Understanding Gas Turbine Performance

UNDERSTANDING GAS TURBINE PERFORMANCE ©

Jim Noordermeer, P.Eng. Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada www.gryphoneng.com

GRYPHON INTERNATIONAL ENGINEERING SERVICES INC. ST. CATHARINES, ONTARIO, CANADA www.gryphoneng.com GRYPHON is a Canadian multi-discipline, full-service engineering design firm specializing in cogeneration, combined-cycle and thermal power plants and related equipment and systems, including level I, II and III feasibility studies, conceptual/schematic design, project development assistance, detail-design engineering, commissioning and startup, testing & project management. GRYPHON clients include Lake Superior Power, Northland Power, Cornell University, Florida Power Corporation, PB Power, TransCanada Pipelines / Energy, Union Gas Power, Kimberly-Clark Forest Products, Great Lakes Power, 3M Canada, Potter Station Power, NRC, AECL, Ontario Hydro / Ontario Power Generation, EnWave/Toronto District Heating Corporation, ICS-State, R. V. Anderson, Wascana Energy, Bayer Inc., NOVA Pipelines Ventures, NOVA Corporation, West Windsor Power, Orenda Aerospace, Henderson Hospital, BFC Industrial and Nicholls-Radtke Ltd., Gas Energy Development Corporation, ConEd Development Corporation, KeySpan Energy Development Corporation, Innovative Steam Technologies, Acres International, ABGS Inc., Bowater Pulp and Paper Canada, Ormat Industries, Consorcio Skanska Conciviles, Donohue Inc. / QUNO.

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UNDERSTANDING GAS TURBINE PERFORMANCE© ABSTRACT The performance characteristics of a gas turbine engine or Gas Turbine Generator package (GTG) depends upon the type and model of engine being examined, the location at which it will be installed, the ambient conditions under which it will operate, and the fuel(s) and NOx suppression methods which will be utilized. This paper is a primer presenting an explanation of typical gas turbine and GTG package rating methods and why and how they are corrected, so that an accurate real-life picture of the performance envelope of a unit can be determined for the examiner's evaluation.

UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

ABSTRACT

UNDERSTANDING GAS TURBINE PERFORMANCE© TABLE OF CONTENTS 1.0 1.1 1.2 1.3

WHAT ARE ISO and NEMA RATINGS ? .............................................. 1 ISO Ratings .................................................................................................... 1 NEMA Ratings .............................................................................................. 2 GTW Ratings ................................................................................................. 2

2.0 2.1

SITE CONDITIONS and OTHER DEFINITIONS ................................. 5 What Site Conditions Should be Considered ................................................ 5 2.1.1 Site Altitude ......................................................................................... 5 2.1.2 Ambient Temperature ......................................................................... 5 2.1.3 Inlet Losses .......................................................................................... 5 2.1.4 Exhaust Losses .................................................................................... 6 2.1.5 Fuel Requirements ............................................................................... 7 2.1.6 NOx Suppression Requirements ......................................................... 7 2.1.7 Relative Humidity ............................................................................... 8 Other Common Definitions and Expressions ................................................ 8 2.2.1 New and Clean Condition ................................................................... 8 2.2.2 Expected Performance ......................................................................... 9 2.2.3 Guaranteed Performance ..................................................................... 9 2.2.4 Degraded Performance ........................................................................ 9 2.2.5 Gross and Net Power Output ............................................................. 10 2.2.6 Gross and Net Heat Rate ................................................................... 10 2.2.7 Fuel Higher Heating Value and Lower Heating Value ..................... 12 2.2.8 Auxiliary Losses ................................................................................ 12

2.2

3.0 3.1

3.2

GTG PERFORMANCE CALCULATIONS from VENDOR INFORMATION ........................................................................................ 18 Calculations Using Vendor Curves and Data .............................................. 18 3.1.1 Nominal GTG Baseline Conditions .................................................. 18 3.1.2 Site Conditions .................................................................................. 19 3.1.3 Correction Factors and Calculation ................................................... 19 Other Methods of Obtaining Performance .................................................. 21 3.2.1 Manufacturer's Cycle Decks ............................................................. 21 3.2.3 Proprietary Software Programs ......................................................... 21

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UNDERSTANDING GAS TURBINE PERFORMANCE© TABLE OF CONTENTS - continued 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7

PERFORMANCE COMPARISON DEMONSTRATION ................... 22 ISO Rating vs. Actual Site Rating at Various Ambients ............................. 22 Site Ratings at Various ambients with Varying Exhaust Loss .....................23 Natural Gas vs. No. 2 Fuel Oil vs. Heavy Fuel Oil ..................................... 24 Dry Unabated vs. Steam Injection vs. Water Injection vs. Dry Low NOx . 25 New & Clean Expected Performance vs. Degraded Performance .............. 26 New and Clean Expected Performance vs. Guaranteed Performance ........ 27 Gross Power vs. Net Power vs. Power after Transformer Loss .................. 28

5.0

SUMMARY ................................................................................................ 29

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UNDERSTANDING GAS TURBINE PERFORMANCE© 1.0

WHAT ARE ISO, NEMA and GTW RATINGS ?

1.1

ISO Ratings

The International Organization for Standardization (ISO) is a worldwide federation of national standards bodies which prepares and issues standards which are accepted around the world. ISO 2314 Gas Turbines - Acceptance Tests is the international standard defining the basis and procedures for rating and testing Gas Turbines. The ISO standard contains definitions of the parameters to be measured and defines the appropriate procedures. Vendors of gas turbines publish ISO ratings in order to provide comparative performance at a standardized baseline point. The Standard Reference conditions contained in ISO 2314 for all Gas Turbine testing and rating are defined as: A. For the Intake Air at the compressor flange (alternatively the compressor intake flare): a) A total pressure of 101.3 kPaa (14.69 psia) b) A total temperature of 15 °C (59 °F) c) A relative humidity of 60% B.

