Availability and Maintainability of Advance Gas Turbinesmedia.caspianmedia.com/document/8362d5426d015f7e61c96e9e59606fd6.pdfAdvanced Gas Turbines Availability and Maintainability

Advanced Gas Turbines Availability and Maintainability

Advanced Gas Turbines

Availability and Maintainability

By

Dr. Meherwan P. Boyce, P.E. Fellow ASME,

and Fellow the Institute of Diesel and Gas Turbine Engineers

INTRODUCTION

A new generation of Combined Cycle Power Plants has replaced the large Steam Turbine

Plants, which were the main fossil power plants through the eighties. Combined Cycle

Power Plants, not new in concept, have been in operation since the mid-fifties. The new

high capacity and efficiency gas turbines have made the combined cycle plants the plant

of choice for the new millennium.

The new generation of Combined Cycle Power Plants operates at thermal efficiencies

between 53% -58%. The new Combined Cycle power plants use many twists to the

ordinary combined cycles by using concepts such as solar heat for preheat, water flashing

for inter-cooling in between compression stages on the gas turbine compressor, reheating,

and steam for gas turbine hot component cooling schemes. The high efficiencies attained

by the Combined Cycle Power Plants are due, in no small part, to the introduction of a

new generation of Gas Turbines with thermal efficiencies between 45%- 48%.

Dr. Meherwan P. Boyce, P.E. 1


The performance of this new generation of Gas Turbines in combined cycle operation is

complex and presents new problems, which have to be addressed. The new gas turbines

operate at very High Pressure Ratios, and Turbine Firing Temperatures. Thus variation in

this firing temperature significantly affects the performance and life of the components in

the hot section of the turbine. The compressor pressure ratio is high which leads to a very

narrow operation margin, thus making the turbine very susceptible to compressor fouling.

The turbines are also very sensitive to backpressure exerted on them by the heat recovery

steam generators. The pressure drop through the air filter also results in major

deterioration of the performance of the turbine.

The last twenty years has seen a large growth in Gas Turbine Technology. The growth is

spear headed by the increase in compressor pressure ratio, advanced combustion

techniques, the growth of materials technology, new coatings and new cooling schemes.

The Industrial Gas Turbine has always emphasized long life and this conservative

approach has resulted in the Industrial Gas Turbine in many aspects giving up high

performance for rugged operation. The Industrial Gas Turbine has been conservative in

the pressure ratio and the firing temperatures. This has all changed in the last ten years;

spurred on by the introduction of the “Aero-Derivative Gas Turbine “the Industrial gas

turbine has dramatically improved its performance in all operational aspects. This has

resulted in dramatically reducing the performance gap between these two types of gas

turbines.



The high temperatures result in increasing the NOx emissions from the gas turbines. This

resulted in initially attacking the NOx problem by injecting water or steam in the

combustor. The next stage was the development of Dry Low NOx Combustors. The

development of new Dry Low NOx Combustors has been a very critical component in

reducing the NOx output as the gas turbine firing temperature is increased. The new low

NOx combustors increase the number of fuel nozzle and the complexity of the control

algorithms.

The new gas turbines have seen a great increase in the technology in the following

components over the past few years:

1. The Axial Flow Compressor

2. The Combustors

3. The Materials used in the high temperature region of the Gas Turbine

4. The Controls

AVAILABILITY AND RELIABILITY

The Availability of a gas turbine is the percent of time the gas turbine is available to

generate power in any given period at its acceptance load. The Acceptance Load or the

Net Established Capacity would be the net electric power generating capacity of the gas

turbine at design or reference conditions established as result of the Performance Tests



conducted for acceptance of the plant. The actual power produced by the gas turbine

would be corrected to the design or reference conditions and is the actual net available

capacity of the gas turbine. Thus it is necessary to calculate the effective forced outage

hours which are based on the maximum load the plant can produce in a given time

interval when the plant is unable to produce the power required of it. The effective

forced outage hours is based on the following relationship:

d

ad

MW

MWMWxHOEFH

)( −= (1)

where:

MWd = Desired Output corrected to the design or reference conditions. This must be

equal to or less than the gas turbine load measured and corrected to the design or

reference conditions at the acceptance test.

