Asset management – gas turbine power stations: open cycle and combined cycle gas turbines

This, part 2 of a four-part overview, focuses on the salient asset management operational and maintenance aspects of industrial GT/heat recovery steam generator units.

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Sep 27, 2017
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Author(s): Douglas Hutchinson

Abstract

This overview was written primarily with graduates, engineers and electrical, mechanical, chemical, instrumentation and control and computer technicians in mind who wish to enter the power generation industry at large and gas turbine (GT) power generating plants in particular, since combined cycle GT power plants constitute the largest output and most efficient turbine power generation stations operating today. 

Large output gas turbine generator (GTG) start-up

To demonstrate the start-up superiority of large output industrial GTG units, depending on design, a typical start-up time sketch is shown, for circa 150–250 MW (Megawatt) units. The sketch indicates turbine/generator shaft rotational acceleration rates and the time taken to get to full speed no load (FSNL) prior to ‘auto’ synchronising with the local grid network.

Note: All GT generating unit operation is now almost completely computer controlled. GT run-up to FSNL and synchronising is computer controlled, as is online load changing and plant shut-down (Fig 1).

Fig 1: GT Run-up time to full speed no-load (synchronous speed)

Typical large output GTG start-up time to FSNL (large output GT generator unit FSNL)

Depending on design, an equivalent sized multi-cylinder steam turbine (ST) generating unit in the ‘cold’ or ‘warm’ rotor metal temperature condition (discussed in Part 3) would be started and reach synchronous speed in readiness for synchronising between one-and-a-half and two-and-a-half hours.

GTG loading rates are also higher than that of STs; however, a significant constraint becomes apparent in combined cycle GT (CCGT) power plants – that of allowable steam pressure raising rates and high-pressure (HP) turbine, and if reheat is used, intermediate pressure (IP) turbine, rotor metal temperature. Thus, during CC start-up – the first GT generating unit is the heat source to its associated heat recovery steam generator (HRSG) – thus controlling allowable steam pressure raising rates together with steam temperatures for ST start-up and loading.

From the commencement of GT ignition and start-up, the engine exhaust gasses are ducted directly in to the machine’s HRSG – thus pressure raising, in keeping within allowable steam drum pressure raising rates, together with steam temperature control to match ST metal temperatures, means GT loading rates are initially constrained to allow CC ST start-up and loading.

GT ‘hot gas path’

Exotic, high cost, difficult to machine and process material makes up the GT ‘gas path’ components. These components rotating at high rotational (synchronous) speeds, or in gearbox applications, at even higher speeds – have a finite lifespan before metal fatigue and expensive component replacement or extremely expensive failure in service occurs (Fig 2).

Fig 2: CCGT ‘Watch and maintain’ areas

Diagrammatic and notional CCGT ‘watch and maintain’ areas

Failure of a metal component in service means debris (some very large and some very small) is swept by the compressor air plus combustion gases mass flow throughout the machine, thus severely damaging and in some cases completely destroying, downstream blading, vanes, seals and other segments of the overall machine. In fact, in service failure and ‘foreign object’ ingress – of whatever type or for whatever reason/s – into the axial-flow compressor and/or the GT units, let them be in open or CC configuration, has always remained the No. 1 Scourge, curse, enemy and topic of great concern for all GT power generator operators.

GT maintenance regime

Working at natural gas, middle distillate or heavy (crude) flame temperatures – GT maintenance is governed by a ‘running hours’ and ‘number of starts’ maintenance regime.

Running hours is self-explanatory; however, the number of starts plays an enormous part in GT operations because, at start-up, ignition and the entry of hot gasses into the turbine creates very large temperature gradients across the machine blades and discs whilst being rapidly accelerated to synchronous speeds and then maintained at synchronous speeds (50 or 60 Hz plants run at 3000 or 3600 rpm, respectively). After synchronising, rapid loading (high temperature and increasing gas mass flow) also creates large heat transfer and thermal gradients within the GT. Thus, GT rotors, blades and gas path components are initially subjected to high heat transfers before attaining their normal working temperatures to become safely heat soaked with blade air cooling being maintained at optimum levels throughout the start-up and in-service process.

Depending on the designs of different manufacturers, the number of starts is equated to machine equivalent running hours, because these starting very high thermal gradients contribute to blade and other gas path component metallurgical deterioration and ultimate metal fatigue failure. Thus, each GT start is considered to be equivalent to an additional number of running hours.

Actual machine running hours + machine number of starts = total equivalent running hours to gas path overhaul and/or blade change-out per original equipment manufacturer (OEM) formula/recommendations.

Sketch – uneven flame spread

Starting can also be most damaging and contributes to the reduction in shelf life of GT gas path blades and components (Fig 3).

Fig 3: Sketch – GT uneven flame spread

Indicative sketch of combustion chambers arranged around GT inlet.

Flame tubes connect combustion chambers, thus allowing and assisting the flame to almost instantly spread upwards to all combustion chambers on ignition and machine start-up.

