Machinery-health monitoring helps plants achieve financial goals

By Ned Endress, RoMaDyn

Machinery-health monitoring (MHM) systems continuously record and analyze turbine vibration data (see sidebar). They take raw data from vibration sensors and other process-related indications and convert it to actionable information about machinery condition—thereby enabling better asset management. In today’s merchant power business, high plant availability is critical to success and the continuous assessment of machine condition provided by MHM is critical to achieving demanding performance goals.

Combined-cycle/cogen plants come in many sizes, shapes, and operating budgets. So do MHM systems. The challenge for plant operators is to select the grade—the level of sophistication—of MHM that is right for their gas and steam turbines and operating paradigm. There is no one-size-fits-all answer. This article focuses on five levels of MHM sophistication. Plant operators can use this discussion to evaluate their current MHM system, and make the business case for investment in any changes or improvements. Such an evaluation should:

  • Review current business objectives and contractual obligations.
  • Quantify past and present unit performance, maintenance history, and unit availability.
  • Review and quantify previous operational, maintenance, and machinery failures, as well as their impacts on unit availability.
  • Review the existing MHM system data in relation to the failures, maintenance history, and unit availability.
  • Evaluate current MHM system status and capabilities.
  • Quantify the expected return-oninvestment of possible changes or improvements.

Keep in mind that each level has progressively more sophistication, automation, and protection. A Level 1 system, for example, offers fundamental machinery protection with very little diagnostic capability. Level 2 and 3 systems have all the machinery protection capabilities of a Level 1 system; plus they offer trending and some basic statistical analysis capabilities involving vibration and process-related data. A Level 4 system provides comprehensive machinery protection and full machinery diagnostic capabilities. Level 5 systems have all of the functionality of Level 4 systems; however, they also include advanced automated machinery diagnostic capabilities.

For older peaking turbines running only occasionally, a Level 1 system would be sufficient for basic machinery protection. By contrast, a Level 3 system would provide minimum machinery protection, diagnostics, and management for a typical combined-cycle plant. The choice of which level of sophistication to deploy should depend heavily upon an evaluation of business risk verses system capability, protection, and maintenance management.

Level 1: Basic protection

A Level 1 system consists of vibration and process-related sensors connected to a machinery protection system (Fig 1). This configuration provides rapid response for machinery protection. Depending on the type of hardware, it also can provide options for high-pass, low-pass, and band-pass filtering of the vibration signal.

Within the machinery protection system, independent setpoints for “alert” and “danger” levels typically are specified. Alert and danger levels can be established for the direct (unfiltered) vibration level and the probe-gap level, as well as for the vectors, amplitudes and phases at synchronous (1×) and twice synchronous (2×) shaft speeds. Changes in synchronous and twice synchronous vectors can indicate: changes in load; unbalance of the shaft; bowed or bent shaft; foreign material accumulation; misalignment; shaft crack; or several other potential problems.

Gap voltages can indicate that the average shaft centerline has moved to an undesirable position within the journal bearing clearance. The type of bearing and direction of its rotation (clockwise verses counterclockwise) usually will determine the expected location of the shaft centerline. A location that is high within the bearing clearance might indicate misalignment, or that the loading of the bearing is reduced hence an increased potential for oil-film instability might be present. Additional diagnostic tools would be needed to determine which is the root-cause.

Trending of hot-shutdown proximity- probe gap voltage levels can be a useful tool for monitoring bearing wear. Gap voltage alert levels can be established to warn when excess bearing wear has occurred and corrective action should be taken. Trip delays and relay latching also are valuable features to have available in the protection system.

Typically, vibration data are not retained by the machinery protection system. This means that with a Level 1 system almost no post-mortem analysis of vibration events can be performed to determine the root cause of those events. If any data are retained, it likely will be static values or levels, or a very limited number of dynamic waveforms. Static values, if retained, can be trended and correlated with each other to yield useful information. A trend in direct vibration amplitudes might be an indication of an incipient vibration problem, and the rate of change of that trend may give an indication of the worsening severity of the problem and urgency to take corrective action. Correlating direct and synchronous vibration levels with bearing metal temperatures also can provide clues into the condition of the journal bearings.

Major advantage of a Level 1 MHM system is that it enables rapid response—including automated unit trips, if desired—to changes in vibration and process-related conditions. Cost of this type of system can be approximately $3000 to $4000 per point. For a GE-7FA, the standard vibration-sensor suite comprises two proximity probes on each turbine and generator bearing, dual thrustproximity probes, and one seismic sensor on each turbine and generator bearing. That’s a total of 14 points, meaning a Level 1 system should cost approximately $42,000 to $56,000. The installation of transducers and protection system hardware is a manageable process; however, configuration of the protection system hardware and software may require additional vendor expertise and support.

