Hi

Appreciate if someone could tell me what is the changeover load for GE 9FA gas turbines when switching from distillate to natural gas? Thank you in anticipation.

 

I read your mail,

For DLN combustion units like GE 9FA and 7 FA gas turbines,fuel change over from fuel oil to fuel gas at load of < 30MW i.e. GT operating with out fuel gas performance heater.

 

This is a very "loaded" question (pun intended!). I'm going to qualify my answer by saying: <b>The answer is appropriate only under ideal conditions</b>. And the ideal conditions are that the liquid fuel system was properly purged after it was last operated on liquid fuel, that the liquid fuel and purge check and water injection check valves are working properly (some units still use check valves; others use or have switched to air-operated valves in place of the check valves), that the liquid fuel supply piping is free of air, that the liquid fuel supply pressure is per design and doesn't decrease when liq fuel starts flowing or liq fuel supply pressure doesn't increase as liq fuel flow decreases. Also, the gas fuel supply pressure must be constant and capable of remaining relatively stable during increasing and decreasing gas fuel flows while transferring fuels.

As has been said before on control.com, the liquid fuel system has many components which are not controlled nor monitored by the Speedtronic turbine control system and which must be configured to work properly and be working properly for successful fuel transfers. So, this is a big concern.

Lastly, there is a process referred to as fuel matching in which the fuel stroke references for gas and liquid fuels are matched so that load is stable during and at the end of a fuel transfers, and this answer presumes that fuel matching was properly performed.

In general, a GE-design heavy duty gas turbine should be able to transfer fuels (again, <b>under ideal conditions</b>) at any load, or even at FSNL (Full Speed No Load). This presumes the fuel supply systems were properly designed, capable of maintaining stable pressures and flows during increasing and decreasing fuel flow-rates during fuel transfers, that all components are working properly (check valves, or isolation valves, pressure regulators, etc.), and that fuel matching was performed properly during commissioning.

But, I really want to stress that the fuel supply systems, the fuel system components, and the turbine control system have to all be properly designed, configured, and proven (tested during commissioning), to allow this ideal capability to transfer fuel at any point while at rated speed (unloaded or loaded).

It's also not recommended to transfer fuels while operating on CPD- or CPR-biased exhaust temperature control (Base Load), simply because if there is a significant bobble in fuel flows which results in an exhaust over temperature the unit could be tripped, it's usually recommended to reduce load about 10% of rated from Base Load when attempting to manually initiate a fuel transfer, just to be prudent and cautious. The caveat here is that with IGV temperature control active, the unit is still at or near the exhaust temperature reference limit anyway, so it's still an uncomfortable situation even with reduced load.

Answering this kind of question without understanding the context of why it's being asked or how the answer will be used is very distressing. We're talking about relatively large machines (225+ MW or more) and some relatively high fuel flow-rates that accompany these kind of loads. Most fuel supply systems were not really designed for low fuel flow-rates that are experienced during fuel transfers, and even have difficulty providing stable pressures and flow-rates during start-up (acceleration) and shutdown (deceleration), much less during fuel transfers. Most sites have such awful experiences with fuel transfers during commissioning when fuel matching is being attempted that the control system is rarely every properly configured to limit load swings or load differences after fuel transfers are complete. Lastly, most sites never perform a fuel transfer unless there is a problem with the fuel that is currently being burned (low pressure or flow), and then the systems can't usually respond quickly enough to limit load swings or even loss of flame. Combine this (decreasing fuel supply pressure/flow) with all of the system components which must be 100% configured and working properly, and, well, the success rate for fuel transfers isn't very good. In fact, it's downright awful.

So, expectations must be realistic when expecting a unit to successfully transfer fuels. Under ideal conditions, most every unit should be able to transfer fuels at any load while at rated speed. Again, that's under ideal conditions. And those don't exist at many turbine sites, or if they do, they don't persist for very long at many sites. If it's expected that the unit will be able to transfer fuels at any load with minimal load swings, GE recommends periodic (once per week) fuel transfers just to ensure all the components and systems are working properly. And very few sites ever do that, regardless of Frame size or combustor type (conventional or DLN). And, again, in an emergency situation, depending on how fast the current fuel supply pressure/flow is decreasing, it just may not be possible for the unit (turbine control system and auxiliaries) to successfully transfer fuels without large load swings and/or loss of flame.

I hope this helps, but I'm more concerned about how the answers will be used. Please be sure to include the qualification "under ideal conditions" when quoting this answer to anyone. And take the time to review all the relevant piping system drawings (P&IDs) to understand how everything must work together and how many components are simply not controlled or even monitored by the Speedtronic, and how quickly fuel supply pressures/flows can change during an emergency, and what the turbine control system has to do to initialize (pre-fill) and begin a fuel transfer sequence, and how long the fuel transfer sequence requires, before stating that fuel transfers can't or don't work. It's not a simple question, nor does it have a simple answer.

 

AS ABRS says, if the unit(s) at your site have gas fuel heaters (performance or start-up), this is another consideration which must be taken into account.

