Phase 2: Development and Testing of Monitoring Applications
Researchers from the University of North Carolina at Charlotte developed and tested a wireless mesh sensor network at the Gaston plant to perform some specific monitoring functions. The deployed wireless mesh sensor network (WMSN) included temperature-sensing nodes and vibration-sensing nodes. Although there are many potential uses for the two types of wireless sensor nodes developed for this project, two specific applications were highlighted for demonstration purposes: feedwater heater emergency drain valve monitioring and condition monitoring for boiler feed pumps.
Application 1: Feedwater Heater Emergency Drain Valve Monitoring. One issue affecting thermal performance in steam plants is valve leakage. One possible way to detect such leaks is to measure the temperature on both sides of the valve. If a leak occurs, the two temperature readings will begin to approach each other. If the valve is properly seated, the temperature on the downstream side should remain much lower than the upstream temperature.
Application 2: Condition Monitoring for Boiler Feed Pumps. Failures in boiler feed pumps and other critical rotating machines are extremely problematic in any power plant. A shutdown can be costly, particularly if it means that the entire unit must be shut down. The most common components to fail in any rotating-machine application are the bearings. For that reason, the project team constructed nodes that could be used to monitor the condition of the fluid bearings on all three components of the pump-motor-drive combination.
When monitoring bearing condition, both temperature and vibration measurements are very useful. To monitor boiler feed pumps at Gaston, the researchers designed a monitoring application that used a combination of thermocouple nodes and vibration plus sound-sensing nodes. Thermocouple nodes were deployed on both the lube oil piping for the various fluid bearings as well as on the bearings themselves.
Two types of vibration nodes were designed. The Type-1 vibration nodes were designed using the accelerometer in the integrated sensor board. Because the sensor board needs to be directly attached to the mote, these nodes must be physically attached to the surface where the vibration is to be measured. The Type-1 vibration nodes were also programmed to measure average sound intensity at the location using an on-board microphone. Because the physical size of the node enclosure precludes its use for measuring vibrations in hard-to-reach locations, a Type-2 vibration node was also designed that used a small external vibration probe attached to the node enclosure by a 6-foot cable.
The Type-1 vibration- and sound-sensing node determines the intensities of vibration and sound signals as perceived from a set of rapidly taken samples. The vibration intensity is obtained by computing the standard deviation of 20 consecutive samples of the x-axis accelerometer output that were obtained at 25-ms intervals, an effective sampling rate of 40 Hz.
The sound intensity was obtained by applying a slightly modified process to the microphone signal. In this case, the sampling interval was set to 50 ms, a rate that was experimentally determined to generate the best results. Furthermore, in order to allow the sound amplifiers to stabilize after each turn-on, the first 5 seconds of recorded data were discarded during each measurement period.
The Type-2 vibration-sensing node was designed using an external vibration probe with a magnetic holder that could be attached to locations where the Type-1 vibration-sensing node enclosure would not fit (Figure 5).
5. Vibration sleuth. The Type-2 vibration sensor is equipped with an external sensor, a custom sensor interface board, and a high-speed sampling mote. Courtesy: Southern Co.
This node was equipped with the following components:
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External vibration sensor. An external piezoelectric device originally used with a commercial, handheld vibration meter.
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Custom sensor interface board. Piezoelectric sensors are high-impedance devices, and they must be interfaced with signal-conditioning circuits that have high input impedance. A custom interface board was designed to match the high impedance of the sensor and transform the vibration signal to appropriate voltage variations for the data acquisition board.
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High-speed sampling mote. The data acquisition board performed two functions. First, it was used to generate the DC power to activate the interface board at intervals of 15 minutes, when vibration samples were taken. Second, during the sampling period, it obtained 20 samples at intervals of 50 ms for obtaining the vibration intensity.
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