Electrical Systems / Energy Management

Keeping air filters clean is the key to energy efficiency

In this installment of What Works, off-site management and analysis helps power plant increase efficiency.

Centro Energia Teverola started operations at its 150-MW cogeneration combined cycle power plant in 1998. The plant in Teverola, Italy, has two gas turbines, two heat recovery steam generators and a steam turbine.

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Although the plant ran smoothly for the most part, filters on the gas turbines underwent unexpected performance degradation from clogging. These filters clean the inlet air to the turbine’s compressor section.

The inlet air must be as clean as possible, particularly as such large volumes of air are passed through the filters, to prevent compressor blade fouling, which has a large effect on performance. Impurities such as dust otherwise build up quickly in the compressor and degrade the polytropic efficiency of air compression.

“Filter clogging often resulted in emergency downtime to install replacement filters, which was especially costly during peak production periods,” says Vincenzo Piscitelli, general manager at Centro Energia. “When the filters clogged, the plant almost always was forced to shut down a turbine because of the serious performance degradation caused by high-pressure drop across the filters.”

Centro Energia and Emerson Process Management had been collaborating on plant performance analysis for several years. Initially, Centro Energia wanted to understand what benefits Emerson could provide to improve plant operations. When the filter problem became significant, Centro Energia engineers visited an Emerson client in England, which already had successfully adopted the AMS Performance Monitor. This trip eliminated doubts about the benefits that could be obtained.

After cleaning, the data goes through a first-principles thermodynamic model, which has been tuned to represent that particular asset with its unique characteristics and behaviors.

To solve the filter problem and to improve overall performance, Centro Energia installed an AMS Performance Monitor system remote analysis of performance data on two gas turbines, two heat recovery steam generators (HRSGs) and one steam turbine. “After analyzing the data remotely, Emerson could inform operators when problems existed,” explains Piscitelli. “Remote analysis would allow us to schedule filter replacements at a more convenient time, such as during low-demand periods.”

The AMS Performance Monitor provided a rapid return on investment. A single filter change, scheduled proactively instead of reactively, paid for the leased AMS service for two years.

Remote monitoring and analysis

AMS Performance Monitor takes data from the Centro Energia historian, so it’s completely independent of the plant’s control system — a legacy Bailey Infi-90 control system — and its instrumentation. The AMS Performance Monitor typically uses as inputs the hourly averaged values from the historian.

For the gas turbines, AMS monitors several variables, not just filter pressure drop, to provide a range of benefits.

  • Inlet temperature — This is needed to correct the analysis to normalized performance, so comparisons are based on known conditions.
  • Inlet pressure — This also is used to correct to normalized conditions.
  • Shaft power — This is a measurement of the gas turbine’s power output from the associated generator.
  • Shaft speed — This generally is assumed to be constant for electricity-generating turbines.
  • Fuel flow — The system measures fuel flow to the machine in energy units. If mass or volume units are preferred, assumptions about fuel properties can be specified.
  • Additional measurements — Air flow, compressor temperatures and other variables provide for more in depth and comprehensive analysis.

The historian provides similar data for the two heat recovery steam generators and the steam turbine.

“The Emerson data collection tool formats the historian data into Excel files, which are then transmitted by e-mail to Emerson’s Performance Centre in Teesside, England,” explains Piscitelli. “For security reasons, no explanatory information is transmitted with the values. Instead, the data is stripped of its units of measurement. The team in the UK knows the proper data format, so it can perform the analysis unhampered.”

Monitoring gas turbine variables by extracting data from the plant’s data historian allowed remote comparison of live data to a computer model to determine when filters needed service.
Monitoring gas turbine variables by extracting data from the plant’s data historian allowed remote comparison of live data to a computer model to determine when filters needed service.

On receipt in Teesside, the data undergoes a thorough analysis and cleaning, which involves automatic and manual steps. Initially, data is inspected manually for obvious corruption, blank data and other problems. The data is then put through an automated SPC-type analysis to identify missing and suspect data points (outliers or data marked as bad). If necessary, agreed values, such as last good or mean values, can be substituted for missing values, or data can be re-collected.

