Common Metals & Materials Used in Butterfly Valve Manufacture

Many different materials can be used in the design of a butterfly valve. A typical butterfly valve will use several different materials within its construction to achieve the best combination of functionality and cost. The following briefly describes the most common materials that may be used in the design of an effective butterfly valve, highlighting their relative advantages and disadvantages. 

Advantages of materials used for Butterfly Valves

Carbon Steel

Carbon steel is an alloy of iron where the main alloying element is carbon. Generally, no other alloying elements are added to control the properties of the material. For butterfly valve construction, carbon steel is most often used to form the body and disc of the valve using the sand casting process.

Carbon steels are available in several different grades. The most common grades used for valve bodies and discs are cast grades ASTM A216 WCB (Weldable Cast B-grade) and LCC (Low Carbon Content) steels. WCB material is most suitable for high temperature use, whereas LCC can be used at low (sub-zero) temperatures.

Main Advantage of carbon steel:

The cost - carbon steels are relatively cheap, and valves produced from carbon steels provide a cost-effective solution in environments where other factors are considered less important than cost.

Main disadvantage of carbon steel:

Poor corrosion resistance. This can be overcome by surface protection such as paint, provided that the line media does not corrode the valve from the inside.

Stainless Steel

The definition of a stainless steel is an alloy of iron with a minimum chromium content of 10.5%. The effect of the chromium is to form a self-healing layer of chromium oxide on the surface of the material. When the surface is broken by mechanical damage such as scratching, the chromium quickly reacts with oxygen in the air, so preventing the oxygen from reacting with the iron and forming iron oxide (rust). An ever-increasing number of stainless steels are available, of which the simple iron-chromium-nickel grades are most often termed 'stainless steel'.

Stainless steels can be further classified as ferritic, austenitic, martensitic, duplex and precipitation hardenable. This classification is based on the microstructure that is developed in the material by varying the alloying elements present. For valve construction, the most common grades used are austenitic and duplex. These are described briefly below.

Austenitic stainless steels

Austenitic stainless steels, in addition to chromium, contain elements such as nickel, which have the effect of retaining the high temperature face-centred-cubic austenitic structure at temperatures where it would normally have transformed to the ferritic body-centred-cubic structure. This face-centred cubic structure gives the material improved toughness and ductility compared to the ferritic grades. Depending on the nickel content, the tough austenitic structure can be retained even at extremely low temperatures, allowing the material to be used in cryogenic applications. Improved resistance to pitting corrosion can be achieved by adding molybdenum to the alloy.

Duplex stainless steels

Duplex stainless steels contain a balanced structure of both the austenitic face-centred cubic and ferritic body-centred structure of iron. This structure is developed by carefully controlling both the alloying elements and the heat treatment performed on the alloy to obtain a structure consisting of 50% austenite and 50% ferrite. The result is an alloy that combines the higher strength of ferrite with the improved toughness of austenite. The super duplex grades contain higher levels of chromium and molybdenum to enhance their resistance to pitting and crevice corrosion.

Nickel Alloys

Nickel alloys are used in valve components where severe service conditions are encountered. These alloys are particularly useful in harsh corrosive environments, which would attack stainless steels by breaking down their protective oxide layer.

Hastelloy nickel-based alloys are most often used in valve construction. There are several different forms of Hastelloy, with each being tailored by adding specific alloying elements is varying proportions to suit particular service conditions.

The main disadvantage of nickel alloys is their weight and cost. Nickel alloys have a high density and their cost can be many times that of basic stainless steels.

Titanium Alloys

Titanium alloys combine high strength with light weight and excellent corrosion resistance, having the highest strength-to-weight ratio of any metal. In a similar manner to stainless steels, titanium alloys gain their corrosion resistance by the development of a protective oxide layer on its surface. They are particularly resistant to corrosion by seawater, in particular in systems where hypochlorite is present to prevent biofouling.

The main disadvantage of titanium alloys is their cost, being around ten times as expensive as basic stainless steels or nickel aluminium bronze. The material is also difficult to process due to its highly reactive nature. Special casting techniques are required to prevent reaction with oxygen during melting and pouring.

Nickel Aluminium Bronze

Nickel aluminium bronze is a copper-based alloy containing approximately 10% aluminium, 5% nickel and 5% iron. The nickel aluminium bronze alloys provide excellent corrosion resistance, particularly in seawater environments. They also strongly resist the formation of a bio-film, which can cause increased corrosion problems in stainless steels.

Nickel aluminium bronze has a cost approximately equivalent to that of basic grades of stainless steel. One disadvantage of this material is that it is more anodic than most materials, so that if a nickel aluminium bronze valve is coupled to a stainless steel pipe, then corrosion of the valve in seawater could be fairly rapid.

The above shows that many different materials can be used in valve construction. The ultimate choice depends on several factors including service conditions, cost and required life expectancy. Choice can also be strongly influenced by the material of construction of the pipeline, where galvanic corrosion issues must also be taken into account.

Maharashtra Govt. approves 1600 cr desalination project

 Maharashtra Chief Minister Uddhav Thackeray has approved to set up a desalination project with a capacity of 200 MLD in Mumbai.The project will entail an investment of Rs 1,600 crore. The project will be set up over 30 acre at Manori, Malad, and is expected to be completed in four years. After tapping about six possible locations, the Brihanmumbai Municipal Corporation (BMC) has finalised Manori for setting up the project.The capacity of the plant can be raised to 400 MLD in the future, which can make up for the shortfall of 10 percent of the total water consumption of the city of 3,800 MLD when there is poor rainfall.

The detailed project report will be prepared in around nine months and take another three months to complete the tendering process.The actual completion of the project will need around three more years. The site has been finalised by the technical team as it has no mangroves around it, good quality of water and connected by roads, among other reasonsBMC had tapped five other locations, including Versova, Gorai, Malad, among others for the project. The project will help to make up the shortfall of potable water in summer due to the late arrival of monsoon or shortfall in expected water stock. The operational cost of the project will reduce significantly if operated on solar power.The operational cost of the desalination plant is higher than the dam-based water-supply projects and has been one of 

the reasons behind the delay to the project, which has been planned for years. BMC expects it to be 50% more than its existing system of dam-based water supply.

Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance,as in soil desalination, which is an issue for agriculture.

Saltwater (especially sea water) is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water sources.

There are approximately 16,000 operational desalination plants, located across 177 countries, which generate an estimated 95 million m3/day of freshwater.Currently, desalination accounts for about one percent of the world’s drinking water.Desalination is particularly prevalent in countries located in the Middle East and North Africa region, such as Saudi Arabia, the UAE, and Kuwait.Desalination is also an important source of water in the Small Island Developing States.

Due to its energy consumption, desalinating sea water is generally more costly than fresh water from surface water or groundwater, water recycling and water conservation. However, these alternatives are not always available and depletion of reserves is a critical problem worldwide. Desalination processes are usually driven by either thermal (in the case of distillation) or electrical (in the case of reverse osmosis) as the primary energy types.

Currently, approximately 1% of the world's population is dependent on desalinated water to meet daily needs, but the UN expects that 14% of the world's population will encounter water scarcity by 2025.Desalination is particularly relevant in dry countries such as Australia, which traditionally have relied on collecting rainfall behind dams for water.

Kuwait produces a higher proportion of its water through desalination than any other country, totaling 100% of its water use.