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Flue Gas Flow Measurement Techniques : Week 2/1/2013

In our last article, we discussed about Flue Gas Flow Monitoring in any variety of Boilers. In this article, we will discuss about available techniques of measurement & also about new upcoming techniques .

What are the available techniques?

The commonly used gas flow monitoring techniques are :

  • Ultrasonic Pulse Detection
  • Differential Pressure
  • Thermal Detection

In all these four techniques, the flow monitors (including controller,pressure & temperature transmitter) are stack or duct mounted

1) Ultrasonic Flow Monitors :

In case of Ultrasonic Flow Monitoring, the volumetric flow rate of stack gas is measured by transmitting ultrasonic pulses across the stack in both directions. The time required to traverse the distance of the stack traveling with and against the flow is a function of the sound velocity and the effluent velocity. The angle at which the ultrasonic pulses need to traverse the stack or duct is important & is critical to the success of this type of measurement

2) Differential Pressure Flow Monitors :

The flow meters that work on the principle of measuring differential pressure: include S-type pitot tubes, Dual-manifold pitot probe, and annubars. All these differential pressure flow rate systems comprise of an electronic, flow-indicating transmitter that receives pressure and temperature signals from the stack, calculates the exhaust gas flow rate, and automatically performs electronic drift checks and system purging. These are lower in terms of cost, but not so good in terms of measurement accuracy, and are susceptible to signal distortion and failure. One of the main limitation is the possible need to include valves between primary & secondary devices to facilitate removal of secondary devices for repair or replacement.

3) Thermal Flow Monitors :

These flow monitors measure the electric power required to maintain a constant temperature of around 24 to 38 Deg.C. above the exhaust gas temperature in a flow sensor. Both single-point & multi-point analysis options are available in the market. However, the use of these types of flow meters is limited to clean, non abrasive fluids. Presence of moisture or droplets can lead to measurement inaccuracy. It is essential to know thermal properties of the gases, as variation from the calibrated values can result in inaccurate readings.

Are there any new technologies available?

There are new techniques coming up & are becoming more popular due to the reliability, ease of operation & decreased maintenance. This latest generation of unique flow monitors provide non-contact measurement of gas velocity using a time-delay correlation of flue gas infrared emissions received by two detectors placed at a fixed distance apart.

In our next article, we will discuss more about this new technology.