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Figure 1. Butterfly valves up to 96” 150 class for SWCC Figure 2. Butterfly valve concept

sealing system where elastomers are mostly used.
The overall weight of the valve is approximately less
than 50% compared to other valve models, which
gives a substantial cost advantage when using al-
loys with corrosion resistance to the environment to
be handled. Moreover, the end-to-end dimension
of the Butterfly valve is the smallest from all valve
model, which provide a substantial advantage in
term of space. Finally, the absence of body cavity
is reducing the risk of accelerated corrosion due to
deposition of dissolved and suspended solids pre-
sent in saline water such as carbonates, silicates
and hydroxides. (figure 1 - 2).
Figure 3. K-LO Butterfly Control Figure 4. Dyna-Lok Butterfly
Control Valves Control valve 72’’ for
This category of valves includes globe, check, but- Corrosion mechanism Hyundai valve 94’’ for Maire
Tecnimont
terfly, control ball, safety relief, regulating valves. Crevice corrosion
This is from far the category of valves subject to
the most severe service condition due to their pri- Crevice and pitting corrosion extensively affect car-
mary function to regulate the flow in the line and to bon steel material if not adequately protected but
control cavitation. This includes pressure reduction also austenitic 316 and martensitic 431 stainless
and flow reduction or control in case of throttling steel materials, which are materials still in use in
service and expose valve components to high fluid the seawater application because of economics
velocities. (figure 3 - 4). considerations. These materials have to be pro-
perly protected to resist to see water environment,
e.g. rubber lining or coatings such as FBE (Fusion
Material selection Bonded Epoxy) for carbon steel, or providing ade-
quate cathodic protection to components of the
valve in contact (body – stem) in the case of 316 or
As fluids handled in desalination are corrosive to 431 materials. In the case of stainless steel mate-
carbon steel and cast iron, two low cost widely rial, crevice corrosion occurs under deposit of any
used materials, it is necessary to either provide corrosion products on component’s surface. So-
protective coatings to these materials or use cor- metimes these deposits work as semi-preventive
rosion resistant ones. Because valves often disturb layer between the medium and the metal surface
the flow in piping systems where they are installed, but as soon as corrosion ions find their path to a
they can generate turbulence, with associated high small surface defect such as a micro crack or a
fluid velocities, within the valves and downstream hole, stagnant conditions will subsist allowing the
from them. In some cases cavitation can be gene- creation of concentration cells. The difference in
rated which can damage the valves and the down- ions concentration at the component surface is
stream piping. In selecting the valve material, it is then generating anodic and cathodic areas leading
therefore necessary to consider their resistance to to material destruction. Crevice corrosion on these
fast flow and to cavitation. materials could be controlled by:
• Adequate cathodic protection
In selecting the valve material, • Increased flow velocity that will prevent depo-
it is therefore necessary to • sition on the component surface
Decreasing the aggressiveness of the medium
“consider their resistance to fast such as Cl ions concentration, acidity and
-
flow and to cavitation temperature
• Periodical cleanliness of the component surface

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