Safely immerse motor compressors in process gas without risking costly damage.
by Richard Shultz, Waukesha Magnetic Bearings

Active magnetic bearing (AMB) reliability and availability levels have surpassed oil bearings after 10 years of technological advancements. These advances have made an impact on the industry, drawing attention from major original equipment manufacturers globally.

In 2002, an oil and gas processing company required a bearing for a hermetically sealed integral motor compressor. The natural gas from the well contained hydrogen sulfide, which lead the company to pursue a corrosion-resistant canned bearing. Finding a reliable bearing for this hermetically sealed integral motor compressor was a challenge—but one worth the cost. The solution included sealed bearings and electrical connectors, which eliminated the need for a costly enclosure surrounding the motor compressor. Installing the motor compressor outdoors, without a building or enclosure, saved the company significantly in capital expenditures. The hermetic sealing of the bearings and motor compressor also ensured that emission limits were not exceeded for sites in which operating licenses limited hydrocarbon emissions.

Process Gas Immersion

Magnetic bearings are an effective solution for many oil and gas applications because they can be immersed in the process gas. A canned magnetic bearing with metallic lined stators segregates the electrical connections and windings from the corrosive gas. Canned bearings allow for the placement of the magnet core and windings behind an impervious pressure-rated barrier constructed of corrosion-resistant alloy or other nonmetallic material.

Canned magnetic bearingsBecause they can be immersed in process gas without corroding the electrical connections, canned magnetic bearings offer oil and gas end users a significant advantage. (Article images and graphics courtesy of Waukesha.)

Special alloys ensure that the bearings will not corrode, protecting the inside of the machine from serious degradation.

With canned AMB designs, the metallic can separates the pressurized volume inside the machine from the cavity and ambient pressures and protects against leakage to the machine’s exterior. The metallic can must withstand the maximum pressure inside the machine, which is equal to the can’s differential pressure.

A proper design of the metallic can and backing system is crucial to survive all the temperature and pressure conditions that the AMBs will encounter during operation.

In one 2006 oil and gas installation, the system has operated safely in its environment. After 25,000 hours, the units’ operation provides 99.9 percent availability to its end user. In addition, the canned bearings’ temperature ratings 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. The bearings’ electrical components are protected from process conditions, including chemical attack by process gas and condensates. The 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 defined by reference to sulfide stress cracking (SSC). 
Figure 1 shows in what levels of sour gas service sealed and canned designs may be applied while still expecting a reasonable service life, based on documented corrosion rates. The dividing line corresponds to a concentration of about 600 parts per million of sulfides.

Plot of in situ pH against partial pressure of hydrogen sulfideFigure 1. A plot of in situ pH against the partial pressure of hydrogen sulfide shows increasing corrosion from Regions 0 (no effect) to 3 and beyond (highest propensity for SSC).

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 auxiliary bearings offer a benefit to the oil and gas industry, in which environmental regulations continue to increase.

These bearings can be applied in the manufacturing of integral motor compressors, externally driven compressors and turbo expanders.