Track 3 – Session 4: Combined cycles, Combustion Turbines, Steam Turbines and Generators Atlanta, Georgia, USA - June 16-19, 2003

Elected “The Best ASME Paper” Of 2003 Track 3 ASME papers

Paper Number IJPGC2003-40114

CYCLING TOLERANCE - NATURAL CIRCULATION VERTICAL HRSGS

Ir. Pascal Fontaine Senior Engineer CMI – HRSG Avenue Greiner, 1 4100 Seraing - Belgium [email protected]

ABSTRACT The US market is currently making a double jump in its HRSG requirements. Heretofore, HRSGs were used largely in industrial size cogen applications. According to the PURPA (Public Utility Regulatory Policy Act), public utilities were required to purchase that electric power generated in excess of the steam host's needs. Thus, HRSGs were relatively small and operated under constant conditions. Now, HRSGs are much larger (utility size) and also more complex due to the introduction of triple pressure plus reheat behind powerful heavy duty gas turbines. With the onset of deregulation and consequent merchant power, combined cycle plants are now required to supply electrical power to the grid as and when needed with consequent day/night and weekday/weekend cycling. Those merchant plants have to come on and off line with minimal notice and be run sometimes at partial loads. Even units which were originally designed for base load are all eventually forced to cycle as new more efficient power plants are built. Thus, substantial changes in basic HRSG design are needed to cope with these changes. Coincidentally, the types of service projected for USA HRSGs have been in effect in Europe for over two decades. For this reason, European HRSG manufacturers/operators have adopted cycling tolerant Vertical HRSGs based on designs which permit the tubes to expand/contract freely and independently of one another, as distinguished from the more rigid horizontal gas pass design. Thus, fatigue stresses related to load following swings are minimized. This is just an illustration of the specific features of the Vertical European HRSGs for minimizing damages due to cycling related fatigue stresses. Vertical HRSG design shall be considered not only in terms of smaller footprint, but also as a solution to cycling related problems.

As generally recognized, the cycling criterion is an integral part of HRSG design. This paper presents solutions to HRSG design issues for cycling tolerant operation. It relates to published data on problems observed with cycling Horizontal HRSGs, and it describes how these problems can be overcome. Concepts, design features and calculation methods applied to cycling tolerant HRSGs are reviewed in detail. Vertical HRSGs have been criticized because of their need for circulation pumps. Interestingly, the need for such pumps was eliminated a decade ago, with the advent of natural circulation for Vertical HRSGs up to 1800 psia (124 bar A) operating pressure. KEYWORDS Cycling; Vertical HRSG; start-up curves. INTRODUCTION A recent study published by Power magazine [4] reveals that the overall cost of a single cold plant start-up could range between 70 000 $ and 240 000 $ including direct and indirect repair costs. This is quite dramatic. This paper focuses on HRSG cycling related problems as reported mainly for daily start-ups rather than load following. Reports of those problems have been published by TETRA Engineering [2] and EPRI [3]. Also, operator feedback from HRSGs’ Users Group and published [11] in USA have been useful in preparing this paper. Herein a way for addressing cycling issues is presented. The article is structured as follows. First, construction features for HRSG cycling tolerance are reviewed. Second, HRSG fatigue analysis using the German code TRD will be mentioned. Finally, HRSG operating procedures and adequate water chemistry survey methods for cycling duty are presented.

1

Copyright © 2003 by ASME

CONSTRUCTION FEATURES FOR HRSG CYCLING

R

F

Top supported vertical HRSG design The basic concept of the European Vertical HRSG design is to have horizontal tubes which are top supported from the boiler steel structure. This allows free thermal expansion downwards. The heat exchanger modules are suspended one to another by suspension tube sheets. Load of pressure parts is transferred the steel structure at the boiler outlet (cooler part). Thus, the Vertical HRSG design allows vertical thermal growth with no restraint (Fig.1).

Thermal Expansion F: to Front D: to Bottom R: to Rear

D

Fig. 2 - Heat exchanger construction analogous to flexible trombone. Header box Headers are located outside the gas flow. They are within nearly gas tight header boxes on either side of the Vertical HRSG ducting (Fig.1). As headers are not exposed to gas flow, they will warm up and cool down under the steam media heat transfer. Thus, the thick header at the outlet of the HP superheater is somehow protected against the rapid gas temperature rise during start-up.

Outlet Duct Fix Point

Tube to Header Connections

ECONOMISER

EVAPORATOR

SUPERHEATER

IN

Thermal expansion

Inlet Gas Duct

Typically for Horizontal HRSGs, rigid tube/header connections do not allow for relative movements when tubes expand/shrink [2][6]. In contrast , the Vertical HRSG tubes are bent at the tube/header welds thus allowing for better tube linear expansion (Fig. 3). Also, as no intermediate headers are required, the bottom header is the drainable lowest point, and the top header is the ventable upper point of the complete heat exchanger.

