Liquid fFuel Flowdivider Magtic Pick Up Is Showing Value

B

Thread Starter

barindra75

What is working method of flow divider magnetic pick up?
Why due to vibration spurious flow shows?
can anybody give me links for the details of manuals or working principles?
 
Look up "passive speed pick-up" or "passive speed sensor" using your preferred Internet search engine--because that's what GE uses for sensing the speed of the rotating toothed wheel(s) of a liquid fuel flow divider.

Flow divider internals (bearings; etc.) can become worn over time and become subject to slight oscillations due to nearby machinery (Accessory Gear Boxes; Aux. pumps; etc.) and cause the output of the speed pick-up(s) to output a frequency to the Speedtronic.

This is ESPECIALLY true of older machines that have had their control systems upgraded to newer Mark VI and Mark VIe Speedtronic turbine control systems. The speed sensing inputs of the Mark VI and the Mark VIe are <i>MUCH</i> more sensitive that earlier Speedtronic turbine control systems (yes--even the Mark V). And, it's especially true of the liquid fuel flow divider speed sensors. The teeth are smaller and the toothed wheel(s) of the liquid fuel flow divider are smaller diameter than the speed sensing rings on the turbine shaft--and even though the exact same speed pick-up (or same manufacturer/type, with slightly different dimensions) are used for both turbine shaft speed sensing and liquid fuel flow divider speed sensing the gap setting is usually different for the two applications.

The turbine shaft speed pick-ups are usually to be set at 0.050 inches, and the liquid fuel flow divider speed pick-ups are usually to be set to 0.010 inches. This is because at low liquid fuel flow rates and because of the smaller teeth and smaller diameter of the liquid fuel flow divider toothed wheels it was necessary to use a smaller gap to be able to sense low liquid fuel flow-rates with the older Speedtronic turbine control systems.

Now that the Speedtronic turbine control system speed sensor inputs are so much more sensitive the same speed pick-ups can appear to be failing or problematic and output frequency even when there's no liquid fuel flowing. And, in most cases, all that's required to solve the problem is to increase the gap to approximately 0.030 inches--but not much more than that. If it's necessary to increase the gap by much more than 0.030 inches, then the liquid fuel flow divider needs refurbishment. (Liquid fuel flow dividers take a beating if fuels other than distillate (diesel; HSD (High Speed Diesel); etc.) such as HFO (Heavy Fuel Oil), bunker fuels, crude oil, and naphtha are used and should be regularly refurbished (approximately once every two years, on average).

Now, having said that--some GE-design Frame 9E heavy duty gas turbines have been experiencing leakage of the liquid fuel check valves at the fuel nozzles causing hot combustion gases to push liquid fuel back through the lines and the flow divider and even to open the Liquid Fuel Stop Valve. So, this could actually be a reason for the liquid fuel flow dividers to be outputting frequency--because they don't know which way the toothed wheels are rotating, just that teeth are passing in front of the face of the pick-up.

There's very little magic in the sensors and field devices used on GE-design heavy duty gas turbines. Even the mythical and much misunderstood and maligned electro-hydraulic servo-valves aren't really that magical and mysterious. It's just that people don't understand the bipolar nature of the device and just want everything to be <i>easy</i> and <i>simple</i>, like 4-20 mA positioners.

Anyway, hope this helps! Please do try to tell us about the circumstances regarding these kinds of problems when posting for information. It would be helpful to know if this is on a new unit, or a unit with a new/upgraded control system, what the control system is, what Frame size the unit is, what you've done to try to troubleshoot the problem, etc. It's admirable that you're trying to determine how the devices work in general, but it's sometimes much more helpful to the respondents and to other people who read these threads (and a LOT of people do, even though they never write) to read about real problems and resolutions. We always try to provide a little extra information here at control.com, such as the general workings of field devices and instruments, when it's deemed helpful. But, again, there really aren't any magical devices used on most GE-design heavy duty gas turbines.
 
B
R' CSA,

thank you for your response.

In our case, it is an old (more than 20 yrs) frame-III machine. You are right, we have gone thru recent up-gradation of mark-IV to mark-VIe.

Now we are getting unstable fluctuating liquid flow (HSD) (upto 0.9 %). we have confirmed the flow still comes even after completely isolating the liquid line.

Normally the machine run in NG. Adjusting the gap has also attempted several times but not solved completely.
 
Top