Introduction

This report provides a clear and practical explanation of how the GE 9FA gas turbine behaves across its operating range in a combined-cycle power plant.
It focuses on:

Load vs firing temperature

CPD (compressor pressure ratio) influence

IGV and mass flow relationship

Why exhaust temperature is high at part load

Why exhaust temperature decreases toward base load

The difference between simple-cycle and combined-cycle firing logic

The role of IBH (Inlet Bleed Heat) in part-load operation

The explanations here are intended to give engineers a strong understanding of how the turbine’s thermodynamic characteristics and control philosophy interact.

2. GT Load Control Modes (Mark VIe Overview)

GE Mark VIe uses three main control regions:

2.1 Speed/Acceleration Control (0–3000 rpm)

Fuel flow controlled to maintain acceleration profile

IGVs remain near minimum position

Reaches 3000 rpm

2.2 Droop Load Control (Synchronization → Near Base Load)

Begins immediately after synchronization

Fuel increases based on droop (frequency-sensitive control)

IGVs open gradually to increase air mass flow

Unit responds to grid frequency deviations

2.3 Temperature Control (High load → Base load)

When firing temperature hits its limit, fuel stops increasing

Load becomes a consequence of firing temperature

IGVs at or near full-open

3. Relationship Between Firing Temperature and CPD

The GE 9FA does not use a fixed firing temperature.

Instead, it uses a CPD-biased firing temperature reference (TTRF1):

Low CPD → Higher firing temperature allowed

High CPD → Lower firing temperature allowed

This protects turbine hot-gas-path components because cooling air pressure increases with CPD.
Thus:

TTRF1 = function of (CPD), not a constant.

3.1 Firing Temperature Control Logic

The Mark VIe controller manages load and exhaust temperature as follows:

The controller reads CPD and determines the corresponding TTRF reference (TTRF_REF).

If the governor/load demands more power, fuel is increased, causing actual exhaust temperature and TTRF to rise.

If TTRF approaches or exceeds TTRF_REF, the FSR2 (firing temperature limiter) reduces fuel or prevents further increase.

A new equilibrium is established where CPD, mass flow, fuel flow, and TTRF satisfy both the load demand and TTRF limits.

This explains why, as CPD increases, the turbine initially adds fuel to maintain load and exhaust temperature until the firing limit is reached, after which the temperature controller constrains further increases.

4. Why Exhaust Temperature Is High at Part Load

At part load:

Mass flow is lower

CPD is lower

Firing temperature must be raised for combustion stability

HRSG needs hotter exhaust to maintain steam generation

This results in exhaust temperatures around 640–660°C at part load.

4.1 Combined-Cycle Requirement

GE intentionally maintains high exhaust temperature at part load because:

HRSG steam generation collapses if exhaust temperature falls

Combined-cycle efficiency depends heavily on exhaust heat

This is a design requirement, not a fault.

5. Why Exhaust Temperature Decreases at Higher Loads

As the turbine approaches base load:

Mass flow increases

More energy is converted into shaft power

Less heat remains in exhaust

CPD increases → firing reference decreases

Exhaust temperature typically drops to ~600–620°C

This is normal GE 9FA behaviour.

6. Simple-Cycle vs Combined-Cycle Firing Philosophy

Simple Cycle

Exhaust temperature can be reduced (even to 550–580°C)

Lower firing temperature acceptable

No HRSG to feed

Combined Cycle

Requires higher part-load exhaust to maintain steam production

Uses a different firing schedule optimized for HRSG efficiency

This explains why you cannot reduce exhaust temperature in combined cycle the same way as in simple cycle.

7. IGV, Mass Flow, and Compressor Work

7.1 IGV Function

More IGV opening → more air mass flow → higher CPD

Less IGV opening → less air mass flow → lower CPD

7.2 Compressor Work

Compressor power depends on both mass flow and pressure ratio.
Thus, compressor load increases significantly as IGVs open and airflow rises.

8. Why You Cannot Reduce Exhaust Temperature in Combined Cycle

Even if the gas turbine could run at lower exhaust temperatures, this would:

Reduce steam production

Reduce steam turbine output

Lower total plant efficiency

Move operating point outside GE combined-cycle design philosophy

Therefore:
Combined cycle requires higher exhaust temperature at part load — by design.

9. Real Plant Observation Interpretation

Your measurements confirm GE design behaviour:

150 MW → Part-load conditions

CPD ≈ 10.5 bar

Exhaust temperature ≈ 650°C

Exhaust flow ≈ 460 kg/s

~180 MW → Transition point

CPD ≈ 11.8 bar

IGV ≈ 63°

Mass flow ≈ 502 kg/s

Exhaust temperature starts decreasing

250 MW → Near base-load conditions

CPD ≈ 15.2 bar

Exhaust temperature ≈ 613°C

Exhaust flow ≈ 638 kg/s

This shows the transition point at which:

Exhaust temperature starts to fall

CPD rises

IGV and mass flow increase toward base-load values

These real values perfectly match expected 9FA characteristics and demonstrate how firing temperature, CPD, and airflow interact to control exhaust temperature.

