DCS Real-Time Calculations of Specific Work Performance
By Tom McAuliffe – POWER Engineers
and Juan Escalante and Josue Hernandez – POLARIS Energy Nicaragua
This paper describes the real-time continuous online Distributed Control System (DCS) calculation, display and trending of a number of steamfield and plant performance parameters. The calculations begin with a mass and energy balance around the steam separator in order to calculate the reservoir enthalpy and temperature. Next, an equation for calculating the efficiency of the condensing steam turbine is developed. Then formulas for the ideal and real turbine specific work are established along with their related optimal flash pressures for maximizing the plant’s gross output. Finally, the effect of changes in the condenser backpressure on the plant’s gross output is determined. These calculations have been implemented in the POLARIS Energy Nicaragua single-flash 72 MW (net) geothermal power project DCS. The essential lesson is to maximize the worth of the owner’s investment by implementing calculations in the DCS, which provides value to the operators, engineers and plant management. Note: most DCS platforms today have steam property algorithms available as standard function blocks.
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