A Low Cost, On-Site Performance Monitoring System

Transcription

1 79-GT-21 r^i Copyright 1979 by ASME $3.00 PER COPY author(s). $1.50 TO ASME MEMBERS The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation. Full credit should be given to ASME, the Technical Division, and the A Low Cost, On-Site Performance Monitoring System H. I. H. SARAVANAMUTTOO Professor and Chairman, Dept. of Mechanical and Aeronautical Engineering, Carleton University, Ottawa, Canada Mem.ASME A simple thermodynamic analysis for on-site performance monitoring of shaft power gas turbines is described. The method is simple, requires minimal extra instrumentation, yet permits the operator to determine the critical cycle parameters of turbine inlet temperature and airflow. Computing requirements can be handled by a programmable calculator at minimum cost. Experimental verification of the system showed very good agreement with engine tests. Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the Gas Turbine Conference & Exhibit & Solar Energy Conference, San Diego, Calif., March 12-15, Manuscript received at ASME Headquarters December 8, Copies will be available until December 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 A Low Cost, On-Site Performance Monitoring System H. I. H. SARAVANAMUTTOO ABSTRACT A simple thermodynamic analysis for on-site performance monitoring of shaft power gas turbines is described. The method is simple, requires minimal extra instrumentation, yet permits the operator to determine the critical cycle parameters of turbine inlet temperature and airflow. Computing requirements can be handled by a programmable calculator at minimum cost. Experimental verification of the system showed very good agreement with engine tests. NOMENCLATURE '1 temperature m air flow mf fuel flow cpspecific heat at constant pressure ii efficiency rl cc combustion efficiency n ccompressor efficiency n tturbine efficiency INTRODUCTION temperature correction factor The availability of an on-site performance monitoring capability would clearly be a great asset to users of industrial gas turbines, permitting them both to check when performance has deteriorated to an unacceptable level and also to ascertain that critical operating parameters remain within allowable limits. These requirements can obviously be met by installing a sophisticated data gathering and analysis system, but the associated costs may be prohibitive to the smaller industrial user. This paper describes a simple, low cost, on-site performance monitoring system which can be used in conjunction with manual data gathering; the only hardware required is a programmable calculator, readily available at a cost of between $500 and $1,000. SYSTEM REQUIREMENTS The basic requirement of the system is that it should be capable of giving a rapid and reliable indication of engine performance using manual data collection. In the past, many organizations have found manual data collection to be totally unreliable, largely due to the fact that recorded data may not be analyzed until much later, with the result that data which initially seemed sensible may show a totally unacceptable scatter. A data validation package with the capability of checking field data for consistency is described in (1); this package was based on the use of an already available digital computer used for station control, and is considerably more sophisticated than desirable for smaller users. Another major problem with manual data collection is that the log sheet supplied to the operator usually requires the reading of a large number of items, only a few of which are critically required for performance measurements; a result of this can be that the time taken to collect the data is such that the engine operating point has changed, and critical performance parameters such as power, fuel flow and turbine temperature may have been recorded at different power settings. The data collection sheet must be organized so that the operator can gather the few required items of 1