For the Exhaust Gases at the turbine exhaust flange: a) A static pressure of 101.3 kPaa (14.69 psia)

The above A & B criteria essentially equate to a gas turbine theoretically installed at sea level – with no inlet air filtration, silencing or ducting; and with no exhaust plenum, heat recovery device, exhaust silencer or discharge stack. In actual practice, there are probably no stationary gas turbines which are installed in this manner, and that operate under these exact conditions. According to ISO, the Power Output of the gas turbine may be expressed in terms of output at the turbine coupling or electrical power at the generator terminals. In addition, ISO states that the gas turbine's Thermal Efficiency or the Specific Heat Consumption shall be based on the net specific energy of the fuel at constant pressure. The specific energy used shall be based on a pressure of 101.3 kPaa and a temperature of 15 °C (59 °F). The Sensible Heat above 15°C (59 °F) shall be taken into account. UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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1.2

NEMA Ratings

Early on during the introduction of Industrial Gas Turbines into common use, the National Electrical Manufacturers Association (NEMA) in the United States introduced a Standard based generally on the use of the Gas Turbine as a prime mover driving electrical generators, thus within their sphere of interest. The common Standard used as a basis for the rating of Gas Turbines in the 1960’s was NEMA Standard SM 30-1959. Since NEMA was an American organization, the standard was not metric and had the following basis: A. Inlet air temperature 80 °F B. Inlet pressure 14.17 psia C. Exhaust pressure 14.17 psia The NEMA inlet and exhaust conditions were measured at the compressor inlet flange and the turbine exhaust flange, respectively. Fuel consumption and efficiency were based on the higher heating value (HHV) of the fuel. The above rating conditions essentially equated to a theoretical gas turbine installed at 1,000 feet AMSL, with no inlet air filtration, silencing or ducting, and with no exhaust plenum, heat recovery device, exhaust silencer or discharge stack. With the growing use of the Gas Turbine in the natural gas transportation business in the late 1960’s, and its increasing use outside North America, it became obvious to the industry that a more universal standard was required and it needed to be one which was metric. In April of 1971, all of the NEMA Standards then in use relative to the Industrial Gas Turbine were canceled and this rating system is now only of historical interest. 1.3

Gas Turbine World Ratings

The most readily-available and complete listing of industrial gas turbine ratings are published annually in the Gas Turbine World (GTW) Handbook. Gas turbine manufacturers and packagers are invited to provide current data for all their machine configurations, in a form common to all. The GTW asks that the data be provided for natural gas fuel operation at 15 °C and sea level ISO conditions, with suitable account for gear losses and generator efficiencies, but no account for inlet and exhaust losses, nor for auxiliary power requirements, nor for NOx abatement via steam or water injection. For an example, refer to the attached typical listing of Electric Power Gas Turbines from the 1998-99 GTW Handbook. This particular example (page 54) shows both aero-derivative and heavy-duty industrial gas turbine generators. UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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The various columns provide the following information: a)

The manufacturer’s model number and/or series designation.

b)

The first year the unit was available for purchase.

c)

The ISO Base Load rating, the rating at which it can be operated for at least 6000 hours a year without causing a reduction in the interval between overhauls.

d)

The gas turbine Heat Rate in btu/kw.hr at the Base Load rating, stated in terms of the lower heating value (LHV) of the fuel

e)

The ISO Peak Load rating, the output power rating at which it can be operated for up to 2000 hours a year. When considering operation at this rating, consult the OEM carefully to determine what effect this operation would have on the overhaul interval. It is quite possible that a considerable reduction in the period between overhauls would occur.

f)

The gas turbine Heat Rate in btu/kw.hr at the peak load rating, again stated in terms of the lower heating value (LHV) of the fuel

g)

The nominal Pressure Ratio across the compressor section of the gas turbine. In general, the higher the pressure ratio, the higher the thermal efficiency of the engine.

h)

The Mass Flow through the engine at the base load rating. The higher the mass flow, the greater the amount of energy available to be extracted from the exhaust gas by a Heat Recovery Steam Generator (HRSG) to generate steam.

i)

The Turbine Speed data indicates the speed of the turbine's power output shaft and helps you to deduce more about the package design. In general, most aero-derivative machines will require speed-reduction gearboxes, since they operate at speeds higher than 2-pole or 4-pole generator synchronous speeds (3600 and 1800 rpm respectively for 60 Hz duty, 3000 and 1500 rpm respectively for 50 Hz duty). There are exceptions, including larger aero-derivative machines where the nominal speed of the LP section of the original turbo-fan engine was close enough to 3600 rpm to be used as-is with "minor" modifications, or where the design speed of the power turbine fitted to an industrialized jet engine is deliberately selected to be 3600 rpm. The smaller heavy-duty industrial gas turbines also generally operate above synchronous speeds, and require speed reduction gearboxes. In the larger

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classes of heavy duty machines, there are both high speed gear-drive machines and synchronous speed direct-drive machines. In the largest frame sizes, most manufacturers deliberately design 3600 rpm single-shaft machines suitable only for direct-drive 60 Hz duty (e.g. GE Frame 7 and Westinghouse W501). Then they may produce a "scaled-up" 3000 rpm direct-drive variation solely for 50 Hz duty (e.g. GE Frame 9 and Westinghouse W701). Or vice versa for European manufacturers. j)

The Turbine Inlet Temperature at the base load rating is the average combustor exit temperature into the first stage of the turbine section. The various manufacturers report this number differently, sometimes at the inlet to the 1st stage stationary blade, sometimes at the inlet to the 1st stage rotating blade, or sometimes it is not reported at all (ABB). In general, the higher this temperature, the more efficient the machine, and the more NOx will be produced (in unabated form).

k)

The Exhaust Temperature together with the mass flow, can be used to calculate the exhaust heat energy available for production of steam from the exhaust gas by an HRSG. The higher this temperature, the higher the steam temperature which can be achieved, without additional supplementary firing.

l)

Watch the Approximate Weight information given. Some weights will be for the gas turbine engine alone, and some will be for a complete package installation.

m)

The information in the Approximate L x W x H column similarly must be considered carefully as to whether it is for a complete package or only the gas turbine.

n)

The Comments column typically lists information about model variants or, for packagers, the manufacturer of the gas turbine. Some manufacturers will provide additional information on steam injected or liquid fuel variations.

