CPD Pressure


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Hi all

As U know in full speed no load CPD is approximately 5 bar (GE frame9), and when unit is in base load it is approximately 10 bar. now my question is how it is possible with considering RPM is the same and IGV angle is the same?

This is a great question, and one that few people take note of--or care to ask about. It's just kind of accepted, but rarely questioned.

First of all, axial compressors are pretty unique machines. If you plot CPD versus speed you will see that CPD doesn't really start to increase very much until the unit gets up pretty close to rated speed. They are NOT very efficient below their rated speed. As you noted, they run at a constant speed and push a "relatively" constant (read below) amount of air when at rated speed (if the IGV angles doesn't change).

Second, when the downstream pressure (the "back-pressure) of an axial compressor increases the axial compressor continues to move the same amount of air, but it has to do so by increasing the discharge pressure. In this way, it's kind of like other compressors--but still not very much.

Third, most "newer" GE-design Frame 9E heavy duty gas turbines have variable IGVs (Inlet Guide Vanes), that modulate (change position) from a minimum operating angle at FSNL (prior to and for some time after synchronization) to a maximum operating angle at or near Base Load (the position is is not "stepped", rather it's infinitely variable between min- and max operating angles, depending on the operating condition and IGV position reference).

(There are machines with what are called "bang-bang" IGVS--meaning they are either "open" or "closed", and no position in between. There are even some very "mature" (old) machines, though no Frame 9E's that I'm aware of or have encountered, that have fixed IGVs that never change position from zero speed to Base Load (or from Base Load to zero speed). So, without knowing more about your machine and it's configuration it's very difficult to say--and I'm going to presume the unit has VIGVs (Variable Inlet Guide Vanes), because that's more the norm for GE-design Frame 9E heavy duty gas turbines (produced since about the early-to-middle 1990s, anyway.)

As the IGV angle is varied when the speed remains constant the air flow-rate through the axial compressor also changes. Since no other mechanical component internal to the machine varies with load during operation (that is, the turbine nozzles don't change angle/opening; the axial compressor blades (stationary and rotating) don't change angle/position; the exhaust diffuser is fixed and doesn't change position; the openings in the combustion liner are fixed and don't change size; etc.) as air flow increases there is naturally an increase in back-pressure--albeit slight, but it is there, again because the machine internals remain constant and present as a "orifice" acting as kind of a restriction (but withing the operating limits of the machine; the axial compressor can almost never flow too much air).

Fourth, some GE-design heavy duty gas turbines also do not close the axial compressor bleed valves until the generator breaker closes during synchronization, some not even until the unit reaches some load (around 5 MW). There is a pretty significant increase in CPD when the axial compressor bleed valves close (even at 95% speed, when they are typically closed on most non-DLN combustor-equipped GE-design heavy duty gas turbines).

Fifth, when the machine is synchronized and loaded, the fuel flow-rate into the machine increases. For fuel to flow into the combustor it must be at a higher pressure than the pressure inside the combustor--and this, also, very slightly increases the back pressure on the axial compressor.

Sixth, as fuel is burned there is a pressure rise inside the combustor--a pretty significant pressure rise, just due to the expansion of the fuel/air mixture as it burns and increases the temperature of the gases inside the combustor. This is the biggest cause for the increase in CPD as the machine is loaded. If you were to plot CPD when the machine was running, you would see that as fuel flow-rate increases or decreases CPD follows pretty closely (even when IGV angle is also changing).

(When fuel is burned (ignited) in a piston engine it is the pressure rise that creates the force the drives the pistons to create the torque to propel the vehicle. In a gas turbine, there's no "bang", just a constant burn when the unit is operating normally--which does cause the back-pressure against the axial compressor to increase which causes the axial compressor pressure (CPD) to increase while trying to maintain air flow.)

If you were plotting CPD while the unit was synchronized and producing power and the unit tripped (meaning the fuel stop valve was closed very suddenly) you would see a marked decrease in CPD--this is due primarily to the loss of pressure due to the extinguishing of the flame (loss of combustion). Yes, the turbine speed does start to decrease very quickly when flame is lost and at some point the axial compressor bleed valves will also open (either when the generator breaker opens or at approximately 94.5% speed), but the loss of pressure due to the loss of flame (combustion) is the primary reason for the loss of CPD.

So, all of these reasons, to one extent or another, cause the axial compressor discharge pressure (CPD) to increase as load increases. The axial compressor is running at a constant speed, so the air flow is constant (unless the IGV angle is changing), and as combustion increases (more fuel is being burned) the back-pressure against the axial compressor increases which causes the axial compressor discharge pressure (CPD) to increase. The other factors (variable IGVs, axial compressor bleed valves) also factor, slightly, into the increase in CPD as load increases.

