Why atomizing air compressor discharge pressure is to be maintained at around 1.2 times compressor discharge pressure ?

Frame 6 GT: PG 6541
Make: GE
Mark V control system

Atomizing Compressor discharge Pressure around 220 psi

 

Denice Thomas,

You posted another question about gas valves, so this question must be about why AA pressure remains above CPD when the unit is running on gas fuel.

There are two basic reasons. Most Frame 6B units have an accessory gear-driven AA compressor, and it runs all the time the unit is running. There is no clutch, so it's always running. You should have an Atomizing Air (Compressor) Bypass Valve, which should open to, 1) reduce the AA pressure when running on gas fuel, and, 2) reduce the energy consumed by the AA compressor when running on gas fuel.

In the first case, when the unit is running on gas fuel there is no need to provide air to assist with atomizing liquid fuel. But, because the compressor is always running (no clutch) there is no need for the same pressure/flow of atomizing air when running on gas fuel as when running on liquid fuel. There is, however, a need to keep a slight positive flow through the AA system to prevent backflow of hot combustion gases through the AA system, so there only needs to be sufficient flow to 1) prevent the backflow of hot combustion gases, and, 2) to maintain flow through the AA system for cooling purposes.

In the second case, when running on gas fuel with no need for AA other than for preventing the backflow of hot combustion gases AND for cooling, there is no need to run the AA compressor at the "full" output when running on liquid fuel. Another positive aspect of reducing the load on the AA compressor is to reduce the power lost to running the AA compressor, leaving a little more power output for the driven device.

In my experience, when a dual fuel GE-design Frame 6B heavy duty gas turbine is operating on liquid fuel, the AA compressor ratio is usually around 1.2, which is roughly the amount of AA which is necessary for proper atomization of liquid fuel. When the unit is operating on gas fuel, the AA compressor ratio is usually around 1.1.

So, my question to you is: Is the AA compressor ratio 1.2 on gas fuel or liquid fuel?

One last thing, the ratios I mentioned are *absolute*, which is calculated by adding atmospheric pressure to the numerator and denominator of the ratio calculation before dividing. It's only a slight change to the quotient, but it's the way GE calculates AA pressure ratios.

 

please mostly time during start up turbine i face the AA pressure ration Low which the causes of alarm

 

Dear Maint,

Does the alarm comes up during startup and stays or cleared after some time?

Did you check the pressure switch (I believe its 63AD) operation and is the setpoint of the switch is correct?

 

yes, during startup only

 

Maint,
Maint,
Maint,

Your posts tend to be short and one line, regardless of the thread. You provide very little in the way of information with which to provide a quick and concise response. As much as a Frame 6 is a Frame 6 (turbine-wise), the options and auxiliaries and packaging and control systems can be very, Very, VERY different.

You say you have a persistent AA pressure ratio low alarm during start-up, but you don't tell us if the unit is single fuel (liquid fuel only) or dual fuel (gas/liquid) and if the alarm is present on every start or just when starting on liquid fuel (if the unit is dual fuel).

When a GE-design heavy duty gas turbine with liquid fuel is starting-up, the axial compressor discharge pressure is low until the unit gets near rated speed. Since the suction of the AA compressor is fed from the discharge of the axial compressor, the output of the AA compressor won't be high enough with a low input to exceed the required differential during start-up. Also, the IGVs are usually closed until around 80% speed, which also tends to limit the axial compressor discharge pressure and therefore the AA compressor discharge pressure. Finally, the compressor bleed valves are also open until at or near rated speed.

But, once the unit is at rated speed, you seem to say that the low AA differential pressure alarm clears. Which indicates the pressure switch is likely calibrated correctly and the differential pressure is likely correct when the unit is at rated speed.

SO, what's usually done is to "block" the alarm until the unit is near or at rated speed to allow the differential to be at or near required.

It sounds like the "permissive" which blocks the alarm from being inappropriately displayed during start-up might be missing. When does the alarm go away/clear on your unit during the starting sequence? At 95% speed? At 100% speed/breaker closure? At 85% speed?

In the future, please provide as much information as possible when posting or when adding to another's post for for help with your problem. We don't know the particulars of your site and your unit, so if you want a quick and concise response please provide as much information as possible, including anything you've done to try to troubleshoot the problem or to understand the issue you're writing about.

 

sorry, i think Mr CSA when i write brief i think get my Question exactly. Thank You for your All your answers in this website. i real i follow your answer and explanation.

My unit working only by Liquid Fuel (Diesel). AA pressure ration Alarm coming during startup After 60 % of speed no problem that i mean.

Thank You

 

Maint,

I´m having some problems to start my turbine on liquid fuel. It´s a Taurus 60 turbine SOLAR. During start cycle we use air assists 120 cfm @ 100 psi; after 65% turbine should continues increasing speed without external air assists, however turbines shutdown occurs due to DP low pressure (Pressure of air assist - PCD) at 80% speed.

For this kind of turbine We will require a compressor all the time? (Increasing air assist pressure)

After 65% turbine should take PCD air to atomize? If yes, how does it happens? It´s a tubing taken out from the axial compressor? or it happens using flow through axial compressor - combustor?

 

CSA,

Are you saying that after turbine got rated speed Air assist only works as prevention?

I have got 100% speed on idle, I have seen that air assist pressure in the package was 125 psi same than compressor (external air source) pressure, it means that there was not any flow consumption from the air assist or It was minimum. When consumption exists there should be air pressure drop between source (compressor) and point to be fed.
It appear to be that at this speed, turbine is not depending of air assists.

I would appreciate your comments

Thanks

Regards,

 

Angel Albornoz,

I have very limited experience with Solar gas turbines, so I'm not saying anything about how any model of Solar gas turbine would operate on liquid fuel, or what the axial compressor discharge pressure should be, or the atomizing air compressor discharge pressure should be.

I have limited experience with GE aero-derivative gas turbines running on liquid fuel. My experience with the single annular combustor (non-DLE) versions of these turbines is that there is no atomizing air compressor. Liquid fuel is atomized by the fuel nozzles.

Sorry I can't be of more help with your specific issue.