Abrasive Flow Machining of Boiler Feedwater Closed Impeller Pumps
In this article for the Abrasives Hub, Kennametal Extrude Hone explains how abrasives flow machining (AFM) of feedwater closed impeller pumps provides the benefit of increased efficiency. Multi-stage closed impeller, boiler feedwater pumps are critical components in a modern coal powered plant. Inner flow passages of the impellers and diffusers benefits from AFM’s selective material removal and surface finish improvement to contribute to improve pump efficiency.
In this particular application, a 2% increase in pump efficiency has been achieved translating to lower energy consumption at the power generating station. Kennametal says that it can be estimated that for a coal powered plant equipped with three boilers and six boiler feed pumps, parasitic loss i.e. the energy required to drive the pumps, will be reduced by at least 1% of the power generated. This translates to annual saving of US$500,000 to the utility, based on the coal price needed to generate the amount of power saved.
Indeed, AFM surface technology solutions are delivering savings throughout the energy sector by supporting pump efficiency improved. AFM is a well-established process for material removal at points of greatest restriction and surface improvements of passages not accessible by other methods. So closed impellers as well as diffusers are a natural application for AFM. Here a ‘two-way’ flow AFM process is used to realize the improvement on the internal passages.
This specific case deals with multi-stage, horizontal, ‘between-bearing’ pump components which are in the 300-400 mm range for impellers and 500-600 mm range for diffusers. Investment or sand cast stainless steel impellers and diffusers are typical for this application. Many closed impeller applications in brass, aluminium and various stainless steels in sizes 100 -800 mm, have benefitted from the AFM process. There are a wide range of pump and compressor applications using closed impellers.
Foundries are working hard to deliver better products to pump manufacturers. However, the casting process alone cannot meet the needs of today’s pump and compressor manufacturers or their customers. The importance of more efficient units drawing less power to deliver equivalent or improved results has gained increased awareness as energy costs continue to increase. In order to deliver a consistent, polished surface, AFM is used as repeatable process to address the inner passages of the closed fluid flow paths. Customers’ requirements are to improve roughness
drastically, sometimes by up to 10 times. With AFM, Kennametal can deliver a final roughness in the range Ra 0.8 µm-0.2µm, depending on the starting surface condition.
Growth in Generation
Electricity generation is forecast to grow considerably over the next 20 years; doubling. Nuclear is not the only generating technology and optimising all form of power generation will be considered and fossil fuels, coal, oil and natural gas, will remain important. In order to generate energy with a better efficiency through the thermodynamic process related to these fuels, power absorbed by high energy consumption components like boiler feed pumps are definitely a focus.
When processing with AFM the aim is to:-
*Improve flow rate capability of the closed impeller,
*Improve quality by providing more consistent material removal over the entire inner closed impeller surface and consistency from part to part;
*Improve productivity when finishing closed impellers, processing the part in around an hour and,
*Improve efficiency generating valuable savings.
For this application, the throat or inner section of the closed impeller is an important factor as well as the overall diameter. The AFM machines have to be sized to deliver the best efficiency in regards to the size and section of the impellers to be processed. Large impellers, having a fixture containment ring over 1m; require a machine with high force clamping cylinders as well as a large volume capacity of media to ensure the process is efficient and quick.
The Kennametal Extrude Hone machine is based on an abrasive flow machining process where the media loaded with abrasive is flowed though the closed impeller under pressure. Simple fixtures are required to hold the impeller clamped between the upper and lower media assembles of the machine. The media is a visco-elastic polymer that is blended to the correct viscosity, abrasive size, shape, type (usually silicon carbide) and concentration to meet the needs of the particular application. Process parameters like pressure, media volumetric rate, temperature and media volume are controlled during the AFM process.
Abrasive media crucial
Abrasive flow media is the crucial ‘cutting tool’ in the AFM process. The composition, quality and consistency of the AFM media provide the unique behaviour and dynamical performance that truly controls the results. Specially designed media equipment, extensive testing and analysis, using ISO certified manufacturing procedures provide repeatable, quality materials AFM process. Kennametal has a centre of excellence in Irwin, Pasadena, where it performs feasibility studies with customer specific parts to determine the optimal media for each process and the group also develops new media for special applications there.
Kennametal Extrude Hone invented and patented the AFM process in 1966. Alongside its equipment it also markets its Autoflow control software which controls media flow rates and back pressure for the best processing results.
Kennametal Inc delivers productivity to customers seeking peak performance in demanding environments by providing innovative custom and standard wear-resistant solutions. This proven productivity is enabled through our advanced materials science and application knowledge. Kennametal’s customers range from companies operating in a wide range of industries, from airframes to coal mining, engines to oil well and from turbochargers to construction. In 2010, customers bought around US$ 1.9 billion Kennametal products and services delivered by 11,000 employees in more than 60 countries. Half of its revenue is outside North America. For more information visit www.kennametal.com.