Fig. 1 – Schematic view of the Vertical HRSG top supported. Serpentine tube arrangement Typically, hole diameters in tube sheets are 0.078 inches (2 mm) larger than external finned tube diameter, thus allowing tubes to move on their supports. Tube sheets themselves are hung from one another allowing free vertical and lateral thermal expansion. Such "three-way" free expansion is the key to flexibility for the Vertical HRSG. Headers are mounted on either end of each exchanger module. The serpentine tube arrangement is fully flexible (analogous to trombones), allowing each tube to expand independently of its neighbors without header constraints (Fig.2). That is particularly advantageous when 2 economizers share the same inlet/outlet headers because each HP/IP section can expand/shrink independently. Otherwise, severe undue tensions would occur when the first cold feedwater is admitted in the HP header section. Also, it is to be noted here that any finned tubes rubbing on their support tube sheets will neither affect HRSG performance, nor tube thickness integrity as fins protect tubes from wearing. To minimize fin abrasive wear, the fin density could be increased at those locations where the tubes are lying on their supports.

Fig. 3 – Tubes/headers flexible connection of Vertical HRSGs Finned tube diameter Compared to the usual American standard up to 2 inch diameter tubes, Vertical HRSG manufacturers use smaller tube diameters standardized on either 1 ½” (38 mm) or 1 ¾” (44 mm) depending on pressure drop. By using smaller tube diameters, wall thickness is also reduced for the same design conditions, thus reducing thermal stresses under cycling. This is particularly important for the HP superheater outlet working at the highest pressure and temperature and also subject to the

2

Copyright © 2003 by ASME

greatest temperature variations. Finned tubes are seamless. Also, the fin height of only 3/8” (15 mm) increases fin efficiency at the fin tip. As a consequence, the overall heating surface is more effectively used, This is the reason why the Vertical HRSG heating surface can be smaller than that for the Horizontal HRSG for the same boiler performance. In addition, the The Vertical HRSG design features vertical fins on horizontal tubes, and therefore less subject to gas side deposition and fouling. Tube & Header materials In Europe, boiler pressure parts are often designed according to ASME code. As ASME leaves the choice, boiler manufacturers prefer greater creep strength grade material SA 213 T91 over SA 213 T22. This material selection is particularly important for the highest temperature superheater parts in order to reduce both thickness and tubes/header weights. Although P91 is more expensive and requires great care in welding, this single step in material selection reduces those thicknesses by a factor of two. The HRSG cyclic capability is then greatly improved by both comparable thickness between welded walls and thinner walls.

Fig. 4 - Partial penetration fusion weld detail (socket welding)

Flexible interconnecting piping Interconnecting piping between headers and drums must be flexible enough to accommodate tube differential temperatures. In this respect, the Vertical HRSG features drum locations separate from boiler pressure parts on the steel structure extension. Consequently, several directional changes in the interconnecting piping are needed which help to enhance piping flexibility. This extra flexibility is demonstrated by piping stress calculations carried out at the engineering stage. Also, along with traditional load cases which are base load thermal expansion combined with wind or seismic forces, isometrics stress calculations also include a dedicated load case for startups and/or shut downs. Indeed, experienced differential expansion during start-up and shut down can be completely different and can even be the design load case for pipe supports and guides. This point is particularly important when internal or mixed internal/external insulation is used as it is mainly the case for Horizontal HRSGs. Full penetration welding Although the ASME code accepts partial-penetration welds (Fig. 4), full-penetration welds (Fig. 5) on at tube/ header joints of superheaters and reheaters are preferred by European boiler manufacturers. Indeed, full penetration welds are not subject to stress-concentration effects at their roots and therefore greatly resist cycling stresses and related fatigue. Therefore, full fusion penetration is highly recommended for superheater and reheater sections where the greatest expansion growths are experienced. However, experience has also demonstrated that partial welding remains quite acceptable for economizer and vaporizer tubes.

