Certificiranje, propisi i norme - Ex- · PDF fileEx-Bilten 2012. Vol. 40, br. 1-2 Certificiranje, propisi i norme Pretpostavljeni obujam potencijalno eksplozivne atmosfere u kontekstu

Ex-Bilten 2012. Vol. 40, br. 1-2

Certificiranje, propisi i norme

Pretpostavljeni obujam potencijalno eksplozivne atmosfere u kontekstu norme IEC 60079-10-1

Hypothetical volume of potentially explosive atmosphere in the context of IEC standard 60079-10-1

Predrag Peršić, dipl. ing. stroj.

INA Oil Industry, Zagreb, Croatia e-mail: [email protected]

Sažetak— Pretpostavljeni obujam potencijalno eksplozivne atmosfere (Vz) veličina je koja se koristi u klasifikaciji ugroženoga prostora za određivanje tipa zona opasnosti. Ovaj tekst daje kritički prikaz jednadžbi za određivanje Vz u normi IEC 60079-10-1 (kod nas HRN EN 60079-10-1), detaljno razmatra njihove nedostatke, predlaže alternativni pristup za određivanje Vz te opisuje jedan broj pokusa u stvarnim tehnološkim uvjetima u svrhu potvrđivanja novo predloženih jednadžbi.

Ključne riječi— pretpostavljeni obujam potencijalno eksplozivne atmosfere, donja granica eksplozivnosti, količina ispuštanja zapaljive tvari, izvor ispuštanja, prividni izvor ispuštanja, intenzitet provjetravanja, raspršivanje plina/pare, turbulencijsko uključivanje zraka, koncentracija zapaljivoga plina ili pare, pozadinska koncentracija, doseg zona opasnosti, zamašni mlaz ispuštanja, pasivni oblak, jednadžba kontinuiteta, prostorni kut radijalnoga širenja mlaza.

Abstract— Hypothetical Volume of Potentially Explosive Atmosphere (Vz) is the value used in hazardous area classification for estimating type of the zones. This text displays a critical review to the current equations for estimating Vz set in IEC standard 60079-10-1, scrutinizes their inherent deficiencies, proposes the alternative approach to Vz and describes number of field tests performed to validate the new proposed equations.

Keywords— Hypothetical Volume of Potentially Explosive Atmosphere, Lower Explosive Limit, Release Rate of Flammable Material, Source of Release, Pseudo Source of Release, Ventilation Rate, Gas / Vapour Dispersion, Air Entrainment, Concentration of Flammable Gas or Vapour, Background Concentration, Extent of Zones, Momentum Jet, Advective Plume, Continuity Equation, Spatial Angle of Radial Expansion.

I. TEORIJA I. THEORY

A. Uvod A. Introduction

Već izvjesno vrijeme problem pretpostavljenogaobujma potencijalno eksplozivne atmosfere Vz (rizičniobujam u daljnjem tekstu) predmet je rasprava međustručnjacima koji se bave klasifikacijom a prostora.Pojam je uveden u prvo izdanje norme, tadaIEC 60079-10, kao sredstvo za određivanje tipa zona iod tada je ostao izvor prijepora i ponekih ozbiljnihkritika. Pojam je po sebi u stvari apstrakcija kojaprikazuje obujam atmosfere s prosječnomkoncentracijom zapaljive tvari koja odgovara donjojgranici eksplozivnosti ili ispod toga, za odgovarajućifaktor sigurnosti. Naglasak je ovdje na prosječnojkoncentraciji. To podrazumijeva da unutar Vz postojičitava skala koncentracija, od 100% u blizini izvoraispuštanja do gotovo 0% na vanjskim rubovima obujma.

Taj obujam nije definiran ni geometrijski ni položajemu odnosu na izvor ispuštanja. To je ponajprije mjerakontaminacije koja "naznačava" koliki može biti stvarniprostor koji zauzima potencijalno eksplozivna atmosfera.Zbog toga su još u ranim danima postojanja norme

For quite some time the issue of Hypothetical Volume of Potentially Explosive Atmosphere Vz (hazardous volume in the further text) has been the subject of discussions among the experts dealing with hazardous area classification. The notion was introduced in the first edition of then IEC standard 60079-10 as the means for estimating the type of the zones and since then has been the source of controversies and some heavy critics. The notion by itself is actually an abstraction depicting the volume of atmosphere with the average concentration of flammable gas or vapour being at the Lower Explosive Limit or below for a certain safety factor. The emphasis here is on average concentration. That implies that there is a whole scale of concentrations within Vz, from 100% near the source of release of the flammable gas or vapour, to near 0% at the boundaries of the volume. This volume is defined neither by geometry nor by the position relative to the source of the release. This is more the measure of contamination that gives an idea

Predrag Peršić: Pretpostavljeni obujam potencijalno eksplozivne atmosfere... (a2 - a32)


nastali neki nesporazumi. Korisnici su bili u iskušenju dapromatraju Vz kao veličinu ugroženoga prostora pa sustoga započeli s praksom pretvaranja numeričkevrijednosti Vz u pravilne geometrijske oblike (običnokuglaste ili valjkaste) oko izvora ispuštanja zapaljivetvari. Pogrešna uporaba ovoga koncepta postala jeočigledna već tijekom prvoga perioda održavanja normepa je stoga u drugo izdanje uvedeno sljedeće upozorenje:"Nije namjera da se ovi proračuni (misli se za Vz)koriste za izravno određivanje dosega ugroženihprostora".

Međutim, koncept Vz ostao je sporan do današnjeg dana, ponajviše zbog metode izračuna navedene u

normi:

on how large the area of potentially explosive atmosphere really is.

Therefore, some misunderstandings emerged in the early days of the standard. The users had been tempted to see Vz as the size of the hazardous area and therefore started with the practice of transforming the numerical value of Vz in some regular geometrical shapes (usually spherical or cylindrical) around the source of release of flammable substance. The misuse of the concept became evident during the first maintenance cycle of the standard and therefore in the second edition the following warning was introduced: "It is not intended that these calculations (means for Vz) are used to directly determine extent of the hazardous areas".

