By Steve Blankinship, Associate Editor
The need to adhere to strict NOx emissions is forcing power plant operators to make sure fuel mixtures fall within tight parameters.
As they strive to meet optimum NOx emission levels, combustion turbine (CT) power plant operators need a system to monitor pressure frequencies in turbine combustion chambers. “When you try to get the mixture just right in combustion turbines, you end up running them very close to the point where dynamics in the combustion chambers can cause problems,” says Doug Dieruff, applications engineer for Intech, supplier of combustion pressure monitoring systems. Combustion dynamics monitoring reports conditions inside the chambers to the power plant control system and alerts the control room operator if a potential problem exists. The challenge is to provide a reliable system for continuously monitoring turbine combustion pressure. Such combustion pressure monitoring systems typically consist of PC-based data acquisition (DAQ) interfaces controlled by LabWindows/CVI.
Although the newest combustion turbines come equipped with combustion pressure monitoring, the majority of combined cycle plants in operation must have systems added on. Their purpose is to allow the utility to constantly monitor the pressure dynamics within the combustion turbine and use that information to extend turbine life and stretch the intervals between plant maintenance. “We find that when they can monitor the precise dynamics occurring inside the turbine and can see that things are fine and performing well, companies can confidently make the decision to push a planned outage out for a month or so if conditions warrant,” says Dieruff. “These systems allow you to have that flexibility, which in today’s energy market, can be very valuable.”
Dieruff says that if a CT is operated a few hours a year, such monitoring is of negligible value. “Combustion monitoring can provide extremely useful information for plants running longer hours in the spring and fall when you have a good swing in temperature between day and nighttime. When you start getting swings in air temperature and density that affect the combustion, fine tuning gets to be very complicated.”
He explains that when fuel is adjusted for maximum NOx control, air temperature swings and density changes can cause the flame to sputter. This may result in high pressure dynamics. Pressure can get bad enough to cause damaging vibration resulting in cracked linings and components.
Over the past 10 years, Intech has installed combustion monitoring systems on more than 60 GE and Siemens-Westinghouse units in the United States. They are currently operating in plants as far south as Florida, as far north as Rhode Island and as far west as Texas. From a utility’s system operations center, the company’s designated combustion turbine expert can look at all CTs operating in the fleet and see the dynamics occurring within their combustion chambers. “That person can be on the phone with someone at the plant telling them how to tweak things,” says Dieruff. “You can have one expert watching over all CTs from one location and making sure the dynamics are acceptable.”
Although potential turbine-damaging dynamics can occur as a plant operator tries to adjust the fuel/air ratio without the benefit of such a system, Dieruff stresses that the systems themselves don’t tell a utility what to do or how to operate their CTs. “The combustion monitoring system collects data, processes it and reports the dynamics being detected. It’s up to the company to interpret that data. Then the operator uses it to tweak the conditions in the combustion chamber based upon his own knowledge.”
Combustion monitoring requires little maintenance itself. Since the combustor heat would destroy the transducer if it were placed at the combustor, a tube runs from the combustor port to the transducer. Condensation can build up in the tubes and coils associated with the transducer. Because of this, nitrogen must be periodically blown through the system to remove the moisture. “We have an option in our system that provides an automatic purge allowing the operator to determine a time each day when he wants the purge to occur,” says Dieruff. “And for 10 seconds it will blow air through the line.”
He says that most of the GE turbines on which Intech installs systems are 7FAs. Since the newer 7FAs have built-in dynamic monitoring there is no market for the newest models. But with most CTs having been built over the past 20 years, hundreds of units may lack pressure dynamics monitoring. Pressure dynamics monitoring can be installed on any CT as long as a port exists on the combustor. The cost for a single 7FA unit would be $30,000 to $50,000. The price of the system is based on the turbine type, whether or not the site already has transducers in place and which system options are selected.