PGEN: Overview of Gas Turbine Control philosophy

Mon May 4 22:26:25 EDT 2009

==> Automation List post by CSA...
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Re: PGEN: Overview of Gas Turbine Control  philosophy
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      L4S         L94T          L4T            L4
------| |----------|/|----------|/|------------( )
             |
             |
       L4    |
------| |----|

      L94XZ (Excessive shutdown time)          L94T
------| |------------------|-------------------( )
                           |
                           |
      L28FD       L83RB    | (Flame is lost before
------|/|----------| |-----|  L2RBT times out
                           |  but below K60RB)
                           |
    L2CANT                 |
------| |------------------| (Chamber Flamed Out
                           |  During Shutdown)
                           |
    L2RBT                  |
------| |------------------| ("Normal" shutdown)

L94T is the normal shutdown "trip" logic signal. It will be a logic "1" when fuel is to be shut off during a normal fired shutdown. A fired shutdown is a deceleration from operating speed while maintaing fuel flow and flame as low as possible. This is as opposed to an emergency trip during which fuel flow is immediately shut off and the unit decelerates from operating speed without flame.

The normal course of events during a fired shutdown from load for a generator drive unit with a Mark IV, Mark V, Mark VI, or Mark VIe control system is as follows:

TNR is reduced until reverse power is detected and the generator breaker is opened

Fuel is reduced as the unit decelerates

If flame is lost *before* the unit reaches L60RB then usually the process alarm "Chamber Flamed Out During Shutdown" is annunciated and if multiple flame indications are lost then fuel flow is shut off

If flame continues as the unit decelerates below K60RB (after L60RB is a logic "1") then the L2RBT timer starts timing

If flame is lost before L2RBT times out, L94T goes to a logic "1" and fuel flow is shut off by dropping out of L4

If flame is still present when L2RBT times out, L94T goes to a logic "1" and fuel flow is shut off by dropping out L4

Most units running on gas fuel should be able to maintain flame to below approximately 20% TNH (speed).

Most units running on liquid fuel cannot support flame below approximately 40-50% speed (varies depending on unit and atomizing air/booster atomizing compressor configuration)

Prior to Mark IV Speedtronic control systems, it was common to shut off fuel flow when the unit decelerated below 14HS drop-out (usually 94% TNH (speed)). This results in something of a thermal shock to the unit. Beginning with Mark IV (or thereabouts), a scheme called 'Low Cycle Fatigue' was implemented and one of the things it sought to do was to reduce thermal stresses on the hot gas path parts during starting and shutdown, primarily during shutdown by maintaining flame during coastdown much lower than approximately 94% speed. The logic you are wanting to discuss is all about this: trying to reduce the thermal stress caused by cutting off fuel flow at low speeds.

If flame goes out before the K60RB setting, then it's usually alarmed to the operator as it's not a desirable condition.

If the unit doesn't decelerate to the K60RB setting before L94XZ times out after the generator breaker opens, then L94T is picked up. (A cause would be incorrect settings of the FSKMIN control constant arrays, or excessive fuel flow during shutdown (control valve actuator or servo problem, etc.).)

Many liquid-fueled units cannot get below even 60% TNH without excessing smoking during fired shutdown, so it's necessary to raise the K60RB setting for liquid fuel firing just to prevent excessive smoking (it's a no-no in some parts of the world). Some dual fuel machines have one K60RB for gas fuel, and one for liquid fuel.

I hope this answers your questions and clears things up somewhat.

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