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Vibration Monitoring System Enhances Troubleshooting

Issue 9 and Volume 103.

Networked Vibration-based Condition Assessment Systems have been in Operation at PECO Energy’s Peach Bottom Atomic Power Station (PBAPS) since the end of 1996. The plant has documented multiple benefits with the systems, ranging from early crack growth identification to shorter turbine run-up times to enhanced troubleshooting.

HISTORY

PBAPS is a boiling water reactor plant driving two 1,150 MW GE steam turbines; the facility produces 25 percent of PECO’s total capacity. An extensive reliability-centered analysis conducted in 1992 of the PBAPS condition monitoring systems determined that upgrades to the turbine supervisory instrumentation and motor vibration monitoring would improve the depth and quality of data needed for accurate diagnosis. The systems were unable to capture data during machine transients, and several critical, continuously running pieces of plant equipment were without any kind of vibration monitoring. Shaft-rider probes were prone to wear and provided limited vibration data, and the vibration trip logic on the reactor feed pump/turbine offered an easy opportunity for an inadvertent turbine trip.


This steam turbine rotor is instrumented to collect and feed data into Peach Bottom’s NVIS.
Click here to enlarge image

Perhaps the most important problem was that the systems were not integrated, making it difficult to correlate vibration changes to pressures, temperatures and flow levels. In 1994, plant management decided to install a networked vibration information system (NVIS); installation was completed in late 1996.

OBJECTIVES

PBAPS identified several technical objectives to increase plant safety and equipment availability:

* Optimize diagnostic capabilities by enhancing transient capture and on-line vibration monitoring, and significantly improve recirculation pump shaft crack monitoring;

* Utilize state-of-the-art sensor technology to provide increased signal integrity and more reliable data; and

* Integrate with the plant-wide data acquisition network and data base to eliminate islands of automation.

Peach Bottom’s key business objective was a payback in less than five years, representing a 20:1 cost benefit ratio. The plan identified major O&M savings and reductions in the number and duration of planned and unplanned outages amounting to more than $60 million over the 20-year planned plant life, with immediate savings of $50,000 in instrument cabling and installation costs. Many of the new cables could be installed in existing trays, reducing costs.

ARCHITECTURE

The NVIS system is integrated into the existing PBAPS Data Acquisition System Ethernet ‘backbone,’ which links various plant areas to a communications panel in the main control room (Figure 1). NVIS continuously monitors and analyzes plant systems across the existing data acquisition infrastructure, routing vibration data to the existing plant computer, and extracting related operational data. More than 270 vibration analysis channels are monitored, concurrently diagnosing 18 separate pieces of rotating equipment.


Figure 1
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The system incorporates alarm and trip contacts for the main turbine/generator and the reactor feed pumps and turbines, and collects, stores, analyzes and displays vibration data for predictive maintenance on these units plus the condensate and recirculation pumps/motors. A backup machinery monitoring system monitors the main turbines and the feed pump turbines, and provides alarm and trip functions independent of the computer network.

At the heart of NVIS are seven Solartron 1051 monitoring systems feeding data via Ethernet to the workstations. Each 1051 provides up to 64 parallel dynamic analyzer channels and 120 static analog or digital data acquisition channels, enabling it to relate machinery vibration levels to plant status and other static parameters, such as temperature, pressure and flow. This is the key feature that facilitates integration of the complete condition monitoring system, providing invaluable information during plant run-up, on-load and coast-down states. An important strength of the monitoring hardware is its ability to simultaneously acquire vibration data from multiple rotating shafts and static plant data in parallel with processing spectra. This power, combined with a built-in machine life-cycle database of critical plant equipment, simplifies fault diagnosis, helping detect and identify potential equipment problems well in advance, avoiding unnecessary shutdowns and optimizing the use of scheduled downtime.

Fiber optic Ethernet cabling, installed to ensure EMI (electromagnetic interference) immunity, also provides an open architecture networking platform and ensures high data reliability, simplified cable pulls and reduced power drain. NVIS is an excellent demonstration of the benefits of standardization: the 1051, for example, is built on a VME bus-based hardware platform, UNIX/OSF operating systems with X-Windows/MOTIF graphical interfacing, and OSI and TCP/IP networking.

Plant operators are provided with a machine-state sensitive graphical interface. The password-protected graphical environment for the vibration analyst provides several software functions-such as user-configurable alert and alarm functions, data management and a wide range of advanced analytical displays-with which to assess machine condition and diagnose problems. The ‘filtered’ orbit plot, for example, removes unwanted information, providing an uncluttered display and allowing analysts to focus clearly on specific machine problems.

The multi-plot display can highlight hidden relationships between seemingly unrelated vibration and static plant parameters, such as interactions between turbines and auxiliary equipment. For instance, when vibration problems with an outboard pump bearing were detected, the multi-plot display allowed the shaft orbits from three similar pumps to be compared side-by-side, immediately highlighting the differences in each pump.


Figure 2
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Figure 2 shows a multi-plot display that was used to troubleshoot abnormal condition measurements from a reactor feed pump turbine at Peach Bottom. One of the three feed pumps had tripped on the Unit 2 main turbine generator, causing an abrupt speed change on the two remaining feed pumps and a high vibration alarm on the turbine supervisory system. The system manager for the feed pumps used the multi-plot display to confirm that the pump was only in alarm for a very short period and that it was safe to continue operating the pumps.

PAYBACK

The first real savings with NVIS came from tracking down the cause of a repeated intermittent high vibration trip on a reactor feed pump that had been a persistent annoyance. Vibration data analysis revealed that the pump was operating too close to its first critical frequency. Next, the system provided early indication of a failing primary probe on a reactor recirculation pump, enabling proactive maintenance to be scheduled and avoiding a critical alarm-induced trip. The biggest opportunities for cost savings are in turbine restarts following maintenance outages. PBAPS’s monitoring system has enabled turbine rotor bow elimination procedures to be slashed, cutting four hours off run-up times and saving $85,000 per restart. NVIS has also expedited the ability to recognize shaft crack growth in the vertical reactor recirculating pumps, and to record and document dynamic response information for ongoing comparison.

One of the most critical benefits at Peach Bottom is the ready availability of reliable data in an immediately useable format that allows vibration analysts to resolve crisis events in a short time, enabling them to quickly eliminate the unlikely and just focus on the likely. In a recent incident, Peach Bottom’s recirculating pump set alarm tripped. Immediate examination of the vibration data showed no problem, convincing plant personnel to bring the pump back on-line and search for the problem elsewhere. Subsequent analysis eventually traced the cause to a circuitry problem on the alarm trip. This type of troubleshooting decision support, termed ‘negative diagnosis’ by PBAPS, has reduced analysis time during a crisis event by some 85 percent.

Authors-

Wade T. Mackey is a supervisor in the Maintenance/I&C department at Peach Bottom.

Roy Fenner is a product manager in Solartron’s Marketing Department.

Acknowledgement:

Ted Hopenwasser, with Solartron’s system integrator, Zeefax Inc., assisted in the preparation of this article.