Critical Flow Measurements for Optimizing Gas Turbine Thermal Efficiency and Environmental Compliance

Transcription

1 Critical Flow Measurements for Optimizing Gas Turbine Thermal Efficiency and Environmental Compliance Robert D. Carrell President, Hoffer Flow Controls, Inc. Sandra L. Kelly Vice President, North American Sales, Hoffer Flow Controls, Inc. Presented April 13, 2017 Electric Power Conference Chicago, IL

2 Abstract: Gas turbines come equipped with many different sensors to monitor the health and performance of the unit. Two flow sensors in particular are critical to achieving optimal thermal efficiency of the turbine as well as important feedback and verification of emission levels that are directly tied to compliance with the plant's operating license. These two measurements are the primary combustion fuel supply and the water supply for a combustion air fogging system. Both measurements are critical to active control, balance of plant calculations and documenting emissions compliance. Advances in measurement technology and calibration options in the last several years provide the knowledgeable gas turbine OEM and their operator clients with the opportunity to extract more data from these two measurements at higher levels of accuracy and with higher confidence levels resulting in the ability to optimize performance while remaining in compliance with plant emission license requirements. This paper will explore the state of the art in measurement technology for these two applications and define the critical specifications needed by those responsible for selecting these mission critical sensors. Other factors such as calibration schemes and options and ancillary electronic options and control interface options will be addressed. Additional content will focus on maintenance and calibration recommendations to assure long-term effective and accurate measurement. Real-world examples and data will be included with analytical tools needed to systematically evaluate the ongoing performance of the sensors used in these measurements. Air Inlet Fogging System Demineralized Water Flow Measurement: The original concept behind air inlet fogging systems was to reduce the inlet air temperature by means of evaporative cooling to increase the combustion air density. This increased density produces a boost to gas turbine output for the same fuel input. Advances thereafter allowed for its use in even high humidity climates which further expanded the geographic reach of the technology. In addition the development of overspraying techniques expanded further the options available for designers to utilize the technology in gas turbine plant design. Optimizing the evaporative cooling benefits at a given site is a constant control problem. Monitoring ambient atmospheric conditions (dry and wet bulb temperatures and relative humidity) must be done in real time as they are subject to real time fluctuations during the course of the day, change in local weather and change of seasons. All of these affect the potential cooling benefit that can be produced by the fogging system. The measurement of the atmospheric conditions is just the front end of the controls problem. They are critical input variables to the control algorithm. The power produced by the turbine compared to the algorithm s predicted output closes the basic control loop but the flow rate measurement of the fogging system s water supply is required to produce the feedback input for the adjustments to that flow rate that must be made to achieve the optimal inlet air condition at a given moment in order to produce the expected increase in power output. The relationship of the accuracy of the flow measurement of the

3 demineralized water is proportional to the efficacy of the process and the resultant boost to the turbine s thermal efficiency. Provided all other variables are known and/or controlled then flow measurement accuracy determines the probability (or uncertainty) of achieving the desired atmospheric condition within the air inlet system. Accuracy is, of course a relative term. What may be accurate enough to achieve the desired control or result in one process may either be inadequate or excessive in another. In some applications such as legal for trade or custody transfer measurement where the flow meter s reading is used to calculate the cost of a business transaction involving the sale of the product being measured, either the two parties involved with the transaction may determine the accuracy needed or regulatory authorities may have jurisdiction to decide the accuracy requirement. In the case of an industrial process measurement not otherwise regulated then typically the facility owner would have the option to determine what accuracy is needed. Though a blanket better accuracy is always desired may seem like the best answer it must be understood that improvements in accuracy usually come at a cost. That cost may be in the initial investment, the ongoing maintenance and calibration of the measuring device, the life expectancy of the device or some combination of these and other variables. As with many choices in business and life, selection of process measurement and control devices and systems often comes down to a compromise of the various competing advantages and disadvantages. The measurement of water in its different phases and its wide range of generic and specialized forms are, collectively, the most common flow measurements made in the consumer and industrial worlds. With that breadth and scope of flow measurement applications there is a similarly broad scope of technologies available to make the measurements. Those technologies range in measurement accuracy from a small fraction of a single percent of the indicated flow rate to several percentage points of the indicated flow rate. Accuracy is also a term that requires definition as it is used in different ways depending on the technology employed. Without trying to get too deep into the metrological weeds some basics need to be defined and are summarized below as they apply to flow measurement. Accuracy The deviation of a measurement made compared to a known standard reference flow rate or total. Percent of Reading: The deviation of the instantaneous flow rate or total indicated by an instrument compared to a known standard instantaneous reference flow rate or total expressed as a percentage of the instantaneous indicated flow rate or total indicated by the instrument.