On GT start-up, retractable electric powered spark igniters – are interlocked to be inserted after the GT purge sequence has been completed, and before fuel admission. With igniters in service and heavily sparking, when atomised fuels/or gas fuels are admitted, ignition occurs immediately and the flame spreads upwards to each combustion chamber via flame tubes, thus achieving complete and almost simultaneous ignition in all combustors; thus, no large temperature differences and extreme thermal gradients are allowed to occur.

  • Note 1: should for any reasons, ignition not occur in any given combustion chamber/s – the flame spread temperature difference – would cause very large temperature differences and hugely varying thermal gradients across the turbine rotor discs and blade rows. As GT rotors commence to rotate, these components are subjected to high then low flame temperatures (thermal cycling) at an ever-increasing speed change rate. Thus, flame spread temperature difference – when detected – will cause machine shut-down.
  • Note 2: flame spread temperature difference protection is provided on all modern, large output GT units.
  • Note 3: When flame spread temperature difference protection operation occurs and GT shut-down is initiated, it is classed as a GT failed start, and thus contributes to the equivalent running hours count.

GT maintenance frequency

Reams have been written, and will continue to be written about the economic maintenance of open cycle (OC) and CC GT units. GT units now operate not only in the power utility sense, or simply for emergency back-up power situations, they now operate on off-shore oil rigs, in remote desert locations, in military applications and in a multitude of co-generation and desalination applications.

GT technology requires a different type of thinking and management based on the plant design, principal technologies involved, degree of computerisation and automation employed and hot gas path (HGI) component ‘running hours’ and ‘number of starts’ regimes. The thermal and speed factors are so critical, continuous online monitoring and trending are now accepted asset management practise. As soon as undesirable or dangerous trend conditions are detected – stations are alerted!

These key areas must be accurately detailed and monitored, be it solely by the operating company, the OEM Company, or some agreed combination of the two. Whatever the chosen maintenance management approach, generally speaking, GT OEMs manufacture, supply, fit and sign-off on all hot gas path inspections (HGIs) and overhauls.

The spares quantities manufactured and held in stock are invariably in keeping with their (OEMs) ‘recommended HGI’ maintenance and major overhaul frequencies. OEMs keep very little in the way of extra spare part stocks!

Component failures outside their maintenance schedules could conceivably result in there being no parts available in stock! Worst still – the parts could conceivably be on 6 months delivery!

Thus, asset management, from the commencement of new plants or the acquisition and take-over of existing power plants, absolutely demands – very careful ‘spares parts analysis’ together with very careful running hours, flame spread temperature difference, number of starts and failed starts analysis – together with an analysis of the OEM’s total supply capability – that is – the OEMs existing parts stock levels, spare parts delivery schedules and the OEMs response times and costs to supply parts and services if called on to do so in emergencies.

The power supply company, whether it be experienced utility owned or some ‘independent power provider’ company – has to negotiate and come to firm contractual arrangements (maintenance and parts supply contracts) – for parts and maintenance services with GT engine manufacturers. Thereafter, the ‘asset manager’ has to also manage and undertake all other balance of plant (BOP) maintenance, with or without the assistance of BOP OEMs.

The power supply company has to also perform the same detailed analyses and have firm arrangements in place to safeguard the HRSG and generator unit together with the CC STG unit/s.

Note: modern, large output GTs drive their own direct coupled generators – and it is possible in the highly competitive commercial engineering, procurement and construction (EPC) contractual world – the GT generators–exciter units may be of a different supply from the ST generators–exciter unit/s.

Trending and the development of parts and special tools inventories and appropriate maintenance records should be maintained to assist timely procurement and rigorously guard against failures in service of GT main plant or auxiliary BOP. Thus, an extremely detailed equivalent running hours, maintenance parts and service, coupled with real-time online monitoring contracts are necessarily called for.

To present the complexities of operations and maintenance of CCGT plants, a diagrammatic example of a large output (700 MW) reheat, STG CCGT plant is shown.

Whilst the key asset management thinking required for GTs operation has been presented, GT units are nevertheless rotating machines – and as such, require extra short-term operational tactical thinking and longer-term trend and strategic ‘condition monitoring’ maintenance thinking. Other operational and maintenance (O&M) methods such as reliability-based maintenance thinking can also be profitably applied just as in all large rotating machine maintenance management regimes. GTG units together with STG units and BOP OEM specific recommendations must be studied and included in all on-going O&M philosophy, practises and procedures.

Plants are run year round, in all weathers and thermal conditions in accordance with the OEM, power purchase agreement (PPA), insurance, national standards and other industrial legal requirements; furthermore, CCGT plants are operated in ‘hot’ ‘warm’ and ‘cold’ thermal conditions.

Note: large output ‘reheat’ steam cycle CCGT plants usually generate three steam systems – and hence three low pressure (LP), IP and HP boiler drums and steam systems plus the reheat system are incorporated into the ST design.