However, the concept of Vz has remained controversial until today, mostly due to the calculation

methods referenced in the standard:

C

)dt/dV(fV min

z

(m3) (0.1)

gdje je: (dV/dt)min najmanji obujamski protok svježega

zraka (m3/s) potreban za razrjeđivanje atmosfere do donje granice eksplozivnosti (DGE), odnosno ispod nje za određeni sigurnosni odmak;

C broj izmjena zraka u jedinici vremena (s-1);

f (ne)učinkovitost provjetravanja/ventilacije, u pravilu od 1 to 5.

Najmanji obujamski protok svježega zraka izračunava se pomoću sljedeće jednadžbe:

where: (dV/dt)min is the Minimum Volumetric Flow Rate

of Fresh Air in (m3/s) required to dilute the atmosphere to Lower Explosive Limit (LEL);

C is the number of air changes in time unit (s-1);

f is (in)efficiency of ventilation, typically from 1 to 5.

Minimum Volumetric Flow of Fresh Air is calculated by the following equation:

293

T

LELk

)dt/dG()dt/dV( a

m

maxmin

(m3/s) (0.2)

gdje je: (dV/dt)min količina ispuštanja zapaljivetvari (kg/s);

Ta temperatura okoline (K); LELm donja granica eksplozivnosti (kg/m3); k faktor sigurnosti primijenjen na LELm

where: (dV/dt)min is the Maximum Release Rate of

Flammable Material (kg/s); Ta is the ambient temperature (K); LELm is Lower Explosive Limit (kg/m3); k is the safety factor applied to LELm

C je veličina kojom je definiran intenzitetprovjetravanja zatvorenoga prostora. Određuje sedijeljenjem obujamskoga protoka svježega zraka srazmatranim obujmom (V0). Pod određenim uvjetima tajobujam može biti i obujam prostorije koja se razmatra.

C is the value that defines ventilation rate of an enclosed space. It is obtained by dividing volumetric flow rate of fresh air with Volume under Consideration (V0). Under the circumstances, this volume may be considered the volume of the room concerned

0V

dt/dVC (0.3)

Očigledno je da uz isti obujamski protok svježegazraka, C ovisi samo o razmatranome obujmu. Što je većitaj obujam, to je manji C i obrnuto.

Postavlja se pitanje ovisi li Vz, definiran jednadžbom

It is obvious that with the same volumetric flow rate of fresh air C, depends only on size of the Volume under Consideration. The larger the volume, the lower C and vice versa.



(0.1), doista o C u fizikalnome smislu? Odgovor jepotvrdan ukoliko pretpostavljamo da postoji brzo iravnomjerno miješanje zraka s ispuštanim plinom/paromu cjelokupnome razmatranom obujmu. Međutim, tamogdje npr. postoji točkasti izvor relativno slabogispuštanja u velikoj prostoriji, primijenjeno ćeprovjetravanje jedva imati ikakva utjecaja na prostor ublizini ispuštanja i na sam oblak plina/pare. U velikimprostorijama učinak provjetravanja u smislu kretanjazraka osjeća se samo u blizini ulaznih i izlaznih otvora,ponajprije ovih drugih. Tako se u većini slučajevautjecaj provjetravanja ne osjeća tamo gdje bi trebalo, tj.u neposrednu prostoru ispuštanja. To znači da u velikombroju slučajeva veličina C nije relevantna za procesmiješanja / raspršivanja koji se zbiva u blizini izvoraispuštanja zapaljive tvari. Drugim riječima, razmatraniobujam može biti prevelik za razmatrano ispuštanje pastoga rezultati za Vz mogu ispasti suviše konzervativni udanim okolnostima.

Ista poteškoća nastaje i kod ispuštanja na otvorenome.Radi prevladavanja problema s provjetravanjem, koje nemože biti primijenjeno na otvoreni prostor, autorikoncepta u normi uveli su pojam virtualne kocke sastranicama od 15 m i pretpostavili da je "provjetravana"vjetrom brzine 0,5 m/s. Iz jednostavna izračuna proizlazida je broj izmjena zraka u takvoj virtualnoj kocki oko0,03 s-1. Za manja ispuštanja takva je virtualna kockaprevelika pa rezultati za Vz, koji su obrnutoproporcionalni broju izmjena zraka, postaju prevelikiodnosno prekonzervativni. U stvari, u najvećem brojuslučajeva koji se susreću u praksi virtualna će kocka bitiprevelika i stoga će rezultati biti daleko od stvarnosti nakonzervativnoj strani. To znači da postoji svojstvenapogreška u razmišljanju kada se pretpostavlja stalan,nepromjenljiv razmatrani obujam na otvorenome. Naprvi pogled čini se da količina ispuštanja i razmatraniobujam nisu funkcionalno uvjetovani, no ustvari ipakjesu. Rezultat je toga da su jednadžbe za izračun Vz unormi zasnovane na pogrešnoj pretpostavci. Autoriopisana koncepta bili su vjerojatno svjesni toga, novjerojatno su smatrali da je bolje koristiti jednostavne,razumljive iako ne sasvim točne jednadžbe koje će ipakdati rezultate gotovo uvijek na sigurnoj strani. Tako surazlozi sigurnosti i jednostavnosti prevladali nadrazlozima veće fizikalne ispravnosti.

Opisana logika bila je sasvim zadovoljavajuća uvrijeme kada je norma nastala, međutim tijekomposljednja dva desetljeća mnogo se toga promijenilo napodručju protueksplozijske zaštite. Tehnologijaproizvodnje opreme i instalacija za ugrožene prostoreznačajno je napredovala. Danas postoji mnogo opremekoja je projektirana i izrađena samo za zonu 2(kategorije 3). Dok je ranije bilo uobičajeno ugrađivatiopremu za zonu 1 (kategorije 2) u zonu 2 radi

Now, the question is whether Vz as defined by equation (0.1) really depends on C in physical terms? The answer is positive if we assume that there is a quick uniform mixing of air with the released gas/vapour throughout the whole volume. However where there is e.g. a point source of relatively small release in a large room, the ventilation applied will hardly have any impact on the area near the source of the release and the gas / vapour cloud itself. In large rooms the ventilation effect in terms of air movement is felt only near the inlet and exhaust openings. So, in most cases it is not felt where it matters, i.e. in the area of the release. That means that in many cases the value C is not relevant for the mixing/dispersion process that takes place near the source of the release of flammable material. In other words, the volume under consideration may be taken too large for the release concerned and therefore the result for Vz may be too conservative under the circumstances.

