iron or ductile cast iron bodies.
This low-pressure process also
requires a large number of butterfly
valves ranging in size from 3 inches (80
millimeters) to 110 inches ( 2,500 millimeters). These valves may be specified
in a wide range of configurations and in
a variety of seat and disc materials,
including Viton, Halar, Hypalon and
PTFE, as well as other plastics and elastomers. Valve bodies are typically
offered in seawater-resistant materials
such as aluminum bronze, duplex stainless steel and super-duplex stainless
steel. Valve sizes to 24 inches (600 millimeters) will have lug-style bodies,
while larger sizes are normally flanged.
To clean the water even more, chemical
pretreatment is employed. This process
further protects the relatively delicate
membrane equipment as well as related
process equipment and increases the
efficiency of the reverse osmosis
process. Oxidizing biocides, pH adjustment, anti-scalants, etc., use valves typically found in any chemical dosing operation. Depending upon the corrosiveness
of chemical agents and operator preference, the valves used here typically
range from relatively inexpensive plastic
diaphragm and check valves to more
robust PTFE-lined plug, ball and butterfly valves.
the SWRO process is during desalination,
which occurs in the high-pressure membrane area. Here, both the pressure (up
to 1,015 psi or 70 bar) and the chloride
content of the water are high (Figure 2).
The high-pressure system requirements dictate use of ASME Class 600-
rated valves, typically plug valves, for
membrane isolation service. Although
the seawater is very clean at this stage,
its high chloride content necessitates use
of super-duplex stainless steel (grade
5A) for all wetted parts of the valve.
Typically, these valves are butt welded rather than flanged, as most users
want to avoid the expensive ASME
Class 600 super-duplex stainless steel
valve and pipe-mating flanges. To protect valve investments against corrosion, these valves can be specified with
an encapsulated PFA plug that rides
directly on a machined body seat. These
high-pressure membrane isolation
valves are commonly available in sizes
to 16 inches (400 millimeters).
To better protect the membrane from
over-pressurization during startup,
some SWRO plant operators use a
bypass line. Industry standards dictate
the reverse osmosis system pressure
should gradually increase to reach full
process pressure in about two minutes.
Some installations achieve this by using
a variable speed drive pump or control
valves. A simpler, less expensive solution
involves a bypass line on the main isola-
tion valve. This line includes a smaller
ball valve with a seat capable of with-
standing high-pressure drops. At the
time of startup, the main valve would be
closed and the bypass valve opened. This
bypass valve is sized to ensure the sys-
tem pressure is increased at the recom-
mended rate. Once the system has
reached full operating pressure, the
main valve would open.
After the reverse osmosis process is completed, two streams emerge for post-treatment. The clean water is disinfected
either by UV lamps or chlorination
before further transmission. A brine discharge containing high salt levels, chemicals from the treatment processes and
toxic metals is either discharged into the
ocean or treated in a sewage plant with
disposal to a landfill. Because of the high
salt levels, chemicals and metals, rubber-seated butterfly valves with ductile iron
bodies or full port ball valves are typically deployed in these services.
In processing chloride-laden seawater,
using the best materials prevents localized forms of corrosion such as pitting,
crevice corrosion and stress corrosion
cracking. The SWRO industry has experimented with various materials, such as
aluminum bronze, 904L austenitic stainless steel, duplex stainless steels and
super-duplex stainless steels.
A growing body of materials research
and failure analyses within the industry