- By Scott A. Rouse
- Automation Basics
Know your options when selecting the appropriate flowmeter technology to measure natural gas, water, and steam in power generation.
In many chemical plants, the electricity the plant uses is derived from a natural gas power plant or a cogeneration plant burning waste gas streams. In large boilers (figure 1), power plants bring together air and fuel (natural gas, waste gas, oil, or coal) for combustion, which creates heat. The heat boils the water, creating steam. The steam runs through a turbine, which causes the turbine to spin, thus generating electricity.
Measuring the flow energy—flows of fuel that cost money—in these boiler applications is critical for improving energy efficiency, identifying waste, and minimizing the greenhouse gases (GHG) going into the atmosphere. Only with accurate flow measurement can users make informed decisions to improve energy efficiency.
How do users decide which flowmeter technology is best to measure the gas, water, and steam for boiler applications? Choosing the right flowmeters depends on the fluid being measured. When discussing boiler efficiency improvements, three primary applications are involved:
- Accurate inlet air and fuel (natural gas, waste gas, oil, or coal) measurement for efficient combustion.
- Inlet feedwater measurement to determine steam production efficiency and to identify waste.
- Measurement of outlet steam production.
Optimize fuel-to-air ratio
Power generation requires inlet air and fuel for combustion. Engineers must measure the air and gas ratio accurately for efficient combustion in the boilers. Too much gas is wasteful, dangerous, and costly; too little creates insufficient flame to boil the water efficiently.
Orifice and turbine meters. Traditionally, monitoring fuel gas to the boiler units is accomplished with an orifice or turbine meter. However, these are not the best measuring devices for this application because they are subject to failure and require frequent skilled maintenance to provide an accurate and reliable measurement. Constrained piping conditions also can give engineers headaches. For example, an orifice meter requires 10 to 50 diameters of upstream piping to eliminate the effect of flow disturbances. Because long straight pipe runs are hard to find, most flow measurement systems are affected adversely by varying flow profiles within the pipe.
The biggest cause for concern is that orifice and turbine meters measure volumetric flow. Additional pressure, temperature, and differential pressure sensors, as well as a flow computer, are required to calculate or infer mass flow (figure 2). This not only degrades the flow measurement accuracy, but the installation and maintenance costs with this type of compensated measurement increase the cost of ownership.
Thermal mass flowmeters. In contrast, thermal mass flowmeters are suitable for direct mass flow measurement of gases, not volumetric flow. Because thermal mass flowmeters count the gas molecules, they are immune to changes in inlet temperature and pressure, and measure mass flow directly without compensation. In inlet air and gas flow boiler applications, thermal flowmeters perform well because the optimal fuel-to-air ratio for efficient combustion in boilers is calculated on a mass basis, not a volumetric one (figure 3).
In a thermal flowmeter’s simplest working configuration, fluid flows past a heated thermal sensor and a temperature sensor. As the fluid’s molecules flow past the heated thermal sensor, heat is lost to the flowing fluid. The thermal sensor cools down, while the temperature sensor continues to measure the flowing fluid’s relatively constant temperature. The amount of heat loss depends on the fluid’s thermal properties and its flow rate. By measuring the temperature difference between the thermal and temperature sensors, the flow rate can be determined.
New developments in four-sensor thermal technology, coupled with stable “dry sense” sensor technology as well as advanced thermodynamic modeling algorithms, enable some thermal flowmeters to attain ±0.5 percent reading accuracy, rivaling Coriolis flowmeter accuracy at less cost. Onboard software apps also enable gas-mixing capability, in situ validation, and dial-a-pipe.
Measure inlet feedwater accurately
Water is also an expensive flow energy and limited resource. In boiler applications, it is important to measure the inlet feedwater flow to the boiler accurately, because users need to measure the efficiency at which the boiler turns this feedwater into steam (figure 1).
Clamp-on ultrasonic flowmeters. Although users could measure inlet water with a volumetric vortex flowmeter, clamp-on ultrasonic flowmeters are ideal for water flow applications due to their ease of use and application flexibility. They achieve high accuracy at low and high flows, save time with no pipe cutting or process shutdown, and are not affected by external noise. Advances in ultrasonic technology now have onboard software and apps that make the meter easy to install, providing a visual signal that it has been done correctly.
Optimize steam production
The boiler’s steam must be measured accurately to determine whether the boiler is producing the expected amount of steam or needs to be tuned for increased efficiency (figure 1). Traditionally, steam flow has been measured with a differential pressure device, typically an orifice plate.
However, such devices are inherently volumetric flow measurements. Changes in pressure and temperature will change the steam’s mass flow rate. Even a “small” change of 10 percent in steam pressure will result in a 10 percent error in noncompensated mass flow. This means that, in a typical differential pressure measurement installation, the volumetric flow rate must be compensated by measuring temperature and pressure. These three measurements (ΔP, T, and P) are integrated with a flow computer to calculate mass flow.
Insertion multivariable vortex flowmeters. Insertion multivariable vortex flowmeters measure steam output production from boilers more accurately. One insertion vortex flowmeter with one process connection measures mass flow rate, temperature, pressure, volumetric flow rate, and fluid density simultaneously. Saturated steam’s density varies with either temperature or pressure, while superheated steam varies with temperature and pressure, so multivariable vortex flowmeters ensure the flowmeter’s density calculations are correct, and therefore, the mass steam flow measurements are correct.
Case Study: Thermal Flowmeters Improve Boiler Efficiency at a Chemical Plant in China
Purified terephthalic acid (PTA) is the precursor to polyethylene terephthalate (PET), the ubiquitous material used worldwide in plastic bottles, textiles, and elsewhere. A PTA chemical plant in China generates steam and electricity from its on-site power plant using coal as a fuel. It also has a wastewater treatment station that produces methane, which was flared off. Both processes are major GHG emitters.
New government regulations required the company to reduce its CO2 emissions. The plant decided to modify its four boilers to burn both coal and the previously flared-off waste gas (methane), for a savings of approximately $0.5 million in coal each year. Working with a single-source supplier, engineers reworked the boilers’ designs and installed industrial insertion thermal flowmeters to measure its combustion air and waste gas fuel.
One thermal flowmeter measures the waste gas flow, while the other four thermal flowmeters provide submetering of this gas stream to each boiler. Another four meters measure preheated (200°C, 392°F) combustion air to each boiler, allowing the boiler control system to optimize the fuel-to-air ratio. The flowmeters provided both precision flow data for complying with government regulations and helped the company reduce waste while increasing efficiency.
Other potential metering applications are under review, including:
- Feedwater to the boilers using clamp-on ultrasonic flow: Because this is a preexisting feed piping system, a clamp-on ultrasonic meter is a flexible solution.
- Steam flow measurement: Measurement of steam flow delivered from the boilers to the turbine generator and submetering to the other plant processes.
All images courtesy of Sierra Instruments
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