• MWa = Actual maximum acceptance test produced and corrected to the design or

reference conditions.

• HO = Hours of operation at reduced load.

The Availability of a gas turbine can now be calculated by the following relationship,

which takes into account the stoppage due to both forced and planed outages, as well

as the forced effective outage hours:

(2)

where:


PT

EFHFOPMPTA

)( −−−=


• PT = Time Period (8760 hrs/year)

• PM = Planned Maintenance Hours

• FO = Forced Outage Hours

• EFH = Equivalent forced outage hours

The reliability of the gas turbine is the percentage of time between planed overhauls and

is defined as:

PT

EFHFOPTR

)( −−= (3)

Availability and Reliability have a very major impact on the plant economy. Reliability is

essential in that when the power is needed it must be there. When the power is not

available it must be generated or purchased and can be very costly in the operation of a

plant. Planned outages are scheduled for non peak periods. Peak periods are when the

majority of the income is generated as usually there are various tiers of pricing depending

on the demand. Many power purchase agreements have clauses, which contain capacity

payments, thus making plant availability critical in the economics of the plant. A 1%

reduction in plant Availability could cost $500,000 in loss of income on a 100MW plant.

Reliability of a plant depends on many parameters, such as the type of fuel, the

preventive maintenance programs, the operating mode, the control systems, and the firing

temperatures. Another very important factor in a gas turbine is the Starting Reliability



(SR). This reliability is a clear understanding of the successful starts that have taken place

and is given by the following relationship:

)( failuresstartingofnumbersuccessesstartingofnumber

successesstartingofnumberSR

+= (4)

The insurance industry concerns itself with the risks of equipment failure. For advanced

gas turbines, the frequencies of failures and the severity of failures are major concerns. In

engineering terms, however, risk is better defined as:

Risk = Probability of Failure x Consequences of Failure (5)

where: The consequences of failure include the repair/replacement costs and the

lost revenue from the downtime to correct the failure.

Actions taken, which reduce the probability and/or consequences of failure, tend to

reduce risk and generally enhance insurability. Because of the high risks associated with

insuring advanced gas turbines, demonstrated successful operation is important to the

underwriting process.

The benefits of advanced gas turbines and their technologies are easily quantified. The

gas turbines produce more power, use less fuel, provide higher combined cycle



efficiencies, and reduce emission levels significantly. The advanced gas turbines have

developed very high efficiencies of between 40-45% due to high pressure ratio (30:1 for

frame and 40:1 for aero engines) and high firing temperatures (2400ºF, 1315ºC). The

advantages of advanced gas turbines have been eclipsed by the following major problems

experienced in the operation of these turbines:

• Lower Availability (up to 10% Lower)

• Lower Life of Nozzles and Blades (averaging 15000 hrs)

• Higher Degradation Rate (5% - 7% in first 10,000 hours of operation)

• Instability of Low NOx Combustors

Meetings with users have indicated that the users are satisfied with the efficiency of these

turbines but would like to see an improvement in the overall operation and maintenance

of the turbines. The survey of users indicated that the following were the major concerns

of the users, regarding the operation of gas turbines:

a. Low Availability and Reliability

b. Repair of Single Crystal Blades

c. Stability of Low NOx Combustors

d. Surge in Compressors, and excessive tip rubs

e. Bearings and Seal Problems



From an Availability and Reliability point of view there is a significant down side to

today’s new turbines. New advanced turbines are run at higher firing temperatures, are

physically larger in size, have larger throughput (airflows and fuel flows), and have

higher loadings (pressure and expansion ratios, fewer airfoils, larger diameters) than

previous gas turbine designs. The large size of these gas turbines is one inherent cause

of a lower availability and reliability as it takes much longer to do the various inspections

and overhauls.

New advanced turbines are run at higher pressure ratios (as high as 30:1). This creates

a very narrow operating margin (surge-choke). Thus any deposits on the blade could

lead to degraded performance and surge in the compressors. The close tolerances

between the casing and the compressor blades lead sometimes to excessive rubs.

