This case study concerns a large induced draft fan that ran for 15 years without any significant operating issues or mechanical problems. However, prior to a recent outage, the plant operators reported step changes on both the drive end (DE) and non-drive end (NDE) bearing housing vibrations, and a step change in the DE bearing temperature. But, after a short period, the vibrations and temperature returned to approximately normal values. They reported that they had seen this a few times in the past, but the vibrations and/or temperatures would not stay elevated long and would always return to normal values. They were not sure if this was a serious issue, so they raised it as a concern to the responsible Machinery Engineer. The trends were reviewed and additional vibration data and bearing housing temperature data was taken, but the issue could not be explained. It was decided to inspect the bearings at the next planned outage. The inspection revealed significant damage to the DE bearing and shaft journal. The purpose of this article is to describe the incident and present the lessons
The subject machine is a double-width, double-inlet centrifugal fan driven by a 1,750 HP, 1,175 RPM induction motor. The motor operates at a fixed speed and a disc pack type spacer coupling connects the motor to the fan. The impeller is mounted on a shaft between two Dodge RTL pillow block type bearing housings. The pillow blocks are bolted to steel pedestals, which are mounted on a concrete foundation. The NDE bearing is a split, Babbitt lined sleeve-type journal bearing. The DE bearing consists of a split Babbitt lined sleeve type journal bearing and split, Babbitted fixed pad-type thrust bearings. The fan rotor includes a single thrust collar that is integral with the fan shaft. The arrangement and a rotor picture are shown in Figures 1 and 2.
The DE and NDE bearing housings incorporate self-contained oil reservoirs. As shown in the fan arrangement, each journal bearing is comprised of two separate running surfaces. Each section incorporates its own slinger ring to supply oil to the bearing for lubrication. The inner liner of the thrust bearing also has a center cavity that provides flooding lubrication to the thrust plates. The oil that is used in this machine is an ISO VG46 turbine oil. The bearing housings are water cooled using plant cooling water. Each journal bearing has a single thermocouple that is used to continuously monitor bearing temperature. The liners are drilled so that the thermocouple extends into the liner, near the Babbitt surface. However, the thermocouples are located on the side of the bearings, not on the bottom where the load and temperature are the highest. Also, as noted above, each journal bearing is comprised of two sections and the temperature of only one of the sections is monitored.
The bearing housings also are fitted with a single flat surface, bull’s-eye oil level gauges and Trico constant level oilers. The shaft oil seals are a floating labyrinth type. Finally, each bearing housing is fitted with a single velocity transmitter mounted in the horizontal direction to continuously monitor vibration.
The fan was started in 1995 and was put into continuous service. Planned plant outages occurred every 2-3 years. During these outages, the oil was changed, but unless there was a problem or issue, the bearings were not typically disassembled and inspected. The typical NDE bearing housing vibration levels were below 0.080?/second (IPS) and the typical DE bearing housing vibrations were below 0.13 IPS. In December 2009, there was a step change in vibration following a plant outage.
The NDE vibration increased to 0.16 IPS, while the DE increased to 0.20 IPS. During the next two years, the NDE vibration increased and varied between 0.18 and 0.20 IPS, while the DE increased and varied between 0.21 and 0.26 IPS. API 673, “Centrifugal Fans for Petroleum, Chemical and Gas Industry Services,” is a good reference for evaluating vibration amplitudes. Per API 673, the unfiltered (overall) bearing housing vibration limit is 0.16 IPS and the filtered vibration limit is 0.10 IPS. Although the operating levels were above the API limits, the vibration levels were relatively stable. During the 2009 outage, the main drive motor was replaced and the increase was attributed to either shaft misalignment or impeller fouling. The bearing temperatures did not change after the outage. The DE and NDE bearing temperatures typically operated between 100°F and 130°F, based on fluctuations in ambient and cooling water temperatures. This was well below the manufacturer’s recommended temperature alarm limit of 180°F.