This column presents a discussion between myself and pump users regarding some of their questions, concerns and preferences about materials of construction that they consider for their applications. Some of this feedback comes from previous articles published in Pumps & Systems.
Dear Dr. Nelik,
I enjoyed your recent article about using a specialty material to help improve the efficiency and reliability of a multistage pump. I was interested in your selection of the Graphalloy material. I have used Vespel CR6100 to reduce running clearances for efficiency gains and rotor stability improvements. Do you have experience using Vespel? If so, what comparisons can you make between this and Graphalloy? When would you recommend one over the other? Any experiences you can share would be greatly appreciated.
Lee M. Posey
Lev Nelik responds:
A great multitude of materials can be used for pump bushings, including metals (bronze, hardened steels), hard coatings, non-metals (thermoplastics, thermosets, composites) and ceramics. All have benefits and shortcomings, such as resistance to temperature, thermal or mechanical shock, machinability, galling resistance, dimensional stability, swelling, chemical resistance, abrasive resistance and price.
It would be good to have a three-dimensional tabulation listing these materials on one column (say, horizontal), versus properties on the other (vertical), and application (pumped fluid) inside the table, for some sort of visual pick-and-choose guide. I am not aware of such table.
In practice, it is difficult to select an all-around best choice. The main reason is unfamiliarity of the end users with the multiple products that exist and users being cautious against over-zealous salesman trying to sell them a bushing material for their application which may not be appropriate for what they pump.
Pump manufacturers have a less biased approach to this because they are willing to apply any material that the end user asks for (reflective in price). They also may go on their own experience with (usually a limited number of) material(s). If they do not hear anything bad about the material from the field, good enough. No news is good news!
One needs to have a good survey of the cross-section of interested parties: material manufacturers, pump suppliers and end users to produce and compile such a table. During my days with Goulds Pumps Engineering, we produced such tabulation, trying to reflect our experience and knowledge. Although it was limited, it provided us with some degree of judgment when applying one material or another to different applications. Other pump companies probably have similar, internally-generated selection guides, but little is actually published. To be truly unbiased, such compilation would have to include a wide spectrum of parties, with often unavoidably biased commercial interests.
My personal choice of Graphalloy, as presented in the article you read, was based on what I knew from my own experience. With limited knowledge of alternatives, some of which perhaps could work just as well but unfortunately not known to me, I made this choice.
In my younger days with Ingersoll-Rand, Graphalloy was also used for liquid CO2, at multi-stage pumps, with high pressures and low temperatures and with poor lubricity and viscosity. Knowing it handles CO2 is a good reason for me to review its potential for other applications. That is how it works for most people: we use what we know works. I have a section for feedback at:
www.pumpingmachinery.com/pump_magazine/maintenance/maintenance.htm (section: Materials) which will be used to collect feedback and is available for the end users as. It is in its infancy, but as the information develops, it might evolve into a useful materials comparison resource. Thank you for your question. Perhaps you have started a good information exchange on materials.
At the request of Lev Nelik, Gilberto Lunardi—applications engineer (Kalrez® & Vespel®), DuPont Performance Polymers—responds:
Your inquiry on Vespel® has been forwarded to us by Dr. Nelik. Among the different families of Vespel® products, CR-6100 is the one specifically recommended for use in centrifugal pumps. Process conditions are important factors to consider when selecting a bushing material in each application. Vespel® CR-6100 exhibits a unique combination of properties and attributes like non-galling operation, self-lubrication, high wear resistance, ability to reduce running clearances, large temperature range, broad chemical resistance, non-brittle, shock resistance and ease of machining. This unique differentiation compared to other nonmetallic materials allows Vespel® CR-6100 to perform in a broad variety of process conditions. Some of these properties are also found in other nonmetallic bushing materials. It is important to understand the process conditions in each application to select the best material for each individual case. For example, you may not use Vespel® CR-6100 in temperatures above 550 degrees F, and may not be able to use some metal-graphite composites in acids that would attack the metal or use PEEK-based composites in contact with alkaline media.
Applications Engineer – Kalrez® & Vespel®
Dear Dr. Nelik,