Logo Gas Path Analysis Limited
Answers to some questions from GPAL simulator users

I tried to decrease the ambient pressure in the simulation program. I found that the efficiency did increase. Please explain why is that?

This only happens when the power demand from the generator is below the maximum power available from the gas turbine (i.e. no EGT or Power alarm are displayed), and the ambient temperature is constant during this (decreasing) ambient pressure transient.

As the ambient pressure decreases, the air mass flow rate through the engine also decreases. However, the engine is not on an operating limit (i.e. no EGT or Power alarms). Therefore, the fuel control system responds by increasing the gas turbine specific work to compensate for the loss in air flow rate in order to maintain the power demand from the generator. To increase in specific work, the turbine entry temperature (TET) must increase, which also increases compressor pressure ratio (due to component matching). The increase in these two parameters improves the thermal efficiency as observed with the simulator. Thus, when engines operate at low power demand conditions (i.e. no alarms) a low ambient pressure is very desirable.

At high power demands when the engine continues to operate on an alarm limit, low ambient pressure is undesirable as the power output decreases primarily due to the decreases in air flow rate though the engine. The thermal efficiency, however, remains essentially constant.

This is discussed in the e-booklet enclosed with the simulator.


Please provide me with the definition and explanation about the "turbine creep life usage". Is this the same as the Larson-Miller parameter?

The time for the turbine to fail in creep for a given turbine material depends on the turbine blade stress and temperature. Higher these parameters smaller is this time. Gas turbines can operate at various operating conditions and will influence how long the blade will last before it fails in creep. However, the “actual” creep of the turbine is fixed and cannot increase during operation – that is the strain (plastic deformation) of the blade in creep before the blade is considered to fail is fixed. But the usage of this creep “life” will depend on operating conditions. As an extreme example, if the engine is shutdown the turbine blade life with respect to creep will be infinite. What we are really saying is that the turbine creep life usage is zero. On the other hand if we operate the gas turbine at very high turbine temperatures the blade may only last a few hours – the creep life usage is very high, although the creep strain (plastic deformation) is the same for both cases. The Larson-Miller parameter gives us an elegant means of determining this life and it is this time that is shown in the trends for turbine life in the simulator.


where LM Larson-Miller parameter, t time (Hours) and T is temperature (K)


Why did you say "the turbine creep usage increases during the period of constant EGT operation. This is primarily due to the increase in compressor pressure ratio and thus turbine pressure ratio while operating at a constant EGT. The turbine entry temperature (TET or T3) and hence the turbine blade metal temperature increase as the ambient temperature decreases". I thought that the TET remains constant in the region where the unit is EGT limited. The increase in power is strictly caused by an increase in mass flow. Please note that the volumetric flow remains constant as the ambient temperature changes".

This corresponds to the case when the ambient temperature is decreasing and we are operating the gas turbine at a high enough power level such that the gas turbine remains on the EGT operating limit. As the ambient temperature decreases the compressor non-dimensional speed will increase. The increase in compressor non-dimensional speed will increase compressor inlet non-dimensional flow and thus the airflow rate through the engine. If we assume that the maximum to minimum temperature (T3/T1) remains constant (by adjusting the power output as the ambient temperature decreases), the compressor pressure ratio (P2/P1) must increase to satisfy the flow compatibility (assuming a choked turbine i.e. turbine non-dimensional flow is constant).


The higher compressor pressure ratio will also result in a higher turbine pressure ratio. For a fixed turbine exit temperature (EGT limit), an increase in turbine pressure ratio must necessarily increase the turbine entry temperature (T3). Therefore, a decrease in ambient temperature will result in an increase in the temperature ratio T3/T1 when operating at constant EGT, which increases the compressor pressure ratio and thus turbine pressure ratio further to satisfy the flow compatibility equation. This will also increase the turbine entry, T3, temperature further.

The increase in T3 will increase the turbine blade temperature and therefore the turbine creep life usage as discussed above. The increase in T3/T1 will increase the specific work. Hence, the increase in power output as the ambient temperature decreases, while operating at constant EGT, is due to the increase in mass flow rate through the engine and increase in specific work.

Regarding the volume flow rate remaining constant during ambient temperature changes, I cannot establish this. Unlike process compressors, we normally do not work with volume flow rates in gas turbines and compressors used in gas turbines rarely exhibit fan law relationships often found with process centrifugal compressors.


Please provide me with the definition of the following terms: PZTemp, EGT-VIGV, Fault Indices (%), and AFR.

PZTemp is the combustor primary zone temperature. The primary zone is where the fuel is burnt and is the major heat release zone in gas turbine combustors. It is also where the maximum temperature occurs in the combustor. This temperature is important because it is in this zone of the combustor where emissions such as NOx and CO, including CO2 and H2O, are formed.

The single shaft gas turbine simulator is fitted with a variable inlet guide vane (VIGV). The control and operation of the VIGV is determined by an exhaust gas temperature (EGT) value, which is the set point for the VIGV control system. EGT-VIGV is this temperature set point and the default value is 650K. If this temperature set point is below the EGT then VIGV will open and will become fully opened during the normal power output range of gas turbine (i.e. the EGT is never below 650K during normal operation). At low loads and starting conditions, the EGT is below 650K and the VIGV will be fully closed and this reduces starting power requirements because the closure of the VIGV will reduce the compressor airflow rate. Thus, at certain power outputs the VIGV will modulate between 0 and 100% while maintaining the EGT at 650K, which is the EGT-VIGV set temperature set point.