4 Percent of Full Scale: The deviation of the instantaneous flow rate or total indicated by an instrument compared to a known standard instantaneous reference flow rate or total expressed as a percentage of the maximum rated flow measurement capacity of the indicating instrument. Repeatability: The deviation between measurement values indicated by in instrument taken under the same conditions and over a short period of time. Precision: In the context of repeatability and reproducibility, precision is the closeness of agreement between independent measurements of a quantity under the same conditions.¹ Reproducibility: The precision determined when the same methods, but different equipment and operators are used to make measurements on identical specimens.¹ This definition typically is modified within the context of flow measurement to incorporate the use of the same flow measurement device or technology in making the measurements. Linearity: The deviation at any point over a defined flow measurement range from a mean or average calibration factor exhibited by an instrument. Note that not all flow measurement devices utilize or exhibit linear behavior. Devices that relate flow rate to a change in head or differential pressure obey Bernoulli s principle and thus exhibit a square law relationship between flow rate (velocity) and pressure. It has been the experience of the authors in the combined 50+ years of experience in flow measurement that the greatest confusion regarding the application of these terms occurs most often between the definitions for percent of reading versus percent of full scale. The percent of reading specification applies normally to devices that exhibit a near-linear relationship between flow rate and their produced flow indication or signal. Take for example, a device with a linear flow range of say 5 to 50 GPM and a stated accuracy of +/-0.5% of reading. That device would then be expected to produce an indication of +/- 0.5% of 5 GPM when the flow rate was 5 GPM (+/ GPM deviation) and +/-0.5% of the indicated reading for any value of flow rate greater than 5 GPM up to 50 GPM. Another device with the same +/-0.5% numeric specification but as a percent of full scale looks quite different. Assuming the same flow range, 5 to 50 GPM, then +/-0.5% of full scale translates into 50 GPM x +/ = +/-0.25 GPM. The anticipated +/-0.25 GPM at 50 GPM would then be the same anticipated absolute error at 30 GPM or 10 GPM or 5 GPM. An error of +/-0.25 GPM at a flow rate of 5 GPM is really an error of +/-5.0% at that flow rate. The greater the span between the maximum or full scale value and the minimum measureable flow rate then the greater the absolute error becomes with decreasing flow rate.

5 A third device for the same flow range may have a stated accuracy of +/-0.2% of full scale. Plus or minus 0.2% of full scale then would be +/-0.1 GPM (50 GPM x.001). That would be the anticipated deviation in accuracy then at all flow rates between 5 and 50 GPM +/-0.1 GPM at 5 GPM, +/- 0.1 GPM at 10 GPM, +/- 0.1 GPM at 20 GPM, etc. In the above examples then which device is the most accurate? The answer depends on the flow rate being measured at any given moment. If the flow rate various significantly over time and covers a large part of the device s 5 to 50 GPM flow range specification then very likely the device with the +/-0.5% of reading will produce the more accurate total result over time. If the flow rate is fixed and is in the top end of the scale for flow rating then the percent of full scale device may produce the more accurate result over time. This concept is critical to understand when considering or specifying any flow measurement device. By the same token, repeatability is of significant importance as well in determining the applicability of a given flow measurement technology for a particular process and task. Repeatability is often a function of certain characteristics of the fundamental flow measurement technology. Here the user may find it necessary to ask questions about the effect of any changes in environmental or operating conditions seen in the process which may have an effect on repeatability. Such conditions most often manifest themselves in their effect, if any, on the devices ability to maintain it stated repeatability or reproducibility under the variability in those conditions. Specific terms that may apply in this analysis would include zero stability (or zero drift) and hysteresis. Zero stability is the ability of the device to return to the same state at zero flow rate under the changing conditions or over time. Hysteresis is the lag time between a change in flow rate and an indication of that change in flow rate by the device. In general, most modern flow device specifications include an accuracy statement expressed in terms of +/- percent of reading or +/- percent of full scale as well as a repeatability specification for both. Again, typically the better these specifications are the better the quality of the measurement that can be expected. In the specific application of measurement of fogging system supply water flow rates, hysteresis should also be taken into account due to the need to be able to adjust for changes in the inlet air atmospheric conditions in real time. If the system operates over a wide range of flow rates then devices with accuracies expressed as a percentage of reading often produce a better control situation by reducing the change in measurement uncertainty as the flow rate changes to meet the changing inlet air conditions. Typically for this specific application, wide flow ranges or turndown ratios are common as the relationships between air temperature and humidity levels are non-linear.² Another consideration in flow meters used for this application is the ability to operate at the elevated pressures associated with fogging systems. In general it is not recommended to install flow meters on the suction side of a pump such as might seem desirable in this case to avoid having to deal with the high pressure in the piping from the pump to the fogging nozzles. Upstream placement can create measurement issues cause by pump

