WO2003089770A1

WO2003089770A1 - Water injection for gas turbine inlet air

Info

Publication number: WO2003089770A1
Application number: PCT/US2003/011650
Authority: WO
Inventors: Thomas R. Mee; Ross A. Petersen
Original assignee: Mee Industries, Inc.
Priority date: 2002-04-15
Filing date: 2003-04-15
Publication date: 2003-10-30
Also published as: AU2003224986A1

Abstract

A gas turbine has an inlet (14) compressor section (10), combustion section (12), and turbine section (11). Water droplets are injected (16) into an inlet air stream for the compressor section, and means (21, 22, 19, 23) are provided for controlling the quantity of water droplets injected. Air mass flow rate through the compressor is measured and provides feedback for adjusting the means for controlling water to achieve a desired air mass flow rate. For example, the compressor discharge air pressure is measured by a pressure transducer (21) near the compressor discharge, and used for determining the air mass flow rate through the compressor.

Description

WATER INJECTION FOR GAS TURBINE INLET AIR

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No. 60/372,658, filed May 15, 2002. The subject matter of the Provisional Application is hereby incorporated by reference.

BACKGROUND

This invention relates to control of water injected into an inlet air stream for a gas turbine for raising the humidity of the inlet air and/or injecting liquid water droplets into the compressor section of the gas turbine.

The term gas turbine is used throughout. These are turbines where the working fluid comprises combustion products as distinguished from a steam turbine where steam is the working fluid. Fuel for a gas turbine may be a gas or liquid. Such a turbine is also referred to as a combustion turbine. Exemplary large gas turbines are available from companies such as General Electric, Siemens/Westinghouse, and Asea Brown Bovari (ABB), for example. Such gas turbines are widely used for driving generators in electric power plants. Electric power generator turbines are run at constant rotational speed to keep the generator in synch with the

60 cycle (in the US) power grid.

The basic construction of a gas turbine has a compressor section 10 at the upstream end and a turbine section 11 at the downstream end, as shown schematically in FIG. 1. Intake air is first compressed in the compressor section and then heated in a combustion section 12, and these gases then expand through the turbine, which in turn powers a shaft 13. Some of the shaft power is used to drive the compressor and the balance is available to drive a load such as an electric generator.

Gas turbines suffer a loss of output power when ambient conditions are hot. On a typical summer day, for example, a gas turbine may produce up to 25% less power than on a cold winter day. Gas turbines are constant volume machines, which means that at a given shaft speed they move the same volume of air, but the power output of a turbine depends on the mass flow of gases through the turbine. During warm weather when the air is less dense, the total mass flow of air through the turbine is reduced and therefore the output falls off. Furthermore, the work required to compress air increases as the temperature of the air increases. This means that the compressor is consuming more of the power generated by the expansion turbine, and available shaft power is further reduced. Gas turbine power output and efficiency are improved by cooling the compressor inlet air, thereby increasing its density, which results in reduced compressor work and increased mass flow. Gas turbine operators have, for many years, cooled compressor inlet air by injecting water droplets (or fog) into the inlet air stream for the compressor. Evaporation of the water droplets cools the air, thereby increasing its density and the mass flow of air into the compressor.

Gas turbine manufacturers have adopted a standard rating point of 15 °C and 60% relative humidity at standard barometric pressure for rating gas turbine output (usually referred to as ISO conditions). The power output of a gas turbine decreases by about 0.5 to

0.9% for every degree Celsius increase in inlet air temperature. Increases in output power of as much as about 20% and fuel savings of up to about 3% can be attained by cooling gas turbine inlet air.

It is possible to compute a new air mass flow after injecting water droplets by computing what the new density of the air will be and adjusting the ISO mass flow value through the turbine accordingly. One can also calculate the amount of water to be injected to achieve a target temperature the new air mass flow after water injection. The amount of water to be added depends on the initial inlet air conditions (e.g., temperature and relative humidity) and the desired final conditions of temperature and (usually) 100% relative humidity. Such calculations are normally done in mass units of water rather than volume (e.g. pounds or kilograms), and using specific volume, the reciprocal of density (e.g. ft³/lb or m³/kg). From such calculations, the engineer routinely selects the maximum amount of water to be added as droplets into the inlet air stream.

Water droplets are injected by an array of nozzles in the inlet air duct to the compressor. In exemplary installations for power plant gas turbines there are from five to thirty manifolds of nozzles. These may be charged with purified water by one or several pumps, e.g. up to about eight. Typically, any number of the several manifolds of nozzles can be selectively turned on to inject differing quantities of water. An exemplary manifold may have enough nozzles to accomplish 5 °F (2.8°C) of cooling, so five manifolds give a total cooling capacity of about 25 °F (13.9°C). The more banks of nozzles used, and the smaller the cooling capacity of each bank, the finer is the increment of control of the quantity of water injected and the amount of air temperature reduction.

