Water in a thermal power plant is a crucial element, not only as the working fluid in the turbine but also for the operation of other systems. The entire water consumption is usually referred as thermoelectric water consumption and is defined as the water evaporated or incorporated into by-products as result of the production of electricity from heat. 
The demineralized (DM) water is used to generate the steam that enters the turbine to produce power. Regardless of the technology used to produce such steam, the main losses attained to the total of DM water is due to evaporation, start up and shut down bypassing/venting, valve passing and blow down. 
These water losses have to be addressed. The DM make up water is the compensating water entering the cycle. Therefore, the DM make up water can be easily measured with a flow meter installed in the feed water inlet to the DM tank. The tanks are kept at a certain level at which the operation of the power plant is safe. The feed water works with the feedback from the tank level to either increase or decrease feed water flow.
It is important to perform a characterization of the DM water consumption at different loads and different ambient conditions, in order to keep a level of efficiency by detecting increments in losses. In recent years, the DM water consumption has been reduced as a result in better metallurgic processes, monitoring and condensate polishing units.
If a flow meter is not available one simple way to roughly estimate the DM water consumption is to close the DM make up inlet and run the unit for a certain time period. It is important to record the initial and final level at the DM tank, so with basic geometry the tank’s volume can be calculated using the difference in height from initial to the final water level. Depending on the grade of uncertainty desired, the calculation can be improved by sampling the water to determine its density and then calculate the mass of the DM water consumed.
|||USGS National Water Census and National Streamflow Information Program, “Methods for Estimating Water Consumption for Thermoelectric Power Plants in the United States,” U.S. Department of the Interior, 2013.|
|||P. Kaushik and D. Khanduja, “DM Make up water reduction in thermal power plants using Six Sigma DMAIC methodology,” Journal of Scientific & Industrial Research, vol. 67, pp. 36-42, 2008.|