GT running under alarm conditions 2

[jetman1]

Hello,

This is my first thread, so here goes. Is it possible to run an aeroderivative or frame GT under low fuel pressure alarm conditions? Tripping the turbine is not an option on low pressure. What would be the consequences, e.g. -ve impact to MTBF or shortening of GT life, etc.? Thanks.

 

[toothless]

An alarm is an alert. It is telling you that you have a problem that needs attention, but the condition isn't so bad that a trip is warranted. If the pressure goes lower, an interlock will trip your turbine.

Depending on the type of gas valves you have, you may see the unit begin to lose load before the unit trips. This will occur when the gas valve(s) are wide open.

Tripping a CT from full-load is not catastrophic but certainly isn't desirable. It creates lots of stresses and prematurely "ages" your equipment.

 

[ScottyUK]

Tripping the turbine is not an option on low pressure.

It is absolutely an option. If your fuel input is inadequate to meet the load, it is the only option you have. The alarm is the warning that you are approaching the trip condition. If your pressure continues to fall or you do not reduce load then tripping is the likely consequence. You can consider load-shedding if the connected load is an electrical generator.

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Sometimes I only open my mouth to swap feet...

 

[byrdj]

I've only walked by CTs on may way to power hall so I'm also asking here. If the fuel pressure decays below alarm, the CT could not make full load and wouldn't it gradually unload itself until flame out trip? So with a low fuel pressure alarm, you would not expect full load capibility?

Or is there some fuel injection compressor (for gas) or pump (for liquid) that might have cavitation problem?

 

[unclesyd]

Just a tidbit of information on our gas turbine.

The turbine is frame 7 and runs on natural gas. During the second year of operation the site received a request from the gas pipeline company to start curtailing usage. Our site essentially did nothing. The second notice came with a right now clause for a 25% reduction. Within a hour the line pressures started dropping. Another request for a 50% reduction again with the right now clause. Evidently the pipeline company didn't like our approach to the problem as the line pressure started dropping again and within a hour the turbine tripped with a flame out, with bang, that stripped half the fish scales out of HRSG.

The turbine was only half the story as we lost all steam boilers and Therminol vaporizers.

 

[rmw]

The flame patterns and proper mixing of the fuel and combustion air in your combustion cans is a function of many things among them the gas pressure. When the gas pressure is not adequate, flame fronts begin to move to places where they are not designed to be and components begin to operate at different temperatures than originally designed, higher or lower, and that's when things start to go badly and damage occurs.

Your alarm is telling you that you are close to having to do something as others have pointed out. Take it seriously and do what is required to remedy the problem.

rmw

 

[jetman1]

Thanks all for replying. My recollection from previous experience with GE Frame type GTs is that a low pressure interlock will trip the turbine for reasons given above. However, I am also informed that aeroderivative units can operate at much lower fuel pressure than a frame GT. Reason being that you wouldnt want to trip the engine in flight. Comments?

 

[ScottyUK]

Bear in mind that an aero-derivative and a true aero engine are very different animals: one is optimised to burn dense warm air at ground level; the other is designed to burn bitterly cold low density air being rammed into its compressor at 550mph. The two engines are cousins, but they're designed with very different operating parameters and different burner designs, and radically different requirements to continue running under contingency conditions. The aero engine has to keep turning however bad things get otherwise the plane falls out of the sky; most aero-derivative users would rather shut down their expensive engine rather than wreck it.

Why can't you maintain proper fuel supply conditions to the engine?


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Sometimes I only open my mouth to swap feet...

 

[abeltio]

some comments...
first Industrial Type Units (also called "frame units")

the manufacturer provides in the logic the conditions that cause a TRIP.
if the condition causes: ALARM ONLY, it is up to the operator to decide whether to run or shutdown (normal shutdown) - tripping the unit is the last resort in case the unit does not respond to the controls.
I WOULD NEVER TRIP A UNIT IF THE UNIT RESPONDS TO THE CONTROLS.
UNLOADING THE UNIT UNTIL THE VALVES ARE NOT 100% OPEN IS ONE OPTION TO AVOID WORKING WITH THE SERVOVALVES IN MAX SATURATION (100% OPEN)

in case of dual fuel units, this alarm initiates an automatic change over to liquid fuel operation - nasty stuff...

GAS ONLY
with the frame units we need to make a distinction between standard combustor with or without wet NOx control (steam/water) or Dry NOx control.

frame with std comb w/o NOx control...
typically there will be a 2 stage control of gas supply the first stage controls the gas fuel pressure to a certain setpoint based on the speed of the unit, the second stage controls the flow based on load demand.
when the gas fuel pressure drops... both valves compensate opening more and more... if the pressure falls so much that the valves are 100% open... and keeps dropping the unit will unload and eventually flameout (loss of flame trip)

if the cause of the pressure drop is DIRTY FILTERS... beware... the situation descripted will also take place.

The filters will eventually collapse due to extremely high differential pressure.
A huge pressure surge will take place with the valves fully open.
The unit will trip on overtemperature and you will have created what is popularly known as a "dragon" - flames coming out of the stack.
A mesmerizing sight, but a short lived hot gas path.

frame w/std combustion w/NOx control
the unit will flame out much faster due to the water/steam injection.

frame with dry NOx control
unit will flame out almost immediately... the low gas fuel supply pressure is a trip - and also a permissive to start.

Aircraft derivatives
I've seen units that require 400psig gas fuel supply.
these units have 2 trips, one for low and one for high gas fuel supply pressure.
And, e.g. the GE LM units have only one valve controlling gas fuel supply.
the control of units running fed from the same gas main requires careful study of the trip situation: if one GT trips this creates a surge of pressure in the header that may activate the HIGH PRESS trip of the other units, thus having a domino effect...
one solution we used was to install pressure transducers just before the gas control valve on each unit and a logic that will activate/reset a pressure relief solenoid based on rate of change of pressure (psi/sec) and pressure level (psig) - worked as a charm.

pheewww. hope you are still awake.










saludos.
a.