grbcic ,mrakovcic, hodak

7/25/2019 Grbcic ,Mrakovcic, Hodak

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POMORSTVO ScienticJournal of Maritime Research 27/1(2013) str./pp. 39-54 39

Luka Grbi, studentDr. sc. Tomislav Mrakovi /Ph. D.Sveuilite u Rijeci / University of RijekaTehniki fakultet /Faculty of Engineering RijekaVukovarska 58, 51000 Rijeka

Goran Hodak, mag. ing. mech.Croatia osiguranje d.d.Podrunica Rijeka /Branch ofce RijekaKorzo 39, 51000 RijekaHrvatska / Croatia

Izvorni znanstveni radOriginal scientifc paper

UDK / UDC:629.5.045.4

66.096.1

Primljeno /Received:7. travnja 2013. / 7thApril 2013

Odobreno /Accepted:10. travnja 2013. / 10thApril 2013

NUMERIKA SIMULACIJA INERTIRANJA TANKOVA TERETA

NUMERICAL SIMULATION OF CARGO TANKS INERTING

SAETAK

U radu je prikazana problematika inertiranja tankovana brodovima za prijevoz tekueg tereta. Opisan jesustav za proizvodnju inertnoga plina te su navedenizahtjevi koje takav sustav mora ispunjavati. Istaknutisu imbenici koji utjeu na kvalitetu inertiranjatankova tereta te karakteristike razliitih pristupa priupuhivanju inertnoga plina. Nadalje, je prikazanmatematiki model koriten u numerikoj simulacijiinertiranja te njegova primjena na primjeru tankera zaprijevoz tekueg tereta. Numerika simulacijainertiranja provedena je za sluaj prvog inertiranjatanka nakon primopredaje broda ili inertiranja nakondokovanja broda. Obraeno je vie sluajeva srazliitim smjerovima upuhivanja inertnoga plina u

tank tereta sa svrhom odreivanja optimalnog smjera.Provedeno je i istraivanje utjecaja brzine upuhivanjainertnoga plina na brzinu promjene koncentracijepojedinih plinova. U zakljuku je dan osvrt nadobivene rezultate uz smjernice za poboljanjepredstavljenoga simulacijskog modela.

Kljune rijei: inertiranje, tankovi tereta, numerikasimulacija, metoda konanih volumena

SUMMARY

The topic of cargo tanks inerting on board liquid cargocarriers is presented in this paper. The inert gasproduction system and requirements which such asystem has to full are described. Factors that inuencethe quality of cargo tank inerting as well as differentapproaches to cargo tanks inerting are pointed out.The mathematical model used in the numericalsimulation of cargo tanks inerting and its applicationon the example of a liquid cargo carrier is described inthe paper. The numerical simulation of inerting isperformed for a case of the rst cargo tank inertingafter the ship delivery or after docking. Severalexamples of different inert gas injection directions wereanalyzed with an intention to nd out an optimal inert

gas injection direction. Furthermore, the inuence ofthe inert gas injection velocity on the gas concentrationchange rate inside the cargo tank is investigated. Anoverview of the obtained results and guidelines for theimprovement of the presented simulation model aregiven in the conclusion chapter.

Key words:inerting, cargo tanks, numerical simulation,nite volume method


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L. Grbi, T. Mrakovi, G. Hodak: NUMERIKA SIMULACIJA INERTIRANJA TANKOVA TERETA

40 POMORSTVO ScienticJournal of Maritime Research 27/1(2013) str./pp. 39-54

1. UVOD

Tijekom eksploatacije brodova za prijevoztekueg tereta postoji poveana opasnost odpoara i eksplozije zbog specinih karakteri-stika tereta koji prevoze. Takvi su brodovi opre-

mljeni sustavima inertnoga plina koji omoguu-ju odravanje inertne atmosfere u tankovimatereta, ime se opasnost od eksplozije smanjujena najmanju moguu mjeru. Obveza ugradnjesustava inertnoga plina donesena je tek 1980-ihgodina kao posljedica niza eksplozija na tanke-rima diljem svijeta uz gubitak mnogih ljudskihivota [1, 2]. Brzina i kvaliteta inertiranja tan-kova ovisi o kapacitetu i izvedbi sustava iner-tnoga plina te geometriji tankova koji se ispu-njavaju inertnim plinom. Inertiranje tankovatereta provodi se postupkom mijeanja inertno-ga plina s parama ugljikovodika ili postupkomistiskivanja para ugljikovodika iz tanka tereta.

Zbog niza utjecajnih uvjeta te kompleksnogstrujanja uida unutar tankova, analitikim jepostupcima vrlo teko izraunati vrijeme po-trebno za kvalitetno inertiranje tankova tereta.Primjenom odgovarajuih numerikih metodamogue je predvidjeti tijek inertiranja tankovatereta uz zadane rubne uvjete poput okolinihuvjeta, geometrije tanka, vrste inertnoga plina,

njegove brzine i smjera strujanja na ulazu utank, i sl. Takva saznanja mogu se iskoristiti priprojektiranju sustava inertnoga plina s namje-rom da se u to kraem vremenu postignu uvje-ti koji osiguravaju maksimalnu sigurnost priplovidbi ili manipuliranju teretom.