A typical water injection control system monitors ambient temperature and humidity to determine how much cooling can be accomplished with current climatic conditions. When only cooling is desired, the maximum number of manifolds of nozzles are activated to cool the air without exceeding 100% relative humidity. There is a limit to safe inlet air cooling. If the air temperature goes too low, ice can form on structures in the inlet air duct, on the inlet struts, or inlet guide vanes of the compressor itself, and the ice can flake off and do mechanical damage to compressor blades. Thus, it is recommended that inlet air (after cooling) be maintained at a temperature well above the freezing point of water. The initial air mass flow to be adjusted by cooling may not be known. For a new or refurbished gas turbine going on-line for the first time, the manufacturer's rating for that gas turbine at ISO conditions may be used. There are other factors, however, which affect air mass flow of the compressor and which cannot be readily calculated. For example, when the inlet air filters become plugged, or the compressor blades become fouled with dirt, or the compressor blades become eroded, the compressor will move less air. It is not uncommon for a gas turbine to lose 10% of its output due to compressor fouling before it is shut down and the blades are cleaned. Thus, the actual air mass flow may be significantly different from what one would calculate based only on ambient conditions and the manufacturer's rating of the gas turbine. It may also be desirable to inject more water droplets into the inlet air stream to the compressor than required to reach 100% relative humidity of air entering the compressor. In other words, more water is injected into the inlet air stream than can evaporate before the inlet air reaches the compressor. The un-evaporated portions of the water droplets are carried by the air stream into the compressor. Operation in this mode is typically called "overspray" or "fog intercooling" or "wet compression".

During overspray, the inlet air is cooled from ambient conditions until roughly 100% relative humidity is reached. The remaining water droplets enter the compressor and evaporate as the air is heated by compression, thereby extracting heat and cooling the air mass, further increasing its density and thereby reducing the work of compression. (For such reasons, the addition of excess water droplets is sometimes called intercooling or wet compression instead of overspray.) Since the compressor normally consumes up to 50% of the work output of the turbine, any reduction in the work of compression means more work is available at the output shaft of the turbine. The principal effect of the overspray is on work of compression, but there is also a small amount of additional mass flow since the water droplets are appreciably more dense than the air they displace. This is a secondary effect.

The use of overspray can achieve increases in power output from the gas turbine substantially greater than is possible by evaporative cooling alone, and may also enhance fuel efficiency. The amount of water to be added as overspray is subject to the same uncertainties caused by inlet air filter and compressor degradation as water injected solely for evaporative cooling of the inlet air. Furthermore, the amount of water the operator may wish to add to the inlet air stream changes as ambient atmospheric conditions change during the course of the day and night, either for simply reducing temperature or for also adding or adjusting overspray. It is, therefore, desirable to provide some means for near real time control of the quantity of water droplets injected into the inlet air stream for the compressor.

BRIEF SUMMARY OF THE INVENTION

There is, therefore, provided in practice of this invention a method for controlling the quantity of water injected into the inlet air stream to the compressor section of a gas turbine by measuring a parameter related to the air mass flow rate. Feedback from the air mass flow rate measurement is used for adjusting the quantity of water injected so as to achieve a desired air mass flow rate.

Preferably the measurement for feedback is obtained by measuring pressure near the discharge of air from the compressor section. The compressor discharge pressure is a function of air mass flow rate, so the air mass flow rate is thus determined.

A suitable water injection system for a gas turbine has means for injecting water droplets into an inlet air stream for the compressor section of the gas turbine. A pressure transducer measures compressor discharge pressure, and a feedback controller connected between the pressure transducer and a water quantity controller adjusts the means for injecting water droplets into the inlet air stream. Other parameters may be measured for input to a feedback controller.

DRAWINGS These and other features and advantages of the present invention will be appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates schematically a gas turbine feedback control system for water injection; FIG. 2 is a graph showing an exemplary compressor discharge pressure as a function of air mass flow rate in a typical turbine; and

FIG. 3 illustrates schematically a gas turbine control system with additional transducers for feedback.

DESCRIPTION

A typical gas turbine has a compressor section 10 connected to a turbine section 11 by a shaft 13. Air is drawn into the compressor through an inlet duct 14. Fuel is added in a combustion section 12, and the combustion products pass through the turbine for producing power. The gas turbine illustrated in FIG. 1 is shown schematically, since this invention may be used with any of a broad variety of conventional gas turbines and for gas turbines yet to be developed. Several manifolds of water injection nozzles 16 are arrayed in the air inlet duct for injecting water droplets into compressor inlet air. (Four manifolds are illustrated schematically, although ordinarily several additional manifolds may be used.)

