BY RICHARD SHULTZ
WAUKESHA MAGNETIC BEARINGS
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reached 165 C. Other canned bearings have reached 130 C in similar applications.
Managing Corrosion Risks
Magnetic bearings must conform to the corrosion safety standards for oil and gas applications, such as National Association of Corrosion Engineers (NACE) MR0175. h e bearings’ electrical components are protected from process conditions, including chemical attack by process gas and condensates. h e motor compressor can eliminate the use of dry gas seals, avoiding natural and sour gas damage to the environment.
Figure 1 has been adapted from NACE MR0175. It details regions of increasing sour gas corrosion to metal alloys used in oil and gas processing equipment. Corrosion is dei ned by reference to suli de stress cracking (SSC).
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Because they can be immersed in process gas without corroding the electrical connections, canned magnetic bearings offer oil and gas end users a signifi cant advantage.
(Article images and graphics courtesy of Waukesha.)
Figure 1. A plot of in situ pH against the partial pressure of hydrogen sulfi de shows increasing corrosion from Regions 0 (no effect) to 3 and beyond (highest propensity for SSC).
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SPECIAL SECTION
SEALS & BEARINGS
Figure 1 shows in what levels of sour gas service sealed and canned designs may be applied while still expecting a reason- able service life, based on documented corrosion rates. h e dividing line corre- sponds to a concentration of about 600 parts per million of suli des.
A rotor system inside a pressure vessel also helps cool the motors and bearings and provides electrical connections that penetrate the pressure vessel.
Erosion and corrosion can cause costly damage to a machine’s interior. Canned and corrosion-resistant auxil-
iary bearings of er a benei t to the oil and gas industry, in which environmen- tal regulations continue to increase.
h ese bearings can be applied in the manufacturing of integral motor com- pressors, externally driven compressors and turbo expanders.
A canned magnetic bearing segregates electrical connections and windings from the process gas.
Richard Shultz is the design engineering manager for magnetic bearing systems at Waukesha Magnetic Bearings. He has 20 years of industrial experience designing magnetic bearing systems and auxiliary bearing systems, specializing in rotor- dynamics and control system design. He recently instructed at the Mag- netic Bearing Short Course at the Texas A&M Turboma- chinery Symposium and the Middle East Turbomachinery Symposium in Qatar.
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W
ithin the oil and gas industry, the rotating shafts of equipment—such as pumps, motors, compres- sors, gearboxes and turbines—perform an essen- tial function in both upstream and downstream applications. h is rotating equipment ensures process l ow and the safety of employees and the surrounding community.Bearings are critical to the reliability of rotating equipment. If they fail, the equipment has to be repaired or replaced. Because they support the shaft, bearings are prone to wear and damage. Lubrication starvation and contamination are the principal factors that cause premature bearing failure, so systems have been developed to prevent these issues.
Methods for sealing the gap between a rotating shaft and the stationary bore have evolved with advances in lubrication and lubrication delivery systems. Selecting the right seal for a lubrication system will yield the best bearing protection. Lip seals and labyrinth seals, also known as bearing isolators, are the most common solutions.
Lip seals are unidirectional contact type seals. h eir geo- metry is important for maintaining an optimal radial load on the shaft to develop a meniscus of oil on which the lip can ride, which reduces friction and wear on the shaft and lip (see Fig- ure 1). Depending on the direction of the installed seal, it will either prevent egress of lubrication or ingress of contamination (see Figure 2).
Bearing isolators are non-contact seals that provide bidi- rectional sealing capability. h eir construction includes two