Blowdown6.3 BlowdownContents6.3.1 What should TDS of my boiler be?.......................................................................................26.3.2 What is the purpose of blowdown?.......................................................................................26.3.3 What problems are caused by carry over?..........................................................................3ase Study!1..............................................................................................................................36.3." What is the difference between intermittent and continuous blowdown?.......................46.3.# $ow should % minimi&e blowdown?..................................................................................76.3.6 $ow much blowdown is needed?..........................................................................................8ase Study!2..............................................................................................................................96.3.' What is the impact of condensate recovery on TDS?......................................................11ase Study!3............................................................................................................................116.3.( When should % recover heat from blowdown?...................................................................12ase Study!"............................................................................................................................12ase Study!#............................................................................................................................136.3.) What care should be ta*en while blowdown?...................................................................14Page 1 of 14Blowdown6.3.1 What should TDS of my boiler be?The fi+ures below are offered as a broad +uide only for process boilers. The boilermanufacturer should always be consulted for specific recommendations.Maximum TDS tolerated by boiler type Maximum TDS ppm!"an#ashire 1,-,,,$%pass e#onomi# "-#,,&a#'a(e and 3%pass e#onomi# 3-,,,!3-#,,"ow pressure water tube 2-,,,!3-,,,Medium pressure water tube 1-#,,)oil Boilers and *enerators 2-,,,6.3.$ What is the purpose of blowdown?Waterisnotpure.%tcontains a lot ofimpuritiesli*e dissolved solids- suspendedsolids- dissolved +ases and scum formin+ substances. Some of these impurities are carried in the boiler water. When steam is +eneratedthe dissolved solids present in the boiler water continue to remain in the boiler andthe amount of dissolved solids in the boiler water increases. Some of the dissolvedsolids form scales resultin+inlocali&edheatin+whichpro+ressivelyleadtotubefailure. .lso- hi+h level of total dissolved solids /TDS0 encoura+es foamin+ and carryover. Therefore to control the level of concentrations- blowdown is carried out. %n order toPage 2 of 14Blowdownmaintain an optimum level of Total Dissolved Solids /TDS0 the blowdown water isreplaced by fresh feed water.1lowdown is necessary to protect the heat e2chan+er surfaces in boiler.6.3.3 What problems are #aused by #arry o+er?.s the boiler +enerates steam- any impurities which are present in the feedwater and which do not boil off with steam will concentrate in the boiler water..s the concentration of dissolved solids increases- the steam bubbles tend tobecome more stable- failin+ to burst as they reach the water surface of theboiler. There comes a point where a substantial part of the steam space intheboiler isfilledwithbubblesandfoamwhich then+ets carriedwith thesteam. The increased moisture content in the steam is wasta+e of fuel as only steamparticipatesinheat e2chan+eprocess. Theillustrationbelowshowsthedecrease in dryness fraction to an increase in the level of TDS. )ase Study%13arametersBoiler Desi(n &ressure 1, bar+Boiler ,peratin( &ressure ( bar+Boiler -ffi#ien#y ',4.uel /sedoal 56 3,,, *cal7*+.eed water softened! TDS (,, ppmBlow down TDS (,,, ppmSteam Demand 2 T3$0t a TDS le+el of 1222ppm the steam would be 134 dry i.e. there would be a134 moisture #arryo+er5 while at 3322ppm steam is 614 dry.6.3.7 What is the differen#e between intermittent and #ontinuous blowdown?ontinuous blowdown has a calibrated blowdown valve