The same difficulty arises for the releases in outdoor situations. To overcome the problem of ventilation that cannot be applied to open air, the authors of the concept set in the standard have introduced a virtual cube with the sides of 15 m and considered it to be "ventilated" by the wind of 0,5 m/s speed. By simple calculation it appears that the number of air changes in such a virtual cube is about 0,03 s-1. For smaller releases, the virtual cube is too large and the results for Vz that are inversely proportional to the number of air changes appear overlarge, i.e. too conservative. As a matter of fact, for most cases met in practice, the virtual cube will be too large and hence the results will be far from reality on the conservative side. That implies that there is an inherent error in thinking when considering a fixed Volume under Consideration in open air. At the first sight it seems that Release Rate of Flammable Material and the Volume under Consideration are not functionally related, but actually they are. The result is that the equations for Vz set in the standard are based upon an erroneous assumption. The authors of this concept were probably aware of that, but considered that it is better to use simple and understandable although not quite exact equations which will deliver the results that are almost always on the safe side. So, the reasons of safety and simplicity prevailed over the reasons of physical exactness.

The logic described was quite satisfactory at the time when the standard was born however, in the last two decades a lot has changed in the area of explosion protection.

The technology of manufacturing electrical equipment and installations for hazardous areas has advanced tremendously. There is now a lot of equipment designed and manufactured for zone 2 only.



izbjegavanja pomne analize principa sigurnosti, danas sepodrazumijeva da treba zahtijevati opremu i instalacijekategorije 3 za zonu 2 (iako konačna odluka ovisi okorisniku i o njegovu upravljanju rizicima). Međutim,najveća promjena nastala je uvođenjem neelektričnihuzročnika paljenja u kompleks protueksplozijske zaštite.Sve je to promijenilo filozofiju klasifikacije ugroženogaprostora. Odjednom je postalo vrlo važno kakav će bititip zona i njihov doseg pa je kao posljedica togaklasifikacija prostora postajala sve važniji i odgovornijizadatak. To je pridonijelo i sve većoj relevantnostiliterature koja se bavi raspršivanjem plinova i para priispuštanju te konačno i same klasifikacije ugroženogaprostora. U takvim je okolnostima važnost norme IEC60079-10-1 (osnovne norme iz područjaprotueksplozijske zaštite) porasla do stupnja kada jepostalo neophodno pozabaviti se nekim njezinimnaslijeđenim nedostacima. Na kraju, ali ne i najmanjevažno, računalno modeliranje (CFD) postalo je snažanalat u procjeni raspršivanja mlaza plinova/para. Dok je uranim danima postojanja norme konzervativizamsmatran vrlinom, sad je on postao zapreka uravnoteženupristupu protueksplozijskoj zaštiti. Bilo je za očekivatida će korisnici prije ili kasnije početi pomnije promišljatijednadžbe za određivanje Vz i propitivati u kojoj mjerirezultati korespondiraju sa stvarnosti i/ili računalnimmodelima. Nije bilo teško dokazati da nešto nije u redu stim jednadžbama, posebno stoga što sama normapriznaje da "...ova jednadžba (misli se za situacije naotvorenome) općenito rezultira prevelikim obujmom..."

While before it was normal to install the equipment for zone 1in zone 2 to avoid scrupulous analysis of the safety principle, nowdays it is essential to require category 3 of equipment and installations for zone 2.

However, the major change has been the introduction of non electrical sources of ignition into the complex of explosion protection. All that has changed the philosophy of area classification. Suddenly, It has become very important what the type and the extent of zones will be and consequently, area classification is becoming increasingly important and responsible job. That has contributed to the relevance of the literature dealing with gas/vapour dispersion and ultimately the area classification itself. In such a context, the importance of IEC 60079-10-1 (the basic standard in the area of explosion protection) has risen to the degree when it become necessary to deal with certain inherited deficiencies. Last, but not least, CFD computational modelling has become a powerful tool in assessing gas / vapour dispersion. While conservatism was considered as virtue in the early days of the standard, now it has become an obstacle for the balanced approach to explosion protection. It was quite expected that sooner or later the users will start to scrutinize the equations for Vz and ask themselves how the results comply with the reality or CFD modelling. It was not difficult to prove that something is wrong with those equations, especially because the standard itself recognizes that "...this equation (means for open air situations) will generally result in an overlarge volume...".

1) Osnovni pristupi za ponovno promišljanje Vz 1) Basic approaches to rethinking Vz

Postalo je jasno da smo u stvari suočeni s istimproblemom kad računamo Vz za zatvorene prostore kao iza otvoreni prostor, osim što je očitiji onaj za otvoreniprostor. U oba slučaja stvarni problem predstavljarazmatrani obujam V0 koji u svakom slučaju nijedovoljno dobro definiran.

Mogao bi se steći pogrešan dojam da je kodzatvorenoga prostora taj obujam određen zidovimaprostorije. Međutim, isto kao i za otvoreni prostor, ovajobujam može biti prevelik za neka manja ispuštanja.Drugim riječima, postoji nesklad između uprosječivanjaprovjetravanja na razini V0 i onoga što se stvarno zbiva ublizini razmatrana ispuštanja. Provjetravanje(ventilacija) koja opslužuje čitav zatvoreni prostor možese pokazati irelevantnom za prostor u blizini ispuštanja.Da rezimiramo, pod određenim uvjetima ispuštanje "nezna" je li u zatvorenome ili na otvorenome prostoru.

