On-line energy monitoring in a control system

An important component in improving process performance and plant profitability is the exact measurement of purchased energy and the utilization of energy within the process. Such information should be available in real time to gain maximum benefit from this information in day-to-day operations. An accurate gas measurement based on the AGA or ISO standards is often the basis for accurate account of purchased energy. Also, the enthalpies to an accuracy of 0.1% are needed for steam and water at process operating pressures and temperatures to perform energy balances and evaluate the efficiency of unit operations. The effort and time required to engineer and implement such online calculations may be significantly reduced if gas, steam, and water property algorithms are embedded within the DCS or control system itself. In this paper, an example is provided of how these calculations have been implemented as function blocks in a modem, scalable process control system. Details of the nine function blocks for gas, steam, and water properties are provided with examples of how these may be used to address typical energy monitoring and calculation applications.