3 performance data in the shortest possible time; the operators must be made fully aware of this requirement. In many applications a few key parameters, which are observed directly, are trended over an extended period to keep a check on engine conditions. A typical parameter would be the Exhaust Gas Temperature (EGT) or Inter Turbine Temperature (ITT) on a shaft power gas turbine. It should be realized, however, that these are only an indirect indication of the critical temperature, the Turbine Inlet Temperature (TIT), which has a controlling effect on creep life. TIT is seldom measured in practice because of the major difficulties involved, although it should be noted that for future very high temperature turbines with cooled blades it will be necessary to measure the actual Turbine Blade Temperature (TBT) by radiation pyrometry. It is highly desirable from the operator's point of view to know the TIT rather than the more readily measured temperatures lower down the turbine. Another quantity of prime interest which is not normally measured is the engine air flow; a decrease in airflow could be indicative of compressor fouling, compressor Foreign Object Damage (FOG), icing or incorrectly positioned variable geometry. A drop in air flow will cause either a drop in power or an increase in turbine temperature to maintain the required power setting, neither of which is acceptable. Air flow measurement requires either the use of a venturi or a calibrated bell mouth, neither of which are really practical in the field and it is for this reason that air flow is seldom measured. A simple Gas Path Analysis permitting the calculation of both air flow and turbine inlet temperature is presented in this paper; the calculations can be done on a small programmable calculator, e.g. HP 65, 67 or 97, with the program stored on magnetic cards. The measurements required are those normally taken, with the addition of the compressor delivery temperature. GAS PATH ANALYSIS The Gas Path Analysis may be considered at two levels, i) overall cycle performance and ii) analysis at the component level. The method was originally developed for analyzing helicopter power plant (2), but is valid for any shaft power application. The engine considered was a free turbine unit, as shown in Fig and two appropriate temperatures; analysis of component efficiencies requires additional pressure measurement. The accurate measurement of component efficiencies in an engine is very difficult; the prime requirement here, however, is to establish the change in efficiency over an extended period, and we may base our efficiency on any easily measured pressure, e.g. a wall static tapping at compressor delivery. The Gas Path Analysis has been investigated for three different temperature measurement systems, i.e. i) Compressor delivery and exhaust gas temperature ii) Compressor delivery and inter turbine temperature iii) Turbine inlet and exhaust gas temperature The first two cases are the more important and the analysis will be presented for case (i) in detail and case (ii) in outline. Overall Cycle Performance It is assumed that the following measurements are available: T 1, T 2, m f, T 5, Power The air flow is not measured but can readily he deduced as follows: i) Equating the compressor and gas generator power requirements, ml cp12 AT 12 - m 3 cp34 AT 34 n m with the fuel flow approximately equal to the bleed required for disc cooling and sealing m 1 = m 3 and hence cpl2 [AT 34 ] REQD = AT12 c pc4 2m (1) (for an engine with an air cooled turbine it would be necessary to allow for the additional cooling bleed). ii) The combustion temperature rise, and hence turbine inlet temperature, can be calculated from the fuel: air ratio, combustion efficiency and compressor delivery temperature using standard combustion charts. The airflow is not known, but a value can be assumed to obtain the air:fuel ratio; the value assumed is not critical, as the correct value will be obtained by iteration. Thus we have f/a = mf /m l and AT23 = f(f/a, T 2' n cc ) (2) For use on a calculator it can be assumed with small error that AT 23 is a linear function of f/a. iii) With the assumed value of mass flow, the power turbine temperature drop (AT 45 ) can be calculated from ti measured power, i.e. KEY I COMPRESSOR INLET 2 COMPRESSOR DELIVERY 3 GAS GENERATOR TURBINE INLET 4 POWER TURBINE INLET 5 POWER TURBINE OUTLET Fig. 1 Station Numbering The overall cycle performane analysis requires the measurement of power, fuel flow, inlet conditions iv) power = m l c05 AT 45 3 m P AT = Power (3) 45 ml c P45 fl Knowing T 3 (=T 2 + AT 23 ), T 5 and AT 45 the temperature drop available for the gas generator turbine can now be calculated, 3 - (T 5 + AT 45 ) (4) [AT 34 I CALC = 1 v) The value of AT 34 calculated from Equation (4) must he the same as the value of AT 34 previously found from Equation (1) for the compressor power requirement. If the two calculated values of AT 3q do not agree, the assumed mass flow must be altered and an iteration carried out until agreement is reached.

4 CALC AT34R FROM COMPRESSOR WORK ESTIMATE AIR FLOW FIND T3 FROM F/A, jcc AND 12 CALC T45 FROM POWER AND MI MODIFY MI T4=T5-AT45 AT34C=T3- T4-3- ENGINE TESTS The effectiveness of the Gas Path Analysis was verified using shop tests of a 7.3 MW industrial gas turbine. These shop tests, for customer acceptance, included accurate measurement of both power and engine airflow; airflow was measured by means of a venturi meter, checked against the known engine compressor characteristic. Calculations were carried out using a Hewlett Packard 65 Programmable Calculator and an IBM 1620 digital computer. The HP 65, with 100 program steps, has now been replaced by the HP67/97 with 200 program steps, the HP 97 having a printer attached. Airflow measurements over a range of powers from 3.6 to 7.7 MW are shown in Table 1, compared with the Gas Generator Speed (rpm) Air Flow (kg/s) Venturi Calculated Calculated (HP 65) (IBM 1620) AT34C=AT34R? N ^ YES OVERALL CALCULATIONS COMPLETED COMPONENT LEVEL CALCULATIONS Fig. 2 Information Flow MI, T The simple flow chart for this calculation is shown in Fig. 2; the calculation is very simple, even by slide rule, and can readily he programmed on a small calculator. Once the iteration has been completed both the airflow and turbine inlet temperature are known, giving a good indication of overall engine health. In case (ii), with T 2 and T measured, it is not necessary to iterate for the airflow, as 1 3 follows directly from the compressor-turbine power balance. With T 2, T 3 and m known, m I can be found directly. For simplicity of presentation the analysis has been performed with constant specific heats, but variable fluid properties can readily be included using the data from (3). The estimation of component efficiencies was described in (1) and will not be repeated here; the main use of this analysis would be for checking the health of individual components of the engine. Table 1 Validation of Air Flow Measurement airflows calculated using the HP 65 (constant c ) and the IBM 1620 (variable fluid properties). It p can be seen that excellent agreement was obtained, and that the HP 65 results are more than adequate; at the higher powers the error is about 0.8% and even at about 50% power the error is only 2.2%. It is, of course, changes in airflow from the originally installed performance which are of interest in the field rather than absolute values of airflow. The close agreement between the airflow calculations and measurements, however, would suggest that the calculated TIT is also reliable; the calculated TIT at full power was in good agreement with the design value. The calculated airflows as a function of compressor speed are shown in Fig. 3, which demonstrates the need for reducing both airflow and speed to standard conditions, as shown in Figure 4. The variation of TIT/E 1 with N/ 01 is shown in Figure 5. OPERATION IN THE FIELD For each engine, performance curves such as Figures 4 and 5 should be generated when the engine is installed, providing base line performance for both turbine inlet temperature and airflow as a function of gas generator speed. Once provided with these curves, the operator can readily check his performance readings without the necessity of taking the readings at any pre-determined power setting. Increases in TIT without changes in airflow are indicative of hot end distress, but increases in TIT combined with a drop in airflow probably indicate compressor problems which may well be cured by compressor washing. Erroneous readings which result in data far removed from the base line curves can readily be corrected. A notable feature of the Gas Path Analysis is the ability to detect instrumentation (or reading) errors from unlikely thermodynamic observations. As an example, a low reading of EDT will manifest itself as an increase in airflow for a given power, which cannot 3