2. INERTNI PLIN I NJEGOVAPROIZVODNJA

Inertni plin je onaj plin koji pri zadanimuvjetima kemijski ne reagira s drugim tvarima.Poradi svojstva da ne reagiraju u kemijskim re-akcijama, inertni se plinovi esto rabe pri pre-

venciji nepoeljnih kemijskih reakcija. Inertniplin koji se najee koristi na brodovima mje-avina je plinova sastavljena najveim dijelomiz duika, ugljinog dioksida, kisika te manjihudjela vlage i duikovih oksida. Takav sastavinertnog plina postie se izgaranjem goriva ugeneratoru inertnoga plina ili dovoenjem is-punih plinova iz dimovoda.

Na brodovima za prijevoz tekuih goriva injihovih ranata moglo bi se s najmanje troko-

va koristiti ispune plinove koje proizvode po-

1 INTRODUCTION

Due to specic characteristics of liquid car-goes, there is a signicant danger of explosionon board liquid cargo carriers. Such ships areequipped with inert gas systems which providepreservation of inert atmosphere inside cargotanks, what decrease the danger of explosionon the smallest possibility. The installation ofthe inert gas system was imposed in early 1980safter several explosions on board VLCC tank-ers with severe loss of human lives [1, 2]. Quick-ness and quality of cargo tanks depend on thecapacity and layout of inert gas system and ge-ometry of cargo tanks being lled by inert gas.Cargo tank inerting is performed by mixing in-ert gas with hydrocarbon vapours (dilutionprocess) or by the displacement of hydrocarbon

vapours out of cargo tank. Due to many differ-ent factors and complex uid ow inside cargotanks, it is very difcult to calculate the timeneeded for quality cargo tank inerting by ana-lytical methods. The application of adequatenumerical methods provide prediction of cargotank inerting progress by given boundary condi-tions such as environment conditions, cargotank geometry, kind of inert gas, inert gas owspeed and inlet direction etc. Such informationcan be used during the inert gas system designprocess with the intention to create conditionsfor the highest safety during navigation or car-go handling in the shortest time.

2 INERT GAS AND ITSPRODUCTION

An inert gas is a gas which does not undergochemical reactions under a set of given condi-tions. Because of that property, inert gases arefrequently used for the prevention of undesira-

ble chemical reactions. Inert gas which is com-monly used on board ships is a gas mixturecomposed mostly of nitrogen, carbon dioxide,oxygen and small content of moisture and ni-trogen oxides. Such composition of inert gas isobtained by fuel combustion in an inert gasgenerator or by taking exhaust gases from ex-haust pipes.

The exhaust gases from liquid cargo carrierpropulsion plants could be used with the lowestcost if their purity would be satisfactory and the

oxygen content lower than the ammabilitylimit. Although such a system is an economicalsolution for the inert gas production, it is notfrequently used because such inert gas contains


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L. Grbi, T. Mrakovi, G. Hodak: NUMERICAL SIMULATION OF CARGO TANKS INERTING


gonski strojevi kada bi njihova istoa bila za-dovoljavajua, a sadraj kisika ispod granicezapaljivosti. Iako je ovaj sustav ekonomino rje-enje za dobivanje inertnoga plina, nije estokoriten jer takav inertni plin, uz relativno viso-ke udjele kisika, sadri i znatne koliine nei-stoa, ae i sumporovih oksida [3].

U sluaju primjene stroih zahtjeva za kvali-tetu inertnoga plina koristi se generator iner-tnoga plina, koji omoguuje proizvodnju iner-

tnoga plina s niskim sadrajem kisika,sumporovih oksida, ae i ostalih neistoa.Generator, odnosno cijeli sustav za proizvodnjuinertnoga plina prikazan je na slici 1. Ovakavsustav omoguuje proizvodnju inertnoga plinate njegovu distribuciju do tankova tereta uz re-gulaciju svih relevantnih parametara za pouz-dan rad sustava.

Legenda: Legend:1 ventilatori 6 nepovratni ventil 1 fans 6 non-return valve2 generator inertnoga plina 7 P/V ventil 2 inert gas generator 7 P/V valve3 analizator kisika 8 prekotlano vakuumski ventil 3 oxygen analyzer 8 P/V breaker4 regulacijski ventil 9 stanica instr. zraka 4 control valve 9 instrument air supply5 palubna vodena brtva 10 upravljaka jedinica 5 deck seal 10 control unit

Slika 1. Sustav inertnog plina s generatorom inertnog plinaFigure 1Inert gas system with an inert gas generator

Izvor /Source:Autori /Authors

relatively high oxygen content and considerableamount of impurities, soot and sulphur oxides[3].

