Help combined cycle real case question

[Dave001]

In the start up of a combined power plant we have 3 hrsg working each at 100 mw and we are on a position control mode with bypass open . The turbine control valve is set at 98 MW load and the bypass pressure set point is 70 bar . my question is if we change the pressure of set point to 80 bar ( for example), the bypass will closes a little to achieve the new pressure , also the control valve will repond to this new change , but how about flow rates ? I know it will decrease in the path of bypass but will it increase in the path of turbine ? Does the increase of pressure will affects the flow rate from the boilers drums taking in consideration that the load of the gas turbines are constant ?

 

[byrdj]

I didn't work Combine Cycle...But I have a question.

If the steam turbine is in "position control", would that mean the CVs are set XX% open in a high valve gate with any other governing. So why would you expect the CVs to respond to the BPVs and change in pressure

 

[Dave001]

The CV will respond as we gave it the set point at 98 MW so when bypass closes , the flow rate decreases to the bypass and increases in the way to CV so it responds to any change to maintain the constant load that we give to it which is 98 MW

 

[rotw]

Quote
the flow rate decreases to the bypass and increases
Unquote
Why? What is the reasoning behind this inference?

 

[byrdj]

IF, the bypass valves (BPV) are controlling pressure and the pressure set point is increased, the BPV will throttle to raise pressure

IF, the control valves (CV) are at a set position, the increase in pressure will result in increased flow to the turbine and the generator output to increase whild CVs stay constant.

IF, the CVs are in power control, then either the sensed increase in flow to the turbine as a stage pressure or the increase in generator current will throttle the CVs to maintain the power set point.

depending on whether the control loops are propertional or intergal will determne if a small change in flow occures.