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Gas Turbine Cooling Flows and Their Influence in Output PUBLIC ACCESS

[+] Author Notes
Brent A. Gregory, Oleg Moroz

Creative Power Solutions, Fountain Hills, AZ.

Mechanical Engineering 137(03), 48-54 (Mar 01, 2015) (5 pages) Paper No: ME-15-MAR-4; doi: 10.1115/1.2015-Mar-4

This article presents the importance of understanding cooling flow monitoring especially when applied to land-based gas turbines. Cooling flows are necessary for the engine to function; however, too much cooling has a negative impact on the performance and output. 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 order to monitor cooling flows, a good approach is to look at disc cavity temperatures as well as bypass valve positions. It is best to trend both bypass valve position 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. A quick way to determine whether there are cooling issues in an engine or not is to trend disc cavity temperature and bypass valve positions {AQ: Edits have been made in this sentence “A quick…valve positions.” for better readability. Please check and correct if necessary.}

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 components.

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 (amainlineanda 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.

The OEMs may represent the cooling in a diagram often referred to as the worm chart. Figure 1 shows a schematic of the turbine and thevarious amounts of flow as they are reintroduced to the gas path.

FIGURE 1 A “worm chart” showing cooling flows in the first two stages of a gas turbine. (Courtesy VKI lecture series LS1999-02)

Grahic Jump LocationFIGURE 1 A “worm chart” showing cooling flows in the first two stages of a gas turbine. (Courtesy VKI lecture series LS1999-02)

FIGURE 2 The flow circuit diagram (in this case between the first nozzle and the second stator). (Courtesy Rolls-Royce)

Grahic Jump LocationFIGURE 2 The flow circuit diagram (in this case between the first nozzle and the second stator). (Courtesy Rolls-Royce)

Actual representation of the cooling flows can be modeled using a more sophisticated 3-D characterization of the flow as shown in Figure 3.

FIGURE 3 Computational fluid dynamics (CFD) model of cooling flow emerging on the surface of a turbine blade and its influence by the main stream flow. (Courtesy of B&B-AGEMA GmbH)

Grahic Jump LocationFIGURE 3 Computational fluid dynamics (CFD) model of cooling flow emerging on the surface of a turbine blade and its influence by the main stream flow. (Courtesy of B&B-AGEMA GmbH)

To demonstrate the effect of cooling flow on the performance of a gas turbine typical of a simple cycle or combined cycle operation CPS produced a theoretical model in a commercially available code (GasTurb) and changed cooling flows to see the impact on operating point (the model uses a generic compressor and turbine map). GasTurb allows changing the amount of cooling flow and the energy of cooling flow.

As discussed, the main purposes of cooling flows are to maintain saf vane and blade metal temperatures during gas turbine operation. Cooling flows have a net positive result on gas turbine performance. Since turbine cooling flows do not go through the combustor, some of the work is lost. However, cooling flows allow for a higher firing temperature which leads to a net higher gas turbine power output and better efficiency.

At base-load, most gas turbine engines are operated on an exhaust temperature control curve. A sample exhaust temperature control curve is presented in Figure 4.

FIGURE 4 Sample Base-load Exhaust Temperature Control Curve

Grahic Jump LocationFIGURE 4 Sample Base-load Exhaust Temperature Control Curve

The base-load exhaust temperature control curve is usually created by running the engine model through a range of ambient temperatures at some given design firing temperature. Based on the model output and site specific conditions, a control curve for base-load operation is created. The above control curve was created using an F-frame model developed in GasTurb.

The main disadvantage of the curve is that there is no awareness of what the actual firing temperature of an engine is. So if cooling flow increases, firing temperature increases as well in order to maintain the same exhaust temperature output for a certain shell pressure. On the other hand, if cooling flows decrease the exhaust temperature will decrease, so now firing temperature will drop in order to maintain the exhaust temperature called out by the control curve.

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 here on.

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 ofT4 will help assess and make decisions regarding engine performance.

First law of thermodynamics states that energy is always conserved.

Energy in = Energy out

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.

While total airflow into a gas turbine can be computed from 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 setupa 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.

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, GasTurb. The software allows modeling of heavy duty gas turbines along with other turbomachinery. An F-frame engine was modeled based on public data available. GasTurb 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 energy.

FIGURE 5 Effect of Cooling Flows on Engine Performance

Grahic Jump LocationFIGURE 5 Effect of Cooling Flows on Engine Performance

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 GasTurb 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 ofvlew. 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 outages.

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 6 Long-Term Deterioration of Power and Heat Rate Showing Zone of Deterioration versus Operating Hours.