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2.0

SITE CONDITIONS and OTHER DEFINITIONS

2.1

What Site Conditions should be Considered

In principle, when obtaining performance data from a gas turbine manufacturer or GTG packager, or when calculating performance yourself from vendor curves, the following factors must be considered: 2.1.1 Site Altitude The ISO/GTW ratings are given at sea level, but at higher altitudes, gas turbines will swallow less air and deliver less power. The gas turbine heat rate will remain the same, although fuel consumption will decrease proportional to the power decrease. The exhaust gas temperature will generally remain the same with changes in altitude. The typical affect of altitude is about a 4% loss of power per 1000 feet of elevation. 2.1.2 Ambient Temperature Gas turbines perform very differently at ambient temperatures other than the 15 °C (59 °F) used by ISO and GTW. In general, the temperature affects are as follows: < 15 °C (59 °F) Increases

> 15 °C (59 °F) Decreases

Heat Rate

Decreases

Increases

Thermal Efficiency

Increases

Decreases

Exhaust Flow

Increases

Decreases

Exhaust Temperature

Decreases

Increases

Parameter Power Output

Some specific aero-derivative engines do not follow these rules of thumb at the lower ambient temperatures. 2.1.3 Inlet Losses The air entering a gas turbine must be extremely clean, and to remove air-borne contaminants, inlet filtration is always fitted. The type of filtration system required depends upon the type of environment (marine, industrial, desert, etc.) that the unit is being installed in, and can include dust louvers, single-stage or multi-stage filter pads, pulse-clean filters and/or oil bath filters. In addition, further treatment of the inlet air is sometimes required or desired, UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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including heating coils for winter operation, chilling coils for summer operation, evaporative cooling to lower the wet bulb temperature in dry environments, and/or supercharging to overcome the affects of high altitudes. In addition, the high level of intake noise produced by a gas turbine needs to be treated by inlet silencers, with the size of the silencing system generally depending upon the noise permit requirements. All these devices add pressure drop between the prevailing barometric pressure and the actual inlet of the gas turbine's first compressor stage, decreasing mass flow and power output, and increasing the heat rate and exhaust temperature from the idealized ISO/GTW rating. Typical inlet losses for GTG packages are: a) b) c) d) e)

2.5~5.0 inches W.C. for standard pad or pulse-clean filtration systems, silencers and ducting 1.0~1.5 inch W.C. incremental loss for dust louver systems 1.0~2.5 inch W.C. incremental loss for chilling/heating coils or evaporative cooling systems 0.5~2.0 inch W.C. incremental loss for additional inlet silencing +5.0 to +12.0 inch W.C. gain for supercharging, although this is rarely used since it requires a lot of power to operate the fan, and dramatically raises the inlet air temperature, requiring chilling and/or evaporative cooling both before and after the supercharging fan.

2.1.4 Exhaust Losses The ISO/GTW ratings are based upon the turbine's exhaust gases discharging directly to atmosphere (please plug your ears and don't stand in the way). Of course, in practice the exhaust gases need to be silenced and need to discharged safely into either a vertical discharge stack (for simple-cycle duty) or into a heat recovery steam generator (for cogeneration or combined-cycle duty). In addition, pollution control equipment may also be fitted in the exhaust stream. Exhaust losses restrict the ability of a gas turbine to swallow air, and thus reduce power, and increase heat rate and fuel flow. Typical exhaust losses which might be applied are: a) b)

2.5~5.0 inch W.C. for simple cycle discharge stacks, depending upon the silencing requirements 8.0~12.0 inch W.C. for unfired heat recovery steam generators

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c)

10.0~16.0 inch W.C. for fired heat recovery steam generators

d)

2.0~7.5 inch W.C. incremental loss for SCR's and CO pollution control systems.