New advanced gas turbines are pushing the temperature envelope. The technologies

(design, materials, and coatings) required to achieve the benefits are more complex to

concurrently meet gas turbine performance, emissions, and life requirements.

The design margins with these technologies tend to be reduced or un-validated. While

analytical models may be extrapolated to evaluate the new designs, full-scale verification

of the new designs is an absolute necessity. Similarly, the materials being used are

either relatively new or are being pushed to new limits. This leads to temperature

problems in the turbine nozzle vanes, and the turbine blades causing a reduction in the

life of these components.



There is no reliability record for the new designs. While component rig testing (scale or

sector) may help validate some component’s performance, the first time the unit reaches

design conditions is in the owner’s plant. Essentially, the units are considered prototype

or unproven designs for the first three years of operation or until the entire major design

problems are identified and corrected. Most of the advanced gas turbine nozzle vanes

and blades have had to be redesigned as excessive hot spots were being located after

short field operations.

The cost of hardware and subsequent cost of ownership have increased due to the

complex designs, increased size, and higher throughput in the advanced machines.

Gas turbine operation has become more complex and computer driven, thus requiring

new and different skill sets for staffing in plants.

Axial Flow Compressor

The Axial flow compressor in the advanced gas turbine is a multistage compressor (17-22

stages) with exceedingly high pressure ratio. It is not uncommon to see pressure ratios in

industrial gas turbines in the 17 to 20:1 range some units have pressure ratios in the 30:1

range. The more stages the smaller the operational margin between the surge and choke

regions of the compressor.

The trends for compressors are towards fewer, thinner, larger, three Dimensional and

controlled diffusion shaped airfoils (3D/CDA), with smaller clearances and higher



loading per stage. There are also trends towards water injection at the inlet or between

compressor sections which will likely affect airfoil erosion life. The smaller clearances

(20-50 mils) and high pressure ratios tend to increase the probability of encountering

rubs. These tip rubs usually occur near the bleed flow sections of the turbines where

there are inner diameter changes, and the compressor casing could be out of round. The

advanced compressor blades also usually have squealer sections on the blade tips, which

are designed to wear in a safe manner if the blades are in contact with the casing. These

rubs, if severe can lead to tip fractures and overall destruction of the downstream blades

and diffuser vanes due to domestic object damage (DOD).

The very high temperature at the exit of the compressor, which in some cases exceeds a

1000ºF, causes a very hot compression section, which also requires the cooling of the

bleed flows before they can be used for cooling the turbine section. This also limits the

down time between start-ups of the turbines. Design margins are set by Finite Element

Modeling (FEM) at the element level which results in lower safety margins than previous

designs. The costs of these larger, thinner, less-rub tolerant, and more twisted-shape

airfoils are usually higher. When several of the major characteristics of advanced gas

turbines are examined from a risk viewpoint (i.e., probability and consequences of

failure), there are no characteristics which reduce the probability of failure and/or

decrease the consequence of failure.

Table 1 indicates the changes in the compressor blades that are now prevalent on the

advanced gas turbines. The first column represents previous gas turbine designs, the



second column represents new gas turbine designs, and the last column indicates the

change in risk ( represents higher) for the design differences. Most of the comparisons

are self-explanatory.

Table 1 - State of Gas Turbine Technology – Compressors

Figure1 indicates typical blade failures in blades around the fifth to seventh stage of a

typical axial flow compressor in an advanced gas turbine. Note the number of

blades downstream which have suffered from DOD.


Previous Designs New Designs Risk• 2D double circular arc or NACA 65 profiles

• 3D or Controlled Diffusion Airfoil (CDA) profiles

• Large number of airfoils • Reduced airfoil count • Repeating stages/shorter chords • Stages unique/longer chords • Low/ modest Aspect ratios • High Aspect ratios • Large clearances • Smaller clearances • Low/modest pressure ratios (Rc) • Much higher pressure ratios (Rc) • Low/modest blade loading per stage • High blade loading per stage • Wider Operating margin • Narrow operating margin • Thicker leading edges • Thinner leading edges • Dry operation • Wet operation • Bulk safety margins • Safety margins by FEM • Lower costs • Higher costs • •


FIGURE 1

Compressor Tip failures



Axial Flow Turbines

The development of new materials as well as cooling schemes has seen the rapid

growth of the turbine firing temperature leading to high turbine efficiencies. The

stage 1 blade must withstand the most severe combination of temperature,

stress and environment; it is generally the limiting component in the machine.

