Page 102 - 83

SPECIALE VALVOLE

• Limiting or removing by filtration or precipita- terial selection and eventually discuss some selec-
tion suspended particles in the media tion.
• Avoiding “dead cavities” that could produce Each plant has its specific operating conditions so
stagnant conditions and sharp edges in com- the cost – efficiency balance will be different from
ponent’s machining. one to another, consequently the selection of the
type of valves and the materials used will have to
In the case of carbon steel, lifetime of the valve will be optimised.
be linked to the above parameters but mainly to
the quality of the protective layer (rubber or coating) Each plant has its specific
applied on base metal. Bad adhesion or porosity
(pinholes) in the protection will expose directly the operating conditions so the cost
metal surface to heavy crevice corrosion process. “– efficiency balance will
be different from one to another

Erosion corrosion – consequently, the selection
Erosion corrosion combines chemical corrosion of the type of valves and the
and mechanical abrasion creating a synergy effect.
When the fluid velocity is high and contains parti- materials used will have to be
cles, the passive film created by chemical corro- optimised
sion is continuously removed by erosive effect until
the material fails. Erosion corrosion rate depends
on many factors of which the most important are This is what Valvitalia can offer, based on years of
the type of material, the nature of the medium and expertise on valves installed and feedback of cu-
its velocity. The effect of flow velocity on corrosion stomer on the durability of our products.
rate for a number of metals in sea water have been Carbon steel valves with rubber liner of protective
studied (4). It appears clearly that all materials do coatings will not work in every condition and their
not have the same ability to resist to erosion cor- durability is exclusively linked to the quality of the
rosion as the flow increase and that customized application of these products. The criticality of the
protection and adapted geometries will have to be application might impose the use of corrosion resi-
selected to guaranty resistance. stant materials like Nickel Aluminium Bronze alloys
(largely used in our military / naval application) or
stainless steel alloys with high molybdenum con-
Valvitalia approach tent such as Duplex or Super Duplex (5). These
Even if Customer usually specify construction ma- Molybdenum alloyed steels will extend the opera-
terials for the valve during the engineering – feed ting life of the valve by several orders of magnitude
phase of the project, the valve manufacturer shall compared to other materials, making them eco-
verify the operating conditions relatively to the ma- nomically viable. Other solution could consist of

La corrosione delle valvole negli impianti

di desalinizzazione: le condizioni

di servizio come fattore critico nella scelta

della valvola

Le valvole sono un elemento essenziale nel trattamento, nel trasporto e nella distribuzione
dell’acqua. Se le valvole non sono selezionate correttamente, possono verificarsi problemi
di prestazioni nel lungo termine. La scelta sbagliata della tipologia di valvola o del materiale
può essere causa del 40% dei problemi operativi. È sufficiente il malfunzionamento di una
sola valvola per impattare seriamente sull’efficienza dell’intero impianto, nonostante, talvolta,
queste rappresentino effettivamente solo l’1% dei costi di investimento complessivo. Esiste una
correlazione, che il cliente dovrebbe sempre tenere in considerazione tra l’investimento iniziale
e il ciclo di vita del prodotto: la riduzione del CAPEX iniziale potrebbe portare a un aumento
dell’OPEX, da sommare ai danni conseguenti la chiusura dell’impianto.

Questo articolo esamina i diversi meccanismi di corrosione delle valvole utilizzate nell’ambito
marino e descrive l’approccio di Valvitalia, in quanto produttore di valvole, nella selezione della
corretta tipologia di valvola e del corretto materiale di costruzione, sottolineando l’importanza
delle condizioni operative.

98 98 Impiantistica Italiana - Settembre-Ottobre 2020