In case of the application of rigorous re-quirements on inert gas quality, an inert gasgenerator is used, which provides a productionof inert gas with low content of oxygen, soot,sulphur oxides and other impurities. An inertgas system scheme with inert gas generator isshown in Figure 1. Such a system provides inert

gas production and its distribution to cargotanks with the control of all relevant parame-ters for a reliable operation.


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3. INERTIRANJE TANKOVATERETA

Pojam inertiranja tankova tereta podrazumi-jeva uvoenje inertnog plina u tankove tereta sciljem smanjenja koncentracije para ugljikovo-

dika i/ili smanjenja udjela kisika. Inertiranjetankova tereta moe se izvoditi postupkom ra-zrjeivanja ili postupkom istiskivanja para uglji-kovodika [4]. Pri postupku razrjeivanja, pareugljikovodika ili zraka se mijeaju s inertnimplinom koji velikom brzinom ulazi u tank teretate se njihova mjeavina isputa u atmosferukroz otvor na vrhu tanka. Ovaj postupak inerti-ranja tankova je bri od inertiranja istiskiva-njem. Tijekom inertiranja tanka potrebno je iz-mjeriti sadraj kisika i para ugljikovodika na

nekoliko razliitih pozicija u tanku kako bi se sdovoljnom sigurnou utvrdilo da ne prijetiopasnost od eksplozije.

Inertiranje istiskivanjem podrazumijeva iz-mjenu sastava plinova u tanku tereta na teme-lju razlike gustoa inertnog plina, para ugljiko-

vodika i svjeeg zraka. Pritom su tankovi teretaopremljeni cijevima za inertiranje, a brzinauvoenja inertnog plina se odrava dovoljno ni-skom kako bi se minimiziralo mijeanje u tan-kovima (Slika 2.). Budui da je inertni plinma-

nje gustoe od para ugljikovodika, zadrava seiznad para tereta i potiskuje ih prema dnu tan-ka. Prednost inertiranja potiskivanjem je malapotrebna koliina inertnog plina te istovreme-no inertiranje vie tankova tereta.

Slika 2.Postupci inertiranja a) mijeanjem i b) istiskivanjem

Figure 2Inerting processes: a) dilution and b) displacement

Izvor /Source:http://thenauticalsite.com/

3 CARGO TANKS INERTING

Cargo tanks inerting implies the introductionof inert gas in cargo tanks with the intention tolower the hydrocarbon vapour and/or oxygenconcentrations. Cargo tanks inerting can be

performed by the dilution process or by the dis-placement process of gas in cargo tanks [4].During the dilution process, the hydrocarbon

vapour or oxygen are mixed with the inert gaswhich enters the cargo tanks with high velocityand their mixture is exhausted in the atmos-phere through the exhaust pipe on top of thecargo tank. This process is faster than the dis-placement process of gas in cargo tanks. Duringthe cargo tanks inerting, oxygen and hydrocar-bon vapour content must be measured on sev-eral different positions in order to safely ensure

that there is no danger of explosion.The displacement process implies the ex-

change of gas composition in cargo tanks onthe basis of the difference between the densi-ties of inert gas, hydrocarbon vapour and air.

At the same time, cargo tanks are equippedwith inerting pipes while the inert gas inlet ve-locity is maintained sufciently low to minimizethe mixing of gases inside cargo tanks (Figure2.). Since inert gas has a lower density than hy-drocarbon vapour, it retains over cargo vapours

and presses them towards the tank bottom. Theadvantage of the displacement process is asmall required volume of the inert gas what al-lows multiple cargo tanks to be inerted simulta-neously.

Palubni cjevovod/ Deck pipeline Palubni cjevovod/ Deck pipeline

a) b)


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Inertiranje tankova tereta se koristi pri slje-deim situacijama:

Poetno inertiranje tankova tereta nakonprimopredaje broda ili nakon dokovanja;

Obnavljanje zadovoljavajue inertne atmos-fere, ako je zbog nekog razloga kvalitetainertne atmosfere u tankovima tereta bilanaruena;

Smanjenje koncentracije para ugljikovodikau tankovima tereta.

Kada se udio kisika u atmosferi tankova tere-ta snizi ispod 11,5 %, mjeavina para ugljikovo-

dika i zraka u tankovima vie nije zapaljiva.Tank tereta moe se smatrati inertnim ako jevolumni udio kisika u atmosferi tanka nii od8 % [5]. Dijagram na slici 3. prikazuje podrujeeksplozivnosti mjeavine para zraka i ugljikovo-dika te su na njemu ucrtane karakteristine vri-

jednosti i toke podruja eksplozivnosti.