6 pulsations, pressure drops that could create cavitation and internal inefficiencies in fluid transfer or bypassing or leaks within the pump. Measuring flow at high pressures and to do so without deterioration in measurement accuracy are highly desirable characteristics in a flow meter for these applications. Output signal and meter signal resolution (or response time) address the interface between the flow meter s raw flow measurement signal and the higher level control system that utilizes the flow rate input as a part of the flow control algorithm. The control system must not only determine the flow rate required to achieve the desired inlet atmospheric condition for humidity, it must also activate the control elements to adjust that flow rate, determine when the flow rate has been achieved and then monitor and adjust the flow rate to maintain the target flow rate until conditions change requiring the next adjustment and control of the flow rate. In this area the designer is often faced with an embarrassment of riches in terms of the options available from most every applicable flow measurement technology. Analog and digital; open architecture and proprietary networks; wired and wireless; single or multiparameter measurement devices; transmitters only and transmitters with integral local displays the choices are limited almost entirely by the needs, budget and imagination of the designer. Typically the basic parameters for these electronic accessories are driven more by the facilities control system architecture or that selected by the integrator for the fogging system. The process fluid being measured and utilized for this application is demineralized water which possesses certain unique characteristics which can have a bearing on the choice and/or configuration of specific flow measurement technologies. It is a low viscosity liquid which exhibits a reduced electrical charge compared to tap water and virtually no impurities. The fluid has limited lubricity. Though deionized and demineralized waters in and of them-selves are not especially corrosive they will readily form weak acids in solution once in contact with air. It has been the experience of the authors that the use of high carbon steels in flow meter components, especially when in contact with stainless steel other dissimilar metals will cause corrosion of the high carbon steel components and should be avoided in the specification phase. The low electrical conductivity of the fluid also makes it problematic for flow electromagnetic based flow meters (mag meters). Any use of such technology in this application should be discussed in detail with the flow meter manufacturer. Cost-versus-performance as well as installation space available often play significant roles in the selection of flow meters for fogging system applications. Many systems require the use of multiple flow meters to measure flow to different nozzle arrays and/or for multiple fog injection points. This need may drive compromises on the designer s part in order to achieve the desired result at an economically justifiable level of investment for the flow meters. In this situation the designer should consider multiple technology and/or vendor options to select the technology or technologies that best align with the different technical and economic specifications established for the system.

7 This paper includes a relative comparison of several different technologies that are typically used for air inlet fogging system demineralized water flow measurement across these and other parameters discussed above along with brief discussions on each. The actual line size and flow range for these applications tend towards the lower to lower middle end of the scale for both. Line sizes most commonly specified for this application range from ¾ to as large as 2. Flow rates range from a maximum of approximately 3 GPM to as much as 150 GPM with the spread in range from application to application being functions of the gas turbine to be installed and number of nozzles and or injection points in the gas turbine being serviced by the line. These variables are controlled by the system designers. Flow meters for any of the technologies listed herein to meet these line sizes and flow rates are readily available. The same is true for meeting the operating pressures seen for these applications in the 2000 to 5000 PSIG range. A final consideration in selecting a flow meter technology for this application are the relationships between accuracy of measurement and balance of plant calculations as well as its part in acting as one of the cross-references for assuring emissions compliance. The relationships between the increase in power production tied to the air inlet fogging system at each operating condition versus the fuel input (potential energy) and the produced power are defined for each installation. A flow measurement of inadequate accuracy can lead to difficulties in balancing all these factors which in turn can lead to questions regarding the calculated emission levels. Unnecessary confusion in these areas due to the selection of an inadequate flow measurement technology is a situation to be avoided.