and blowdown tap near theboiler water surface. %t draws off water continuously from the top of boiler surface.The rate at which blowdown needs to be carried out is pre!determined and hencePage 3 of 14Blowdownre8uires no re+ular operator intervention.The concentration of impurities in the water should be controlled by blowdown fromsteamdrumand blowdown fromthe mud drumor bottomheader removessuspended solids- mud or slud+e from the boiler.Page 4 of 14BlowdownPage 5 of 14Blowdown 8ntermittent Manual! Blowdown ,peration .ll steamboilerincludeameansformanual blowdownasstandarde8uipment.9anual blowdown allows for removal of solids that settle at the bottom of the boiler.TDSmust becontrolledaccuratelyneartoma2imumlevel- inordertominimi&eblowdown- and to avoid carryover and foamin+ caused by TDS levels. onsider ascenario of manual operation of blowdown valve by operator: The boiler operator opens the blowdown valve when the TDSlevelof boiler water +oes above the set point and closes thevalve when the TDS level +oes below the set point. So- over a period of time there is a lar+e fluctuation in the TDSlevel of theboilerwater.$i+hTDSlevel causesscalin+andcarryover which is bad for boiler and also for e8uipments alon+the steam line.;ow TDS levels causes loss of sensible heat and thus wasta+eof fuel. Thus- manual blowdown control is not ener+y efficientil 3 T&90+(. TDS addition to the boiler 122 ppm 100+102; 33 ppm1lowdown per day 7.$ T&D $.6 T&D6.3.1 When should 8 re#o+er heat from blowdown?.ny boiler with continuous blowdown e2ceedin+ #4of steamrate should considerrecoverin+ heat from blowdown. %f your plant has intermittent /manual0 blowdown systemconsider switchin+ to continuous blowdown coupled with heat recovery.1lowdownheat canberecoveredwithaheat e2chan+er- flashtan*or flashtan*incombination with a heat e2chan+er. The blowdown is flashed in a flash vesselto obtainlowpressuresteam. Thislowpressuresteamistypicallyusedtoincreasefeedwatertemperature.?e#o+erin( heat from blowdown flash1lowdownwaterisatthesamepressureandtemperatureaswaterintheboiler drum. %t ispossibletorecover theheat intheblowdownwater byseparatin+ the flash steam from the blow down before drainin+ it. This flashsteam can be recovered and utili&ed to increase the feed water temperature.)ase Study%7ontinuin+ the ase study!3;i8uid enthalpy hf at 1, bar = 1(6.'' *cal7*+;i8uid enthalpy hf at ,.# bar = 111.'( *cal7*+Page 12 of 14Blowdown;atent heat hf+ at ,.# bar = #31.' *cal7*+4 flash +enerated =hf high pressurehf low pressurehfglow pressure 2 1,,= 186.77111.78531.7 2 1,, = 1".1 4.mount of flash steam +enerated= '6) *+7h 2 1".14 = 1,( *+7hh+ of steam = 6"3."3 *cal7*+Bner+y in flash steam =1,( 2 6"3."3 = 6)"), *cal7*+ Calculator for % flashThisflashsteam canberecoveredandutili&edtoincreasethefeedwatertemperature.?e#o+erin( heat from blowdown water@orlar+eboilersbesidesblowdownflashwecanadditionallyrecovertheblowdown heat via a heat e2chan+er. These methods minimi&e the heat lossthrou+h blowdown- savin+ fuel.)ase Study%3ontinuin+ the case study!"Total blowdown = '6) *+7h with 1,( *+7h flash steam Water flowrate re8uired = '6)!1#' = 612 *+7h-ner(y in waterBnthalpy /hf0 of saturated water at ,.# bar = 111.'( *cal7*+Bnthalpy of water at #, o = #, *cal7*+Bner+y available to heat ma*e up water = 111.'(!#,= 61.'( *cal7*+ Total ener+y that can be recovered = 612 2 61.'( = 3'(,) *cal7hThis ener+y can be utili&ed to increase the feed water temperature.Calculator for BHRSPage 13 of 14Blowdown.6.3.6 What #are should be ta'en while blowdown?Safety #are%n order to prevent the uncontrolled flashin+ of steam in the boiler room- theblowdown water should be dischar+ed into a vessel in which the flashin+ canoccur in a controlled manner.-n+ironment #areBnvironmental lawsstatethat thetemperatureof thedischar+edhot watershould be less than 6, o. $ot water promotes e2cessive +rowth of bacterialeadin+ to de+radation of sewer lines. $enceadditional stepsshouldbeta*entobrin+downthetemperatureofblowdown water from its boilin+ point to 6, o or less.Eneof thewaysistousetheblowdownwaterinheat e2chan+ertoheatincomin+ water which can be used as feed water which in turn cools down thetemperature of the blowdown water to specified norms.Page 14 of 14