Importance of Surfaces for Wear Parts in Pumps
The Quality of the Surface
Tribology is the science and technology of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear.
Throughout history, the search has been on forbetter materials to create tools with which to improve daily life. Those that achieve success in this search gain an advantage, and thus the research conducted in this area by the mechanical engineering department of Sulzer Pumps is crucial to the division’s success in the competitive pump market. Sulzer Pumps successfully combines high efficiency with operational reliability, and this achievement is due in no small part to the commitment to continuous tribological research. As a result, Sulzer Pumps uses a broad spectrum of surface technologies to the benefit of the customers.
Leakage through annular seals is one of the main sources of reduced hydraulic efficiency in centrifugal pumps. The principle criteria on which the pump designer must concentrate are keeping the clearance as small and the flow-resistance coefficient as high as possible, e.g., by the use of labyrinth seals. Tribological properties of the mating annularseal surfaces are therefore of utmost importance (Fig. 1). Transient (i. e. start/stop) operation presents a particular problem, especially for long multistage pumps, as it is during transient operation that the tendency for the rotor to contact the stator is strongest (Fig. 2)
. This behavior is due to the natural sag of the rotor, exacerbated by the fact that during transient operation, no hydraulic restoring forces (Lomakin effect) are generated to help maintain the clearance between rotor and stator.
Selecting the Best Material
The surfaces of annular-seal components are thus required to withstand erosion due to fluid passage—often accelerated by abrasive action attributable to solid particles in the pumped medium—and also exhibit a weak tendency for galling in the event of a rotor/stator contact. Additional practical concerns must also be considered when selecting materials and surface treatments for annular-seal components:
- Corrosion resistance
- Resistance to fluid erosion and particle abrasion
- Galling characteristics
- Cost
- Easy manufacturing/sourcing
Obviously not all these criteria can be fully met by any single solution. A best compromise must always be adopted, with cost and availability often becoming important factors in the selection procedure. In order to reliably quantify the galling performance of a material combination, it is necessary to establish a repeatable test method that sufficiently replicates a centrifugal-pump annular seal with respect to geometry, circumferential speed, load, and fluid conditions such as type of fluid, flow rate, and temperature.
Dedicated Test Rig
To this end, Sulzer Pumps built a test rig in Oberwinterthur (Switzerland) in the early 1990s, which has the ability to test both product-lubricated bearings and annular-seal components (Fig. 3). The test loop is made of stainless steel to allow operation with a variety of different fluids such as demineralised, hot, or cold water, seawater, and representative hydrocarbons (Fig. 4). To simulatereal conditions in a pump, a defined radial force is applied to the rotary wear ring before the test begins (see box). A defined flow of filtered water is circulated through the clearance during the
test. One test cycle consists of a startup, followed by a period of steady state operation before shutdown. A constant radial load is applied throughout the cycle. Several such cycles are conducted, with the applied radial load increasing incrementally.
Suitable Materials Identified
In this manner, the engineers of Sulzer Pumps have used the test rig to quantify the relative galling behavior of approximately 150 rotor/stator combinations. These tests have identified several plastics and surface coatings that exhibit superior performance.
Furthermore, some noncoated metals have been shown to exhibit good properties. As noncoated metals represent the most costeffective solution, particular attention is paid to the best of twwqswdswedxhese combinations, with further tests conducted to better verify suitability.
Sulzer Pumps and Sulzer Metco, Sulzer’s surface specialists, combine their experience and technology to provide a wide range of surface-coating solutions and material combinations.
Extremely Corrosive Fluid
In the process of offshore production of crude oil, specialized pumps inject water in the drill hole to keep up the pressure in the oil well. During the 1990s, stricter ecological controls drove a demand for seawater injection pumps to be rerated to inject produced water, which is the water that is separated from crude oil. Produced water varies considerably depending on the geology of the oil well from which it is extracted. In general terms, it is moreaggressively corrosive than seawater and is often contaminated with H2S and sand. In the most extreme cases, high concentrations of fracture sand are also entrained in produced water. Fracture sand is sharp, hard quartz sand, deliberately injected into some oil formations to create fissures through which oil can flow. For this demanding operation, Sulzer Metco developed Sume®PumpSA, a tungsten-carbide coating sprayed using the high-velocity oxygenfuel (HVOF) process.
Combined Experience
Injecting produced water through a pump designed for seawater injection presents a number of challenges, not least of which is preventing rapid wear of the annularseal surfaces due to the entrainment of abrasive sand particles. Sulzer was able to levy a wealth of combined experience from 3 divisions—Sulzer Pumps, Sulzer Metco, and the R&D knowledge of Sulzer Innotec—to quickly identify, test, and rank numerous materials, coatings, and surface treatments that exhibited the potential to provide a solution for this application.
The result of this joint effort, SumePumpSA, uses optimized spraying parameters and a blend of 2 different powders to provide a unique combination of properties.
- Excellent antigalling characteristics, superior to every combination of materials tested on Sulzer’s tribology test rig (with the exception of some plastic materials).
- Excellent combined erosioncorrosion resistance in a wide range of aggressively corrosive media—including produced water contaminated with H2S
Excellent resistance to both fluid and abrasive-particle erosion- Application thickness of up to 1.2 mm (0.048")
- Thermal-shock resistance, withstands rapid quench of ΔT=200K
- Little substrate heating, substrate temperature is maintained below 120 °C during processing—thus incurring no distortion.
- Purely mechanical bond, thus no heat-affected zone or embrittlement
- Repeated stripping by grinding and reapplication.
SumePumpSA proved so successful in produced-water injection that Sulzer Pumps has extended its use to a multitude of duties, such as cracker bottom, condensate extraction and boiler feed pumps. The fact that SumePumpSA can be applied directly to impeller annular-seal surfaces without incurring distortion or metallurgical corruption of the substrate has been a bonus for Sulzer Pumps as the demand for higher stage pressureshas rendered replaceable wear rings impractical (Fig. 5).
Range of Dedicated Coatings
Following the success of SumePumpSA, Sulzer Pumps has worked closely with Sulzer Metco and Sulzer Innotec to develop a range of more than 20 SumePump coatings—all of which have been adapted to provide the best combination of characteristics for particular applications:
- SumeSol: a blend of HVOF-sprayed tungsten carbide with solid-phase lubricant for product-lubricated bearing sleeves.
- SumePumpGB: an alternative to weld overlays such as Stellite and Colmonoy. The material combines greater hardness and erosion resistance with superior galling characteristics when used with stainless-steel counterparts.
- SumePumpRA: a repair material for corrosion-resistant steels such as Super Duplex. Can be applied up to 3 mm (0.120") thick and provides antigalling for assembly and disassembly of heavy interference fits.
The cooperation of the 3 divisions—Sulzer Pumps, Sulzer Metco and Sulzer Innotec—exploiting synergies to develop and continuously improve the SumePump range, is an outstanding example of the added value Sulzer can provide to its clients.
Gauggel, Werner, et al. "The Quality of the Surface."Sulzer Technical Review. 4/2007: 12-15.