2.1.5 Fuel Requirements The choice of fuels for your application are generally dictated by the type, the security and the cost of fuels available locally for your facility. When clean-burning natural gas is available in adequate quantities and inexpensively, it is the logical choice for a gas turbine. However, costly and power-consuming gas compressors may be required to raise the gas pressure to adequate levels. In general gas turbines operating on natural gas fuels have better power output and better heat rates than those operating on liquid fuels, and usually less maintenance costs. Distillate fuel or No. 2 fuel oil or their variations are frequently less-expensive than natural gas, but do not burn as cleanly and may decrease the time between overhauls of the gas turbine. These liquid fuels are frequently used when a local gas supply does not exist, or if the local gas supply is intermittent, or if the gas supply contract is cheaper for interruptible service, or for startup/shutdown purposes for heavy fuel oil-fired plants. On-site liquid fuel storage, filtration and pumping/unloading equipment will usually be required. If an emergency power plant is being considered, which is required to operate in the event of a main power supply failure, the power plant would be designed to operate solely on a liquid fuel, since the fuel can be readily stored at site ensuring it's availability when required. The most popular fuel for these emergency power plants would be Distillate or No. 2 fuel. Heavy fuel oils or residual oils are generally inexpensive, and when available locally can be used in heavy duty industrial gas turbines. In addition to the fuel storage and pumping equipment, a considerable investment in fuel treatment systems (chemical injection and washing), heating and waste treatment systems will also be required. Due to the high ash content of these fuels, the turbine firing temperature is usually lowered (i.e. the normal rated power and efficiency is much lower than a gas-fired or No. 2 fired machine) and water washing of the turbine section is required very frequently (sometimes on-line once a day plus more vigorous off-line washing once per weekend) in order to maintain a tolerable level of performance. UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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2.1.6 NOx Suppression Requirements ISO ratings, and the majority of GTW ratings are expressed for "unabated" gas turbines, i.e. using conventional diffusion combustion systems without any NOx abatement measures taken. However, "unabated" machines are rarely acceptable, since they produce NOx in the order of 125 to 300 ppmvd on natural gas, and higher for liquid fuel machines, depending primarily upon the firing temperature. NOx can be suppressed in a gas turbine with the addition of steam injection or water injection equipment. The gas turbine manufacturers have also developed complex dry low NOx (DLN) lean-burn combustion systems, for both gas fuels and the lighter liquid fuels. The method of NOx suppression method required will be a function of the environmental permit stipulations, the availability of steam or water for injection, the required system reliability, etc. Compared to unabated machines, steam injected machines have higher power outputs and lower heat rates. Water injected machines have higher power outputs and higher heat rates. DLN machines usually have lower power outputs and higher heat rates. 2.1.7 Relative Humidity The site relative humidity will usually be different than the 60% used by ISO and GTW, and most frequently used by the vendors. As the relative humidity increases, there will be minor improvements in power output and heat rate. 2.2

Other Common Definitions and Expressions

Many expressions and definitions are commonly used when discussing the performance of Gas Turbines. The meaning of each should be clearly understood, before the user applies the data in their evaluation or comparison of units. These expressions include: 2.2.1 New and Clean Condition When a gas turbine or GTG package is "new and clean", no physical degradation of mechanical components has yet occurred, nor fouling of the compressor blading due to air-borne dust, nor fouling or degradation of the combustion system and/or turbine blading due to combustion byproducts, nor fouling of the inlet filters, nor UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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change in exhaust backpressure due to exhaust system/HRSG fouling. A gas turbine in the "new and clean" condition will produce the maximum power, at the best efficiency. Manufacturers and vendors will usually contractually define a gas turbine as "new and clean" for only the first 100~250 hours of operation, usually long enough to complete the initial site commissioning and to perform the on-site performance test. After initial installation and testing, the closest that an installation with considerable operating hours will come back to "new and clean" is immediately after a complete major overhaul and cleaning. 2.2.2 Expected Performance When a vendor states that they are supplying "expected" performance for a gas turbine or GTG package, they generally mean the data represents the average or mean of the many production line models they have produced. Depending upon the stackup of tolerances during manufacturing, the purchaser may actually get an engine which performs better or worse than the "expected" performance. 2.2.3 Guaranteed Performance When a contract for a GTG package is executed, the Vendor will guarantee the performance of the "new and clean" package at a specific set of "Guarantee Reference Conditions", including barometric pressure (representing the site altitude), ambient temperature and humidity, inlet loss and exhaust loss, fuel temperature and composition. During the on-site performance test, the actual operating conditions may be significantly different than the "reference" conditions, and test results will be corrected back to "reference" by using pre-agreed test correction curves and/or computer models. Since the Purchaser's contract will include liquidated damages for any lack of performance, the Vendor's stated "Guarantee Performance" will always be lower than the "Expected Performance", in order to protect himself. The typical power output difference between a Vendor's expected and guaranteed performance, at a specific ambient temperature, may be 2% to 4% lower. 2.2.4 Degraded Performance When a proponent conducts a detailed economic evaluation of a power plant, for a 20-year project life, he must ensure that the economic performance (power sales UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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revenue and fuel costs) reflect the actual long-term performance characteristics of a gas turbine generator. Thus the performance model that the project developer uses for his evaluation should include the reduction in performance which results from the long-term operation of the unit. Fouling of the compressor, erosion of the compressor or turbine blades reducing their aerodynamic effectiveness, changes in the combustion system resulting in less efficient or less uniform combustion, all of these combine to lower the performance of the equipment. Some of this lost performance can be regained relatively easily by cleaning of the compressor to reduce the effects of the fouling. The others unfortunately are more serious and costly and these performance losses can only be recovered by either a partial or complete engine overhaul, the extent depending on the design of the equipment. Each manufacturer should be contacted to supply their individual degradation definitions and values, however, in general, the average long-term degraded performance of aero-derivative gas turbines may show a 2.5~5.0% power shortfall and 1.0~3.0% heat rate increase. Heavy duty industrial engines may experience 2.0~4.0% power shortfalls and 0.5~3.0% heat rate increases. 2.2.5 Gross and Net Power Output The meaning of the power output of a gas turbine or GTG package is fairly obvious, however you must understand what actually pays the bills. The Gross Power Output of a engine or package is usually the amount of power delivered at the output shaft of a mechanical drive engine, or at the generator terminals of a GTG package. The Net Power Output is the effective amount of power at the connection point to the customer, after accounting for all drivetrain losses, auxiliary losses, etc., and is the one that pays the bills. 2.2.6 Gross and Net Heat Rate The heat rate of a gas turbine engine or GTG package is merely an expression of the specific fuel consumption or thermal efficiency, considering the fuel supplied to the engine; and the power output delivered by the engine. For example, a 40 MW GTG package that requires 400 x 106 btu/hr of fuel to operate might have a heat rate of: UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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Heat Rate

= =

400,000,000 btu/hr 40,000 kW 10,000 btu/kw.hr

The efficiency of this same package at these operating conditions can be found by use of the conversion factor 1 kW = 3,413 btu/hr, for example: Efficiency

