INSPECTING FRACTIONA FRACTIONAT TION TOWERS By Juan Manuel Sanchez, Álvaro Valverde, Carlos Di Marco and Eduardo Carosio, YPF S.A. July !, "#!! Dis$illa$ion colu%ns are o&$en 'or(horses in $he che%ical )rocess indus$ries *CP+, and a$ $i%es, i$ is necessary $o )hysically ins)ec$ $he in$erior o& a colu%n. -his ar$icle de%ons$ra$es 'ha$ an an ins)ec$ion can reveal, y sharin/ $he e0)erience o& ac$ual &rac$iona$ion1$o'er ins)ec$ions in a )e$roleu% re&inery. -he )oin$s %en$ioned are in$ended $o )rovide /uidance $o &u$ure $o'er ins)ec$ors, ins)ec$ors, so $ha$ $hey can &ocus on e0$re%ely i%)or$an$ and and relevan$ )oin$s $ha$ 'ill direc$ly i%)ac$ $he o)era$ion o& dis$illa$ion colu%ns.
INSPECTION BASICS Who inspects? 2henever a &rac$iona$ion $o'er is o)ened &or ins)ec$ion, i$ is i%)or$an$ $ha$ i$ e ins)ec$ed and analyzed
y a 3uali&ied )rocess en/ineer, en/ineer, 'ho has een $rained in 'ha$ $o loo( loo( &or. -y)ically, -y)ically, ins)ec$ions are carried ou$ y a nu%er o& di&&eren$ de)ar$%en$s, each one &ocused on i$s )ar$icular area o& concern. Colu%n co%)onen$s are usually ins)ec$ed y $he $ he e3ui)%en$ ins)ec$ion de)ar$%en$. -he )roduc$ion de)ar$%en$, esides chec(in/ i& i$ is necessary $o re)lace or re)air in$ernals, ins)ec$s i& $he $o'er is clean or i& i$ has accu%ula$ed lar/e 3uan$i$ies o& co(e, sal$ or corrosion )roduc$s durin/ o)era$ion. Cleanin/, re)airs and %oun$in/s are $he res)onsiili$y o& $he %ain$enance de)ar$%en$. -he )rocess de)ar$%en$ ins)ec$or is usually res)onsile &or cri$ically analyzin/ chan/es %ade in a colu%n and &or iden$i&yin/ )o$en$ial o)era$ional )role%s. So%e$hin/ as si%)le as $he re)lace%en$ o& a do'nco%er can /enera$e a si/ni&ican$ &rac$iona$ion1e&&iciency loss i& i$ is no$ )ro)erly %oun$ed. For e0a%)le, i& $he clearance under a do'nco%er is $oo s%all, $he colu%n could su&&er &ro% &loodin/ ecause o& $he &lo' res$ric$ion. -he cons$ruc$ion cre', ho'ever, does no$ al'ays unders$and ho' a colu%n 'or(s, and %i/h$ $here&ore %a(e decisions $ha$ are no$ $he es$. +$ is essen$ial $o have )eo)le 'ho are (no'led/eale aou$ $he dis$illa$ion )rocess $o cri$ically analyze and address $hese $y)es o& de$ails in $he colu%n. +n addi$ion $o $he cri$ical analysis, i$ is i%)era$ive $ha$ $he )rocess ins)ec$or avoid re)ea$in/ old )rac$ices and errors $ha$ can e dra//ed on &or years and eco%e )ar$ o& $he 'ay o& 'or(in/. Preparing for inspection. A cri$ical ins)ec$ion is a $edious and o&$en dir$y $as(. +$ is necessary $o re%ain inside $he
$o'er &or hours. A 4o 'ell1done, ho'ever, can iden$i&y )o$en$ial )role%s and 'ill save headaches 'hen $he $o'er is ac( in o)era$ion. +$ is i%)or$an$ $o iden$i&y )role%s e&ore o)enin/ $he &rac$iona$ion $o'er &or ins)ec$ion. Moni$orin/ $he variales in $he colu%n 'ill allo' $he ins)ec$or $o &ocus on sec$ors $ha$ have een iden$i&ied as )role%a$ic in advance. For e0a%)le, i& $he colu%n is &looded a$ $he $o), e$'een $he (erosene su%) and $he $o) o& $he $o'er *iden$i&ied y $he di&&eren$ial )ressure and /a%%a scannin/, $ha$ sec$ion should e ins)ec$ed i%%edia$ely 'hen $he colu%n is o)ened. +n our )ar$icular case, 'e in$ended $o re)lace $he dis$riu$ors and $he 'ash ed o& $he vacuu% $o'er, ecause 'e (ne' &ro% %oni$orin/ )ressure dro) durin/ o)era$ion $ha$ i$ 'as &illed 'i$h a lar/e 3uan$i$y o& co(e.
