Gas Turbines Maintenance Inspections and Calculations of Equivalent Operating Hours
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1.
M A I N T E N A N C IEN S P E C T I O N(3 S .5 – 0 0 2 2 )
Maintenance operations must be performed on the gas turbine at regular intervals. As with any plant operation will lead to wear and tear. It is the function of maintenance to detect and influence wear and tear and to generate new wear and tear reserves by way of repairing (cf. DIN 31 051). Section 3.4-0051, Maintenance Intervals, deals with all operations that can be performed on the gas turbine including its auxiliary systems in operation or on stand-by without impairing availability. Here, as a supplement, time sequences are given for maintenance measures that cannot be performed unless the gas turbine is at stand-still. Since stress on the hot items (combustion chambers and turbine blades) is especially high, it is expedient to base the intervals on the cumulative operating stress of these items. All cycle times given in the following are guide values that may be extended or shortened by up to 20 per cent depending on the site conditions and findings of previous inspections.
Unit
Maint
Interval
Duration (days)
1-2
Combustion Inspection
4000
10
Major Overhauling
24000
60
Combustion Inspection
3000
10
Hot Gas Path Inspection
6000
30
Major Overhauling
18000
45
Minor Overhauling
25000
10
Major Overhauling
50000
60
Combustion Inspection
7500
10
Hot Gas Path Inspection
22500
45
Major Overhauling
45000
45
Minor Overhauling
25000
10
Major Overhauling
50000
45
Combustion Inspection
4000
7
Hot Gas Path Inspection
25000
-
Major Overhauling
50000
60
Minor Overhauling
25000
7
Major Overhauling
50000
60
3-4
9-10 5-8
11-12 13-14
15
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2.
EQ U I V A L E N O T P E R A T I N GHO U R S (3 .5 – 0 0 2 2 )
Rapid changes in the turbine inlet temperature and operating periods at gas temperatures exceeding those of base load cause additional stresses on the hot-gas-path items. The effect of such stresses on the service life of these items is allowed for by determining the equivalent operating hours at base load stress. For this purpose, various process events and operating hours in the various temperature ranges are assigned individual factors which are then totalled. Equivalent operating hours t ae are calculated using the formula given below define the duration of the interval between two major inspections. n3
tae = a1n1 + a2n2 a+ Σ 3
+ b1t1 + b2t2
where n 1 = number of start-ups
a 1 = 10, factor for one startup
n 2 = number of rapid loadings a 2 = 10, factor for rapid loading n 3 = number of rapid temperature changes
a 3 = factor for rapid temperature change
t 1 = operating hours at base load
b 1 = 1, for base load exhaust temperature
t 2 = operating hours above b 2 = 4, for peak load exhaust base load up to peak load temperature Any start-up process at which a pronounced increase in the measured and recorded gas temperatures indicates ignition of the main flames is classified as a start-up (n 1 ). With automatic recording (major inspection clock), one start-up is counted when the speed switching value of the start-up ramp function generator is exceeded at about 1/3 rated speed. The associated weighting factor is a 1 =10 h, identical for all gas turbines. The same speed switching point starts and stops counting of the operating time. Rapid loading (n 2 = 30 MW/min) is counted on depression of the dedicated selector switch during operation. The standard setting of the rapid loading process in the automatic program is assigned the weighting factor a 2 = 10 h. If the possibility of a start-up at reduced start-up power (e.g. black start at synchronous starting) is selected acceleration will be at an increased hot gas temperature. In terms of additional stresses, this process is to be assessed the same as one rapid loading. Other process events n 3 need not be taken into account unless, due to special requirements imposed by the load consumer (e.g. electric smelting furnaces), the turbine is subjected to load changes the gradients of which exceed the specified standard automatic program Prepared by: Fazal-ur-Rehman Babar
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values for loading and unloading. Weighting factor a 3 for rapid temperature changes is shown in Fig. 1 ‘Equivalent operating hours for rapid temperature changes in the turbine’ applicable to a given type of gas turbine. The hours for trip and load rejection are given in Fig. 2 ‘Equivalent operating hours for Trip and Load Rejection’. The decisive factor is the change in the outlet temperature irrespective of the mathematical sign. Pronounced and rapid heating and cooling will increase the maximum stress difference on the item. Changes taking place within 10 seconds are classified as temperature steps. As a rule, the initial and final values can be seen in the temperature record. The temperature at trip is taken on trip initiation, whereas the non-steady-state temperature after flame extinction is taken as 150 °C. The creep dependency of the hot-gas-path items, particularly of the turbine blades, on the operating temperature is allowed for by weighting factors b. After commissioning of the gas turbine, those turbine outlet temperatures are determined at which the assured base and peak loads are reached. Automatic correction of outlet temperature ϑ AT with simultaneously measured compressor inlet temperature ϑ VI using the formula