= =

3,413 10,000 34.1

btu/kw.hr btu/kw.hr %

In the above general example, we have actually left out two (2) very important factors which must always be included when discussing heat rate, efficiency and fuel consumption: a) was the power output expressed on net or gross basis? b) was the fuel consumption expressed in lower heating value (LHV) terms or in higher heating value (HHV) terms? If we assume that the above GTG package had a Gross Power output of 40 MW and an Auxiliary Loss of 250 kW, the net power output would be: Net Power Output = Gross Power Output - Auxiliary Losses = 40,000 - 250 kW = 39,750 kW. Similarly, if the previously stated 400 x 106 btu/hr fuel input was the LHV fuel consumption, the corresponding fuel input in HHV terms (assuming natural gas) would be: HHV Fuel Consumption = =

400 x 106 444 x 106

btu/hrLHV x 1.11 btu/hrHHV

The corresponding gross and net power outputs, heat rates and efficiencies for this GTG package, at this operating condition, expressed in both LHV and HHV terms, is summarized as follows: Gross Power Output = 40,000 kW Net Power Output = 39,750 kW 6 LHV Fuel Consumption = 400 x 10 btu/hrLHV 6 HHV Fuel Consumption = 444 x 10 btu/hrHHV (natural gas) Gross Heat Rate - HHV = 11,100 btu/kw.hrHHV Gross Heat Rate - LHV = 10,000 btu/kw.hrLHV Net Heat Rate - HHV = 11,170 btu/kw.hrHHV Net Heat Rate - LHV = 10,063 btu/kw.hrLHV UNDERSTANDING GAS TURBINE PERFORMANCE © Gryphon International Engineering Services Inc. St. Catharines, Ontario, Canada GTPerf_0900

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Gross HHV Efficiency Gross LHV Efficiency Net HHV Efficiency Net LHV Efficiency

= = = =

30.8 % 34.1 % 30.6 % 33.9 %

This array of varying information (all for the same gas turbine at the same operating point) can be confusing, and you must know what to request, and/or must understand what information you get, in order to use it properly. All references to fuel consumption, heat rate and efficiency should include an annotation defining either LHV or HHV. 2.2.7 Fuel Higher Heating Value and Lower Heating Value These two Heating Value terms generally lead to the most confusion and subsequent errors when discussing and calculating the performance of gas turbines, and when evaluating the subsequent economic return. Most gas turbine performance data sheets state efficiency and/or heat rate, and hence fuel consumption, in terms of the Lower Heating Value (LHV) of the fuel, whether for gaseous fuel or liquid fuel. However, fuel suppliers (in particular those in the natural gas business) work in, state their pricing, and write their contracts in terms of a fuel's Higher Heating Value (HHV). In simple terms, a fuel's HHV energy content is measured on the basis of the chemical energy in the fuel, and is the total heat given up during the combustion of the fuel, and includes the formation of water vapour from the combustion of fuelbound hydrogen. By comparison, a fuel's LHV energy content quantifies the usable energy available from this combustion process. Gas Turbine cycle calculations are generally done in terms of LHV thus, when discussing fuel supply with gas suppliers and transporters, or sizing fuel compression / metering / regulation systems, or sizing liquid fuel storage tank / heating / unloading / forwarding systems, one needs to take into account the difference between the LHV data obtained in your gas turbine calculations, and the HHV terms discussed in the remainder of the fuel business. To convert from an LHV basis to an HHV basis, a multiplication factor in the order of 1.06 to 1.065 is used for liquid fuels and 1.11 for natural gas.

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2.2.8 Auxiliary Losses These auxiliary losses can include drivetrain losses, plus the power required to support the mechanical and controls operation of the gas turbine package, the power required to operate a generator, plus transformer losses, as applicable. The majority of the auxiliary losses are the result of electric motors driving pumps and fans. Manufacturers and packagers are generally able to provide the actual running auxiliary losses in kW, even though the sum total of all motor ratings in the package may seem to be much higher. This is because most motors will be selected to a higher frame size, thus not drawing full power, and many applications may be 2 x 100%, where only one of two motors actually operates. With reference to the attached sketch SK01 - Typical Power Plant Single Line Diagram with Auxiliary Losses, some typical auxiliary losses include: a)

Lubricating Oil Pumps - Every gas turbine generator package has at least one lubricating oil system supplying pressurized oil to the gas turbine bearings and possibly control systems. In some cases, this oil system will also supply oil to the driven gearbox, generator and/or compressor, and in some cases, a separate oil system will do this additional duty. If the oil system pumps are shaft-driven off the gas turbine, the gearbox and/or the generator shaft, then the manufacturer will usually automatically deduct the power consumption used to drive them from the generator power available. If instead the oil pumps are continuously driven by electric motors, then the motor power is a parasitic loss, which will have to be deducted from the gross power output of the generator. If the electric-motor driven pump is used only during start-up or shutdown or in the case of an emergency, it is not a continuous load and need not be included in the continuous parasitic losses.

b)

Ventilation Fans - A gas turbine generator enclosure typically has a forced air cooling and ventilation system with fans moving the required mass of air. Typically, ventilation systems are 2 x 100% duty, with a normal system and a back-up system. When the electrical generator is also air cooled, it too will require ventilation air and this requirement will have no effect on the parasitic load UNLESS there is a separate requirement for motor driven fans to augment the air flow provided by the fans on generator rotor.