Ref. “Performance Deterioration in Industrial Gas Turbines.” Cyrus B. Meher-Homji, Mustapha A. Chaker, and Hatim M. Motiwala.

Grahic Jump LocationFIGURE 6 Long-Term Deterioration of Power and Heat Rate Showing Zone of Deterioration versus Operating Hours.Ref. “Performance Deterioration in Industrial Gas Turbines.” Cyrus B. Meher-Homji, Mustapha A. Chaker, and Hatim M. Motiwala.

An engine completing an outage may have tight blade clearances in the turbine sections as well as new turbine components and sealing (providing these parts were replaced). Clearances will open up as the engine goes through several initial thermal cycles. As shown, seals will also see most degradation during those initial few thousand hours of operation.

This will cause the gas turbine to degrade. Once the gas turbine is degraded it is not able to extract as much energy out of the working fluid (mix of air and fuel) which causes the gas turbine exhaust temperature to go up. The control curve “sees” the exhaust temperature going up and will cause the engine to decrease the firing temperature. If the firing temperature decreases, that’s a big plus for the parts since now their lifecycle is extended. However, it is detrimental to engine performance.

Figure 7 shows how decreased firing temperature due to engine degradation affects an engine’s performance.

FIGURE 7 Performance Behavior with Engine Degradation

Grahic Jump LocationFIGURE 7 Performance Behavior with Engine Degradation

For the above plot, the GasTurb model was maintained on the design exhaust temperature control curve. Thus T4 is dropping as engine heat rate* increases. Engine heat rate increase is modeled based on turbine degradation only. Compressor degradation is not included.

As mentioned previously, because the turbine is not able to extract as much work out of the working fluid, the exhaust temperature goes up and T4 has to drop in order to stay on the control curve.

Figure 8 shows a plot of how an engine performance can be optimized by adjusting the base-load control curve, so that an engine operates at constant T4.

FIGURE 8 Impact of Maintaining Constant T4 through Engine Degradation

Grahic Jump LocationFIGURE 8 Impact of Maintaining Constant T4 through Engine Degradation

Blue and red lines are the same as from Figure 7. Purple and green lines were developed by keeping the firing temperature at the design level as the engine degrades.

In Figure 8, the purple line represents the unrecoverable and expected engine degradation. The blue line represents power loss due to both unrecoverable engine degradation and inability of the control curve to adjust so that the engine is maintained at a constant T4 value.

The benefit of monitoring cooling flows and trending firing temperature Is that when firing temperature drops, the control curve can be adjusted and some of the gas turbine performance (and hence output) may be re-gained.

An example of potential power recovery due to a control curve adjustment is presented here.

Table 1 Sample Potential RecoveryCalculation

Grahic Jump LocationTable 1 Sample Potential RecoveryCalculation

That’s a potential recovery of 4MW at 8,000 hours. This value is only an estimate because the heat rate deterioration is assumed to be due to turbine degradation only. Compressor degradation is not considered.

Cooling flow monitoring along with properly calibrated plant instrumentation allows for trending of engine firing temperature and engine performance using fundamental thermodynamics. Performance Issues arising during engine operation can be accurately assessed and plans for corrections can be made during scheduled outages.

A quick way to determine If there are cooling Issues In an engine Is to trend disc cavity temperature and bypass valve positions. If the bypass valves are either fully open or fully closed that may lead to engine undercooling or overcooling. If disc cavity temperatures are under the limits that they are controlled to then the engine is being overcooled; If they are over the limits then the engine Is being undercooled. The trends should be plotted for a range of ambient conditions and engine load variations.

Neglecting to monitor cooling flows and engine health may lead to underperforming engine, unnecessary forced outages or early replacement of engine components.

*One measure of the efficiency of agas turbine that converts a fuel into heat and into electricity is the heat rate. To express the efficiency of agas turbine as a percentage, divide the equivalent Btu content of a kWh of electricity (which is 3,412 Btu) by the heat rate. For example, if the heat rate is 10,140 Btu, the efficiency is 34%. If the heat rate is 7,500 Btu, the efficiency is 45%.

GasTurb simulates the most Important gas turbine configurations used for propulsion or for power generation. Virtually all gas turbine performance simulation problems can be solved with GasTurb, http://www.gasturb.de/
Copyright © 2015 by ASME
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References

GasTurb simulates the most Important gas turbine configurations used for propulsion or for power generation. Virtually all gas turbine performance simulation problems can be solved with GasTurb, http://www.gasturb.de/

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