Fresh water on-board

  

Fresh water on-bord is Supply fresh water to the vessel ,it use for necessary for the operation of the ship and meeting the daily needs of passengers and the crew on bord regarding the operation of the ship together with the water required.

Large commercial and military working marine vessels, sailboats and power cruisers, furthermore as offshore oil and gas platforms need a dependable and consistent supply of pure water on demand, whenever and wherever they're across the world. Indeed, many boats would require huge amounts of water for drinking, cooking, ice, showers, dishwashers and laundry a day, with a number of the most important cruise ships, as an example, requiring in far more than 260,000 gallons a day.

Fresh water supply is provided in two ways as follows：

1.      From sea water by means of purification system on board :-

 

Freshwater may be obtained from shore mains supply or water barge. Alternatively, the majority of ships employ an evaporator system that uses distillation, or a pressurised filtering system which uses reverse osmosis to convert seawater into potable water.

When the evaporator system is used, seawater is heated and turned to steam, before the fresh water condensate is collected. Alternatively, reverse osmosis forces seawater though progressively finer filtering to produce fresh water. Neither method is without drawbacks: both leave organic compounds such as benzene and trichloroethylene in the water which may be harmful to humans. The process therefore has to be managed carefully.

 

2.      From the harbour dock connection :-

  Due to high sea water purification costs fresh water is mostly supplied at port services.

  

·        Importance of having fresh water on board.

-           Use for, washing, drinking, cooking, and even running other important machinery which use fresh water as a cooling medium.

-  Cruise vessels use for entertainment activities (swimming pools, shops)

 

 

 

 Process of obtaining fresh water on board by cruise vessels and merchant's vessels, discuss separately including the equipment used for that purpose.

 

·         FRESH WATER ON BOARD BY CRUISE VESSELS

 

Cruise ships are beautiful creations, outside and inside, with all their paraphernalia done to artistic perfection in an effort to woo guests. An average-sized cruise liner carries anywhere between 2000-3000 passengers. With these many guests on board, for several days and sometimes even months together, ever wondered how the freshwater requirement of so many people (crew included) are catered to on cruise ships.

The average freshwater consumption on cruise ships can vary anywhere between 500-1000 tonnes per day. It can be even higher on larger ships.

This ships always generate from sea water by means of purification system on board.

On cruise ships, fresh water is required for drinking, galleys, laundries, high-pressure washing & cleaning purposes, steam generation (distilled water), various heating & cooling systems within and outside the machinery spaces, sprinkler and hyper-mist systems for fire-fighting and recreational purposes (swimming pools & water-slides).

 

·         Methods of producing fresh water cruise vessel.

 

-          Reverse Osmosis

 Reverse Osmosis is also a common method of producing fresh water from seawater on- board cruise ships. A typical RO plant on board is a compact unit with a much smaller footprint than a flash evaporator. It has a capacity of 12-15 m3/hr. depending upon seawater quality and the condition of the filters. A cruise ship can have one or two RO plants, depending upon the requirement.

-          Flash Evaporators

 Flash evaporators on cruise ships typically require a considerable amount of space. They have a large footprint and longitudinally extend to nearly the whole length of the compartment in which they are placed. Usually two in number, they are strategically placed close to or inside their respective engine compartments.

The illustration above is just for the purpose of explanation and understanding of the basic working of the flash evaporator. On passenger ships, it is common to find a multi-stage flash evaporator plant incorporating 3 or 4 stages, although here I have chosen two stages only for simplicity.

Feed water, which is seawater, is preheated by HT jacket water from the engines to a temperature of 70-80 degrees Celsius. In many modern evaporators, this temperature can be set and adjusted to increase the production of the evaporator.

Both the flash tanks (1st and 2nd stage) are maintained under vacuum which is created with the help of the air educator. The air educator works on the venture principle.

-          Potable water bunkering

Despite having two evaporators and a similar number of RO plants with large production capacities, the quantity of potable water on-board often poses a challenge as the demand almost always tends to match or exceed the supply of potable water on-board cruise ships.

The main reason for this is that the ships spend quite a large amount of time at ports. An average cruise liner operating in say, the US-Caribbean route can have as many as 5 port days in a week. A cruise ship could be docked for an average of 8-10 hours, which can at times even go up to 12 hours.

The bunker water is usually supplied by a tanker or terminal. As per USPH guidelines, the potable water supply should be checked for quality before supplying to the vessel and a written declaration must be provided to the ship mentioning the Chlorine content and pH..

production water from evaporators and RO, the bunker water too needs to attain a minimum of 2 mg/L (2 PPM) of free chlorine content within the first 30 minutes of the commencement of bunkering and thereafter maintain a value of 2-2.50 PPM at all times during the bunker. This is recorded in a bunker chlorination chart recorder.

