By Teresa Hansen, Associate Editor
Nuclear power plants depend on electric motor-driven pumps to circulate coolant through a closed system of pipes to help dissipate excess heat. These machines, reactor coolant pumps (RCPs), must be monitored continuously to ensure dependable operation. Resistance temperature detectors (RTDs) located on the pump’s bearings measure temperature and send the signal to an automatic alarm and control system that quickly alerts operators of any status change. RCP bearings are likely suspects for failures. One failure mode is a bearing seal that leaks lubricant and can become a fire hazard. Because of their importance, monitoring RCP bearings is a high priority, and usually, multiple sensors are installed on each bearing to provide redundancy.
Because issues with bearing seals have been a reoccurring problem in numerous nuclear power plants, the Nuclear Regulatory Commission has issued a multilevel procedure for powering down facilities when seal problems occur. One company has gone even further, initiating a special program to research the cause of seal failures and developing a method for testing thrust-bearing seal leaks outside the machine. The program uses a simulator composed of a test stand that accommodates a large motor and bearing assembly; hydraulic actuators that place external, vertical and horizontal stresses on the bearing shaft; and a data acquisition system that monitors temperatures, forces, pressures, oil flows and vibration in the motor and bearing.
Stress analysis engineers at EME Associates, Pittsburgh, Pa., designed and developed the test stand and appropriate instrumentation. EME had several dual-channel data acquisition systems from which to choose, but the design eventually called for more than 50 channels of simultaneous data collection. Clearly, the EME team had to find a compact data acquisition system capable of handling that number of channels and flexible enough to accommodate a wide variety of sensor types.
After investigating several possible systems, the EME team selected the IOtech ZonicBook for the application. Although it was the first IOtech system with which they had experience, Christopher Mascaro, an electrical engineer at EME learned how to use it in less than a day. The data acquisition system let him connect 56 channels with a mix of sensors that included RTDs, flow sensors, pressure sensors, tachometers and proximity probes.
Mascaro worked with a thrust bearing assembly that weighs about 12,500 pounds and measures 3 feet high and 5 feet in diameter. The test stand also contains a 500-hp motor coupled to a 600-hp electronic drive and a thrust bearing assembly. The system contains hydraulic cylinders that apply vertical and horizontal thrust to the bearings to simulate loads. The vertical thrust loads are 100,000 to 177,000 pounds and the side loads range from 0 to about 700 pound-force. The thrust load is controlled from an initial load of up to 170,000 pound-force down to 102,000 pound-force as the shaft runs up to operating speed. The shaft ramps up from 0 to 1,190 rpm in 16 seconds and down in 5 minutes to simulate actual field-spin times. The bearing reservoir contains view ports in the top cover and sidewalls. It also contains features that accommodate thermocouples, pressure and flow sensors, three borescopes and five cameras.
“The original design called for about 20 sensor channels,” says Mascaro, “but it expanded to 56 channels as the project developed.” Six WBK18, 8-channel dynamic signal conditioning modules were added to the Zonic Book to expand the channel count, as well as two DBK84 thermocouple modules to handle multiple temperature channels. “The ZonicBook and expansion modules worked out extremely well for us because they are flexible enough to handle the wide variety of variables in our customer’s specifications,” says Mascaro. “We could monitor the data in real time and track changes as they appeared.” The parameters measured included pressure from 0 to 60 pounds per square inch gauge (psig), temperature from 80 to 350 F and shaft speed from 0 to 1,190 rpm.
In addition to the originally specified sensors, proximity sensors were added to measure vibration and the shaft’s axial position. Other sensors were added to measure oil level. Hydraulic cylinders were installed on mounting collars to apply side loads to the bearings and on thrust runner reaction bearings to apply vertical loads. In addition, load cells were installed on these actuators to measure the amount of force applied to the bearings.
“The simulator stand had to be compact,” says Mascaro. “We built a single control panel to contain the ZonicBook, signal-conditioning modules, computer, TV monitor and digital video recorder.” EME also used the IOtech DBK48 Signal Conditioning Module containing non-destructive test relay outputs to shut down the motor in the event of an over-temperature or over-speed condition, excessive vibration, low oil level or excessive vertical or horizontal thrust.
“The eZ-TOMAS software supplied with the ZonicBook was easy to use and let me construct Bode plots of the vibration,” says Mascaro. “We were able to commission the system and turn it over to the customer within three months, and we taught (the customer’s) engineers how to use both the software and hardware in less than a week.”