Design and Troubleshooting of Boilers - Case Studies

K.K. Parthiban and K.K. Jhunjhunwala

Yash Papers Ltd., P.O. Darshan Nagar, Dist. Faizabad-224 135 (U.P.)

• ABSTRACTThe article is focused towards the author's experience in the design and trouble shooting of Boilers relatedto the principle of natural circulation. Inadequate circulation causes tube failures. Poor circulation in aboiler may be due to design defect or improper boiler operation. In this paper, the factors affecting thecirculation are summarized. Six case studies are presented.

INTRODUCTION

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Principle of natural circulation

Boilers are designed with Economiser, Evaporator andSuperheater depending on the design parameters.Economisers add sensible heat to water. Theeconomiser water outlet temperature will be closer tosaturation temperature. The water is forced throughthe economiser by the boiler feed pumps. Superheatersadd heat to steam. That is the heat which is addedto steam leaving the Boiler steam drum/Boiler shell.The steam passes through the superheated tubes byvirtue of the boiler operating pressure.

Evaporators may be multi tubular shell, Waterwalltubes, Boiler bank tubes or Bed coils as in FBC boiler.In evaporators the latent heat is added. The additionof heat is done at boiling temperature. The flow ofwater through the evaporator is not by the pump butby the thermosiphon. The density of the water,saturated or subcooled is higher as compared to thewater steam mixture in the heated evaporator tubes.The circulation is absent once the boiler firing isstopped.

Boiling mechanism

There are two regimes of boiling mechanisms, namely,the nucleate boiling and the film boiling. Nucleateboiling is formation and release of steam bubbles atthe tube surface, with water still wetting the surfaceimmediately. Since the tube surface temperature iscloser to saturation temperature, the tube is alwayssafe against failure.

Film boiling is the formation of steam film at the

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tube surface in which the metal temperature risessharply. This leads to instantaneous OJ; long termoverheating of tubes and failures. Film boiling beginsdue to high heat flux or low velocity or inclined tubes.

Circulation ratio/number

The flow of water through a circuit should be morethan the steam generated in order to protect the tubefrom overheating. The boiler tubes, its feedingdowncomer pipes, relief tubes/pipes are arranged insuch a way that a/desired flow is obtained to safeguardthe tubes. The ratio of the actual mass flowthrough the circuit to the steam generated is calledcirculation ratio.

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Total Flow Through the CircuitCirculation ratio = ----....,.....,.:..--------

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Steam Generated in the Circuit

Depending on the Boiler design parameters andconfiguration of the boiler, this number would beanywhere between 5 and 60. In low pressure boilers,this number is on the higher side as the densitydifference between water and steam is high.

What if the ctrcutattori ratio is less than thatrequired minimum?

Tube deformation/leakage failures/tube to fin weldfailures take place. The Failure mode varies dependingupon the flow, heat input, tube size, boilerconfiguration and water quality.

• Wrinkles seen in tubes

Bulging of tubes

• Wrinkle formation and subsequent circularcrack

IPPTA J., Vol. 14, No.1, March 2002 39

Downcomer

Steam drum

Heat inputtofurnace tubes

Fig. 1Flow due to density difference

Heavy water side scaling inside tubes.

Corrosion of tubes.Prolonged overheating and irregular cracks ontubesSagging of tubes if orientation is horizontal!inclined

• Tube to fin weld crack

(See Fig. 2 for the illustations.)

Factors which affect circulation

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Number of downcomers, diameter, thickness,layout

Number of downcomers are selected depending uponthe heat duty of each section of evaporator tubes.Depending on the length of the distributing header,more downcomers would be necessary to avoid flowunbalance. It is desirable to keep the bends andbranches to a minimum so that the pressure drop isless. The selection of downcomers is so done to keepthe velocity less than 3 m/s.

Heated downcomers

In some boilers the downcomers are subject to heattransfer, e.g. rear section of boiler bank in Bi drumboilers .. The circulation pattern in these boilerevaporator tubes is a function of heat transfer. In case

40 IPPTA J., Vol. 14, No. ~;.Mal:cfl 2002

of heated downcomers, burning of tubes may takeplace if the design is defective. There could bestagnation of water in some tubes depending on theheat pick up.Downcomer location and entry arrangementinside the drum

Depending on the Boiler configuration downcomersmay be directly connected to steam drum or else tomud drum. One should ensure that the entry of sub-cooled water is smooth into the downcomer. Adowncomer directly connected to steam drum isvulnerable to steam bubble entry into the downcomer.In such a case the circulation is affected. Instead ofusing big pipes, more number of smaller diameterpipes would avoid this. Vortex breaker would benecessary to avoid steam entry into the down comerpipe. In case a set of bank tubes are used for takingwater to mud drum one should ensure that the steamdoes not enter these tubes during water levelfluctuation. Proper baffle plates would be necessaryto avoid mix up of steam water mixture from riserssection to downcomer section. Downcomers takenfrom mud drum are very safe. An obstruction in frontof downcomer can cause the poor circulation inevaporator tubes.