5 I I i I ICI I^ II _ i-- 45 p F- a 44 a o 43- J oo _ i a z z 42 -._ - p 41 -r 1 i _- y 40- t 0 o N (RPM) N/,/B (RPM) Fig. 3 Calculated Air Flow Fig. 5 Turbine Inlet Temperature Variation a C 0 z e. e 7 o N//a (RPM) Fig. 4 Non-Dimensional Air Flow Variation be valid for a fixed geometry engine. Analysis at the component level may also be used to detect instrumentation errors which may result in apparent increases in component efficiency, which are clearly invalid. Table 2 shows data for synthesized engine data, as reported in (4). At the Design Point of the synthesized engine, TIT was 1221 K, airflow was 5.A5kg/s and power output was 1097kW. The effects of errors in fuel f1 ow, EGT and compressor delivery temperature are shown in Table 2, with the invalid outputs marked by asterisk. Analysis of the synthesized engine data revealed that fuel flow was the quantity requiring the most careful measurement; 1 error in fuel flow gave 1.40 error in airflow. An error of 1% in power, hoe ever, only caused an error of 0.4 in airflow. The reason for the lower sensitivity of the airflow to errors in power is due to the fact that the Gas Path Analysis deduces the airflow from the gross power produced by both turbines, and typically the power absorbed by the compressor will Se larger than the output power. This is rather fortunate, as fue]. flow is basically easier to measure accurately than power output. The analysis, however, pinpoints the need for accurate fuel flow measurement. CONCLUSIONS A simple but powerful method of Gas Path Analysis has been presented. Implementation requires very little extra instrumentation and computing requirements can be met by an off the shelf programmable computer. The operator can get an immediate readout of the key performance parameters, turbine inlet temperature and airflow, without the necessity of using a fixed power setting. Use of the thermodynamic 4

6 -5- N//S 1 FUEL FLOW EGT CDT(T2) 100% +30% STD -30% STD -50 C -1000C STD -500C -1000C T 2 ( o c) T 3 0 ( c) T 5 ( o c) '. 841 n c % 1.44* n t * m l (kg/s) 8.26* % 7.44* m5 (kg/h) kw 'n i0 Significant quantities marked with an asterisk Table 2 Effect of Gross Errors in Fuel Flow, EGT and CDT analysis can detect instrumentation or reading errors, and the proposed system can he implemented at negligible financial or technical risk. ACKNOWLEDGEMENTS This work was financially supported by the National Research Council of Canada, and this support is gratefully acknowledged. Thanks are also due to N.H. Stowell for reduction of the test data. REFERENCES 1 Agrawal, R.K., Maclsaac, B.D., Saravanamuttoo, H.I.H., "An Analysis Procedure for the Validation of On-Site Performance Measurements of Gas Turbines", ASME Paper 78 -GT -152 (to be published in Journal of Engineering for Power). 2 Saravanamuttoo, H.I.H., Staples, L.J. "An Engine Analyzer Program for Helicopter Turboshaft Powerplants", NATO/AGARD Conference Proceedings 165, Diagnostics and Engine Condition Monitoring, Chappell, M.S., Cockshutt, E.P., "Thermodynamic Data Tables for Air and Combustion Products", NRC Report LR-517, January Saravanamuttoo, H.I.H., "Gas Path Analysis for Pipeline Gas Turbines", National Research Council of Canada, Gas Turbine Operations and Maintenance Symposium, Edmonton, October