In summary, a Level 1 system will provide:

  • Rapid machinery protection.
  • Direct vibration amplitudes.
  • Vibration vectors (amplitude and phase).
  • Gap voltage levels (if trended, shaft radial position anomalies).
  • Alarm or setpoints for direct vibration, vibration vectors and gap voltages.
  • Typically no trending capability or retention of vibration information.

Level 2: Trending capability

A Level 2 system is comprised of the vibration and process-related sensors of a Level 1 system, which are connected to transmitters and then to the distributed control system (DCS) or other plant control system— instead of to a dedicated machinery protection system (Fig 2). Typically, the transmitters provide basic signal conditioning and sensor power; however, they also can provide signal filtering and gap-voltage monitoring.

This system typically passes direct (unfiltered) vibration levels to the control system for monitoring, trending, and alarming. Because process- related sensor data are merged with vibration levels at the control system, the data can be correlated, trended, and used for performance computations. These signals also can be used to compute other derived quantities or values.

Because the Level 2 system

lacks dedicated machinery protection capability, its cost is less than a Level 1 system—approximately $1500 to $2000 per point. Again considering the case of a GE-7FA, the standard 14-point vibration-sensor suite is two proximity probes on each turbine and generator bearing, dual thrustproximity probes, and one seismic sensor on each turbine and generator bearing. At $1500 to $2000 per point, a Level 2 system should cost approximately $21,000 to $28,000.

For a Level 2 system, the installation of transducers and transmitters, along with their integration with the control system, typically is a manageable process, and can be done entirely by the end user or control- system OEM. However, integrating sensor signals and customizing the control-system software might require some additional OEM expertise and support.

Transmitters can offer networking capability to make digital connectivity with the DCS, programmable logic controllers, or other type of control system a simple task. They also can provide a 4- to 20-mA output signal for analog connection to the control system.

These systems typically provide retention of data; however, direct vibration levels—by themselves— have very little practical value for vibration machinery diagnostics. This means that, as is the case for a Level 1 system, only a limited postmortem of vibration events can be performed.

Thus, a Level 2 system will provide:

  • Trending capability.
  • Direct vibration amplitudes.
  • Gap voltage levels (if trended, shaft radial position anomalies).
  • Alarm or setpoints for direct vibration and gap voltages.

Level 3: Protection plus trending

A Level 3 system provides the machinery protection of a Level 1 system, plus the capability of trending and integrating other process-related information found in a Level 2 system. Alarm and trip functions are performed by a machinery protection system, not the plant DCS, but they are communicated to the control system (Fig 3). This achieves more rapid machinery protection, compared to a Level 2 system, while retaining that system’s data-archiving capabilities.

A Level 3 system typically can send direct (unfiltered) vibration levels, proximity-probe gap voltages, and numerous vibration parameters— such as synchronous filtered amplitude and phase—to the control system for monitoring, trending, and archiving. Process-related sensor data can be merged with vibration data at the control system, which enables the correlating and trending of data, as well as the calculation of performance metrics. This information is helpful, although an experienced rotor-dynamics specialist typically is required to convert the data to useful information.

Cost of a Level 3 system can be up to $4000 per point. Installation of transducers and transmitters, and their integration with the control system, can be done by the end user or control-system OEM. It usually is a manageable process; however, integrating sensor signals and customizing the control-system software might require some additional vendor expertise and support. For the 14 points in a GE-7FA standard vibration- sensor suite, a Level 3 system should cost approximately $49,000 to $56,000.

In summary, a Level 3 system will provide: Rapid machinery protection.

  • Trending capability.
  • Direct vibration amplitudes.
  • Gap voltage levels (if trended, shaft radial position anomalies).
  • Alarm or setpoints for direct vibration and gap voltages.

Level 4: Dedicated server allows higher sampling rates

A Level 4 system has most of the characteristics of a Level 3 system, except that a dedicated backend computer— rather than the DCS—handles all of the complex analysis of vibration data. This system consists of vibration and key process sensors connected to a machinery protection and management system, with a substantial data-acquisition server computer that supports data reduction and diagnostics. Vibration sensors are connected directly to the machinery protection system, prior to being routed into the server computer and its machinery management software (Fig 4). Reason for a dedicated server computer: It enables higher dynamic data sampling rates of the vibration sensors.