Arrangement of evaporator tubes

The circulation in each evaporator tube is dependenton how much it receives heat. If there is non-uniformheating among evaporator tubes, one can expect non-uniform flow. At times, even flow reversal can takeplace. In some situations, the water may becomestagnated leading to water with high TDS or high pH.Localized corrosion of tubes would occur.

Improper operation of boiler

Depending upon the boiler capacity, there may benumber of burners/compartments in a boiler. This isrequired in order to achieve the boiler turn down inan efficient way. In FBC boilers number ofcompartments are provided for turn down. Operatingonly certain compartments all the time would causestagnation of water in unheated section of bed coils.The concentrations, dissolved solids and pH could befar different from the bulk water chemistry. This leadsto corrosion of boiler tubes. Similarly, operating sameburner would heat the evaporator tubes in non-uniformway leading to different water chemistry in unheatedsection of furnace tubes.

Feed pump operation

In low - pressure boilers, (pressure below 21 kg/em-g),the feed pump on I off operation is usually linked to

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Tube sagging Fin cracking

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Wrinkles seen in tubes Caustic gouging

Bulging of tubes Cracks on tube

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Circumferential cracks in tubes Heavy scaling in tubes

• Fig. 2 Illustrations showing tube failures

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level switches in the steam drum. When the pulp isin off mode, it is likely that the steam bubbleswould enter the downcomer tubes and cause lossof circulation.

Arrangement of evaporative sections and theinterconnections between sections

In certain configurations of boilers it is possible toobtain better circulation by interconnecting a well -

IPPTA J., Vol. 14, No.1, March 2002 41

Feed nozzle

Downcomer

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Riser

Fig. 3 Case-1

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heated evaporator section to poorly heated evaporatorsection. It would be necessary to separate the poorlyheated section if it lies in paralel to well heatedsection. The downcomers and risers are to be arrangedseparately so that the reliable circulation can beensured. This principle is called sectionalizing forreliable circulation. The inlet headers/outlet headersshall be partitioned for this purpose. However, it isdesirable to arrange the evaporative surface in such a

Blow down stub

Feed distributor

. Fig. 4 Case-2

42 IPPTA J., Vol. 14, No.1, March 2002

way that heat flux and heat duties in various circuitsare more or less same. If tubes are inclined close tohorizontal, the steam separation would take placeleading to overheating of tubes.Number of risers, pipe inside diameter, bends,branches

Number of risers are so selected that the velocityinside the pipes would be 5-6 m/s. The number ofrisers are selected in such a way that the flowunbalance is minimum. It is preferable to adopt longradius bends to keep the pressure drop to minimum.The number of bends and branches should be kept asminimum as possible as these elements contribute forhigh - pressure drop.Arrangement of risers in the drum

The risers are arranged in such a way that the pressuredrop is minimum. The baffles are spaced apart to keepthe obstruction to flow minimum. Instead ofterminating the risers below the water level in thedrum, it would be better to terminate above water levelin the steam drum as it allows free entry.

Feed distributor inside the steam drum

BaffleFeed distributor shall be arranged in such way that thesubcooled water enters the downcomer section. Thiswill ensure that the good hydrostatic head is availablefor circulation.

Drum internal arrangement

Drum internal parts such as baffles, cyclone separator

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Flow obstruction

.-Fig. 5 Case-3

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also form part of the natural circulation circuit. Thebaffles are arranged in such a way that the steam wouldrise easily to the steam space without much resistance .High - pressure drop in the drum internals will retardthe flow through evaporator tubes.

Siagging of furnace tubes

The design of the furnace shall be in such a way thatthe slagging of the fuel ash is avoided. Slaggingretards the heat transfer to tubes and thus the drivingforce for circulation will come down. At locationswhere the tubes are clean, this would lead tooverheating of tubes. If unavoidable, soot blowersshall be so arranged that the uniform heat flux toevaporative sections is not hindered.

Critical heat flux, Allowable steam quality,recommended fluid velocity

In the design of furnace, the heat flux should not behigher than a limit beyond which the tube will burn.Several correlations are available on this. In a circuitthe steam produced divided by the mass flow wouldbe the quality of steam produced in the circuit. Theallowable steam quality has been found to bedependent on the heat flux, mass velocity and thesteam pressure.

Even after ensuring that the heat flux and steamquality are safe, the entry velocity is important toavoid departure from nucleate boiling. For verticalrising circuit, the velocity is in the range of 0.3 mlsto 1.5 m/s. For inclined circuit, the velocity shall be

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in the range of 1.54 mls to 3 m/s.