And, let's not forget: Load (electrical power production--amperes) is directly proportional (related) to fuel flow-rate. More fuel means more torque which means more amperes which means more MW.

Lastly, gas turbines are mass flow machines--meaning that the more air that flows through the axial compressor and turbine the more power they can produce. Yes, fuel flow-rate is important, too, but air flow is extremely important. (And air temperature has a LOT to do with air density, and so air flow-rate, through a gas turbine--all other factors being constant). If you plot CPD and MW and ambient temperature when the unit is operating at Base Load (CPD- or CPR-biased exhaust temperature control), you will see that as ambient temperature changes during the day so will CPD and MW. As ambient temperature increases during the hotter part of the day, CPD will increase because the air is less dense so less air is being moved through the axial compressor (when it's at constant speed and the IGVs are at steady at maximum operating angle). As the ambient temperature decreases CPD and MW will increase, because the air is more dense and more air is being moved through the axial compressor running at constant speed with the IGVs steady at maximum operating angle. (Yes, fuel flow-rate does change slightly--but it's primarily the change is air flow-rate due to ambient temperature change that's responsible for the ability to burn slightly more or less fuel and which is also responsible for the change in torque/MW production.)

It's a great question, and has a lot of variables, but the single biggest reason CPD changes when load changes is because of the change in the pressure inside the combustor as fuel is burned and fuel flow-rate changes. Again, there is a pressure rise when the fuel is burned--but it's not like that in a reciprocating engine which peaks every time fuel is ignited and then drops and then peaks again. In a gas turbine fuel is constantly flowing into the combustors and burning (under normal conditions!) so the pressure rise isn't as noticeable--but it does occur. Again, CPD decreases quickly when fuel is cut off and it's because of the pressure drop when combustion ceases that's primarily responsible for that.

It's interesting to note that some people believe that because "spark plugs" (more correctly called ignitors) are used to establish flame in gas turbines, that the spark plugs operate like those in internal combustion engines and are sparking whenever the turbine is running to maintain flame. The ignitors ("spark plugs") are only energized for a minute or less during starting to establish flame; and fuel flow-rate changes and the pressure in the combustor changes as fuel flow-rate changes--it's not high/low/high/low.... Pressure changes a fuel flowing in a continuously burning combustor changes--we don't actually monitor the pressure inside the combustor (or inside the cylinder of a reciprocating engine), but in a gas turbine CPD is directly proportional (related) to combustor pressure which is a function of fuel flow/burn.

Hope this helps!

(It's a long-winded answer because every time I explain this to people, they immediately start asking about IGV angle changes, and ambient temperature changes. As soon as the combustion pressure thing is explained and understood, all kinds of other things--which have small effects on CPD--are brought up for discussion. So, in an effort to try to limit the back-and-forth on a World Wide Web forum, I've attempted to be as precise as possible, which means there's no brief answer.)
Again thank U and thank U CSA.

These days I dont have enough time to reread your answer and think about it but I hope I'll gain a sense and understanding about it as soon as possible.
It's a pleasure reading CSA responses. Thanks Mr CSA !

> in full speed no load CPD is approximately 5 bar (GE frame9), and when
>unit is in base load it is approximately 10 bar

I'm not familiar with GE Frame 9 but I used to work on Heavy Duty Gas turbines like Frame 5 and 3. the change on the CPD is never so high between the FSNL and Base Load. I think combustion and back pressure will not double the CPD value. i think there is something else which double the Frame9 CPD?

Hope someone could give more clarifications about this CPD changes..


Thank you for the kind words!

GE-design Frame 9E heavy duty gas turbines are rated from 100 MW (older machines) to 125+ MW. The CPD for this range of machines is large, and while the range described by the original poster is fairly large it's not outside the realm of possibility for a machine with new and clean inlet air filters, well-adjusted IGVs, a new and clean compressor and a low exhaust back-pressure. But, basically the planets would all have to be in perfect alignment for the range described for FSNL to Base Load.

Some older machines run at FSNL during synchronization with the axial compressor bleed valves open, closing them when the generator breaker closes. This would make the range described more likely.

Some newer machines with DLN combustion systems operate at FSNL with the compressor bleed valves open and close them when the load increases above approximately 5 MW. But I believe the FSNL CPD for these machines is higher than 5 barg.

Hope this helps! Thanks again!
Dear Mr. CSA,

i am guessing that there was an unintentionally miss-typing in the post (As ambient temperature increases during the hotter part of the day, CPD will increase).
it should read CPD will decrease instead of increase.

Thanks a lot for your usual great answers, it is really pleasure to read them.