Reducing spiral vibration on turbo-generators

Paul Boughton

Richard Livermore-Hardy discusses the capabilities of a bearing for large turbo machinery

Alstom Power is a leading provider in the power generation industry. Close to 25% of the world’s power production capacity depends on Alstom technology or services.

When Alstom identified a phenomenon of spiral vibration close to the trip level on one of its 450 MVA hydrogen-cooled generators, the proprietary predictive tools and extensive industry experience of Waukesha Bearings on equipment of this size made Waukesha the clear choice to design and manufacture a drop-in bearing replacement.

Spiral vibration, also known as the Newkirk effect, can be observed on various types of turbomachinery and is caused by a vibration-induced hot spot on the shaft surface, generated by friction.

To resolve the vibration on the Alstom generator, the new bearing was required to have maximised stiffness coefficients, in particular in the direct coupled horizontal plane. Within the space constraints of the original fixed geometry bearing, the new tilt pad design required a solid load support and acceptable oil inlet flow velocities, whilst also accommodating double insulation to prevent grounding of shaft currents via the bearing.

Waukesha Bearings engineered a Maxalign tilt pad journal bearing with a 400 mm (15.75") inside diameter to meet the challenge. Both the pad geometry and the lubrication method were optimised for the application, and Waukesha’s patented ball and socket pivot design provided a solid load support with superior stiffness qualities.

Using proprietary predictive tools coupled with 30 years of field experience, testing, and development, Waukesha was able to confidently predict steady state and dynamic bearing performance.

The installation successfully eliminated the vibration and shifted critical speeds to well above the rated speed, just as the rotordynamic model of the generator using the optimised bearing characteristics had predicted.

Since the initial spiral vibration phenomenon was detected in 2003, more than 13 generator sets have been commissioned. In total, the Maxalign bearings have seen 600,000 cumulative operating hours in these Alstom generators.

The features of the Maxalign bearing yield benefits that increase availability and reliability in the field. In addition to being highly adaptable to shaft misalignment caused by mechanical loading or thermal effects in large rotating equipment, the ball and socket design eliminates pivot wear commonly associated with traditional pivot designs, thus providing a zero maintenance bearing.

By utilising ‘Directed Lubrication’, the Maxalign bearing also reduces the amount of oil needed, reduces power loss, and keeps operating temperatures within acceptable limits. Waukesha’s optimizsed hydrostatic jacking groove design reduces friction and wear during start-up, whilst also minimising disturbance of the oil film during normal operation, increasing safety margins and machine reliability.

In addition, the Maxalign bearing can include provision for instrumentation to match original equipment fittings and the design of double insulation allows the direct measurement of the electrical resistance of each insulating layer when the bearings are assembled on the shaft.

Richard Livermore-Hardy is Engineering Manager at Waukesha Bearings.