Introduction

In this article, I will explain about the monitors that receive the signal from the sensor that was explained last time. The monitor converts the signal to each monitoring parameter and performs the monitoring.

3. Condition Monitors

The API 670 also contains requirements for condition monitors. It indicates detailed requirements for vibration monitors and axial position monitors. The VM-5 Series monitor (Photograph 3) and the VM-7 Series monitor (Photograph 4) are monitors that were designed to meet the basic requirements of the API 670. They can of course be used to monitor vibration and axial position as defined in the API 670. Furthermore, as I said in Part 1, they can also be used as Turbine Supervisory Instrumentation (TSI) monitors on large turbines in power plants for commercial use.

The condition monitor receives the voltage signal or current signal from the sensors. These signals include items such as vibration waveforms, displacement, and rotating pulse. The condition monitor then converts these signals into monitoring parameters such as vibration amplitude, axial position, and rotational speed. The condition monitor then performs calculations on these monitoring parameters and monitors the condition of the machinery.

The main functions of condition monitors include:

(1) Conversion to monitoring parameters
(2) Indication of measured value
(3) Output of measured value
(4) Detection of danger/alert alarm
(5) Indication of danger/alert alarm
(6) Output of danger/alert alarm
(7) Buffer output of vibration waveform for analysis

etc. Figure 3 shows a conceptual image of the main functions of condition monitors. Also, Figure 4 shows an example of a system where the VM-7 Series monitor is applied as a TSI monitor for a combined cycle power generation system.

In TSI systems for large turbines in power plants, there are special monitoring parameters in addition to the vibration and axial position specified in the API 670. As shown in Table 2, these special monitoring parameters include items such as eccentricity and differential expansion. In Table 2, if a monitor item has a circle marked in the Alarm column, then it is used for alarms. If a monitor item has a circle marked in the Shutdown column, then it outputs a trip contact signal that is used to shut down the turbine.

The eddy-current displacement sensors explained in Part 2 are not used only as a sensor to measure vibration. Of the items shown on Table 2, they can also be used to measure the rotational speed, eccentricity, differential expansion, and axial position. They are not used to measure casing vibration. The sensors used to measure casing vibration include moving coil type velocity sensors, piezoelectric velocity sensors, and piezoelectric acceleration sensors. In addition to using eddy-current displacement sensors to measure rotational speed, it is also possible to use electromagnetic pickups. Also, LVDTs (Linear Variable Differential Transformers) are used to measure the expansion and the valve position.

Table 2. TSI monitoring items

Previously, the basic scope of TSI normally included the following:
· The indication of each monitoring parameter using a condition monitor (a panel PC in the case of VM-7)
· The output of a DC voltage or current signal in response to the measured value
· The alarm contact output
However, systems installed in recent years often include the following as well:
· The performance of phase analysis and frequency analysis based on the vibration waveforms and phase reference signals that are obtained from phase reference and vibration sensors
· An analysis and diagnostic system that performs vibration analysis based on the results of the phase analysis and frequency analysis. The infiSYS RV-200 in Figure 3 corresponds to this analysis and diagnostic system. I will explain the details of this system in the next article.