Edited by David S. Hotter
Green, Tweed & Co.
Pumps with mating components made of cast iron, stainless steel, and bronze have long been a source of problems for manufacturers and end users. The abrasive quality of metals leads to significant wear in pump parts, which frequently results in failures from galling and seizing. As anyone working in a manufacturing environment knows, your worst nightmare can come true when a pump runs dry or a bearing fails, shutting down an entire assembly line or plant operation. And the costs are hardly trivial for repairing all of the pump components destroyed or carrying out periodic maintenance on the equipment to head off such a nightmare.
Engineers are starting to replace metals with advanced plastics for pump components such as bushings, line shaft bearings, and case and impeller wear rings. The resins they use are based on polyetheretherketone (PEEK) chemistry and were initially developed for aerospace and defense applications. Using these engineered resins, pump manufacturers improve performance, boost output, and cut costs by taking advantage of better wear and friction qualities, mechanical strength, and the ability to resist chemicals.
The chemical makeup of PEEK resins helps ease engineers’ concerns about galling and seizing and are safer to work with. Taking advantage of these qualities, along with an understanding of some basic design guidelines, opens new doors in pump performance and reliability.
DESIGNING WITH PEEK
Engineers often choose PEEK resin because of its overall balance of qualities, including hydrolysis resistance, dimensional stability, wear resistance, temperature stability, strength, and chemcial resistance. However, switching from metal to plastic in pump parts requires engineers to change their mindsets.
PEEK can be injection molded, compression molded, or extruded into stock shapes, which are typically machined to final- part dimensions. Most pump components can be injection molded as long as they don’t require tight tolerances. The strict tolerances required for bushings and wear rings require parts to be machined from molded tube stock.
Though manufacturers may be used to machining metals, speeds and feeds are much different for plastics, and the process generally calls fo high-end carbide and diamond tooling. Machining operations vary depending on which fillers and reinforcements are used in each resin grade.
Though plastics can be machined more quickly than metals, it is not as easy to hold tolerance because plastics tend to spring when parted off. Tolerances in the range of ±0.002 in. can be held on parts with diameters up to 10 in.
The most common blends of PEEK resin used in pump applications are carbon filled and carbon-fiber reinforced. Carbon-filled PEEK compounds can be molded in solids or tubes up to 40 in. in diameter.
Carbon-fiber reinforced resin, in contrast, are robotically wound and formed around a mandril. This limits fiber-reinforced resins to hollow shapes no samller than 0.375 in. in diameter. Splitting hollow shapes is not recommended because the strength of the composite relies on the carbon fibers being molded and kept in tension.
Another difference between carbon-filled and carbon-fiber-reinforced PEEK resins is how they react to temperature changes. Carbon- filled compounds have a radial thermal expansion rate of 16 × 10-6 in./in./°F, while that of fiber-reinforced resins is less than 3 ×10-6 in./in./°F. These two values fall on either side of the thermal expansion rate of carbon steel, which is 6 × 10-6 in./in./°F, making the choice between the two resins critical when combining PEEK resins with metal pump housings and components.
Carbon-filled resins are recommended for press-in applications such as bushings and case wear rings. As temperatures rise, PEEK components will get tighter because they are expanding at a faster rate than the steels surrounding the bushings or rings. In contrast, carbon-fiber-reinforced resins work better for press-on applications such as impeller wear rings. The ID of the wear ring fits on the impeller skirt, which has a slightly larger OD than the ring ID. As operating temperatures rise, the steel impeller skirt expands more quickly than the PEEK wear rings, creating a tighter fit.
In summary, when compared to carbonfiber-reinforced resin, carbon-filled PEEK is less expensive, easier to machine, and can be split without compromising physical properties. However, fiber-reinforced resins have the advantage when it comes to higher operating temperatures. Reinforced resins are also stronger than carbon-filled blends. Fiber-reinforced PEEK is a true metal replacement because it combines the physical properties of steel with the processing ease and flexibility associated with thermoplastics.
AT THE REFINERY
One place where metal-to-plastic conversions are reaping benefits is in the petroleum industry where multistage horizontal pumps often experience high vibration levels that frequently cause seals to fail. Vibrations wear down bronze bushings and case wear rings which, in turn, reduces overall pump efficiency and increases maintenance costs.
Clearances between components increase as bushings and rings wear, making pumps susceptible to backflow and leakage. This, in turn, reduces head pressure and the amount of fluid delivered.
Replacing bonze and steel wear rings with carbon-filled or carbon-reinforced PEEK components, lets manufacturers increase pump efficiency by achieving tighter clearances. PEEK resins also reduce vibrations caused by fluid flow across wear rings and excess rotor runout.
The ductility of plastic pump components helps them outperform metal versions when it comes to reducing vibration, as well as resisting impact. For example, a petroleum refinery, that once considered vibrations of 0.525 in./sec acceptable for two multistage pipeline diesel-fuel feed pumps, was able to reduce vibration by more than 80% using carbon-filled PEEK case wear rings and bushings. Vibrations were slashed to 0.084 in./sec in the first-stage pump and 0.094 in./sec in the second-stage pump, which runs at higher pressures.
An added benefit when designing pump components from plastic is safety, particularly for industries working with flammable fluids. Plastics eliminate the risk of sparks when pumps run dry because of momentary suction losses during system upsets, which is a concern with metal-on-metal contact in conventional designs.
BEATING THE BOILERS
Galling and even seizure are common concerns when using pumps with steel components and wear parts. This is especially true in water pumps where, because of the material’s corrosion resistance, stainless steels are used for pump parts as well as wear components. While they protect pumps from corrosion, stainless steels gall more easily than other metals.
Engineers often design boiler-feed water pumps with tight diametrical running clearances to maximize discharge pressures. However, the tight clearances on stainlesssteel wear rings and bushings severely damage pumps during brief periods of cavitation. Cavitation takes place when suction pressures drop below the net positive suction head required by a pump. Momentary cavitations are common in pumps running close to the vapor pressure of a fluid, which is often the case in boiler-feed water pumps.
Wear, or hard rubs, resulting from cavitation often show up initially in the first-stage wear rings and the center-stage bushings. As a result, rotors deflect more which causes pumps to seize. One refinery experienced pump seizures on average every six weeks because of such failures. It cost the company $15,000 each time it had to replace impellers and shafts.
Impeller wear rings and sleeves made of fiber-reinforced PEEK eliminate galling because the material can run for short periods without lubrication. As a result, PEEK wear parts and mating steel components remain undamaged during momentary system upsets.
Under extreme conditions, such as when pumps run dry for long periods of time, PEEK wear-ring clearances will open up as they experience surface melt out. When this happens, pump repairs are limited to replacing sacrificial thermoplastic wear components. In most cases, mating metallic wear components are left unharmed. For the refinery mentioned above, pump failures due to long dry-running periods left impellers and shafts undamaged, and part replacement costs were cut from $15,000 to an average of $3,000 per pump repair.
Without concerns about galling, seizure, or damage to mating parts, manufacturers using PEEK components have the confidence to run pumps with tighter wear-ring clearances. The improved efficiency and reduced vibrations resulting from tighter clearances help reduce pump noise and increase the life of bearings and mechanical seals.