In 2010, AES Alamitos’ Units 3 & 4 boiler circulating pumps and Units 5 & 6 exciters were identified as having reliability issues. These machines were never equipped with vibration probes and only the metal temperatures of the exciter pedestal bearing and common lube oil drains were monitored by the control room recorders. Due to low cost of implementation and software compatibility, it was decided to purchase and install a monitoring system (see Figure 1) to improve the reliability of the AES Alamitos Units 3 & 4 boiler circulating pumps (see Figure 2) and the Units 5 & 6 exciters.
Figure 1: Monitoring system
Figure 2: Boiler circulating pump
BOILER CIRCULATING PUMPS
The vertical boiler circulating pumps have a carbon coated bearing and seal that are cooled and lubricated with water. As the seal fails due to normal wear, it allows hot boiler water to enter the pump bearing, in turn causing it to fail (see Figure 3). The failure mode causes the pump’s upper bearing to have a subsynchronous vibration (see Table 1). In the past, the seal failure (see Figure 4) would go undetected until the motor’s liner, which separates the windings from the pump, ruptured. The rupture would ground the motor, causing a catastrophic motor failure. The repairs would take up to nine weeks at a cost of over $150,000 per pump/motor. With the new monitoring system, they can now detect an impending bearing seal failure at its earliest symptoms and shut down the pump/motor before it ruptures the Inconel liner. The pump can then be removed and the seal and bearing repaired in less than three weeks for a total cost of $30,000, a savings of about $120,000 per pump.
Figure 3: Bearing melted carbon coating
Figure 4: Pump impeller seal/bearing wear area
HP & LP EXCITERS
The Units 5 & 6 HP and LP exciters (see Figure 5) are directly coupled to the turbine/generators with a reduction gearbox using a ratio of 5 to 1 and 2 to 1, respectively. These exciters were not equipped with a monitoring system other than lube oil temperatures wired to recorders in the control room. Some of the reduction gearbox bearings are not accessible with the data collector due to the placement of the exciter housing. Recent catastrophic failures of these units caused major outages with the resulting repairs and loss of unit availability costing over $1 million. With the success of the monitoring system on the boiler circulating pumps, it was decided to place one system (see Figure 6) on each exciter and gearbox. Both the vibration and temperature are monitored with dual mode accelerometers. A new problem was first identified when starting up the new monitors. The vibration was instantly in alarm with the captured spectrum, indicating mechanical looseness on the exciters. During the next outage, the exciter base bolts were checked and found to be loose, saving the company another costly outage and repair.
Figure 5: Exciter bearing accelerometer
Figure 6: Monitor being wired
DATA PATH TO ROSEMOUNT CMMS AND PI HISTORIAN:
The monitors use an Ethernet connection and IP address to communicate with the software. The software is used to define the measurements taken for each input channel and create frequency bands that trend the vibration of critical frequencies representing machine faults. For example, on the boiler circulating pumps, a subsynchronous band is monitored. The subsynchronous band usually trends with low amplitude until the pump seal begins to fail. Any of the bands monitored along with their alarms are automatically assigned a Modbus address in the monitor. Using the Modbus TCP protocol, this data can be accessed through the same Ethernet connector that is used by the software to communicate with the monitor. In our case at AES, we use a server to retrieve the band data via Modbus TCP from the monitor on the different units. The band and alarm data is then moved to an OPS server where the Rosemount CMMS and PI Historian (see Figure 7) systems pick up the data for archiving and analog display in the control room. This provides us with a clean and efficient method of getting data from the monitor to the plant control environment.
Figure 7: PI Historian – Unit 4 boiler circulating pumps in operation
A BRIEF SUMMARY OF THE MONITORING SYSTEM
We are using Ludeca supplied Pruftechnik VIBNODEs with Omnitrend software for the online monitoring system. Our software serves as the platform for our portable data collection program, as well as our online vibration monitoring systems. The monitoring systems are connected to the Alamitos LAN system via the plant’s Ethernet. The software can display trends, spectrums, and overall frequency band specific alarms. The Units 5 & 6 exciters’ vibration probes also have temperature outputs. As previously mentioned, the band amplitudes and associated alarms are displayed on the Rosemount CMMS or PI Historian screens. Monitoring software can be installed and viewed on any plant computer by users with appropriate access rights.
To date, AES Southland has successfully saved six boiler circulating pumps at Units 3 & 4, generating savings of over $100,000 each. In addition, it has identified two serious vibration problems on Units 5 & 6 exciters with preventive savings in the millions of dollars for each event. AES estimates a potential overall savings since 2010 of about $2.6 million on a capital outlay of under $75,000.00. Not a bad return on investment!
James J. Cerda, Southland Technical Services, has 31 years experience in Power Generation. He joined AES in 2000 at AES Alamitos. Since joining AES, Jim has held the positions of Control Operator, CBM Technician and Reliability/Performance Engineer.
Greg Lee is a Senior Application Engineer at LUDECA, Inc. Since the 1980s, he has worked in the field of vibration and field balancing, running programs for IRD, SKF, Prüftechnik and LUDECA. His activities have included product development for hand-held and online hardware and software. www.ludeca.com