-his ar$icle covers $hese e0a%)les, and %ore &ro% $he )oin$ o& vie' o& $he )rocess. -o )u$ $his $y)e o& ins)ec$ion in$o )rac$ice, i$ is necessary $o )re)are a de$ailed )ro/ra% $o (no' 'ha$ $o ins)ec$ and ho' $o loo( &or $he )o$en$ial )role%s. An e0)erienced )rocess en/ineer once said $ha$ a /ood oserver and de$ec$or o& )o$en$ial )role%s inside $he colu%n $urns hi%sel& in$o $he li3uid $ha$ &lo's $hrou/h $he do'nco%ers and in$o va)or, 'hich rises $hrou/h $he $rays.
TIPS FOR TOWER INSPECTIONS Fore%os$, i$ is i%)or$an$ $ha$ an ins)ec$ion is carried ou$ y a $ea% o& )eo)le $ha$ includes a 3uali&ied )rocess en/ineer 'ho is &a%iliar 'i$h $he &rac$iona$ion colu%n and has een $rained in 'ha$ $o loo( &or. 5ere are so%e addi$ional $i)s /ained &ro% e0)erience in $o'er ins)ec$ions6 !. +den$i&y )role%s e&ore o)enin/ $he &rac$iona$ion $o'er &or ins)ec$ion. Moni$orin/ )ressure dro) and o$her variales can )in)oin$ )role% areas in advance o& $he ins)ec$ion ". Pre)are a de$ailed schedule &or ins)ec$ion. 7ee) in close con$ac$ 'i$h $he %ain$enance )ersonnel 8. +$ is i%)or$an$ $o have ade3ua$e $ools $ha$ 'ill e needed durin/ $he ins)ec$ion 9. :-hin( li(e $he li3uid; 'hen ins)ec$in/ and closely oserve 'here $he li3uid 'ill &lo' $hrou/h $he do'nco%ers and 'here $he va)ors 'ill rise u), loo(in/ &or os$ruc$ions or desi/n &la's <. -es$ s)ray nozzles 'i$h 'a$er $o chec( &or )lu//in/ =. Moni$or any chan/es %ade $o $he colu%n, %a(in/ sure ins$alla$ions are done accordin/ $o desi/n >. Chec( $ha$ clearances under $he do'nco%ers are 'i$hin s)eci&ica$ions. +%)ro)er clearances can cause &loodin/ or en$rain%en$ ?. Be&ore closin/ $he colu%n a$ $he end o& an ins)ec$ion, %a(e sure all $ools or o$her i$e%s rou/h$ in have een re%oved @. +ns)ec$ and %ain$ain $he &il$ers )eriodically
THE MANUFACTURING SITE -his ar$icle shares e0)eriences &ro% ins)ec$ions )er&or%ed in $he vacuu% and a$%os)heric1)ressure &rac$iona$ion $o'ers in our re&inery, durin/ rou$ine %ain$enance shu$do'ns o& $he uni$s in ove%er "##?. -he re&inery, o'ned y YPF S.A. is called u4n de Cuyo, and is loca$ed in $he )rovince o& Mendoza in Ar/en$ina.