Fig. 5 - Full penetration fusion weld detail (butt welding) Welding procedure specifications (WPS) Mini cracks at welding locations which may exist from manufacturing process of boiler pressure parts are likely to propagate to complete tube failures. The probability of such progressions is much more important under cycling duty than under base load. As it is known from experience, modified 9% Cr material P91 is prone to form cracks if it cools down too quickly. Over this last decade, P91/T91 material has been used extensively and a lot experience has been accumulated. Attention to quality control and Welding Procedure Specifications (WPS) shall be enhanced for P91, especially if the boiler is manufactured overseas. In particular, welding preheating and post heat treatment according to ASME prescriptions shall be followed. Superheater drainage Condensation occurs in superheaters/reheaters during purges of the HRSG prior to gas turbine ignition. This is because the exhaust gas temperature is below the saturation temperature. Quantities of condensate can be substantial during hot starts. When condensation occurs, water droplets are created as a mist in saturated steam. Those droplets flash on the still hot wall. Due to the latent heat transfer, the internal heat transfer

3

Copyright © 2003 by ASME

coefficient suddenly increases by a factor of 20. This creates an immediate intense heat flux which cools down temperature wall to saturation temperature within a few minutes. Afterwards, the outlet header may be flooded with condensate water. This would result in an internal heat transfer coefficient significantly higher than otherwise with steam only. Condensate quenching in hot superheaters induces very severe low cycle thermal stress fatigue. Tubes shrink at various rates and locations inducing unusually high stresses on tube/header welds, and the outlet headers are severely thermally shocked (thick walls). If the plant is operated at 2 shift duty, such repeated cycles will rapidly limit the material life expectancy. Therefore, water quenching during hot start-ups must not be overlooked as it can be the cause of tube failure in as few as 100 start-ups. Efficient means to collect condensed water shall be provided. The Horizontal HRSG is equipped with complex manifolds on numerous bottom intermediate headers. Those headers collect water as it passes before it is carried away with steam in the vertical tubes. Furthermore, as those intermediate headers are all low points, they could tend to be flooded if the drainage system fails to clear condensate efficiently [13]. By design, the Vertical HRSG configuration is good in this regard. First, its flexible serpentine tube arrangement does not require any intermediate headers, and secondly the lowest point of the superheater is located in the downstream main piping instead of being inside the superheater. Thus, any condensation will be naturally drawn outside with steam flow up to this collecting drain point. This drain at the superheater outlet shall be generously sized and equipped with enlarged collection pots (6 inch diameter or greater) configured as a water trap (Fig. 6). Unlike the Horizontal HRSG, development of an obstructing water plug is impossible with the Vertical HRSG design.

located at about 20 ft above ground level, thus providing ample space for installing and operating the drain systems to the blow down tank at ground level with a continuous downward slope versus steam flow. By construction, this is a feature offered by the Vertical HRSG design. This insures the efficient drainage of superheater/reheaters and drums. This feature is particularly effective during air purging performed after the GT trips when large quantities of steam could otherwise condense in the superheater. The Vertical HRSG is able to easily eliminate the related condensate water without sub-cooling the outlet header and without overstressing the tube/header welds. Drainage of this condensed water is done by gravity to the blow down tank. This tank is simply located on the ground level and does not require water extraction pumps. Of course, automated drain valves from drums and superheaters are motorized on the manifolds of the blow down tank (Fig. 7).

Fig. 7 – Blow down tank 20 ft below boiler pressure parts Start-up vent valve

Fig. 6 – Superheater drain according to ASME TDP-1-1998 ‘Recommended Practices for the Prevention of Water Damage to Steam Turbines in Electric Power Generation’ Blow down tank In case of the Vertical HRSGs, the lowest point of the boiler pressure parts is the superheater outlet header. It is typically

In case of the Vertical HRSG, no restrictions are imposed on the gas turbine loading rate, but the HP drum pressure is rather controlled using dedicated motorized start-up vent lines equipped with adequate controls. Typically, the Vertical HRSG features sizing of those lines at 4 inches on HP and 6 inches on hot reheat circuits. This vent size is calculated by simulation of the HRSG start-up performed at the engineering design stage in accordance with specifically established plant start-up procedures. The purpose of the start-up vents is to evacuate the initial steam production and to control the pressure/ time gradient build up prior to dumping it to the condenser through steam turbine by-pass stations. Typically, the condenser is ready when the HP pressure reaches 116 psia (8 barA ). Then, a start-up ejector can be put into service for about 3 minutes to reach the required minimum vacuum to operate the steam turbine control by-pass stations.

4

Copyright © 2003 by ASME

Drum diameter As thermal stresses during start-up are proportional to the square of the drum wall thickness which in turn is proportional to diameter, smaller drum diameters are better for cycling, everything else being equal. In this regard, internal steam drum dryers shall be as compact as possible in order not to increase drum diameter. European boiler manufacturers have adopted the compact demister mesh instead of cyclones. Comparing the 2 solutions (mesh versus cyclones), the demister mesh is only 12 inches (300 mm) high for the same de-entrainment efficiency, but penalized by a slightly higher pressure drop (Fig. 8).

wall aerated part of the drum is about 10 times higher than the water phase heat transfer coefficient of the lower part. As the upper drum surface is hotter than bottom drum parts, the drum tends to be deformed into a banana shape. This phenomenon is observed only during start-up as otherwise temperatures are uniform in the drum. In case of the Vertical HRSG, drums are supported at 2 points using sliding pads (Fig. 9). This allows this natural phenomenon to occur without restraint as well as allowing lateral and longitudinal thermal expansion of the drum.

