Soot Deposits and Fires in Exhaust Gas Boiler - Martin's Marine ...

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Analogy with snow (soot deposits)Analogy with coal briquettes (ignition)General four main parametersIn a snowstorm at below-zero temperature,the snowflakes (dry soot particulates)will not easily deposit on the ground unlessthe wind (gas) velocity is reduced,as it is for example behind a fence. Thelow wind velocity will cause the snowflakesto deposit and form a snowdrift,and if the wind direction changes(higher velocity), part of the snowdriftmay move. This means that at a certainlow wind (gas) velocity, the snowflakes(dry soot particulates) will deposit.In a thaw, for example, when the snowflakesare wet (wet soot), the snowflakeswill deposit more easily, and a changein the wind direction (higher velocity) willmake only a small part of the snowdriftmove. Thus, the wet snowflakes (wetsoot) will deposit, but will do so alreadyat a wind velocity (gas velocity) which ishigher than the wind (gas) velocity for theabove-mentioned frozen snowflakes(dry soot).In general, therefore, high wind (gas) velocitiesand frozen snowflakes (dry soot) willreduce the tendency towards deposits.Igniting a coal briquette (dry soot) for a grillis quite difficult, as its ignition temperatureis rather high. On the other hand, if thebriquettes have been wetted with oil (wetsoot), the ignition temperature will be lowerand it will be easier to ignite the briquettes(wet soot). The higher the temperatureof the wetted briquettes (wet soot), theeasier they are to set on fire.So in general, the drier the briquettes(soot), and the lower the temperature,the more difficult they will be to ignite.Analogy with putting-out a fonduefire (oxygen)If the oil in a fondue pot has becometoo hot and has been set on fire, theeasiest way to extinguish the fire is toput a cover over the fire and stop thesupply of oxygen.When a soot fire occurs in an exhaust gasboiler, similar action has to be taken. Inthis case the oxygen supply is stopped bystopping the diesel engine, as the engine’sexhaust gas still contains about 14%oxygen.Wrong CorrectGiven the points discussed above, therisk of soot deposits and ignition followedby soot fires may be minimised by respectingthe following four main parameters- valid for both water and smoketube boilers:• The gas velocity in the boiler mustnot be too low,- this reduces the main risk factor forsoot deposits.• The gas temperature on the boilertube surfaces must not be too low,- this reduces the additional risk ofsoot deposits due to the formationof wet soot.• The engine smoke emission shouldnot be allowed to deteriorate,- since this would increase the tendencytowards soot deposits.• The circulation water flow velocity andratio in the boiler must not be too low- this keeps the gas temperature at theboundary layer of the boiler tubes below the ignition temperature of the soot.The first three of these parameters relateto the soot deposits, whereas the numberfour parameter relates to the risk of ignitionof the soot.SootdepositsInletpipingBoilertubesectionRecirculationof gasExhaustgasFig. 22: Exhaust gas boiler – influence of inlet pipingsExhaustgasBoilertubesectionInletpiping UniformgasvelocityRecommended boiler designcriteriaThe boiler design criteria that can berecommended on the basis of the fourmain parameters mentioned in abovesection, with due consideration for theinfluence of the low gas velocity, are thusas follows:Referring to soot deposits:a) The design mean gas velocity of theboiler should be higher than about 20m/s, but the limit may, in fact, dependon how dry and sticky the soot is (fueltype/fuel additive).18This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net
)The pinch point temperature of theboiler should be higher than about15°C or, even better, 20°C.c)The boiler’s exhaust gas outlet temperatureshould not be lower thanabout 165°C as otherwise condensationof sulphuric acid in the exhaustgas could make the soot sticky.d)The inlet piping to the boiler shouldbe designed so that the gas flow velocitydistribution is as uniform as possible,in order to avoid local pointswith a particularly low gas velocity.e)The exhaust gas design pressureloss across the boiler should be ashigh as possible - increasing the gasvelocity in the boiler. This means thatthe pressure losses in the remainingparts of the exhaust gas systemshould be dimensioned as low aspossible (large pipe diameters).f) A dumping condenser should be