55 680 mw coal gas fired power

7/27/2019 55 680 MW Coal Gas Fired Power

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Two-Shift Operation at Castle Peak Power Station OMMI (Vol. 1, Issue 2) August 2002 2

(Note: This Paper was originally presented at the Two Shifting Conference organised by

European Technology Development Ltd. and held at IOM, London, in June 2001. The paper isbeing reproduced here because of the importance of the subject)

SummaryOver 15 years of experience in two-shift operation of four 680 MW units, predominantly incoal-fired regime, has convincingly affirmed that large coal-fired generating units can

perform extensive two-shift operation without requiring replacement of major components ofboilers or turbines. This achievement required a focused attention to thermal and operationalflexibility during the design stage, and a sound understanding of the influence of methods ofoperation on the conditions developed in boiler and turbine during startups and shutdowns.To accomplish this, the more damaging thermal transients that can occur in boiler, turbineand key balance of plant equipment must be avoided through the establishment of sound

procedures for shutdowns and startups.

The overall performance of the Castle Peak B (CPB) units, namely, availability, reliability

and efficiency, has demonstrated that large, extensively two-shifted units can equal or surpassthe performance of base loaded plants, and experience no more significant problems than the

best performing units that operate continuously. Despite the excellent overall performance ofthe CPB units, the additional O & M costs incurred by two-shifting are slightly greater thanthe fuel cost savings achieved by improving the system efficiency of the CLP grid througheconomic load dispatch. Improvements made to lower the minimum load for operation withstable combustion without oil support have reduced the number of units that have to be two-shifted.


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Starts

UnitDate of FirstOperation

RunningHours

EOH

Hot Warm Cold Total

B1 Nov '85 88,778 146,288 1,657 201 59 1,917

B2 Sept '86 98,287 141,307 1,254 148 32 1,434

B3 Sept '87 92,221 136,711 1,317 136 30 1,483

B4 Aug '89 67,665 114,885 1,372 159 43 1,574

Figure 3 Cumulative Unit Starts & Hours for CPB 680MW Units at Dec. 31, 2000

Until 1995, the CPB units on average operated close to 100 starts per year per unit, with atypical offload period of 6 to 8 hours overnight. After commissioning of the natural gas-firedcombined cycle units at Black Point P.S., from 1996, the frequency of two shifting of CPBunits progressively increased to an average of close to 150 hot starts per unit year, with someunits performing up to 170 hot starts per year, and the shutdown periods overnight lengthenedto typically 10 to 12 hours, Fig. 4.


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Confidence in successful two-shift startups to meet dispatch requirements depends onmaintaining a high startup reliability. During the first 2 to 3 years, a few design weaknessesthat delayed some starts were overcome by appropriate modifications to design or methods ofoperation. Since then, start up reliability has remained consistently at or above 99.99%, Fig.5. Startup costs are therefore minimized by delaying the commencement of startups afterovernight shutdowns until there is just enough time to meet the scheduled time forsynchronizing.

Teething Problems Overcome During Early Operating Years

When the 680MW units first entered service at CPB, system operation required them togenerate on occasions up to 35%, but predominantly between 20% and 30% of the prevailingsystem demand. The early attainment of high reliability was therefore a dominant objectiveto reduce unit trips from load to a practical minimum, in order to prevent disconnection ofcustomers by automatic under-frequency load shedding which resulted from the tripping of aunit carrying up to 20%~30% of system generation during minimum load overnight.

Turbine bypass system controls

Each CPB unit is installed with a turbine bypass system intended to provide three functionalcapabilities: - large load rejections, steam pressure shadowing, and unit startups.Observations on the benefits of installing turbine bypass for two-shift units are discussedlater.

The complex interaction of controls and protection associated with interfaces between turbine

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Boi ler feed pumpsEarly problems with rotor locking of the pressure pump after shutdown caused bytemperature stratification inside the pump, and with low cycle fatigue failures of pressure

pump end cover retaining bolts, were successfully rectified by design modifications.

LP turbine bursting diaphragms

After about 100 two-shift cycles, several starts were delayed from 1 to 3 hours whenattempting to draw condenser vacuum by air ingress through circumferential cracks in thelead bursting discs, which are designed to rupture at a condenser pressure of 5psig.Investigations revealed that similar problems had occurred at other installations but had never

been reported to the OEM. The cracks were caused by low cycle fatigue accelerated by stresscorrosion of the lead discs. The environment corrosive to lead is carbonic acid formed after

breaking vacuum. Sealing of the exposed lead surfaces to prevent carbonic acid contactsubstantially reduced the frequency of delayed starts, but has not entirely eliminated the

problem. Sudden rise in pressure in the LP turbine has been a contributory factor which has

been addressed by more stringent vacuum raising procedures during startup. On occasion,inadvertent admission of a large quantity of hot water into the condenser during the shutdown

period when isolating and draining down hot feedwater heaters has cracked some leadbursting diaphragms.