I have seen sites where the sequencing or application code will reduce load during a fuel transfer when switching fuels if there are special requirements for modes/fuels. I have also seen units that did not have this special sequencing/application code. And I have see sites that use the DCS to raise or lower load during a fuel transfer to accommodate any special requirements (usually not very well, I might add).

The intent of most Speedtronic control systems is to make this as "automatic" as possible, but it doesn't always get properly tested or configured during commissioning, especially on these large machines which can cause large problems for grids if they trip (and tripping and load swings during fuel transfers is, unfortunately, all too common).

Again, we don't know the particulars about your machine or the problems you are experiencing or might be encountering.

 

Hi

Switching to distillate at higher loads (premix mode)flame is not stable and 9FA trips. We now reduce load to 30MWs (Diffusion 5 mode) Flame is stable and we have not had problems on changing to distillate since.

 

What is pre-mix operation for Liq. fuel. No separate diffusion and pre-mix operation in liq. fuel.

This terminology of Pre-mix only applicable for Gas operation. Not for liq. operation .

Liq. fuel flame is always stable. So switching from gas to liq. at any load and liq. to gas at lower load.

Main restriction is for switching to Gas is Modified wobbee index.
Normal time period for fuel change over is 1 min .

If you carry out fuel change over from Liq. to gas, Not possible to get the fuel gas temperature in 1 min to satisfy MWI(Modified wobee index--this is applicable for Pre-mix). For fuel oil no -premix and no MWI issues.

 

Thanks for all

I would like to ask about the logic diagram for the fuel transfer, can anyone help.

my Email is
[email protected]

 

It's also not recommended to transfer fuels while operating on CPD- or CPR-biased exhaust temperature control (Base Load), simply because if there is a significant bobble in fuel flows which results in an exhaust over temperature the unit could be tripped, it's usually recommended to reduce load about 10% of rated from Base Load when attempting to manually initiate a fuel transfer, just to be prudent and cautious. The caveat here is that with IGV temperature control active, the unit is still at or near the exhaust temperature reference limit anyway, so it's still an uncomfortable situation even with reduced load.

thanks for explanation
since my unit are not quite similar to datasheet for rating base load. name plate stated 30MW +- 10%,. actual condition was 25MW and FSRT almost equal to FSRN, difference value at 25MW are 0.01,.

while fuel gas outage occurred, it unable to perform fuel change over due to exhaust temperature trip. please advise how to measure safe running megawatt in order to prevent trip during fuel gas outage,.

my unit was frame 6b, control system PLC based.
at the time trip during fuel change over, TTXM indication was 596 degC.

thankyou.
Tresna

 

Tresna,

I really can't say what happened at your site with the PLC-based control system. We don't know how the control system was programmed to perform a fuel changeover or how the control system was programmed to handle Droop Speed Control and CPD-biased exhaust temperature control and switching between them as necessary.

Most sites I have been to with PLC-based control systems had issues with things like fuel change-overs and bumpless switching between Droop Speed Control and CPD-biased exhaust temperature control. Control system integrators performing these kinds of turbine control system replacements don't always have turbine control experience (though a couple of them have LOTS of experience, most don't). And, most aren't good at reading the Mark* logic/sequencing and "duplicating" the functionality without understanding GE-design heavy duty gas turbine controls philosophy (there is a LOT of basic operating and control and protection that is based on decades of fundamental ideas and schemes used by GE when deciding how to program and configure a control system (things like minimum select "gates" and inversion masking and drop-out to run and high-select, etc.). I have seen some very intelligent programming for PLCs used as turbine control systems--bordering on genius--but they just didn't have the basics of how the control scheme was supposed to work so it didn't (work).

And, when talking about fuel change-overs when there is a "loss" of the fuel the unit is operating on--that is something that just can't be always be done. If the loss of fuel is sudden and complete, it's highly likely that any control system isn't going to be able to handle that without tripping. In a normal fuel change-over the running fuel flow-rate is ramped down while the flow-rate of the fuel being switched to is ramped up--and that's after a few seconds (sometimes as much as 30 seconds when changing from gas to liquid fuel) of getting the auxiliaries running and opening valves and performing a pre-fill of the liquid fuel lines (pressurizing them in the hopes of preventing air bubbles in the liquid fuel supply piping)).

And, a LOT of control system integrators don't think understanding the auxiliaries and the P&IDs for the various systems are very important--when they are (very important).

There are just not enough details here to be able to tell you how to handle a loss of fuel situation with the PLC-based turbine control at your site. Sure, a well-programmed and configured PLC control system will reliably operate a GE-design heavy duty gas turbine during normal starting and loading and unloading and shutdown, but it's things like dealing with fuel change-overs and grid frequency disturbances that really test the programming and configuration. And, again--if the loss of the running fuel was sudden, and especially if the unit was running on gas fuel, the transfer to liquid fuel is probably not going to occur without some issues or tripping--whether it's on loss of flame or exhaust overtemperature.

Sorry; wish I could be more helpful, but without a LOT more information and without being able to see the programming in the PLC it's really not possible to say any more.