Dear All,

We are facing a problem of spurious flow divider magnetic pick up feedback during machine running on NG. When the machine is under shutdown, no signal were observed.

We have already replaced the all magnetic pick up and checked the gap setting. On opur request flow divider was also replaced but the spurious signals problem is not resolved yet.

Any one may please help to solve the problem which are facing in our GT. Any possible cause may please elaborate.
Thanks

 

This problem seems to occur most frequently on turbines retrofitted with Mark VI and Mark VIe control systems. The sensitivity of the passive, magnetic speed pick-up inputs has been increased so much that they can be subject to erroneous, intermittent indications.

There are essentially two causes for this problem. The first is primarily found, again, on older units which have had the Speedtronic control systems upgraded to Mark VI or Mark VIe vintage.

The speed pick-up gap for many flow divider speed pick-ups was commonly set to 0.010 inches (0.254 millimeters). Combine this with several other factors as well as the increased sensitivity of the Speedtronic speed pick-up inputs and you have a recipe for nuisance and erroneous flow indications.

One of the other contributing factors has been the age and mechanical condition of the flow divider (gears; bearings; etc.). If the flow divider is old and has not been maintained or recently refurbished then mechanical vibrations from other components (Accessory Gear; Hydraulic Pump; Starting Means; etc.) can cause the toothed wheels of the flow dividers to vibrate and the Speedtronic interprets this as fuel flow.

Many sites have increased the liquid fuel flow divider speed pick-up gaps to approximately 0.040-0.050 inches and this has eliminated the problem. Others have installed some filters (sorry; I don't have any specifics on the component values/tolerances) with some success.

However, there is another more serious possibility that may be occurring depending on the Frame size of the machine. Many Frame 9Es have experienced leaking or failing liquid fuel check valves, and this causes hot combustion gases under pressure (essentially compressor discharge pressure) to enter the liquid fuel tubing and cause the Liq. Fuel Stop Valve (VS1-1) to open. This produces a path for very small back-flows of liquid fuel, primarily, as well as combustion gases (which do cool as they enter and flow through the tubing/piping). These back-flows can be detected as flow (since the neither the speed pick-ups nor the Speedtronic can detect reverse flow from forward flow) can cause air bubbles in the piping, and also primarily cause coking of the liquid fuel in the fuel nozzles and liquid fuel check valves.

So, it's not always mechanical vibrations being misinterpreted as fuel flow causing the erroneous indications when the unit is starting or running on gas fuel. It can also be caused by back-flows when the unit is running on gas fuel.

While not prevalent on smaller Frame-size machines, similar back-flow problems have also been reported on Frame 6B machines with failed liquid fuel check valves, though the liquid fuel stop valves don't seem to be opening under back-pressure as on the Frame 9Es.

Please write back and tell us what Frame-size machine(s) you are experiencing this problem on, if this is a new unit installation with a Mark VIe or a retrofit of an older of an older Speedtronic with a Mark VIe, and what the gap distance that's being used for the liquid fuel flow divider speed pick-ups is.