installedto control steam production/consumption.A gas by-pass valve installed to controlthe steam production would reducethe gas velocity in the boiler - andconsequently increase the risk of sootdeposits - and cannot, therefore, berecommended.The supplementary recommendationsbelow apply only to boilers of the watertube type:g)A by-pass duct with an automaticallyoperated on/off valve (open/closed at50% SMCR) may in certain operatingconditions be recommended for watertube boilers. If, for example, theship is often slow steaming, i.e. thediesel engine operates at low load,such an installation will prevent sootdeposits on the boiler tubes by bypassingall the gas and thereby avoidinglow gas velocities and the associatedrisk of soot deposits in the boiler.h)Automatic soot blowers for frequentcleaning should be installed in watertube boilers in order to clean the tubesof soot. The pressure of the soot blowingmedium should be as high as possibleduring the entire soot blowing sequence.As the possible steam pressure usedis only about 7 barg, and in some cases6 barg, the use of high-pressure air willbe better.i) Fixed water washing system and/ormanual cleaning at regular intervals.Water washing is performed in orderto clean the boiler completely of sootwhich has not been cleaned away bythe soot blowers. The exhaust gaspiping between engine and boiler shouldbe so arranged that the boiler can becleaned more thoroughly from time totime when the engine is stopped inharbour without the risk of flooding theengine/turbochargers with cleaning fluid.Water washing should preferably beundertaken while the tubes are stillhot, making it easier to remove thesoot as it will “crack”.If the above-mentioned on/off exhaustbypass is installed, the boiler can bebypassed. Water washing shouldthen also be carried out during seaservice as often as possible (when theexhaust pressure loss increases), andnot only during stops in harbour. Afterwater washing, it should be checkedthat no soot is left, as remaining wetsoot may increase the risk of soot depositswhen continuing operation.j) The water circulation temperature atthe boiler inlet, for boilers with apreheater section, should be higherthan about 140°C as otherwise toolow temperatures could cause someof the gas constituents, such as fueland lube oil vapour, to condense onthe cold boiler tube surfaces, and thiscould increase the tendency towardssoot deposits.Another advantage of this is that thetemperature of the preheater tubesurfaces can then be higher than thedew point of the sulphuric acid in thegas, thus minimising the risk ofsulphuric acid corrosion.Referring to ignition:k)The circulation water flow velocityand ratio at the boiler inlet should beas high as possible in order to keepthe gas temperature at the boilertube surface as low as possible (incontrast to point j). The water flow ratio(water flow/steam production ratio)is recommended to be equal to orhigher than 6. This should reduce therisk of ignition of possible soot deposits,which can happen at temperaturesabove some 150°C and, underextreme conditions, even as low as120°C.It is therefore also very important toensure the best suction conditions sothat cavitation does not occur in thecirculating pumps under any workingconditions, as otherwise the circulatingwater flow could be reduced oreven stopped.A temperature monitoring systemmounted above the boiler might be recommendableas a means of detecting afire in the boiler as soon as it starts.Recommended operatingconditionsIn view of the damage that can becaused by an extensive soot fire in theexhaust gas boiler, it is recommended,during the operation of the ship, to givedue consideration to the following:Normal operating conditionsa)Soot-blowingIf soot-blowing equipment is installed,we recommend checking its efficiencyand adjusting the number ofdaily soot-blowings accordingly.This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net19
Page 1 and 2: Soot Deposits and Fires in Exhaust
Page 3 and 4: Soot Deposits and Fires in Exhaust
Page 5: Chapter IBasic Information andBoile
Page 8 and 9: OC Temperature TExhaust gas boilerP
Page 10 and 11: oC1501401301201101000Fig. 11: Sulph
Page 12 and 13: 21% O 279% N 297% HC3% S97% HC2.5%
Page 14 and 15: Chapter IIBoiler Experience andDesi
Page 16 and 17: Number of cases15Troubledesign mean
Page 20 and 21: )Preheated feed water during start-

Soot Deposits and Fires in Exhaust Gas Boiler - Martin's Marine ...

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