Gas turbine

What is CPD bias and FSR bias? Our Mark V Frame 6 GT was earliear running on CPD bias. One day CPD reading become 8.6 to 7.6 on same load and machine transfer on FSR bias. Is it ok on FSR bias or not? What should we do? Please tell me about FSR and CPD bias.

From your wording "One day CPD reading become 8.6 to 7.6 on same load and machine transfer on FSR bias.", can I assume that the CPD dropped from 8.6 to 7.6?

The bias is applied to temperature reference TTRX to prevent the firing temperature Tf from exceeding the limit. Tf depends on two parameters - air (CPD) and fuel (fsr). The two biasing lines - CPD bias and FSR bias are parallel to each other with the FSR bias following the CPD bias.

In the case of your unit, the response is normal. The FSR bias took over, because of the drop in CPD.

Control.com has a very powerful 'Search' feature and these two terms (usually presented as being hyphenated words (CPD-biased, FSR-biased)) have been covered in detail in previous threads.

Very simply, CPD (Compressor Discharge Pressure) bias is the means for determining the maximum allowable exhaust temperature reference for running conditions. FSR (Fuel Stroke Reference) bias is a "back-up" to maximum allowable exhaust temperature reference. On some machines, it allows the unit to operate in the event of a failure of a CPD transmitter.

It would appear your CPD transmitter has failed; has anyone compared the CPD transmitter feedback value to the CPD pressure gauge reading?

FSR bias is an approximation of allowable exhaust temperature control limits based on Fuel Strok Reference (the Gas Control Valve reference or the Liquid Fuel Bypass Valve fuel flow-rate reference). Sometimes, it results in a reduced output and sometimes it results in a higher than optimal output. (FSR is the signal that determines fuel flow; for gas fuel, that's a valve position reference, for liquid fuel that's a liq fuel flow-rate reference.)

If the unit has shifted from CPD- to FSR-bias, you should *immediately* determine what the problem with the CPD transmitter is and resolve it. I would recommend that before you return the CPD transmitter to service, if you're going to do it with the unit running, that you reduce load to 70-75% of rated before putting the transmitter back in service, and do so by slowly opening the sensing/isolation valve.

How do you view whether the unit is running on CPD bias or FSR bias?

What are the diffrences toward the machine?

I want to learn about CDP and air flow in GE Frame 5 unit.

My understanding is as follows:
Since the compressor rotates at 5100 RPM, the mass flow of air through the turbine is constant. When load is less, the fuel is less in the combustion chamber and hence more air can easily force itself in the combustion chamber so less air should be available for cooling. In this case, since air can easily enter the combustion chamber the CDP will read less. Is this the correct line of thinking?

My next question is about inlet temperature:
I have read here that as the inlet temperature falls, the CDP improves and hence the power generation increases. But is it right to think as follows: when compressed cold air will enter the combustion chamber the temperature in the combustion chamber will tend to fall (because air is colder due to lower inlet temp), hence more fuel will be needed to make up for the loss of heat energy. So the power generation should remain constant, and the fuel consumption should rise.

Thank you,
Ashutosh

The most common Mark V signal name for CPD-biased exhaust temperature control reference temperature is TTRXP, Turbine Temperature Reference - Exhaust, Primary, since the CPD bias is considered to be the primary exhaust temperature control bias.

The most common Mark V signal name for FSR-biased exhaust temperature control reference temperature is TTRXS, Turbine Temperature Reference - Exhaust, Secondary, since the FSR bias is considered to be the secondary exhaust temperature control bias. I have to caution many people that early on in the product life of the Mark V, the secondary- or back-up exhaust temperature control reference was changed from FSR to megawatt, or load, bias. The signal name didn't change, but the value that was used for the secondary, back-up exhaust temperature reference determination was changed from FSR to DW or DWATT (those are the two most common signal names for the unit output when driving a generator).

These two values (should) feed into a MINimimum SELect block where the lower of the two values becomes the Exhaust Temperature Control Reference, TTRX, which then feeds into the calculation of FSRT and FSR. So, if TTRX is equal to TTRXP *and* the unit is operating at Base Load (exhaust temperature control), then the unit is operating on CPD-biased exhaust temperature control. If TTRX is equal to TTRXS *and* the unit is operating at Base Load (exhaust temperature control), then the unit is operating on the secondary, back-up exhaust temperature control, which might be FSR-biased or megawatt- (load-) biased exhaust temperature control.