ϑ
ATK
=ϑ
AT
– 0.46 • ϑ
VI
ensures that the turbine inlet temperature likewise remains constant at a constant corrected turbine outlet temperature ϑ ATK . Unless the ϑ ATK value, which is thus assigned to the base load, is exceeded, the equivalent and actual operating hours increase uniformly at weighting factor b 1 = 1. At higher ϑ ATK values, a reduced blading service life can be anticipated. The major and maintenance inspection intervals will change accordingly, since then the equivalent operating hours accrue more rapidly than the actual operating hours. This is calculated with weighting factor b2 = 4, by which the operating hours t2 between base load and peak load are multiplied. Due to the fact that the metallic parts of the flame cylinder are protected by a lining of refractory tiles, the fuel-dependent radiant heat of the flames has only a low impact on the metal temperatures. Hence, equivalent operating hours are not dependent on the fuel used, provided the fuel complies with the SIEMENS/KWU fuel specifications. 3.1
C a lc u la tio n o f E q u iv a le n t O p e ra tin g H o u rs
Unit
FO
Gas
HSD
Trip
Start
1-2
1
1
1
100 (750)
10
3-4
-
1
1.5
-
10
9-10
-
1
1
-
25
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5-8
2.5
1
2.5
-
1
11-12
1
1
1
-
13-14
1
1
1
Max. 140
H=5, W=25, C=50 10
15
1
1
1
-
25
3 .2 E q u iv a le n t O p e r a t in g H o u r s fo r F a s t T e m p e r a tu r e C h a n g e s in T u rb in e
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Examples :
a fast change from initial turbine outlet temperature of 540 °C to final turbine outlet temperature of 230 °C (or from 230 °C to 540 °C) is equivalent to ∆ ϑ = 310 °C. This yields 71 equivalent operating hours with open (100 %) inlet guide vanes 29 equivalent operating hours with half-closed (50 %) inlet guide vanes
8 equivalent operating hours with closedis(0 %) inlet In case of a trip, the final turbine outlet temperature about 150guide °C. vanes In case of load rejection with open inlet guide vanes, the final turbine outlet temperature is about 200 °C. In case of load rejection with closed inlet guide vanes, the final turbine outlet temperature is about 275 °C.
3 .3 E q u iv a le n t O p e ra tin g H o u rs fo r T rip a n d L o a d R e je c tio n
Examples :
a trip (sudden temperature decrease) from 540 °C initial turbine outlet temperature corrected with inlet guide vanes open yields about 140 equivalent operating hours.
Prepared Babar a by: loadFazal-ur-Rehman rejection from the same initial temperature yields about 90 equivalent operating hours. a trip from about 540 °C initial temperature with closed inlet guide vanes yields
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Inspection Cycles at peak load and intermediate load operation frequently, and particularly during the weekends time periods will occur, during which the gas turbine is shut down for more than one day. These standstills allow regular inspections without impairing scheduled operation. Thanks to the provision of easily removable access ports, SIEMENS/KWU gas turbines allow easy and fast performance of inspections of the hot-gas-path items in the combustion chamber and turbine areas largely without the need for dismantling of combustion chambers or time-consuming endoscopy which are otherwise required. On this basis, we recommend an inspection interval of 4000 equivalent hours. Special attention in the monitoring operations is given to quantitative recording of the wear and tear in sliding joints which is primarily caused by start-stop cycles and at locations subjected to high temperature gradients. The more frequently such measurements and inspections are performed, the more precise the assessment of the maintenance efforts required at the next extended shutdown will be. If this results in process dependent extended shutdowns, an inspection interval of 2000 hours may be expedient and recommendable. With liquid fuels requiring additives or exceeding the admissible particulates content specified in Section 3.1-0173, inspection of the fuel nozzles at intervals of 2000 hours is recommended. If, in addition to electric energy, thermal energy is also supplied to continuously operating industrial plants, the gas turbine inlet temperature will ideally be constant over an extended time period. Load changes will occur very slowly. These plants are not used for frequency back-up control and are either not operated at peak load at all or for less than 50 hours annually. The operational mode described is also applicable to gas turbines that are operated exclusively for base-load power generation. This operational mode produces a very low detrimental impact on the gas turbine since non-steady-state excessive temperature gradients and relative expansions are minimized. In this case, the inspection interval may be extended to up to 1 year. If interruptions of operation of more than 12 hours provide the opportunity of an intermediate inspection of the hot-gas paths, this opportunity should be taken, provided the last visual inspection was performed more than 2000 hours ago. Prepared by: Fazal-ur-Rehman Babar
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Cycles for Inspections
Hot-Gas-Path
Item
Servicing
and
Major