This configuration supports rapid vibration setpoint alarming for enhanced machinery protection. Additional alarming and machinery protection also can be performed within the server computer machinery management software. The machinery protection system, which can be localized within a single hardware rack or distributed into individual transmitter/relay modules, provides signal conditioning for the vibration and other process sensors.

Because data acquisition is being accomplished by a dedicated computer and specialized software, higher data sampling rates can be used for collection of static as well as dynamic data. Dynamic data are comprised of waveform data acquired during startup, shutdown, or periodically during steady-state operation of the unit.

During steady-state operation, waveform data are collected at specific time intervals—during specified speed variations or during a specific user-determined interval. During startup or shutdown, waveform data are collected at specific speed intervals. The waveform data are collected using synchronous and asynchronous sampling. Synchronous sampling is based on shaft speed—a specific number of samples per shaft revolution. Asynchronous sampling is based on a specific number of samples per second.

Retention of static and dynamic data is critical for comparing current vibration conditions to historical data, or for retrieving data for diagnosing vibration events after they occur. However, significant amounts of the data need to be retained as historical data to document vibration and process conditions. The extent of the data will depend on how frequently the data are collected and how often the unit is started up and shut down. For example, a bearing with dual proximity probes and dual seismic sensors with dynamic data being collected every 10 rpm or every 10 seconds, and static data being collected every 10 minutes, can accumulate 50-100 megabytes of hard diskspace in 30 days. Archiving of such data can become a frequent task. If archiving is performed automatically, the end user will be expected to add fresh storage media—typically a DVD—every six to twelve months. If this is not automated, the end user must initiate the archive backup and also add fresh storage media on a more frequent basis.

A Level 4 system also integrates process-related information with the data from the vibration sensors. By combining vibration and process data within the same management system, a plant operator is able to correlate related parameters from diverse sources—such as vibration, bearing temperature, and generator- output data. Note that vibration data in this system are not restricted to numerical values; they also can include shaft relative orbits, waveforms, shaft centerline or eccentricity, and spectral information.

Advanced notification of vibration- related events also can be included in a Level 4 system. This enables the operator to assign severity levels based on criteria for vibration or process- related events. If the current operating conditions meet or exceed the criteria, the MHM system automatically contacts operations, supervisory, or maintenance personnel concerning the event. Simple advice messages or corrective actions can be included in the message.

Remote access to the system is possible via high-speed internet connection with VPN (virtual personal network) software or dial-up modem. High-speed internet connection is the preferred method because of the sophisticated computer graphics and data manipulation in this system. Existing security measures—such as corporate firewalls—and integration with plant-wide networks can pose serious installation challenges. One advantage of remote access to the system is to allow other plants or outside vendors to perform diagnostics, balancing, performance optimization, or any other task that normally is done locally at the system.

Key vibration indications
For a particular vibration event, several different indicators often must be analyzed together to understand the root causes of that event and formulate corrective actions. These indicators, discussed below, come from vibration data during steady-state, transient (startup and shutdown), slow-roll, and stopped conditions.

  • Direct vibration amplitude are unfiltered vibration levels, either shaft relative motion or seismic casing vibration. Direct vibration amplitudes provide a general indication of the mechanical condition of the unit.
  • Vibration-vector amplitude and phase are important parameters to monitor and trend on both a long- and short-term basis. Generally, changes in synchronous (1X) and twice synchronous (2X) vectors might be caused by changes in load, shaft unbalance, bowed or bent shaft, foreign material accumulation, misalignment, shaft crack, or several other mechanical malfunctions.
  • Shaft radial position anomalies can be observed by changes in shaft radial position on shaft centerline plots. These changes can be caused by changes in bearing static loading or stiffness characteristic changes. These changes in shaft centerline position can be symptoms of common machinery malfunctions like misalignment, fluidfilm instability, or rubs.
  • Vibration frequencies are important in understanding which frequencies are contributing to the overall vibration.
  • Direction of precession provides an indication of whether the direction of vibration or shaft orbit is “with rotation”—so-called forward procession—or if the direction of vibration is “opposite rotation”—reverse precession. Precession is a useful tool for diagnosing specific machinery malfunctions—such as rubs.
  • Slow-roll speed range data provide important information about proximity-probe shaft target area surface quality, typically called “glitch” or “runout.”
  • Slow-roll vectors are important for compensation of synchronous, and integer multiple of synchronous, vibration vectors attributed to shaft target “runout.”
  • Heavy-spot location or angle indicates changes in the location of mass unbalance for a shaft. The heavyspot location is the position on the shaft where balance corrections for a given mode would be applied. Tracking and trending heavy spot location also can tell you whether the residual unbalance of the shaft has changed over time.
  • Anisotropic stiffness occurs when there are unequal stiffness levels in radial directions. Typically, one radial direction will be stronger than other weaker directions. This behavior will lead to split-balance resonance. A splitbalance resonance is two balance resonances that have the same deflection or mode shape, but are separated slightly in frequency (shaft speed). Anisotropic stiffness will affect the behavior of balance resonances.
  • Balance resonance (or “critical”) frequencies are important because at and near these resonant frequencies, vibration response of the shaft is significantly higher compared to other frequencies. Balance resonant frequencies are also referred to as “critical” frequencies. When shaft speed approaches or coincides with a balance resonance frequency, vibration levels increase significantly or are amplified. This amplification is caused by reduced dynamic stiffness and/or damping at this frequency. If the vibration level at each balance resonance frequency is monitored and trended, changes in dynamic stiffness can be identified.
  • Distinguish between modes. The shaft deflection shape that corresponds to each balance resonance can be taken from the response (amplitude and phase) measured by each vibration sensor at each balance resonance frequency. Balance resonant modes can be distinguished by their characteristic deflection shapes and frequencies, such as cylindrical mode verses a pivotal mode. Mode shapes are useful during balancing.
  • Synchronous amplification factor (SAF) is used to quantify changes in vibration conditions that result from changes in stiffness or damping associated with balance resonances. The SAF indicates how strong an effect a resonance will have on shaft unbalance. The higher an SAF, the more of an effect the resonance will have on unbalance.