HP drum

To superheater Wiremesh

Perforated plates High High level High level

Normal level

Low/Start-Up level Low Low level From evaporator From economiser

To evaporator

Fig. 8 – Compact dryer mesh reduces drum diameter Drum material Start-up ramp rates are limited by the wall thickness of HP drums. Typically, drum wall thickness is up to 6 inches (150 mm) for a design pressure of 2320 psia (160 barA). Inasmuch as the overall water inventory of the Vertical HRSGs is less than for equivalent Horizontal HRSGs, the surging water from the evaporator is also reduced, and consequently the drum itself can be smaller, inasmuch as it is designed based on water swell. As a result, temperature/ time gradients can be steeper because of thinner drum walls and still be consistent with ASME code and design criteria. Also, use of material SA302 Grade B (Re=22770 psi – 157 N/mm2 ) for the drum shell and head allows for slightly thinner walls than the usually widely used SA299 (Re = 21320 psi – 147 N/mm2) while still meeting the ASME code. Drum support During start-up, one can observe the so-called banana shape effect on the long HP drum. The condensing heat transfer coefficient taking place where steam condenses on the upper

Fig . 9 – Drum supports of Vertical HRSG on 2 sliding pads Warm ducting The Vertical HRSG ducting is designed based on warm casing (externally insulated). It is hung from the top steel structure with free downward thermal expansion. This is also the case for the pressure parts. The thermal growth of the casing is not restrained and it expands along with tubes at the same gas temperature in both vertical and horizontal directions. The warm ducting expands vertically while vertical piping between drums and headers expands as well: piping expands in the same direction. Therefore, adverse movement between warm casing and pressure parts are much reduced as both expand in the same direction and at the same rate with reduced additional stresses during transients. Of course, the Vertical HRSG concept calls for casing guides and fixed points in the steel structure to take wind and seismic loads. Also, one header of any heat exchanger always remains free while the other is blocked in the casing. Header movements are guided/blocked in rotation or in translation on a case by case basis in order to accommodate forces from connecting pipes.

5

Copyright © 2003 by ASME

Control valve location Steaming in economizer is a generic problem of HRSGs designed only for base load (feedwater control valve located upstream of the economizer) and which are required to run at partial loads according to electrical power demand [11]. Location of the drum level control valve is important for either Horizontal or Vertical HRSGs operating under cycling. While this steaming can be accommodated (up to a certain extent) by the Horizontal HRSG featuring the last vertical row of the economizer tubes in upward direction to the drum, the Vertical HRSG is not designed to tolerate any economizer steaming. This is the reason why the Vertical HRSGs have the feedwater control valves always located downstream of economizer. This design prevents the economizer from steaming at any partial loads as the feedwater pumps will always pressurize the subcooled economizer water. At very high pressure (above 2320 psi – 160 bar), this design can be questionable as the control valve can operate under heavy flashing at partial load if a variable speed feedwater pump is not used. In that situation, the control valve can be upstream, but steaming must then be avoided with economizer recirculation pumps.

thermal shock as cold feedwater is commonly used. This cold water is typically picked up at the economizer inlet at 140°F (60°C). Indeed, this cold water would be injected into the attemperator which is located standstill in superheated steam at 1054°F (568°C). However, this thermal shock can be mitigated by using warm water instead of cold water: desuperheater water is picked up from the economizer outlet instead of the inlet. Also, although the Vertical HRSG can be either designed with insterstage attemperation according to client specification, the Vertical HRSG typically features final attemperation based on current spring loaded multi-spray atomization technology (Fig. 10). In addition to the fact that final attemperation is about $250 000 less expensive for a 3PR behind class F, it is also better for cycling duty as it eliminates two intermediate headers which jeopardize tube flexibility as discussed above.