Starting, operating and shutting down CCGT plants are computerised; thus, the entire CCGT unit plant, all unit BOP Plus the CCGT unit electrical supplies and high-voltage station electrical switchyard – must be – pre – checked and put into service – or – be in a ‘Click-to-go’ … ‘Ready to Start’ press ‘Enter’ to start/stop condition at a control-room computer workstation.

In the commercial times that prevail, all maintenance planning includes permit-to-work preparation times, parts delivery and scheduling, scaffolding scheduling, contractor and manpower scheduling and work scheduling, down to the last nut and bolt levels are commonplaces for all GT/generator/exciter/BOP maintenance outages (Fig 4).

Fig 4: Basic configuration – 700 MW CCGT reheat plant

Basic schematic configuration – 700 MW CCGT reheat installation

Hot, warm and cold plant start-up conditions are specified by the OEMs. The PPA agreement with the grid authority usually includes identical or very similar recommendations. Generally, international best practise summarises and defines these conditions as follows:

  • (a) ‘Hot’ start: is defined as CCGT unit that has been off-load for up to 8 h.
  • (b) ‘Warm’ start: is defined as a CCGT unit that has been off-load for 8–48 h.
  • (c) ‘Cold’ start: is defined as a CCGT unit that has been off-load for more than 48 h.

With OC and CCGT plants locked into running hours and number of start maintenance periods. The maintenance of all other sections must be arranged to fit into these times such that overall maintenance down times are minimised.

GTs are the most critical maintenance item in any open or CC plant.

Depending on the design, unit size, fuel burned, plant locations, running hours and number of starts profiles and without introducing all sorts of relevant and not-so-relevant extraneous factors – around the world, GTs generally demand three types of maintenances:

  • A. Combustor or combustion inspections (CIs).
  • B. HGP inspections (HGPIs).
  • C. Major inspections (MIs).

Again, depending on OEMs design, unit size, fuel burned, locations, running hours and number of starts profiles and without introducing all sorts of relevant and not-so-relevant extraneous factors – in general, around the world, the key stipulated maintenance requirements are:

  1. Combustor or CIs: performed annually at 8000 h.
  2. HGPIs: performed at 24,000 h – within every 2–3 years.
  3. MIs: performed at 40,000 h – every 5 years.

Very careful analysis of the relevant and not-so-relevant extraneous factors is however required, and includes such items as – the number of GT over-speed incidents, possible changes to fuel specification, severe electrical system low-frequency incidents etc. – allcan have an influence on gas path components. Prudent technical analysis can modify and change these general OEM maintenance requirements.

During CIs – the term inspection – means inspection and maintenance, remedying the here and now items whilst the plant is shut-down. CIs are all to do with fuel, burners, atomisers and combustor strip down, inspection (including non-destructive examination (NDE) inspection) and change-out maintenance. Naturally, on CI maintenance outages, generator–exciter and as much relevant BOP systems are checked and maintained in line with their condition profiles. Unjustifiable delays are not applauded!

Also during CIs, and in fact all inspections, other forward-looking inspections are performed noting, recording and where necessary photographing the condition of the combustion and HGP components. Gaining advanced information is important to planning whether the next scheduled maintenance period should be kept as normal or brought forward in view of the photographed condition of HGP components. In this respect, borescope inspections are performed on GTs – looking for and photographing potential hairline cracks, turbine blade coating material erosion/corrosion, minute metal fragment accumulations – indeed, looking for anything unusual, in order to ensure the plant can continue to run safely until the next scheduled maintenance shut-down.

HGPIs: are performed on GT combustor systems. Inspection (including NDE inspection) and remedying the here and now items whilst the plant is shut-down is mandatory. Naturally, on HGPIs, GT internal inspections are undertaken. Generator–exciter and as much relevant BOP systems are also checked and maintained in line with their condition profiles. Forward-looking inspections are repeated in order to ensure the plant can continue to run safely until the next scheduled maintenance shut-down.

MIs: are the largest maintenance outages and require complete change-out of GT combustors/turbine blades/and all other HGP turbine components to the OEMs schedules. Prior to re-synchronising, return to service testing is performed to OEM, insurers, owners and other relevant industry governing body requirements. Within the GT available maintenance time period, extensive generator–exciter and BOP maintenance is also performed.

Note: Large output CCGTs drive air-cooled or hydrogen-cooled generators, and generally air-cooled exciter plants. These machines require the maintenance outage periods of steam cycle generators and can thus usually fit into the short duration 8–14 days CI, HGPI or GT major outage periods. Thus, GT maintenance is designed to form the critical path maintenance activity – all other maintenances are arranged to be completed within the critical maintenance time path of the GT.

Hence, whilst GT maintenance is the primary concern, thorough visual, extensive NDE inspection, shaft alignment and BOP maintenance are performed throughout all inspection and repair outages. The key overall GT plant O&M areas are listed. Where necessary, the essential basics with other relevancies in tabulated form are presented to ensure better understanding.