10. Summary of Key Points

GE 9FA exhaust temperature is intentionally high at part load.

Exhaust temperature drops as load increases.

IGVs directly influence mass flow, CPD, and firing temperature.

Combined-cycle units must maintain high exhaust temperature for HRSG performance.

Firing temperature reference is CPD-biased, not fixed.

Base-load exhaust temperature is lower because turbine efficiency increases.

IBH plays a major role in stabilizing part-load combustion and airflow.

11. IBH (Inlet Bleed Heat) Operation and Impact on Exhaust Temperature

IBH is a key system at part load and is important for understanding turbine behavior.

11.1 What IBH Does

IBH re-circulates hot compressor discharge air back to the compressor inlet.
This increases inlet temperature and lowers compressor pressure ratio.

IBH is mainly used for:

1. Improving Compressor Surge Margin

At low load:

Mass flow is low

Pressure ratio is low

Surge margin becomes tight

IBH helps by shifting the compressor operating point away from the surge line.

2. Improving Emissions Stability

At low load, combustion tends to become unstable.
IBH increases inlet temperature, helping:

Maintain stable flame

Reduce NOx and CO emissions

11.2 How IBH Operates With Load

Below ~180 MW: IBH opens, providing surge protection and emissions stability.

Above ~180 MW: IBH closes, restoring maximum CPD and airflow for base-load efficiency.

11.3 Impact on Exhaust Temperature

When IBH is open:

Inlet temperature increases

CPD drops

Firing temperature reference increases

Exhaust temperature becomes higher

When IBH closes:

Inlet temperature decreases

CPD increases

Firing reference decreases

Exhaust temperature drops

Conclusion:
IBH is a major contributor to high part-load exhaust temperature and its decrease toward base load.

12. Disclaimer

This report was prepared as a personal effort by Mohamed Zaki Nawar, with the assistance of ChatGPT to help organize information and explain technical logic.
The content reflects my current understanding of the GE 9FA gas turbine in combined-cycle operation, but it is not guaranteed to be 100% accurate.
For operational decisions, please refer to official GE manuals and operating guides.

 

@Zikwa,

What combustion system is in use on the 9FA(s) at your place of work? DLN 2.0, or DLN-2.6, or conventional combustors (no DLN equipment/controls)?

How old is/are the machine(s) at your place of work?

Do you have access to the Control Specification provided with the machine or the control system if the control system has been upgraded since original commissioning?

Are you certain the machine(s) at your site don't use CPR (Compressor Pressure Ratio) bias for Base Load Exhaust Temperature control?

What control system is presently being used on the machine(s) at your place of work?

What control system was provided with the machine(s) when they were originally installed and commissioned?

Because a lot of the things you wrote are pretty contrary to the way I've seen GE do them. Droop Speed Control, for example. Droop Speed Control, is first and foremost, the way a prime mover (the 9FA in your case) raises and lowers load on the generator it is connected to between synchronization and Base Load. One of of the definitions of Base Load for GE-design heavy duty gas turbines is that the IGVs are operating at the Maximum Operating Angle (CSKGVMAX). TTRF is typically only used for switching combustion modes on DLN combustor-equipped GE-design heavy duty gas turbines. It is a calculated value that uses several operating parameters--not just CPD--to derive the machine's firing temperature (because it can't be measured using typical instrumentation in a running machine equipped with can annular combustors (as GE machines are)).

If you used AI I don't think the AI you used has had enough data to make proper descriptions of GE-design heavy duty gas turbine control philosophy.

You probably put a lot of work into the document, but how much effort did you put into confirming your ideas by reading the app code or sequencing running in the Mark* turbine control panel?

 

Thank you for your comments.

In my report I was not trying to describe the full GE control philosophy in detail. I clearly mentioned that load is controlled by Droop Speed Control until the unit reaches Base Load, which is exactly the standard GE method.

Our units are GE 9FA, equipped with DLN 2.6+, controlled by Mark VIe, and have been in operation for around 12 years.

I do not have access to the full Mark VIe application code, so my intention in the report was to simplify the physical behavior for operators—not to present a full controls-engineering analysis.
Examples include explaining why Exhaust Temperature rises at part load but decreases again near high load, and how CPD/CPR influences the TTRX reference, which then limits any further load increase.

I did not say that firing temperature “controls load”; I only explained that Firing Temperature Limit—calculated through TTRX, which is related to CPD/CPR—is the main limit that stops further fuel increase as the machine approaches Base Load. This is consistent with operator-level understanding.

If you have deeper access to the ToolboxST logic and can explain the exact sequencing or CPR bias method used in our version of DLN 2.6+, your expertise would be appreciated. My goal is simply to make the operational picture clearer for those who do not have access to the software.
I attached som pictures from manual tiling about Temperature Control and cpd biased temp control curve

 

Thanks for the report. Really interesting. I was always wondering when the operator demands a load increment lets say from 400-500MW (sorry for the values i learned on 9ha) which control mode takes over? And which sequence is being followed? First fuel increment and then igvs opening?