Valve Magazine - Spring 2012

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.

Chemical Pretreatment 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.

Desalination

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.

Post-treatment

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.

SELECTING THE
RIGHT MATERIALS

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.