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Experience Regarding Conformity With Regu-latory Codes in the Planning of Gas Turbine (GT)Installations With Downstream Heat RecoverySteam Generators (HRSG)

H. Chr. Schrder, H. Engesser, G. Scheffknecht, and H. Stierstorfer

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SPECIAL PRINT VGB KraftwerksTechnik79. volume, issue 4/99, page 36-42

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Introduction

This contribution shows problems that arisein the practical application of the pertinentsafety codes, particularly in combined-cyclepower plants, i.e. gas turbine plants withdownstream HRSG (heat recovery steamgenerators). These problems are encounteredin the following areas:

the venting and purging of the flue gasducts,

flame monitoring, and

trails in ignition times.

Several different aspects are to be consideredin deciding to build a combined-cycle powerplant. These include, for example, the reduc-tion of the CO2 emissions, the high efficiencyof the facility, fast start-up times, high flexi-bility of operation, short construction time,low expenditure on investment, and possibil-ities for re-powering existing facilities.

In Germany, the TRDs (Technical Rules forSteam Boilers) of Series 400 apply, the spe-cific code depending on the fuel beingburned. TRD 411 and 412 apply for the usualfuels burned in gas turbines, heating oil ELand gas respectively. TRD 411 calls for a

purging of the flue gas paths with at least50 % of the combustion air flow at full loadover a period of time long enough to producea three-fold air exchange in the flue gas vol-ume being purged.

The volume flows in the gas turbine com-pressor are dependent not only on speed:They are subject to limits due to the start-uppower of the starting device and to bladingvibrations. For this reason, it is not alwayspossible to satisfy this provision of TRD 411.

Combustion in the gas turbine GT 24/GT 26of ABB takes place in sequence: a first com-bustor with a high-pressure turbine, followedby the second combustor with a low-pressureturbine. For the flame monitoring in the sec-ond combustor of a gas turbine (sequentialcombustion), the inlet temperature is thecritical factor inherently determining thecombustion. For the second combustor then,a temperature monitor can be used instead ofthe optical flame monitor called for in thecodes since, in this range of temperature,there is no ensuring that optical flame moni-tors will recognize the flame with certainty.F i g u r e 1 shows the design of the first andsecond combustors in an ABB gas turbine ofmodern design.

The processes of filling the fuel supply linesand cross-ignition of the burners in moderngas turbines with annular combustors meanthat a greater amount of time is required be-fore the flame is burning safely. It is not pos-sible here to maintain the trail in ignitionperiod required according to the TRD be-tween the opening of the shut-off device (tripequipment) and activation of the opticalflame monitor.

This leads to questions in connection withpurging the flue gas paths, flame monitoring,and trail in ignition times when applying theTRDs.

In the following, we present possible solu-tions for dealing with these topics safely, i.e.for ensuring a high degree of safety in theprocess while still maintaining the high avail-ability demanded of gas turbine plants, andfor fulfilling the safety standards called for inthe codes with the use of plant components

that are normally there, without requiring anadditional fresh air fan.

Purging of Combined-Cycle Plants

Requ i r emen t s i n t he Codes fo r t he Pu rg ing o f F lue Gas Pa th sin Combined -Cyc l e Power P l an t s

The procedures followed for purging theHRSG in combined-cycle power plants withand without a bypass stack can differ.

How much purging is needed results essen-tially from the requirements in the individualTRDs, such as, for example, TRD 411 forfuel oils and TRD 412 for fuel gases.

We are also familiar with two further systemsof rules from the USA and England, namelythe NFPA (National Fire Protection As-sociation) 8506 from the USA, and theguidelines from British Gas (Guidance Noteson the Installation of Industrial Turbines, As-sociated Gas Compressors and Supplemen-tary Firing Burners).

In America, the differences in operation anddesign between heat recovery steam gener-ators and conventional steam generators haveled to separate standards for HRSGs (NFPA8606). The current revision of the NFPAcalls for at least a 5-fold exchange of the gasturbine air at a rate of at least 8 % of themaximum mass through-flow. Alternativepossibilities are also offered for demonstrat-ing the effectiveness of the purging of theHRSG.

