Bear the Wear

How technical ceramic-based submersible pump bearing systems help oil producers

Patrick Stone, CoorsTek Inc., USA, highlights how technical ceramic-based electrical submersible pump bearing systems help oil producers optimize efficiency and run life.

Note: This article originally appeared in the July/August 2020 issue of Oilfield Technology magazine. Reprinted by Permission.

Electrical submersible pumps (ESPs) are an efficient, cost-effective method of providing artificial lift in the oil industry. ESP systems have continuously improved since 1927, when the first pump made in Bartlesville, Oklahoma, US, was installed in an oil well near Russell, Kansas, US. Today’s ESP systems create a small environmental footprint because of their optimised power utilisation, minimal production stoppages, and high reliability. These qualities reduce unplanned activity at the wellsite and minimise the need for repairs, as well as maintenance waste streams.

An ESP system comprises several major components (Figure 1). The pump is a multistage centrifugal type wherein most surfaces are directly exposed to production fluids. The focus of this article is primarily on the pump’s radial bearings and, to a lesser degree, the mechanical seal in the seal chamber section and other bearings in the components.

Oil well challenges

Most oil wells require artificial lift at some point to enhance production. As an oil well matures, it is not unusual for it to produce increasing amounts of water, abrasive solids, and gas. Shale oil wells produce these materials as well, sometimes immediately. ESP systems are ideal for lifting large volumes, but abrasive solids and gas pose major challenges for these types of pumps.

Abrasive solids in the pump or intake may introduce a three-body wear process. This type of wear is frequently referred to as abrasion and is defined as the removal of material by particles sliding, rolling, or crushing between two surfaces, as illustrated in Figure 2, or by particles embedded in one surface sliding against a second surface.¹ This type of wear most frequently affects the pump’s radial bearings, impeller-diffuser seal areas, and thrust Bearings.²

When a pump’s radial bearings wear, the resulting increased clearance between the sliding bearing surfaces causes increasing shaft instability (whipping), which can compromise the mechanical seal in the seal chamber section and allow wellbore fluids into the motor to cause an electrical failure. This shaft instability also stresses the motor bearings.

Recognising the importance of bearing systems for long run-life and reliability, the ESP industry has responded with design and material improvements. In particular, manufacturers have implemented different bearing approaches; the biggest step-change has been the incorporation of technical ceramics and cermet (ceramic/ metal composite) materials such as tungsten carbide.

Figure 1. Major components of an ESP system.

Radial bearing system design

An ESP’s radial bearing system consists of a sleeve that is keyed to a rotating shaft that is positioned inside a bushing held in a housing or diffuser bore. Two main styles of bushings are generally used. The compliant style typically incorporates O-rings between the outside diameter of the bushing and the housing or diffuser bore, and they have some means to retain them in the bore. The O-rings serve to minimize rotation, vibrations, and stresses, as well as compensate for thermal expansion and allow proper alignment.³ Figure 3 illustrates this concept.

Another style of bushing design uses an interference fit between the bushing and the housing or diffuser bore. The interference can be created by pressing the parts together or heating the housing or diffuser so the bore expands, enabling the cooler bushing to be placed in the diffuser. However, this design has a limitation in that the bushing can migrate out of the diffuser.⁴

Figure 2. The three-body wear process.

The pump’s shaft sleeves are keyed to the rotating shaft and spin inside the bushing. There are two styles of keyed sleeves. The first style is a simple cylinder with an inner diameter keyway, while the other has a flanged end that imparts the axial forces from the impeller to the thrust bearing in the seal chamber section. Figure 4 illustrates both designs.

Another design approach removes the keyway in the sleeve.³ This design removes the stress concentration created by the keyway. Some manufacturers use different materials for the sleeve and bushing to leverage the strengths of each. For example, common sleeve and bushings combinations include a zirconia sleeve and bushing, a silicon carbide sleeve and a zirconia bushing, a silicon carbide sleeve and bushing, a tungsten carbide sleeve and a zirconia bushing, and a tungsten carbide sleeve and bushing.