8 Chart 1 Comparison of Liquid Flow Meter Technologies for Fogging System Water Supply Measurement Flow Meter Specification Flow Meter Type Differential Vortex Coriolis Pressure Ultrasonic Shedder Turbine Primary Measurement: Mass Volumetric Volumetric Volumetric Volumetric Compensated Volumetric Mass Mass Mass Mass Measurement: (Optional) Accuracy: +/-0.15 to 0.25% +/-0.2 to 1.0% +/-0.25 to 0.5% +/1.0% +/-0.07 to 0.5% of Reading of Full Scale of Reading of Full Scale of Reading MAWP: to ,000 (Typical) (PSIG): Piping None 20 Dia. Up 10 Dia. Up 35 Dia. Up 10 Dia. Up Installation 10 Dia. Down 5 Dia. Down 5 Dia. Down 5 Dia. Down Requirements: Relative High Moderate to Low Moderate Low to Pressure High Moderate Loss: Relative Moderate to Moderate to Moderate Moderate to Fast Dynamic Slow Slow Slow Response: Relative High Moderate High Moderate Moderate Initial Cost: These commonly used flow meter technologies outlined in the above chart are discussed in greater detail below. These technologies are the most commonly used ones seen by the authors in this application. It is not intended to be a complete list of all possible technologies that could potentially be utilized but is rather a compendium of the ones which most typically are used.

9 Coriolis Meters: The Coriolis meter is a direct mass flow measurement technology used in a wide variety of industries and applications. Though it can be used for gas flow measurement under specific circumstances it is far more commonly applied in the measurement of liquids. Most Coriolis meters are designed for use in applications requiring high accuracy. With respect to this application, the primary benefits of the technology is its accuracy and to a lesser degree, its lack of moving parts. Its disadvantages include being typically the most expensive of the technology choices and a high pressure drop. Differential Pressure Based Flow Transmitters: Based on one of the oldest flow measurement principles, differential pressure flow transmitters have the advantage of simplicity and typically the lowest initial acquisition cost. The fact that its accuracy is expressed as a percent of full scale rather than of reading will often limit it to applications requiring relatively narrow operating flow ranges. It can also be more difficult to install based on the need to provide for 20 diameters of straight pipe before and 10 diameters of straight pipe after the transmitter to provide the necessary flow conditioning required for optimum measurement accuracy. Ultrasonic Flow Meters: Transit time based ultrasonic flow meters are designed to work with pure liquids (no solids and no bubbles. They range in accuracy depending on line size and the number of transducers in a given design. They are not always suitable for use in high pressure applications and their use in high pressure should be discussed with the manufacturer. Ultrasonic meters do require 10 diameters of straight pipe upstream and 5 diameters of straight pipe downstream for proper flow conditioning. In terms of initial cost, ultrasonic meters are typically at the high end of range but vary with performance specifications. Vortex Shedding Flow Meters: Vortex shedding flow meters are another type of velocity measuring device. This device contains no moving parts but does also require a minimum of 10 diameters of straight pipe before and 5 diameters of straight pipe after the flow meter for flow conditioning. Its initial acquisition cost is in the moderate category as well as its accuracy being in the mid-range of the technologies presented here with a typical specification for liquids being +/-1% of full scale. The technology is most widely used for steam flow measurement but is applicable for liquids and gases as well as long as certain minimum velocity conditions are met.

10 Turbine Flow Meters: Another technology based on a measurement principle that is among the oldest is the turbine flow meter. Turbine flow meters are among the most accurate technologies for volumetric measurement available. Turbine meters also provide high output signal resolution and the fastest dynamic response time of any flow meter technology. They can be used with equal effectiveness on liquids and gases over extremely wide ranges of temperature and pressure. When used in this application care should be taken to select a non-metallic sleeve bearing for the rotor. Turbine meters require 10 diameters of straight pipe upstream and 5 diameters of straight pipe downstream for proper flow conditioning. Their combination of accuracy as a percent of reading, high accuracy and repeatability, moderate price and flexibility in design often make turbine flow meters among the best choices for this application. Fuel Flow Measurement: Gas turbines are exceptionally flexible in the types of fuels they can be designed to utilize. By far the most common fuel used in aeroderivative and main frame gas turbines for electric power production in the US and in most other installations around the world is natural gas. The increasing availability of the fuel and its cleaner burning properties have contributed substantially to the growth in gas turbine based power production. Other fuel choices tend to be liquid fossil fuel based hydrocarbons or biofuels (liquids or gas). For the purposes of this paper our focus is on natural gas based fuel applications. Though the relationships between the water flow rates in an inlet air fogging system and power production, balance of plant and emissions monitoring may not be immediately obvious to every observer; those same relationships as they apply to fuel flow rates are far more self-evident. Fuel flow measurement is part of determining the potential energy input to a system that converts some portion of that potential energy into electricity. The other part is determining the unit volume or unit mass potential energy content of the fuel. That second part can be achieved as simply as assuming average energy content based on sample based analysis, by contract between the utility and the gas supplier or as complex as direct measurement of the energy content in real time through the use of an inline gas chromatograph or similar analyzer. The selection of the specific technique utilized in determining unit energy content is beyond the scope of this paper and the expertise of the authors. Once the unit energy content is determined then combining it with the flow rate measurement produces the input energy variable required for the other performance and regulatory driven calculations and control system responses that follow. For US-based facilities the starting point in selecting a fuel flow meter for a gas turbine almost always includes the regulatory requirements contained in 40 CFR Part 75 Appendix D of the US Federal Register. This Appendix titled Optional SO2 Emissions Data Protocol for Gas-Fired and Oil-Fired Units defines the critical parameters a flow meter must meet in order to satisfy the requirements of the Clean Air Act as it relates to

