Removing the liquid fuel flow divider from a gas turbine that is presently operating on fuel gas. Trying to determine if disconnecting the speed pickup signals from the flow divider will cause any control issues (Ready to Start) MarkIV.

Daniel_griffin [@] transcanada [.] com

 

Dan,

It should not.

You can easily just determinate the speed pickups from the AIO TB and the check to see if the 'Ready to Start' indication is adversely affected, and if there are no problems during a subsequent START with the speed pickups disconnected everything should be okay.

I suspect the problem you are having is related to worn flow divider bearings. Mechanical vibrations from the turbine and nearby motors (in the Accessory Compartment) can be seen by the Mark IV as "fuel flow." One quick way to check this is to just take a 12-inch adjustable spanner and tap on the tubing the speed pickup wires run through, right next to the flow divider (when the turbine is at zero speed and off Cooldown). Don't dent the tubing. Monitor the flow divider feedback on the Mark IV to see if it jumps. I suggest using the Logic Forcing Display because you should be able to see all three feedbacks at the same time while you can only tap on one spew pickup at a time. You can also try tapping on the flow divider itself. Otherwise you will need to use one of the troubleshooting displays to see all of the speed pickups. If you know the hex address of the flow divider feedback you can also use the Aux. Display (though most sites have failed to maintain the Aux. Display even though it is a POWERFUL troubleshooting tool!).

Have you checked to flow divider speed pickup gaps recently? You're not using liquid fuel, but if the gap gets too large or too small then the above scenario can occur.

Another possible cause if this doesn't work out is poor grounding of the twisted, shielded pair shield drain wires of one or more of the speed pickup signals. Or, possibly an insulation problem (worn/chafed). I've been to sites that didn't use twisted, shielded pair cables for speed pickups--and they worked fine for years, but eventually the Mark IV inputs degraded (filter networks).

If the Mark IV uses the plug-connected cables, many times the plugs weren't properly tightened during installation/commissioning and loosen over time. Also, some times the pins in the connectors get corroded.

Finally, if the Mark IV turbine control panel is not properly grounded, eventually the analog input filter networks can start to degrade and cause this problem. Many (too many) Mark IV panels were not properly grounded during installation. There should be a substantial conductor (wire or cable) connecting the ground bus bar in the lower right side of the panel directly to the station grounding grid. Many people mistakenly believed that if the Mark IV was bolted to its foundation that was sufficient for grounding purposes--but it's not. And if the Mark IV has had 125 VDC issues over the years this can exacerbate the problem.

Hope this helps! Please write back to let us know how you fare!