By William Marscher and Eric Olson, Mechanical Solutions Inc.
Just weeks before the commissioning deadline at a new power plant, problems with one of a pair of steam turbine/generator sets surfaced. Hundreds of thousands of dollars per day in liquidated damages would be due if the problem wasn’t solved.
The plant consisted of two 425 MW steam turbine/generator sets, the first of which had been commissioned without a hitch. The sister unit, however, which was nominally identical in all design details including the piping and foundation, tripped out at the first critical speed every time the original equipment manufacturer (OEM) tried to bring it up to operating speed.
Problems with one of the steam turbine/generator sets at this plant were fixed without disassembling the turbine. Photo courtesy MSI.
The vibration was almost entirely one-times running speed and the OEM had assumed that the issue was simply one of appropriate balancing. But the addition of balance weights, based on information at low speed and at a speed just below where the unit tripped on start-up, was not making any progress, even after 10 trials.
The OEM concluded that the problem was not balancing after all, but rather some machining error or other internal anomaly that could only be determined by full disassembly of the unit in the field, with subsequent repair at the OEM’s shop. Unfortunately, this would lead to many weeks of schedule slippage, at a high contract cost.
Mechanical Solutions Inc. (MSI) was contacted on behalf of the engineering construction firm for the new plant to perform field testing work.
MSI used a 14-channel B&K Pulse Spectrum Analyzer/Digital Recorder to monitor proximity probe, bearing housing and casing accelerometer signals simultaneously. No internal rub was taking place, although a mild rub on the high pressure (HP) outboard bearing was indicated. The shaft orbits suggested that both of the HP bearings were misaligned, such that the shaft was constrained from developing a roughly circular orbit, forced instead into a highly elongated orbit. Furthermore, an inconsistent and roughly 20 percent shift in the HP first critical speed was observed as the shaft approached the trip point, so that the phase angle of the vibration response, as related back to the keyphasor, would vary considerably at any given speed in the trip zone.
MSI concluded that the bearing misalignment situation was leading to significantly different bearing stiffness than the turbine was designed for. The transient thermal growth of the rotor and casing made the bearing “pinch” and therefore the critical speed shift was different from one run-up to the next. This meant that basing the balance correction weight angular positions on the phase information from one run generally would not hold true for what was required for proper balance in a succeeding run.
Working with the OEM’s balance team, MSI got through the first critical speed in two trials using an influence coefficient approach. MSI also advised that balancing efforts could not come close to spec until the bearing problem was fixed.
Based on these findings, MSI recommended a week-long shut-down for inspection and repair of the HP bearing installation, but the OEM felt this was unnecessary. Then it was discovered that the HP outboard bearing had been installed horizontally cocked, forcing the shaft out of the intended operational position on the bearing fixed central pad and into the center of the anti-whirl pocket, greatly shifting the bearing “k.” In addition, the HP inboard bearing pedestal was cocking during the start-up thermal growth because of accidental misplacement of one of the thick shims between the casing and the bearing pedestal.
Fixing these bearing issues eliminated the critical speed shifting near the original problem speed and allowed balancing to quickly progress. Turbine disassembly was avoided and the rigidly coupled machinery train was brought fully within the ISO-7919 vibration specification, one week before initiation of contract liquidated damages.
Authors: William D. Marscher is president and technical director of Mechanical Solutions Inc. (MSI). Marscher wrote the Turbomachinery Structural Design & Analysis chapter of the Third Edition of “Sawyer’s Gas Turbine Handbook.” Eric Olson is the director of sales and marketing at MSI. He is a multi-year short-course speaker at the Texas A&M International Pump Symposium.