Improvements to boiler design and startup procedures for two-shift operation

During the initial operation of the first CPB unit, co-ordinated two-shift startup procedureswere developed as part of the final acceptance trials by the CLP startup team with

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In 1988, concerns stimulated by the reports of widespread extensive fatigue damage in final

superheater outlet headers at other installations, which had required urgent prematurereplacement of some headers after relatively few starts, prompted a comprehensive evaluationof the impact of the planned extensive two-shifting on the superheater headers of the CPB

boilers.

An initial review of extensive measurements from 150 thermocouples, installed primarily forcreep life monitoring purposes on tubes and headers of superheaters and reheater of each

boiler, highlighted that conditions in the superheaters during startups were very differentfrom those assumed. A paramount objective of the original two-shift startup proceduredeveloped with the boiler OEM during final acceptance trials on the first unit had been tolimit the fuel input during the initial period of firing to allow what was thought by the OEMto be long enough for the condensate that had collected during the shutdown and prestart

purge in the pendant tubes of platen and final superheaters to boil dry before it becamenecessary to establish a cooling steam flow through the platen superheater tubes to preventoverheating by opening the HP bypass valve.

However, it was clearly evident that, although steam flow was delayed for 20 minutes afterfiring commenced and with one coal mill in service for most of that time and a second millalso started, there were still substantial quantities of condensate in platen and finalsuperheater pendant tubes when steam flow commenced. This condensate caused rapidtemperature drops, and quench-cooled the closely pitched tube holes when blown into thesignificantly hotter outlet header of the final superheater immediately after steam flow wasfirst established, (Reference 1).


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superheater into its relatively hot outlet header by the pressure drop established through the

superheater when steam flow commenced and each time steam flow rate was increased. Thetests also failed to find a means of significantly lowering the steam temperature ramp rate at

platen outlet

Both key startup objectives for the CPB boilers were achieved by the installation of a largewarming drains at platen outlet to permit steam flow through primary and platen superheaterswithout initiating steam flow through the final superheater. This had not been possible withthe originally CPB boilers because there were no drains between those at the bottom headersof the rear pass cage wall, which are also the inlet headers of the primary superheater, and theoutlet of the final superheater, so that when steam flow was necessary to cool the platensuperheater tubes, it also established flow and pressure drop through final superheaters and

blew condensate into the much hotter final superheater outlet headers.

The platen outlet warming drains are opened immediately after the oil burners are in serviceand condensate in primary superheater tubes and in the platen pendant tubes is blown out

when the platen superheater outlet header is only a little hotter than the condensate atsaturation temperature. Operators are now able to limit the rate of increase in steamtemperature at platen outlet reasonably close to the target rate of 8C/minute. By eliminatingthe carry forward of condensate from primary and platen headers into the final superheater,quench cooling has been eliminated and at worst the final superheater outlet headersometimes experiences a minor chill by the remnants of condensate from some tube loopsthat have not boiled completely dry.

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many large coal-fired boilers. The use of partial sliding pressure control for HP steam further

assists in keeping the HP turbine hot after shutdown, since it reduces the throttling drop insteam temperature across the HP turbine inlet valves.

A typical CPB hot start is illustrated by the trend plots of key parameters from the data loggerfor the two-shift startup of unit B3 on 27/4/2001 after about 9 hours off load, Attachments2A, 2B & 2C. The time from starting fans for the purge to synchronizing and block loadingthe turbine generator was less than one hour, of which almost 15 minutes was required tocomplete the purge. From first oil burner in service to block load applied took about 45minutes. The turbine was run-up to speed and synchronized in 8 minutes and then loaded toabout 150MW over 10 minutes, after which load was added to follow the increase in systemdemand in the early morning. The first coal mill feeder was started as soon as two rows of oil

burners were in service for flame support and the platen warming drain was opened about thesame time after which the platen tube temperature measured in the gas space increased from305C to 490C at 13.2C/minute followed by a slow increase to a tube temperature of 522C.Steam temperature at platen outlet increased initially from 313C to 518C at the very

moderate rate of 6.6C/minute. After firing commenced the final superheater tubetemperature measured in the gas space increased from 286C to 524C at 5.5C /minute. Thetemperature of final superheater outlet tubes, stub header and stub pipe, also of outlet steam,did not increase until the HP bypass opened. When the HP bypass opened, the stub pipeexperienced a small chill from 350C to 345C (caused by the remnants of condensate blownforward) and then ramped up as steam temperature increased. Final superheater outlettemperature increased from 410C to 535C at the moderate rate of 4.0C/minute

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shutdown. Turbine bypass minimizes oil consumption during startups and also the amount of

blowdown and make-up water costs.

