ArticleAlignment-Related Problems On A Re-Rated Steam Turbine
by Richardo Mohammed
This paper is a case history of problems experienced after upgrade of two turbine-driven feed water pumps. Upon OEM upgrade of the two units, a thrust bearing failure was experienced by one of the two turbines. After rebuild of the turbine, initial vibration levels were acceptable, but after operating the unit for about 5 minutes, the vibration levels quickly rose to unacceptable levels. The unit was shutdown. A later solo run of the turbine saw no problems and low vibration levels. Both pipe strain & thermal growth of the turbine were suspected. Adjustment of the exhaust piping to reduce pipe strain was made and the thermal growth of the turbine measured using benchmark gauges. Measured thermal growth was found in significant variation with that expected by the OEM and previously accounted for in the alignment. Once the measured thermal growth amounts were accounted for in the shaft alignment, acceptable vibration levels were consistently measured on the machine whether hot or cold.
“A major requirement of a technical upgrade of an anhydrous ammonia plant was an increased demand for high-pressure boiler feed water. The two high-pressure boiler feed water pumps and their associated steam turbine drivers were revamped.
The pump is a multistage barrel-enclosed centrifugal design. The multistage condensing steam turbines exhaust to one common condenser. Both units were designed and constructed more than thirty years ago and are identical except for slight differences in the governor end bearing arrangement.
The upgrade of the steam turbines was achieved by changing both rotating and stationary components along the steam path of the turbines. The components included a new Curtis wheel, new fifth and sixth stage wheels, new diaphragms, and an upgraded coupling. The bearing design was also upgraded with new tilt pad thrust bearings and two axial groove sleeve journal bearings to handle increased thrust and radial loads.
The purpose of these modifications was to increase both horsepower and steam rate by more than 25%. Before the upgrade, both units were reliable and trouble free. One important feature of the upgrade was to allow one pump to fulfill the boiler feed water requirements of the entire plant.
The upgrade was carried out by the OEM; they were responsible for all design, manufacture, field machining, assembly, and installation of the re-rated units. All the field and hardware modifications were to be completed within one month from the time of plant shutdown to recommissioning. Neither the exhaust piping nor the common condenser was subjected to any design changes as a result of the upgrade of the turbines. Figure 1 shows the layout of the turbines.
After assembly and installation of the upgraded turbines, the units were to be started up as required by the process demands of the plant. Due to problems and delays encountered in other areas of the plant, the unit was not placed in service until a considerable time after completion of the mechanical work. The operating speed of around 3,700 RPM was attained without the presence of the plant’s reliability/condition monitoring personnel; therefore, vibration data were not available at full load.
Unfortunately, the north unit experienced an unexplainable thrust bearing failure while at full load. Overhaul and inspection of the turbine. were performed by maintenance personnel. When the unit was placed on line at an operating speed of approximately 3,700 RPM, all vibration levels on the unit were initially acceptable, with a maximum of 0.20 in./sec.”
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