Since 1950, turbine bucket material temperature capability has advanced

approximately 850ºF (472ºC), approximately 20ºF (10ºC) per year. The

importance of this increase can be appreciated by noting that an increase of 100º

F (56ºC) in turbine firing temperature can provide a corresponding increase of

8% to 13% in output and 2% to 4% improvement in simple-cycle efficiency.

Advances in alloys and processing, while expensive and time-consuming,

provide significant incentives through increased power density and improved

efficiency.



The early stages of the turbine, complex multi-path serpentine cooling designs are

utilized. Higher strength single crystal (SC) blade materials coupled with

oxidation resistant coatings and/or thermal barrier coatings (TBC) are used in the

first stage blades, and directionally solidified (DS) blade materials with TBC are

used in the second and third stage blades to meet turbine life requirements.

Design margins are set by FEM at the element level, but the long-term creep strength

characteristics of the turbine materials are not well-defined. In addition, the

turbine materials utilized typically have reduced temperature margin to melting as

compared to previous designs. As with compressors, the smaller clearances and

higher expansion ratios associated with the new design turbines tend to increase

the probability of encountering rubs. The costs of these larger, complex-cooled,

more twisted-shape airfoils with more sophisticated materials and coatings are

substantially higher per airfoil stage. The trends for the advanced turbines are

similar to the compressor with fewer, larger, 3D airfoils with smaller clearances

and higher expansion ratios (Re) being used. As with compressors, the smaller

clearances and higher expansion ratios associated with the new design turbines

tend to increase the probability of encountering rubs. The costs of these larger,

complex-cooled, more twisted-shape airfoils with more sophisticated materials

and coatings are substantially higher per airfoil stage. Table 2 shows the

difference between the older turbines and the new advanced gas turbines.

Table 2 - State of Gas Turbine Technology – Turbines

Previous Designs New Designs Risk• 2D reaction-type airfoil profiles • 3D airfoil profiles • More airfoils/shorter chords • Fewer airfoils/longer chords • Larger clearances • Smaller clearances • Low/modest expansion ratios (Re) • Much higher Re’s



• Uncooled/simple cooling designs • Complex cooling designs

• Air as the only cooling medium• Air and Steam as cooling

mediums• Equi-axed castings • DS and SC castings • Oxidation coatings and/or TBC used

for extending life• Oxidation coatings and/or

TBC needed to meet life

• Bulk safety margins • Safety margins by FEM • Margin to melting larger • Margin to melting smaller • Lower costs/stage • Ultra-high costs/stage

The Advanced Gas Turbines have been encountering problems with temperature causing

failures in turbine nozzle vanes and blades. The failures have been occurring at blade

tips, and at the base of the turbine nozzle vanes. Figure 2 shows a typical failure in an

advanced gas turbine.



FIGURE 2

A Typical Gas Turbine Failure

The first stage nozzle vanes are encountering air temperatures in range of 2100ºF –

2300ºF blades and the cooling of these blades is very important. The nozzle vanes are

encountering problems at their base with high temperature. Design changes have been

carried out by all OEM’s, these include new cooling patterns for the nozzle vane base,

further cooling of the compressor bleed air with external cooling heat exchangers, the use

of steam for cooling purposes in combined cycle applications.



The first stage turbine blades are usually impulse turbines, and the second to the third and

fourth stages are reaction type (30-60 percent) and usually have shroud tips. These

shroud tips are there to give the blade more support so that the blades do not suffer from

blade resonance problems.

Combustion Systems

The Advanced Gas Turbines all have Dry Low NOx combustors. The typical stable,

simple, diffusion flame combustor has been replaced with barely stable, staged-

combustion systems with multiple injection locations which vary with gas turbine load.

The majority of the NOx produced in the combustion chamber is called ‘thermal NOx’. It

is produced by a series of chemical reactions between the nitrogen (N2) and the oxygen

(O2) in the air that occur at the elevated temperatures and pressures in gas turbine

combustors. The reaction rates are highly temperature dependent, and the NOx

production rate becomes significant above flame temperatures of about 3300ºF (1815ºC).

