Magnetic pickup in flow divider not working

M

Thread Starter

Manjunath

Magnetic pick-ups of flow divider of gas turbine (marc IV) stopped working during normal operation and caused machine trip. and in the next start-ups didn't show any readings and machine is tripping because of loss of flame ( i.e. in the succeeding start-ups).

flow divide was rotated and checked manually for magnetic pick up check up. magnetic pick up showed values
 
Hi,

A single flow divider mag pick-up fault should NOT cause a turbine trip--on either a TMR- or a SIMPLEX Mark IV. So, something else is <b>also</b> amiss. Either another flow divider mag pick-up is or has failed or there is a serious wiring error (or someone has jumpered a single mag pick-up to more than one input, <b>OR</b> one of the other three processors in a TMR panel is trying to trip the turbine for a different reason).

If the turbine is tripping on loss of flame then there it would seem there is liquid fuel flowing.... And a loss is a mag pick-up usually causes that processor (of a TMR panel) to try to put more fuel into the turbine.

What kind of Mark IV is it--TMR or SIMPLEX?

When did this problem start?

You say the flow divider was manually rotated and speed (frequency) was measured--how and where? At the Mark IV? At the JB near the flow divider? How about the other mag pick-ups? What speed did they read when the flow divider is being rotated?

What are the Process Alarms being annunciated when the turbine is being started (<b>ALL</b> of them!)?

What are the Diagnostic Alarms being annunciated when the turbine is being started?

Is the gap setting correct?
 
Dear Manjunath and CSA, I am just wondering if this is too simple an answer.

You say that the magnetic speed pickups stopped working while running which caused the machine to trip. And then on the next start-up the pickups did not show any readings and the machine trips on loss of flame. But the probes read when the flow divider is turned manually.

It sounds to me like maybe you are not getting fuel to the unit. If there is no fuel moving through the flow divider then it won't spin. If there is little or no fuel the unit wont start.

Have you verified proper pressures at the fuel forwarding skid? Proper pressure at the flow divider? What pressure do you show going to each combustion can using the selector valve at the flow divider?

As CSA says if the control system is asking for fuel, and flow feedback indicates little or no fuel, then the control system will ask for more. From my experience a loss of speed indication while running from the flow divider will make the control system ask for enough fuel that the machine will trip on overtemperature.
 
Thanks for the reply,

1) Its TMR type.

2) problem started after 900 hour of successful running.

3) there is cover beside flow divider which was opened and rotated using special tool.all three magnetic pick-up showed values i.e (20, 25 & 38). 2 hour prior to machine trip one magnetic pick-up started reading value greater than other two and at one instant all stopped working.

4) Process alarms annunciated are magnetic pick-up 2 abnormal and at the trip time liquid fuel control fault.

5) gap setting and wiring is perfect.
 
Dear MIKEVI,

Thanks for the reply,

Machine is reaching 39% of its full speed then its tripping because of loss of flame trip.

At the fuel forwarding skid pressure is normal and at flow divider also normal.

using selector valve values read were between 10 to 11 kg.
 
Manjunath,

> 2 hour prior to machine trip one magnetic pick-up started reading value greater
> than other two and at one instant all stopped working.

If all flow divider feedbacks all stopped giving an output at the same time, then something appears to have caused the flow divider to stop turning, or the flow of liquid fuel to stop (are you certain the Liquid Fuel Forwarding Stop Valve is working correctly?) which would result in a loss of flame trip. If the flow divider is a "horizontal" model (all the divider wheels are arranged on a single, horizontal shaft with the speed pick-ups at the two ends) then it's virtually impossible for one end of the shaft to stop turning while the rest of the shaft turns.

If the turbine starts (establishes flame) and accelerates to 39% TNH with little or no exhaust temperature spread (indicating stable flame in ALL combustors--even those without exhaust thermocouples) then suddenly loses flame, that too sounds like something is stopping fuel flow--and possibly the flow divider.

Horizontal flow dividers have been known to suffer a broken shaft (meaning all the divider wheels would not be spinning at the same speed). But usually when that happens the unit trips on high exhaust temperature spreads OR exhaust over-temperature. And from the information provided neither seems to be occurring....

A Liquid Fuel Control Fault can be the result of one or more of several possible conditions; it wouldn't necessarily be the result of the loss of a single flow divider feedback signal, though the other alarm you described would be.

Mark IVs are older control systems, and if one of the three 4 Master Protective Relays is dropped out <B>AND</b> then one liquid fuel ow divider feedback was lost THAT could (depending on how the sequencing was written) result in a "Loss of Flame" trip.

Without being able to see the exact sequence of events and the sequencing running in the Mark IV it will be very difficult to be of too much more help.

Passive speed pick-ups do fail, and do also fail intermittently before failing completely.

Please write back to let us know how you fare in resolving this problem.
 
>Mark IVs are older control systems, and
>if one of the three 4 Master Protective
>Relays is dropped out <B>AND</b> then
>one liquid fuel ow divider feedback was
>lost THAT could (depending on how the
>sequencing was written) result in a
>"Loss of Flame" trip.

WOW!!! That paragraph was harrible!!!

It should have read:

Mark IVs are older control systems, and if one of the three Master Protective "4" Relays is dropped out <b>AND</b> then one liquid fuel flow divider feedback was lost that <b>could</b> (depending on how the sequencing was written) result in a "Loss of Flame" trip.

Sorry for the typo's!
 
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