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Prepared by: Mohamed Zaki Nawar
Machine: GE 9FA + Mark VIe | DLN 2.6+
Subject: Effect of IBH Valve Failure on Compressor Operating Margin, CPR Limits, and Load Reduction
1. Introduction
This report explains the technical reasons why the gas turbine was unable to operate below a certain load when the Inlet Bleed Heat (IBH) system malfunctioned and remained closed. It also describes how IBH influences compressor operating margin, corrected speed, compressor pressure ratio (CPR), and combustion stability. The investigation includes operational data, physical behavior analysis, and the final event where the unit tripped on high exhaust temperature spread during shutdown.
—
2. Function of IBH and Its Relation to Compressor Operating Margin
IBH (Inlet Bleed Heat) extracts hot air from the compressor discharge and mixes it with the compressor inlet air. Its main purposes are:
2.1 Increase Inlet Temperature at Low Load
When IBH is open, compressor inlet temperature (CTIM) increases. This results in:
Lower air density
Prevent ice formation at cold days due to drop in pressure at low igv angles
Lower mass flow through compressor
Reduced compressor pressure ratio (CPR)
Increased surge margin
Stable combustion at low load
Lower NOx emissions (DLN stability)
2.2 IBH Operation Logic
For GE 9FA DLN 2.6+ machines:
IBH opens at ~95% corrected speed before synchronization
It stays open during low load operation
It starts closing when IGV > 45°
It fully closes at IGV ≈ 63° (around ~180 MW depending on ambient)
This system is critical for maintaining compressor surge margin and DLN combustion stability during part-load operation.
---
4. Effect of IBH Remaining Closed at Low Load
When IBH stayed closed, the following occurred:
4.1 Decrease in Compressor Inlet Temperature
Compared with normal operation at the same ambient and same MW, CTIM decreased because IBH was not supplying heated discharge air to the inlet.
4.2 Increase in Inlet Air Density & Mass Flow
Lower CTIM → higher density → more mass flow enters compressor even at the same IGV angle and same speed.
4.3 Increase in CPR Towards Its Limit
More mass flow + same IGV + same speed → CPR increases toward the surge protection curve.
Because CPR limit is a function of:
IGV position
Corrected speed
Ambient temperature
…the unit was operating dangerously close to the compressor operating limit.
4.4 Reduced Surge Margin / Operating Margin
The compressor moved toward the surge line. This condition becomes unsafe below ~160 MW, explaining why:
The machine could not reduce load further
Any attempt to go down in load caused instability
Combustion quality deteriorated (DLN lean blowout risk)
---
5. Impact on Combustion and Exhaust Spread Trip
When IBH is closed at low load:
1. Air mass flow increases
2. Fuel flow decreases (during shutdown sequence)
3. The mixture becomes too lean
4. DLN combustion becomes unstable
5. Large differences develop between combustor cans
6. Exhaust temperature spread increases
7. Trip occurs at high spread limit
This matches exactly what happened:
At 110 MW during shutdown
High exhaust spread alarms on Spread 1 & 2
Final trip on high exhaust temperature spread
The root cause: DLN lean blowout due to IBH failure.
---
6. Why Load Could Not Be Reduced Under These Conditions
With IBH closed:
Lower CTIM → higher corrected speed
Higher corrected speed → higher compressor capability
Higher mass flow → higher CPR
CPR approached the CPR-limit curve
Approaching CPR limit forces the control system to stop unloading
Further unloading increases surge risk
Therefore the unit stabilized only above ~160 MW. Below this point:
Combustion becomes unstable
CPR margin is nearly zero
DLN dynamics cause high spread
Unit eventually trips
Thus, operation below this load is physically unsafe, not just control-limited.
--
In this report we confirm that when IBH remained fully closed, the compressor inlet temperature (CTIM) decreased compared to the same ambient conditions when IBH is operating normally. This reduction in inlet temperature increases inlet air density and mass flow, which in turn raises CPR toward the compressor operating limit. Because CPR limit is a function of IGV angle and corrected speed, and because IBH normally opens at 95% speed before synchronization and closes only when IGV exceeds ~63°, the turbine lost its required operating margin. This explains why the machine could not reduce load further and why combustion instability occurred during shutdown, leading to high exhaust spread and the subsequent trip.
---
8. Conclusion
The root cause of the operational limits and eventual shutdown trip was:
Primary Cause
Failure of the IBH system (valve stem broken), causing IBH to remain closed during part-load operation.