Analyzing for boiler water circulation

In a circuit, the circulation takes place due todifference in density between the cold water in thedowncomer circuit and the density of steam watermixture in the evaporator tube. The flow will increaseas the heat input is more and the density of watersteam mixture decreases in the evaporative circuit. Butpressure loss in a circuit rises as the flow increases .Hence there will be a point of balancing at which timethe pressure loss is equal to the head. In order toimprovel retard the flow, the circuit may be rearrangedduly considering the above discussed factors. UsingMSEXCEL, practically any circuit can be analyzed forthe circulation .

CASE STUDIES

This boiler (Fig. 3) was converted for fluidized bedfiring. There were wrinkle formations in bed coiltubes. It was felt that the Downcomer and risers wereinadequate and several modifications were done inorder to reduce the pressure drop in the circuit still thefailures continued. Suspecting boiler expansionproblem, the refractory work was reconstructed withadequate provision for expansion. Yet the failurescontinued. The two drums were provided with feednozzles at dished ends with separate nonreturn valves.It was noticed that the feed water was not going into

Heated downcomers

Fig. 6 Case-4

IPPTA J., Vol. 14, No.1, March 2002 43

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Downcom

Fig. 7 Case-5

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one of the drums, as the NRV was defective. It ispossible that flow reversal was taking place in thedowncomer in the drum where the NRV was notfunctioning. The NRV at each steam drum inlet wasremoved and a common NRV was provided in the feedline. Also a feed distributor was added in each drumto distribute the water to downcomer area. This waythe flow reversal in the downcomer was eliminated andthe failures stopped.

The bottom rows of the bank tubes of this crosstube boiler were sagging. There were no druminternals. Feed distrubutor was added to improve thecirculation. Further in order to have saturated waterinto the downcomer baffles were added in the steamdrum to promote circulation The blow down stub wasvery close to the bottom row of tubes. Continuousblowdown was recommended so that loss ofcirculation could be averted.

Case 2

In this case, Fig. 4, the water wall and bed coils werefailing after bulging and overheating. Thick roughedge cracks were observed wherever the failure tookplace. There were several locations at which thefailures had taken place. There was severe scaling inthe boiler. Hence the water quality was suspected for

44 IPPTA J., Vol. 14, No.1, March 2002

a long time. By removing the flue tube immediate tothe down comer, the failure stopped.

Case 3

Similar failures Fig. 5 were noted when there was lotof accumulation inside the headers due to improperpost cleaning operation after a chemical cleaning ofthe boiler.

Case 4

The boiler Fig. 6 was provided with heateddowncomers. There were no baffles inside the drumto separate the steam water mixture from downcomersection. When the load in the boiler increased beyonda point the downcomers started bursting. This provedthe possibility of steam water mixture entering thedowncomers. Boiler drum internals with cycloneseparators were added.

Case 5

The above is a Fluidized bed combustion boilerFig. 7 with three compartments. A pin hole failure wasreported in the 12 O'clock position of the bed coiltube. On cutting the tubes, the inside was found tohave gouging mark for the throughout the inclinedportion of the tube. Several adjacent tubes are

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Corrosion failure due to caustic

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Idle compartment

• Fig. 8 Case-6

•inspected with D meter. Four adjacent tubes showedless thickness 12/0-clock position. The tubes werecut, inspected and these tubes were also found to havethe same marks as the leaked tube: On suspicion, thesymmetrical tubes about the boiler axis were alsochecked with D meter. The tubes were found to havesimilar gouging attack. The boiler water log sheetssince commisioning were analyzed and found that thewater chemistry had deviated in the past three months.The boiler was operated at pH of 11, resulting in freehydroxide. The water inside the idle compartment wasstagnant, as the compartment was kept idle. Causticattack had been the cause of failure. Customer wasadvised for alternate activation of compartments sothat the circulation in all tubes would be good. Theabove case is clearly a circulation - related failure dueto operational defects.

Case 6

The illustration shows a boiler Fig. 8 converted forFBC firing. In this, boiler vibration of riser tubes wasexperienced. Even after a snubber support wasprovided, the vibration continued. The circulationcalculation showed a velocity of 7 mls in riser tubes.The vibration problem vanished after one of the riserswas removed. The velocity in the riser was thenestimated to be less than 6 m/s.

CONCLUSION

The design of the boiler is not necessarily such a merecalculation of heat transfer surfaces. It is much beyondthat. One such subject of importance is undoubtedlythe circulation.

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Solutions to mill problems, troubleshooting etc. are invited for publication InIPPTA Journal.

IPPTA J., Vol. 14, No.1, March 2002 45