FRESH WATER ON BOARD BY MERCHANT'S VESSELS

Merchant’s ships not need much of fresh water of, beacuse they have maximum 30 crew members .so they most of the time fresh water is mostly supplied at port services.

So ship crew member use fresh water for drinking, cooking and most important works, but other works they use purification water.

In a merchant ships other activities (bath rooms, washing machines, and operating for machineries) on that works use for sea water purification water.

 Fresh water generated using desalination plants (fresh water generator) or reverse osmosis process on ships should be transferred to the fresh water tanks only when the ship is 25 miles away from the shore or 50 miles from an estuary.

·         Process of Converting Seawater to Freshwater on a Ship

 -        Fresh Water Generator Working Principle.

Basic principle of all low pressure freshwater generator is that, boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it. By maintaining a low pressure, water can be boiled at low temperatures say 50 degree Celsius. The source of heat for the fresh water generator could be waste heat rejected by main engine jacket cooling water.

 Hence using energy from a heating coil, and by reducing pressure in the evaporator shell, boiling can takes place at about 40 to 60 degree Celsius. This type of single effect plant is designed to give better economy than obsolete Boiling Evaporators.

 

-         Fresh water Generator Starting Procedure

 

1.      Make sure seawater ejector pump suction, discharge and overboard valves are open. Start the ejector pump. Seawater pressure at the air ejector must be 3 bar or more.

2.      Wait for vacuum to build up inside fresh water generator shell. (About 92 % vacuum).

      3.      Open the feed water valve to feed seawater to the evaporator. Adjust the feed water pressure. Normally marking is provided on the pressure gauge for desired feed water pressure.

     4.      Open main engine jacket cooling water inlet and outlet to the evaporator gradually.

     5.      Open the air vent clock at the top of the evaporator to make sure the evaporator is filled with jacket cooling water. Air must be purged out if any.

    6.      Switch on the salinity alarm panel for measuring purity of the freshwater produced.

    7.      There will be a sight glass provided at the suction line for the distillate pump. Make sure condensed water is coming to the suction line. Now start the distillate pump and open discharge valve to lead generated water to specified storage tanks.

   8.      Do checks While Running Fresh water Generator.

   9.      Through the sight glass provided in the evaporator shell, observe flashing of water.

  10.  Also check for the brine level inside. It should not be too high or too low.

   11.  Shell temperature must be around 50 deg cel..

   12.  Make sure shell vacuum is more than 90% from the vacuum gauge.

   13.  Check seawater inlet and outlet temperature to the condenser.

   14.  Ensure seawater pressure at air ejector inlet more than 3 bars.

   15.  Check for distillate pump pressure and water flow meter.

   16.  Check salinity of fresh water produced.

   17.  Check level and flow of dosing chemical.

   18.  Check ampere of ejector pump and distillate pump motor.

 

 -          Fresh water Generator Stopping Procedure

Operation of freshwater should be carried out in consultation with bridge watch keeper. Following procedure may be adopted for stopping fresh water generator

 

 

1.      Slowly open bypass valve for main engine jacket cooling water.

2.      Ensure that main engine jacket cooling water temperature is within normal limits.

3.      Close jacket cooling water inlet and outlet valves for the freshwater generator respectively.

4.      Close the feed water chemical dosing valve.

5.      Stop the distillate pump and shut discharge valve.

6.      Switch off salinity meter.

7.      Close filling valve to freshwater tanks.

8.      Wait for evaporator shell temperature to drop below 50 deg cel.

9.      Close the feed water valve to evaporator.

10.  Stop ejector pump. Shut fresh water generator overboard valve.

11.  Open the vacuum breaker valve to make shell side pressure equal to atmospheric pressure.

12.  Open the drain valve of the evaporator to drain all the seawater from the fresh water generator.

Main rules and regulations related to freshwater on-board.

 

-          International maritime labour organization. (Ex: - Title 3. Accommodation, recreational facilities, food and catering.

-          Flag state regulations. (Ex :-  sri lanka –Maritime Environmental protection Authority )

 

 Ways of reducing freshwater consumption on -board.

 

-          Advice Crew Not To Waste Water

-          Install pressurized or vacuum-assisted flush toilets.

-          Check the Pressure of Hydrophore.

-          Repair Accommodation Leakages Immediately.

-          Reduce the Flow of Shower Head.

-          Avoid using freshwater in deck washing.