Osnovno je pitanje kako postupiti u ovoj naslijeđenojsituaciji i unaprijediti normu da postane relevantankodeks tehničke prakse. Mnoga su istraživanja obavljenai mnogo je truda uloženo da bi se proizvele jednadžbekoje će u razumnoj mjeri korespondirati s fizikalnom

It has become clear that we actually face the same problem when calculating Vz for enclosed spaces and for open air situations, except that the one for open air is more obvious. In both cases the real issue is V0 which is not well defined anyway.

One may get the wrong impression that for enclosed spaces it is obvious that it is a volume defined by the walls. However, same as for open area, this volume may be too large for some smaller releases. In other words, there is a conflict between the averaging of ventilation at the level of V0 and that what is really happening at the release being considered. The ventilation that serves the whole enclosed space may prove to be irrelevant to the space in close proximity of the release. To summarize, at certain conditions the release "does not know" whether it is in an enclosed or whether in an open space.

The basic question now is, how to deal with this inherited situation and upgrade the standard to become a relevant code of practice. A lot of researches have been done and a lot of effort made to produce equations that will reasonably correspond with reality. Not an



stvarnosti. Doista, nimalo lak zadatak uzimajući u obzirfizikalnu složenost bilo kojeg ispuštanja plina/pare ičinjenicu da u praksi ne postoje dva potpuno ista slučajaispuštanja. Postalo je očigledno da u potrazi za "boljim"jednadžbama treba preispitati čitav postojeći koncept.Matematičku apstrakciju nazvanu Vz lišenu fizikalnogasadržaja trebalo je preispitati i podvrći temeljitoj analizi.Zaključak je da Vz ipak treba promatrati kao manje iliviše realističan mlaz ili oblak kako bi se predvidjelanjegova veličina. Tijekom ovoga promišljanja pojavilasu se dva osnovna pristupa:

a) Smatrati Vz kao grubo geometrijsko približenje uobičajena oblika točkastoga ispuštanja plina/pare u interakciji s vjetrom, odnosno gibanjem zraka i primijeniti jednadžbu kontinuiteta na taj stožasti kontrolni obujam,

b) Smatrati Vz kao dinamički, zamašni mlaz koji se samorazrjeđuje čistim mehanizmom turbulencijske difuzije na svojim rubovima, bez ikakva utjecaja atmosferske nestabilnosti okoline.

Oba pristupa predstavljaju samo grubo približenjeinače složena procesa raspršivanja. Međutim, oba sudaleko stvarnija u pretpostavkama i rezultatima negopostojeći pristup u normi.

easy task indeed, taking into account complexity of any gas/vapour release and the fact that there are no exactly two same cases of release. It has become obvious that in searching for "better" equations, the whole concept has to be reexamined. The mathematical abstraction called Vz deprived of the physical content has been revisited and scrutinized.

The conclusion is that Vz has to be regarded as a more or less realistic jet or plume in order to estimate its size. In the course of this rethinking process two major approaches emerged:

a) To consider Vz as a rough geometrical approximation of a gas/vapour release in interaction with wind / air movement and apply Continuity Equation to this conical control volume,

b) To consider Vz as a momentum jet that is self diluted by the sheer mechanism of turbulence diffusion at the boundaries without any impact of atmospheric instability.

Both approaches are just rough approximations of the otherwise complex dispersion mechanism. However, both are far more realistic in assumptions and the results than the current approach set in the standard.

a) Procjena Vz pomoću jednadžbe kontinuiteta a) Estimating of Vz by Continuity Equation

Koncept je prikazan na slici 1. Zapaljiva tvar (plin ilipara) izlazi pod tlakom kroz otvor atmosferskogaispusta. Kao kod bilo kojeg ispuštanja pod tlakom, mlazima tipičan stožasti oblik označen prostornim kutemradijalnoga širenja φ. Idealno je približenje ovoga oblikaisječak kugle polumjera r. Vjetar određene brzinenastrujava na tu virtualnu kuglu i prolazi kroz nju. Naslici 1. smjer vjetra prikazan je paralelno u smjeruispuštanja, no u svrhu ovoga razmatranja to nije bitno.Može postojati čak i situacija s vjetrom u suprotnusmjeru od ispuštanja, samo bi u tom slučaju prostorni kutbio nešto veći. Međutim, za potrebe izračuna Vznormalno, računa se s najnepovoljnijim scenarijima, a tipodrazumijevaju male do umjerene brzine vjetra kojejedva da imaju utjecaja na oblik tlačnoga mlaza, aliunatoč tome mogu odnijeti zapaljivu tvar na udaljenostna kojoj mlaz prestaje biti konzistentan i ponaša se višekao pasivni dim. Tako je čitav koncept neutralan uodnosu na vanjske čimbenike koji utječu na procesispuštanja.

he concept is displayed in Figure 1. Flammable material (gas or vapour) is being released under pressure, e.g. through a vent. As by any pressurized release, the jet has a typical conical shape with a spatial angle of expansion φ. The ideal approximation of this shape is a sector of a sphere having radius r. Wind of certain speed is streaming across this virtual sphere. On Figure 1, the direction of the wind is downstream but for the purpose of this elaboration it is not important, it may even be an upwind situation. It is just that for an upwind situation the angle φ would be greater. However, for the purpose of calculating Vz we normally deal with the worst case scenarios and those imply small to moderate wind speeds which have hardly any impact on the shape of the pressurized jet but can nevertheless transport the flammable material away at the distance where jet is not consistent any more and behaves rather like an advective plume. So, the whole concept is neutral towards the external factors that influence the release.