ur vacuu% $o'er, CV!#, is 8> % lon/ 'i$h a dia%e$er o& ?." % in i$s lar/es$ sec$ion. +$ )rocesses around @,### % 8d o& a$%os)heric residue. ur crude dis$illa$ion a$%os)heric $o'er, CV!, is 8@.= % lon/ and 9 % in dia%e$er. +$ )rocesses around !!,### %8d o& crude oil. Full ins)ec$ions &or each o& $he $o'ers had een )lanned. Previous )lannin/ 'as necessary $o avoid delays and $i%e e0$ensions o& $he scheduled shu$do'n. Al$hou/h a co%)le$e ins)ec$ion o& each one o& $he $o'ers 'as conduc$ed, $his ar$icle )resen$s only $he )oin$s o& %a4or in$eres$, 'hich 'ill e use&ul &or )rocess ins)ec$ors. 2e &ocus on $he &ollo'in/ )oin$s o& concern6 Vacuum tower
+ns)ec$ion and $es$in/ o& dis$riu$ors +ns)ec$ion and $es$in/ o& su%)s and chi%ney $rays +ns)ec$ion o& dual1ele%en$ &il$er screens Desi/n )role%s Atmospheric distillation tower
+ns)ec$ion o& $rays *valves, holes, %easure%en$s o& 'eirs and clearances under $he do'nco%ers +ns)ec$ion and $es$in/ o& su%)s Desi/n )role%s -es$ o& /ravi$y1&lo' dis$riu$or
VACUUM DISTILLATION TOWER Light vacuum gas oil (LVGO) pump around distriutor.A%on/ a $o$al o& !@ s)ray nozzles, only one 'as &ound $o e
)lu//ed. +$ 'as a/reed 'i$h $he %ain$enance s$a&& $o rando%ly selec$ "#G o& all nozzles, dis%oun$ and chec( $he%. +& <#G o& $hese nozzles 'ere )lu//ed or dir$y, all nozzles 'ould e dis%oun$ed &or cleanin/ and ins)ec$ion.
F+HIE !. Chec( nozzles &or )lu/s. -his one is loc(ed 'i$h a )iece o& )ac(in/
F+HIE ". 2a$er $es$in/ 3uic(ly iden$i&ies nozzle )lu//in/ -he nozzle $ha$ 'as &ound $o e )lu//ed *Fi/ure !, 'as loc(ed 'i$h a )iece o& )ac(in/ *Fle0irin/. -o shed li/h$ on our 3ues$ion o& ho' i$ could have /o$$en $here, 'e 'en$ $o ins)ec$ $he dual1ele%en$ &il$er screens $ha$ are e0$ernal $o $he $o'er. 2e &ound $'o di&&eren$ )ossile e0)lana$ions6 ! 2e &ound no %eshes inside $he &il$ers, and $ his 'ould allo' $he )ac(in/ )iece $o y)ass $he &il$ers, and " ne o& $he dual1ele%en$ &il$ers 'as dis%oun$ed $o connec$ a se/%en$ o& )i)in/ $ha$ y)assed $he dual &il$ers. So%eone had decided $o dis%oun$ i$ $o /ive o)era$ional &le0iili$y $o $he uni$. !istriutor testing . -o veri&y $ha$ no s)ray nozzles had
een )lu//ed, $hey 'ere $es$ed 'i$h 'a$er. Fi/ure " sho's 'ha$ ha))ens 'hen $he &lo' $hrou/h $he nozzles is no$ ade3ua$e. -his is due %ainly $o o)era$ional &ailures, or in $he 'ors$ case, )lu//in/ o& $he nozzles. A )lu//ed nozzle )roduces a )oor &lo' &or 'ashin/ on a )or$ion o& $he ed.
LVGO"heav# vacuum gas oil ($VGO) fractionation ed distriutor. 2e have a /ravi$a$ional dis$riu$or in $he
VH5VH &rac$iona$ion ed *Fi/ure 8. -he VH &ills $he )re1dis$riu$or channel $hrou/h a &eed )i)e. -he li3uid over&lo's in$o a )ar$in/ o0. -hen, $rou/hs are &illed. A$ $he $o) o& each $rou/h $here are ori&ices, 'here $he li3uid &lo's $o dri) $ues. -he li3uid arrives $o s)oons ases *$he li3uid drains $hrou/h $he corners o& a ase, called $he s)oon ase K seeFi/ure 8 and dis$riu$es li3uid $o $he ed.
F+HIE 8. -his sche%a$ic sho's $he con&i/ura$ion o& $he /ravi$a$ional dis$riu$or
-he &irs$ $hin/ $ha$ 'as oserved durin/ $he ins)ec$ion o& $his dis$riu$or 'as $ha$ $he cen$ral )ar$in/ o0 'as uneven.Fi/ure 9 sho's a %issin/ ol$ necessary $o hold $he o0. Inevenness in $he cen$ral )ar$in/ o0 'ill a&&ec$ li3uid dis$riu$ion $o $he ed elo', since $he li3uid 'ill &lo' )re&eren$ially $o so%e $rou/hs. A sec$ion o& $he )ac(in/ 'ill e dry due $o reduced irri/a$ion, and a )oor end )oin$ *$he hi/hes$ $e%)era$ure recorded durin/ dis$illa$ion or cloud )oin$ *$he $e%)era$ure a$ 'hich $he li3uid has a cloudy a))earance si/ni&yin/ $ha$ 'a0 is e/innin/ $o crys$allize 'ould resul$. +n cer$ain sec$ions, dri) $ues 'i$h s3uare s)oons ases 'ere no$ in )lace, u$ 'ere ins$ead on $he ed. Also, as sho'n inFi/ure <, so%e 'ere en$ so $ha$ $he li3uid dis$riu$ion $o $he ed 'as no$ /ood.
