Cooling Water System Jacket Cooling Water System and Piston Cooling Water System


Cooling Water System

Jacket Cooling Water System and Piston Cooling Water System

Cooling of Engine is required to enable the engine metal to retain their mechanical properties.

Fresh water is used as the coolant in the jacket cooling water system.

Fresh water is circulated around through internal passages within the engine

Fresh water is then cooled by sea water in a cooler.

Sea water is not used directly as a coolant because of its corrosive action and tendency of scale formation in the narrow cooling passages.

Fresh water is used as the coolant for piston cooling water system

Lubricating oil may also be used as the coolant for piston cooling system

Fuel Oil System:

Fuel Oil System for a large diesel engine has two systems

Fuel Oil Supply System and Fuel Oil Injection System


Fuel Oil Supply System:

This system involves supply of fuel oil from bunker/storage tanks to the main engine fuel pumps

Fuel Oil from bunker/storage tanks is pumped into the settling tank by using transfer pumps

In the settling tanks fuel oil is heated and huge amount of water is separated and drained

Fuel oil is pumped from the settling tanks to service tank by purifier / centrifuge pump through heaters and purifiers

Purifier removes water and suspended solid impurities (sludge) from the Fuel oil

Fuel oil is pumped from service tank to main engine fuel pumps using booster pumps through flow meters, mixing column/tank, heaters, viscosity regulators and filters

Reasons for doing Chemical Treatment of Cooling Water (CW) System / Jacket Cooling Water (JCW) System

Distilled Water (Demineralised Water / Water produced from FWG) is used as CW / JCW for Diesel Engines

Untreated Distilled Water absorbs Carbon-di-Oxide from the Air and becomes Corrosive

pH less than 7 is acidic and causes corrosion

pH 7 to 9, slightly alkaline, is ideal

Hardness salts like CaCOз causes scale deposit formation

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

Hence Chemical Treatment of Cooling Water (Distilled Water) is done for Preventing Corrosion and for preventing scale formation of the CW System and also for Effective Cooling


Sea water or fresh water contaminated by sea water is not used as CW. Since SW is highly corrosive and it causes deposit formation in the CW system

Rainwater is not used as CW. Since Rainwater is heavily contaminated and is highly corrosive

Tap Water (Drinking Water / Mineralised Water) is not used as cooling water. Since it causes chalk deposit formation in the CW system

However Tap Water may be used as cooling water after softening and after chemical treatment (based on the ingredients in the Tap Water)


Method of Chemical Treatment of CW / JCW system 

Chemical Treatment involves adding Corrosion Inhibitors / Chemicals to the CW System

Prepare the solution of Inhibitor / Chemical according to the instruction from the maker and add the solution into the expansion tank

Some inhibitors/ chemicals may be toxic and hazardous. Safety precautions should be followed while handling inhibitors / chemicals


Only Nitrate- Borate based Corrosion Inhibitors are added

Oil based Inhibitors are not used. Since these inhibitors adhere to the Cooling Surface which will reduce the cooling Efficiency



Testing of CW / JCW

Property of the CW changes during service due to contamination or evaporation

Therefore, CW should be checked periodically during service (once a week)

CW Sample is draw from the system

CW sample is tested using test kits supplied by the Inhibitor / Chemical Maker

In addition to testing the CW every week, it should be checked in a Laboratory once in three months

CW test results should be recorded and kept for trend evaluation.

If test result shows that the property / 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


Tests carried out on JCW Permitted Values
pH 7 to 9
Total Hardness (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)

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