Combined Cycle Plant Benefits

Aug 3 2017

Knowing the water concentration at the HRSG outlet in combined cycle power plants can benefit the plant in 3 areas. The areas are corrosion protection during outages, tube leak detection and improved feed water temperature control.

Many plants during outages need to eliminate water condensation so the tubes within the HRSG are not exposed to corrosion. Some plants insert balloons to eliminate humid, ambient air entering the HRSG during outages.  Another option is to monitor the dew point in real time and control the fans as the dry bulb temperature approaches the dew point temperature.

The second benefit of measuring the moisture concentration at the exhaust of the HRSG is to pick up tube leaks.  A gradual increase in the moisture concentration will alert plants that there is a small leak occurring.  The moisture concentration for natural gas combustion turbines will be very constant so all that is needed is the ambient humidity value.  The ambient humidity can be subtracted from the HRSG exhaust value and the resultant number will stay fairly constant at given load conditions.  If the number is trending higher then a leak is happening.  The use of advanced pattern recognition software like EtaPro and Prism will also help determine where the leak is located based on temperature variations.

Lastly, the water dew point and the sulfuric acid dew point in combined cycle plants are closely related. For feed water temperature control, some experts say that all you need is the water dew point while others believe the acid dew point needs to be known. The sulfur content in natural gas is low and somewhat constant. The water dew point can be used as a direct control point for the feed water temperature or a correlated acid dew point can be derived from the real time water content and the known sulfur content of the gas.

Power plants are predicting H2SO4 dew points using our sensor as a real-time input and a worst case SO3 concentration utilizing the Verhoff equation:

Tds = 1000/(2.276 – 0.0294 ln(p H2O) – 0.0858 ln(p SO3) + 0.0062 ln(p H2O) ln( p SO3))

Since moisture is the largest variable in the process it is the main parameter affecting the acid dew point temperature variation. Moisture variations in the flue gas can be caused from ambient conditions, load size and tube leaks. Running without the measurement can cause corrosion or for the plant to run inefficiently by running at a conservative feed water temperature.  If the average feed water temperature could be dropped 3 degrees (F), the energy savings could be up to $4,000 / month on an average sized combined cycle plant.

One sensor at the economizer outlet will provide all three benefits listed above and have a strong impact on the plants overall efficiency.  If you want to learn more on how the sensor can be applied in power plant applications, please visit for more information.

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