Turbine bypass facilitates more repetitive startup procedures with greater confidence ofsynchronizing on schedule. It also reduces cyclic life expenditure in critical boiler andturbine components. During two-shift starts, steam temperatures at the turbine are close toideal with virtually no cooling of rotors and casings.

The capacity of an HP bypass system required only for startup assistance depends on whatsteam temperatures the boiler can produce at low steam flow rates. At CPB, where steamtemperatures produced at low steam flows are relatively high, an HP turbine bypass systemcapacity of about 25% at full boiler pressure would suffice, but the LP bypass capacity atCPB of 40% of BCMR flow at design hot reheat pressure cannot be reduced withoutintroducing problems with windage heating of the HP turbine during startups. Otherinstallations with natural circulation boilers that produce lower steam temperatures at lowsteam flows would require larger capacity bypass systems to be able to match HP steam

temperatures to turbine metal temperatures during hot startups.

A turbine bypass system sized just for startup duty can, with additional controls, protectionand validation systems, also be utilized for continuous pressure shadowing of HP pressurewhen the unit is on load. Should an external fault cause HP pressure to rise rapidly, the HP

bypass can be arranged to detect this and open and modulate to control HP pressure beforeany superheater safety valves lift. This reduces the risk of trip and also avoids outages torepair leaking superheating safety valves or to repair a safety valve that fails to reseat.


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are designed to produce the rated steam temperatures down to significantly lower loads than

many boilers can achieve, which further reduces the amount by which heat rate increaseswith reducing load due to the lower thermodynamic cycle efficiency with lower steamtemperatures. Thus at CPB, the startup costs for make-up water, fuel oil, coal and auxiliary

power consumed while returning the unit to service are not recovered by the fuel cost savingsfrom more efficient operation of the units that remain synchronized unless the duration of theshutdown exceeds 8 hours.

Benefit of Part-loading

HK$/Cycle______Non Heat Cost:

- Maintenance 13,000

Heat Cost Overnight - 23,500

Start-up Cost :

- Make-up Water 1,000- Fuel Oil 20.000- Fuel Coal 4,000- Auxiliary Power 1,400

Total 15,900Figure 6 Cost benefit of Part-loading two 680MW instead of Two-shifting one 680MW

unit


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with 150 permanent thermocouples on tubes, pipes and headers of the higher temperature

parts of platen and final superheaters and the reheater. These were used during earlyoperation to locate those elements of platen and final superheaters and reheater that operatewith highest temperatures across the furnace width. Those headers, manifolds andinterconnecting pipes that operate at highest temperature and with a significant rate of creepstain accumulation are connected to a creep life monitoring system that records, for each

point monitored, the metal temperature and operating pressure every few seconds and byreverse design calculation calculates the creep life consumption, which is continuouslysummated. The rate of creep life consumption, which is highest at platen superheater outlet,

indicates that all parts of the boiler should exceed 200,000 hours, possibly significantly sobecause of the conservatism built into the design code stress limits for parts subjected tocreep.

The

component

Possible defects or

failure modes

Inspection

in

overhauls

Inspection

in life

assessment

Defects

found

Remedy

Superheateroutletheaders

LCF crackingassociated withcreep at ligamentsfrom nozzle holes

No CCTV Nil Honing untilreplacement isnecessary

Economiserinlet headers

LCF cracking atligaments from

nozzle holes

No CCTV Nil Honing untilreplacement is

necessary

Water tube LCF cracking at MPI MPI Yes Weld repair,


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creep were found in either boiler. Figure 7 highlights that only minor problems have been

experienced in specific parts of the furnace walls that are fatigue associated with two-shiftoperation, and these minor problems were repaired during normal planned outages. Thecontribution of close attention to water chemistry control, especially during startups and shutdowns, should also be emphasised. There have been no failures at CPB that result from awater chemistry problem.

