A SUPPLEMENT TO MECHANICAL ENGINEERING MAGAZINE
...Continued from previous page
temperature will decrease, so now firing temperature will drop in
order to maintain the exhaust temperature called out by the control
Firing temperature in this case is referred to as the temperature
coming out of the combustor. The industry standard nomenclature
for firing temperature is T4, and this nomenclature will be used from
To predict engine cooling flows, site specific gas turbine models
are needed. Generating these will usually require performance
software and a good amount of reliable engine data. However,
measuring cooling flows is not as complicated. By recording
temperature, pressure and pressure drop across a known orifice, it
can be calculated to keep track of cooling flow measurements.
One of the main reasons to track cooling flow measurements
is to have an idea of how T4 is changing with time. Knowing the
exact value is not the goal here. With engine variations and inability
to track vane 1 cooling flow, the actual T4 value is very hard to
determine. However, knowing the trend of T4 will help assess and
make decisions regarding engine performance.
First law of thermodynamics states that energy is always
Energy in = Energy out
While total airflow into a gas turbine can be computed from
A T4 estimate can be calculated by creating a heat balance
around the combustor. Total airflow into the engine can be
computed from a heat balance created around the engine. Proper
plant instrumentation has to be installed and calibrated in order to
accurately determine inlet air flow.
a heat balance around an engine, air flow into a combustor will
depend on how much cooling flow was taken off of a compressor
prior to air reaching the combustor.
In order to have an accurate representation of cooling flows,
instruments that are used to calculate flows have to be regularly
calibrated. It’s best to set up a calibration plan for the whole gas
turbine to make sure that the instrumentation shows what is actually
going on with the engine.
With accurate data and a bit of engineering “know-how” engine
performance can be assessed.
Benefits of Measuring Cooling Flows
Effect of Cooling Flows
on Engine Operation
The benefits of monitoring engine cooling flows are being able to
better assess engine performance and engine operation.
Upcoming plots were created using commercial software,
Gas Turb. The software allows modeling of heavy duty gas turbines
along with other turbomachinery. An F-frame engine was modeled
based on public data available. Gas Turb uses generic, publicly
available compressor and turbine maps.
The plot shown in Figure 5 shows the impact of cooling flow
changes on firing temperature, power output, heat rate and exhaust
In the plot above, total engine cooling flow is being changed by
+/- 4%. Based on experience, total cooling flow can account for up to
20-25% of total engine flow.
The plot above was created by keeping an engine on the base-
load exhaust temperature control curve that was generated based
on the Gas Turb model. This way the changes presented more closely
resemble actual changes that may be seen during operation.
The power and heat rate impact is significant from the
performance point of view. However, just as significant is the impact
that firing temperature may have on the lifecycle of the turbine
parts. Based on experience, increasing the firing temperature by + 40
deg F may decrease the life of hot section parts by 50%!
If turbine firing temperature is being trended any spikes or falls
in T4 can be identified and immediately investigated. That way there
are no surprises that may cause extended scheduled or even forced
Maintaining Optimal Engine Performance
Another benefit of keeping track of cooling flows and trending T4
is maintaining optimal engine performance output. As mentioned
before, the base-load exhaust temperature control curve is created
based on design firing temperature. It is not aware of what the
actual engine firing temperature is.
It is well known that turbine degradation has an inverse
exponential profile. Most of the degradation is seen in the first few
thousand hours, with degradation leveling out as more time goes on.
A sample degradation curve is presented in Figure 6.
Figure 5 – Effect of Cooling Flows on Engine Performance