11 the use of the flow meter s output as part of the reporting requirements and permit compliance of the facilities sulfur dioxide emissions. An abbreviated description of the requirements contained in this appendix comes down the fuel meter used in this application is contained in paragraph Initial Certification Requirements for all Fuel Flowmeters and reads as follows: For the purpose of initial certification, each fuel flowmeter used to meet the requirements of this protocol shall meet a flowmeter accuracy of +/-2.0 percent of the upper range value (i.e. maximum fuel flow rate measurable by the flowmeter) across the range of fuel flow rate to be measured at the unit. To use some of the terminology described in the first part of this paper, the flow meter must have a minimum accuracy of +/-2% of full scale in order to comply with this regulatory requirement. There are a number of other requirements that apply to the calibration of the meter and the re-calibration of the meter that will be discussed later in this paper. For the present, however, the above defines that absolute minimum performance specification the flow meter must meet in order to satisfy Appendix D when it is invoked for a given facility. In general, the accuracy specifications for gas flow meters have traditionally been less demanding than their liquid counterparts across the full spectrum of applications. Liquids are typically quite homogenous in nature as well as being incompressible. Their viscosities and densities most often follow defined relationships versus temperature though some liquids (not water or liquid fuels) can have a change in viscosity due to shear rates. Gases, and especially natural gas, tend to be blends of various constituent gases with often varying compositions, are most definitely compressible, rarely follow the Ideal Gas Law at elevated pressure and are easily subject to pressure fluctuations which create changes in the instantaneous density and velocity in often random patterns. A further complication that has often hampered the accuracy of gas flow measurement has been the lack of gas calibration facilities for elevated pressures as well as for higher flow rates. These historical limitations on the accuracy of gas flow measurement can be seen in the acceptance and maintenance tolerances given in the National Institute for Standards and Testing s (NIST) Handbook 44: Specifications, Tolerance and Other Technical Requirements for Weighing and Measuring Devices. Section 3.31 of the Handbook provides guidelines of acceptance and maintenance tolerances fuel dispenser flow meters such as those used in an automotive fuel dispense. Table 1 of that section shows that for the size dispensers most commonly used for fueling non-commercial vehicles the acceptance tolerance is +/-0.3% of reading and the maintenance tolerance is +/-0.5% of reading. Section 3.33 of the Handbook provides similar guidelines for over and under registration for hydrocarbon gas-vapor flow meters. The acceptance and maintenance tolerance is +1.5% for over registration and -3.0% for under registration. In effect, the

