Singapore's seawater desalination solution
May 2010
Analytical Instrumentation & Environmental Monitoring
Singapore began tackling its drinking water supply problem a few years ago. The first large seawater desalination system ‘TUAS Desalination’ was designed and built by Hyflux. The system operates according to the reverse osmosis principle and produces more than 136 000 cubic metres of drinking water per day – enough to fill about 55 Olympic size swimming pools.
Technology and process
Seawater desalination runs in several process steps. The first of these is the drawing of the seawater and feeding into the system. This is followed by a multistage filtration, flotation and ventilation to remove suspended matter and undesirable substances dissolved in the water. The filtered water is conditioned with ferric chloride, sulphuric acid, hydrochloric acid, caustic soda and other additives for reverse osmosis and set to a specific pH. Without these important water treatment steps, the downstream osmosis diaphragms would become useless very quickly.
After conditioning of the water, it is fed to the osmosis diaphragm modules by high-pressure pumps. The systems have a modular structure so that the diaphragms can be backwashed at regular intervals. In a system with 10 modules, for example, one of them is always in backwash mode whilst the others produce drinking water. In the last part of the system, after the reverse osmosis, which supplies almost distilled water, the water is brought back up to drinking water quality mineral content. This is the final process step before the water is fed into the distribution pipes and sent on its way to the consumers.
The GEMÜ solution
GEMÜ supplied more than 760 valves in nominal sizes from DN 15 - 1400 for this project. To offer the customers a package solution, valves were also purchased from partner companies. The customer could therefore be offered everything from a single source.
Butterfly valves with the different disc materials in 1.4408, bronze or ECTFE coated are mainly used in the seawater inlet, in the filtration/flotation and in the supply (tanks and pump stations). The nominal sizes are between DN 65 – 1400 and the operating pressure is 2,5–18 bar depending on the process area. Positioning is via pneumatic and electric solutions as well as manual operation using hand levers and gears.
In water pre- and post-treatment, highly chemically resistant diaphragm valves made of PVC or PVDF are used for the chemical dosing processes. The nominal sizes are DN 15 - 40, operating at pressures from 3-5 bar. Float flowmeters of nominal sizes DN 20 – 40 are also installed. Chemical dosing is fully automatic so the valves are usually controlled pneumatically.
The high pressure and non-return valves on the pump stations in front of the osmosis modules are made of special materials such as Super Duplex A890. They are controlled pneumatically and equipped with intelligent positioners. The differential pressure in the start-up phase of a pump is up to 60 bar. The operating pressure is usually between 5–84 bars the valves are designed in DN 300 throughout.
For more information contact Keith Hedges, GEMÜ Africa, +27 (0)11 462 7795, [email protected], www.gemue.de
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