Assisted circulation versus Natural circulation First generations of Vertical HRSG were based on the assisted circulation design. Currently, Vertical HRSGs can be designed, at the customer's preference for either assisted or natural circulation. Indeed, European boiler manufacturers have natural Vertical HRSG references in service for more than a decade. The reason for the move from assisted to natural circulation had been dictated by the worldwide power market place, requiring that the Vertical HRSG technology to be adapted to natural circulation. In terms of cycling, assisted circulation is still regarded as better performing than natural circulation because the pumping reduces the temperature differences between drum and vaporizer. On the other hand, swell effect occurs more gradually in natural circulation with less thermal shock as explained in following paragraph. The net advantage of the assisted over natural circulation is then minor. Even more so, the latest generation of the natural Vertical HRSG is designed without any start-up pumps. Instead, a low point of at least 2 meters below the inlet header is created in the vaporizer downcomers. This implies that the downcomer is rising toward the vaporizer inlet header insuring that the swell effect is always in the forward direction rather than backwards to the drum. Swell effect is discussed in depth in a following paragraph. In any event, if assisted circulation is selected by the client, the typical consumed electrical power of the HP, IP and LP circulation pumps is minimal, i.e., respectively about 120 kw, 30 Kw, 15 Kw for a HRSG behind GE 9FA. Attemperator Under cycling duty, the HRSG control system can call suddenly for an automatic attemperation on either HP superheater or reheater sections. If the thick attemperator nozzle was not initially in service, it will undergo a severe

Fig.10– Final desuperheater featuring spring loaded multi spray HRSG instrumentation monitoring Unlike traditional fired boilers, HRSGs do not typically have extensive metal temperatures measurements on headers and drums as the temperature range is much lower. No special additional instrumentation is required to monitor the Vertical HRSG under cycling duty except for metal thermocouples on the superheater tubes in case of duct firing. Cogeneration merchant plants must sometimes be duct fired at maximum rate to satisfy steam demand while the GT is at partial load according to electricity grid demand. Then, the GT exhaust gas mass flow is reduced, but the maximum firing temperature 1470°F (800°C) occurs concurrently. This puts the HRSG under unfavorable operating conditions as the superheater tubes could be ‘undercooled’ because of the reduced steam production. As superheaters are made of different creep strength materials as T91, T22, and T11, lowest alloyed materials could be subject to creep damage unless tube metal temperature is properly monitored. Also, it is to be noted that flame impingement under these conditions is more likely to occur. In any event, the limitation curve on the duct firing rate versus GT load shall be introduced into the DCS based on

6

Copyright © 2003 by ASME

boiler heat balance simulations performed during the HRSG engineering stage. Also, it is worth noting here that for such cogeneration plant applications, interstage attemperation shall be used because desuperheating is then necessary not only for steam temperature control, but also to cool down the most exposed superheater tubes. Weather Damper The weather damper is typically made of 2 butterfly flaps equipped with an electrical actuator. This equipment is located in the chimney at low gas temperature around 212°F (100°C). Interestingly, unlike the Horizontal HRSG, the weather damper (Fig. 11) is used as standard design on the Vertical HRSG because, recognizing this boiler configuration, it prevents rain from falling on the boiler tubes. The configuration of the Horizontal HRSG is different in this regard as its chimney is equipped with a drain at its bottom on ground level. A good side effect of the vertical design is that the weather damper is also used to close up the boiler during outages. Therefore, the HRSG can be kept under pressure much longer by reduced heat losses to atmosphere. Typically, an overnight shut down therefore results in a warm start-up the next morning whereas it could have cooled off to cold otherwise.

Internal boiler accessibility Beyond the fact that the Vertical HRSG is designed for cycling duty, the HRSG construction provides very easy access to boiler internal parts. Some Horizontal HRSG have little facilities or spaces to inspect header internals. Thus fatigue damage may not be evident until a crack propagates through the header wall [6]. The Vertical HRSG offers an advantage with good accessibility to the reduced number of headers which can be inspected from inside ducting without a need for cutting the outer casing. Header boxes are accessed from inside casing through dedicated visit doors provided at each interbloc. The headers are all equipped with boroscope peep holes for monitoring, thus keeping outage time to a minimum. Also, the Vertical HRSG features horizontal tubes. Thus, internal inspections can be performed by merely walking on finned tubes rather than using expensive and cumbersome scaffoldings. This facility is provided at each interbloc. Boiler stack insulation European boiler manufacturers generally insulate the HRSG exhaust stack. The chimney is externally insulated to avoid inside gas condensation during the daily start/stop. Insulation is cladded with aluminium sheets which help also to enhance the architectural aesthetics, as the boiler stack is often the predominant visual impact source of the boiler (Fig. 12).

Fig.11 - Weather damper under installation on a Vertical HRSG Warm-up burner Due to the vertical gas path feature, the Vertical HRSG can be fit with a small burner in its transition inlet duct for temperature conservation purposes. This burner would automatically switch on and off to provide enough heat to maintain a predetermined HP drum pressure during outage. Even a very small burner will provide sufficient heat, evenly distributed due to the natural draft within the serpentine tube bundles. Interestingly, this is also a very efficient means for boiler freeze protection in case of extended shutdown. For this latter purpose, a small electrical heater can also be temporarily placed in the bottom duct and can function according the same concept.

