Tripping Security of GE Frame 5 Gas Turbine on GCV Reference and Feedback Difference


Thread Starter

Mohsin Raza

In GE Frame 5 Gas Turbines operated by MARK-VI Control System, there is tripping security on difference between GCV Reference Command and GCV Feedback i.e. turbine will trip if difference exceeds 5%. What is the reason/significance of this tripping security?
Every valve has a position/flow characteristic. This means that for a certain position of the valve you expect a certain amount of fluid (gas or liquid) to pass through it for a certain amount of time.

When you have a positioning error on the valve (difference between position command and position feedback), this means you expect a certain flow through it and you get something else. In the case of the GCV, for example, you get more/less gas than you expect.

Small positioning errors are not a problem. But 5% is normally considered the maximum acceptable limit. If you go there, you should consider checking/recalibrating your valve.
Dear Mohsin Raza,

to add to MK6TA's comments. When a valve command vs actual position differs by more than a certain percent, then the fuel/water/steam or whatever the process, is not flowing in the proper amount per the calculation of logic designed for the system. In some machines, like your older Frame 5 unit, the problem will not cause any major issues usually.

In a newer machine that may have some sort of dry low nox system, you may see the machine emissions go out of compliance.

The issue of protection you refer to has more to do with protection the hardware of the machine. If the calibration of your valve is slightly off then it may be a sign that the "LVDT" of the valve needs calibration. If the valve difference changes as the machine is running, which might indicate a problem with the valve or servo or other part of the system. The issue is, if the difference between commanded and reference were to get to lets say 10%, and then suddenly the valve travels to the commanded position, the gas flow may increase or decrease by a very large amount. This will most likely cause a large shift in machine output, and cause an even larger change in combustion temperature which could cause damage to machine hot gas system parts.

By tripping the machine on a difference in position of 5% or so, you hopefully indicate to the operator that there is a problem with the system, and protect the machine. That is unless someone changes the difference number to keep the machine from tripping, instead of repairing the real problem. Hope this helps.