12 tolerance range then for gas flow meters according to NIST is 4.5-times greater for a hydrocarbon gas meter than for a liquid hydrocarbon fuels meter. These standards were created decades ago as a basis for legal-for-trade transactions and as can be seen are very similar to that required under Appendix D. That said, many if not most gas turbines used for base or even peak power production consume vast quantities of natural gas in the course of a year s operation. The purchasing agreements between the turbine builders and their utility clients grow ever more competitive in terms of energy conversion rate claims and contracted guarantees. Flow meters that can only measure the fuel consumption with +/-2.0 of full scale accuracy do not align with the demands produced by this competitive environment. Fortunately there have been significant advances in not only gas flow meter technology but also in gas flow meter calibration capabilities since the NIST standards and the EPA regulations were developed. Multiple technologies coupled with advanced calibration techniques can produce gas flow meter measurement performance that rivals that of their liquid flow measurement cousins. These advances in technology come with a higher cost but the improvements in measurement accuracy allow those costs to be recaptured by the user often within a matter days in operation. Turbine flow meters, Coriolis flow meters and multi-path ultrasonic flow meters now have the ability to achieve gas flow measurement accuracies in the range of +/-0.1% to +/-0.35% of reading or better. Differential pressure based flow transmitters still are limited to percent of full scale accuracy specifications and in the range of +/-0.5% of full scale for gas flow measurement. Historically large bore gas flow meters such as those typically seen used with gas turbines in power generation, were calibrated most often with water and in limited cases with air at low pressure. The calibration techniques varied from manufacturer to manufacturer and from calibration lab to calibration lab. Master or reference flow meters and gravimetric based calibrations were the most popular and common of the calibration techniques utilized. The problem with these techniques varied with the flow meter technology being calibrated. For turbine flow meters and ultrasonic flow meters that are velocity measurement based technologies the lower velocities achievable with water calibration and the lower densities achievable with low pressure air relative to the conditions seen in a high pressure natural gas supply line to a gas turbine created uncertainties in the calibration. For mass Coriolis flow meters the issues are similar. For differential pressure transmitter based measurement the significant difference in densities between low pressure air or water and high pressure natural gas produce different differential pressure signal outputs that have to be mathematically adjusted to obtain a best case calibration. Similar adjustments between the calibration data and the meter calibration used for high pressure natural gas were necessary for all the other technologies as well with the resultant uncertainties.

13 One of the largest calibration laboratories in North America is operated by Colorado Engineering Experimental Station, Inc. (CEESI) ( Established originally in the early 1950 s as a flow lab by the University of Colorado it became a privately held calibration and research facility in in The facility has operated a limited high pressure gas calibration standard at its Nunn, CO for many years. This facility is limited by the fact that the supply gas must be compressed and stored in large high pressure tanks. The supply of gas is thus limited which makes multi-point calibrations problematic and the compression costs make the overall costs unattractive for production meter calibrations. Thus this facility has more often been used for research and development testing. In 1999, CEESI opened a high pressure, large bore flow meter calibration facility located near Garner, Iowa. The source of supply for the high pressure gas is a large natural gas pipeline operating at just over 1,000 PSIG. The facility has a calibration flow range of 6 to 20,000 actual cubic feet per minute (ACFM) with a flowrate uncertainty of +/-0.23% and for line sizes from 4 to 36. This facility helped usher in a dramatic improvement in high pressure natural gas flow meter calibrations with a proportional improvement in demonstrable flow meter accuracy specifications. Data gathered by this facility by many different flow meter manufacturers contributed to their ability to not only document the accuracy of their existing designs to a far greater level of certainty but produced information that led to various performance improvements. Specifying large bore flow meters in high pressure applications, liquid or gas, carries with it considerations that are often less pronounced in smaller meter sizes such as those previously discussed for the fogging system water supply application. Though many of the installation considerations are exactly the same, meter sizing in particular as it relates to pressure drop and maximum meter capacity often require closer scrutiny. Gas turbine manufacturers are rather specific about the inlet conditions and quality of the fuels used with their units. Often the turbine designs are specifically designed around certain of those conditions and inlet pressure entering the combustion zones is one such criteria that must be kept in mind when sizing a flow meter. For vortex shedding flow meters and for ultrasonic multipath flow meters, pressure loss through or across the flow meter is usually minimal. For mass Coriolis meters, differential pressure based meters and turbine meters pressure loss is an important variable to control. The dynamic measuring range is sizing dependent for all of these flow meter technologies. On this latter point, most flow meter manufacturers will follow American Gas Association (AGA) guidelines in terms of sizing flow meters so that the maximum required flow rate for the application does not exceed approximately 80% of the flow meter s maximum rated flow measurement capacity. That specification, however, should be cross-checked against the maximum allowable pressure loss available for a given installation. It is not unusual for a given gas turbine to require these flow meters be sized to a much lower percentage of their maximum capacity versus the maximum flow rate anticipated in order to keep the pressure loss within the limits established for the specific gas turbine. Such restrictions have been known to drive the