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c)

Fuel Gas Compression Systems - When a natural gas fueled gas turbine is supplied with fuel at a pressure below that required for the manufacturer's fuel gas control system, then on-site compression will be required. Such gas compression equipment includes not only rotary or reciprocating compressors, but also cooling fans and/or inter-cooler and after-cooler water pumping systems. All this equipment is generally electric motor driven, and the power required must be obtained via an auxiliary transformer from within the powerplant itself. Thus, this will be an additional parasitic loss to the power plant. As a general rule, when required, fuel gas compression will be the largest parasitic loss for any installation. Several hundred horsepower is not an uncommon requirement for larger gas turbines. For illustration, on sketch SK01, the gas compressor load is 526 kW. This auxiliary load will change with the output power and the gas supply pressure, and will be maximum at the low ambient temperatures. If the gas transmission / supply system provides the natural gas at the required or at a higher pressure, then no on-site gas compression is required.

d)

Fuel Oil Pumps and Fuel Oil Systems - Gas turbines which include fuel oil systems may have the system main high-pressure pumps either shaft-driven or more frequently, electric motor-driven. Again, if the main fuel oil pump is driven from the gas turbine or a gearbox, the manufacturer will generally account for the parasitic loss when he calculates his generator performance, and it will have no further effect on the parasitic losses of the installation. If the main fuel oil pump is electric motor driven via an auxiliary transformer, the parasitic loss needs to be deducted from the gross power output of the generator. A fuel oil system will also generally include forwarding pumps to move fuel from the main storage tanks to a daytank, and a boost pump moving the fuel from the daytank to the inlet of the main high-pressure pump, plus frequently, tanker off-loading pumps. The power consumption for these devices should be included when calculating net power output. Some fuel oil systems may require heating due to elevated pour points or high levels of paraffin. Most heavy fuel oil systems will require heating and large fuel treatment systems to make the fuel suitable for combustion. The

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parasitic losses for the heaters and pumps in these systems should be included when calculating net power output. e)

Lube Oil Cooling Systems - Oil coolers are used to reject the heat collected by the lubricating oil system(s). These coolers can be either water-cooled via shell and tube heat exchangers, or air-cooled via electric motor driven fans driving a stream of air across finned coils, hence the general term "fin-fan cooler", or a combination of both. Shell and tube systems generally require a water/glycol pumping system and a heat sink (which can be either a fin-fan cooler or perhaps a condensate system in a plant with a steam system). In either case, the parasitic losses for the fans and/or pumps need to be deducted from the gross power output of the generator when calculating the net power output.

f)

Generators - The efficiency of an electrical generator, as provided by the manufacturer, takes into account losses from a number of sources including: • •

• • •

Electrical resistance losses in the windings Windage losses resulting from the friction between the rotating rotor and the air in the airgap between the rotor and stator, windage through the ends of the rotor coils which protrude from the main cylindrical section of the rotor, and the friction between the cooling air and the walls of the cooling air passages in the body of the rotor Friction losses in the bearing system The power required to drive the cooling air fans, located at the two ends of the rotor, which force the cooling air through the machine Losses in the generator excitation system (for permanent magnet generator (PMG) systems) which are a combination of electrical resistance losses, bearing friction, windage and cooling fan losses similar to those in the main machine but of much smaller magnitude.

Generator efficiency curves take all of these factors into account. There will be additional affects due to changes in load, and with the power factor at which the machine is operated. g)

Static Excitation System Losses - When static excitation systems are specified for generators due to utility system requirements, the power for the exciter is obtained from the generator output system via a step-down transformer, typically 13.8kV - 300V. The excitation system can draw up to

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0.2~0.3% of the generator gross output, depending upon the generator operating point. For illustration, on sketch SK01, the static excitation transformer draws 117 kW, or 0.23% of the 50,395 kW of gross power produced by the generator. h)

Transformer Losses - Every transformer has losses associated with magnetic and current induced resistance losses, and cooling system losses, usually associated with some sort of cooling system which will include temperature-controlled electric motor-driven forced-air fans. The transformer manufacturer will provide efficiency data on the specific piece of equipment but typical input-to-output efficiency values are in the order of 98.0 ~ 99.7%, depending upon the transformer's size and design, and the operating and ambient conditions. These transformer losses will apply to both the plant step-up and plant step-down transformers. For illustration, sketch SK01 shows that the main step-up transformer T1 loses about 148 kW of the 49,106 kW input power (a loss of approximately 0.3%), while the auxiliary transformer AT-1 and AT-2 losses are about 11/537 = 2% and 7/635 = 1% respectively. The efficiencies of these transformers are part of the parasitic losses of a total powerplant system.

i)

Controls - The parasitic loss for the controls will be one of the smaller powerplant parasitic loads, and should be based on the normal continuous steady state operating load from the control power supply system. In the case of co-generation or combined cycle projects where the control systems for the steam side may be integrated with that for the gas turbine, it will be rather difficult to split the parasitic load for attribution to the gas turbine and the steam system.

j)

Miscellaneous Auxiliaries - Other devices which can cause parasitic losses are the battery chargers for the DC power supply system for the emergency lube pumps, air compressors, lighting, HVAC - heating, ventilation and air conditioning, and in the case of co-generation or combined-cycle plants, feedwater and condensate pumping systems, water treatment systems, condenser cooling water and/or cooling tower systems.

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For a final illustration, sketch SK01 shows that for a generator gross power output of 50,395 kW, the auxiliary losses directly attributable to the GTG are: a) b)

117 kW - for the static excitation system, and, 120 kW - for the GTG services (via motor control centre MCC-3). 237 kW

The resultant 50,395 - 237 = 50,158 kW will be the value that the GTG packager will typically show as the "Net Power Output" of his package. The other auxiliary losses associated with the operation of this powerplant include: a) b) c) d) e)

526 kW 313 kW 195 kW 18 kW 148 kW 1,200 kW

for the gas turbine's fuel gas compressors. for building services (via MCC-1). for essential services (via MCC-2). in auxiliary transformer losses. in main step-up transformer losses.

The resultant 50,158 - 1,200 = 48,958 kW will be "Net Plant Power Output", and is about 97.1 % of the GTG package's initial gross power output of 50,395 kW.