Slika 1. Ispuštanje plina na otvorenom Figure 1 Gas release in open air

Slijedom ove šire slike ispuštanja, na slici 2. prikazanje stožasti oblik kao "kontrolni obujam" za jednadžbukontinuiteta. Zapaljivi plin ili para promatrani su na dvauzastopna presjeka: kod prividnoga izvora ispuštanja,gdje je koncentracija još uvijek 100%, i na udaljenojperiferiji, gdje je mlaz razrijeđen do gotovo zanemarivekoncentracije. Pretpostavljamo da će ista količina plinaili pare koja uđe u "kontrolni obujam" na diskuprividnoga izvora polumjera rs, napustiti ga krozvanjski/periferijski disk polumjera rd. To je u stvariprincip osnovnoga zakona održanja. U ovomrazmatranju zanemareni su mehanizmi raspršivanja jerza potrebe ovoga izračuna nije važno je li mlaz biorazrijeđen turbulencijskom difuzijom ili gibanjem zrakauzrokovanim vjetrom ili utjecajem oba čimbenika.Ovdje "hvatamo snimku" dva već razvijena stanja mlazana dva različita presjeka niz struju ispuštanja plina. Toće nas dovesti do konačne jednadžbe s osnovnimveličinama koje određuju veličinu bilo kojeg oblakaplina ili pare. Prednost ovakva pristupa jasna je sama posebi jer nam omogućava da "preskočimo" sve složeneaspekte razvoja mlaza koristeći činjenicu da njegovoblik malo varira u okviru uobičajenih meteorološkihuvjeta.

Further to this broader picture, Figure 2 displays the conical shape as the "control volume" of Continuity Equation. The flammable gas or vapour is considered at two cross sections: at the pseudo source distance from the opening where the concentration is still 100% and at the far periphery where the release jet has been diluted to almost negligible degree. We assume that the same amount of gas or vapour that enters this control volume at the pseudo source disk of radius rs will leave it through the peripherial disk of radius rd. This is actually the basic of Conservation Law. In this consideration, the mechanisms of dispersion have been neglected because, for the purpose of this calculation, it does not matter whether the jet has been diluted by the turbulence diffusion or by the wind induced air movement or by the both. We just "screen capture" the two already developed situations at the two different cross sections downstream. This consideration will bring us to the final equation with the basic values that determine the size of any gas/vapour cloud. The advantage of such an approach is selfevident since it allows us to "by-pass" all the complex aspects of jet development using the fact that its shape varies very little within the usual meteorological conditions.



Slika 2. Koncept "kontrolnoga obujma" za ispuštanje plina Figure 2 Concept of the control volume for a gas release

Jednadžba kontinuiteta za zapaljivi plin koji prolazikroz "kontrolni obujam" prikazan približno kao kuglinisječak, može se opisati kao:

Maseni protok na ulazu = Maseni protok na izlazu (na periferijskome disku)

Continuity Equation for the flammable gas through the "control volume" displayed as a near sphere sector can be described as:

Mass flow of gas at the inlet = Mass flow of gas at the outlet (at peripherial disk)

2d

2sss rv)r(ru (1.1)

2d

2sss rv)r(CruC

gdje je: Cs koncentracija zapaljive tvari na prividnome

izvoru (kg/m3); C(r) koncentracija zapaljive tvari na vanjskom/

periferijskom disku (kg/m3). Pretpostavlja se da će brzina istjecanja zapaljivoga

plina na udaljenosti r cos φ/2 od izvora, a pri izlasku izkontrolnoga volumena, biti jednaka brzini vjetra sobzirom da se plin tamo ponaša kao pasivni dim. Tada,transformacijom lijeve strane, jednadžba poprimasljedeći oblik:

where: Cs is the concentration of flammable material at

pseudo source (kg/m3); C(r) is the concentration of flammable material at

peripherial disk (kg/m3). It is assumed that the velocity of the flammable gas

at the distance r cos φ/2 will be equal to wind speed since the cloud there behaves as an advective plume. Then, by transformation of the left side, the equation

gets the following form:

2dmax rv)r(C)dt/dG(

v)r(C

)dt/dG(r max2d

Također se pretpostavlja da će koncentracijazapaljivoga plina na udaljenosti r cos φ/2 od izvora bitiniža od DGEm, npr. k x DGEm. U svrhu ovogarazmatranja, k može poprimiti zanemarivo malevrijednosti, a ne nužno samo tipične vrijednostinavedene u normi (0,25 ili 0,5).

It is also assumed that the concentration of the flammable gas at the distance r cos φ/2 will be less than LELm, e.g. k x LELm. For the purpose of this elaboration, k may take a negligibly small value and not necessarily the typical values set in the standard (0,25 or 0,5).

To podrazumijeva sljedeću transformaciju: That implies following transformation:

vLELk

)dt/dG(r

m

max2d

2/3

m

max3d vLELk

)dt/dG(r

(1.2)

Ukoliko Vz odgovara isječku kugle kao "omotaču"potencijalno eksplozivne atmosfere, tada je Vz = f (rd)kao što je prikazano niže:

If Vz equals a sphere sector as an envelope of potentially explosive atmosphere, then Vz = f (rd) as displayed below:



2cos1

3

r2V

3

z

(obujam kuglina isječka / volume of a sphere sector))

2sin

rr d

(polumjer kugle / radius of the sphere))

2cos1

2sin3

r2V

3

3d

z

2sin

2cos1

r3

2V

3

3dz

3)( dz rV

)(

Vr z3d (1.3)

Tada, izjednačavanjem (1.3) i (1.2): Then, by equalizing (1.3) and (1.2):

2/3

m

maxz

vLELk

)dt/dG(

)(

V

2/3

m

maxz vLELk

)dt/dG()(V

)()/( 3

2/3

max mvLELk

dtdGV

mz

(1.4)

ZAKLJUČCI: CONCLUSIONS:

Jednadžba (1.4) izvedena je iz jednadžbe kontinuiteta istoga zadovoljava osnovni zakon dinamike fluida.Zaključci se zasnivaju na sljedećim premisama:

a) Mehanizam uključivanja zraka u rubnoj zoni mlaza zanemaren je u svrhu pojednostavljenja. Uzete su u obzir samo posljedice uključivanja zraka, tj. razrjeđivanje oblaka plina duž osi ispuštanja. U tom smislu konačna jednadžba ne predstavlja ni zamašni mlaz ni pasivni oblak iako je dakako više orijentirana prema principu dima. To nije nikakav nedostatak jer će oblak plina, čak i kada ga generira zamašni mlaz ispuštanja, poprimiti daleko od otvora svojstva pasivnoga dima.