Proces s Techno log i ca l Requ i r e -men t s f o r t he Pu rg ing o fCombined -Cyc l e P l an t s

The maximum temperature of the purgingmedium during the purging process is limitedby the theoretical ignition temperature of thefuel being used. The ignition temperature ofordinary natural gas is on an order of ap-proximately 600 C. Coal gas, as a mixture ofhydrogen and carbon monoxide, likewise hasan ignition temperature of approximately550 C. Liquid fuels such as light heating oil(HEL) or naphtha are at approximately220 C.

Experience Regarding Conformity WithRegulatory Codes in the Planning ofGas Turbine (GT) Installations WithDownstream Heat Recovery SteamGenerators (HRSG)By H.Chr. Schrder, H. Engesser, G. Scheffknecht and H. Stierstorfer

Dipl.-Ing. H.Chr. Schrder

TV Sddeutschland,Mannheim.

Dipl.-Ing. H. Engesser

ABB Power Generation Ltd.,Baden/Switzerland.

Dr.-Ing. G. Scheffknecht

EVT Energie- und Ver-fahrenstechnik GmbH, Stuttgart.

Dipl.-Ing. H. Stierstorfer

Siemens AG, Bereich Energieerzeu-gung (KWU), Erlangen.

For a relative safety margin from the theoret-ical ignition temperature, the maximum tem-perature for purging should provide a marginof approximately 20 % from the above-men-tioned temperatures, i.e., a margin of ap-proximately 100 C for gas and ap-proximately 40 C for oil.

In practice this means that, for gas turbinesfired on natural gas, it is possible to purgewith the exhaust flow, provided that the tem-perature of the flue gases remains far enoughbelow the ignition temperature of the givenfuel after taking into account any temperatureslopes and peak temperatures that might oc-cur. It must be noted, however, that naturalgas often contains among the components inits mix longer-chained hydrocarbons such aspropane, butane, etc., whose ignition tem-peratures are less than 600 C. In order torule out even those uncertainties with regardto the venting temperature that result in con-nection with temperature peaks and lows, itis recommended not to purge with an exhaustgas temperature of more than 400 C.

The situation is different when using liquidfuels. Here it is not readily possible to purge

the downstream units with the exhaust flowfrom the gas turbine because the exhaust gastemperature from the gas turbine is higherthan the ignition temperature of the liquidfuel. The possibility of explosions could notbe ruled out if there were any remnants ofunburned liquid fuel present in the HRSG.

In addition, it should be mentioned that thepertinent TRDs neither make any referenceto the air temperature nor do they indicatewhether the purging volumes are in terms ofmass flow or volume flow.

In actual practice, purging the gas turbineitself presents no relevant problem since thegas turbine, regardless of the operating con-dition from which the firing is begun, is auto-matically purged before every firing start.This means that no discussion is needed ofany renewed light-off after a malfunction inoperation or shutdown in operation.

The situation is different in the downstreamcomponents of the plant. The volume flowejected from the gas turbine is still present inthe downstream flue gas system and in theHRSG.

Due to the small volume flows in the gas tur-bine in its lower speed range while the com-pressor is being turned by the generator(turning operation), it is impossible toachieve a purging flow in the HRSG of morethan 50 % relative to the total flue gas flowproduced from the combustion. The purgingflows that can actually be attained underthese operating conditions lie at about 10 to15 % of the maximum compressor air massflow. Section Verification of SuccessfulPurging below shows this to be sufficientfor a safe purging.

Imp lemen ta t i on o f Pu rg ing P roces se s i n Combined -Cyc l ePower P l an t s

Necessity of Purging Combined-CyclePower Plants

It can be necessary to purge an HRSG in thefollowing cases:

prior to commissioning,

after a failed ignition (e.g., where theflame monitor has not taken over),

following an interrupted start of the gasturbine after the ignition process,

after an emergency trip from operationunder load.

The TRDs regulate the first of these cases ap-propriately. The TRD expressly allows dis-pensing with a purging of the flue gas pathsif a fail-safe monitoring of the fuel supplysystem has been provided.

While the next two cases urgently require apurging of the flue gas paths, the emergencytrip from load operation and even the ordi-nary shutdown require closer examination.

During an ordinary shutdown of the gas tur-bine, i.e., a shifting from operation underload to idling and a subsequent shutting offof the fuel supply, the flue gas system is ad-equately purged by the volume flow of ex-haust gas coming from the gas turbine while

4 VGB PowerTech 4/99

Temperature > 1000 C

Sequential burner