14 flow meters to be over-sized in terms of maximum flow measurement capacity for a given installation in order to remain within the maximum allowable pressure loss specifications. Turbine flow meters do present additional flexibility for situations such as the one just described. Most turbine flow meter manufacturers offer at least two different rotor blade angles for their gas turbine flow meters. Our company offers four standard angles ranging from 15-degrees to 30-degrees with custom angles available for critical applications. The wide range of densities that characterize gas flow measurement applications is the driving force behind the use of multiple blade angles. In general the shallower the blade angle the wider the turndown range (i.e. the ratio of maximum to minimum measurable flow rate for a flow meter) will be. The steeper the blade angle the narrower the turndown range but the lower the minimum flow rate the meter may measure. Shallower blade angles tend to produce lower pressure losses as well. It is this flexibility in flow range and pressure loss combined with high accuracy that often make turbine flow meters a popular choice for this application. The following chart provides a relative comparison of the gas measurement versions of the various flow meter technologies from the prior discussion on fogging system supply water flow measurement.

15 Chart 2 Comparison of Gas Flow Meter Technologies for Gas Turbine Fuel Supply Measurement Flow Meter Specification Flow Meter Type Differential Vortex Coriolis Pressure Ultrasonic Shedder Turbine Primary Measurement: Mass Volumetric Volumetric Volumetric Volumetric Compensated Volumetric Mass Mass Mass Mass Measurement: (Optional) Accuracy: +/-0.25 to 1.0% +/-1.0 to 2.0% +/-0.1 to 0.5% +/-2.0% +/-0.1 to 0.5% of Reading of Full Scale of Reading of Full Scale of Reading MAWP: to ,000 (Typical) (PSIG): Piping None 20 Dia. Up 10 Dia. Up 35 Dia. Up 10 Dia. Up Installation 10 Dia. Down 5 Dia. Down 5 Dia. Down 5 Dia. Down Requirements: Relative High Moderate to Low Moderate Low to Pressure High Moderate Loss: Relative Moderate to Moderate to Moderate Moderate to Fast Dynamic Slow Slow Slow Response: Relative High Moderate High Moderate Moderate Initial Cost: 40 CFR Part 75, App. D Compliant Flow Meter Calibration Considerations: As mentioned at the beginning of this discussion regarding natural gas fuel flow measurement, elements of Appendix D (latest edition dated ) extend beyond just the minimum accuracy requirements of the flow meters used in this application. To begin, paragraph serves as an exhaustive list of the various deign and calibration standards acceptable for the service across a wide range of flow measurement technologies. A note of caution regarding these listings Most of the standards listed are generated by technical societies and technically oriented business trade associations. Examples include the American Society of Mechanical Engineers (ASME), the American Gas Association (AGA) and the American Petroleum Institute (API). Virtually all the

16 standards listed are issue-date specific and most have been superseded by newer editions that most manufacturers and/or calibration providers will have already adopted. Though this divergence between the Appendix and actual flow meter design and calibration has never been known to the authors to create an issue with any installation, the user should be aware of this anomaly should such a question arise. It should additionally be noted that the paragraph does allow for the EPA Administrator to approve alternate standards or procedures. Assuming that a given flow meter was designed or calibrated to a later version of one of the listed standards and a regulator should raise a question on the point then be aware of this avenue to resolve the matter if it cannot be resolved directly at the local or regional level. Should the user wish to use a technology or calibration procedure not otherwise referenced under this paragraph then the user would be well-advised to seek Administrator approval in advance of construction. Under this same paragraph there are references to a specific formula (Eq. D-1) and a specific form for reporting flow meters calibration test results (Table D-1-Table of Flowmeter Accuracy Results). It is important to specify that for a new flow meter the supplier should provide you with a calibration report that is generally compliant with the details and form described in this section. This is generically referred to in the flow meter industry as a 40 CFR 75 Appendix D compliant calibration report. It has been the experience of the authors and is often their recommendation to clients to have the initial and follow up calibrations for the flow meter conducted at the three flow rates described in this section (including the repeats) plus additional points in between and/or above and below these flow rates to confirm the flow meter s total performance. The Appendix requires that the flow meter be tested three times at three different flow rates corresponding to the maximum and minimum designed operating conditions plus a third point located approximately midway between those two points. The report should also include a reference as to the percentage each of these flow rates represents versus the maximum flow rating of the meter under test. The additional test points in between and/or above the Appendix mandated points provides assurance that the flow meter is indeed operating in accordance with its published specification over its full range of flow. At this point the authors wish to address certain points from the Appendix that tend to create confusion among a not insignificant percentage of end users. These have to do with the allowable period from initial calibration and startup of the flow meter and the frequency of recalibration. A part of this confusion arises from the fact that the Appendix does allow for field or in-situ initial and follow-on calibration of the flow meter using either a reference flow meter or portable flow meter proving system. Field proving and calibration of flow meters is a long-established practice especially in the petroleum and natural gas industries. Metering stations in those industries are often equipped to accommodate portal flow meter provers, include a fixed flow meter prover or include a master or reference flow meter all intended to provide a more convenient method of checking flow meter calibrations. Most of these facilities operate at relatively steady flow rates for extended periods of time. Some, such as gas pipeline metering stations see variations driven by the season and by weather events. If needed, the operators can conduct calibrations and calibration checks at most anytime of their choosing.