b) Obujam eksplozivne atmosfere Vz ima stožasti oblik što je razumno očekivati pri bilo kojem ispuštanju iz točkastoga izvora. Stožasti oblik očekivan je čak i u slučajevima kad vjetar puše uz struju ispuštanja. Računalno modeliranje pokazuje da u takvim slučajevima zamah mlaza prevladava i da vjetar utječe samo na prostorni kut radijalnoga širenja mlaza. Samo u rijetkim slučajevima s jakim nasuprotnim vjetrom oblak može poprimiti

Equation (1.4) is derived from Continuity Equation and hence satisfies the basic law of fluid dynamics. Following premises have been set:

a) The mechanisms of air entrainment at the boundaries of the jet have been neglected for the purpose of simplification. Just the consequences of air entrainment have been taken into account and that means, the dilution of the gas cloud along the release axis. In that sense, the final equation is neither momentum jet, nor plume, However, it's form is certainly plume oriented. This is not a deficiency since any gas cloud, even that generated by momentum jet, will have plume characteristics downstream.

b) The volume of explosive atmosphere Vz has a conical shape what is reasonably expected by any release from a point source. Conical shape is expected even in cases when wind is blowing opposite to release direction. CFD modelling shows that the jet momentum usually prevails in such cases and that the wind affects only the spatial angle of radial expansion of the jet. Only in



valjkasti oblik. Međutim, slučajevi s vjetrom takve jačine nisu predmet ovih razmatranja.

c) Osnovni je problem koeficijent Ф koji ovisi o velikom broju varijabli sažetih u pretpostavljeni prostorni kut radijalnoga širenja. Može se staviti prigovor da koeficijent Ф nije dovoljno dobro definiran i da zahtijeva empirijsko potvrđivanje. Unatoč svemu, uz neke razumne premise možemo ga pretpostaviti na sigurnoj strani. Na primjer, znamo pouzdano da je prostorni kut širenja mlaza/dima negdje između 220 i 600 ovisno o uvjetima ispuštanja te da koncentracija plina na periferiji mora biti sasvim zanemariva. Odgovarajući koeficijent Ф možemo pokušati naći između tih graničnih vrijednosti, ovisno o zahtijevanu sigurnosnome odmaku.

Jednadžba (1.4) također je primjenljiva i za zatvoreneprostore. Međutim, tada valja razmotriti drukčiji faktor(ne)učinkovitosti provjetravanja/ventilacije (f > 1), zbogtoga što je u zatvorenome prostoru vjerojatnijenakupljanje plina uslijed njegova ponovnogauključivanja u mlaz ispuštanja.

rare instances, with the strong opposing wind, will the gas cloud take a cylindrical shape. However, the cases with wind of such magnitude are not the subject of these considerations.

c) The main issue is the coefficient Ф which depends upon a large number of variables compressed in assumed spatial angle of radial expansion. Objection could be submitted that the coefficient Ф is not well defined and that it requires a proper empirical validation. Nevertheless, under some reasonable premises, it can be assumed on the safe side. E.g., we know for sure that the spatial angle of jet/plume expansion is somewhere between 220 and 600 depending on the release conditions and that the gas concentration at the periphery must be quite negligible. Between those border values we may try to find an appropriate coefficient Ф depending on the safety margin required.

Equation (1.4) is applicable for enclosed spaces as well. However, then a different coefficient of ventilation (in)efficiency (f > 1) must be considered because gas build up due to gas reentrainment is more likely.

b) Procjena Vz konceptom zamašnoga mlaza b) Estimating of Vz by Momentum Jet concept

Ovaj koncept zasnovan je na dinamičkoj analizistvarnoga mlaza ispuštanja koji se raspršuje u okolnojatmosferi inducirajući turbulenciju na svojim rubovima.Okolni se zrak uključuje u mlaz duž rubne zone(difundira u mlaz) pa se na taj način mlazsamorazrjeđuje uslijed vlastita zamaha. Nakon izvjesnaperioda na vremenskoj skali mlaz će poprimiti konačanoblik i veličinu koja će se zatim održavati tijekom čitavatrajanja ispuštanja. To je dinamički-vremenskiuprosječen model ispuštanja pod stalnim tlakom i uznultu brzinu vjetra. To podrazumijeva da je gibanjezraka/atmosferska nestabilnost posve izvan razmatranjau opisivanju procesa raspršivanja iako ovaj model vrijedii uz umjerene brzine vjetra. Ideja iza ovoga pristupa, jestda nulta brzina vjetra predstavlja u stvari najnepovoljnijiscenario u kontekstu protueksplozijske zaštite. Međutim,razumno je pretpostaviti da će takva idealna, vremenskiuprosječena koncentracija plina/pare prije ili kasnijenestati nakon što prestane ispuštanje. Zbog toga je ipakpotrebna bar neka atmosferska nestabilnost da oblakbude odnesen/uklonjen. Tako se ovaj model zasniva nanekoj vrsti dvojna pristupa:

1. Atmosferska nestabilnost nema nikakva utjecaja na razvoj mlaza/oblaka jer zamah mlaza prevladava nad bilo kojim drugim vanjskim utjecajem,

2. Atmosferska nestabilnost utječe na proces raspršivanja tek nakon što je ispuštanje prestalo.

U kasnijim razmatranjima vidjet ćemo da model nakraju ipak zahtijeva atmosfersku nestabilnost jer ne

This concept is based upon the fluid dynamics analysis of a real gas release jet which disperses in the surrounding atmosphere by inducing turbulence at it's boundary. The surrounding air is entrained along the boundary zone and thus the jet is actually self diluted by the momentum of the release. After certain period on a time scale, the jet will take it's final shape and size which will then be sustained during the whole release duration. This is a dynamic - time averaged model of the release under constant pressure at zero wind speed. That implies that air movement/atmospheric instability is completely out of consideration in describing the dispersion process although the model is valid for the range of zero to moderate wind speeds. The idea is that a zero wind speed situation is actually the worst case scenario in the context of explosion protection. However, it is reasonable to assume that such an ideal, time averaged gas/vapur concentration will sooner or later disappear after the release has stopped. Therefore, at least some kind of atmospheric instability is needed to transport/remove the cloud away. So, the model is based on a kind of dual approach: 1. Atmospheric instability does not have any impact

on the development of the jet/cloud because the jet momentum prevails over any other external influence,

