Cost

For many water and other noncorrosive services, bronze satisfies these criteria and, as a result, is the most widely used impeller material for these services. Bronze impellers should not be used for pumping temperatures in excess of 250°F (120°C). This is a limitation imposed primarily because of the differential rate of expansion between the bronze impeller and the steel shaft. Above 250°F (120°C), the differential rate of expansion between bronze and steel will produce an unacceptable clearance between the impeller and the shaft. The result will be a loose impeller on the shaft.

Leaded bronzes have been used extensively in the past as impellers, especially in less demanding applications. The lead addition to bronze enhances its castability and machin-ability. In recent years, environmental concerns associated with lead have caused many nonferrous foundries to stop producing these alloys and pump manufacturers are increasing their use of nonleaded bronzes for impeller applications.

It should be noted that bronzes have velocity limitations above which they will suffer accelerated erosion corrosion. The maximum velocity, which will correspond with the periphery of the impeller, is higher in fresh water than in salt water. The most resistant bronzes, able to tolerate the highest velocities, are the nickel aluminum bronzes. These alloys are often used as impellers in salt water applications because they combine high mechanical properties, good corrosion resistance, and the capability to be weld-repaired. A nickel aluminum bronze impeller can be designed for a higher speed than any other bronze impeller alloy.

Cast-iron impellers are used to a limited extent in small, low-cost pumps. Cast iron is inferior to bronze in corrosion, erosion, and cavitation resistance. It also cannot be welded to repair damage due to wear or erosion. For these reasons, a low initial cost is usually the only justification for selecting a cast-iron impeller.

Martensitic stainless steel impellers are widely used where bronze will not satisfy the requirements for corrosion, erosion, or cavitation resistance. The alloys most commonly used are CA-15 and CA-6NM. These alloys can be used for pumping temperatures above 250°F (120°C), as the differential expansion problem no longer exists with a steel impeller on a steel shaft. Martensitic stainless steel impellers are used in a wide range of applications, including boiler feed water, many cooling waters, and a variety of hydrocarbon applications. It does not have sufficient resistance to pitting corrosion for use in sea water.

Martensitic stainless steels are heat-treatable alloys. The specified mechanical properties are developed through a quench and temper heat treatment. Quenching can be in oil or, as is more common, in air. The cooling rate in air is sufficiently rapid that the high temperature austenitic structure will transform to the metastable martensitic structure, which can subsequently be tempered to the desired hardness. The designer should specify that tempering be done at a minimum temperature of 1100°F (600°C) in order to assure that the casting has adequate toughness. It is also important that these alloys be heat-treated after weld repairs. This can present a problem in the case of a finish machined casting, which would suffer distortion if heat-treated. Welding techniques have been developed, however, that do not require a post-weld heat treatment, but these are, in most cases, unsuitable for use on martensitic stainless impellers.

Oil and refining industry applications often involve exposure to hydrogen sulfide, which may be present as a trace contaminant in hydrocarbon fluids. Martensitic stainless steels are susceptible to a form of SCC in this environment and should be specified with a special double-temper heat treatment designed to limit hardness and thereby prevent cracking.

Austenitic stainless steels are used for impellers in applications requiring a higher level of corrosion resistance than can be obtained from the martensitic grades. A number of different alloys make up this group. The most widely used are CF-8M and CF-3M, which are the cast versions of the well-known 316 and 316L wrought materials. The cast alloys have a slightly different chemistry than the wrought grades. This difference accounts for the presence of 5 to 15% ferrite in the castings, which makes them slightly magnetic. The ferrite also enhances the resistance to SCC and hot shortness, a casting problem associated with fully austenitic cast grades. These alloys provide corrosion resistance over a wide range of pH and have reasonably good resistance to pitting and crevice corrosion in aqueous chlorides.

Higher alloyed austenitic cast grades are also available for applications requiring a greater degree of corrosion resistance. Alloy 20 contains about 30% nickel and was developed for sulfuric acid applications. The high nickel makes the alloy fully austenitic (without ferrite). Consequently, it is difficult to cast and suffers from hot shortness, which may manifest itself as fine cracking at the intersection between the vane and the shroud in an impeller. The high nickel content also makes Alloy 20 very resistant to SCC.

Austenitic grades containing 6% molybdenum have been developed for use in salt water and other high-chloride applications such as acidic brines used in oil field waterflood injection. The high level of molybdenum makes these alloys fully resistant to pitting in stagnant seawater, which will be present when a pump is not in operation. The 6% molybdenum grades are more expensive and therefore not frequently used for most applications. These alloys are usually considered only for critical, demanding applications where a high level of corrosion resistance is needed.

Austenitic stainless steels with unique properties have been developed for specific applications. A chrome-manganese alloy, discussed in the section on cavitation erosion, can be employed to mitigate or entirely eliminate cavitation damage in problem applications.

A high-strength austenitic stainless grade, CF10SMnN, can be used where the mechanical properties of CF-8M are inadequate. Some pump manufacturers also offer nitrogen-enriched austenitic grades that have corrosion resistance and mechanical properties better than CF-8M.

Duplex stainless steels offer a combination of higher mechanical properties and better corrosion resistance than the standard austenitic grades. The original duplex casting grade, CD4MCu, was developed in the 1950s. Use of this material was limited by problems with castability and weldability. Improved steelmaking technologies now enable the addition of precise amounts of nitrogen to duplex stainless steel. The nitrogen addition improves castability, weldability, and also corrosion resistance. Numerous duplex stainless grades have been developed in recent years, all having a specified nitrogen addition. These duplex grades all outperform the old CD4MCu grade, which did not have a nitrogen addition. Many foundries now make CD4MCu with nitrogen.

Duplex stainless impellers are extensively used in mining, flue gas desulfurization, and similar applications that require a combination of resistance to corrosion and abrasion. Duplex stainless steels also have better corrosion resistance than the standard austenitic grades and are used in a variety of applications in the chemical industry, the pulp and paper industry, and the marine industry. Duplex stainless pumps are standard for offshore high-pressure water injection pumps in the oil industry. Published corrosion data indicates that, for acceptable resistance to seawater, a duplex stainless should contain a minimum of 25% chrome, 3% molybdenum, and 0.15% nitrogen.

Casings The following criteria should be considered when selecting material for centrifugal pump casings:

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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