Multi-Stage Flash for Water that’s Fresh

The principle of MSF is similar to that of boiling a kettle. Steam is produced, condensation occurs, and pure water is collected. Apply vacuum to the system and the heated water spontaneously turns to water vapor, in other words it ‘flashes’. While the process is relatively simple, MSF distillation plants use an enormous amount of power, so many of them, especially large ones, are paired with conventional power plants in a cogeneration configuration. Waste heat from the power plant is used to heat the seawater, providing cooling for the power plant at the same time. This reduces the energy needed by one-half to two-thirds, making MSF plants more economical.

In further efforts to make MSF plants more economical, there is research going on into the use of solar thermal power generation plants to operate in tandem with the desalination plant, a process that could make MSF plants in areas like Southern California and Florida even more practical.

In a multi-stage unit, the plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector. In each stage, the salt water is heated and then placed in a lower pressure vessel, which causes a portion of the salt water to instantly vaporize (flash). This steam is then collected and condensed as desalinated water. As only a portion of the water is vaporized, the remaining brine goes through a series of stages at lower and lower pressure, causing more water to be vaporized.

Photo copyright Doosan.com

Throughout the process, valves are used to control the flow of the original incoming salt water, the purified water, and the brine.

Corrosion from all sides

The major problem in the efficient functioning of a multi-stage flash desalination plant is corrosion. This plant is exposed to seawater, salt-air, corrosive gases, very fast or extremely slow moving liquids and particulates in the water. All of these create a number of corrosion related problems. Besides general corrosion and mechanically and chemically-induced erosion-corrosion, localized corrosion such as pitting, intergranular corrosion, selective leaching and stress-induced corrosion are a big problem, not just for the flash chambers, but also for the pumps, valves, piping and even the venting systems.

Therefore, material selection is paramount when choosing valves for this process. Because valves by their very nature disturb the flow in piping systems where they are fitted, they can generate turbulence within the valves and downstream from them. In some cases cavitation can be generated which can damage the valve and the downstream piping. Therefore, in addition to corrosion from the salt and brine, it is also necessary to consider the valve’s resistance to fast flow and to cavitation.

For intake from the sea, diaphragm valves and butterfly valves are used, occasionally in stainless steel, but also in PVC-U or coated carbon steel. The real test for the materials comes during the distillation process, especially if more than one metal is used.

Stainless Steel the ideal

Stainless steels have a nearly negligible corrosion rate in flowing seawater above a velocity of about 1 m/s (3ft/s) and up to nearly 40m/s (130 ft/s). While conventional austenitic SS 304 and 316 are the most common materials because they are very strong, have high ductility, workability and weldability, the best choice for the control valves and ball valves normally utilized to control the flow between stages is superaustenitic and duplex stainless steels, which were designed to resist chloride-induced corrosion.

For the article on SWRO desalination and the valves involved therein by Ben Lee, be sure to watch for the Spring edition of Valve Magazine, due out in April. Subscribe here.

Kate Kunkel is Senior Editor of Valve Magazine. Reach her at kkunkel@vma.org