A cloud of water droplets less than about 40 micrometers diameter is called a fog, whereas larger droplets are called mist or rain droplets. For injecting water into a gas turbine compressor it is beneficial to make a true fog, not a mist. True fogs tend to remain airborne due to Brownian movement, while mists tend to descend relatively quickly. In a typical gas turbine inlet duct, there is only a limited time for the evaporation process to take place (typically less than two seconds and often less than one second). Small droplets are desirable to speed evaporation for efficient air cooling. Per unit mass of water, the surface area of water increases in inverse proportion to droplet diameter. For instance, water atomized into 10 micrometer droplets yields ten times more surface area than the same volume atomized into 100 micrometer droplets, and surface area plays a key role in evaporation rate.

Average droplet diameters are often expressed as Sauter mean diameter. This is a number used to express the average droplet size of a spray in terms of the average ratio of volume to surface area of the droplets. Since it deals with surface area, Sauter mean diameter is a good way to describe a spray that is to be used for processes involving evaporation. Sauter mean diameter is the diameter of a hypothetical droplet whose ratio of volume to surface area is equal to that of the entire spray.

A high pressure pump 17 (or pumps) provides water to the nozzles by way of separate valves 18 for the respective manifolds. A water controller 19 is connected to the valves for selectively opening or closing valves to the manifolds. Suitable water injection systems and components of such systems, including water filters and purification means, pumps, valves, manifolds, nozzles, computerized controllers, and associated plumbing and electronic components, are available from Mee Industries, Inc., Monrovia, California. During operation of the water injection system for a gas turbine, one or more pumps supply water to the valves, and hence to the nozzles, with sufficient volume and pressure to emit water droplets from all of the nozzles connected to the pump(s), and preferably with sufficient capacity to emit overspray into the air inlet duct. The quantity of water injected is controlled by opening the valves to an appropriate number of manifolds to emit the desired amount of water. Preferably, the injected water droplets are very small, i.e. with an average diameter up to about 25 micrometers, for efficient evaporation into the inlet air, and when a gas turbine is used in overspray mode, to minimize erosion of compressor blades. A preferred nozzle for injecting water droplets into the inlet duct of a gas turbine emits droplets with diameters ranging from about 5 to 25 micrometers.

Alternatively, instead of simple on-off valves, one may use gate valves or the like with proportional control for a continuous range of water injection quantity from each or some of the manifolds. Similarly, one may vary pump pressure (and hence flow rate) with or without proportional valve opening to provide infinitely variable water flow rate, as compared with the incremental control provided by the illustrated embodiment.

It will be apparent that the schematic illustration in FIG. 1 is elementary and has only enough detail that might be needed for one skilled in the art to make and use this invention.

There are clearly many collateral features which are not illustrated or described. For example, there are inlet screens and filters for the air to prevent damage to the gas turbine. There are usually water purification and/or filtering apparatus for the water supply to the pump. The turbine is connected to some type of power utilization device, such as, for example, an electricity generator. The gas turbine has a fuel supply .and controls, which are also not illustrated nor needed for an understanding of this invention.

A pressure transducer 21 is ordinarily provided by the original equipment manufacturer near the outlet of the compressor section of the gas turbine. For example, a conventional transducer produces a 4 to 20 milliamp signal as a measurement of the compressor discharge pressure (often called "PCD"). In conventional operation, the compressor discharge pressure and exhaust gas temperature are measured to compute how much fuel should be injected in the combustion section.

FIG. 2 illustrates the relationship of air mass flow and compressor discharge pressure for an exemplary, arbitrarily selected gas turbine (GE 7-EA)and for various inlet air humidity .and temperature conditions as well as various conditions of inlet pressure drop, barometric pressure, compressor fouling, etc. As can be seen, air mass flow is very nearly proportional to PCD. Air mass flow as a function of PCD is readily determined from a gas turbine computer model called GT-PRO, available from Thermoflow, Inc., Sudbury, Massachusetts. The model allows one to change different operating parameters and to see what effect they have on the operating condition of the gas turbine. Engineers use the program to optimize a design for a particular site or application.

Compressor efficiency is an editable parameter in the GT-PRO computer model that allows the operator to model the effect of changes in efficiency that would be associated with fouling or blade damage, for example. In other words, a new and clean compressor would operate at 100% efficiency, while a fouled compressor or one with damaged or eroded blades, may operate at less than 100%. Another parameter affecting air mass flow rate is the air pressure drop through the inlet filters and ducts. Increasing the pressure drop input to the computer model shows the effect of plugged filters or design changes that would result in higher inlet pressure drops.

Increased pressure drop and/or compressor inefficiencies reduce the mass flow of air at given atmospheric conditions. Decreased air mass flow decreases the compressor discharge pressure proportionately along a line similar to that shown in FIG. 2. Cooling of inlet air by water additions, either to nearly 100% relative humidity or with overspray, increases air mass flow rate and this can be sensed directly by a proportional change in compressor discharge pressure. The PCD can therefore be used to calculate how much water to inject into the air Met duct to achieve a desired air mass flow rate.