Fig.12 – Vertical HRSG feature insulated/cladded exhaust stack

7

Copyright © 2003 by ASME

8 7 6 FSR PT001A/01B

PRESS GRADIENT (BAR/MIN)

ASME considers continuous operation at design conditions, but it does not mandate assessment for fatigue analysis. European boiler manufacturers perform this analysis using the German code TRD 301 which is more stringent in terms of cycling to calculate the acceptable pressure/ time gradients. Even when the HRSG is designed according ASME, it is in addition checked for fatigue analysis using code TRD 301. Most recently, a new European code EN 12952-3 for life time computation has been introduced. Future editions of ASME should include recommendations in that regard. Input data of the TRD 301 are basically the drum diameter, material, thickness and operating pressure. Two methods can be applied, either the number of cycles is calculated for given start-up and shut down rates, or those allowable rates are calculated for a given number of cycles. Interestingly, acceptable ramp rates vary versus operating pressure (Fig. 13): allowable temperature ramp rates increase with increasing HP drum pressure. In practice, TRD 301 fatigue analysis is applied to the thick HP drum wall, but the outlet headers of RHT/SHT can also be affected by material fatigue.

No limitations are imposed on LP and IP as explained previously. Such optimized variable set pressure control during start-up is commonly used by European boiler manufacturers. SETPOINT FOR POSITIVE HP DRUM

ASME versus TRD for cumulative damage assessment

5 4 3 2 1 0 0

20

40

60

80

100

120

140

H P D R U M P R E S S U R E (B A R G ) F S R P T 0 0 1 A /0 1 B

Fig. 14 – Optimized variable pressure control during start-up Start-up time for the cold Vertical HRSG During the early engineering phase, the HRSG start-up shall be simulated by computer in order to determine its behavior and to optimize the best HRSG start-up sequence (Fig. 15). For the usual US application of a 3PR at 1800 psia (124 bar A), such typical optimized sequence of the Vertical HRSG start-up time is about one hour between the complete cold status up to full load. Typically, restrictions come usually from the steam turbine warm up gradients rather than from the HRSG.

Fig. 13 – Ramp rates according TRD 301applied for a HP drum Drum pressure control set points The calculated temperature/time gradients need to be converted into pressure/time gradients as they are more reliable/accessible controllable parameters during transients. As a result of the TRD 301 (Fig. 13), a non linear control pressure/ time gradient can be used for an optimized start-up time without consuming any extra cyclic material lifetime. This pressure control (Fig. 14) is implemented into DCS as a variable set point curve to be followed by the steam turbine control by-pass stations and by the dedicated vent valve used earlier in the start-up. It is to be noted that only the HP drum pressure needs to be controlled.

Fig. 15 – Computer start-up simulation of a cold Vertical HRSG 3P behind GT 13E2 started-up directly without by-pass stack Swell effect The so-called swell effect occurs during start-up when the first steaming takes place in any evaporator. Water above the steam bubble in formation is then expelled to the relevant drum whose capacity is always designed accordingly. However, this surge occurs differently between Horizontal and Vertical HRSGs. Indeed, as the Horizontal HRSG features evaporator tubes in parallel configuration, the drum water level will rise rather

8

Copyright © 2003 by ASME

gradually as the most exposed vaporizer tubes will swell one after the other. That means also, that the water circulation can establish itself in most exposed tubes while water remains at standstill or even flows backwards in others. In case of the Vertical HRSG, vaporizer tubes are arranged differently in serpentine pattern. A much larger amount of water is then expelled at once during swell effect resulting in a quicker drum water level increase. This means also, that natural circulation of the Vertical HRSG will establish itself thoroughly at once in all serial tubes of the vaporizer upon swell effect occurrence. HRSG air purging before start-up Prior to GT ignition, NFPA 85 mandates for an HRSG air purging to renew several times the HRSG ducting volume in order to and evacuate any unburned combustible from the previous shut down. Typically, the GT compressor is then put on start-up motor for about 8 minutes feeding the HRSG with cold air. In case the plant is equipped with a gas by-pass stack, it is advised to stop and start the GT on open cycle. Otherwise, this HRSG purging cannot be avoided, although this air purging can rather be performed at GT idle load in some cases. However, detrimental consequences of such cold air purging on a hot HRSG can be limited with a prescribed shut down procedure in case of an immediate restart. Indeed, it is recommended to ramp down the superheater outlet temperature and pressure to their minimum in a controlled fashion during the GT unloading. Thus, during the subsequent HRSG restart, the superheater will experience less thermal shock and reduced condensation. Forced cooling Within a very narrow working window, a boiler may be force cooled to enter the HRSG for making repairs. The GT compressor is then put on start-up cranker motor as above mentioned, but this time cooling will last for hours down to atmospheric temperature. As the air mass flow is much reduced compared to normal operation, and as the gas pressure drop depends on the square of this mass flow, cold air will not distribute uniformly into the HRSG bundles. This situation can be damaging for the Horizontal HRSG due to the likely gas stratification over the high ducting: as nearly no counter pressure is offered to this minimum air mass flow, cold air would preferably flow in the duct bottom of the Horizontal HRSG [13]. As the cooling gradient progressively builds up over the tall duct height, tubes and ducts will shrink at different rates creating high thermal tensile stresses on components. Such stratification as high as 255°F (123°C) depending on the steepness of the angle of the Horizontal HRSG transition duct has been reported [13]. By construction, the Vertical HRSG cannot present such gas stratification over tube length. In any event, the number such forced cooling episodes shall be limited. The plant manager shall well assess the benefits of a forced cooling for quick repairs compared to the additional stress fatigue imposed on the HRSG. If a forced cooling is deemed justified, a special procedure shall be followed by the operator to limit its consequences. This procedure shall consist mainly of first bringing the pressure down to limit water condensation, and secondly to evacuate water through superheater drain to avoid damaging water quenching.