4. NUMERIKA SIMULACIJAINERTIRANJA TANKA TERETA

Numerika simulacija inertiranja tanka tere-ta provedena je na primjeru tankera za prijevozsirove nafte nosivosti 47 000 tona. Izabrani tan-

Eksplozivno

podrujeExplosive

range

Koncentracija kisika, % vol Oxygen content, % vol

Koncentracijaugljikovodika,

%v

ol

Hydrocarbongasconte

nt,%v

ol

Neupaljivo podruje

Non flamable range

Premalo kisikaToo low oxygen

Krit. granica razrjeivanjaCritical dilution line

Slika 3.Dijagram eksplozivnosti smjese para ugljikovodika i kisikaFigure 3Explosive range of hydrocarbon vapour and air mixture

Izvor /Source:http://thenauticalsite.com/

Cargo tank inerting is performed in the fol-lowing conditions:

Initial cargo tank inerting after the ship deli-very or after docking

Refreshment of the satisfactory inert atmosp-here if its quality in cargo tanks was distur-bed for some reason

Reduction of the hydrocarbon vapours con-centration inside cargo tanks.

When the oxygen content in cargo tanks isreduced below 11.5%, the mixture of the hy-drocarbon vapour and air is no longer amma-

ble. The cargo tank can be considered as an in-ert one when the oxygen content inside thetank is lower than 8% [5]. The explosive rangeof hydrocarbon vapour and air mixture withcharacteristic points and values is shown in Fig-ure 3.

4 NUMERICAL SIMULATION OFCARGO TANK INERTING

The numerical simulation of cargo tank in-erting is performed on the example of a crudeoil carrier of 47,000 dwt. The chosen tanker has12 cargo tanks or 6 pairs of cargo tanks where


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ker ima 12 tankova, odnosno 6 pari tankova, teje za simulaciju izabran jedan od tankova obuj-ma 5.158 m3. Na slici 4. prikazane su osnovnedimenzije tanka tereta te smjetaj i dimenzijeprikljuaka za ulaz inertnog plina i izlaz smjesepara ugljikovodika i zraka. Inertiranje tankovatereta provodi se postupkom inertiranja mijea-njem.

Radi pojednostavljenja geometrije unutra-njosti tanka, izostavljena je uronjena pumpa te-reta, zdenac ispod pumpe tereta, silazna cijevtereta i pratea armatura, a korigirane pregra-de i krov tanka tereta predstavljeni su kao rav-ne glatke plohe.

Numerika simulacija provedena je s pomo-u metode konanih volumena te je izraentrodimenzionalni model tanka tereta. Prostor,odnosno domena tanka tereta omreena je ne-

strukturiranom mreom tetraedara te su namodelu postavljeni odgovarajui rubni uvjeti.

Za analizu strujanja plinova unutar tanka te-reta potrebno je denirati matematiki modelkoji ukljuuje jednadbe ouvanja mase, kolii-ne gibanja te ouvanja energije [6, 7].

Jednadba ouvanja mase moe se zapisati usljedeem obliku:

, (1)

gdje je gustoa uida, a u brzina strujanjakroz granice kontrolnog volumena.

Slika 4.Dimenzije tanka tereta te smjetaj i dimenzije prikljuakaFigure 4Cargo tank dimensions with the location and size of ttings


simulation is performed on a 5158 m3capacitytank. The main dimension of the cargo tank

with the location and size of the inert gas inletand gas mixture exhaust pipes are shown in Fig-ure 4. The cargo tank inerting is performed bythe dilution process.

Toward the simplication of the inside tank

geometry, cargo pump, drop line and ttingswere omitted, while the corrugated bulkheadsand tank cover are represented as plain surfac-es.

A numerical simulation was performed byapplying a nite volume method and the cargotank was represented by a three dimensionalmodel. The cargo tank space was meshed witha non-structured tetrahedral mesh and ade-quate boundary conditions were set on themodel.

To analyze the ow of gases inside the cargotank, the mathematical model which includesmass, energy and momentum conservationequations had to be dened [6, 7].

The mass conservation equation can be writ-ten in the following form:

, (1)

where is the uid density, and u the ow ve-locity through the control volume boundaries.

The momentum conservation equation canbe written as:

Inlet pipeExit pipe

Exit pipe

Inlet pipe


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Jednadba ouvanja koliine gibanja moe seprikazati kao:

, (2)

gdje jepstatiki tlak,gubrzanje sile gravitacije,S

M

izvorni lan kao posljedica djelovanja sila, a

ijtenzor viskoznih naprezanja deniran kao:

, (3)

pri emu su dinamika viskoznost, V volu-menska viskoznost, aI

ijjedinini tenzor.

Jednadba ouvanja energije izraena je pre-ko ukupne specine entalpijeh0:

, (4)

gdje je k koecijent provoenja topline, a Shizvorni lan koji opisuje dotok topline. Ukupnaspecina entalpija h0 denirana je kao

, pri

emu je iunutarnja energija.

Za opis turbulentnog strujanja odabran jestandardnik-model turbulencije koji je opisans dvije transportne jednadbe za opis kinetikeenergije turbulencijeki energije disipacije tur-bulencije . Detaljni opis standardnoga modelaturbulencije dan je u [7].