A suitable proportionality of PCD versus air mass flow rate may be available from the gas turbine manufacturer. Such a proportionality may be estimated from recognized pressure drops and compressor inefficiencies using the GT-PRO program. Alternatively, one may "calibrate" a gas turbine by taking several readings of PCD and air mass flow at different operating conditions. These values are then used to generate a correlation between air mass flow rate and PCD for that turbine.

Use of such a proportionality for computing air mass flow rate as a function of the transducer signal, i.e., the compressor discharge pressure, will necessarily have some uncertainty. In other words, instead of a narrow line as illustrated in FIG. 2, the correlation is more of a narrow band. Although the air mass flow rate based on PCD used as a feedback input to the water controller with a scaling factor may not be precise, it is certainly far more accurate than the approximations previously used to estimate the quantity of water that should be injected into the compressor air inlet duct based primarily on atmospheric measurements. Furthermore, PCD may be used for feedback input on a near real time basis, thereby allowing changes in water input as operating conditions or atmospheric parameters change.

A feedback controller 22 is connected to the pressure transducer at the compressor discharge. It determines air mass flow rate as a function of the transducer signal which is a function of the compressor discharge pressure. The water controller operates in a conventional manner to achieve a desired set point for air mass flow rate as determined by a conventional feedback controller. If the air mass flow rate is more than the design point, the feedback controller causes the water controller to open a valve further or open additional valves to increase the amount of water injected, so as to maintain the same ratio of water mass to air mass Similarly, if the air mass flow falls below the design point, the feedback controller sends a signal to the water controller telling it to reduce the water injection rate, again so that the proportion of water mass to air mass is kept constant. If the system operator desires increased power output, such as for peaking conditions in an electric power grid, the water injection set point can be changed to introduce or increase overspray into the compressor. Temperature of the compressor discharge air stream is also available from conventional measurements as a parameter to employ in the feedback controller but PCD is considered sufficient. By more or less continually measuring a compressor or turbine parameter related to mass air flow, the quantity of water injected may be continually adjusted, if needed, to achieve a desired air mass_. flow rate. The preferred measure is of air mass flow rate through the compressor section, and more particularly measurement of compressor discharge air pressure. Overshoot, hunting and similar irregularities are avoided by setting a suitable dead band in the set point of the feedback controller, as is conventional in feedback systems. One may also intermittently measure air flow rate, either on a regular schedule or in response to change in some parameter, such as a change in ambient temperature or humidity. Measurements are more frequent during and after changes in turbine operating conditions if not normally continuous. An upper limit on the amount of water injected into the air inlet duct may sometimes be desirable to avoid ice formation at the inlet. Thus, ambient temperature and relative humidity measurements from a conventional weather measuring station 23 may be fed to the feedback and/or water controllers to determine a maximum amount of water that will avoid formation of ice under the then existing ambient atmospheric conditions. Upper limits and set points may be set manually or automatically.

In an exemplary situation where water droplets are injected into the inlet air stream, ambient temperature and humidity are measured and wet bulb temperature calculated. To provide a conservative margin of safety to prevent icing, the amount of water added will not bring average air temperature into the compressor below an operator set point, usually about 60 °F (about 15 ° C), the idea being to assure that there are no places in the air stream where conditions are such that ice would form. Such a limit has been used without feedback control and remains appropriate when feedback is used for controlling the amount of water injected. Feedback enables that limit to be observed automatically if atmospheric conditions should change. A number of factors may affect the air mass flow rate through a compressor. Increased pressure drop due to fouled inlet air filters and compressor inefficiencies have already been mentioned. It may also occur that an older turbine may have worn turbine section inlet nozzles. This effectively means there is a larger orifice for discharge air. In some modern industrial turbines air is taken from the middle stages of the compressor section and routed to hotter section blading where it is injected into hollow blades to keep them cool. The amount of bleed air taken off will affect the compressor discharge pressure. The feedback algorithm for injection of water droplets as a function of air mass flow rate may need such factors included as adjustments. The amount of bleed air at any given time may be found from the bleed air valve position, or by measuring pressure drop across some known bleed air passage. In the event there is a shift in the proportionality between air mass flow rate and compressor discharge pressure, a scaling factor for the actual air mass flow rate through the compressor may be determined by "calibrating" the gas turbine. Calibration of air mass flow rate can be accomplished by measuring pressure drop across the bell mouth of the turbine or by an array of air flow sensors across the air inlet duct. In order to increase the accuracy of the feedback control, one may also wish to calibrate a new turbine since its inlet air mass flow rate may differ significantly from the air mass flow rates published by the manufacturer. Such a scaling factor is applied to the signal from the pressure transducer to find a final value for the actual air mass flow of the compressor.