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3.0

GTG PERFORMANCE CALCULATIONS from VENDOR INFORMATION

3.1

Calculations Using Vendor Curves and Data

We will run through an example performance calculation using a Vendor's typical corrections curves, based upon a hypothetical 75 MW heavy-duty industrial gas turbine generator. These correction curves can be obtained from the manufacturers or are available in their standard product binders which also include the descriptions of their complete product and scope of supply. 3.1.1 Nominal GTG Baseline Conditions For the purposes of assisting a proponent in calculating performance at his site conditions, the vendor will normally start by stating the "baseline" condition for their GTG package, in a manner similar to the following: Load: Generator Type: Fuel: NOx Suppression: Elevation: Temperature: Relative Humidity: Standard Inlet Loss: Standard Exhaust Loss: Net Power Heat Rate* Exhaust Flow Exhaust Temperature Fuel Flow* *

Base Load TEWAC generator Natural gas Without steam or water injection Sea level 59 deg F 60 % 4.5 inch H2O 5.0 inch H2O 74,750 kW 10,300 btu/kw.hr 2,054,000 lb/hr 987 deg F 36,733 lb/hr

based upon a natural gas heating value of 20,960 btu/lbLHV

You will note that this "baseline" condition is very similar to the ISO and GTW ratings, with some additional standardization on inlet filtration system losses and exhaust losses, based upon the Vendor's standard package design. The vendor will also have a set of "baseline" conditions for Peak Load and Part Load operation, Distillate Fuel operation and for other types of generators (open air cooled, hydrogen cooled, etc.).

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The Net Power already includes a deduction of 250 kW for the Vendor's auxiliary power losses, and thus the Heat Rate can be understood to be a "Net" heat rate. Although it doesn't specifically say so, the heat rate is probably based upon the LHV of the fuel, because of the Vendor's note at the bottom of the tabulation. 3.1.2 Site Conditions If we now wish to determine what this GTG's performance will be at the following site conditions and ambient temperature: Site Elevation: Ambient Temperature: Relative Humidity Load: NOx Suppression: Inlet System: Exhaust System:

1,000 feet AMSL 35 deg F 60 % 100% (Base) Load Steam Injection to 42 ppmvd NOx Assume the vendor's standard inlet system Unfired HRSG, with an estimated exhaust loss of: 10.0 inch H2O (i.e. an excess of 5.0 inch H2O)

3.1.3 Correction Factors and Calculation By reference to the attached typical correction curves, we find the following correction factors (for power and heat rate):

Elevation

Power 74,750 kW P1 0.965

Heat Rate 10,300 btu/kw.hrLHV HR1 1.00

Figure 1

Ambient Temp

P2

1.09

HR2

0.985

Figure 2

Inlet Losses

P3

1.00

HR3

1.00

Figure 3

Exhaust Losses

P4

0.993

HR4

1.0075

Figure 4

Stm/Fuel Ratio

-

1:1

-

1:1

NOx Curve

P5

1.081

HR5

0.972

Figure 8

Baseline Value

Stm Injection Corrected Value

84,400 kW

9,935 btu/kw.hrLHV

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Figure

Calc'd per below

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The above two calculations were performed as follows: Net Power

= = =

Baseline Power x P1 x P2 x P3 x P4 x P5 74,750 x 0.965 x 1.09 x 1.00 x 0.993 x 1.081 84,400 kW

Net Heat Rate

= = =

Baseline Heat Rate x HR1 x HR2 x HR3 x HR 4 x HR5 10,300 x 1.00 x 0.985 x 1.00 x 1.0075 x 0.972 9,935 btu/kw.hrLHV

As a note, the changes in exhaust flow and exhaust temperature can be calculated in a similar manner. By further calculation, we can see that the Net LHV Efficiency of the GTG unit at this operating condition is: Net LHV Efficiency

=

3,413 / 9,935

=

34.3

%

and that the required fuel flow will be as follows: Fuel Flow

=

84,400 x 9,935 20,960

kW x btu/kw.hrLHV btu/lbLHV

=

40,005

lb/hr

or, in Gas Compressor terms (mmscfd - million standard cubic feet per day): Compressor Capacity = Required =

40,005 x 20,960 x 24 905 x 1,000,000

lb/hr x btu/lbLHV x hr/day btu/scfLHV

22.2

mmscfd

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3.2

Other Methods of Obtaining Performance

3.2.1 Manufacturer's Cycle Decks All the major gas turbine manufacturers have computer programs (cycle-decks) that they can use to quickly provide proponents with performance information for a variety of operating and site conditions. Two examples from GE and S&S are enclosed. In the case of frequent users (e.g. engineering design firms and major developers), some manufacturers (e.g. General Electric, Stewart & Stevenson, Westinghouse) will make available either a dial-up connection to their cycle deck, or will load licensed cycle decks onto the user's server. The results from these are similar to the salesmen's cycle decks. 3.2.2 Proprietary Software Programs In addition, there are several proprietary software vendors who have prepared proprietary performance programs such as GT PRO, GT MASTER, et. al. by ThermoFlow Inc.; SOAPP-CT by SEPRIL; the Gate Cycle™ by ENTER Software Inc.; and GrypHeat™ by Gryphon International Engineering Services Inc. These programs allow the proponent to model the performance of gas turbines and virtually any other component within a power plant, including HRSGs, STGs, boilers, condensers and cooling towers, heat exchangers and piping systems.

