E AME ICA SOCIE Y O MEC A ICA E GI EE S G 222 4 E. 4 S ., ew Yo k, .Y. 00
C C e Socie y s a o e es o si e o s a eme s o o i io s a a ce i i, a e s o iscussio a mee i gs o e Socie y o o i s i isio s o Sec io s, m ® o i e i i s u ica io s. iscussio is i e o y i e a e is u is e i a ASME ou a . a e s a e a ai a e om ASME o mo s a e e mee i g.
i e i U.S.A. Copyright © 1993 by ASME
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A S AC Later concepts increased the number of hot gas path thermo- Thermocouples have been installed in gas turbine exhaust gas couples in a single plane to obtain a more accurate indication of paths for decades for control functions. Since the late 1970s, such energy input by enabling data averaging. Still later arrangements thermocouples have also been used for combustion monitoring. placed blade path thermocouples in two planes in the exhaust The purposes for these thermocouples and the use of the path. The upstream plane thermocouples were used for moni- information they provide are sometimes misunderstood. When toring combustion system performance. The downstream plane properly situated and incorporated into a control system, these devices, benefiting from gas mixing, were used for fuel control devices provide data that can ensure a properly operating information. The data being provided by later design arrange- combustion system, prevent catastrophic hot parts failures, extend ments can be used to ensure a properly operating combustion component life, and increase availability. Although the use of system, prevent hot parts failures, extend component life, and blade path thermocouples for combustion monitoring has generally increase availability. been associated with later model gas turbines using cannular combustors, their advantages can also be realized by the modifica- tion of older cannular gas turbines as well as turbines with annular or silo combustors. This paper reviews the concept and ISCUSSIO summarizes the benefits of blade path monitoring and the use of Combustion systems used in most gas turbines are of cannular, the temperature information obtained. A recommendation is made annular, or silo configurations. In each, hot gases are generated for the retrofit of older engines and the enhanced instrumentation in the combustor primary zone, cooled in the secondary or diluent of some later frames. section, and directed to the first turbine stage through a suitable transition arrangement. Considerable effort is expended in the design of gas turbines to ensure that the hot gas entering the ACKG OU turbine section is as uniform in temperature as possible to prevent For many years, thermocouples have been located in the damaging excitation of the rotating blades and to prevent hot exhaust gas path of combustion turbines for control and over- erosion or corrosion of blades and vanes. temperature protection. Ideally such thermocouples would be Figure 1 depicts a typical single combustor test rig used by a located at the hot gas entry to the first stage nozzles. In this manufacturer to determine the performance, including profile and location they could measure actual turbine inlet temperatures. pattern factor, of a candidate cannular combustor under test. Each The temperatures present at this location, however, exceed the of these parameters is an index of the uniformity of the hot gas operating limits of long-life thermocouples. Consequently, it is exiting the transition section. Each directly affects the operational necessary to locate them downstream of the last turbine stage. or service life of the turbine section components. Since pattern Various and increasingly sophisticated hot gas path thermo- factor and profile are well established for a production gas couple schemes have been used. Earlier arrangements employed turbine, thermocouples can be used to reliably detect excursions relatively few devices for control and temperature protection. from normal operation.
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CA U A COM US IO SYS EMS ment of blade path thermocouples to monitor the performance of In gas turbines with cannular combustors, the energy release to each combustor. These thermocouples can then provide assurance the turbine section is intended to be divided equally among the that all combustors within a set are operating within acceptable several combustors. Fuel, for example, may be directed through limits. They can also provide trend information that can detect flow dividers to ensure that flow differences between combustors progressive or sudden changes in individual combustor do not exceed 0.5%. In practice, some combustors in an engine performance. will inherently behave differently than others for reasons as subtle as a cooling pipe in the combustor shell affecting air entries. These differences in performance are usually acceptably small and consistent between engines of a given type or frame. The hot gas from each combustor will be directed to the first turbine stage M x M n — through a transition piece. t p r t r t p r t r Figure 2 shows a typical isotherm pattern from each of the several transition pieces. The example shows that first stage nozzles are individually affected by the behavior of the combustor and transition directly ahead. Rotating blades, by comparison, 00 traverse each transition discharge and are affected by differences in temperatures both radially and circumferentially during rotation. 0 20 0 If the energy release of each transition piece discharge is not within acceptable limits, turbine blading will be subjected to damagingly high cycle fatigue and can fail in a period of a relatively few hours. The gases discharged by each transition piece will pass through 028 .00 0 the turbine section relatively intact, but with some angular displacement, or swirl, as shown by Figure 3. This swirl will r n t n f Ex t I th r vary with load and may be as much as 90° at idle and be on the order of 45 0 at the baseload conditions. This permits the place- r 2 Com us o E USE O A E A E MOCOU ES 0 0 aceme As discussed, the hot gas flow through the several turbine a si io stages will occur with a consistent swirl and without undue u i e mixing of the gases. Consequently, if thermocouples are located immediately behind the last turbine stage, the data they provide
