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,
First, thank you for encouraging the dialogue about nonmetallic materials. I have been working with DuPont Vespel CR-6100 for the past 13 years—for four years as one of the first customers when I was a rotating equipment engineer at a refinery in California and for the past seven years representing Vespel CR-6100. I currently represent Vespel CR-6100 in EMEA (Europe Middle East Africa).
The most important thing is for users to recognize the value of installing nonmetallic wear rings with reduced clearance. Even after 20 years of experience with nonmetallic wear rings, 98 percent of pumps used in industry continue to use metal wear rings with larger clearances. These pumps with metal rings run with lower reliability, less efficiency and are not as safe as pumps fitted with nonmetallic wear rings. I would encourage users to recognize the value of nonmetallic wear ring upgrades for their pumps, learn about the various options and select the material they believe will provide them with the best reliability.
As for providing users a guide of which materials to chose, I would point to API610 11th Edition, Table H.3. It includes the most commonly used composite materials for industrial pump applications: Vespel CR-6100 (under the cumbersome, non-commercial description of “PFA/CF reinforced composite 20% mass fraction random x-y oriented carbon fiber”), polyetheretherketone (PEEK) based composites and impregnated graphite materials. The table provides acceptable operating conditions in terms of temperature and differential pressure. These limits are based upon feedback from the pump users' community, and the API committee is working to expand and improve the table with each successive edition of the standard.
Beyond the temperature and pressure limits in table H.3, users should consider the following material characteristics: Coefficent of thermal expansion, chemical compatibility, limiting PV (pressure-velocity ratio), creep resistance, wear rate, impact resistance, thermal shock resistance, machining and installation characteristics, and for composite plastics, glass transition temperature. For a specific application, other properties may come into play, but the above list should be a good starting point for most centrifugal pump applications. Users need to consider the full range of properties as the long-term performance of nonmetallic materials is not driven by one or two factors but a combination of a wide range of properties.
One final note, in your original response to the user, you suggest that users look to the pump OEM as an unbiased source of information. In some cases, this will be true, but not always.
Graphalloy has been used by Valero facilities for many years for various applications—including pumps for boiler feed service, hydrocarbons, liquid gases and vacuum tower bottoms. Compared to plastics, Graphalloy has some key advantages:
- It has dimensional stability and heat-transfer capabilities.
- It is not just dimensionally stable in one or two directions but experiences no creep, even under high loads.
- It is self-lubricating and has run-dry capabilities. It can outperform plastics as it is a run-dry material that survives loss of pumpage operation for prolonged periods without damage or preventing pump restart
- Graphalloy is non-galling.
- Graphalloy has a constant coefficient of friction.
- Clearances using Graphalloy can be reduced to lower vibration levels and reduce cavitation with no risk of galling.
- It does not swell or cold flow (in any direction) and can be submerged in liquids.
- It has a wider temperature range than plastic. It performs in cryogenic temperatures and is used in pump applications up to 750 degrees F (and much higher in non-oxidizing environments).
- Graphalloy can also be machined to your specifications or semi-finished sizes to minimize subsequent machining.
- It has been used for more than 50 years in multistage pumps, moving fluids from super critical ethylene to heater drain condensate to bottoms pumps.
Graphite Metallizing Corporation
Pumps & Systems, October 2011