F+HIE 9. +ns)ec$ion uncovered a %issin/ ol$
F+HIE <. So%e dri) $ues 'ere en$, causin/ uneven li3uid dis$riu$ion
F+HIE =. Even $hou/h dir$ 'as &ound in $he dis$riu$or, $he desi/n co%)ensa$ed &or $he &oulin/
F+HIE >. -his loc(a/e re3uired $ha$ $he dis$riu$or e dis%oun$ed and cleaned
F+HIE ?a *$o) and ? *o$$o%. -his desi/n %is$a(e caused a )i)e $o e 'e$$ed &ro% a nozzle, 'hich could )o$en$ially cause corrosion
F+HIE @a and . A lon/er nozzle %i/h$ have caused a 'orse corrosion )role% $o $he )i)e 'eld
F+HIE !#. -he :Y; &il$ers 'ere ins)ec$ed
F+HIE !!. -his de&or%ed %esh $a(en &ro% $he Y &il$er 'ould allo' solids $o )ass $o $he s)ray nozzles and )o$en$ially cause )lu//in/
F+HIE !". +ns)ec$ion uncovered de&or%ed colu%n in$ernals, such as $his /rid
F+HIE !8. Modi&ica$ions need $o e %ade
accordin/ $o desi/n. A correc$ /rid arran/e%en$ is sho'n here
F+HIE !9a *$o) and *o$$o%. So%e valves on $he $rays 'ere &ound $o e %issin/ or corroded
F+HIE !<. +%)ro)er asse%ly o& colu%n in$ernals can lo'er &rac$iona$ion e&&iciency
F+HIE !=. -his )anel 'as ro$a$ed aou$ i$s nor%al )osi$ion, renderin/ $he inle$ 'eirs ino)erale
F+HIE !>. Chec( $ha$ do'nco%er clearances are 'i$hin s)eci&ica$ions
F+HIE !?. Durin/ ins$alla$ion o& $he dis$riu$ors in $he colu%n, do'nco%ers 'ere da%a/ed
F+HIE !@. 2hen &lo' is increased aove desi/n values, )oor dis$riu$ion o& li3uid resul$s in $he /ravi$y &lo' dis$riu$or Even $hou/h $o en$er $he colu%n i$ 'as *and should e 'ashed &or hours, $here 'as dir$ inside $he side $rou/hs o& $he dis$riu$or *Fi/ure =. As $he holes &ro% 'here $he li3uid en$ers are aove 'here $he dir$ 'as in $he $rou/hs, $here should have een no os$ruc$ion $o $he )assa/e o& li3uid. -his sho's $he e&&ec$iveness o& $his desi/n in $er%s o& an$i&oulin/. Even 'i$h $he advan$a/es o& $his $echnolo/y, ho'ever, $he ins)ec$ion uncovered a clear loc(a/e in $he cen$er )ar$in/ o0 *Fi/ure >, 'hich hindered $he )assa/e o& li3uid $o $he la$eral $rou/hs. -his 'as discovered $hrou/h a 'a$er $es$ o& $he dis$riu$or. As a resul$, $he dis$riu$or 'as dis%oun$ed and cleaned. $VGO pump around. 2e &ound a desi/n %is$a(e in $his sec$ion. Fi/ures
?a and ? sho' ho' a ranch )i)e 'as 'e$$ed y li3uid issuin/ &ro% $he s)ray nozzle. -his occurred ecause $he )i)e 'as inclined. Hiven $ha$ $he values o& $o$al acid nu%er *-A and sul&ur con$en$ o& $he curren$ 5VH are !.8 (/ *%/ 75 )er /ra% o& hydrocaron and #.