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The same holds true for the middle segment,
4,000-5,500, but at a slower rate.
It is the opposite, however, in the large
segment, beyond 5,500. The trend is for axially
split machines, Reliability, availability and
efficiency are the most important factors on
these large installations, generally under BOO/
BOT financing schemes. Equipment first cost is
secondary.
As for energy recovery, today's plants feature
turbines of the Pelton type. Initially, reverse
running pumps and, in some cases, Francis
turbines were used. However, as they became
more reliable, the more efficient Pelton turbine
became the element of choice. The Pelton turbine
allows a more simple plant design and simplifies
the operation.
Other energy recovery devices are being con-
sidered but, to date, some are restricted to small
plants or retrofits and others need to gather expe-
rience and operating hours on larger units.
3. Materials of construction. Alloy 885
Metallurgy is one factor that makes special
the specification and construction of a seawater
reverse osmosis high-pressure pump.
Typical seawater where reverse osmosis plants
are installed has salt contents ranging from
20,000 to 40,000 ppm and temperatures from 15
to 35°C. Dissolved oxygen levels are moderate.
Fluid velocities within the pump are rather high;
values of 40m/s are representative. Fortunately,
inorganic suspended solids levels are very low.
Simplistic material recommendations are,
therefore, impractical and the designer has to
consider, among other things, localized corrosion
(pitting and crevice corrosion) and stress
corrosion cracking (SCC). Castability and weld-
ability are also to be taken into account.
The families of materials under consideration
are: 300 series austenitic stainless steels, duplex
stainless steels, super austenitic stainless steels
and specialized alloys.
3.1. 300 series austenitic stainless steels
Their susceptibility to localized corrosion
(pitting and crevice) and SCC make them a
marginal material for this service. Improved
service life has been experienced with steels
containing molybdenum in colder seawater. The
chrome oxide layer is very tenacious and, there- fore, the corrosion rate is not velocity sensitive.
They are readily available and easily weld
repaired (the low carbon grades) in the field.
3.2. Duplex stainless steels
Materials with a ferrite and austenitic micro-
structure, hence the name "duplex"; ferrite is the
matrix phase. Higher levels of chrome, molyb-
denum and nitrogen make them more resistant to