Combustion Air Flow Monitoring at Yiyang Power Plant in Hunan Province, China
Winters in China can be bone-chillingly cold with a thick haze hovering in the sky on most winter days. The haze is there because most of China’s power plants use coal as fuel.  Driven by stricter government regulations, new coal-fired power plants are regulated to be as clean and efficient as possible and many must reinvest in improved instrumentation in new construction projects.  In order to meet these increasingly more stringent environmental control regulations, the Hunan Electric Power Design Institute designed a state-of-the-art coal-fired power plant in Yiyang City, Hunan Province called Hunan Yiyang Project Two, a substantial improvement over Project One built ten years earlier.

In coal-fired plants, combustion air is pre-heated to temperatures approaching 500 °F (260 °C) prior to being mixed with pulverized coal and burned in the boiler to produce steam for electricity.  Adhering to the standards previously used in coal-fired plants in China, Hunan Yiyang Project One used annubar-type flow meters for air flow measurements.    However, use of these annubar flow meters had proven to be problematic. As you might imagine, the combustion air is heavily laden with coal dust.  The small holes in the annubar flow meters were prone to plugging and, once the holes started plugging, meter performance and accuracy began to degrade.  These blockages led to increasing imbalances in the fuel (pulverized coal) to combustion air ratio, which in turn led to incomplete combustion, decreased efficiency and increased emissions.  Yiyang Power Plant One had difficulties with maintenance, customer service, and accuracy.

Sierra Delivers Customized High Temperature Immersible Thermal Instrumentation
In 2007, Erwang Song, Managing Director of Sierra Asia, approached the engineers designing Yiyang Project Two explaining the virtues of thermal technology. Not being familiar with thermal technology, the engineers were hesitant to take a chance even though their annubar technology was fraught with problems.  Erwang Song and his team visited the engineers at the plant on several occasions to explain the virtues of thermal technology--direct mass flow measurement of combustion air with one device, a wide 100 to 1 turndown, high accuracy and    a self-validating capability that leads to lower maintenance costs.

Finally convinced, the engineers at Hunan Yiyang Project Two decided to give Sierra’s Immersible Thermal Mass Flow Technology a try.  One of the major design challenges in this project was that the combustion air in this plant was pre-heated to temperatures approaching 500 °F (260 °C).  Traditional thermal meters are not designed to handle this type of heat.  Sierra Asia and Sierra North America banded together, partnering closely with the engineers at Hunan Yiyang Project Two, to design a high-temperature version of Sierra’s Immersible Thermal Mass Flow Meter.  Within 6 months, Sierra’s team had completed a customized product to meet the needs of the project and had delivered and installed six meters for testing.  Pleased with their performance, engineers at Hunan Yiyang Project Two placed their first order for 32 high-temperature Immersible Thermal Meters. 

Sierra Asia’s Customer Service Wins Business in China
When their 32 new Thermal meters arrived at Yiyang Project Two, the engineers were at a loss as to how to install and set-up the meters.  Realizing that the engineers had only installed annubar meters and were unfamiliar with thermal technology, Erwang and his team visited the Yiyang plant, installed all 32 meters for the engineers, and trained the engineers in how to operate and validate the meters.  Michael Ma, a Sierra-Asia Applications Engineer, visited the Hunan Yiyang power plant at least ten times over the course of 2007, an unprecedented level of support.  “We sent our distributors to the field.  It gave our customers confidence,” explains Erwang. “Sierra works in the field not in the office. We take responsibility. It’s the Sierra culture.”

With Project Two now on-line, the advancements over the Project One design became clear.  At Yiyang Project Two, combustion air is routed through a series of pre-heaters. By using otherwise wasted heat to pre-heat the air used in the boiler, greater efficiencies were obtained.  Additionally, all air flows are measured and balanced using high temperatureImmersible Thermal Mass Flow Technology, resulting in more precise combustion control and thus far lower emissions from the cleaner-burning coal.

Dry-Sense™ Technology: The Deciding Factor
In evaluating immersible thermal flow meters for use in Project Two, one clear Sierra advantage stood out to the engineers at the Hunan Electric Power Design Institute: the use of patented Dry-Sense™ Technology in Sierra’s Model 640S HT flow meter. 

Most manufactures of thermal flow sensors use an epoxy or glue to hold the sensing element into place in a thermal flow sensor’s thermo well.  The unfortunate result of velocity sensors designed with epoxies or glues is that they can expand or shrink over time, leading to the formation of air pockets or cracks.  This changes the flow calibration, resulting in what is commonly called “flow drift”. This means recalibration.   Immersible thermal mass flow meters using glues in the velocity sensor generally require an annual factory recalibration since drift is on the order of 1% of reading per year at a minimum.  Calibration in the factory is required because the meter needs a sophisticated flow loop to insure accurate calibration.

Sierra’s patented Dry-Sense™ uses NO epoxies or glues, instead using a proprietary swaging process to press the velocity sensor into the thermo well.  This leads to a very stable sensor that does not require frequent recalibration.  Sierra’s sophisticated calibration laboratory, coupled with the unique Dry-sense™ Technology and a  field validation program supplied the Project Two engineers with a very stable, low maintenance flow solution and ultimately led to the specification of Sierra’s Model 640S into Yiyang and other future projects.  “Sierra is much better than our competitors at maintaining accuracy with Dry Sense™.  Others can’t maintain. We can maintain,” explains Erwang Song. With this lower maintenance, customers don’t need to send the instrument back to the manufacturer, sometimes located in Europe or the US, for calibration or repair.  Engineers at the plant can use the self-validation features to do their own troubleshooting and maintenance rather than spending countless dollars on sending the instrument for the factory for maintenance and wasting hours of production time.

Sierra’s 640S High Temperature Mass Flow Meter Sets the Standard in China’s Coal Fired Power Plants
Due to the excellent performance exhibited over the last two years at the Yiyang plant, and Sierra-Asia’s unparalleled customer support, China’s Power Plant Design Institute has specified Sierra’s Steel-Mass Model 640S High Temperature Thermal Mass Flow Meter as the instrument of choice for measuring pre-heated combustion air flows in China’s coal-fired power plants.

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