  • GT section 1: Air inlet system.
  • GT section 2: Axial-flow compressor.
  • GT section 3: Overall GT unit and support BOP.
  • GT section 4: HRSG and gas passes.
  • GT section 5: HRSG steam and water circuits.
  • GT section 6: GT exhaust to atmosphere control damper (open cycle operation).
  • GT section 7: High-speed gearbox unit (if applicable).
  • GT section 8: Generator and exciter units.

GT section 1: Air inlet

Essential basics: The inlet air to compressors must be clean, that is, clean of erosive particulate and corrosive chemical substances. Plants operating in (say) desert locations, would without air filters, ingest huge quantities of dust, sand, grit and create a mini-dust storm throughout the entire axial-flow compressor/GT combination, perpetually sand-blasting all parts of the compressor, combustion chambers and GT.

Silicates and other substances melt at fuel flame temperatures and deposit on turbine blades and vanes to change the profiles of these components, and hence adversely affect the efficiency of these machines. Silicates and other substances also melt to form corrosion sites to the detriment of the machines.

Dust build-ups also accumulate, constrict and then markedly reduce turbine blade cooling air flows within their cooling air systems. As cooling air flows reduce, the choking dust build-ups accelerate further, reducing cooling air flows to blades and this ultimately leads to the overheating and blade failure.

Similarly, GT installations in the proximity of coastlines ingest salt-laden air and sodium and chloride deposits at flame temperature melt to form corrosion sites, which weaken high rotational speed blades leading to very costly replacement, or worst still, failure in service with all the implications of downstream component wipe-out damage. Similarly, atmospheric industrial pollutants drawn into GTs have very undesirable, profile changing and erosion/corrosion effects on GT engine units.

Thus, the correct selection of primary and secondary air filter materials and their rigid design structures are paramount prerequisites to obtain the maximum protection for GT machines. Primary and secondary air filters which remove particulate and chemical pollutants are mandatory in all GT installations be they small, medium or mega-large installations.

The key method of detecting filter over-burden is the measurement of differential pressure (DP) (Δ P) across both primary and secondary filter banks. To prevent compressor operational instability and damage, in the event of massive filter choking conditions, ‘implosion doors’ are installed in compressor inlet areas. Un-checked and ever-increasing DP (Δ P or DP) across filters will lead to the automatic ‘implosion’ doors opening (‘inwards opening’ doors), thereby reducing and preventing any possibility of compressor air-starvation, instability and damage.

Industrial filter manufacturers’ work very closely with GT manufacturers to optimise primary and secondary filter material performances to encounter the world’s seasons, geographic, industrial and other locations into which GT units are located (Table 1).

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primary and secondary air filters

mandatory – correct selection of air filter materials and rigid and secure fitment

correct selection of air filter materials is crucial  necessary for all locations – particularly for all industrial/desert/coastal locations and other atmospheric polluted environments

coarse or primary air filters

routine physical checking of inlet filter installations

depending on filter design material selection, check all filter packs for any obstructions drawn toward or into filter intake areas institute routine primary air filter washing/cleaning routines  ensure local and remote DP monitoring of primary and secondary filters. From day one, commence DP trend recording charts

secondary air filters

institute routine visual and physical checking at every convenient shut-down opportunity.  commence local and remote secondary air filter DP monitoring and trending

secondary air filters require extremely careful checking since they provide fine filtration and are in the direct path to compressor inlets  maintain local and remote monitoring of DP trend records charts  any indication of air inlet filters choking-up requires urgent attention, special cleaning or replacement  note: if filter choking is allowed to go too far – implosion doors will operate inwards and render the GT unit open to completely unfiltered air, thus exacerbating turbine damage and failure mechanisms

Table 1: Primary and secondary air filter checks

GT section 2: Axial-flow compressor

See Table 2.

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compressor inlet guide vanes (IGVs)

constantly check position at shut-downs, and during on-load operation via remote indication .  maintain and check IGV opening/closing calibration records

check position constantly – to ensure all IGVs are in an open position consistent with the GT operational off-load or on-load conditions. Note and trend IGV operation during:  1. unit start-up  2. during FSNL, running prior to generator synchronising  3. on-load during progressive increasing/decreasing load conditions  IGV opening (controls air mass flow to compressor and hence GT) – also affects GT exhaust temperatures. Ensure air mass flow to GT does not elevate or depress GT exhaust temperatures at known load/temperature relationship points  incorrect IGVs operation during start-up or at any load condition, for whatever reason, can cause compressor surging and stall, causing severe damage

compressor air bleed valves

depending on design, check air bleed valves are open with machine off – load and at speeds below (say) 95% FSNL.  Note: different manufacturers have different air bleed valve settings

air bleed valves must be open at start-up to allow minimum circulation of air through the compressor during machine run-up to FSNL  mal-operation of air bleed valves, for whatever reason, can cause severe compressor damage  ensure air bleed valves automatically close at appropriate speed-times during GT start-up

compressor performance

leaking air bleed valves + bad IGV positioning – poor compressor performance

leads to lack of power turbine output + lower than normal compressor discharge pressures – lack of cooling air to turbine blades – potentially leading to turbine blades thermal degradation and failure  bad IGV positioning can lead to compressor blade failures due to high cycle vibration and fatigue if IGVs wrongly positioned.

