We are having GE-9FA machine with MARK 6E control system. Both GCV and SRV servo valves are connected to the PCAA card. But for IGV servo valve is connected to servo "PSVO" card. Why do we connect to servo valve? Why not to a core analog "PCAA" card? Is there any particular reason?

 

NVK,

The GE EPE (Energy Products Europe) operation (a.k.a, 'The Belfort Bunch') don't need a reason for anything they do. They just make a decision to do something and everyone else just needs to get out of their way because that's the way it's going to be done. Full stop. Period. End of discussion.

Most F-class turbines have a DLN combustion system, meaning there would be more than two gas valves on a gas fuel system. If I recall correctly, the TCAT/PCAA only has six (6) servo-valve outputs. They might have decided to keep all of the fuel control valve servo outputs on the PCAA and put the IGV output on a TSVx/PSVO (which would be cheaper than adding another combined analog card set just because they needed another servo-valve output or two).

Another example would be that some time back there was a prevailing (but incorrect) feeling that some servo-operated functions shouldn't be driven by the TCAT/PCAA. Some engineers, especially ones from a large continental European nation, get something in their mind and NEVER let it go. If your unit can burn liquid fuel, they might have put a TSVx/PSVO in the system for the liquid fuel bypass valve servo-valve, and then just decided to move the IGV servo-valve output to the TSVx/PSVO and leave all the gas fuel servo-valve outputs on the TCAT/PCAA. With Belfort, one never really knows. (I don't think Belfort ever really knows, either. Things just seem to "happen" there, because they can.)

Having said the above (which is fully true and correct), I'm hearing that GE EPE are experimenting with electric actuators for IGV control. Are you certain the IGV is using a hydraulic actuator with an electro-hydraulic servo-valve? If so, what is the current required by the IGV servo-valve?

I believe the current output variation available from the TCAT/PCAA card is very limited--in fact, if I recall correctly it's only +/-10 mA, whereas the TSVx/PSVO can drive +/- 10 mA or +/- 200 mA and a couple of other ranges.

So there are at least three possible reasons:

1) GE EPE Engineering decision, for whatever reason deemed appropriate;

2) The IGV is a non-hydraulic actuator and requires a different mA output than the TCAT/PCAA can provide;

3) The IGV actuator is using an electro-hydraulic servo-valve that requires a different servo current value (mA output) than the TCAT/PCAA can provide.

So, my questions should have been: How many other servo outputs are connected to the TCAT/PCAA? And what are they? And, what are all the servo-valves connected to the TSVx/PSVO?

[By the way, a PSVO is an I/O Pack, not a terminal board. The TSVx, where 'x' could be a O or C or some other character reflecting a particular revision, is the terminal board. PCAA is the integral I/O Pack on the TCAT, which is one of the terminal boards for the combined analog I/O card group (the other being the TCAS). I/O Packs start with "P" and terminal boards start with "T"--generally!]

 

NVK,

The TCAT/TCAS/PCAA "core analog" card group was designed to replace several other cards with just a minimum number of cards. This group of cards can handle seismic vibration pick-ups (which would require a TVIB/PVIB), and thermocouples (which would require a separate TB and I/O Pack), milliamp inputs and outputs (which would require several terminal boards and I/O packs), and speed pick-ups and servo-valve outputs and LVDTs (which would require TSVx/PSVOs). So, the core analog card group replaces many cards and I/o packs with a smaller number of cards and I/O packs, and does so with a much smaller "footprint" than the larger number of terminal boards would require for the same number of inputs and outputs.

And, when it's necessary to add just a few more of a certain type of input or output, then GE usually do so with that particular I/O type's terminal board and I/O pack instead of adding the entire TCAT/TCAS/PCAA card group.

So, that's another possible reason--and some background behind the core analog I/O card group.