Consequences
Low CTIM
Increased mass flow
High CPR near limit
Reduced surge margin
DLN combustion instability at low load
High exhaust spread during shutdown
Final trip
Restoring IBH operation is necessary to:
Maintain compressor margin
Ensure DLN stability
Allow safe load reduction
Prevent future spread trips
—
9. 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
Machine: GE 9FA + Mark VIe | DLN 2.6+
Subject: Effect of IBH Valve Failure on Compressor Operating Margin, CPR Limits, and Load Reduction
1. Introduction
This report explains the technical reasons why the gas turbine was unable to operate below a certain load when the Inlet Bleed Heat (IBH) system malfunctioned and remained closed. It also describes how IBH influences compressor operating margin, corrected speed, compressor pressure ratio (CPR), and combustion stability. The investigation includes operational data, physical behavior analysis, and the final event where the unit tripped on high exhaust temperature spread during shutdown.
—
2. Function of IBH and Its Relation to Compressor Operating Margin
IBH (Inlet Bleed Heat) extracts hot air from the compressor discharge and mixes it with the compressor inlet air. Its main purposes are:
2.1 Increase Inlet Temperature at Low Load
When IBH is open, compressor inlet temperature (CTIM) increases. This results in:
Lower air density
Prevent ice formation at cold days due to drop in pressure at low igv angles
Lower mass flow through compressor
Reduced compressor pressure ratio (CPR)
Increased surge margin
Stable combustion at low load
Lower NOx emissions (DLN stability)
2.2 IBH Operation Logic
For GE 9FA DLN 2.6+ machines:
IBH opens at ~95% corrected speed before synchronization
It stays open during low load operation
It starts closing when IGV > 45°
It fully closes at IGV ≈ 63° (around ~180 MW depending on ambient)
This system is critical for maintaining compressor surge margin and DLN combustion stability during part-load operation.
---
4. Effect of IBH Remaining Closed at Low Load
When IBH stayed closed, the following occurred:
4.1 Decrease in Compressor Inlet Temperature
Compared with normal operation at the same ambient and same MW, CTIM decreased because IBH was not supplying heated discharge air to the inlet.
4.2 Increase in Inlet Air Density & Mass Flow
Lower CTIM → higher density → more mass flow enters compressor even at the same IGV angle and same speed.
4.3 Increase in CPR Towards Its Limit
More mass flow + same IGV + same speed → CPR increases toward the surge protection curve.
Because CPR limit is a function of:
IGV position
Corrected speed
Ambient temperature
…the unit was operating dangerously close to the compressor operating limit.
4.4 Reduced Surge Margin / Operating Margin
The compressor moved toward the surge line. This condition becomes unsafe below ~160 MW, explaining why:
The machine could not reduce load further
Any attempt to go down in load caused instability
Combustion quality deteriorated (DLN lean blowout risk)
---
5. Impact on Combustion and Exhaust Spread Trip
When IBH is closed at low load:
1. Air mass flow increases
2. Fuel flow decreases (during shutdown sequence)
3. The mixture becomes too lean
4. DLN combustion becomes unstable
5. Large differences develop between combustor cans
6. Exhaust temperature spread increases
7. Trip occurs at high spread limit
This matches exactly what happened:
At 110 MW during shutdown
High exhaust spread alarms on Spread 1 & 2
Final trip on high exhaust temperature spread
The root cause: DLN lean blowout due to IBH failure.
---
6. Why Load Could Not Be Reduced Under These Conditions
With IBH closed:
Lower CTIM → higher corrected speed
Higher corrected speed → higher compressor capability
Higher mass flow → higher CPR
CPR approached the CPR-limit curve
Approaching CPR limit forces the control system to stop unloading
Further unloading increases surge risk
Therefore the unit stabilized only above ~160 MW. Below this point:
Combustion becomes unstable
CPR margin is nearly zero
DLN dynamics cause high spread
Unit eventually trips
Thus, operation below this load is physically unsafe, not just control-limited.
--
In this report we confirm that when IBH remained fully closed, the compressor inlet temperature (CTIM) decreased compared to the same ambient conditions when IBH is operating normally. This reduction in inlet temperature increases inlet air density and mass flow, which in turn raises CPR toward the compressor operating limit. Because CPR limit is a function of IGV angle and corrected speed, and because IBH normally opens at 95% speed before synchronization and closes only when IGV exceeds ~63°, the turbine lost its required operating margin. This explains why the machine could not reduce load further and why combustion instability occurred during shutdown, leading to high exhaust spread and the subsequent trip.
---
8. Conclusion
The root cause of the operational limits and eventual shutdown trip was:
Primary Cause
Failure of the IBH system (valve stem broken), causing IBH to remain closed during part-load operation.
Consequences
Low CTIM
Increased mass flow
High CPR near limit
Reduced surge margin
DLN combustion instability at low load
High exhaust spread during shutdown
Final trip
Restoring IBH operation is necessary to:
Maintain compressor margin
Ensure DLN stability
Allow safe load reduction
Prevent future spread trips
—
9. 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
