Valves for Seawater Service
Why Use NAB Valves ?The advantages of NAB valves are significant. They are particularly suitable and useful for seawater service where corrosion performance, particularly where its resistance to chloride pitting is excellent. The techniques of producing consistent castings of good quality are well understood and there is little need for the extensive non-destructive tests that are required in the case of 6Mo, duplex and super duplex steels.
Mechanically, NAB is comparable with other popular corrosion-resistant alloys, but to take full advantage of these properties, specially determined pressure-temperature ratings have to be used. The excellent galling and wear properties help ensure longevity and good performance of valves in NAB.
The limitations of NAB valves are that it should not be used in sulphide environments and account must be taken of its flow limitations. The rival cast iron and steel valves need some form of protection to compete, and even then the quality and durability of this protection determine the life. Stainless steel valves suffer from severe crevice corrosion and pitting in seawater and the 6Mo, duplex and super duplex stainless steel valves have a temperature limitation of 20 oC for seawater service with a maximum chlorine content. The expense of the more exotic higher alloys becomes the critical factor, needing a special reason for justification.
While market prices vary, despite recent increases in the copper price,
NAB remains a cost effective valve material for
seawater service. However, compatibility with piping material is likely
to be the factor that determines the valve material, the advantages suggest
that NAB is capable of more than this