Main Steam Flow Measurement Using Turbine 1st Stage Pressure Measurement


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We are using nozzle type DP transmitters in steam turbine for measuring the main steam flow. We are getting the Square root value of DP and we are having doing density compensation by taking both pressure & temperature with some calculations in DCS program. I have also come across main steam flow can be measured from turbine 1st stage pressure measurement. I just want to understand how the 1st stage pressure is used in measuring steam flow and implement in our process to compare the accuracy of DP type flow measurement.

Could you help me in sharing your knowledge about that
First stage pressure for a given turbine has a fairly linear relationship with steam flow through the turbine, as long as the overall condition of the turbine doesn't change, and the steam cycle doesn't change. If a steam seal clearances open up due to a rub, or steam flow passages get bigger due to erosion, the turbine will pass more steam flow for a given amount of first stage pressure. Taking feedwater heaters out of service, plugging tubes, etc, will change extraction flows and will have an effect on the first stage pressure vs steam flow relationship. Superheat temperature, reheat temperature, and use of reheat sprays for reheat temperature control will have an effect as well.

First stage pressure is very useful as a control parameter to represent main steam flow. But use of first stage pressure for heat rate calculations would be very course.
Dear Krush,

I have a boiler where the steam drum has 2 PSVs and 1 on superheater outlet. In order to do on-line test while boiler in operation, we gag the super heater PSV while pop testing the drum PSVs one by one. Will this cause overheating of the superheater tubes due to starvation. Steam flows out from silencers while testing?

The fact that your PSV is on the outlet of the Super heater means you are safe from tube starvation.