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High Exhaust Spread in 6B
Reason for high exhaust spread in frame 6B machine

in frame 6B machine exhaust spread is becoming very high. At 23MW of load it is going up to 53degC with spread limit 69.6degc.

May the nozzle choking be the only reason?

or VS1 valve passing and allowing 7-8 kgcm2 pressure which in turn allows liquid fuel to enter in combustion chambers, may also be one of the reason?

All the Ex TCs are healthy. I don't see any other reason. Please tell if there may any other reason too. Also please explain reason regarding VS1 passing.

what is the problem if some liquid fuel is continuously going inside CCs as these machines are designed to operate in mix fuel mode also?

1 out of 1 members thought this post was helpful...

khoriwal000,

The subject of exhaust temperature spreads has been covered many times before on control.com. Have you used the 'Search' feature at the far right corner of the Menu bar at the top of every control.com webpage to look for exhaust temperature spread-related threads?

As for the subject of liquid fuel dribbling into the combustors while running on liquid fuel, that is bad. Because while a dual-fuel machine can run on mixed fuel, the mix should be such that the fuel pressures to the combustors are sufficiently high enough to permit proper atomization (pressure atomization) of the fuel (atomizing air is reduced when operating on gas fuel).

And dribbling fuel can be carbonized in the liquid fuel nozzles, which can cause spread issues or starting issues when starting on or transferring to liquid fuel.

Now for 'how does VS1 (the liquid fuel stop valve) leak'--well, that valve has a pretty big spring keeping it closed when the Liquid Fuel Stop Valve Solenoid, 20VS-1is de-energized and there's no hydraulic pressure to overcome the Stop Valve's spring tension. So, liquid fuel from the Liquid Fuel Supply System shouldn't be getting through the Stop Valve--because Liquid Fuel Supply Pressure should be zero.

However, what can happen--and this is NOT good, and can be difficult to troubleshoot, is that one or more of the liquid fuel check valves can be leaking (passing) and allowing CPD to flow backwards into the liquid fuel system. This can cause the pressure in the liquid fuel system--downstream of the Stop Valve to build up high enough to cause the Stop Valve plug to be pushed up and overcome the valve's closing spring tension and allow CPD to get back even further into the liquid fuel supply system. CPD is HOT and can cause the liquid fuel to carbonize in the lines, the liquid fuel flow divider (because it has to flow through the flow divider and the LFBV to get to the Stop Valve). If there's sufficient air (and CPD is air) the liquid fuel can even ignite in the system.

Also, there should be Purge Air flowing into the liquid fuel nozzles to purge them while running on liquid fuel, and if the liquid fuel check valve(s) is(are) leaking that air is hot, also, and is another source of air (at a slightly higher pressure than CPD) which could cause eventual ignition.

The first thing that happens when one or more liquid fuel check valves start leaking is that it pushes the liquid fuel in the system downstream of the Stop Valve through the liquid fuel flow divider into the other combustors--but this is only a small amount of liquid fuel but still enough to cause carbonization of the fuel in the liquid fuel nozzles and even in the liquid fuel flow divider.

Eventually, if the pressure is high enough to open VS-1, then the pressure upstream of VS-1 will also rise. And CPD and Purge Air can flow into liquid fuel canisters and even as far back as the liquid fuel supply tank. And it can even cause a fire in the liquid fuel supply system. (That would be a very drastic case--but still a possibility, nonetheless.)

If you suspect VS-1 is leaking you should close the Manual Isolation (Stop) Valve upstream of VS-1, the valve closest to the Accessory Base Liquid Fuel inlet. If the pressure in the liquid fuel nozzles builds up to CPD pressure you can be fairly certain one or more liquid fuel check valves is leaking--but the only way to know which one(s) is to shut the machine down and remove the check valves and inspect and test them.

Now, this shouldn't cause high spreads while running on gas fuel, but it may. The volume of liquid fuel in the piping downstream of VS-1 isn't that much, and even if the Liquid Fuel Forwarding Pump was running, the Liquid Fuel Supply Pressure (which should be no more than 4-4-1/2 bath) should be less than the Liquid Fuel Check Valve cracking pressure (approximately 6-1/2 - 7 bath) and still less than CPD.