Standard ESP pumps use two radial bearings: one in the head and one in the base. For wells that produce abrasive solids that lead to bearing wear and shaft instability, manufacturers started increasing the number of radial bearings to reduce the unsupported shaft span. This design advancement has shown that increasing shaft stability extends the mechanical reliability. As a result, some manufacturers place a radial bearing in every stage for increased run-life.³

Pump bearing materials

Technical ceramics and cermets are inorganic solids that are very hard and strong in compression, which makes them ideal materials for ESP radial bearing systems. The technical ceramics commonly used in these radial bearing systems are zirconia and silicon carbide, while tungsten carbides are the typical cermets.⁵

Figure 3. Compliant-style bushing.

An important factor in selecting a bearing design and associated materials is price. In general, zirconia is priced lower than silicon carbide or tungsten carbide. The price of tungsten carbide fluctuates based on global supply and demand factors (e.g. for cobalt and nickel binders that are also used in electric vehicles batteries), so the relative price difference between silicon carbide and tungsten carbide varies with other factors.

Friedrich Mohs developed a scale that shows hardness of minerals and it is shown in Figure 5 compared against a vertical scale for Knoop hardness. Figure 5 shows that bearing materials are harder than produced oil well solids, as well as the metal components used in ESP systems. In most cases, softer materials do not wear harder materials. However, ESP radial bearings are an exception because solids are present and cause a complex grinding abrasion. Several factors determine the rate of material removal: hardness of the bearing and particle material, ductility of the surface, and the size, shape, and toughness of the particle.¹

Therefore, in the case of ESP pumps bearings with grinding abrasion occurring, both the harder and softer materials experience wear. Nevertheless, harder materials have demonstrated improved wear resistance.

Silica sand (quartz) is commonly used as a proppant to increase conductivity. Though its hardness varies, it is likely the hardest material found in production fluids. Its varying hardness is shown in Figure 5 on the high side of its hardness range.

Figure 4. Keyed sleeve design options.

 

Material properties

Selected material properties are shown in Table 1.

Hardness

The hardness measurement tests noted in Table 1 are different, but the values are comparable. As can be seen, the hardness of common bearing materials is much greater than steel: silicon carbide is the hardest, followed by tungsten carbide and then zirconia.

Toughness

The low fracture toughness of technical ceramics and cermets introduces special design considerations. The two bushing designs mentioned previously somewhat compensate for low fracture toughness. In the compliant design, the component is held with shock-dampening O-ring(s), while the interference fit design puts the part in compression and thereby increases its resistance to cracking.

For some components, such as shaft sleeves and motor rotor bearings, compensatory design changes are not possible. In these cases, it is necessary to avoid sudden and large impacts of the components and assemblies. One manufacturer developed special shipping boxes with added rigidity and foam cradles, as well as shock tags and digital shock loggers, to minimise and monitor shock loads.⁶

Zirconia is almost two times tougher than tungsten carbide and nearly three times tougher than silicon carbide. As a result, it is much more resistant to shock or impact, which is desirable in an oil tool application. Therefore, when the hardness of zirconia is adequate for abrasion wear, it has a reduced risk of damage resulting from handling.

Figure 5. Material hardness scale.

Maximum use temperature

During ESP tool tear-down, indications of high temperatures are frequently observed in the bluing of the steel diffusers. Some diffusers’ bluing suggests temperatures approaching 315°C.⁴ Since the technical ceramics used in ESP bearing systems have much higher maximum use temperatures, this is not an application limitation.

Thermal shock

Thermal shock resistance is also high for the technical ceramics, which maintain their strength with rapid temperature changes of 300°C and higher. For the values shown in Table 1, the test was run by quenching samples into water from various elevated temperatures. The change in temperature, where a sharp decrease in flexural strength is observed, is listed as the Delta Tc, or temperature change.

