Vessel sea water system explained…

This post is going to be short and concise as the sea water system is quite simple and easy to operate. Sea water is used onboard vessel mainly for cooling purpose, but also for fresh water production and adjusting vessel trim and its stability.

The sea water intake is possible through a crossover main pipe connected to the high and low sea chests located on each side of the ship. Each sea suction chest has an associated suction filter and this must be maintained in a clean condition, in order to ensure adequate water flow to
the sea water crossover main pipe and each one of them is provided with air and steam connections for weed and ice clearing.

It is important to note that when lifting the cover off a suction filter casing the joint must be broken with nuts still attached to the cover as this allows the cover to be bolted back into position should sea water actually be leaking into the casing.

Sea chest suction on the ship side
Crossover main pipe inside view

A Marine Growth Prevention System (MGPS) is fitted inside each sea water filter, which provides chlorine and copper injection into the sea water suction main in order to inhibit marine growth in the system and must be operational at all times when a sea suction chest is operational and, on modern vessels, a flow switch at each sea chest monitors sea water flow and regulates operation of the MGPS for that sea chest.

The sea suction valves at each sea chest are manually operated, but on large vessels with big valves they are operated by means of local hydraulic actuators below floor plate level, same as the discharge valve for the main sea water circulating system. Which sea water suction is used depends upon operating circumstances. Normally at sea the low suction is used, but when operating in rivers, estuaries and harbors, where is a danger of drawing mud and silt into the suction manifold, the high sea suction is used.

The main cooling sea water pumps discharge to a common sea water pressure manifold which supplies sea water to the central fresh water coolers. The fresh water generator sea water pump operates the vacuum ejector on the FW generator, provides cooling water to cool the vapour produced during operation and supplies the FW generator with feed water. On container vessels, the reefer cooling sea water pumps supply sea water to the two container fresh water coolers. On some type of vessels the auxiliary sea water cooling pumps discharge to a common sea water pressure manifold which supplies sea water to the auxiliary central fresh water coolers, the sewage treatment system, the ballast water transfer system and the vacuum condenser sea water cooling pumps supply cooling sea water to the vacuum condensers and vacuum pump coolers.

All these pumps take suction from the sea water suction crossover main pipe. Other pumps taking suction from the sea water crossover main pipe are Fire pumps and Ballast pumps.

On some of the vessels one of the main SW cooling pump is provided with a priming unit because it has an emergency bilge suction, the valve spindle, which is painted red, extending above the floor plate level.

The SW cooling pumps are started and stopped remotely from the engine control room (ECR) or locally at the local selector switch panels. The pumps may be selected for RUN, OFF or ST.BY. When the selector switch is in the ST.BY position, the pump will start automatically should the running pump fail to maintain the correct system pressure. One of the SW cooling pumps will normally be selected as the standby pump to start if the operating pump cannot maintain the pressure. Pressure switches on the discharge side of the pumps provide the start signal for the standby pump, which would happen if the operational pump failed for any reason. On some systems there are high speed and low speed starting pushbuttons and the selected running pump may be started in the high or low speed condition as required and on modern vessels the pumps are equipped with frequency converters which will adapt the pump speed as required.

It is important to note that when starting any pump remotely from the ECR, ensure that the pump is operating on the system, and that all relevant suction and discharge valves are open. On the main switchboard, check the motor amps for pump load. On the monitoring system screen, check the suction and discharge pressures of the pump, if available.

At sea when the sea water temperature is below 26°C one of the main sea water cooling pump will be used at high speed as will provide enough sea water flow. If the sea water temperature exceeds 26°C both main sea water cooling pumps will be used at high speed giving a higher sea water output at a higher pressure.

The central fresh water coolers are fitted with a back flushing system on the sea water side and this is used to remove debris from the cooler surfaces in order to maintain them in a clean condition and should be carried out whenever the flow of sea water through the cooler is restricted. Basically the back flushing operation means shutting down the cooler and forcing sea water into the cooler through the outlet connection and allowing the sea water to flow overboard via the inlet connection. Valves are provided at each cooler to allow for back flushing. Back flushing should be carried out when the cooling load is low enough to enable one cooler to meet the cooling demand. To prevent damage to the sea water cooler line filter during backflushing, the filter insert must be removed. Before strainers are removed for cleaning the sea water inlet and outlet valves for the cooler must be closed and sea water in the cooler will be drained to the bilge.

Central fresh water coolers
Filter insert on central fresh water cooler

The interval between cleaning of the in-line filter (and backflushing) depends upon the nature of the sea water in which the vessel is operating. An increase in the sea water pressure drop across the cooler indicates fouling and cleaning of the in-line filter is necessary, but if this cleaning does not reduce the pressure drop, the cooler should be backflushed. In-line filter cleaning at monthly intervals should maintain the cooler sea water surfaces in a clean condition.

The procedure for backflushing of the Main Central Fresh Water Coolers is as follow:

  • Monitor the pressure drop across the cooler until increases to an unacceptable level. The pressure drop will increase when debris becomes lodged in the sea water channels of the cooler, this debris must be removed in order to restore the operational efficiency of the cooler.
  • To prevent damage to the sea water cooler line filter during backflushing, the filter insert must be removed. Ensure that the other cooler is in use, then isolate the sea water side of the cooler that need to be backflushed by closing the sea water inlet and outlet valves. Drain the cooler, remove the filter insert, and refit the filter housing cover.
Example of cooler’s backflushing diagram
  • With the auxiliary cooling sea water circulation system operating normally, open the cooler backflushing valves CW018 and CW019 for the cooler being backflushed (in this example No.1 cooler), and close the normal sea water inlet and outlet valves to the same cooler being backflushed, in this case, valves CW031and CW032.
  • The backflushing sea water will enter the cooler via the outlet pipe and leave the cooler via the inlet pipe. The backflushing water flows to the overboard discharge valve via the cooler outlet backflushing valve CW019.
  • Leave the backflush system operating for about 15 minutes.
  • When backflushing is complete, open the cooler’s inlet valve CW031 and outlet valve CW032, and close the cooler backflush valves CW018 and CW019.
  • Check the sea water flow through the cooler. The cooler is now back in operation and the other cooler may be backflushed, if required.

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