Overview of marine sea water pumps and system
The main propulsion & other auxiliary machineries generate heat due to the combustion and other processes involved in performing of their operations. This heat has to be transferred to some other medium, so that the machineries can function properly within the safe operating parameters. The ships main & auxiliary machineries are cooled by fresh water (as sea water causes corrosion), and this fresh water is in turn cooled by the seawater in huge heat exchangers. Thus the heat is extracted from the machineries and heat balance is maintained. For the extraction of heat, the sea water has to be conveyed at a particular pressure and volume. This is done by having 2 or more main sea water pumps in the engine room. The sea water pumps sufficiently cater the needs for the proper cooling of the fresh water and in turn the machineries. Some of the components of the sea water system are
1. High & Low sea suctions.
2. Sea suction Filters and corresponding isolation valves.
3. Main sea water pumps.
4. Heat Exchangers.
5. Temperature adjusting valve(auto/manual) &
6. Overboard valve.
Some systems may be different but these above mentioned components are common for all types.
Attached here with, is the simple main sea water circulating system, onboard a ship.
The system consists of usually 2 sea suctions. One is high and the other is low sea suction. These act as a gate so that sea water enters the engine room through pipelines & valves in a controlled manner. Then after the respective isolation valves, the sea water line has a filter called as "sea suction filter", to filter out the fishes, weeds, Plankton, shells etc..These organisms when they enter into the sea water system, they cause fluctuation in the sea water pressure and erosion of the impeller of the sea water pumps. They also chock the central coolers, causing heat imbalance, making the parameters of the engine to exceed its limit. Thus it is highly essential to filter out all these marine organisms so that the sea water cooling efficiency is maintained at optimum always. After the filter, ships generally have various lines branching out for various shipboard services. But the main huge line goes to the main sea water pumps.
The main SW pumps
These are high capacity centrifugal pumps, two or three in number, circulate the sea water throughout the plant, and responsible for throwing out the same sea water after it extracts the heat from fresh water(from the engine). Thus these centrifugal pumps have to develop a minimum pressure of at least 2 bar (a value which is minimum level onboard almost all ships, where we get low pressure alarm at 1.8 bar). These pumps have their own isolation valves, which can be used to isolate the pump to carry out maintenance on it. These pumps are capable of running in parallel and individually, depending on the heat balance requirement. One of these pumps, sometimes may act as dual purpose pumps, used as General service or even Ballast pump.
One of the important aspect of the capacity of the sea water pump is decided based on the "Emergency Bilge Suction". An Emergency Bilge Suction, is a valve, which is connected to the suction line of the main
sea water pump, thus when it is opened, the sea water pumps taken suction from the engine room. This valve is distinguished by marking it with red/orange color, and kept locked as this must be used only in emergency situation of engine room getting flooded.
Sea water usage in ship’s engine room
Sea Water is typically used for various purposes in the engine room. It is the only source which is always available with no extra cost. They are used for
1. Cooling for fresh water in central coolers,
2. Cooling of condensate (return steam) in auxiliary/atmospheric condensers,
3. Cooling of exhaust/gland/condensate steam from the turbines(cargo pumps),
4. Cooling of air conditioning/refrigeration condensers,
5. Cooling of lubricating oil/ stern tube oil/ intermediate shaft bearing oil ,
6. Sea water feed for the production of fresh water in fresh water generators,
7. In Oil Tankers, sea water is used in scrubber towers & deck seals,
8. The main use is for the Fire main line, for fire fighting purposes. . and many more…!!
Cleaning procedure for sea chest filters
Since there are two or more sea suctions, before opening a sea suction for maintenance, other sea chest must be put in use with proper isolation of valves. After change over, the sea water pressure has to be monitored for any abnormality. If everything normal, the sea chest can be opened up and filter element can be taken out and cleaned.
1. Put the unused sea chest in use by opening the valves in proper sequence.( purge the sea chest out of air by keeping the vent open)
2. When all the air is purged, after ensuring sea water coming out of the vent, then close the vent and open the other valve to put the sea chest in use.
3. Isolate the sea chest on which the maintenance is to be carried out.
4. Open the vent and drain valve on the sea chest.
5. Then with proper tools,equipments & lifting gears open the top cover of the sea chest and pull the filter element out. Take out all the debris like fish, shell, sea weeds, barnacles etc..
6. Put back the filter element and properly close the top cover. Tighten the cover by tightening the bolts of exactly opposite side.
7. After tightening, shut the drain and keep the vent open to purge all the air out of the sea chest by opening the sea chest valve.
8. After purging, shut the vent and keep the cleaned sea chest as stand-by.
Marine Auxiliary Machinery by McGeorge
Introduction To Marine Engineering by D.A.Taylor.