Cost of a Level 4 system can bequite substantial. Depending on the number and type of vibration and process sensors involved, costs can range from $4000 to over $5000 per point. Installation of the system is rather complex and requires expert assistance. Typically, system OEM vendors will install these systems, configure and do system optimization, and also offer system hardware and software maintenance agreements— at an extra cost, of course. For the example GE-7FA unit, the standard vibration-sensor suite of 14 points would costs approximately $56,000 to over $84,000.

In summary, a Level 4 system will provide all of the capabilities of a Level 3 MHM system, plus the following:

  • Monitoring and alarming for a significant amount of detail vibration information (key vibration indicators) is available.
  • Integration of vibration and process information.
  • Long-term retention of detailed vibration information and process data—an archival system.
  • Smart notification of vibrationrelated incidents.
  • Remote system access to perform diagnostics, balancing, performance optimization, or other tasks.

Level 5: World-class MHM system

A Level 5 MHM system has all of the characteristics of a Level 4 system, plus the capability to automate most—if not all—of the machinery diagnostics and performance tasks— such as performance mapping and performance optimization.

In addition to providing automated diagnostics, it also provides end-user notification of critical and pending machinery events. Automated diagnostics typically are performed by artificial intelligence (AI) software— typically a rule-based expert system or a neural network system.

A rule-based expert system uses embedded knowledge from experts, which is incorporated into algorithms and logic to help diagnose events. In contrast, a neural network uses prior events and expert interpretations of those events to recognize and establish patterns in the data, from which the network can classify new events and determine solutions. An AI system monitors and reduces the current data, uses historical information and expert knowledge, diagnoses the current operating situation, and provides constructive feedback and advice to the operator. If the AI system cannot reach a conclusion, a significant amount of diagnostic data are available for a vibration analyst to diagnose the situation.

Major advantages of an AI system are its speed and consistency in reducing large amounts of data and coming up with solutions. It also can assist an inexperienced end user reach meaningful conclusions and also provide accurate decisions or corrective actions. Some AI systems also can be used to optimize performance.

Cost of the Level 5 system, not surprisingly, is significantly higher: $5000 to $6500 or more per point. For the GE-7FA unit’s standard 14-point vibration-sensor suite, the total system cost can exceed $91,000.

In summary, a level 5 system will provide all of the capabilities of a Level 4 system, plus automated:

  • Diagnostics of vibration related events.
  • Optimization of performance.
  • Archiving of information.

Economies of scale

Because of computer software and hardware capabilities, considerable cost savings can be realized when a Level 4 or 5 system is deployed across several units. Once the server and software are installed, adding more points from more units spreads out their cost, driving down the cost per point. For example, a combined-cycle plant with four gas turbines and one steam turbine—GE-7FAs and a GE-D11, respectively—typically has about 78 points. If a Level 4 MHM system is installed, the approximate cost is $312,000 to $468,000, or more. If two additional gas turbines are added to that system as peaking units, the increment cost per point for those additional 28 points would be only $1500 to $2000 per point—or an additional total cost of $42,000 to $56,000. ccj oh