The important parameters in the reduction of NOx are the Temperature of the flame, the

Nitrogen and Oxygen content and the resident time of the gases in the combustor.



In 1977 it was recognized that there were a number of ways to control oxides of

nitrogen:-

1. Use of a rich primary zone in which little NO formed, followed by rapid dilution

in the secondary zone.

2. Use of a very lean primary zone to minimize peak flame temperature by dilution.

3. Use of water or steam admitted with the fuel for cooling the small zone

downstream from the fuel nozzle.

4. Use of inert exhaust gas re-circulated into the reaction zone.

5. Catalytic exhaust cleanup.

‘Wet’ control became the preferred method in the 1980’s and most of 1990’s since ‘dry’

controls and catalytic cleanup were both at very early stages of development. The

catalytic converters were used in the 1980’s and are still being widely used; however the

cost of rejuvenating the catalyst is very high.

Advances in combustion technology now make it possible to control the levels of NOx

production at source, removing the need for ‘wet’ controls. This of course opened up the

market for the gas turbine to operate in areas with limited supplies of suitable quality

water, e.g. deserts or marine platforms. Although water injection is still used, ‘dry’

control combustion technology has become the preferred method for the major players in

the industrial power generation market. DLN (Dry Low NOx) was the first acronym to be



coined, but with the requirement to control NOx without increasing carbon monoxide and

unburned hydrocarbons this has now become DLE (Dry Low Emissions).

The DLE approach is to burn most (at least 75%) of the fuel at cool, fuel-lean conditions

to avoid any significant production of NOx. The principal features of such a combustion

system is the premixing of the fuel and air before the mixture enters the combustion

chamber and leanness of the mixture strength in order to lower the flame temperature and

reduce NOx emission. Controlling CO emissions thus can be difficult and rapid engine

off-loads bring the problem of avoiding flame extinction, which if it occurs cannot be

safely reestablished without bringing the engine to rest and going through the restart

procedure.

The DLE injector has two fuel circuits. The main fuel, approximately 97 percent of the

total, is injected into the air stream immediately downstream of the swirler at the inlet to

the premixing chamber. The pilot fuel is injected directly into the combustion chamber

with little if any premixing. With the flame temperature being much closer to the lean

limit than in a conventional combustion system, some action has to be taken when the

engine load is reduced to prevent flame out. If no action were taken flame out would

occur since the mixture strength would become too lean to burn. A small proportion of

the fuel is always burned richer to provide a stable ‘piloting’ zone, while the remainder is

burned lean. In both cases a swirler is used to create the required flow conditions in the

combustion chamber to stabilize the flame.



The Dry Low NOx combustor system has to be monitored and tuned precisely for

stability from starting to full load while maintaining low emissions and avoiding

flashback and high pressure pulsations which could damage combustor and turbine

components. The principal features of such a combustion system is the premixing of the

fuel and air before the mixture enters the combustion chamber and leanness of the

mixture strength in order to lower the flame temperature and reduce NOx emission. The

management of air in the combustion process and for cooling of the combustor is

particularly critical; this requires that dry low NOx (DLN) combustors have complex

combustor fuel nozzle, cooling, and TBC coating systems to provide adequate life for

both the can-annular and annular combustion systems. The fuel nozzles are more

complicated and larger in number due to the multiple injection locations. When dual fuel

is involved or water injection is used to further reduce emissions, the purge systems for

the multiple injection points are complex and can be a significant source of problems

with fuel nozzle plugging and localized hot section damage. As with new design

compressors and turbines, the costs and the risks of these complex combustion systems

are high in Table 3.



Table 3 - State of Gas Turbine Technology – Combustors

The maj

Flame temperatures are much closer to the lean limit than in a conventional combustion

system, some action has to be taken when the engine load is reduced to prevent flame out.