compressor performance

unexplained lack of MW output

could be attributed to compressor blade fouling with attendant change in compressor blade profiles leading to lower performance – and drop in GT generator MW output. In many locations, compressor blade washing, with OEM approved detergents, is undertaken at GT purge speeds to ensure compressor peak performance

Table 2: Axial flow compressor checks

GT section: 3 GT unit – lubricating oil

Overall compressor/GT/generator–exciter unit bearing lube-oil

Essential basics: The lubricating oil must be of the correct oil type – there are standard specifications for GT unit lubricating oil and all matters relevant thereto will be supplied by the GT OEM and the lubricating oil manufacturers. Lubricating oil is a manufactured material and as such has a material specification which must be adhered to in all respects.

Depending on design, all GT/generator–exciter plants have their thrust and journal bearings forced lubricated from a TG central lubricating oil storage tank. Closed-circuit lubricating oil is pumped through oil filters and oil coolers before proceeding to the inlet oil supply side of all bearings.

In passing through and lubricating the bearings, the pressurised oil maintains the oil clearances within the bearing, thus preventing the high-speed shafts from coming into contact with the white metal lined bearings. The oil is thus subjected to frictional heating within the bearings as well as the residual heat of turbine shafts and so carries away a considerable quantity of heat.

In certain designs, magnetic filters are positioned in the outlet oil pipes from the bearings to remove potential magnetic particulate material or fragments from entering and contaminating the overall bearing oil system. As with all turbo-generator plants, lubricating oil is also used for certain turbine ‘fail-safe’ applications as well as for large output hydrogen-cooled generator seal-oil systems.

Since the generator seal oil necessarily comes into contact with air and hydrogen – the two oil systems are kept apart.

  1. turbo-generator lubricating oil and
  2. generator seal oil.

Make-up replenishment oil and in cases of emergency back-up seal oil is supplied in one direction only – from the turbine lubricating oil installation to the generator seal-oil installation – where hydrogen is detrained and removed from the generator seal-oil system.

Depending on design, plant location and other peripheral matters – GTG forced lubricated oil systems may be air cooled or water cooled (Tables 3 and 4).

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overall compressor/GT/generator–exciter bearings

lubricating oil supply to/from bearings

the oil supply and lubricating oil system must be of the correct oil type and be clean and free from all contamination and polluting materials. All CCGT units are supplied with online lubricating oil purifier and filtration units

bearings

oil temperature to bearings

problems with oil-cooling systems whether they be fan-assisted air cooling, compressor air cooled or water cooled – all must be dealt with and treated with urgency/emergency – oil inlet temperatures to bearings must not be allowed to continue to rise without reason. Remedial follow-up action is urgently required

Table 3: Axial flow compressor checks

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bearings

bearing oil and bearing metal temperatures

high bearing oil outlet temperatures from bearings suggests – insufficient oil supply to the bearing – or oil film too thin in the bearing, thus overheating oil and overheating the entire bearing housing  bearing outlet high oil temperature will contribute to high bearing metal temperatures and then ultimate disastrous melting/failure of white metal in the bearing  high bearing oil temperature and high and continually rising metal temperatures could possibly mean break-down of bearing oil film resulting in metal-to-metal contact inside the bearing must be dealt with and treated as an emergency – particularly if associated with high bearing vibration

bearings

bearing vibration levels

mechanical longitudinal and radial clearances are very fine in axial-flow compressors and GT units. Mechanical clearances also fine in all generating unit bearings and generator oil seals  therefore – vibration levels could be a result of marginal or severe contact within bearings or between fixed and moving parts of compressor or GT in either the axial or radial directions  high/rising bearing vibration levels must be treated with urgency/emergency

Table 4: GT unit bearing & lubricating oil checks

GT section 3: GT oil fuel

Essential basics: All gas, middle distillate and heavy oil GT generating unit operation is to a very large extent computer controlled. Interlocks, plant sequencing, alarms, tripping and other safety protective systems are built into the overall computer control system.

Natural gas, middle distillate and heavy oil fuel is supplied per fuel specification and is comprehensively sampled and monitored to ensure compliance with the fuel specification; however, middle distillate and heavy oil fuels naturally present handling and O&M problems and require more maintenance of oil fuel system components:

  • (a) Oil fuel storage tanks.
  • (b) Oil fuel pipelines: especially long pipelines (kilometres) from distant oil storage facilities.
  • (c) Fuel pumps.
  • (d) Fuel filter installations.
  • (e) GT fuel manifold and fuel measurement devices.
  • (f) GT fuel combustor fuel nozzles etc.

Depending on the design, all are operated and maintained based on OEM recommendations and/or on actual problems encountered at unique plant locations, where oil storage tanks are used. Lengthy pipelines will over time will accumulate moisture and water deposits, and hence develop rust deposits which choke oil fuel filters, GT fuel manifolds, GT fuel measurement devices and combustor burner nozzles.

