i am representing you a data of our power plant:

1.rpm-5151 load-21.5 MW cpd-8.71 bar
2.rpm-5155 load-32.12 MW cpd-9.46 bar

my question is why cpd is increasing? how it is possible that at same speed cpd is increasing because compressor is rotating at same speed only the load is different?

 

me,

Most GE-design heavy duty gas turbines have variable Inlet Guide Vanes (IGVs) on the axial compressor inlet. These are used to control the air being drawn into the axial compressor, which also affects the axial compressor discharge pressure. As the IGVs open or close, the axial compressor discharge pressure will also increase or decrease, even with little or no change in fuel flow-rate.

But, even if the IGVs are not variable or are in a stationary position as a GE-design heavy duty gas turbine is loaded (by increasing the fuel flow-rate to the turbine combustors) the axial compressor discharge pressure will increase. This happens for two reasons; first, the mass flow through the unit is increasing because the fuel flow-rate is increasing.

The second, and more important, reason is that as more fuel is burned the pressure inside the combustor increases--the same thing that happens when the fuel flow-rate to the cylinder(s) of a reciprocating engine is increased. The more fuel being combusted, the more pressure that's generated by the act of burning the additional fuel in a confined space. (A gas turbine combustor is a confined space--not is the safety sense, but in a physical sense.)

In the case of gas turbines, this increases the back-pressure on the axial compressor--which as you said, is rotating at a constant speed (and in this example we are considering the IGVs are not moving as fuel flow-rate is increasing; they are either stationary for some reason, or they are fully open as fuel flow-rate is increasing). The axial compressor has to work harder--at the same speed and with the same air inlet flow-rate--to push the air into the combustor in which the internal pressure is increasing because more fuel is being combusted.

So, primarily, it's caused by the increased fuel being combusted, both the increasing mass flow-rate of fuel AND the increased pressure that results as more fuel is being combusted. And, it's a characteristic of axial compressors (remember: they are NOT centrifugal compressors and as such have different operating characteristics) that the discharge pressure will increase as the pressure in the combustor (which is effectively a "back-pressure) increases.

If you want some additional evidence of this, just look at a high-speed trend of CPD when the turbine is tripped (emergency closing of fuel stop valves). The CPD drops very quickly when there is no fuel being burned and the combustor pressure decreases very suddenly, and then decreases as the shaft decelerates.

This is one reason combustor liners can fail--excessive turbine trips from high loads (high fuel flows). The sudden decrease of pressure inside the combustor, as well as the sudden thermal stress caused by the loss of temperature as flame is extinguished, as well as the change of pressure gradient across the combustor can all cause combustion liners to collapse or fail prematurely.

Fuel flow is directly proportional to load: as fuel flow-rate is increased, the load increases. As fuel flow-rate decreases, the load decreases. As fuel flow-rate increases, the pressure inside the combustors increases. As combustor pressure increases, the axial compressor has to work harder (develop more pressure) for the same flow (assuming the IGVs are stationary).

Hope this helps!

 

I would like to add.

which machine is there? If DLN equipped machine then consider the IBH. What is the position of IBH Control valve, It will also tend to increase the CPD.

Hope this will help

 

me,

Most GE-design heavy duty gas turbines have variable Inlet Guide Vanes (IGVs) on the axial compressor inlet. These are used to control the air being drawn into the axial compressor, which also affects the axial compressor discharge pressure. As the IGVs open or close, the axial compressor discharge pressure will also increase or decrease, even with little or no change in fuel flow-rate.

But, even if the IGVs are not variable or are in a stationary position as a GE-design heavy duty gas turbine is loaded (by increasing the fuel flow-rate to the turbine combustors) the axial compressor discharge pressure will increase. This happens for two reasons; first, the mass flow through the unit is increasing because the fuel flow-rate is increasing.

