What you are looking at here is a fuel valve signal control . There is one signal given to the fuel valve and that is on the lower right side marked FUEL SIGNAL. You can’t have two control loops in control so to allow multiple inputs to control one output you must use some form of selection. The “Minimum Error” makes that selection. It looks at the difference between the set point of each of the five signals trying to control the valve and their set points. The control system passes the signal with the minimum error to the “P+I Controller” block which will select the appropriate proportional control value to be used in the MIN GATE. At the Minimum Gate you have two additional values (tunable) that are part of the selection for the Minimum Gate. The intent of the logic is to use the lightoff and start ramps to give you a smooth lightoff. Adjustments are needed to allow for differences in build, operating conditions etc. Once the turbine was lit-off the start ramp would normally take control of the turbine as it accelerated.

Before the introduction of electronic fuel control valves GAS DP control did not exist. All turbine manufacturers had to review the way flameout was detected. Electro-hydraulic actuators were used before the electronic actuators and they were nowhere near as quick as electronic valves. With quicker operating PLC and electronic valves the fuel valve now responded much quicker than before. You don’t appreciate this until you see how quickly an IGV electronic actuator moves the IGV ring – it is frightening how quickly it moves. What caused the concern for every turbine manufacturer was that when the turbine flamed out the fuel valve would react by opening up to allow more fuel to try to try to return to the set point before the flame out. This allows a huge amount of raw fuel into a hot combustion chamber resulting in an explosion.

One of the control changes that Solar made was “GAS DP” or “LIQUID DP” control (depending on the fuel). For fuel to arrive in the combustion chamber, atomize and mix before combustion it needs to be at a suitable pressure. Too low and it will not be able to get in or too high where it may be difficult to light off or control. The fuel is kept at a specified differential pressure higher than PCD and all works well. If the turbine flames out, the positive difference between Fuel Pressure and PCD will be lost as the fuel pressure will stay constant while the PCD will fall off quickly. In this situation GAS DP and LIQUID DP control takes over the control and prevents the fuel valve from being opened further. This DP control is not only used while at service speed, it is also present during starting.

The electronic PECC and CCC valves were introduced while Turbotronic 3 was still the control for new equipment sales. This new GAS DP and LIQUID DP control has a tendency to take over control before the turbine even lit off. If a turbine is not lighting off it would be normal practice after exhausting any other possibilities to adjust the gains on the Lightoff and Start Ramp. This will allow the fuel valve to open a little quicker giving the turbine longer time with more flow to light off within the time limit. But if GAS DP or LIQUID DP takes over control preventing any additional fuel, any changes you make to the Lightoff or Start Ramp would be futile. This tended to happen more often on CCC than on PECC fuel valves. The fix was to modify the logic to force the control to stay in Lightoff Ramp until it lit off. The logic for Turbotronic 4 does not have this issue. There are probably still units out there that are poor at starting and this may be the problem.

It is difficult to know for sure unless you record all the address to see exactly what is happening in the selector gates. A simple change to the logic is all that is required but being safety critical it needs to be done be a competent technician.

See article ART195 for video on same subject.