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4.0

PERFORMANCE COMPARISON DEMONSTRATION

To demonstrate the importance of obtaining the correct gas turbine performance information, and understanding what it actually means, we provide a series of graphical examples of the differences in power output for various operating conditions. The examples only show power output considerations for an aero-derivative General Electric LM6000 engine (as packaged by Stewart and Stevenson Energy Products) and a heavy duty industrial General Electric Power Systems Frame 6B engine. Heat rate, exhaust flow and temperature also need to be considered. 4.1

ISO Rating vs. Actual Site Rating at Various Ambients 50

Example 1A - LM6000 Pure ISO rating on natural gas vs. Sea Level

Output - MW

45 ISO

Natural gas – Unabated combustion Simple cycle – 4 in. H2O exhaust loss Temperature units are in deg F

40

35 40

50

60

70

80

Example 1B – Frame 6B Pure ISO rating on natural gas vs. Sea Level Natural gas – Unabated combustion Simple cycle – 4 in. H2O exhaust loss Temperature units are in deg F

Output - MW

45

ISO

40

35 40

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4.2

Site Ratings at Various ambients with Varying Exhaust Loss 48

Example 2A – LM6000 Site ratings at various ambients with varying exhaust loss

Output - MW

4in 46

10in

44

Simple Cycle - 4 in. H2O exhaust loss Unfired HRSG - 10 in. H2O exh. loss Fired HRSG - 14 in. H2O exhaust loss

42

500 ft AMSL Natural gas – Unabated combustion Temperature units are in deg F

14in

30

35

40

45

50

43

Example 2B – Frame 6B Site ratings at various ambients with varying exhaust loss Output - MW

Simple Cycle - 4 in. H2O exhaust loss Unfired HRSG - 10 in. H2O exh. loss Fired HRSG - 14 in. H2O exhaust loss

4in 42 10in 41

14in 40

500 ft AMSL Natural gas – Unabated combustion Temperature units in deg F

39 30

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4.3

Natural Gas vs. No. 2 Fuel Oil vs. Heavy Fuel Oil 50

Example 3A – LM6000 Unabated combustion systems

48

Output - MW

Gas

Natural Gas vs. No. 2 Fuel Oil (Distillate)

46 No.2 44

500 ft AMSL Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

42

40 30

40

50

Example 3B – Frame 6B Unabated combustion systems Natural Gas vs. No. 2 Fuel Oil (Distillate) vs. Heavy Fuel Oil

60

42 Gas 41 No.2 Output - MW

20

500 ft AMSL Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

40 Heavy 39

38 40

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Dry Unabated vs. Steam Injection vs. Water Injection vs. Dry Low NOx Example 4A – LM6000 Dry Unabated vs. Steam Injection vs. Water Injection vs. DLN

50

48 Steam Output - MW

Unabated

Water

46 DLN 44

42

500 ft AMSL Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

40 30

35

40

45

Example 4B – Frame 6B Dry Unabated vs. Steam Injection vs. Water Injection vs. DLN

50

45

Steam Output - MW

4.4

43 Water Unabated

41 DLN

500 ft AMSL Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

39 30

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4.5

New & Clean Expected Performance vs. Degraded Performance

55

Output - MW

Example 5A – LM6000 New and Clean - Expected vs. Degraded

Clean

45 Degraged

500 ft AMSL Temperature units in deg F Natural gas – Unabated combustion Simple cycle – 4 inch H2O exhaust loss

35

25 0

20

40

60

80

100

Example 5B – Frame 6B New and Clean - Expected vs. Degraded at various ambients

45 Output - MW

500 ft AMSL Temperature units in deg F Natural gas – Unabated combustion Simple cycle – 4 inch H2O exhaust loss

50

Clean 40

Degraged

35

30 0

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60

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4.6

New and Clean Expected Performance vs. Guaranteed Performance 50

Example 6A – LM6000 New and Clean - Expected vs. Guaranteed

Output - MW

48

46

500 ft AMSL Temperature units in deg F Natural gas – Unabated combustion Simple cycle – 4 inch H2O exhaust loss

44 Clean 42

Guaranteed

40 45

50

55

60

65

45

Example 6B – Frame 6B New and Clean - Expected vs. Guaranteed Output - MW

500 ft AMSL Temperature units in deg F Natural gas – Unabated combustion Simple cycle – 4 inch H2O exhaust loss

43

41 Clean 39

Guaranteed

37

35 45

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Gross Power vs. Net Power vs. Power after Transformer Loss Example 7A – LM6000 Gross Power Output vs. Net Power Output vs. Power Output after Transformer Loss (Utility)

50

48

Output - MW

Gross 46 Net 44

Utility

500 ft AMSL Natural gas – Unabated combustion Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

42

40 30

35

40

45

50

Example 7B – Frame 6B Gross Power Output vs. Net Power Output vs. Power Output after Transformer Loss (Utility) 500 ft AMSL Natural gas – Unabated combustion Temperature units in deg F Simple cycle – 4 inch H2O exhaust loss

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43 Gross Output - MW

4.7

41

Net Utility

39

37

35 30

35

40

45

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50

5.0

SUMMARY

In this paper, we have tried to outline the major items which you must remember as you assess the performance of a Gas Turbine or GTG package. Probably the most important is that you need to question the information presented to you, and you must be certain to understand the basis used in the presentation. Without that knowledge, you cannot make an intelligent evaluation of the equipment proposed. Always remember that the performance of a Gas Turbine or GTG package will change from the initial nominal values, due to the following factors: • Site elevation. • Ambient temperature. • Relative humidity. • Inlet and exhaust conditions. • The type of driven equipment. • The associated auxiliaries losses (some belonging to the Gas Turbine or GTG package, and some from the exterior supporting equipment). • The type fuel used. • The type of NOx suppression method used. • New & Clean, Expected, Degraded, Guaranteed. Don't be afraid to ask questions. Questioning information given at the proposal stage and during the initial assessment process is the smart thing to do. If you are not comfortable with the data given, keep asking questions until you are. Remember that it is your money, or your client's money, which the manufacturer is trying to access. It is far easier and less costly to change the configuration or equipment selection before construction than afterwards, and your customer will be happier.

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