Most of the process industries and power plants use steam as an operating medium for various applications. The thermodynamic properties of steam make it an effective heat transfer medium in a variety of heating applications, including high-temperature and heat-treatment processes. In thermal or geothermal power plants, steam is the working fluid for steam turbines. During the generation, transportation, and utilization in processes, the heat capacity of steam falls, resulting in a rise in moisture content and a decrease in steam dryness. As the quantity of moisture in the steam grows, both scaling and erosion have a negative influence on the process efficiency. Additionally, the flow of saturated steam with low dryness in the steam line results in water hammer, early failure, and overall poor system performance, ultimately negatively affecting revenue streams. As a result, it is critical to developing methodologies for detecting steam dryness to ensure the effective functioning of steam-based operations in process industries and power plants.
There are two methods for measuring dryness: (I) Sample-based (separating calorimeter, throttling calorimeter) and (II) Online (Spectroscopy). Sample-based measurements require time to achieve thermal stability once the operating condition changes from one state to another. Hence, it is not a suitable technique where frequent data reporting is required. Online measurements can be used for the measurement of a steam parameter in transition states with high accuracy. Potential online measurement methods explored are based on optical absorption and scattering spectroscopy. These methods are limited to a range of dryness fraction and operating pressure. The absorption-based technique is used for water vapor concentration measurement in atmospheric and remote sensing. The scattering-based sensing is used for the measurement of water droplets. The optical methods for dryness measurement are based on either absorption or scattering or both techniques. The limitation with the existing techniques is either due to interference from other absorbing or scattering species at the absorption transitions or limited by the operating conditions. The scattering-based water droplet concentration becomes more complex and challenging in the case of low scattered intensity variation. Therefore, a method is proposed which is independent of scattering intensity variation and eliminates the interference from the other absorbing molecules. The method is based on scanned wavelength spectroscopy using Tunable Diode Laser (TDL) for dryness measurement. The system is designed, implemented and tested for measurement of the water vapor concentration in real-time from saturated steam. A thermally insulated steam cell is designed and fabricated with optimum optical path length for the measurement of the range of dryness at different pressure. The dryness of saturated steam is calculated from the water vapor concentration and total mass of steam present in the steam cell for a specific pressure. The accuracy of the steam quality measuring system using the optical method is determined to be ±1% of the true value in the range of dryness and low pressure of the steam. The measurement technique and accuracy are limited due to the pressure broadening of the water vapor transition line at high-pressure saturated steam. A new method for dryness measurement is proposed based on Wavelet Transform, which can eliminate the limitations in TDL based measurement system. An experimental study has been carried out based on wavelet transform for the concentration measurement of individual absorbing molecules in the mixture of SOx and NOx gas using a broadband UV Lamp. These molecules absorption characteristics are broadband in nature. The measured concentration using this technique shows good linearity with input concentration. Therefore, further studies are being carried out for dryness measurement based on the wavelet transform.