The next important boiler life assessment is to be performed when each unit reaches either130,000 operating hours, or 5,000 hot starts, plus 1000 warm starts, plus 150 cold starts,

whichever comes first.Operating experience and the results of life assessments have concluded that two shiftoperation has not caused any identifiable distress of significance in the CPB boilers. TheCPB boilers have experienced no more fatigue-related problems than the boilers of any base-loaded units.

Turbines

The timing of major inspection outages has been mainly influenced by requirements forinternal inspections of the turbines. The turbine OEM recommended an interval of 50,000Equivalent Operating Hours (EOH) between major turbine inspection outages. The timing ofEOH is calculated by adding 30 hours per startup cycle to the unit operating hours, Fig. 8.


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inspection, Fig. 9. The similarity of the generally very good conditions found in all HP and

IP turbines inspected, the absence of turbine casing joint leaks or bolt failures, and the smalldegradation experienced in turbine efficiencies, all motivated the trend towards longerintervals between turbine inspections.

UnitNo

Year CylinderInspected

Operatinghours

Total Numberof starts

EquivalentOperating hours

B1 1999 HP/IP/LP 84,775 1,692 135,535B2 1991 HP/IP 31,739 446 45,119

2001* HP/IP/LP 103,000 1,500 148,000

B3 1994 IP 51,325 765 74,275

2000 HP 88,412 1,396 130,292

2001 LP 92,866 1,495 137,716

B4 1999 LP 61,054 1,196 96,934

2000 HP/IP 67,661 1,573 114,851* B2 outage planned for October 2001

Figure 9 Major turbine inspections

Some degradation was found in HP turbines which is associated with two-shifting. Lowcycle fatigue (LCF) cracks were found in the fillet radii of diaphragm grooves in all HP inner


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shot-peened to improve surface hardness and obliterate any small surface stress raisers. The

first re-inspections were after a further 450 starts which was then increased to 800 starts afterre-inspections had found fewer and shallower cracks after shotpeening, as expected.

Erosion of the LP last stage blades has been accelerated by two-shifting. The extent has beenrecorded to determine the rate of erosion at future inspections and decide when the bladesmust be refurbished.

LCF cracking has been found at the stellited area of the valve seats of HP stop and control

valves and the IP stop valve on some units. One valve seat was replaced and cracks found inthe others were recorded for comparison with findings at future inspections planned for 2003.The cracks do not affect valve operation and the risk of detachment of material has beenassessed to be small. .

The CPB turbines are the fleet leaders on numbers of starts for Alstom turbines of similardesign. The distress found related to two-shifting is not considered to be a forced outage risk

and, with the exception of the LP last stage rotor serrations, does not impact on future outageplanning. The defects related to two shifting operation, - the fatigue cracks identified at theHP inner casing and inlet end inner casing key block and cracks in valve seats, - can berepaired at future planned outages with minimal maintenance costs.

Future turbine inspections are planned to take place at intervals of not less than 100,000Equivalent Operating Hours, and possibly at longer intervals if no more serious distress isfound at future turbine inspections.


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ConclusionsEach of the four 680MW units at CPB have already completed between 70,000 and 100,000operating hours and from 1,450 and 2,000 starts without significant problems in boilers orturbines that are attributable to thermal stresses developed during startups. Those problemsrelated to two-shift operation that have occurred in HP and IP turbines can be managed byrepairs at little extra maintenance cost at major turbine inspections performed at intervals ofat least 100,000 Equivalent Operating Hours startups. Current expectations are that boilersand turbines appear to be capable of at least 200,000 operating hours with 5,000 hot starts,

1,000 warm starts and 200 cold starts before consideration may need to be given to possiblereplacement of some major components at some point in time after further two-shiftoperation.

Despite the extensive two-shifting performed, the availability and reliability of the CPB unitscompares favorably with that of the best of the large base-loaded coal-fired plants which have

performed few startups.

Acknowledgements

Castle Peak Power Station is owned by CAPCO, (Castle Peak Power Company), a jointventure of ExxonMobil Energy Limited and CLP Power Co. Ltd, Hong Kong. CLP Power isresponsible for management of construction and operation of Castle Peak Power Station. Theauthors wish to record their appreciation for the approval by CLP Power to publish this paper.The assistance of C.M. Lui and S.K. Ho and other CLP Power colleagues with preparation ofsome of the contents of the paper is also acknowledged.


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Attachment 1. Unit Shut Down


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Attachment 2A. Unit Start Up Sequence


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Attachment 2B. Final Superheater Temperatures During Hot Start Up


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55 680 mw coal gas fired power

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