The GT combustor fuel nozzles can thus become choked leading to poor combustion (smoke in GT exhaust) and ever-increasing flame temperature spread and eventual machine unscheduled shut-down (Tables 5).

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fuel systems

oil tanks, oil pipelines, oil filters, GT fuel manifold and measurement distributor systems plus combustor oil fuel nozzles

on the basis of design and actual site conditions – routines must be developed for the inspection and cleaning of oil fuel storage tanks, filters, GT fuel manifolds and GT combustor nozzles  routines for the daily ‘de-watering’ of oil fuel storage tanks and filter installations must be instituted  routines for fuel filter cleaning are essential to avoid GT fuel manifolds and fuel measurement and distributor to oil burner fouling – leading to some burners getting more fuel than others – hence GT blade path temperature spread increase – causing extremely bad thermal operating conditions for the turbine blades which will ultimately fail in service  depending on design, generally assume maximum allowable blade path temperature differential spread in the order of 60–70°C maximum

fuel systems

atomising air or atomising steam

bad fuel atomisation means bad combustion – un-burned fuel in nozzles leads to choked up and carbonation of burner nozzles tips – leads to flame movement, toward the burner nozzles  exacerbating bad combustion conditions in GT combustors

fuel systems

incorrect atomiser and burner nozzles tips

using the wrong types of atomiser and burner nozzles means poor combustion. Always check atomiser and nozzles tips are of correct type

fuel systems

GTG loading rates – maintain at OEMs normal rates

generator loading rates are a function of fuel burn rate – fuel burn rates increases heat transfer and thermal stress in all GT HGP internals  recommended loading rates by OEM must be adhered to wherever possible

Table 5: GT unit oil fuel checks

GT section 3: GT combustion

See Tables 6 and 7.

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combustion

initial combustion is dependent on spark igniters being brought into service after the GT start-up purge sequence and before fuel is admitted

listen and look! – when starting up GTs – dull explosion or ‘Puff’ of smoke from exhaust stack – means too much fuel not being ignited sufficiently and early enough – dangerous explosive situation in GT turbine and/or exhaust area

combustion

fail to start – and/or several failed starts

very dangerous – leaving accumulated fuel inside the turbine. Gas accumulations and/or pools of liquid fuels can accumulate inside the GT after several failed starts – finally, getting a start could potentially result in re-ignition of unburnt fuel – explosion – in turbine or turbine exhaust area  note – GT purging removes oil vapour or gas concentrations, however, accumulations of liquid fuel can potentially still remain and would potentially vaporise when GT ignition was ultimately achieved resulting in explosion. GT purging systems are now installed and used before all GT starts. If there is a sequence of failed starts – apart from exhaustive fuel ignition systems checking – inspect turbine exhaust area – before – re-initiating turbine start sequence

combustion

flame spread temperature difference, depending on design, usually set at 69°C

flame must very quickly spread to all combustion chambers – otherwise, there will be a discrepancy between temperatures emanating from each combustion chamber and presented to the GT blades, thus giving a large and uneven spread of flame path temperatures. Results in damaging thermal shock and thermal cycling of turbine blades as machine increases in speed leading to quicker turbine blade deterioration and failure  oil firing – oil nozzles can conceivably get choked or carboned up – leading to a restriction of fuel to some burners – turbine blades encounter hot flame zones and cooler flame zones – in rapid succession as rotors accelerate to synchronous speed and run at synchronous speeds  this leads to blade thermal cycling and ultimate quicker blade deterioration and failure

Table 6: GT combustion equipment checks

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combustion

turbine exhaust smoke

indicates insufficient air supply – check IGVs restricting air mass flows – air or steam atomising systems, complete fuel measurement and distributor system, poor compressor performance – check air to fuel ratio plus turbine exhaust O 2, CO 2 and CO levels  computer systems now control air/fuel ratios whilst limiting combustion temperatures to reduce and minimise NO x emissions

Table 7: GT combustion equipment checks

GT section 4: HRSG gas passes

Essential basics: HRSGs are heat exchangers which are horizontally laid out directly behind the GT exhaust outlets (see sketch below) and are exposed to the considerable GT exhaust gas mass flow and gas temperatures (Fig 5).

Fig 5: HRSG gas passes illustration

Superheater, reheater and any process intermediate and/or LP superheaters are thus arranged to be nearest to the GT high-temperature exhaust zone; natural circulation and/or forced circulation evaporator tube banks follow after the superheater zone and the economiser tube banks are located in the latter sections of the overall HRSG collecting as much heat as possible from the exiting GT exhaust gasses prior to stack discharge to atmosphere (Table 8).

