11. September 2023

Stable gas bearings

The operation of a gas lubricated bearing depends on the gas properties in the bearing clearance. These are primarily defined by the ambient conditions and thermal design. Therefore, for stable operation of the gas bearing, both ambient pressure and temperature range as well as cooling temperatures have to be taken into account in the design stage. Celeroton considers the required operating range of its gas bearing turbo compressors, or other applications with gas bearings, in an early phase of the system design and thereby avoids long and costly trial-and-error design loops. This allows for fast time to market in customized systems. Furthermore, Celeroton applies validation of the gas bearing stability not only in type tests, but also for serial production. Therefore, at Celeroton, each compressor comes with a gas bearing quality control.

In a dynamic (also called self-acting) gas bearing, the pressure in the gas film is built up by the movement of the rotor relative to the bearing bushing. If the rotor is displaced from its center, the pressure increases in the converging gap, which results in a force on the rotor. The force usually has both a radial and a tangential component. While the radial component restores the rotor back to the bearing center, the tangential component can result in a self-excited whirl instability. Depending on the amplitude and the amount of damping provided by the bearing film itself, the bearing is working stably or it may become unstable. In the stable case, i.e. with a small tangential component and high damping the rotor orbits inward to the bearing’s center. In the unstable case, i.e. with a high tangential component and insufficient damping the rotor orbits outward until it touches down on the bearing bushing. It is clear, that the touch down has to be avoided by a proper design under all operating conditions.

The operation of a gas lubricated bearing depends on the pressure and the dynamic viscosity in the gas bearing clearance, the viscosity being strongly depending on the temperature. These gas in the bearing clearance is connected to the ambient and the outlet of the compressor with a gas bearing purge and vent system. Furthermore, the thermal design including cooling system influences also the bearing temperature. In a gas bearing compressor, the gas bearing stability and therefore safe operating range of the gas bearing turbo compressor is thereby dependent on the turbo compressor ambient and inlet conditions, the operating point and other boundary conditions such as the cooling water temperature.

Stable operation: The rotor is restored to the bearing’s center on an inward spiral orbit.
Unstable operation: Outward spiral orbit eventually resulting in a rotor touch down into the bushing: e.g. if gas pressure or temperature is out of design range.

A gas bearing turbo compressor has to operate over a range of inlet temperatures and pressures, cope with different cooling water temperatures, and cover a compressor map with operating points at different pressure ratios. Celeroton’s spiral groove (herringbone) gas bearings are designed to run stably over the required operating range, avoiding trial and error as with some other gas bearing types. Celeroton’s proprietary gas bearing design and optimization toolchain checks the required operating range including different gas parameters, calculates the stable operating range, and guarantees the required stability margins.

Safe operating area of gas bearing
Example of safe operating area of gas bearing with left side border defined by the gas bearing stability limit.

For type tests, the bearings are pushed to the limits, beyond the required operating range, until onset of instability. The onset of an unstable orbit can be captured, which allows to verify the gas bearing design. Furthermore, pushing a gas bearing to the stability limit allows for a quality check of the full gas bearing system. This is of primary importance as purely geometric quality control of the bearing parts for manufacturing tolerances is not always possible as certain tolerances can be below the measurement accuracy of serial production equipment. Therefore, Celeroton has developed and introduced a gas bearing stability check that is lean and therefore can be executed during serial production.

Measured waterfall plot of a stable bearing.
Measured waterfall plot of a bearing becoming unstable: onset of sub-synchronous self-excited whirl instability.

With its proprietary design toolchain, type test methods and serial production tests Celeroton can achieve the following customer benefits:

  • Avoid trial and error for customer specific operating ranges, allowing for a fast time to market
  • Gas bearing validation during type tests
  • Deliver products in series with verified gas bearing quality