the system. 

Rainwater is heavily contaminated and highly corrosive in general, which is also not recommended as cooling water. Tap water (drinking water) is not recommended as cooling water due to risk of chalk deposit formation inside the cooling system. However, if the distilled water, for example from fresh water generator, is not available, tap water may be used as cooling water after softening and some other treatments according to the ingredients.

The cooling water before adding corrosion inhibitor should be checked and treated to satisfy following requirements:

pH	7 to 9
Total hardness as CaCOз	max. 75 ppm(mg/l)
Chloride	max. 50 ppm(mg/l)
Sulphate	max. 100 ppm(mg/l)
Silicate	max. 150 ppm(mg/l)
Residue after evaporation	max. 400 ppm(mg/l)

Note: Chloride and Sulphate are corrosive even in the presence of an inhibitor.

Treatment of Cooling Water

Cooling water should be treated properly and corrosion inhibitor should be added. The analysis and treatment of cooling water are recommended to be carried out by a famous and familiar specialist. Otherwise, keep the treatment procedures strictly according to the instructions from the supplier.

Some recommended products are listed as follows:

Note: Oily inhibitors adhere to cooling surface and influence cooling efficiency, which are not recommended for cooling water. Only nitriteborate based inhibitors are recommended.

Note: Do not mix the inhibitors of different types or properties. 

Note: Some inhibitors may be toxic and hazardous. Strict control is required when handling inhibitors. 

Checking Cooling Water and the System

The property of the cooling water may be changed during service due to contamination or evaporation. Therefore, the cooling water itself and the system should be checked periodically during service, preferably once a week. These tests may be done by means of test kits from inhibitor maker with sample water from the circulating system. However, laboratory test of the sample water by specialist is also recommended regularly at least every three month.

All checking results should be recorded and kept for trend evaluation, which contribute to reliable engine operation with right cooling water treatment.

If test result shows that the contents of cooling water changes suddenly or gradually, the cooling water system should be checked to trace the cause.
Some of the changes may indicate the cause as follows:

Chloride content increasing:

• Check possibility of seawater penetrating into cooling water.
• Check the system which includes sea water, for example fresh water cooler cooled by sea water.

pH value decreasing or sulphate content increasing:

• Check if cooling water is contaminated by exhaust gas.
• Check cylinder head by hydraulic pressure test.

Note: If the quality of the cooling water after checking exceeds control limit by water treatment, the cooling water should be replaced completely by newly treated water.

Cleaning of Cooling Water System

If any deposit or rust is abnormally detected in the cooling water system, the system should be cleaned thoroughly and then the cooling water also should be refilled up completely by newly treated water.

The cleaning of the cooling system includes degreasing and descaling procedures which need special chemicals. As the chemicals may be hazardous, the cleaning of the cooling water system is recommended to be carried out by reliable specialist firm. Otherwise it should be done strictly in accordance with instructions from the supplier of cleaning chemicals.

Note: Descaling process by acid is hazardous, which needs protective equipment for human body, for example, everybody near the system should put on protective glasses and gloves at least.

Note: Careful attention is required to avoid contamination of lubricating oil by acid during descaling process. Check the acid content of lubricating oil of the engine directly after descaling work of the cooling system by acid and check again next day.

Filling-Up of Cooling Water

Directly after cleaning process of the cooling system, flush the system thoroughly with water until pH value to be about 7, and then drain the water. Fill up the system with distilled water until the water level of the expansion tank to have margin for treatment.

Prepare the solution of inhibitor according to the instruction from the maker and add the solution into the expansion tank. Add distilled water more to the normal level of the expansion tank. Run the engine for settlement according to the inhibitor maker’s recommendation or at least for 24 hours and then check the quality of the water for confirmation.

If the amount of cooling water is reduced due to evaporation or leakage or drainage for maintenance, the water level of the tank should be maintained by adding water. Water for compensating evaporation should be distilled water, while the water for loss due to leakage or drainage should be same treated water. After adding the water, checking of the quality of the water should be carried out to confirm the correct concentration of the ingredients.

Replace cooling water completely with newly treated water with interval of at least every two years. Waste cooling water should be treated in compliance with governing laws.

Reference:

Himsen Instruction Book by HYUNDAI HEAVY INDUSTRIES CO., LTD. Engine & Machinery Division