Wet Steam Measurement and Boiler Efficiency

Chemical processing plants typically convey energy to their processes through saturated or superheated steam containing high thermal content. Steam is generated in boilers by heating and pressurising water until it transitions to the vapour phase. It is then distributed via pipelines to every point in the facility where it is needed. Flow measurement is carried out on steam lines to determine the volume of steam and consequently the amount of energy it carries. For this measurement to be accurate, it must be expressed in mass terms. Two common measurement principles are used in steam flow measurement: Differential Pressure and Vortex. Vortex flowmeters are so closely associated with steam that they can also be referred to as steam meters. The amount of energy carried by the steam in the line is actually directly related to its dryness fraction. However, one of the most common errors made is that facility operators assume their steam is completely saturated and 100% dry at every point. Although this can be accepted theoretically, operational reality does not support this assumption. For this reason, it is necessary to have information about steam moisture content in order to determine the production efficiency of the steam boiler and the actual amount of energy carried by the steam. For example, the amount of energy carried by saturated steam at 11 bar with a dryness fraction of 100% is 2000 kJ/kg. As can be seen in the images below, as steam becomes wetter, there is a significant drop in the amount of energy it carries.

Endress+Hauser Prowirl F 200 Vortex Flowmeter

The Endress+Hauser Prowirl F 200 Vortex flowmeter is capable of measuring steam dryness fraction in the range of 80% to 100%. This feature was added to the device following a special experimental study conducted in Switzerland. Additionally, when the dryness fraction drops below 80%, an alarm can be triggered from the device. With this feature of the Prowirl F 200, flow measurements performed are converted into accurate and reliable data. As stated at the beginning of the article, steam measurement must be expressed in mass terms. For this reason, volumetric measurements must be converted to mass flow through temperature and pressure compensation. For saturated steam, compensation using either temperature or pressure value is sufficient. For superheated steam, both values are required. Since the Prowirl F 200 has a built-in temperature sensor, it performs temperature compensation directly. Additionally, it has an internal 4-20 mA input for the pressure compensation required in superheated steam measurement. By connecting the output of the pressure device to this 4-20 mA input, pressure compensation can also be performed directly through the Prowirl F 200. Since steam represents energy for facilities, efficiency in steam production processes is of critical importance. However, many facilities do not have the systems to collect data from the field that would allow them to evaluate the current status of their equipment and analyse it. When considering the savings these systems would provide, it becomes apparent that investment costs are recovered in a short period of time. In a steam production system, the boiler is the most critical equipment. For this reason, boiler efficiency must be monitored closely as a priority. Boiler efficiency calculations can be performed using the RSG45 calculator device. The primary measurements required for efficiency calculations can be listed as follows: inlet fuel flow rate, inlet water temperature, steam flow rate and temperature. By increasing the number of measured parameters, the accuracy of the efficiency calculation is also increased. These measurements are sent to the calculator and boiler efficiency can be calculated as a percentage. Using this data, information is obtained regarding whether operations are proceeding normally in the boiler and predictive maintenance options, among other information. Cem Özen / Industry Manager / Endress Hauser A. Ş.