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

)Preheated feed water during start-upIn order to avoid the condensation ofsome of the gas constituents, preheatedfeed water should always beused (temperature higher than about140°C) during start-up and during lowload operation, especially if the boileris not fitted with an on/off by-passduct/valve which can be activated inthese running conditions.c)Water circulation, correct functioningIt should be ascertained that theboiler’s water circulation system andits control system are functioningproperly.d)Water circulation after engine stopAfter the engine is stopped, the boiler’swater circulating pump should be keptrunning until the boiler temperature hasfallen below 120°C, because wet oilysoot may catch fire at temperaturesas low as this.On the other hand, it is recommendednot to stop the circulating pump inharbour unless the boiler has beenchecked and is clean.e)Heavy smoke from engineIf excessive smoke is observed, eitherconstantly or during acceleration, thisis an indication of a worsening of thesituation. The cause should be identifiedand remedied. Excessive smokecould be caused by defective fuelvalves, a jiggling governor, incorrectadjustment of the governor fuel limiter,or the malfunctioning of one (of two)auxiliary blowers, etc.The boiler should be checked andcleaned if necessary.Operating conditions in waterleakage situationsDnV recently informed about a casewhere a water leakage was discoveredfrom a water tube type exhaust gas boilerRef. [6]. In order to get to port, the watercirculation was shut off. When arrivingat anchorage, the exhaust gas boiler overheated,and the crew found that a hightemperature soot fire had occurred.The above case shows how important itis to cool the tubes, i.e. that the watercirculating through the tubes always functionscorrectly, in order to avoid ignition ofthe soot.In this case, the water circulation couldnot continue because of the waterleakage. Therefore, in such a situationthe below actions are recommended.Actions to be taken prior to dry running:a)When shutting off the water circulation,the main engine should also beshut down so that the exhaust gasboiler can cool down and any smoulderingof soot deposits on the boilertubes can die out.b)The heating surface should be inspectedcarefully for soot deposits,and water washing performed, bothfor cleaning and cooling.c)Make every effort to re-establish thewater circulation to the boiler andthereby reduce the dry running periodto a minimum.d)Boiler manufacturers allow dry runningof exhaust gas boilers only in thecase of emergency and with a cleanboiler. In addition they emphasize thatevery possible precaution must beobserved to prevent soot fire.Actions to be taken during dry running:e)Increase the frequency of soot blowingconsiderably, and perform sootblowing several times prior tomanoeuvring.f) Inspect the boiler frequently and, ifany soot is present, then water washthe boiler and increase the soot blowingfrequency.g)The boiler’s instruction manual mustbe read carefully and its instructionsare always to be followed.Operating in soot fire situationsOn the other hand, if a soot fire doesstart after all, we recommend either ofthe following two types of measures,depending on the level of fire:Fire level 1, where an initial soot firehas just been discovered:a) Stop the main engine, and therebythe oxygen supply to the fire.b) Continue operating the watercirculating pump.c) Never use soot blowers for firefighting, as air will feed the fire withoxygen, and steam will involve a riskof high temperature fire.d) Stop the air circulation through theengine, and thereby the air supply tothe fire, i.e. keep air pressure on thediesel engine’s exhaust valve closingmechanism (closed valves).e) Water washing, if fitted, may beused to extinguish the fire. This isnormally connected to the ship’s firefighting water system.In a well-run plant any fire that startswill be small, and if the above emergencyaction is taken immediately, thefire will be damped down quickly, andwater circulated by the pump will helpkeep the tubes cool and reduce anyheat damage caused by the fire, Ref. [2].If the soot fire has turned into an ironfire, this can be indicated by a loss ofwater, for example, if the feed waterconsumption increases very much and/or if a low level alarm in the steamdrum is activated. A temperature sensor(normally max. 400°C) will not normallybe able to measure the high temperatures.20This document, and more, is available for download at Martin's Marine Engineering Page - www.dieselduck.net