For example, a gas turbine that moves 100 pounds of air per second at ISO conditions, might have a signal from a compressor discharge pressure transducer that reads "19 milliamps." Applying the scaling factor to this signal might result in an actual air mass flow of only 98 pounds per second instead of the 100 pounds per second predicted. This new adjusted air mass flow rate is then used in the calculation of how much water to inject into the air inlet duct to achieve a desired air mass flow rate.

Such a scaling factor may be estimated from recognized pressure drops and compressor inefficiencies using the GT-PRO program or may be based on performance curves provided by the OEM or on a best-guess approximation of air mass flow based on evaluation of turbine operating parameters such as turbine output, PCD, inlet duct pressure, fuel flow, etc. For example, one may calibrate a gas turbine by taking several readings of PCD and turbine output at different operating conditions. These values are then used to generate a new correlation between air mass flow rate and PCD, e.g. a new line shifted relative to the line for a new and clean turbine, such as the line illustrated in the example of FIG. 2. As mentioned earlier, a turbine may also be calibrated by measuring pressure drop across the bell mouth of the turbine using ports already installed by the manufacturer.

"Corrections" in measured PCD may also be made in the event of air flow changing due to a change in the proportion of bleed air. Changes to be expected by changing a bleed orifice, for example, may be provided by the OEM, or estimated by noting the change in PCD, for given inlet conditions, that occurs after changing the bleed air orifice. One may also make direct volumetric measurements of the bleed air for use in the feedback algorithm. Although a measurement of air mass flow rate with the already-installed PCD transducer is desirable, other measurements of air mass flow rate may be used for determining feedback for control of water injection. For example, pressure drop across the bell mouth of the turbine may be used. This technique is illustrated in the schematic drawing of FIG. 3. In this drawing, the same reference numerals as in FIG. 1 are used for like parts.

The manufacturer of the turbine ordinarily includes pressure transducer ports 24 in the bell mouth 26 of the compressor. Pressure transducers in these ports are connected as inputs to the feedback controller 22. Knowing the area of the bell mouth and the pressure drop across it, the air mass flow rate is readily determined. This input parameter for feedback must be used with care during overspray, but can be used directly when there is only humidifying of the air to lower temperature. Also, temperature rise across the compressor varies non-linearly in proportion to air mass flow rate. Since inlet temperature variations may be appreciable, using only compressor discharge air temperature may not give a sufficiently accurate result. However, one may use this temperature and a computed or measured inlet temperature for an accurate feedback control. A computed inlet temperature is preferred because of difficulties of measuring actual temperature of the flowing inlet air. Air temperature may be noticeably non-uniform across the air inlet and there is risk in assuming that measurements taken at one or two points are representative of average air temperature. This uncertainty is exacerbated during fogging, whether for evaporative cooling or overspray since some of the water droplets may have not yet evaporated. Instead, ambient conditions, including air temperature and relative humidity are measured with conventional sensors in the weather station (which may be outside of or inside the inlet air duct). These data may be used directly when there is no water injection. During water injection, the amount of water injected is known or measured and a temperature drop due to evaporation of injected water is computed. This computed inlet air temperature is included in the calculation of air mass flow if one is looking at temperature rise across the compressor.

Additionally, one may wish to employ an algorithm to estimate water evaporation in the given time frame, to further refine the predicted inlet air temperature. Such algorithms are available in the technical literature. In this way, variations in compressor discharge temperature measured in the compressor air discharge which are due to variations in the inlet temperature can be factored out of the calculation and temperature rise across the compressor may be used for determining air mass flow rate for feedback control of water injection.

Compressor outlet temperature can be found by considering the transducer 21 in FIG. 1 to be a temperature transducer. It might be thought that inlet pressure conditions should also be used when PCD is measured for feedback control. This is not deemed important since inlet pressure variations are quite small by comparison and are safely ignored- What one is actually measuring for air mass flow rate is compression ratio of the compressor, i.e., a difference between discharge and inlet pressure. Changes in inlet pressure are minor, whereas changes in inlet temperature

(used for measuring temperature rise across the compressor) have a relatively large effect. Thus, it is preferred to use PCD instead of using extra input parameters when measuring compressor discharge temperature for the feedback algorithm.

To make a more accurate calculation of the relationship of air mass flow to either temperature or pressure rise, one way also wish to take into account the total water vapor content of the air as this will affect the final result. Due to the higher specific heat of water vapor, as compared to air, the temperature rise will be less and the pressure rise greater with higher levels of water vapor in the inlet air.