a e a will be directly representative of the flow into the first stage Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78903/V03AT15A073/2403274/v03at15a073-93-gt-222.pdf by guest on 02 October 2021 S age e mocou e nozzles. The location of the temperature measurements must be : > , S age 2 closely controlled to obtain accurate readings. In an engine with ^, Stage 3 cannular combustors, for instance, the temperature can normally vary as much as 100°F from one point to another in the exhaust path. Therefore, it is essential that each thermocouple be equally Swi spaced about the turbine centerline to correspond with individual c26 .00 0 . is aceme combustors, and that the insertion depth of each be identical. The exact placement of the thermocouples in engines with annular or E gi e silo combustors is less sensitive, but there should be a sufficiently q_ large number of devices to fairly sample the energy input to the Swi a e turbine for the full circle of rotation. A minimum number con- sidered would be eight thermocouples, located 45° apart. Figure 3 Com us io Mo i o i g In a new or recently refurbished combustion turbine, the A U A COM US IO SYS EMS differences in blade path thermocouple readings will be Annular combustion systems in industrial gas turbines built to minimized, since there will be no degradation of combustion date have usually been associated with aircraft-derivative frames. system components. In a new or refurbished cannular combustor The annular combustion system has an inherently more consistent gas turbine, the difference in readings should be approximately hot gas path energy release to the turbine section since there are 50°F with natural gas fuel, and approximately 80°F with distillate no discrete combustors. There are, however, usually more fuel fuel. In all combustion turbines, the readings of the individual nozzles in annular-type combustion turbines than in cannular or thermocouples will be consistent with load and repeatable from silo versions. Fuel to individual nozzles is normally supplied by run to run if the combustion system is functioning normally. a common rail system, or manifold. Since this type of fuel Individual readings will conform to a unique pattern for a system has no flow divider, fuel flow can vary between nozzles, given engine. For instance, the topmost thermocouple might be particularly if nozzle fouling develops. The annular combustor the third hottest of the set; the next the sixth hottest, and so on. may also have nonuniform airflows caused by disturbed entries or This is normal and acceptable so long as all readings are within developing component distress. For these and other reasons, the prescribed limits. Since the temperature measurement of each temperature profiles of the gas into the turbine section will vary thermocouple can be used to infer a turbine inlet temperature, it radially and circumferentially and may change with time. Again, is essential that the gas turbine not be run if any blade path the use of an array of blade path thermocouples can assure thermocouple reading exceeds a trip limit. acceptable performance and provide notification of changes from Since turbine blading can be subjected to high cycle fatigue if normal operation. excessive blade path spread over the range of readings exists, a turbine trip function is also normally provided in the combustion monitoring system for this situation. Of course, the thermocouple readings should be used to provide high suitable temperature SI O COM US IO SYS EMS alarms or blade path spread alarms to permit appropriate action to Although silo combustion systems usually have one or two preclude the engine from reaching trip limits. In practice this is very large combustors, the size of the combustors and the normally accomplished by operating the gas turbine at reduced complexities of turning and distributing the hot gas flows into the load until an outage can be used to determine and correct the turbine section complicate the problem of providing uniform cause of the high temperature or blade path spread condition. radial and circumferential temperatures. The temperature patterns, Catastrophic hot parts failure can quickly result from either however, as with other combustor types, are predictable and nd t n. repeatable. Again, the hot gas will pass through the turbine stages Overtemperature conditions can quickly result in failed vanes with a consistent, known swirl for each running condition. The or nozzles in any turbine stage. High blade path temperature use of blade path thermocouples can provide the advantages spreads may be indicative of an outfired or underfired combustor. previously discussed. An example exists of a gas turbine losing a last row turbine blade as the result of a combustor that outfired on natural gas. In this C rr t d case the operator failed to respond to the reported alarm b t r condition, believing that an outfire on natural gas could not occur l t n 2 4 6 at load. The blade separation occurred in the 58th hour of 2 2 4 48 operating under alarm. In this unit, the combustion monitoring 2 4 48 4 2 4 4 provision accurately reported the excessive blade path temperature 8 4 42 6 spread for the full 58 hours, but a trip function had not been 4 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78903/V03AT15A073/2403274/v03at15a073-93-gt-222.pdf by guest on 02 October 2021 elected by the user. High blade path temperature spread should 4 6 8 4 be normally implemented as a trip function since the protection 6 64 6 6 64 6 6 of the turbine otherwise is totally dependant upon the operator. 