Back-up exhaust temperature control is only intended as a means of keeping the unit running *UNTIL* the CPD transmitters can be repaired/replaced and returned to service. The back-up exhaust temperature control reference is supposed to closely mirror the primary exhaust temperature control reference but be slightly greater than the primary exhaust temperature control reference. So, when a properly-configured unit is operating on back-up exhaust temperature control, the turbine inlet temperature (the temperature of the combustion gases entering the first stage of the turbine) are slightly higher than optimal, meaning the hot gas path components are being exposed to slightly higher than desired temperatures, which in turn decreases the parts life. That's why the condition which caused the primary reference to "fail" be corrected as quickly as possible.

I recommend looking at the Dynamic Rung Display to see these two signals feeding the MIN SEL block and the output of the MIN SEL block and follow it through the CSP to become more familiar with the CSP and logic and sequencing. The answers to all these kinds of questions are in the CSP, it just helps to understand some of the signal names and where to start looking, but people should take the opportunity to look and learn once some of the signal names are "exposed".

This is probably one of the hardest things to explain and understand, because it changes depending on the operating mode of the turbine. And, axial compressors don't really behave exactly as most other types of compressors (reciprocating, centrifugal) behave. Also, I believe we all want to believe that when one thing is changing in a system everything else is constant, but in reality, that doesn't always happen. I also believe that when mathematicians and physicists and machine designers try to explain things to us lay people that they like to make statements to the effect that this or that can be attributed to a change in this one single condition or value. Well, the real world is seldom quite so simple. (CDP and CPD are the same
thing: Compressor Discharge Pressure, and Compressor Pressure -
Discharge.)

Let's consider part load, steady-state operation, *without* Pre-Selected Load Control enabled. The speed of the unit is constant (presuming it's a generator drive and the generator is connected to a fairly stable electrical grid) and the IGV angle (Inlet Guide Vane), should be constant. (Some machines have variable, or, modulated, IGVs; others have IGVs that are just "opened" or "closed"; for the purposes of this discussion it doesn't make any difference because for most units operating at part load the IGV angle is fairly constant for a particular load, but there are exceptions to every rule.) The fuel is being held constant when the unit is at Part Load with Pre-Selected Load disabled. If the ambient temperature drops then the air entering the compressor becomes more dense (denser?), and this requires more energy to move the air through the unit. The actual power output of the unit will decrease, every so slightly, because of the increased energy required to move the denser air and because of the cooling effect of the increased air flow as you have noted. The decrease might be so slight, depending on the change in ambient air temperature, that it would be almost imperceptible, but it occurs. The increased air flow actually causes the CPD to increase slightly. But, at Part Load, the fuel flow is being held constant.

Now, let's consider CPD-biased exhaust temperature control, or, Base Load operation, which is really another way of saying 'constant firing temperature', which some turbine manufacturers use to mean the temperature of the hot combustion gases entering the first stage of the turbine. By definition, a unit cannot be at Base Load unless the IGVs are fully open. (To be specific, I'm talking about when an operator selects Base Load and the unit is producing "rated power" on (CPD-biased) Exhaust Temperature Control.) When the ambient temperature drops and the air becomes more dense, the compressor discharge pressure increases. The increased air flow would tend to cause the firing temperature to decrease but the turbine control system is programmed to increase the fuel flow slightly to keep the firing temperature constant. The net effect of the increased air flow due to the decreased ambient temperature is to increase the power output of the unit.

Only mathematicians and physicists and scientists can tell us how much of the increased power output is due to the increased air flow because of the increased density or how much is because of the increased fuel flow. But, I believe that most formulae tell us that the efficiency of the unit increases, meaning the power output per unit of fuel, increases when the mass flow through the unit increases. Which is why evaporative coolers, foggers, and chillers are cost-efficient and
common: cooler air means more power output and more efficiency. Also,
the ability to burn fuel is primarily a function of the amount of air that can be run through any internal combustion engine: increase the air and you can increase the fuel, and the power output will increase.

