Page 100 - 83

SPECIALE VALVOLE

Corrosion of valves in desalination

plants: operating conditions

as critical factor in valve selection

Valvitalia’s approach
as valve manufacturer in selecting • open position or fully closed position.
Flow throttling: this type of valves is called
the valve type and “Control Valves” as they are operated to con-
construction materials trol the flow at a determined level.
Corrosion attack on valves, like other parts of the
desalination unit, depends mainly on the design of
valve, type and location of valve in the process, na-
ture of fluid being handled or media, materials of
William Toqueboeuf, Corporate Vice President construction of the valve parts and operating con-
Michele Carlini, Vice President Engineering ditions.
Paolo Donini, Senior Project Manager
Valvitalia S.p.A, Italy Various types of corrosion
processes can affect valves
“installed in desalination plants
cross the water industry, valves
are an essential element in the tre-
atment of water in its transportation Different types of corrosion processes can affect
and distribution process. If valves valves installed in desalination plants, e.g. erosion-
are not correctly specified, perfor- corrosion due to suspended particles or high fluid
A mance problems can occur over the velocities, selective corrosion such as de-zincifica-
long term. The wrong selection of valves or mate- tion and graphitization. Another type of corrosion,
rials can be the cause of up to 40% of operational which covers the metal surface uniformly, is general
problems. Even though valves may only represent corrosion. Concentrated corrosion can lead to pit-
1% of the plant investment costs, it only takes a ting or crevice corrosion.
single valve to seriously affect plant production.
There is a trade-off between financial and product
lifecycle customer has to make, keeping in mind Valve type selection
that lowering the CAPEX upfront will lead to OPEX
increment in addition to the consequential dama- On/Off Valves
ges of a plant shut down. The most common on/off valve type requested in
This article surveys the different mechanism of the water industry are certainly butterfly. Ball valves,
corrosion of valves used in seawater environment plug valves and gate valves are remotely requested
and describes Valvitalia approach as valve manu- because of their usual higher cost compared to the
facturer in selecting valve type and construction previously mentioned design, except for specific
materials underlining the importance on operating requirements in term of sealing mechanism (dou-
conditions. ble isolation for ball valves for example). This sim-
ple and effective design whereby the disc rotates
Introduction around an axis at right angles from the flow line
coupled with the absence of a body cavity makes it
Valves are used in desalination plants to control the a cost / quality effective solution for some key fea-
flow in the different processes involved in two di- tures. In effect, this design solution is more adapted
stinctive ways: to low pressure application (usually ANSI 150 – 20
• Flow shut off: this type of valves is called “On- bars and 300 class – 50 bars, up to ANSI 600 class
Off Valves” as they are operated or in fully – 100 bars in remote cases) given the nature of the

96 96 Impiantistica Italiana - Settembre-Ottobre 2020