5. ANALIZA REZULTATASIMULACIJA

U ovome radu simulacija inertiranja tankaprovedena je za sluaj prvog inertiranja tankanakon primopredaje broda ili inertiranja nakondokovanja broda. Na poetku simulacije, unu-tranjost tanka ispunjena je zrakom pri normal-

nim okolinim uvjetima, a potom je u tank upu-hivan plin duik. Pretpostavljeno je da je duik

jednake temperature kao zrak u tanku tereta.Izmjena topline izmeu stijenki tanka tereta iokoline je zanemarena.

Simulacija inertiranja tanka tereta provede-na je za tri razliita smjera upuhivanja duika, as ciljem istraivanja kako pojedini smjer utjeena brzinu postizanja inertne atmosfere. Analizi-rani smjerovi upuhivanja duika prikazani su naslici 5. pri emu je struja duika usmjerena oko-mito prema dnu tanka (sluaj 1), u smjeru pro-storne dijagonale (sluaj 2) te u smjeru dijago-nale bone stijenke tanka tereta (sluaj 3). Sve

, (2)

where p is the static pressure, g is the gravity,SM is the source term as the consequence offorce actions, and

ijis the viscous stress tensor

dened as:

, (3)

whererepresents the dynamic viscosity, Visthe volumetric viscosity, and Iij is the unit ten-sor.

The energy conservation equation is ex-pressed by the total specic enthalpyh0:

, (4)

where k is the heat transfer coefcient, and Shthe source term which desribes the heat ow.The total specic enthalpy h0 is dened as

, whereiis the internal energy.

The turbulent ow is described by the stand-ardk-turbulence model which consists of twotransport equations for the turbulent kineticenergykand the disipation of the turbulent ki-netic energy . A detailed desription of the k-

turbulence model is given in [7].

5 SIMULATION RESULTSANALYSIS

In this paper, the simulation of the cargotank inerting was carried out for the case of therst inerting after the ship delivery or after theship docking. At the beginning of the simula-tion, the cargo tank interior was lled with airat normal ambient conditions and nitrogenblow into the cargo tank started thereupon. It

was assumed that the nitrogen temperature isequal to the air temperature inside the cargotank. The heat exchange between the cargotank walls and the ambient was neglected.

The cargo tank inerting simulation was car-ried out for three different nitrogen stream di-rections with the intention to investigate howcertain stream directions inuence on reachingthe inert atmosphere. The analyzed stream di-rections are shown in Figure 5 where the nitro-gen stream is aimed straight to the tank bottom(case 1), towards space diagonal (case 2) andtowards cargo tank side wall diagonal (case 3).


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simulacije provedene su za brzinu strujanja du-ika od 20 m/s.

5.1. Analiza inertiranja tanka za sluaj 1

Numerika simulacija inertiranja tanka zasluaj 1 ukazuje da se duik najprije rasprostra-njuje uzdu bridova i stijenki tanka te nakon800 sekundi upuhivanja duik jo nije dosegaootvor izlazne cijevi (Slika 6.). Ovakav nainupuhivanja duika formira neku vrstu plata

koja okruuje sredinju zonu tanka s veomkoncentracijom zraka.

Slika 5. Smjerovi upuhivanja duika u tank teretaFigure 5Nitrogen injection directions into the cargo tank

Izvor/ Source:Autori /Authors

Slika 6.Koncentracija duika nakon 15 minuta

Figure 6Nitrogen concentration after 15 minutes


All simulations are carried out for the nitrogenstream velocity of 20 m/s.

5.1 Cargo Tank Inerting Analysis Case 1

The numerical simulation of the cargo tankinerting for case 1 shows that nitrogen owsrstly along cargo tank edges and walls and didnot reach the exhaust pipe 800 seconds afterblowing into the tank (Figure 6). This mode ofthe nitrogen injection forms some kind of a

jacket which surrounds the central zone of thetank with a higher concentration of the air.

Ulazna cijev/ Inlet pipe1. sluaj/ 1stCase

Ulazna cijev/ Inlet pipe2. sluaj/ 2ndCase

Ulazna cijev/ Inlet pipe3. sluaj/ 3rdCase


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Smjetaj cijevi te smjer upuhivanja imaju zaposljedicu vee brzine strujanja uz rubove tanka,pa nakon to duik ispuni sve rubove i kutovetanka tereta, sredinja se zona relativno spororazrjeuje. Na slici 7. prikazana je koncentracijazraka u tanku nakon 2 sata od poetka inertira-

Slika 7.Koncentracija zraka u tanku tereta nakon 2 h (vrh, sredina i dno tanka)Figure 7Air concentration after 2 hours (top, middle and bottom of the tank)


Slika 8.Koncentracija zraka u tanku tereta nakon 4 h (vrh, sredina i dno tanka)Figure 8Air concentration in the cargo tank after 4 hours (top, middle and bottom of the tank)