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HRSG gas passes

turbine exhaust

monitor GT exhaust temperatures – HRSG cubic expansion occurs and changes as GT exhaust gas temperature and mass flow increases/decreases  boiler drum pressure rate of rise governs all HRSG boiler pressure raising rates to OEMs instructions  (see HRSG manufacturers’ instruction manuals)

turbine exhaust temperature

turbine exhaust temperature and mass flow – controls:  1. all HRSG boiler drum pressure raising rates  2. superheater (and reheater) steam temperatures  3. in complex HP, IP and LP steam generation designs GT exhaust controls all HRSG drum and pressure raising rates together with steam temperature and steam output quantities  (see HRSG manufacturers’ instruction manuals)

Table 8: GT exhaust checks

GT section 5: HRSG steam and water circuits

Essential basics: HRSGs require understanding and careful O&M attention. When the CCGT era arrived, HRSGs were a revelation, they were designed from the ground up to suit completely new steam raising applications which fitted directly to a combustion turbine high-temperature exhaust!

HRSGs are very compact heat exchangers and their development had to keep pace with GT prime mover OEMs. In this development mode to get even higher outputs, oil and gas firing directly into the HRSG units along with GT exhaust – has been used in certain installations.

Thus GT exhaust and further oil/gas firing, along with the addition of much demanded ‘reheat steam cycles’ makes HRSGs worthy of considerable study. Their expansion arrangements with reference to three axes in space, along with advanced steam cycle reheat steam supply generally meant the inclusion of LP steam – IP steam – HP steam and reheater steam. This led to the inclusion of complex LP, IP, HP boiler drums and reheater steam systems.

With newer steam output systems all ST operations must adhere to the strict use and understanding of the steam tables and the applications thereof.

The pressure parts of HRSGs are handled, operated and maintained in the same ways as for all steam power plant. Depending on design, these various steam raising systems nevertheless require very careful operational study. CCGT computerised systems thus now necessarily integrate GT operation with HRSG and STG plant operations together with all BOP systems.

Direct spray water de-superheaters: Since these de-superheaters spray water directly into the steam flowing to the STs the danger ofwater-logging the de-superheater is ever present.

Water-logging a de-superheater in the direct steam flow path to an ST can be extremely damaging – the more dense water, at high impact velocities, would not only thermally shock turbine blades and discs (water being ‘colder’ than steam for any given pressure) – the greater density water is sufficient to mechanically slug and break-off turbine blades. Turbine shaft distortion and ‘hogging’ and ‘quenching’ may occur to add to the considerable damage to the ST machine.

CC power plants use start-up sliding pressure regimes. Unit operations where steam pressure (and steam saturation temperatures) change in unison – (but only steam pressure is indicated) present interpretation difficulties. If the ratio of spray water flow to steam flow is excessive, a sudden change of state will occur, and the de-superheater will become instantly water logged. The water so formed flows directly into the turbine (or other steam application) associated with that de-superheater.

Ratio comparison and a safer degree of superheat methods should be used.

Since pressure drops occur in all steam systems – for HP main steam de-superheaters to HP turbine – use HP drum pressure (control-room indicated) – from the steam tables, the corresponding saturation temperature is obtained – always ensure HP de-superheater steam outlet temperature is 30–50°C above boiler drum saturation temperature.

Similarly, for reheater de-superheaters – using IP turbine exhaust pressure (control-room indicated) and from the steam tables the corresponding saturation temperature is obtained – always ensure – reheater de-superheater outlet steam temperature is 30–50°Cabove IP turbine exhaust saturation temperature.

This phenomenon is evident on all CCGT HP, IP and LP steam systems. For operational safety, always use two methods – de-superheater steam and water flow ratios and degree of superheat above saturation temperatures should be utilised (Tables 9 and 10).

Item

Check/s

Actions/comments

HRSG inlet water

condensate and feed flow

condensate and feed flow – monitor flow rates at all loads – above normal average flow rates or excessive flow rates will indicate water leakage and/or tube leaks in HRSG economiser, evaporator or superheaters

HRSG

boiler drum water level/s – particularly if there are multiple steam pressure systems

must constantly control boiler drum water levels– at all times – at all loads – in all circumstances  high/high and low/low drum water level – leads to generating unit tripping and complete loss of power generation  high drum water level can also lead to water carry over into superheater and then on into ST. Extremely damaging  high drum water levels can also exacerbate silica carry over into STs. At HP, silica is soluble in steam and can deposit and change profiles in ST – lessening efficiency – silica deposits form very hard, difficult to remove, scales in turbines

Table 9: ST feed & condensate flow checks

Item

Check/s

Actions/comments

HRSG

boiler drum water level/s – if there are multiple steam systems

must constantly control boiler drum water levels – at all times – at all loads – in all circumstances  falling boiler drum level (inability to maintain drum level) usually indicates severe economiser, evaporator or superheater tube leak/s … or loss of boiler feed water pumps for whatever reasons. Anticipate imminent machine computer shut-down

HRSG

direct contact, spray water de-superheaters

can be very dangerous – must monitor and control – using extreme care and intelligence … applicable to the boiler pressure and saturation temperature at the particular sliding pressure time of operation  sliding boiler pressure conditions – means sliding boiler saturation steam temperature conditions. Use the ratio of de-superheating spray water flow to steam flow  also use degree of superheat methods to constantly regulate de-superheating spray water flows to lower steam temperatures  Dramatic falls in de-superheater outlet steam temperatures means water-logging and water carry over into STs and/or co-gen steam applications  anticipate imminent machine shut-down