Modeling the plant

After cleaning, the data goes through a first-principles thermodynamic model, which has been tuned to represent that particular asset with its unique characteristics and behaviors. Emerson’s engineers in Teesside develop these ASME models for each machine.

“Running data through the appropriate model identifies further data problems, such as instrumentation inaccuracies,” says Piscitelli. “This provides a check on field instrumentation effectiveness and identifies malfunctions or calibration that otherwise would be impossible to detect.”

By monitoring turbine variables and combining them with thermodynamic models, Emerson provides an analysis of filter performance and other operating characteristics. Emerson also monitors overall thermal efficiency before and after a cleaning or maintenance event to determine how effective maintenance has been.

Temperature spread charts on the exhaust of the gas turbines determine how well the heat is spread throughout the unit. This is an important factor in improving overall efficiency and ensuring long machines life by reducing hot spots.

Emerson monitors thermal efficiencies separately during peak and off-peak periods, as well as overall thermal efficiency. This is useful so the plant can aim at being more efficient during peak periods, even if it’s not possible at all times. Experts at Teesside analyze the output from the thermodynamic model and other sources and provide a written summary report. The report highlights problems, unusual items or significant degradation. The report also includes likely causes and suggestions for resolution and improvement. Detailed results, charts and analysis are published to the Centro Energia dedicated secure website for immediate viewing. “Because they’re browser-based, our staff, no matter where they are, can view the information immediately as long as they have their personal secure accounts and passwords,” says Piscitelli.

The UK center contacts Centro Energia either via telephone or email if the analysis reveals urgent problems. In addition, the website provides a red/amber/green alert system that indicates overall asset condition on its summary page. Alerts also can be forwarded automatically to Emerson’s AMS Asset Portal for additional annunciation. Emerson provides periodic performance reports and results based on key performance indicators displayed graphically. Graphs and reports are placed on the website, accessible with a login and password.

“At one point, we had considered a traditional asset monitoring system, with software installed on-site,” says Piscitelli. “The big difference between remote analysis and on-site software was the experience of Emerson’s Teesside people. An on-site software package alone doesn’t come with such support and benefits.”

Significant savings

AMS monitors the gradual filter performance deterioration and calculates the cost of the resulting reduction in turbine performance. By comparing this with the cost of the required maintenance, Centro Energia can determine the most appropriate point to replace the blocked filter. Maintenance is now scheduled for periods when energy production is less profitable and when penalties for being off-line can be avoided. This improved planning enables Centro Energia to reduce average repair times from seven hours to two hours.

Particularly interesting were the results obtained from filter wash optimization. This is important because Centro Energia is promoting a culture of efficiency. “It’s necessary that employees work together to achieve a common goal,” explains Piscitelli. “Therefore, everyone must be aware of what the company is doing and why. For example, if a production manager is directed to make a wash, he might give that task a low priority. But if the amount of benefit is known, then it can become a priority for improved machine efficiency.”

While the main purpose of the AMS Performance Monitor was to detect filter problems, another benefit was that overall plant efficiency increased by 1%. In a 150-MW power plant, a 1% improvement is quite significant. Centro Energia generates 120 million euros worth of power every year, so 1% represents a savings of about 1.2 million euros.

The AMS Performance Monitor is a service that Emerson supplies for a monthly fee, which is based on the number and types of assets managed. Once the lease agreement expires, it can be cancelled. “We intend to continue using the AMS Performance Monitor, because in its first two years of operation it achieved all of our objectives,” says Piscitelli. “It also became obvious that additional major improvements are possible. We realize that we must become even more efficient to compete in the future, so continued investment in AMS Performance Monitor is essential to extending the life of the plant.”

Plans are to monitor other assets and variables more thoroughly on a variety of rotating and fixed equipment including gas turbines, steam turbines, compressors, heat recovery steam generators, boilers and heat exchangers. “We expect our experiences will be similar to the current system, with significant savings realized from expert analysis of key operating variables,” says Piscitelli.