Jacket Water System for a Large 2-Stroke Marine Diesel Engine (A Basic Article for Trainee Marine Engineers)

Jacket Water System

Shown above is a typical cooling water circuit for a slow speed engine.
Water is pump via one of two centrifugal pumps. One is normally in use with the others stand-by. The water passes through to the distributing manifold on the engine side.

Jacket Water Heater In the line is a steam jacket water heater. When the engine is shut down steam heating maintains the engine in a state of readiness reducing the time needed for starting. Attempting to start the engine without heating can lead to poor combustion, poor lubrication and thermal shocking. A modern variation on this is the "blend" water from the stand-by auxiliary alternator engines into the main engine circuit increasing plant efficiency

The water enters and leaves the engine via a series of cylinder isolating valves. In this way each cylinder may be individually drained to prevent excessive water and chemical loss. In addition dual level drains may be fitted which allow either full draining or draining of the head only. A portion of the water is diverted for cooling of the turbocharger.

De-aerator Was an essential part of engines incorporating water cooled pistons were air was deliberately introduced in to the system to aid the "cocktail shaker" cooling action. Air or gas entering the system can lead to unstable and even total loss of cooling water pressure as the gas expands in the suction eye of the circulating pumps. In the event of gas leakage via the head or cracked liner rapid loss of jacket water pressure can occur. The de-aerator is a method to try to slow this process sufficiently to allow the vessel to be placed in a safe position for maintenance. This system also allows the vessel to operate with minor gas leakage.

Jacket Water Cooler The hot water leaving the engine passes to a temperature control valve were a portion is diverted to a cooler. Temperature is controlled using both a feedback signal (temperature measured after the cooler) and a feed forward signal (temperature measured at outlet from the engine). In this way the system reacts more quickly to engine load variations.

Evaporator Increases plant efficiency by utilizing heat in jacket water to produce fresh water. Modern systems sometimes rely on the evaporator to supplement a reduced size main cooler.

Expansion or header tank Maintains a constant head on the circulation pumps reducing cavitations at elevated temperatures. Allows the volume of water in the system to vary without need for dumping. Acts as a reserve in the event of leakage.

High Temp/Low Temp System:

Few Drawings for Easy Understanding: