Cast Valve Materials for Seawater Service:

Nickel-aluminium Bronze and its Rivals

J R C Strang, Shipham Valves
(Abstract of Presentation given at Valveworld 2006)

©Shipham valves 2006

Nickel-aluminium Bronze (NAB) has been used for seawater service for many years. It is also widely recognised that it is excellent at this. However, with the pressures on either purchase cost reduction or minimum through-life cost, it often appears to sit uncomfortably between the cheapest solution (cast irons) at the low end and the more expensive super duplex, nickel alloys and titanium at the other. The aim of this paper is to show that the NAB solution is cost effective, avoiding the poor performance of the low-end materials and the extreme expense of the high-end ones.

There are very few comparisons of materials that cover the full range of material options, from standard to titanium, including the copper alloys, and the duplex stainless steels. This paper will provide comparisons covering mechanical properties, performance in various corrosion conditions as well as costs associated with valve manufacturing. The review will draw on Ship ham's extensive experience of manufacturing valves in most of the materials considered, as well as making reference to published literature from a variety of sources.

The materials include cast iron, carbon steel, 316 stainless, mow, duplex and super-duplex stainless steels, nickel alloys and titanium as well as a selection of high performance nickel-aluminium bronzes. The common corrosion types will be covered.

Additionally, the pressure-temperature characteristics of Nickel Aluminium Bronze will be discussed. On the one hand, everyone knows that Class 150 has a maximum operating pressure of "about 20 bar" and some are aware that for bronze alloys there is a maximum of 15.5 bar. The truth lies somewhere between these two limits. There is no standard that both maximises the potential of NAB as well as recognising its limitations, enabling an economic and a safe design.

In conclusion, NAB is shown to be particularly useful for seawater service, despite elevated temperature and sulphide environment limitations. The main advantages are that:

1) It is cheaper than the exotic stainlesses, and so cost effective;

2) Its performance on general corrosion, pitting and cavitation is comparable to superduplex alloys and significantly better than the standard alloys;

3) It also has beneficial properties of good heat conduction, does not gall, and has excellent anti fouling properties, and

4) It can have a pressure temperature rating well above the bronze standards.

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