The location of the inlet pipe and the inject-ing direction caused higher gas velocities alongcargo tank edges and after the tank was lled

with nitrogen, the central zone was relativelyslowly diluted. Figure 7 shows the air content inthe cargo tank after 2 hours of inerting which is


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nja, a koja iznosi oko 50 %. Koncentracija zao-staloga zraka prikazana je u trima presjenimravninama razmjetenim na vrhu, u sredini i pridnu tanka tereta. Sredinja se zona najsporije ra-zrjeuje te nakon isteka puna 4 sata, prosjenakoncentracija prvobitne atmosfere jo uvijek

iznosi 20 % (Slika 8.).Promjene koncentracija zraka u tanku tereta

tijekom cijele simulacije inertiranja prikazanesu dijagramom na slici 8. za presjene ravninena vrhu, sredini i na dnu tanka tereta. U skladus trendovima dobivenih rezultata, za postizanjekoncentracija zraka niih od 10 %, uz ovakveuvjete upuhivanja duika, trebalo bi jo barem1,3 h. Vrijednosti koncentracija zraka u gornjoji donjoj zoni gotovo su identine, dok sredinjidio ima neto vie vrijednosti, to je obrazloe-

no u tekstu iznad.

5.2. Analiza inertiranja tanka za sluaj 2

Smjer upuhivanja inertnoga plina u ovome jesluaju usmjeren prema sredinjem dijelu tankatereta te se ve na samom poetku inertiranjamoe oekivati intenzivnije mijeanje i raspri-

vanje struje duika. Struja duika ne uspijevadostii do dna tanka, pa se duik ne distribuirapreko donjih rubova kao u prethodnom sluaju.U usporedbi sa slikom 6, uoljiv je nedostatakduika u donjem desnom kutu tanka, a to jeposljedica smjera upuhivanja duika.

Slika 9.Promjena koncentracije zraka u ovisnosti o trajanju inertiranja

Figure 9Change in the air concentration depending on the inerting duration

Izvor/Source:Autori /Authors

around 50%. The concentration of the retainedair is shown in three section planes placed ontop, in the middle and at the bottom of the cargotank. The dilution of the middle zone is the slow-est and the average concentration of the originalatmosphere is still 20% after 4 hours of inerting

(Figure 8).The changes in the air concentration in the

cargo tank during the whole simulation for thesection planes on top, in the middle and at thebottom of the tank are presented in Figure 9.

According to the trends of the obtained resultsand under such nitrogen blow conditions, theair concentration below 10% could be achievedin an additional 1.3 hours at least.

The values of air concentration on top and atthe bottom of the tank are almost identical

while in the middle of the tank the values areslightly higher as explained in the text above.


In this case, the nitrogen injecting was direct-ed towards the middle of the tank and a moreintensive mixing and dispersion of the nitrogenstream could be expected immediately after thebeginning of inerting. The nitrogen stream didnot reach bottom of the tank and nitrogen wasnot distributed along the edges as in the previ-ous case. As compared with Figure 6, the lack of


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Distribucija duika po rubovima tanka uspo-stavlja se tek nakon duljeg vremena, s time da usredini tanka opet ostaje zona vee koncentra-cije zraka. Prosjena koncentracija zraka u tan-ku tereta smanjuje se sporije nego u sluaju 1 tee po isteku 4 sata inertiranja dosei vrijednostod oko 28 % u sredini tanka tereta (Slika 11.).

Slika 11. Promjena koncentracije zraka u ovisnosti o trajanju inertiranjaFigure 11Change in the air concentration depending on the inerting duration


Slika 10.Koncentracija duika nakon 15 minutaFigure 10Nitrogen concentration after 15 minutes


nitrogen in the lower right corner is noted as theconsequence of the nitrogen injecting direction.

The nitrogen distribution along the cargo

tank edges was established after a longer peri-od whereby the middle of the tank still re-mained with a higher air concentration.


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Ako bi se eljela dostii koncentracija od 10 %zraka u tanku tereta, bila bi potrebna jo skoro2 dodatna sata inertiranja.

5.3. Analiza inertiranja tanka za sluaj 3Ovim nainom upuhivanja duika nastoji se

postii bolji efekt ispiranja tanka te postupnijeinertiranje tanka od ulazne prema izlaznoj cije-

vi. No, analizom dobivenih rezultata utvrenoje da zbog velikih dimenzija tanka tereta mlazduika ne dosee suprotnu stranu tanka. Nakon800 s inertiranja, minimalna koncentracija dui-ka iznosi 2,7 %, to je najmanje od svih trijuanaliziranih sluajeva. Distribucija duika utanku tereta nakon 800 s prikazana je na slici

12. Ovakav smjer upuhivanja nije ostvario elje-ni efekt i situacija je slina sluaju 2, ali sa slabi-

jim uinkom. Na slici 13. prikazani su rezultatinakon 4 sata inertiranja, pri emu prosjenakoncentracija zraka u sredini tanka tereta izno-si oko 32 %.

