Fresh Water Generation and Distribution
This report is about a CAIRD & RAYNER CLARK Reverse Osmosis(RO) plant that is used to produce fresh water. Each RO plant is capable of producing 60m3/24Hr of fresh water.
Fresh Water Production
The production of fresh water begins with the intake of sea water, the sea water booster pump draws in sea water from the port sea chest via an inline strainer to a pump and then into a chemical cleaning tank (CIP tank). The sea water strainer is used to filter out any marine life and/or debris which may damage the pump. The CIP tank has the ability to contain an immersion heater; this can be used in colder climates to heat the sea water to the optimum working temperature of 17oC. The sea water or feed water is then pumped, using one of the two low pressure feed pumps, through the pre-treatment stages of the process at a pressure of approximately 3.5 bar and a rate of 8.32m3/hr.
The pre-treatment process can be broken down into two stages, the first stage being two sand /manganese dioxide filters. The sea water is pumped at a low pressure through the top of the filter and flows through the sand and manganese dioxide and it then leaves the filter at the bottom. The sand will remove any particulates that are larger than approximately 80 microns and the manganese dioxide will remove any iron that is present in the water. A low pressure is used to pump the feed water through the filters to ensure maximum efficiency of the filtering process.
The second stage of the pre-treatment process is chemical dosing, this is to prevent the build-up of scale within the plant membrane. The feed water is injected with a solution of anti-scalant that is diluted with fresh water, this is pumped from a dedicated dosing tank and the level in this tank is maintained by the watch-keeping engineers. Polysperse A is used as the anti scalant, it is a commercial product that prevents the build-up of scale caused by hard salts in pipes and machinery where sea water is used.
Once the feed water has gone through large particulate filtration and has been chemically dosed it will then pass through a cartridge filter that will further remove any particles that are larger than 5 microns, this is the acceptable level for entry into the Reverse Osmosis Membranes. Anything larger the 5 microns will cause damage to the membranes and cause them to deteriorate, which will reduce productivity. Now the feed water has been filtered and treated accordingly it is now suitable to pass through the RO membranes where salt is removed from the water via reverse osmosis to produce fresh water.
Reverse osmosis is the process of turning salt water into fresh water using high pressure and fine membranes that only allow water molecules to pass through. To create a high pressure a positive displacement pump is used. The pump is capable of a maximum discharge pressure of around 60 bar and is also capable of producing a flow rate or approximately 8m3hr. To prevent damage of the pump it is fitted with low suction and high discharge pressure cut outs, a pressure relief valve can also be located on the discharge side to prevent any damage to the rest of the system.
In order to prevent any cavitation and minimise any vibration within the pump, the suction line is fitted with a captive acceleration tube and the discharge line is fitted with a pulsation dampener which is designed to smooth out any high pressure pulses produced by the HP pump. The pulsation dampener also ensures that the high pressure feed through the membranes is smooth and uniform by using a 50 bar charge of nitrogen to absorb any vibrations. The high pressure feed then passes through each of the four membranes which are connected in series. To generate the high pressures needed within the membranes a brine pressure regulating valve is fitted to the discharge of the last membrane, this is effectively a relief valve and by increasing the pressure it is possible to set the lifting pressure of the valve. This will determine the pressure within the membranes.
Although the plant is fitted with 4 membranes it is not always possible to use all membranes and the plant is often run on 2 or 3, some factors which determine how many membranes should be used are the temperature, salinity and TDS (total dissolved solids) of the sea water. The high pressure forces the water to diffuse through the semi-permeable membranes whilst leaving the salty brine behind to be discharged overboard.
To ensure the quality of the water, it is passed through a conductivity meter, if the conductivity of the water is above the set-point (500µs) then the solenoid dump valve will open and the water will be discharged overboard. If the conductivity of the water is within range then the valve will remain closed and the water will be allowed to pass on to the next stage of the process.
Now that we have made good quality fresh water it passes through a neutralite mineralising filter to improve its palatability.
The final stage of the process is to sanitise the product water; this is achieved by injecting the product water with a chlorinating chemical, sodium hypochlorite, into the product water. This ensures that the fresh water does not become contaminated whilst being held in a storage tank.
The fresh water is stored in one of the four fresh water storage tanks. In order to prevent a build-up of bacteria and organisms the fresh water must have 0.6ppm of chlorine to maintain a sterile environment inside the tanks.
Fresh Water Distribution.
Four hydrophore supply pumps draw water from the desired tank, the supply pumps are multi-stage, self priming centrifugal pumps each capable of pumping water at 13.5m3/hr. The water is pumped through four carbon filters which are connected in parallel, the filters remove any contaminates and any residual chlorine. The water is then further dosed with sodium hypochlorite, this is to maintain a minimum chlorine level of 0.2ppm (measured at the tap) to prevent any build-up of bacteria within the system. The water is then pumped into two hydrophore vessels.
The hydrophore vessels are maintained at a pressure of around 5 bar by the ships working air system. The two vessels prevent frequent cycling of the supply pumps. From the two hyrdrophore vessels water is pumped to both the hot and cold fresh water system.
The cold water is continuously circulated around the system using two in-line centrifugal pumps using one as duty and the other as stand-by.
The cold water that is drawn from the hryrophore vessels is then heated using three plate heat exchangers using hot water from the central heating system to heat the cold water. Thermostatic valves are used to regulate the temperature of the water and maintain it at around 66oC in order to prevent any bacterial growth. Just like the cold water the hot water is continuously circulated around the system using two in-line centrifugal pumps, one as duty and one as stand-by.