2. Atmospheric instability influences the dispersion process only after the release has stopped.

In the later considerations we will see that the model



može postojati izvan konteksta gibanja zraka/vjetra. Sobzirom da je gibanje zraka posljedicaprovjetravanja/ventilacije, tada ventilacija postaje odpresudne važnosti, ne samo za kontrolu veličinerizičnoga obujma, već i za kontrolu postojanostieksplozivne atmosfere. (Za više detalja vidi: D MWebber, M J Ivings, R C Santon, Ventilation theory anddispersion modelling applied to hazardous areaclassification; Journal of Loss prevention in the ProcessIndustries, 2011).

eventually requires atmospheric instability because it cannot exist out of the context of air movement/wind. Since air movement is the consequence of ventilation, then the ventilation becomes of paramount importance, not only for control of size of the hazardous volume but also for the control of persistence time of explosive atmosphere (For more details see: D M Webber, M J Ivings, R C Santon, Ventilation theory and dispersion modelling applied to hazardous area classification; Journal of Loss prevention in the Process Industries, 2011).

Slika 3. Koncept dinamičkoga, zamašnog mlaza ispuštanja Figure 3 Concept of momentum release jet

)m(XX

X1

16

r9V 3

3

bcrit

b2/3

s

b3

sZ

(1.5)

critb XX

Na otvorenom je Xb = 0 i stoga: In open air Xb = 0 and hence:

)m(X

1

16

r9V 3

3

crit

2/3

s

b3

sZ

(1.6)

gdje je: where:

B

p

pK1rr

a0s efektivni polumjer prividnoga izvora (m) / is the effective radius of the pseudo source (m)

Slika 4. Koncept polumjera prividnoga izvora ispuštanja Figure 4 Concept of pseudo source radius

gdje je: where: r0 efektivni polumjer izvora (m); K konstanta za koju je K = 0,5 dobro približenje;

r0 is the effective radius of the source (m); K is a constant, for which K = 0,5 is a good

approximation;



p apsolutni tlak u spremniku (Pa); pa atmosferski apsolutni tlak (Pa);

p is the absolute pressure in containment (Pa); pa is atmospheric absolute pressure (Pa);

1

1

2B

(npr./e.g., B = 1,9 za/for = 1,4)

politropski eksponent adijabatske ekspanzije ili

omjer specifičnih toplina; α koeficijent uključivanja zraka u procesu

turbulencijske difuzije; ρ gustoća smjese zraka i plina (kg/m3);

s

ss TR

pM

gustoća čistoga zapaljivog plina na prividnome izvoru (kg/m3);

M molarna masa (kg/kmol); ps apsolutni tlak na prividnome izvoru (Pa); ps pa

R opća plinska konstanta 8315 J/kmol K; Ts apsolutna temperatura plina na prividnome

izvoru (K); Xcrit kritična vol./vol. koncentracija (0,25 LELv ili

0,5 LELv);

0

sb VC

qfX

pozadinska vol./vol. koncentracija u prostoriji nastala provjetravanjem/ventilacijom (za otvoreni prostor: Xb= 0)

qs obujamski protok ispuštanja zapaljivoga plina (m3/s);

C broj izmjena zraka u jedinici vremena (s-1).

is the polytropic index of adiabatic expansion or ratio of specific heats;

α is the jet air entrainment coefficient; ρ is the density of air gas mixture (kg/m3);

s

ss TR

pM

is the density of pure flammable gas at pseudo source (kg/m3);

M is the molar mass (kg/kmol); ps is the absolute pressure at pseudo source in (Pa);

ps pa R is the universal gas constant 8315 J/kmol K; Ts is the absolute gas temperature at pseudo source

in (K); Xcrit is the critical vol/vol concentration (0,25 LELv

or 0,5 LELv);

0

sb VC

qfX

is the background vol/vol concentration in the room arising from ventilation (for open air situations: Xb = 0);

qs is the volumetric release rate of flammable gas (m3/s);

C is the number of air changes per unit of time (s-1). Gustoća (kg/m3) pozadinske smjese zraka i plina u

prostoriji može se izračunati na sljedeći način: The density in kg/m3 of the background air gas

mixture in the room can be calculated as follows:

baab C)M/M1(

gdje je: where:

1b q

f

dt

dGC pozadinska koncentracija plina (kg/m3) / is the Background Concentration of gas (kg/m3);

q1 obujamski protok smjese zraka i plina koja napušta prostoriju (m3/s).

q1 is the volumetric flow of air gas mixture leaving the room (m3/s).

Jednadžba (1.5) mogla bi se učiniti jednostavnijom zakorištenje uz pretpostavku da ab te stoga

prikazujući /1/ sb , gdje je ρ bezdimenzionalnarelativna gustoća-veličina koja se uobičajeno koristi prianalizi plina. Jednadžba bi se mogla daljepojednostavniti uvođenjem konstantnih vrijednosti za K iB koje su prethodno predložene te bi tako proizašlasljedeća jednadžba:

Equation (1.5) could be made easier for use by assuming that ab and thus displaying

/1/ sb where ρ is the dimensionless relative density, a value commonly used in gas analysis. The equation could be further simplified by introducing fixed values for K and B proposed above and thus the following relations would be obtained:

2/1

a0s p

pr71,0r

)m(XX

X1r63,0V 3

3

bcrit

b2/3

s

a3

0Z

(1.7)



Za situacije na otvorenom Xb = 0: For open air Xb = 0:

)m(X

1r63,0V 3

3

crit

2/3

s

a3

0Z

(1.8)

ZAKLJUČCI:

Jednadžba (1.5) izvedena je računalnim modeliranjemraspršivanja mlaza (CFD). Zaključci se zasnivaju nasljedećim premisama:

a) Uključivanje zraka u rubnoj zoni mlaza ključni je mehanizam razrjeđivanja. Atmosferska je nestabilnost zanemarena kako bi se ostvario najlošiji scenarij raspršivanja plina. Ventilacija, po sebi, uopće ne pridonosi raspršivanju.