The thing which really confuses most people is that the slope of the CPD-biased exhaust temperature control curve is negative, which means that as fuel flow increases when CPD increases, the exhaust temperature decreases, which is completely opposite to what one would expect (more fuel, more exhaust temperature). But, because of the increased air flow *at a constant firing temperature*, the exhaust temperature actually goes down as power and fuel flow increases. This is the other really hard part of understanding gas turbines; it doesn't seem to make sense until you really consider everything that's happening. Not all of the air that goes through a gas turbine is used for combustion air; quite a lot of it is used for cooling and dilution.

Aren't you really trying to ask in your first question: Why does CPD increase as the unit is being loaded even though the compressor speed is constant and the IGVs are not changing position during loading?

Now, I have a question for you which is kind of related to what you're alluding to in your first question: Presume the gas turbine IGVs are full open for most of the loading of the unit (and for many older Frame 5s this is very true), which means that the air flow through the compressor is fairly constant regardless of the amount of fuel being burned because the speed of the compressor is constant. Further, presume that the ambient temperature is fairly constant (as it is) when the unit is being loaded (from no load to Base Load). CPD increases as fuel flow increases, right? But why? If the air flow isn't changing because the axial compressor is rotating at a constant speed and the IGVs are full open, why is the CPD increasing?

And my question is the basically the same as yours: Why does CPD increase when the unit is being loaded, when the speed of the axial compressor is constant and the IGVs aren't changing? (Hint: What is changing as the unit is being loaded? And the answer is not the exhaust temperature.)

This is not a trick question, nor a "test question" (CTTech, I'm finally beginning to understand your reference!).

Thank you for explaining so nicely in non-mathematical terms. I dont hate maths, rather loved it cos it was the most scoring subject but its always very difficult to mathematically understand the physical process.

I understand two basic things about air flow from your post. First, that at part load operation, the fall in ambient will NOT necessarily boost output, rather might reduce it - albeit too slightly - cos the fuel flow remains constant and cooler air will tend to cool the combustion chamber. Second, that only in temperature control mode the power output will increase as a result of fall in ambient temperature because the control system will again try to make up for the fall in combustion temperature by supplying additional fuel. This will lead to increase in power output.

I think, I may take the 'test' question because I read it in some post by mark5guy on this forum itself that despite the constant speed of the rotor, the cdp changes due to change in gas volume inside the combustor chamber. From FSNL to base load, the only thing that changes is the gas flow in to the combustor cans. Hence this will affect the ease with which the compressor discharge air, enveloping the combustor cans, can penetrate the combustion chamber. Less the load and hence less the volume of gas hence more the ease and subsequently less the CDP and vice versa.

Regards,
Ashutosh

You are right, fuel flow-rate increases as the unit is loaded. When fuel is burned in the combustor, there is a pressure rise inside the combustor. That means the back-pressure against the compressor increases, but the air flow remains basically the same (we're talking about increasing fuel without a change in IGV angle). So, as more fuel is added to the combustor (the fuel has to be at a higher pressure than the pressure inside the combustor) the pressure inside the combustor increases.

When the unit trips (ememgency shutdown), CPD deceases very quickly as the fuel is shut off, then ramps down as speed decreases and the IGVs close.

How quickly can the system reciprocate between the CPD bias and the FSR bias?

Does this relate to Allen-Bradley or PLC 5 systems?

Can you email me at eggman1812 @ live. ca?

How does this information translate to the firing/gas turbine operation on a Allen Bradley SLC 500 or a PLC5 system?

There's absolutely no telling how the integrator programmed the A-B system. I've seen PLCs used for turbine control that didn't have any kind of exhaust temperature control, only load control. The unit would reach a certain output, and that was it. Funny how the output was the ISO rated output of the unit; funnier how it hit that same output regardless of ambient temperature or compressor cleanliness or inlet filter differential pressure. The fun stopped at the first combustion inspection when the first stage turbine nozzles and combustion liners were all found to have extensive cracking.

Does the A-B have a primary and/or back-up exhaust temperature control?

In a Speedtronic turbine control panel, the transition is seamless and bumpless.