Another input for a feedback system for fogging is a direct measurement of humidity of air after all water droplets have evaporated. A turbine manufacturer has alleged that the amount of overspray or humidity of inlet air after fogging cannot be measured accurately. One may, however, measure the humidity of the air after the compression process or far enough into the multistage compressor so that all of the injected water has evaporated. The air to be measured may be drawn downstream from the compressor, or may be from bleed air diverted from the compressor for blade cooling. Turbine manufacturers provide various taps on a compressor and any convenient one of these may be used. Bleed air is diverted at least part way through the compressor and has been heated well above ambient temperature due to compression. By that stage of compression all water droplets may have evaporated. A continuous near real time measurement is made of water condensed from a sample stream of the bleed air in a water cooled volumetric condenser, and the quantity of condensate used as an input to the feedback controller. Or, a suitable commercially available humidistat may be used to measure the humidity of bleed air removed from the compressor or of the compressor discharge air itself.

Measuring compressor discharge (or bleed) air humidity provides a direct feedback control. This tells the operator exactly how much water has been added, above and beyond the known ambient humidity. It is not necessary to know air mass flow because the ratio of starting water vapor (ambient absolute humidity) and the ratio after water injection (absolute humidity in the compressor air after fogging) are known. This direct feedback is applicable for both evaporative cooling, where one wishes to limit the humidity to 100% or less, and overspray.

A difficulty in measuring overspray or evaporative cooling fog is that some of the water droplets injected into the inlet air stream may not enter the compressor, either in the form of water vapor or droplets. Droplets collide with each other in the air stream, or collide with walls and other structural elements such as injection manifolds downstream from where some of the water is injected. The proportion of injected water lost before entering the turbine varies with the total quantity of water injected, temperature, humidity and other variables.

One can monitor the rate water is injected and simultaneously monitor the condensate and other water draining from the inlet duct to approximate the total water evaporated, either in the duct or in the compressor. One may readily (and automatically) calculate the amount of injected water that is evaporated in the inlet air duct to reach saturation based on inlet air temperature and humidity. This and the amount of water draining from the inlet duct can be subtracted from the total quantity injected to yield the amount of overspray. Such measurements are not necessary for most of the input parameters for feedback to control water injection.

When demand for electricity is low, a turbine operator often wants to run the turbine at a reduced load. Sometimes it is preferable to reduce inlet air mass flow .and fuel proportionately to keep the turbine exhaust temperature high. (It is also possible to reduce load by reducing the firing temperature but this results in lower exhaust temperature and the steam cycle suffers in combined cycle plants.)

For reduced load the operator adjusts inlet guide vanes 27 (FIG. 3). These are non- rotating blades at the inlet of the compressor which have a variable pitch. Turning the inlet guide vanes can restrict inlet air flow to keep high exhaust temperature at reduced operating load. Since this directly affects air mass flow rate, a sensor 27 measures inlet guide vane position and using information provided by the OEM thereby determines the amount of reduction in air mass flow rate. Instead of a sensor, a signal may be derived from the vane setting control and delivered to the feedback controller 22. Inlet guide vane position may be used in combination with other measured air flow parameters. Adjusting air flow rate for inlet guide vane position is not necessary when the feedback controller is based on temperature or pressure rise through the compressor, or pressure drop across the bell mouth because the change in air mass flow rate is reflected in the measured parameter. Likewise, when a fog system is controlled by measuring the humidity in the compressor air, after all the fog has evaporated, it is not necessary to account for drain water as one has a accurate measurement of the total amount of water actually injected. However, using inlet guide vane position as input for feedback may be preferable in some cases where fogging during part load operation is desirable but where the accuracy of the other methods is not warranted. However, using inlet guide vane position as input for feedback may be preferable in some cases where fogging during part-load operation is desirable but where the accuracy of the other methods is not warranted.

Clearly, more than one parameter related to air mass flow rate may be used as inputs for the feedback algorithm. Feedback control of water injection based on measured air mass flow rate may also be used in split shaft or multi-shaft turbines. Some turbines have two compressors, one of which may not be running at a constant speed. The inlet pressure and rotational speed of the second stage are parameters already available from existing turbine control sensors. These additional parameters may also be included in the feedback algorithm.

In absence of a feedback system based on measurement of air mass flow rate, it has been considered impractical to use a feedback type control system with an inlet fogging system. Such control has formerly had to be "predictive". The high air velocities and lack of perfect mixing of fog and dry air make it essentially impossible to accurately measure the temperature and humidity downstream from the fog generating nozzles. Furthermore, since there is almost always some unevaporated fog in the air stream, a temperature sensor can be inadvertently wetted and will tend to read ambient wet bulb temperature; or a humidity sensor will read 100% relative humidity. Thus, prior systems consist of temperature and humidity sensors which measure ambient air (before fogging), and a computer with software that calculates the evaporative potential of the ambient air, and then turns on the required number of nozzles to deliver the required amount of fog. This calculation requires several inputs including ambient temperature and humidity and estimated air mass flow of the compressor. There are uncertainties in these inputs which affect the calculation and make predictive control less accurate than is ideal. A primary uncertainty derives from variations in the air flow due to fouling or erosion of compressor blading, inlet filter condition, and pressure and density of the air prior to and after fogging. Measurement of a parameter affected by air mass flow rate makes at least partial feedback control practical. Measurement of the humidity in the compressor, after all fog has evaporated makes true feedback control practical.

