A SUPPLEMENT TO MECHANICAL ENGINEERING MAGAZINE
In a modern gas turbine engine, up to 20% of the main compressor
(inlet) flow is bled off to perform cooling and sealing of hot section
Cooling flows are necessary for the engine to function, however
too much cooling has a negative impact on the performance and
output. To optimize performance one needs to know what cooling
flows to monitor and control.
This article presents the importance of understanding cooling
flow monitoring especially when applied to land-based gas turbines.
Aircraft engines are strictly hands off to access and control cooling
flows but this is not so with land-based units. Strategically placed
instrumentation in the cooling flow delivery system can monitor the
health and hence the output of the gas turbine generator utilized in
a simple or combined cycle operation.
In some four stage turbines the design will usually provide
cooling to the following components (in order of decreasing flow)
1) Vane 1 – Cooling to vane 1 is provided internally to keep
materials at a safe temperature.
2) Rotor and blade cooling – An external pipe will take air from an
engine compressor, cool this air in an external heat exchanger
and provide the air for cooling turbine blades and the rotor.
3) Vane 2 – Two pipes (a main line and a bypass) will provide
cooling for vane 2.
4) Vane 3 – Two pipes (a main line and a bypass) will provide
cooling for vane 3.
5) Vane 4 – Two pipes (a main line and a bypass line) will provide
cooling for vane 4.
Some OEMs have different delivery systems than pipes. For
instance much of the cooling flow may be channeled through the
major internal skeletal structure of the engine.
Vane 1 and rotor and blade cooling are usually non-adjustable.
They are determined by the design of the engine. When the rotor
and blade cooling is provided via an external pipe, that flow can be
easily monitored. Vane 1 cooling flow is generally not measured
although a thermocouple in the region of the vane “box” will help
determine flow from an algorithmic model.
The easiest flows to measure and control are Vane 2 through
4 flows. The main line carries most of the flow which is determined
by pressure difference between the compressor (where the flow is
extracted) and the turbine (where the flow is introduced). The flow
is controlled by an orifice that’s in the pipe. The bigger the orifice
throat area, the more flow will pass.
There is also a bypass line that is smaller than the main line. It
has a valve that will modulate the cooling flow to adjust for different
engine operating conditions. The adjustment is forced by engine
control. Cooling flows for Vanes 2-4 are controlled by disc cavity
temperatures. If more cooling flow is needed to bring down disc
cavity temperature to the control limit the valve will open up to bring
in more cooling flow. Disc cavity limits are used to keep turbine parts
at a safe operating temperature.
If an engine is having trouble staying within disc cavity
temperature limits, cooling flows can be adjusted by changing orifice
plates in the main or bypass lines. This, however, should be done
with care since changing cooling flows will have an impact on engine
life and engine performance.
To monitor cooling flows a good approach is to look at disc
cavity temperatures as well as bypass valve positions. Comparing
actual disc cavity temperature with what it’s being controlled to will
give an idea if an engine is being overcooled or undercooled.
Another useful exercise is to look at bypass valve position for
vane 2, 3 and 4 cooling flows. If the bypass valve is fully open that
would mean that not enough cooling flow is being provided. If the
bypass valve is fully closed, that would mean that the engine is being
overcooled. It should be noted that some control systems treat 100%
as fully open and some as fully closed. A quick check in the control
manual should clear up this issue.
It’s best to trend both bypass valve positions and disc cavity
temperatures over a range of temperatures and engine load
operation to get a better idea if the orifice plates in the main lines
are sized properly.
The impact of cooling flows on engine life and engine
performance is presented later in the article.
Engine upgrades often are a result of increasing the firing
temperature, improvements of the turbine and compressor
efficiency and an increase in the safety margin of the engine
components by better cooling or new technology, such as coatings or
internal blade geometry.
Gas Turbine Cooling Flows and
Their Influence in Output
by Mr. Brent A. Gregory and Mr. Oleg Moroz, both of Creative Power Solutions, Fountain Hills, AZ.