In this column, we will proceed with a continuation of the previous theme, entitled “Condition Monitoring System for Large Rotating Machinery.”

5. Monitor System

　Regarding monitor systems used for critical machines in petroleum-related plants, the API 670 specifies detailed requirements for monitors to measure machine condition parameters such as vibration, axial position, and others. The VM-5 and VM-7 series monitors (see Figure 3-1) are designed in accordance with the requirements of the API 670. However, these systems are applicable not only for machinery protection systems in petroleum-related plants as specified in the API 670, but also for large turbine-generator sets as TSI (Turbine Supervisory Instruments) systems in power plants.

　The condition monitor receives voltage or current signals from sensors such as vibration waveforms, displacement, and rotation pulses, and converts and calculates these signals into monitoring parameters such as vibration amplitude value, shaft axial position, and rotational speed to monitor machine conditions. As shown in Figure 3-2, the primary functions of the condition monitor are “Conversion to monitoring parameters”, “Display of measured values”, “Output of measured values”, “Detection of Danger and Alert status”, “Indication of Danger and Alert status”, “Output of Danger and Alert status”, and ” Buffered output of vibration waveform for analysis”.

　Figure 3-3 shows a system example using the VM-7 series monitor is applied as the TSI system for a combined cycle power generation system.

　In the TSI systems for large turbines in power plants, there are not only vibration and axial position as specified in the API 670, but also unique monitoring parameters such as eccentricity and differential expansion, as shown in Table 3-1. A check mark in the “Alarm” column indicates that an alert relay contact output is used as an alert. A check mark in the “Shutdown” column indicates that a danger relay contact output is used as a shutdown signal for the turbine.

　Eddy current displacement sensors are used for shaft vibration, and electrodynamic velocity sensors and piezoelectric velocity sensors are used for casing vibration. Eddy current displacement sensors are used for a variety of applications, including measuring shaft vibration, rotational speed, eccentricity, differential expansion, and axial position. In addition to eddy current displacement sensors, magnetic pickups are also used for measuring rotational speed. Furthermore, LVDTs (linear variable differential transformers) are used to measure case expansion and valve position.

Table 3-1: TSI monitoring parameters

6. Vibration Analysis and Diagnosis System

　There are two types of vibration analysis and diagnosis systems for rotating machinery: those for general-purpose machines supported mainly by rolling element bearings and those for large rotating machines supported mainly by fluid film journal bearings. While there are many similarities in the basic analysis methods, there are also many differences in the intervals of data collection and the type of analysis graphs used. In the subsequent column, we will address vibration analysis and diagnostic systems for general-purpose rotating machinery. In this article, we will focus on vibration analysis and diagnostic systems for large rotating machinery supported by journal bearings. These large rotating machineries are typically flexible rotors operated at speeds that exceed the critical speed of the rotor. In addition to performing frequency analysis of the measured shaft vibration waveform data, phase analysis of the vibration waveforms is conducted. This analysis incorporates a phase reference signal of one pulse per revolution.

　The basic components of this system are shown in Figure 3-4. A keyway-shaped notch on the rotor is used as the mechanical phase reference position of the rotor, and an eddy current displacement sensor is installed here to obtain a phase reference signal of one pulse per revolution.

　The phase reference signal and vibration waveform signal detected here are then entered into the condition monitors described in Chapter 5, where they are converted into rotational speed and vibration amplitude values. If the vibration amplitude exceeds the alarm setting value, an abnormal vibration alarm is triggered. Concurrently, the signal inputs to the condition monitors are output as buffered signals that are unaltered analog replicas of the input signals through the buffer amplifiers, and the buffered signals are used for vibration analysis. The buffered signal outputs from the condition monitors are input to the DAQpod analysis data acquisition unit, where they are A/D-converted and processed for phase analysis, frequency analysis, and other analyses. In the case of the VM-7 series monitors, the built-in vibration analysis board performs the same signal processing as the DAQpod, so there is no need to install an external DAQpod.

　The processed data is transferred to an analysis PC installed with analysis software (Analysis View VM-773B), which performs the data plots and data storage necessary for abnormality analysis, including trend plots, spectral plots, orbit plots, Bode plots, and polar plots.

　Figure 3-3 shows the overall system configuration of the condition monitors and the infiSYS RV-200 permanent analysis and diagnosis system is represented. Figure 3-5 shows an example of the infiSYS RV-200 display screen.

　In addition to the conventional monitoring of machine abnormalities using condition monitors, the introduction of the analysis and diagnosis system, as described here, is expected to have the following effects.

• Abnormalities that cannot be detected by monitoring overall vibration amplitude (OA) alone can be detected.
  　Vibration monitors typically monitor the overall amplitude (OA)* of vibrations. However, during rated speed operation of machinery, abnormalities in rotors, such as rotor component detachment (e.g., turbine blades), may not be detected as a significant increase in OA, or in some cases, may appear as a decrease in OA due to improved imbalance, making it impossible for vibration monitors to detect such abnormalities. However, even under such abnormal conditions, sudden changes in the vibration vector (consisting of amplitude and phase angle) of the rotational synchronous component (1X) occur. Therefore, by setting area alarms on the polar plot (vector diagram), it becomes possible to detect abnormalities.

* Overall Amplitude (OA) is a vibration amplitude value obtained from a broadband vibration waveform, which is a representative value indicating the intensity of the vibration and is expressed as peak-to-peak value, peak value, or root mean square (RMS) value.

• It can be used to support the diagnosis of abnormal causes.
  　The execution of the diagnostic program of the analysis and diagnosis system enables the identification of potential causes of abnormal vibration. This process involves systematic allocation of weights to different causes, presenting of these causes in order of priority. This approach allows for the estimation of abnormal causes. This can be used as reference information or support for maintenance engineers and vibration analysts when diagnosing abnormal causes.

• It can be utilized to accumulate diagnostic know-how for each piece of machinery.
  　By retaining abnormal data not only as amplitude values but also as analytical data such as vibration waveforms and spectra, it is possible to quantify and qualify diagnostic results based on the senses and experience of maintenance technicians, as well as the results of mechanical disassembly investigations, by linking them with analytical data. This enables the accumulation of diagnostic know-how for each piece of machinery and equipment. By database-izing this information and constructing an interactive search system, even junior maintenance personnel with limited experience can access the extensive experience accumulated within the plant (or enterprise) to obtain appropriate diagnostic hints and support for response methods.

• Enables accurate fault diagnosis and countermeasure consideration through data sharing with experts.
  　In the event of a machine malfunction, providing and sharing various analysis data not only with in-house diagnostic engineers but also with external experts such as the SHINKAWA Remote Sensing Center* and/or the machine manufacturers will enable more accurate fault diagnosis and countermeasure planning.

* SHINKAWA Electric provides the “infiSYS V-Assist” service, which connects the analysis and diagnostic systems such as infiSYS RV-200 and infiSYS 3.0 installed at the user’s site to the “SHINKAWA Remote Sensing Center” online, and sends alarm notification e-mails, generates diagnostic reports, etc.

　In the next issue, we will discuss condition monitoring and analysis diagnosis of general-purpose rotating machinery supported mainly by rolling element bearings.