Maintenance operations such as renewal of the protective surface coating of turbine blades require opening the gas turbine casings and removing the rotor. When work has to be performed on the compressor and turbine blading, the moving blades can be removed without having to disassemble the rotor. Hot-gas-path item maintenance shall be performed at intervals of 25,000 equivalent hours. For continuously operating plants, this results in a cycle of three years. For plants which operate only a fraction of the time, a cycle should not exceed 6 calendar years. Items not in contact with the hot gas such as the rotor items (discs, hollow shafts, tie rod) and the compressor blading for which creep stress is of little or no importance, are subjected to only minimal stress. Such items will not even be adversely affected by hot gas temperature step changes and peak load operation. After the first major inspection, the inspection interval encompassing all items can thus be twice that of the cycle for hot-gas-path items. The major inspection interval is therefore 50,000 equivalent hours. For continuously operating plants, this results in a cycle of 6 years. As a rule, inspection of the abovementioned rotor items can additionally be limited to a few areas which are subjected to higher loadings. If the total of all start-ups during the following major inspection interval is expected to exceed 3000 however, all rotor items should be subjected to nondestructive examination. The major inspection interval of plants in operation only a fraction of the time, where suitable counter measures prevent standstill corrosion to a large extent, may be extended to up to 12 calendar years. The major inspection after 75,000 equivalent hours should be accompanied by an investigation of the residual service life of the item parts, since at the next major inspection interval the design service life of the turbine blading will be exceeded. The components in the hot gas path between the burners and the turbine blading must be inspected at regular intervals. KWU gas turbines use film cooling within the range of the flame tube plate and ceramic liners in the cylindrical section of the flame tube, and operate with relatively low stressing of the combustion system. A periodic replacement of the hot gas path components is thus not necessary. Being enclosed by considerably colder outer casings, the hot inner parts between the flame tube and the turbine move relative to each other and to the outer casing in the sliding-motion guides provided for this purpose. A completely uniform thermal expansion of the inner parts cannot be achieved by design measures. For this reason, the inner parts are subject to a certain wear due to frictional forces and low-cycle fatigue. Prepared by: Fazal-ur-Rehman Babar
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The maintenance inspections are intended to keep a check on this wear phenomenon at regular intervals (see under Intervals for Maintenance Inspections and Major Inspections) and to prevent its progress by suitable remedial measures. The execution of this work is greatly facilitated by the very simple access through manholes designed for quick opening. Direct visual examination of all burnes, hot gas duct walls and first stage turbine rotor and stator blades minimises manpower and time requirements (as compared to borescope inspection or flame tube disassembly).
Approximately eight hours after shutdown, the gas turbine on the turning gear will have cooled down to a temperature at which the combustion chambers can be entered without special heat protection. During the cooling-down period, other maintenance work on the compressor and external to the gas turbine can be performed. In addition, the waiting period can be considerably reduced by running up the gas turbine several times to approximatel, 1/3 of rated speed without ignition using the starting equipment. Depending on the number of personnel, two to three 10-hour shifts are required to perform the work in the hot gas duct. Turning gear operation is only interrupted for inspection of the turbine blading, and the gas turbine is ready for restarting immediately after closure of the manholes. The work should preferably be performed during an outage and will take an average number of 50 man-hours. If the unit operates mainly or exclusively on liquid fuel, an additional inspection of the oil burners will normally be required. This work, which involves disassembly of the oil burners, can be performed in parallel with other work and requires approximately 20 man-hours. With high wear of the injection nozzles due to a higher contaminant content of the fuel, it may become necessary to inspect the oil burners after half the normal inspection interval. The periods indicated are based on the employment of manufacturer's experienced maintenance personnel. Following suitable training, the maintenance inspection can also be perforrned by the user. The accuracy required in performing the visual examination and the correct interpretation of the findings can however, for the extensive knowledge and experience of a suitably qualified inspector. It is therefore recommended to have at least this part of the work performed by the
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manufacturer. Irrespective of this requirement, the inspection facts and remedial measures should be advised to the manufacturer in each case and evaluated when planning the next maintenance inspection or major inspection.
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