Maintenance cooling Heavy duty gas turbines can impose a so-called maintenance cooling after shut down. In this instance also, the GT is kept on start-up cranker motor at very low speed. The purpose of such maintenance cooling is to keep the GT rotor moving in order not to leave this still hot rotor at a standstill. Otherwise, the rotor would bend on its own heavy weight. Consequences of such maintenance cooling are not as severe as the forced cooling, but still the same special procedure must be followed. Also, it can be advantageous to install a by-pass chimney of about 1 meter diameter between GT and HRSG. During maintenance cooling, this small by-pass is opened and the HRSG weather damper is closed in order to exhaust this cold air directly to atmosphere. Cycling water chemistry Boiler water quality maintained rigorously within prescribed limits shall be regarded as ‘boiler lifetime insurance’. As a matter of fact, cycling an HRSG greatly affects boiler water quality. For instance, within the first 2 hours after start-up, feedwater deaeration is not yet fully complete to reach the required 10 ppb O2. Thus, under the most common ‘two-shift’ cycling duty, feedwater oxygen content can be out of range for more than 10% of operating hours. If this situation is also combined with high feedwater conductivity and low pH, corrosion conditions are then encountered . Under continuous base load operation, it is rather easy to monitor boiler water quality and to adjust either ammonia or phosphate injection accordingly. However, due to the time delay between chemical injections and actual effect on water quality, this is much more complicated for cycling boilers as all parameters are fluctuating. Consequently, keeping cycling water chemistry within prescribed limits is a challenge. The plant chemist must attempt to limit those excursions in time and amplitude as far as possible to mitigate damaging effects. In particular, combination of poor water quality and cycling stresses promotes Stress Corrosion Cracking (SCC), and too high pH due to excess of hydrazine promotes Flow Accelerated Corrosion (FAC) occurrences. It shall be noted here that stainless steel material is more subject to SCC than carbon steel due its higher thermal expansion factor, and therefore, it shall preferably not be used for cyclic HRSG construction. Unlike some Horizontal HRSGs, the Vertical HRSG does not use stainless steel preheaters because flue gas condensation cannot be tolerated in Vertical HRSGs. Cascading blow down Usual blow down rate is typically between 0.2% and 0.5%. Thus, heat recovery by cascading blow down from HP to IP, and IP to LP is quite negligible, but it brings even more complexity to the hereabove mentioned cycling chemistry. As those water blow down concentrations have high conductivity, they could interfere substantially with drum phosphate injections. Cascading blow down is common place in US, but it is never prescribed in Europe. It could be used for an All Volatile Treatment (AVT) for HRSGs running on base load. However, for cycling HRSGs, European boiler manufacturers