It will be recognized that since the apparatus described and illustrated herein is schematic and exemplary, there are many equivalents to the elements specifically mentioned. For example, the gas turbine has been mentioned as if for a stationary gas turbine in an electricity generating plant. Gas turbines are also used in vehicles such as for ship propulsion and in aircraft. Feedback for control of water injection is quite suitable for such applications.

One example of a nozzle system for injecting water in the air inlet duct has been mentioned. Clearly, there are other vendors of water injection or fog generating systems that may be adapted for injecting water into the air inlet duct for a gas turbine.

Certainly, the most convenient pressure transducer is one customarily installed by the original gas turbine manufacturer. One might prefer, however, to install a different pressure transducer to obtain a different output signal better adapted to a specific feedback controller. The transducer might also be located in a different position from that used by the original equipment manufacturer.

Claims

WHAT IS CLAIMED IS:

1. A water injection system for a gas turbine comprising: a turbine having an inlet compressor section and a combustion section; means for injecting water droplets into an inlet air stream for the compressor section; means for measuring air mass flow rate through the compressor after water droplets have evaporated; means for controlling the quantity of water droplets injected; and feedback means for adjusting the means for controlling in response to measured air mass flow rate.

2. A water injection system according to claim 1 wherein the means for measuring air mass flow rate measures compressor discharge pressure.

3. A water injection system according to claim 2 wherein the means for measuring compressor discharge pressure comprises a pressure transducer near the outlet of the compressor.

4. A water injection system according to claim 1 wherein the means for measuring air mass flow rate measures temperature rise across the compressor section.

5. A water injection system according to claim 4 wherein the means for measuring temperature rise comprises means for measuring compressor air discharge temperature and applying a correction for estimated inlet air temperature after water droplets have evaporated.

6. A water injection system according to claim 1 wherein the means for measuring air mass flow rate measures temperature of air discharged from the compressor section and compares the measured temperature with a measured or computed compressor inlet temperature.

7. A water injection system according to claim 1 wherein the means for measuring air mass flow rate measures inlet guide vane position.

8. A water injection system according to claim 1 wherein the means for measuring air mass flow rate measures pressure drop across the bell mouth of the turbine.

9. A water injection system according to claim 1 wherein the compressor section has a bell mouth between the means for injecting water droplets and compressor, and the means for measuring air mass flow rate measures pressure drop across the bell mouth of the compressor.

10. A water injection system according to claim 1 wherein the compressor section has movable inlet guide vanes and the means for measuring air mass flow rate measures inlet guide vane position.

11. A water injection system according to claim 1 wherein the means for injecting water droplets comprises a plurality of manifolds of nozzles for emitting water and the means for controlling the quantity of water droplets includes a separate valve for each manifold.

12. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; at least one water nozzle manifold in the inlet duct; a water control valve for such a manifold; a water controller connected to the valve; a pressure transducer at the outlet of the compressor; and a feedback controller connected between the pressure transducer and the water controller.

13. A water injection system according to claim 12 further comprising means for measuring inlet guide vane position.

14. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; a plurality of water nozzle manifolds mounted in the inlet duct; a water control valve for each manifold; a water controller connected to the valves for opening or closing selected valves; a pressure transducer at the outlet of the compressor; and a feedback controller connected between the pressure transducer and the water controller.

15. A water injection system according to claim 14 wherein the feedback controller determines air mass flow in the compressor as a function of compressor outlet pressure.

16. A water injection system according to claim 14 wherein the water controller opens a valve completely or closes a valve completely.

17. A water injection system according to claim 14 comprising a water supply connected to the valves, wherein the water supply and manifolds are sufficient for providing overspray into the air inlet duct.

18. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; at least one water nozzle manifold in the inlet duct; a water control valve for such a manifold; a water controller connected to the valve; a temperature transducer at the outlet of the compressor; and a feedback controller connected between the temperature transducer and the water controller.

19. A water injection system according to claim 18 further comprises a temperature transducer near the compressor inlet.

20. A water injection system according to claim 18 further comprising means for measuring inlet guide vane position.

21. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; at least one water nozzle manifold in the inlet duct; a water control valve for such a manifold; a water controller connected to the valve; a pressure transducer at the bell mouth inlet of the compressor; and a feedback controller connected between the temperature transducer and the water controller.

22. A water injection system according to claim 20 wherein the means for injecting water droplets comprises a plurality of manifolds of nozzles for emitting water and the means for controlling the quantity of water droplets includes a separate valve for each manifold.

23. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; inlet guide vanes for the compressor; at least one water nozzle manifold in the inlet duct; a water control valve for such a manifold; a water controller connected to the valve; a sensor measuring inlet guide vane position; and a feedback controller connected between the sensor and the water controller.

24. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct to the compressor; a plurality of water nozzles mounted in the inlet duct; a water controller connected to the nozzles for selectively emitting water from more or fewer nozzles; at least one transducer measuring a property affected by air mass flow rate through the compressor; and a feedback controller connected between the transducer and the water controller.

25. A water injection system according to claim 24 wherein the nozzles are connected in separately controlled manifolds.

26. A water injection system according to claim 25 comprising a valve for each manifold and wherein the water controller selectively opens or closes each valve.

27. A water injection system according to claim 25 wherein the transducer comprises a pressure transducer near the compressor discharge.

28. A water injection system according to claim 28 wherein the feedback controller determines air mass flow in the compressor as a function of compressor discharge pressure.

29. A water injection system according to claim 24 wherein the transducer comprises a temperature transducer in the compressor discharge air.

30. A water injection system according to claim 29 further comprising ambient temperature and humidity sensors connected to the feedback controller.

31. A water injection system according to claim 24 wherein the transducer measures pressure drop across the bell mouth of the turbine.

32. A water injection system according to claim 24 wherein the transducer measures compressor discharge air temperature.

33. In a gas turbine including an inlet compressor and an outlet turbine connected to the compressor, an air inlet duct to the compressor, a plurality of water nozzles mounted in the air inlet duct, and a water controller connected to the nozzles for adjusting the quantity of water emitted by the nozzles, the improvement comprising: a pressure transducer near the outlet of the compressor; and a feedback controller connected between the pressure transducer and the water controller.

34. In a gas turbine including an inlet compressor and an outlet turbine connected to the compressor, an air inlet duct to the compressor, a plurality of water nozzles mounted in the air inlet duct, and a water controller connected to the nozzles for adjusting the quantity of water emitted by the nozzles, the improvement comprising: a temperature transducer near the outlet of the compressor; and a feedback controller connected between the temperature transducer and the water controller.

35. In a gas turbine according to claim 34, ambient temperature and humidity sensors connected to the feedback controller.

36. A method for controlling the quantity of water injected into the inlet air stream to the compressor section of a gas turbine comprising: measuring air mass flow rate through the compressor section; and adjusting the quantity of water injected as a function of the air mass flow rate to achieve a desired air mass flow rate.

.

37. A method according to claim 36 wherein measuring comprises measuring compressor discharge air pressure.

38. A method according to claim 36 wherein measuring comprises measuring compressor discharge air temperature.

39. A method according to claim 38 wherein measuring comprises also measuring ambient air temperature and humidity and combining this measurement with measurement of compressor discharge air temperature.

40. A method according to claim 36 wherein measuring comprises measuring pressure drop across the bell mouth of the turbine.

41. A method according to claim 36 wherein measuring includes sensing inlet guide vane position.

42. A method according to claim 236 wherein measuring includes sensing bleed air valve position.

43. A method for controlling the quantity of water injected into the inlet air stream to the compressor section of a gas turbine comprising: measuring pressure at discharge of air from the compressor section; determining air mass flow rate as a function of said pressure; and adjusting the quantity of water injected as a function of the air mass flow rate to achieve a desired air mass flow rate.

44. A method according to claim 43 comprising injecting water in the form of droplets having an average diameter up to about 25 micrometers.

45. A method according to claim 43 comprising supplying water to a plurality of nozzles in an air inlet duct to the compressor section and controlling the number of nozzles emitting water droplets.

46. A method according to claim 43 comprising supplying water to a plurality of manifolds of nozzles in an air inlet duct to the compressor section and controlling the number of manifolds receiving water.

47. A method according to claim 43 including employing ambient air temperature and relative humidity measurements to determine a maximum amount of water injected.

48. A water injection system for a gas turbine comprising: a turbine having an inlet compressor section and a combustion section; means for injecting water droplets into an inlet air stream for the compressor section; means for measuring humidity of air from the compressor after water droplets have evaporated; means for controlling the quantity of water droplets injected; and feedback means for adjusting the means for controlling in response to measured humidity.

49. A water injection system according to claim 48 wherein the air is measured downstream from the compressor discharge.

50. A water injection system according to claim 48 comprising a bleed port part way through the compressor, and wherein the air is measured from the bleed port.

51. A water injection system for a gas turbine comprising: a gas turbine having an inlet compressor and an outlet turbine; an air inlet duct for the compressor; a plurality of water nozzle manifolds mounted in the inlet duct; a water control valve for each manifold; a water controller connected to the valves for opening or closing selected valves; a humidity measuring device at the outlet of the compressor; and a feedback controller connected between the humidity measuring device and the water controller.

52. A water injection system according to claim 51 comprising a bleed port part way through the compressor, and wherein the humidity measuring device is connected to the bleed port.

53. A water injection system according to claim 51 wherein the humidity measuring device is connected downstream from the compressor discharge.