Content area
Full text
ROTOR CENTERLINE PLOTS CAN HELP OPERATORS DIAGNOSE FAULTS, SUCH AS MISALIGNMENT, RUBBING, AND FLUID-INDUCED INSTABILITY
Often turbomachinery operators ignore shaft centerline data changes in average radial position of the shaft - and focus exclusively on vibration data to gauge machinery health. But in some cases, problems in the rotor system may show up as serious abnormalities in shaft centerline plots, not in vibration data, (see www.turbomachinerymag.com for more on shaft centerline plots).
Changes in shaft radial position are caused by changes in the static radial load or in the stiffness characteristics of the rotor system. When a rotor system with fluid-film bearings changes speed or load, its stiffness characteristics change. Some malfunctions, such as misalignment, rub, and fluid-induced instability, can produce noticeable changes in centerline behavior.
Excessive rotor lift
A 1,150 MW steam turbine at a power generation facility consists of a doubleflow High Pressure (HP) turbine, three double-flow Low Pressure (LP) condensing turbines, a generator, and a mechanical exciter with a single bearing. Bearings 1, 2 and 11 are tilt pad bearings (4 pad, load-between-pad), with a 20-mil diametral clearance. Bearings 3 and 10 are elliptically bored bearings with a 15mil vertical and a 20-mil horizontal clearance. The generator is a 4-pole unit with synchronous operation at 1,800 rpm.
Steam admission is through two lifting-bar valve racks; one rack on the bottom of the turbine case, with the second valve rack on the top (Figure 1). The lower valve rack opens first, followed by the upper valve rack as load increases.
During a recent start up of the turbine, a slight amount of asymmetry was observed in the support stiffness for the rotor system, which is reflected in the first balance resonance (critical speed) varying from 1,130 to 1,230 cpm (Figure 2). At 1,800 rpm (full speed), the balance condition of the...