Slika 12.Koncentracija duika nakon 15 minutaFigure 12Nitrogen concentration after 15 minutes


The average air concentration was decreas-ing slower than in case 1 and its value wasaround 28% in the middle of the tank after 4hours of inerting (Figure 11). To achieve theconcentration of air in the cargo tank below

10%, inerting should be extended for an addi-tional 2 hours.


This mode of the nitrogen injection tends toachieve a better tank scavenging and progres-sive tank inerting from the inlet to the exhaustpipe. But, the analysis of the obtained resultsshowed that the nitrogen stream did not reachthe opposite tank side because of the large tankdimensions. After 800 s of inerting, the minimalnitrogen concentration was 2.7%, what is thelowest value of all the three analyzed cases. Thenitrogen distribution in the cargo tank after 800s is shown in Figure 12. This direction of the ni-trogen injection did not achieve a desirable ob-

jective and the situation is similar to case 2, butwith an inferior effect. Figure 13 shows the re-sults after 4 hours of inerting where an averageair concentration in the middle of the tank hada value of around 32%.


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5.4. Analiza inertiranja tanka uz poveanebrzine strujanja

Od triju prethodno razmatranih sluajeva,pokazalo se da je sluaj 1 najefektivniji nain

inertiranja tanka tereta. Uz pretpostavku da bipoveanje brzine strujanja duika ubrzalo iner-tiranje tanka, provedene su simulacije za brzinestrujanja od 25 m/s i 30 m/s. Pritom je analizi-ran samo sluaj sa strujom duika usmjerenomokomito prema dnu tanka, koji se je pokazaokao najbolji nain inertiranja tanka.

Dobiveni rezultati prikazani su na slikama14., 15. i 16. za svaku od triju karakteristinihzona u tanku tereta. Iz prikazanih dijagrama

vidljivo je da se najbolje ispiranje, odnosno

inertiranje tanka tereta postie najveom brzi-nom strujanja inertnoga plina, pri emu se na-kon 4 sata u tanku tereta postie koncentracijazraka nia od 10 %. I ovdje se uoava podjed-nako dobro ispiranje gornje i donje zone, dokse u sredini tanka tereta uvijek zadrava neto

via koncentracija zraka.

Prema preporukama iz literature [4], savjetu-je se odravanje dovoljno visoke brzine struja-nja inertnoga plina u tank zbog boljeg mijea-nja s medijem u unutranjosti tanka tereta.Takoer, ispusni otvor na tanku tereta moraimati dovoljno veliku povrinu poprenog pre-sjeka kako pad tlaka na izlasku iz tanka ne bi

Slika 13. Promjena koncentracija zraka u ovisnosti o trajanju inertiranjaFigure 13 Change in the air concentration depending on the inerting duration


5.4 Analysis of Cargo Tank Inerting withHigher Stream Velocities

Among the three previously analyzed cases,case 1 approved as the most effective mode of

cargo tank inerting. With the presumption thatthe increase in the nitrogen injection velocity

would speed up the tank inerting, additionalsimulations for velocities of 25 m/s and 30 m/s

were carried out. The case with the nitrogenstream aimed straight to the tank bottom wasfurther analyzed because it approved as themost effective mode of cargo tank inerting.

The obtained results are presented in Fig-ures 14, 15 and 16 for each of the three charac-teristic zones in the cargo tank. The results pre-sented show that the best cargo tank inerting

was achieved by the highest velocity of the inertgas stream, when the air concentration in thecargo tank below 10% was achieved after 4hours of inerting. It can be noticed that the topand the bottom of the tank were evenly scav-enged while the middle of the tank always con-tained a slightly higher concentration of air.

According to reference [4], it is recommend-ed to maintain high velocities of the inert gasdue to a better mixing with the media inside thecargo tank. In addition, the exhaust opening onthe cargo tank must be of a sufcient area sothe pressure drop on the tank exit would not


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Slika 14.Promjena koncentracija zraka u ovisnosti o trajanju inertiranja vrh tankaFigure 14Change in the air concentration depending on the inerting duration top of the tank


Slika 15.Promjena koncentracija zraka u ovisnosti o trajanju inertiranja sredina tankaFigure 15Change in the air concentration depending on the inerting duration middle of the tank


bio prevelik, to bi nepovoljno utjecalo na sma-njenje brzine strujanja upuhivanja inertnogaplina. Ovisno o kapacitetu sustava inertnogaplina na brodu, potrebno je voditi rauna i obroju tankova koji se istodobno inertiraju, jere to takoer imati znaajan utjecaj na duljinutrajanja postupka inertiranja.

become too high what can decrease the velocityof the inert gas injecting. Depending on the in-ert gas system capacity, the number of tanks

which are simultaneously inerted has a notable

inuence on the duration of the inerting proc-ess.


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6. ZAKLJUAK

U radu je prikazana problematika inertiranjatankova na brodovima za prijevoz tekueg tere-ta kao zatita od eksplozije ili nastanka poara.