VIGV control as described above is also employed in dry low emission combustors (DLE). In this case, we try to maintain the combustion temperature (PZTemp) as the load of the engine is reduced. This is achieved by maintaining the EGT at its limiting value of 850K by closing the VIGV as the power output is reduced. To achieve this we set the EGT-VIGV (temperature set point) to the maximum limiting value of 850K (i.e. increase the EGT-VIGV set point from 650K to 850K). Thus, as the power output reduces the VIGV closes to maintain the EGT at 850K and doing so the combustion temperature is also maintained. In a DLE engine the maximum primary zone temperature, which is also know as combustion temperature, is maintained at a much lower temperature, typically 1850K where NOx and CO emissions are very low.

Both these control strategies can be simulated using the single shaft simulator.

Fault Indices (%)
Many factors affect gas turbine performance deterioration and include compressor fouling. All these factors alter the component characteristics of the affected engine component/s. For example, compressor fouling shifts the compressor speed lines to the left while the efficiency lines are reduced, as shown in Figures 4.1a and 4.1b. Therefore fault indices enable us to simulate performance deterioration by altering the component characteristics by the specified percentage.

If we wish to simulate compressor fouling we set the compressor fouling fault index to -3% and set the compressor efficiency fault index to -1% over a ramp time, say, 3000 seconds. This will result in the compressor speed lines shifting to the left by 3% while the compressor efficiency curves are reduced by 1% in this time period. Thus the fault indices effectively reduce the flow capacity of the compressor by 3% while reducing the compressor efficiency by 1%. Similarly, we can simulate turbine hot end damage by increasing the turbine fouling index by 3% while decreasing the turbine efficiency fault index by 2% over a suitable ramp time.

Trends will now show the change in engine parameters, such as pressure, temperature flows, emissions, life, etc due to such deteriorations.

AFR is the overall air to fuel ratio, which is the ratio of the primary + dilution air to fuel on a mass basis, but excludes turbine cooling air. When operating using the VIGV control strategy 2, where we modulate to the VIGV to maintain the EGT at its maximum value (850K), the AFR remains essentially constant at low loads and this is the requisite for DLE combustion.


Does the red line in Fig 2.5, in the e-booklet provided with the simulator, show the variation of the CO2 emissions Index? This figure states that it is CO2 emissions. If it is the CO2 emissions index, then why would it decrease when the ambient temperature drops while the unit is "EGT Limited"? The mass flow through the machine increases as the ambient temperature drops. The fuel flow increases as well as the ambient temperature drops in this region. Please explain.

Yes, you are right; the red line is indeed the CO2 emissions index (kg/MW-Hour).

At constant EGT, the power output and thermal efficiency increases as the ambient temperature decrease. However, the increase in power output is greater than the increase in thermal efficiency. Thus the fuel flow (heat input) has to increase to satisfy the increase in power output as the ambient temperature decreases. This increase in fuel flow increases the CO2 emissions on a mass basis (Tonnes/Day), at constant EGT, as shown in Figure 2.5 (blue line). The increase in air flow will also increase the CO2 in the exhaust flow; however the increase in CO2 emissions in the exhaust is primarily due to the burning of fuel because the CO2 produced by burning fuel is many orders of magnitude greater than the CO2 in the air (atmosphere). Therefore the CO2 emissions on a mass basis; is largely proportional to the fuel flow. The increase in thermal efficiency now decreases the specific fuel consumption – i.e. kg of fuel per MW-Hours of net power output. Since the CO2 is largely proportional to the fuel flow, the increase in thermal efficiency will decrease the CO2 emissions on an index basis – i.e. CO2 kg/MW-Hour, as seen in Figure 2.5 (red line). Thus the decrease in the CO2 emissions index indicates that CO2 emissions is decreasing in real terms.


Why in section 5.1 you indicated that the NOx emissions will increase during peak rating due to the increase in combustion pressure? I was unaware that the combustion pressure increase during peak rating.

During peak rating we increase the turbine entry temperature (T3) by increasing the EGT limit. In other words we have increased the maximum to minimum cycle temperature ratio (T3/T1). From the flow compatibility equation (please see above), increasing T3/T1 will increase the compressor pressure ratio as the compressor and turbine non-dimensional flows are essentially constant. The increase in compressor pressure ratio will increase the compressor discharge pressure, which will result in an increase in combustion pressure. Therefore, peak rating also increases thermal efficiency (i.e. reduces heat rate) but increases turbine creep life usage.

Note :- In multi-shaft gas turbines operating with free power turbines, there is also an increase in gas generator speed during peak rating. This increases the air mass flow rate through the engine and therefore further augments the power output of gas turbine during peak rating.


In section 4.2 of the manual, you indicated that the compressor isentropic efficiency and flow will be affected by rubbing. I thought that the compressor flow will increase due to a drop in isentropic efficiency. Please confirm that the compressor flow will increase due to rubbing.

Rubs would increase the tip clearance between the compressor rotor and annulus. This increases the secondary loss and also introduces an increased blockage effect on the flow. Therefore, we would expect a decrease in both flow capacity and isentropic efficiency due to rubs. However, the flow effect will be most profound when rubs occur in the LP stages as these stages control the flow at high compressor speeds.

GPAL Home Page

2013 Gas Path Analysis Ltd