9

Copyright © 2003 by ASME

recommend using independent drum blow down instead, and to go for drum phosphate treatment rather than AVT. Also, it is recommended to keep the blow downs continuously in operation rather than intermittently in order to stabilize boiler water cycling chemistry as much as possible. Load rejection When a plant is suddenly disconnected from the electricity network, power production must go down immediately as a plant load rejection is initiated. Then, the steam turbine is usually tripped and the gas turbines go on frequency control mode. Typically, the GT load is ramped down rapidly from full load to idle load to provide just enough power to the plant auxiliaries while waiting for future restart. Under those conditions, gas temperatures through the superheater could be close to or even below saturation temperature. This is a risky condition. As soon as a plant load rejection is forced, European boiler manufacturers recommend to follow an automatic procedure to decrease the HP drum pressure down to the minimum operating pressure using ST control by-pass stations. Conservation during standstill Short standstill periods are very common for cyclic HRSGs, and proper procedures must be adhered to. After HRSG stoppage, pressure will decay naturally to reach atmospheric pressure in a matter of 2 or 3 days. Upon this occurrence, it is particularly important to avoid air ingress into the boiler. Rather than opening drum vents to prevent internal depression, European boiler manufacturers recommend to cap with a nitrogen blanket under slight overpressure 17 psia (1.2 bar A). Inert nitrogen is then drawn naturally into the HRSG as temperature drops from 212°F (100°C) to 68°F (20°C). At the subsequent boiler restart, motorized drum vent valves will be automatically opened to vent nitrogen capping to atmosphere with minimum air ingress. Operator training To illustrate how important operator information can be, we have the following story. Regarding an HRSG built in 1992, the HRSG manufacturer was called upon to repair leaks on tubes to the superheater header. As this damage had already resulted from several occurrences, investigation was undertaken to determine its root cause. The conclusion was amazing. During HRSG start-up, an alarm was always set off in the control room for high steam temperature. Considering the start-up phase, this automatic alarm was quite acceptable, but the operator was untrained and not aware of this. Therefore, the automatic HRSG control logic and the HRSG protection were overridden. The operator systematically took the DCS on manual mode to control this steam temperature by opening widely the attemperator control valve. Huge amounts of cold water were then injected creating sub-cooling in the steam header resulting in tubes shrinking and severe header thermal shock. By his manual intervention, the operator was severely damaging the HRSG without realizing it. Experienced operators well aware about steam processes and start-up procedures are a valuable investment to protect HRSG durability under cycling duty.

CONCLUSIONS With the advent of cycling duty in the USA, HRSGs must be optimized differently from which prevails for simple continuous base load duty. Cycling effects on HRSG components must be duly considered by use of rigorous fatigue analysis. Fatigue damages are cumulative and cannot be reversed. If overlooked, it results in premature HRSG failures, some within months of commissioning. Flexible construction, along with HRSG start-up simulations, adequate operating procedures and material fatigue calculations generally beyond ASME requirements are keys for designing and operating cycling tolerant HRSGs. In Europe, large Combined Cycle plants had to be cycling for decades. European boiler manufacturers have designed HRSGs with these cycling issues in mind since the first HRSG more than 30 years ago. The Vertical HRSG concept is widely adopted in Europe. It was mandatory that this flexible construction cope effectively with cycling duty. ACKNOWLEDGMENTS The author wish to thank Mr Xavier d’Hubert, CMI Vice President CMI – USA and Dr. Marvin Baker, CMI office in USA, for their contribution and support in writing this paper and also introducing it to the ASME review committee. REFERENCES [1] CMI web site www.cmi.be/utility-boilers [2] David S. Moelling, P.E., Frank L.Berte, Ph.D.; Peter S. Jackson, P.E., Ph.D.; Tetra Engineering Group, Inc., Power Gen 2002 Brussels ‘Cycling Experience of Large Heat Recovery Steam Generators in the New England ISO Market’ [3] Dooley R.B., Todd A.K., Mc Naughton W., Paterson S.R., Pearson M., Shields K.J; 2002; EPRI, HRSGs Tube Failure [4] Leafton S.A.; Besuner P.M.; Grimsrud G.P. Aptech Engineering Services Inc; Platts Power Magazine; December 2002; ‘The real cost of cycling power plants’ [5] Collier J.G., Thome J.R., 2001 ‘Convective boiling and condensation’ [6] Michael Pearson, Power, February 1997, ‘Warning: cycling HRSGs can be dangerous to your health’ [7] Martinelli R.C. and Nelson D.B., 1948, ‘Prediction of pressure drop during forced circulation boiling of water’ [8] Claxton K.J., Collier J.G., Ward J.A., November 1972 ‘H.T.F.S. correlations for two phase pressure drop and void fraction in tubes (H.T.F.S. design report 28). [9] Fontaine P., 1990 ‘Modélisation Dynamique d’une chaudière de récuparation’, BSc Thesis, Faculty of Applied Sciences, University of Liège, Belgium. [10] Galopin J-F, 1996, ‘ Dynamic Constraints on HRSG drum design’ [11] Robert Swanekamp, Platts Power Magazine, October 2002, ‘Users group to publish guidelines for operation, maintenance of HRSGs’ [12] TRD 301 Code, April 1979, ‘Zylinderschalen unter innerem Uberdruck’ [13] Michael Pearson, P Eng, J Michael Pearson & Associates Co Ltd. And Robert W Anderson, Florida Power Corp, Power Magazine, July/August 2000, ‘HRSGs – Questions about condensate quenching, prestart purging’

10

Copyright © 2003 by ASME