HRSG and BOP chemistry

condensate and feed water and boiler water chemistry surveillance

specialist water chemistry surveillance – chemistry group will make recommendations for all monitoring, surveillance, dosing, conditioning and reporting on site

boiler water chemistry

specialist subject – follow HRSG OEMs and all station chemistry boiler water recommendations

HRSG evaporator section forced circulation systems

depending on designs – the off-load and in-service operation of HRSG evaporator section forced circulating pumps must be in accordance with pump OEM and HRSG OEM's instructions

Table 10: HRSG boiler drum checks: GT section 6: GT exhaust to atmosphere damper

See Table 11.

Item

Check/s

Actions/comments

depending on design – open cycle stack damper positioning

mechanical (spade) damper

depending on design – for small-to-medium MW output range GT units, this damper is manually positioned and must be correctly secured before GT/HRSG/generator operation is normally undertaken

electrically operated damper

when HRSG is in operation – damper is electrically interlocked for opening when HRSG is to be taken out of service, thus leaving GT/generator running in open cycle mode  these dampers must be cooled (usually cooling air) when in closed position

Table 11: HRSG stack damper position checks

GT section 7: high-speed gearbox (if applicable)

See Table 12.

Item

Check/s

Actions/comments

gear ratios

aero-engine derivative turbine drive inlet speeds to gearbox can be very high, circa > 12,000 rpm or greater – however, gearbox outlet speed is synchronous speed  50 Hz, alternating current (AC) generator–exciter unit speed is 3000 rpm from gearbox  60 Hz, AC generator–exciter unit is 3600 rpm from gearbox

depending on design – gear ratios to accommodate high aero-engine inlet speeds means more than just one gear train inside the gearbox. Designs in order to keep the physical size of the gearbox within reasonable dimensional limits  gearboxes are lubricated by GT generator lubricating oil system. gearbox design may incorporate oil sumps with oil level indicators ……… carefully monitor  gearbox vent vapour emissions and vapour generation treat with urgency. Vapourising oil leads to the formation of oil sludge and gums – investigate soonest  excessive vapour generation can lead to vapour explosion within gearbox. Treat with urgency/emergency  gearbox bearing oil inlet and outlet temperatures, together with gear box (G/B) bearing metal temperatures and vibration must be monitored and trended

gearbox bearings

oil temperature to or before the bearings

problems with oil-cooling system must be dealt with and treated with urgency – oil temperatures must be reduced to normal levels quickly – all rising lube – oil temperature situations must be treated with urgency

bearings

bearing oil and bearing metal temperatures

watch and maintain bearing oil inlet and outlet plus bearing metal temperatures at their normal levels  gearbox lube – oil, bearings and vibration level complications must be dealt with and treated with urgency

Table 12: GT unit gearbox checks (if installed)

GT section 8: generator and exciter units

Essential basics: Two types of generators are used:

  • A. Air-cooled generators: These generators use air-cooled exciters.
  • B. Hydrogen-cooled generators: Also use air-cooled exciters.

Note: Air-cooled exciters are generally used in conjunction with both types of generators; only in exceptional circumstances are hydrogen-cooled exciters specified and used:

  • A. Generally, GTG–exciters unit sizes below 60 MW are air cooled. There are larger air-cool generator installations but they are used to a lesser extent. There are various designs; however, the filter/cooler installations are generally below the generator–exciter units and consist of inlet air filters and water coolers.
  • B. In general, for generator sizes above 60 MW – hydrogen-cooled generators are used. Historically, since hydrogen cooling came after air cooling, generator designs became very much more compact (to minimise expensive hydrogen consumption) – hydrogen cooler installations became an integral part of the overall generator casings.

Cooled hydrogen from the coolers is circulated into and throughout the generator stator and rotor windings; heated hydrogen is channelled back via generator casing internal ducting to water coolers before re-circulation into the generator stator and rotor windings.

Hydrogen has to be sealed within the generator casings and this is accomplished by hydraulic sealing arrangements utilising turbo-generator lubricating oil.

The essential basics and more information concerning generator and exciter units are listed in the final Part 4 of this overview specifically dealing with generator/exciter units.

Go to the profile of Doug Hutchinson

Doug Hutchinson

Director, Power Generation Services Pty Ltd

Career achievement, - working as Power Company Operations Manager on the US$ 540 million, 700 MW, joint venture, Shajiao 'B' power generation project in Guangdong Province PRC, the World Bank / IFC came to study the project and Doug was later asked to write and present a paper to The World Bank / USAID organisations for their " Private Sector Power in Asia" conference held in Kuala Lumpur, Malaysia, on 27 - 29 October 1992 covering the private power experience in China. Author – Central Electricity Generating Board – A Method Study Approach to Power Station Operation. Author - IET Eng/Ref - Overview, Asset Management, Gas Turbine Power Stations. STEM Ambassador – UK

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