No, the cause of the high spread is likely something else--a cracked combustion liner or transition piece, or a leaking side seal, or plugged (choked) gas fuel nozzle passages. Poor gas fuel quality, including entrained liquids in the gas stream such as compressor seal- or lubricating oil, gasoline, etc.,, can cause gas fuel nozzle issues and high spreads.

The above description is primarily for conventional combustors, but can also apply for DLN-I combustion systems as well. A "leaking" VS-1 can be very bad for any unit.

So, you should see that it should not be possible for substantial amounts of liquid fuel to flow into the unit if VS-1 is opening (should only be just slightly opening) unless the Liquid Fuel Forwarding Pump is running, and even then liquid fuel supply pressure is lower than CPD and liquid fuel check valve cracking pressure. And the liquid fuel supply pressure when running on gas fuel should be zero.

Again, if you suspect VS-1 is leaking, you should immediately close the Manual Isolation Valve upstream of the Liquid Fuel inlet to the Accessory Base, and the unit should be shut down and the fuel systems inspected for the possible cause of the high spreads. And, while the fuel nozzles are out the liners and transition pieces should also be inspected. As well as the Liquid Fuel Check Valves and the Purge Air Check Valves (these can leak and cause problems with starting on or transferring to liquid fuel, and/or cause high spreads when running on liquid fuel.

Please review the Liquid Fuel- and Liquid Fuel Purge P&IDs, and find the cut-away drawing showing the internal passages of the fuel nozzles for the details of the systems and the nozzles. But if you have pretty good suspicions VS-1 is leaking you need to take action sooner rather then later.

Hope this helps! Please write back to let us know what you find!!!

Dear CSA,

thanks for reply

we have initiated proposal to replace check valve in liquid fuel line and purge air line both. Job is almost done also. I will tell you the results afterwards.

new Liquid fuel check valves have forward direction operating pressure 7.5 kg/cm2 and purge air check valves have forward direction operating pressure 1.5 kg/cm2. forward direction being direction towards nozzles entry. new check valves are tested on reverse pressure up to 55kg/cm2 and found non operating. So i hope new check valves should be reliable enough.

i have doubt about entrance of CPD into liquid fuel line (in case check valves are passing).

CPD exits from 17th stage of compressor and enters into reverse flow combustion canes liners. how come CPD enters into liquid fuel line?

1 out of 1 members thought this post was helpful...

khoriwal000,

>i have doubt about entrance of CPD into liquid fuel line (in
>case check valves are passing).

>CPD exits from 17th stage of compressor and enters into
>reverse flow combustion canes liners. how come CPD enters
>into liquid fuel line?

The liquid fuel lines discharge through the liquid fuel nozzles into the combustion cans. There is nothing to prevent flow backwards through the liquid fuel nozzle--except the liquid fuel check valve. If the liquid fuel check valve fails and allows the pressure in the combustion can (which is just a couple of psig below CPD, but is usually referred to as CPD pressure...) to enter the liquid fuel nozzle and flow backwards into the liquid fuel system.

Purge air should be flowing into the liquid fuel nozzle--against the liquid fuel check valve which should be blocking purge air from getting into the liquid fuel lines. The main purpose of purge air (which has to be at a slightly higher pressure than the "CPD" inside the combustion can to flow through the liquid fuel nozzle and into the combustion can) is to push out any liquid fuel from the nozzle into the combustion to prevent the carbonization of the liquid fuel due to the heat in the combustion can. Carbonized liquid fuel can cause serious problems when starting on, transferring to or operation on liquid fuel. (The secondary purpose of purge air is to cool the internal liquid fuel nozzle passages; it's not much cooling, but it does help.)

Purge air (through VA-19, I believe) comes from CPD--so it's at "CPD" pressure, which is also a little lower than the pressure inside the combustion can because of the pressure drop across the purge air check valve. If the liquid fuel check valve fails and allows flow in the reverse direction, it can be purge air that initially enters the liquid fuel lines.

