We have Mark V system for Frame6B machine.As due to crisis of NG, the machine was change over to IFO and GCV Not following reference alarm was came. Operation group informed to Force the tripping point L86GCVT to zero as a precautionary method.

Now we have to adjust the LVDT feedack which are showing -3.5 %.

May please elaborate the detailed procedure for adjust the LVDT feedback and how to check with stroking during 100% IFO.

Thnaks

 

The 'Not Following Reference' alarm is NOT related to the -3.5% indication that comes when the GCV (or SRV, for that matter) is "closed". That alarm should be able to be reset with a Master Reset at this time.

This topic has been covered many times on control.com. A <b>properly calibrated</b> gas valve assembly as is typically used on a GE-design Frame 6B heavy duty gas turbine will indicate a position less than 0%. In other words, a negative position.

Please see thread:

http://www.control.com/thread/1296479865

and thread:

http://control.com/thread/1294809227#1295331639

for more information, presuming you have the typical combined gas valve assembly (SRV and GCV in one cast assembly, usually manufactured by Young & Franklin).

As for stroking the GCV during liquid fuel operation, don't try it. There are too many potential problems even if it were easily possible.

 

Actually, I just realized that B K Meena was the originator of the second thread.

And, we're <b>STILL</b> having this same discussion. About the <b>SAME</b> condition.

Give me strength!

 

Dear CSA,

Negative feedback greater than 3% will lead the alarm and greater than 5% for 5 secind will lead tripping. The same situation we have already faced twice in our frame vi b machine of mark vi.

 

bkm,

Depending on how the LVDTs were calibrated and the gap between the valve stems and the actuator rods the negative feedback could be less than -3%, and, even less than -5%--even if the LVDTs were properly calibrated!

So, that would mean that in order to prevent erroneous alarms and trips you would need to change those two setpoints to values that would prevent the alarm- and trip conditions but still protect against a loss of feedback condition.

I don't know how to make the concept clearer. The valve stem, to which the LVDT cores are attached, is <b>***NOT***</b> physically attached or connected to the valve plug. <b>The LVDTs need to be calibrated such that they reflect the valve plug position, NOT the valve stem position.</b> Because the valve stem can "drop" from the valve plug when the plug is closed (because of the gap between the valve stem and actuator rod) it can indicate the plug is in a negative position. <b>That is to be expected because of the design and configuration of the valves and actuators.</b>

The setpoints for the alarm and trip are there to prevent a loss of LVDT feedback from allowing a unit start or to trip the unit if there is a loss of feedback. If conditions (valve wear; re-assembly; etc.) cause the gap to be larger than expected then the magnitude of the negative feedback will be larger than expected. And to prevent nuisance alarms and trips one must change the setpoints (they are field-adjustable Control constants) to something that still provides some measure of protection but prevents nuisance alarms and trips based on site conditions.

It's as simple as that.

When one is working on any piece of equipment, one must understand how that piece of equipment works or is designed or being used. Just because the majority of control valves in the world are manufactured such that the valve stem is physically attached to or connected to the valve plug doesn't mean that every control valve is exactly the same. In this case, the valve stems of both the SRV and the GCV are not attached to or in any way part of the valve plugs. Because of the intentional gap between the valve stem and the actuator rod the valve stem can be moved up and down when the valve plug is in the closed position and is stationary.

The LVDT cores are attached to the valve stems, not the valve plug. But they are supposed to measure valve plug position. So, when properly calibrated the valve stem can "drop" from the valve plug 0% position and indicate a negative valve plug position (which isn't physically possible). If the gap is such that the negative position magnitude is greater than the magnitudes of the alarm or trip setpoints and it's felt that the LVDT feedback is properly calibrated then it's necessary to change the setpoints to prevent nuisance alarm and/or trip conditions.

Nothing more and nothing less.

You seem to also have some Frame 9Es at your site (or at least one). The gas valves of GE-design heavy duty Frame 9Es (ones without DLN combustion systems) are also housed in a combined assembly, but only <b>one</b> of the valves has an intentional gap between the valve stem and the actuator rod. Unfortunately, I can't remember which one, but I believe it's the GCV--but I may be wrong. (I've been wrong many times in the past, and I will be wrong many times in the future. I'm only human, after all.)

And the same principles apply to that valve as well.