In gas turbine what is relation between SRV and P2 pressure?

In gas turbine, the purpose of SRV and GCV valves are understandable. Closed loop of GCV with setpoint given by FSR and feedback is from LVDT.

But, in SRV the valve setpoint is given by Turbine speed (converted to pressure) and feedback is taken from both Pressure transmitter and LVDT.

Doubts:

*set point is speed converted to pressure.this is understood that during start up from 0 to 100% Speed; to make fuel flow inside the pressurized combustion chamber inter valve pressure is increased.

But after synchronization, speed is constant, so setpoint will be constant. When Load is increased, fuel flow should be increased. So, inter-valve pressure should be increased. how this factor is included in the control logic?

*why two feedback for SRV.

-pressure Transmitter feedback-LVDT feedback

why this two? and how its used in control logic.

*When speed is stable, the P2 pressure is stable. When the GCV opens as the machine is loaded, the P2 pressure would tend to decrease--but the SRV opens to maintain the P2 pressure. As the machine is unloaded and the GCV closes the P2 pressure would tend to increase but the SRV closes to maintain the P2 pressure. The SRV is NOT stable when the machine is being loaded or unloaded; it moves as necessary to maintain the P2 pressure as the GCV opens/closes. 

* One of the reasons the P2 pressure is held stable when the machine is at rated speed is because flow through the GCV is proportional to stroke with a constant pressure upstream of the GCV. This makes flow-rate proportional to stroke, a nice linear relationship.

*When the actual P2 pressure equals the P2 pressure reference the error between the two signals is zero--and the valve does not need to move from its current position. As the actual P2 pressure changes OR the P2 pressure reference changes, the error between the two signals changes which means the valve needs to move. When the actual P2 pressure is again equal to the P2 pressure reference, the error is zero and the valve stops at its current position.

The LVDT is used for additional stability; the position loop is an "outer" loop to the "inner" pressure loop--which is the primary control loop for the SRV.

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P2 Pressure is nothing but the pressure in between the SRV (Stop Speed ratio valve) and the GCV (Gas Control Valve). According to the fuel demand which is set by the FSR (fuel stroke reference), GCV opens or closes. Accordingly, P2 Pressure drops or increases respectively. To maintain the P2 Pressure requirement, SRV Opens or closes.

In Short,

Fuel demand is more -- GCV Opens -- P2 Pressure drops -- SRV Opens more to meet the P2 Pressure demand.

Fuel demand is less  --- GCV Closes -- P2 Pressure increases -- SRV Closes to maintain the P2 Pressure.

nice explanation from Rajendran 

The precent of srv open control p2 which cntrl gcv 

Direc Relationship