Na kvalitetu inertiranja, odnosno ispiranja tan-kova tereta utjee niz imbenika poput geome-trije tanka tereta, sastava smjese plinova u tan-ku, vrste plina za inertiranje, brzine i smjerastrujanja plina za inertiranje, temperature oko-line itd. Uz pomo numerike simulacija dobi-

veni su rezultati koji mogu pomoi pri odabiruoptimalnih parametara za inertiranje tanka. Uradu su analizirana tri sluaja upuhivanja dui-ka u tank tereta, pri emu je smjer struje duikabio usmjeren okomito prema dnu tanka, u

smjeru prostorne dijagonale te u smjeru dijago-nale bone stijenke tanka tereta. Upuhivanjeduika okomito prema dnu tanka pokazalo sekao najbolji postupak koji daje najbolje rezulta-te od svih analiziranih sluajeva. Takoer, kaoto je i oekivano, najvea brzina upuhivanjainertnoga plina omoguuje najbre i najkvali-tetnije inertiranje tanka tereta.

Ovu numeriku simulaciju mogue je proiri-ti i za druge reime rada, pri emu se posebnozahtjevni uvjeti rada za sustav inertnoga plinapojavljuju tijekom iskrcaja tereta kada inertniplin mora nadomjestiti obujam ispumpanog te-reta. U tom sluaju u model je potrebno ugra-

Slika 16.Promjena koncentracija zraka u ovisnosti o trajanju inertiranja dno tankaFigure 16Change in the air concentration depending on the inerting duration bottom of the tank


6 CONCLUSION

The issue of the cargo tanks inerting as aprotection from explosion or re on board liq-uid cargo carriers is presented in this paper.

The quality of cargo tanks inerting or scaveng-ing is affected by a number of factors such ascargo tank geometry, gas mixture compositioninside the tank, kind of the inert gas, velocityand direction of the inert gas stream, ambienttemperature etc. The numerical simulationprovided results which could help in the selec-tion of optimal parameters for tank inerting.Three cases of the nitrogen injection were ana-lyzed where the nitrogen stream was aimedstraight to the tank bottom, towards the spacediagonal and towards the cargo tank side walldiagonal. Furthermore, and as expected, thehighest velocity of the inert gas injecting pro-

vides the fastest and the best cargo tank inert-ing.

This numerical simulation is possible to wid-en on other working regimes where particularlydemanding working conditions appear duringhe cargo unloading when the inert gas must re-place the volume of the pumped liquid cargo.In that case, the properties of the hydrocarbon

vapours and diesel oil combustion gases shouldbe added to model what will affect on the owpattern and progress of the cargo tank inerting.

An additional problem is the design of cargo


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diti svojstva para ugljikovodika i inertnoga plinanastalog izgaranjem lakog dizelskog goriva, toe utjecati na prol strujanja i tijek inertiranjatanka. Dodatni problem predstavlja konstrukcijatankova tereta koji nemaju glatku unutranjost,pri emu rebra i ostali dijelovi brodske strukture

predstavljaju prepreke za nesmetano strujanjeinertnog plina koji se upuhuje.

Bez obzira na gore navedene potekoe, nu-merike simulacije ovoga tipa mogu pruiti ko-risna saznanja o strujanju uida unutar tankovatereta te dati uvid u oekivani tijek procesainertiranja. Kvalitetna inertna atmosfera ubrodskim tankovima tereta od velike je vanostiza sigurnost posade, broda i tereta te ova inje-nica predstavlja stalnu motivaciju za prikuplja-nje novih saznanja iz ove problematike.

LITERATURA /REFERENCES

[1] Devanney, J., The Strange History of Tank Inerting, Center for Tankship Excellence, 2005. [2] Devanney, J., The Tankship Tromedy The Impending Disasters in Tankers, 2nded., The CTX Press, 2006. [3] Ozreti, V.,Brodski pomoni strojevi i ureaji, Split, Ship Management, 1996. [4] Standard for Inert Gas Systems, TP 04295E, Government of Canada, 1984. [5] Bjerketvedt, D., J. R. Bakke, K. Wingerden, Gas Explosion Handbook, Journal of Hazardous Materials, 52

(1997), 1150.

[6] Patankar, S. V.,Numerical Heat Transfer and Fluid Flow, Taylor & Francis, 1980. [7] Versteeg, H. K., W. Malalasekera, An Introduction to Computational Fluid Dynamics: The Finite Volume

Method, Prentice Hall, 2nded., 2007.

tanks which have not a smooth interior, whileframes and other parts of the ship structurerepresent obstacles to the inert gas ow.

Regardless of the above mentioned difcul-ties, the numerical simulations can provide use-ful information on the uid ow inside cargo

tanks and give us an insight into the expectedprogress of the inerting process. A quality inertatmosphere in the ship cargo tanks is very im-portant for safety of the crew, the ship and thecargo and this fact represents a permanent mo-tivation for collecting new discoveries on thistopic.

grbcic ,mrakovcic, hodak

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