Once air gets into the liquid fuel system it pushes remaining liquid fuel in the other lines to flow through the liquid fuel flow divider working liquid fuel check valves into the combustion cans. And, when the pressure builds up high enough in the liquid fuel system and flow divider then it also pushes backwards on the liquid fuel in the high-pressure liquid fuel pump and through the LFBV (Liquid Fuel Bypass Valve)--which is wide open when running on gas fuel--and against the bottom of the plug of the liquid fuel stop valve, VS-1.

When there is sufficient pressure against the bottom of the plug of the liquid fuel stop valve it will eventually overcome the closing spring tension and begin to open the liquid fuel stop valve. (When liquid fuel is flowing there is pressure, from the liquid fuel supply system, which acts on the top of the liquid fuel stop valve plug to help the closing spring to close the valve when it needs to close. Hydraulic pressure is used to open the liquid fuel stop valve against the force of the liquid fuel supply pressure combined with the liquid fuel stop valve closing spring to open and keep the liquid fuel stop valve open.) When the unit is running on gas fuel, there is no liquid fuel supply pressure helping the closing spring of the liquid fuel stop valve to keep the valve closed. And there is no hydraulic pressure to close the liquid stop valve, either--only closing spring tension. And, the pressure of "CPD" (be it purge air or combustion can pressure) working on the surface of the bottom of the liquid fuel stop valve plug can slightly open the liquid fuel stop valve.

But this can only happen when one or more liquid fuel check valves are leaking in the reverse direction. The more that are leaking, the worse the problem is. And hot air entering the liquid fuel lines is NOT good--be it purge air, or "CPD" from the combustor. (Remember, CPD is at 700 deg F as it leaves the 17th stage of the axial compressor and enters the reverse flow combustion cans and combustion can liners.)

Find a cutaway drawing of the liquid fuel "cartridge" and you will see that while the internal passages are small there is nothing to prevent flow in either direction--into the combustion can, or out of the combustion can.

I've also been told that since the flow of purge air is small, once reverse flow of purge air through a leaking liquid fuel check valves is established that "CPD" from the combustion can will begin flowing backwards through the fuel nozzle and the leaking check valve, and this is a much higher flow.

So, "CPD" is kind of a catch-all term for purge air pressure and combustion can liner internal pressure, both of which are just slightly less than CPD but are virtually at CPD pressure (for all intents and purposes). Sorry if my reference to "CPD" caused any confusion.

I find looking at two or more P&IDs can sometimes be confusing, because several of them are all related. The Liquid Fuel System P&ID refers to the Liquid Fuel Supply P&ID, and often to the Atomizing Air P&ID and the Purge System P&ID as well as the Hydraulic System P&ID. Make your own drawing of one liquid fuel nozzle, showing the purge air check valve and the liquid fuel check valve (the fuel nozzle portion doesn't have to be detailed--it's the upstream portion you're concerned about). Show the liquid fuel flow divider, and the high-pressure liquid fuel filter and the LFBV (which is a normally open valve--and is fully open when running on gas fuel, making for an open path around the high-pressure liquid fuel pump) around the high-pressure liquid fuel pump and the liquid fuel stop valve. Show the purge air line from VA-19 (if I recall correctly) to the purge air check valve. Show the supply to VA-19 (indicating where it gets is supply/pressure from). You will now be able to see the path more clearly than trying to follow it on several P&IDs.

Or, print large copies of all the relevant P&IDs and make notes on them showing more detail of the associated P&IDs (systems) to get the "bigger picture."

Hope this helps!

I am unable to comprehend on how can the CPD work on the surface of the bottom of the VS1 plug?

khoriwal000,

CPD pushes on the liquid fuel in the system (distillate fuel is non-compressible!) and that pushes on the bottom of the liquid fuel stop valve plug.

Find the drawing of the liquid fuel stop valve in the Parts section of the Operation & Service Manuals provided with the unit, and it will become immediately clear.