Coefficient of thermal expansion

Zirconia has a coefficient of thermal expansion close to steel, so if the shaft outer diameter or bushing bore size changes with temperature, the zirconia sleeve and bushing will substantially change with it. However, tungsten carbide and silicon carbide have lower coefficients of thermal expansion. With these materials, the sleeve may crack if the shaft size increases with temperature, or the bushing may become loose from the bushing bore and migrate out, as mentioned previously. Therefore, when other properties are adequate, zirconia reduces application concerns caused by coefficient of thermal expansion mismatches.

Table 1: Selected material properties

Thermal conductivity

Zirconia is highly thermally insulating, whereas silicon carbide is highly thermally conductive. In other words, zirconia limits the heat transfer to metal components, and silicon carbide and tungsten carbide quickly transfer heat (even faster than steel).

It is not certain which is the more desirable property: thermally conductive or insulating. In normal pump operations it is unlikely to make a significant impact on performance, though zirconia may be preferred to insulate the heat generated at the sliding bearing surfaces from the metal components. However, in cases of gas slugging, it may be desirable to quickly remove heat generated at the unlubricated sliding bearing surfaces. In these cases, a carbide may be preferred.

Chemical resistance

Technical ceramics are inorganic non-metallic solids with high levels of chemical stability. As a result, they are extremely resistant to chemical corrosion. For the chemicals typically injected or produced in an oil well, zirconia and silicon carbide are chemically inert. However, the cermet tungsten carbides are susceptible to corrosion because the binder material can be leached out; a cobalt binder is more susceptible to corrosion than a nickel binder, especially with acids.

Pressure velocity values

A common wear test for hardface materials such as technical ceramics is a pressure velocity (P-V) test. P-V values are determined in a sliding contact test using a rotating ring on a washer and varying rotational speed and applied loads. For hardface material applications without abrasive solids, P-V values are used to predict wear and service life. The presence of abrasive solids introduces other wear factors, but P-V values are indicators of relative performance.

Figure 6 compares the P-V values for different hardface combinations using tap water. SiC, shown in the image, is an acronym for silicon carbide and TC stands for tungsten carbide. SiC1, SiC2, SiC3, and SiC4 are different grades of silicon carbide. Note that the Series 2 value for tungsten carbide is less than half of the Series 8 value for the third grade of silicon carbide (SiC3). It should be noted that surface finish differences in Series 5 and 6 had a minimal influence. The figure illustrates that not all silicon carbide grades are equal with respect to wear and, depending on the grade, wear for silicon carbide can be similar or significantly different than tungsten carbide.

Figure 6. Pressure velocity values for hardface material combinations.

Other ESP system components

Technical ceramics and cermets also provide beneficial properties for additional components in ESPs, including the radial bearings in the intake, the mechanical face seals and thrust bearings in the seal chamber section, and the motor thrust bearings and rotor bearings. Several materials are used in these applications, but the most common are silicon carbide and tungsten carbide due to their wear resistance.

Reduce cost of operations

Other factors aside, an ESP system will eventually fail from vibration caused by radial instability brought about by abrasive wear.2 Some oil producers want ESP systems with a 10-year run life⁷ that are ultra-reliable.⁸ However, not all oil producers need an ESP system with such a long life – or are willing to pay for it. For example, an ESP system for a Permian unconventional well, where it may be replaced in 15 months with a different lift system, should have an optimal bearing design, material selection, and configuration. In contrast, the optimal bearing system for a North Sea well needing a six-year run life will likely be different.

Selecting the right bearing system can reduce the total cost of operations and simultaneously reduce the environmental impact by minimising power needs, preventing unplanned maintenance/repairs, and decreasing their associated waste streams. In addition, new materials are under development with potentially better abrasion resistance than current materials, so one should not accept the status quo – especially since ESP bearing systems play an important role in enabling safe, clean, and affordable energy.

Note: The figures are generalised information and for illustrative purposes only. The charts are intended to illustrate typical properties. Property values vary with method of manufacture, size, shape of part, and test method. Data contained herein is not to be construed as absolute and does not constitute a representation or warranty for which CoorsTek assumes legal responsibility.