=> '$.G res)ec$ively, and $he o)era$in/ $e%)era$ure e0ceeds "##LC, $he 4e$ &lo' &ro% $he nozzle could ne/a$ively i%)ac$ and con$riu$e $o corrosion o& $he &eed )i)in/. -o resolve $his issue, 'e $ried )lacin/ a lon/er nozzle $o s$o) $he li3uid &ro% 'e$$in/ $he ranch )i)e. -he $ue )laced 'as as lon/ as 'e could %a(e i$, $a(in/ $he inclina$ion o& $he )i)e in$o accoun$. Fi/ures @a and @ sho' ho' $he s)ray dis$riu$ion 'ould have 'e$$ed $he ranch1)i)e 'eld i& 'e decided $o carry ou$ $he %odi&ica$ion. nce 'e sa' $his, ho'ever, 'e eca%e concerned aou$ corrodin/ $he 'eld. Any lea( in ranch )i)e 'ould cause )oor dis$riu$ion over $he en$ire ed, leadin/ $o )oor hea$ $rans&er and )ossily co(in/ on $he )ac(in/. -here&ore, 'e decided $o /o ac( $o $he ori/inal arran/e%en$, and 'e %oun$ed $he shor$er )i)e ac(, 'i$h $he co%)ro%ise $o re)lace i$ durin/ $he ne0$ shu$do'n o& $he uni$. 2hile no nozzles 'ere )lu//ed, i$ is a))ro)ria$e $o co%%en$ on ano$her desi/n error as $o $he dual &il$ers o& $he 5VH )u%)1around loo). -hey are loca$ed u)s$rea% o& $he con$rol valve. Dual &il$er screens should e do'ns$rea% o& any e3ui)%en$ e&ore en$erin/ $he colu%n, $o avoid )lu//in/ in $he nozzles. -his ano%aly da$es &ro% $he cons$ruc$ion o& $he )lan$ in !@>#. 2e 'ill &i0 $he% durin/ $he ne0$ shu$do'n o& $he uni$.
Wash ed distriutor . Be&ore en$erin/ $he $o'er &or ins)ec$ion, 'e oserved &ro% $he %an'ay $ha$ an ar% o& $he
dis$riu$or 'as ro(en, so li3uid ca%e ou$ e$'een $he $'o sec$ions. -he visual ins)ec$ion, inside $he colu%n, sho'ed $ha$ ei/h$ ou$ o& =! nozzles 'ere )lu//ed. +n $his )ar$icular case, all $he s)ray nozzles 'ere re)laced y ne' ones, so all o& $he% 'ere dis%oun$ed. 2hen 'e ins)ec$ed $he dis%oun$ed nozzles, 'e 'ere sur)rised $o &ind $ha$ %ore $han ei/h$ nozzles 'ere ac$ually )lu//ed. Aou$ <#G o& $he nozzles 'ere )lu//ed in$ernally. -his 'ould have )roaly a))eared durin/ $he 'a$er $es$ o& $he s)ray nozzles. -he lesson in $his case is $he i%)or$ance o& $he s)ray1dis$riu$or 'a$er $es$. Each &lo's$rea% $ha$ circula$es $o $he colu%n con$ains $'o :Y; &il$ers * Fi/ure !#. -hese dual &il$ers are %ean$ $o s$o) any solids $ha$ could )lu/ $he colu%n in$ernals. 2e &ound only one &il$er 'hose %esh 'as de&or%ed * Fi/ure !!, 'hich allo'ed $he )assa/e o& dir$ $o $he s)ray nozzles. +n ins)ec$in/ $he u))er /rid o& $he ed, 'hich 'as raised $o $he hei/h$ o& $he %an'ay, 'e &ound a ro(en $ie rod. Fi/ure !" sho's ho' $he /rids 'ere de&or%ed. ne ed 'as en$irely re)laced as )lanned. ne $hin/ $o consider is $ha$ 'hen $he %ain$enance s$a&& %oun$ed $his ne' ed, $hey 'ere doin/ so 'i$hou$ dra'in/s. Accordin/ $o desi/n dra'in/s, each ed layer should e a$ a 9<1de/. an/le $o $he layer elo', and $his 'as no$ ein/ res)ec$ed in $he ins$alla$ion. So, 'e had $o in$ervene $o )ro)erly reins$all $he layers. -he lesson here is $o $ry $o (ee) areas$ o& all chan/es %ade. Fi/ure !8 sho's a correc$ &lo' /rid layer arran/e%en$.