If no action were taken flame out would occur since the mixture strength would become

too lean to burn. The major problems with DLE combustors is the flash back problem in

which the flame moves form the main combustor to the pre-mix chambers. This causes


Previous Designs New Designs Risk• NOx, high emissions • Very low emissions on gas • Diffusion flame with stable

combustion• Premix/DLN with instability

(pulsations)

• Single injection points/fuel nozzles simpler

• Multiple injection points/fuel nozzles more complex

• Simple operation with simple controls

• Staged operation with complex controls/tuning

• Combustor construction/cooling designs simpler

• Combustor construction/cooling designs complex

• Combustion thermal life long with or without TBC

• TBC required but life reduced from flashback/distortion damage

• Dry, water, and steam injected

• Dry and wet injected

• Low costs • High costs


the burn out of those chambers as well as damage to the main section of the combustor

can as shown in Figure 3

Figure 3

A DLE Combustor Showing Damage to the Combustor



CONCLUSIONS

Advance Gas Turbines have increased efficiency by over 15-20 percentage points,

however availability of the gas turbines have dropped by 10-15 percentage points.

Partially the large size of the gas turbines are the cause of this drop as it takes a much

longer time to open up the casings of these turbines which are usually over 220MW in

size.

Compressor problems have greatly increased as the turbine ratings increase. In fact, the

compressor problems are slightly larger than the turbine problems, as seen in Figure 4.

This is related to the high flow and high pressure in the larger turbines, and the air coolers

for compressor air used for turbine cooling. The failures of the gas turbine in the

compressor section are usually in the Inlet Guide Vanes, and at the transition areas

between the low pressure and high pressure compressors and due to rubbing in stages

usually upstream of the bleed flow sections. The larger units have very high pressure

ratios, and have both low pressure and high pressure compressors and have high exit

temperatures all leading to problems in the compressor section. The compressor exhaust

air coolers in some systems are part of the Heat Recovery Steam Generator Systems

(HRSG), adding further complexity to the system. Problems such as failure and/or major

rubs of compressor blades and vanes; oxidation damage and failure of turbine airfoils and

nozzle vanes, coating failures; failure of DLN combustors due to flashback, pulsation,



distortion and/or, control systems, and transition sections, have plagued these new

advanced turbines.

Compressor35%

Combustor29%

Turbine28%

Auxillary4%

Rotor4%

Compressor Combustor Turbine Auxillary Rotor

Figure 4 – Major Failures in Gas Turbines Larger 220 MW

The risks and costs of these new design technologies present real problems for

investment bankers, insurers, OEMs, and owners. The providing of Long Term Service

Agreements (LTSA) has spawned a new profit center within all OEM’s. The LTSA is the

fastest and most profitable growth sector for the OEM’s. The owners in most cases

cannot get the investment or insurance without a LTSA with the OEM’s for these

advanced gas turbines. Insurer’s appetite for new designs is limited. The advanced gas

turbines, such as the later “F“, plus the “G”, and “H” technology designs and the new

reheat GT 24, and 26, have had poor early reliability experience. There have been

problems with these designs from all OEM’s as they push the design envelope to its limit.



LTSA’s increase the maintenance costs but is a comfort factor for both investors and

insurance companies.

Advanced Gas Turbines over the past few years have seen considerable improvement in

the design of the major components. These changes have increased the Availability, and

Reliability of these units, and increased the life of some of these parts. Therefore with

the advancements of various components in the last few years the operation of these

turbines have greatly improved and in conjunction with the high overall turbine

efficiency the New Advanced Gas Turbines are here to stay.

REFERENCES:

1. “Condition Monitoring and Its Effect on the Life of New Advanced Gas Turbines”, Boyce and Latcovich, Gas Turbine Users Association Conference, Houston, Texas, April 21-25, 2002

2. “Gas Turbine Engineering Handbook”-Meherwan P. Boyce, P.HD., P.E., Fourth Edition, Butterworth-Hienemann-November 2011

3. “Handbook for Cogeneration and Combined Cycle Power Plants” Meherwan P. Boyce, P.HD., P.E. ASME Second Edition March 2010.


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Availability and Maintainability of Advance Gas Turbinesmedia.caspianmedia.com/document/8362d5426d015f7e61c96e9e59606fd6.pdfAdvanced Gas Turbines Availability and Maintainability