6 40 8 0 8 0 8 0 880 Blade path thermocouples should be situated at peak temper- 6 4 6 ature locations in the hot gas path. The maximum temperature is 8 40 8 40 4 40 4 normally at the intended preferential location in the profile Spr d 2 2 8 4 8 presented to the first stage blades, usually being two-thirds or more of the span above the blade platform. This location ensures t : r bl d p th t p r t r nd t . 6 that the higher temperature gases are present at a lower stressed b t r r r tt nt n. Al r pr d l t region of the blade. As combustion system components degrade, f 80° h b n x d d there will generally be a reduction in firing temperature or a t28e7.002 01-93 deviation from the intended hot gas exit profile. Either of these Trend Data conditions will result in a reduced temperature reading by the corresponding blade path thermocouple. Conditions that can Figure 4 reduce such readings include fouled fuel nozzles, holes or cracks in combustors or transition pieces, and leaking transition piece seals. blading is not being damaged by the lighter loading and the Figure 4 illustrates a daily record of blade path temperatures changing speeds. If a combustor is actually outfired, the control for a base-loaded engine. The figure shows a decay in blade path system will normally provide an alarm indication or trip the unit temperature for one combustor, indicating that attention will be as the motor nears synchronous speed. Most of these points are required. Higher than normal individual blade path thermocouple also directly applicable to noncannular combustion turbines. readings are usually the result of overfueling. The overfueling of Important trending and diagnostic information is provided by some fuel nozzles in a manifolded set can result from the fouling blade path thermocouples. As stated earlier, temperatures of others. The overfueling of all fuel nozzles is usually the result recorded will assure a repeatable day-to-day pattern. If one of the loss of fuel regulation. High temperature readings have thermocouple or several adjacent thermocouples begin to report also been observed as the result of outfired combustors. This can different operating conditions with time, changes in combustion occur when fuel not burned in the combustor bums in the turbine hardware or fuel distribution are indicated. The proximity of section. blade path thermocouples to the turbine section enables the Since overfueling and resultant overtemperature can quickly necessary correlation of specific thermocouple readings with the damage turbine section components, blade path overtemperature location of combustion components requiring attention. protection is applied during all phases of operation, including acceleration. In a properly operating gas turbine, the blade path temperature is predictable within a small range for specific load FURTHER A ICA IO S or acceleration conditions. Blade path spread monitoring, by Blade path monitoring is provided as a standard capability with comparison, is usually provided during acceleration but not active many new combustion turbines. Such capability can be readily as an alarm or trip function since the combustion system is incorporated into older units, particularly those being retrofitted operating considerably off of design point conditions, and higher with computer control systems. Thermocouples can easily be spread indications will be inevitable. Blade path temperature fitted to the low alloy or steel exhaust ducts with compression spread alarms and trip functions are typically activated at high fittings, located and traversed as discussed previously. The turbine speeds and remain active for all idle and load conditions. monitoring, alarm, and trip functions can be readily accomplished Blade path thermocouples can play a significant role during with simple programs. startup. Fuel ramp scheduling is often initiated by flame confirm- ation from UV-detectors. Since UV-detectors are normally located in two combustors in a cannular system, it is possible to CO C USIO accelerate an engine without all combustors firing. An outfired All industrial combustion turbines can benefit from the use of combustor will typically be indicated by a blade path thermo- blade path thermocouples. The number and location of the couple reading of 200°F less than those for adjacent combustors. thermocouples depends on the configuration of a given frame, but Such a reading is not cause for stopping acceleration since it is the thermocouples should be relatively close to the last turbine possible for fired combustors to have 200°F spreads during stage. The number of thermocouples should be sufficient to acceleration. This is not a cause for concern since the turbine monitor the energy inputs to the turbine section for the full circle of rotation. The information provided by the devices can provide E E E CES significant engine protection by detecting combustors which have Gabriel, F. K.. and V. Donato. "Telemetry Measurement of lost flame or are late in achieving ignition, by reporting changes Combustion Turbine Blade Vibration in a High Temperature in temperatures indicative of component degradation, and by the Environment," ASME Paper No. 86-GT.207, 1992. sensing of any overtemperature conditions. The real-time Scalzo, A. J. "High-Cycle Fatigue Design Evolution and information provided enables a timely operator decision to reduce Experience of Free-Standing Combustion Turbine Blades," load, shut down, or take other appropriate action. It is proposed Transactions of the ASME, Vol. 114, April 1992. that all industrial combustion turbine users consider the benefit Scalzo, E. J., J. M. Allen, and R. J. Antos. "Analysis and Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1993/78903/V03AT15A073/2403274/v03at15a073-93-gt-222.pdf by guest on 02 October 2021 afforded by blade path temperature monitoring relative to the Solution of a Nonsynchronous Vibration Problem in the Last Row nominal cost of incorporating such protection. Turbine Blade of a Large Industrial Combustion Turbine, ASME Journal of Engineering for Gas Turbines and Power, Vol 108, 1986.