ATMOSPHERIC DISTILLATION TOWER -his colu%n is co%)osed o& 8@ valve $rays and s$ruc$ured )ac(in/. Fro% $o) $o o$$o%, $he con&i/ura$ion is $he &ollo'in/6 -rays ! $o !8 -ray !9 *(erosene su%) -rays !< $o "8 -ray "9 *a$%os)heric li/h$1/as1oil su%) -rays "< $o 88 -ray 89 *a$%os)heric heavy1/as1oil su%) Bo$$o% $rays 8< $o 8@ *s$ri))in/ sec$ion e$'een )e$roleu% &eed and s$ri))in/ s$ea% inle$ -rays ! $o !#, are $'o1)ass, and !! $o 8@ are &our1)ass $rays. -he ins)ec$ion 'as conduc$ed in de$ail &or each o& $he $rays and s$ruc$ured )ac(in/. -his ar$icle includes only $he hi/hli/h$s. %ra#s inspection. 2e &ound %issin/ valves in so%e cases alon/ $he colu%n, and so%e
*Fi/ures !9a and !9.
valves 'ere &ound $o e corroded
+%)ro)er asse%ly o& $he )anels $ha$ co%)ose $he $rays, )roduced holes * Fi/ure !< $ha$ allo'ed 'ee)in/, 'hich 'orsened $he 3uali$y o& &rac$iona$ion. Besides $he &or%a$ion o& holes, i%)ro)erly )lacin/ a )anel %ay ne/a$e $he &unc$ionali$y &or 'hich i$ 'as uil$. Fi/ure != sho's a )anel ro$a$ed aou$ i$s nor%al )osi$ion. As can e seen, i& $he )anel is ro$a$ed, $he inle$ 'eir 'ill no$ 'or(. -he inle$ 'eir &unc$ion is $o )reven$ $he li3uid &ro% $he do'nco%er &ro% li&$in/ $he valves closes$ $o $he inle$ do'nco%er and 'ee)in/ $hrou/h $he inle$ ro' o& valves. +n each o& $he $rays 'e $oo( %easure%en$s o& $he 'eirs, and $he clearances under $he do'nco%ers $o ascer$ain a li3uid seal on $he $ray. +& $here is no li3uid seal, /as %ay rise u) $he do'nco%ers, )roducin/ li3uid en$rain%en$ and &loodin/. Conversely, i& $he clearance under $he do'nco%er is $oo s%all, i$ 'ill increase $he hei/h$ o& li3uid in $he do'nco%er. 2hen $he do'nco%er ac(u) reaches $he $ray aove, &loodin/ 'ill occur. +$ is su&&icien$ $o have one undersized clearance $o o$$lenec( an en$ire &rac$iona$or. +n $he )ar$icular case sho'n in Fi/ure !>, $he clearance under $he do'nco%er 'as "@ %%. Accordin/ $o $he desi/n dra'in/, ho'ever, $his clearance should e 98.< 8 %%. -his )role% 'as solved y cu$$in/ $he o$$o% o& $he do'nco%er $o acce)$ale values. Be$'een $rays !@ and "# are $he a$%os)heric li/h$1/as1oil *AH )u%)around inle$ )i)e dis$riu$ors. +$ 'as no$ed durin/ $he ins)ec$ion $ha$ durin/ $he ins$alla$ion o& $he dis$riu$ors in $he colu%n, do'nco%ers 'ere da%a/ed * Fi/ure !?. -his caused si/ni&ican$ losses o& &rac$iona$ion ca)aci$y. &umps. 7erosene, AH and a$%os)heric heavy1/as1oil *A5H su%)s 'ere $es$ed 'i$h
'a$er. -hey 'ere &illed 'i$h 'a$er $o a cer$ain level. -hen 'e cu$ o&& $he 'a$er and 'ai$ed aou$ "# %inu$es $o loo( a$ $he &all o& $he 'a$er level. -he conclusion o& $he $es$ 'as $ha$ no )role%s 'ere oserved 'i$h su%) lea(a/e. Anecdo$ally, durin/ $he &inal visual ins)ec$ion o& $he $o'er, 4us$ e&ore ein/ closed, 'e &ound a )i)e sca&&old in $he A5H su%). +$ 'as hidin/, so $ha$ i& 'e had no$ )ro)erly )er&or%ed $he &inal ins)ec$ion, $he sca&&old deris %ay have /enera$ed si/ni&ican$ da%a/e in $he area. Gravit# flow distriutor.-he /ravi$y &lo'
dis$riu$or, loca$ed in $he 'ash sec$ion, 'as $es$ed 'i$h 'a$er $o see 'ha$ ha))ens 'hen $he &lo' increases. Fi/ure !@ sho's ho' i$ &ills $he cen$ral )ar$in/ o0, and $he )oor dis$riu$ion o& li3uid $ha$ resul$s 'hen $he &lo' is increased aove desi/n values. Edited by Dorothy Lozowski