17 Such an option for a gas turbine power plant is more problematic. In order to conduct the initial meter calibration or the roughly annual calibration check as described in 2.1.6, subsection (a) of the Appendix, for example, several things must have happened. First, a reference meter is the only practical method for conducting a field calibration on a large gas flow meter regardless of the technology employed. That is due in no small part to the fact that the Appendix requires that the flow rates be checked between the two meters for a period of no less than 20 minutes for each of the three flow rates. Next the reference meter must have been calibrated within the prior 365-days before the field calibration is conducted. Next the associated temperature and pressure transmitters that are also needed to conduct the field calibration must have been calibrated immediately prior to the calibration. Finally, all testing must be concluded over a period not to exceed 7 consecutive unit operating days. These constraints plus the need to conduct the calibrations at all three of the aforementioned flow rates makes field calibration exceptionally challenging to perform. The above mention of it being necessary for the reference meter to have been calibrated in the 365-days prior to conducting a comparison or calibration of the fuel meter is sometimes misread as requiring that the fuel flow meter itself be calibrated within the prior 365-days before first use. The Appendix makes no such requirement. Under section of the Appendix (Quality Assurance) it requires that, following initial calibration the fuel flow meter s calibration be checked at least once every 4 fuel flow QA operating quarters as defined in 72.2 of this chapter. A fuel operating QA quarter is defined as any quarter during which the gas turbine burns fuel for 168 hours with any fraction of an hour being rounded up to a full hour. The longest total elapsed time allowed between the completion of the initial calibration or a subsequent successful calibration check of a fuel meter and the next time it must be checked is 20 consecutive quarters. That, again, assumes the unit was not operated a total of four fuel flowmeter QA operating quarters at any time during the applicable 20 consecutive quarter period. In short then, a fuel flow meter can sit idle (assuming proper storage and agreement from the meter supplier) for a period of 20 quarters without the need for re-calibration. Please note that the above comments and sections cited do not apply to differential pressure type flow meters (orifice, nozzle or venturi). The procedures for calibration, calibration checks and inspections for those devices are detailed in paragraphs through of the Appendix. It would be to the benefit of the user or gas turbine manufacturer to discuss with the flow meter supplier the impact of these regulations would play in the life cycle support and cost for the flow meter. In the vast majority of the cases, the recalibration costs will exceed the other maintenance and spare parts costs for the flow meter over the life expectancy of the flow meter and/or turbine unit. Planning for how to best handle the recalibrations (especially if on site calibration is to be seriously considered) will yield significant economic benefits to the user. The same discussions should also be

18 undertaken with respect to emergency situations in the event unscheduled maintenance or replacement of a flow meter is required. The fines for failing to make these measurements due to a disabled flow meter can easily exceed those of the precautionary steps which could have been taken to mitigate such a failure. Summary: Accurate measurement of the water supply for gas turbine inlet air fogging systems and for the primary fuel supply to the gas turbine is of critical importance for optimal operation as well as compliance to applicable environmental regulations within the United States. Multiple flow meter technologies are available to the designer to choose from for either application. The final choices can be based on evaluations of each candidate technology against a set of parameters and specifications that most directly relate to any given project and to any specific gas turbine design. A clear understanding of the advantages and potential disadvantages of each technology is also critical to arriving at the best choices. Additional attention in project planning and in meter specification is required for the initial and subsequent calibrations of the fuel flow meters in order to maintain compliance with the applicable sections of 40 CFR Part 75, Appendix D of the Clean Air Act. The cost for reliable and accurate flow measurement in these applications pales in comparison to the potential liabilities should a technology selected solely on initial cost fail to achieve the required measurement accuracy or suffer from premature failure while in operation. Acknowledgements: 1. P.R. Glassel & Associates, Inc. Engineering 101: Repeatability and Reproducibility 2. Gas Turbine Inlet Fogging For Humid Environments, Thomas R. Mee III, PowerGen Asia, 2014