The second, and more important, reason is that as more fuel is burned the pressure inside the combustor increases--the same thing that happens when the fuel flow-rate to the cylinder(s) of a reciprocating engine is increased. The more fuel being combusted, the more pressure that's generated by the act of burning the additional fuel in a confined space. (A gas turbine combustor is a confined space--not is the safety sense, but in a physical sense.)

In the case of gas turbines, this increases the back-pressure on the axial compressor--which as you said, is rotating at a constant speed (and in this example we are considering the IGVs are not moving as fuel flow-rate is increasing; they are either stationary for some reason, or they are fully open as fuel flow-rate is increasing). The axial compressor has to work harder--at the same speed and with the same air inlet flow-rate--to push the air into the combustor in which the internal pressure is increasing because more fuel is being combusted.

So, primarily, it's caused by the increased fuel being combusted, both the increasing mass flow-rate of fuel AND the increased pressure that results as more fuel is being combusted. And, it's a characteristic of axial compressors (remember: they are NOT centrifugal compressors and as such have different operating characteristics) that the discharge pressure will increase as the pressure in the combustor (which is effectively a "back-pressure) increases.

If you want some additional evidence of this, just look at a high-speed trend of CPD when the turbine is tripped (emergency closing of fuel stop valves). The CPD drops very quickly when there is no fuel being burned and the combustor pressure decreases very suddenly, and then decreases as the shaft decelerates.

This is one reason combustor liners can fail--excessive turbine trips from high loads (high fuel flows). The sudden decrease of pressure inside the combustor, as well as the sudden thermal stress caused by the loss of temperature as flame is extinguished, as well as the change of pressure gradient across the combustor can all cause combustion liners to collapse or fail prematurely.

Fuel flow is directly proportional to load: as fuel flow-rate is increased, the load increases. As fuel flow-rate decreases, the load decreases. As fuel flow-rate increases, the pressure inside the combustors increases. As combustor pressure increases, the axial compressor has to work harder (develop more pressure) for the same flow (assuming the IGVs are stationary).

Hope this helps!

I have a query here. Since, the combustion in the GT is a constant pressure combustion (total pressure), then how does fuel addition increases the chamber pressure (or back pressure)..might be it is static pressure that rises, but again, with increase in temperature due to combustion, the temperature and hence velocity increases (there by increasing the dynamic pressure). so, with this both static and dynamic pressure is increasing (nothing but total pressure)..which is in contradiction with constant pressure combustion......can you please help me in understanding the theory.....Thanks in advance

 

Well, the "constant pressure" thing is just that--it's a thing. An ideal thing. In reality, when you add more fuel to the combustor--even if you hold the air flow constant--the pressure in the combustor increases from the combustion of the fuel. It's not a huge increase, but it is an increase that increases the back-pressure against the axial compressor discharge.

If the fuel is immediately shut off (as in an emergency trip) and the flame goes out, the CPD decreases pretty rapidly--it's pretty much a step-change (decrease) in CPD. And, that's often a couple of seconds before the speed starts to decrease and the IGVs start to close. And, before you say, "Wait a minute--if the fuel is shut off the unit will start to decrease immediately!" Well, for many heavy duty (and aero-derivative) gas turbines when the fuel is shut off during loaded operation because of an emergency trip the generator breaker remains closed for a very brief of time--which keeps the turbine and axial compressor spinning at rated speed. The breaker is almost always opened by reverse power--after a short period of time (often the time is inversely proportional to the amount of reverse power; sometimes it's just a second or two). This is done to help prevent overspeeding of the turbine-generator which would likely occur if the breaker were opened at the instant the fuel was shut off--because there is still residual fuel in the fuel line(s) downstream of the fuel stop valve which must dissipate into the combustor and without a load could cause an overspeed.

There's ideal, ivory tower mathematical models of internal combustion engines (which a combustion turbine is), and there's real-world losses and factors which happen.

Hope this helps!

 

Thank You...as expected answer from CSA