b) Proces raspršivanja u zoni ispuštanja razdvojen je od makro procesa ventilacije. Međutim, teorija ventilacije indirektno je prisutna u pozadinskoj koncentraciji Xb. Pozadinska je koncentracija rezultat ventilacije i označava sveopće/uprosječeno stanje atmosfere u zatvorenome prostoru "iza " paradigme ispuštanja. Tako se teorije raspršivanja i ventilacije susreću u jednadžbi (1.5), ali nema uzročno-posljedične veze između njih. Ventilacija je nadređena paradigmi ispuštanja kao neki okvir unutar kojeg se ispuštanje zbiva tako da turbulencija na rubovima mlaza uključuje smjesu zraka i plina koncentracije Xb, a ne čisti zrak.

c) Geometrija rizičnoga obujma Vz nije definirana, međutim pretpostavljamo da je na otvorenome to određena ekstrapolacija zamašnoga mlaza, a ne samo matematička apstrakcija kako je zapisano u normi. Stoga Vz definiran jednadžbom (1.5) ima dvojnu prirodu, onu stvarnoga mlaza koji djeluje silama svog zamaha i drugu, koja je definirana samo njegovom prosječnom koncentracijom. U slučaju zatvorenih prostora prepoznajemo ovu dvojnu prirodu mnogo lakše. Pretpostavljamo da će se kroz dulje vrijeme uspostaviti neko stacionarno stanje u kojem će zapaljive tvari biti po čitavu prostoru u količini određenoj s koncentracijom Xb i mali prostor u blizini izvora ispuštanja s koncentracijom 100% što će rezultirati određenim gradijentom skalarnoga polja koncentracija.

d) Osnovni je problem koeficijent uključivanja zraka α koji zahtijeva odgovarajuću empirijsku potvrdu. Tipično, koeficijent se kreće negdje između 0,05 i 0,1 pri čemu su veće vrijednosti na sigurnijoj strani.

CONCLUSIONS:

Equation (1.5) is derived through fluid dynamics dispersion modelling.

Following premises have been set: a) Air entrainment at the boundary zone of the jet is

the key mechanism of the dilution. Atmospheric instability has been neglected to comply with the worst case scenario of the gas dispersion. Ventilation by itself does not contribute to the dispersion at all.

b) The dispersion process in the area of the release has been separated from the large scale ventilation process. However, ventilation theory has been indirectly introduced through Background Concentration Xb. The background concentration is the result of the ventilation and it depicts the overall / averaged status of the atmosphere in an enclosed space "behind" the release paradigm. So, dispersion and ventilation theory meet in Equation (1.5) but there is no causal connection between the two. Ventilation is superimposed to the release paradigm as a kind of framework so that the turbulence entrarains air gas mixture of concentration Xb and not pure air.

c) The geometry of the hazardous volume Vz is not defined, however it is assumed that in open air situations it is an extrapolation of the real momentum jet and not only an abstraction as defined in the standard. Therefore Vz as defined by Equation (1.5) has a dual nature, the one of the real jet which acts with the forces of it's momentum and the other that is defined only by it's average concentration. In the case of enclosed spaces we recognize this dual nature much easier. It is assumed that in the long run a steady state situation will be established in which there will be flammable gas all over the place in the amount defined by Xb and the small area near the source of release with 100% concentration resulting in a kind of gradient of a scalar field of concentrations.

d) The main issue is the coefficient of air entrainment α which requires a proper empirrical validation. Typically, the coefficient is somewhere between 0,05 and 0,1 with larger values being on the safer side.



c) Sinteza

Nakon utvrđivanja osnovnih činjenica u pogledu ovadva pristupa, možemo se poslužiti nekim veličinama izjednog pristupa da bismo rasvijetlili postavke iz onogadrugog. Uzmimo jednostavan, shematski prikazprostorije s točkastim izvorom ispuštanja i osnovnimkonceptom ventilacije s ulaznim i izlaznim otvorima.Situacija u takvoj prostoriji bila bi u osnovi turbulentna,s koncentracijama plina daleko od ravnomjerneraspodjele unutar prostora. Možemo čak očekivati nekedžepove visoke koncentracije, ne samo u blizini izvoraispuštanja, već također i u kutovima prostorije.Međutim, za potrebe izračuna možemo uprosiječitivrijednosti koncentracija i brzine zraka preko presjekaokomita na glavnu struju.

c) Synthesis

After establishing the basic facts regarding the two approaches, we can use certain values from one approach to enlighten the facts about the other. Let's take a simple schematic display of a room with a point source of gas release and a basic ventilation concept of inlet and outlet openings. The situation in such a room would be turbulent with the concentration of gas far from being uniformly distributed over the place. We may even expect some pockets of high concemtration not only near the source of the release, but in the corners as well. However, for the purpose of calculation we may average the values of the concentration and air velocity over a cross section perpendicular to the main stream.

A1

A2

v2

v1

A0

v

speed profile

vmax

gas release

V0, Xb, Cb

L

fresh air

air gas mixture

A1

A2

v2

v1

A0

v

speed profile

vmax

gas release

V0, Xb, Cb

L

fresh air

air gas mixture

Slika 5. Ispuštanje plina u provjetravanoj prostoriji

Figure 5 Gas release in a ventilated room

22b0bmax

AvCAvCdt

dG

je jednadžba kontinuiteta / is continuity equation

gdje je: where:

maxdt

dG

max. količina ispuštanja zapaljive

tvari (kg/s)

Cb pozadinska koncentracija (kg/m3); v prosječna brzina zraka na presjeku okomitu na

smjer strujanja (m/s);

0A površina presjeka prostorije okomita na smjer strujanja zraka (m2);

v2 brzina strujanja smjese zraka i plina na izlaznom otvoru (m/s);

A2 efektivna površina izlaznog otvora (m2).

maxdt

dG

is the max. release rate of flammable

material (kg/s)

Cb is the Background Concentration (kg/m3); v is the average air speed at the cross section

perpendicular to the air flow direction (m/s);

0A is the area of the cross section perpendicular to the air flow (m2);

v2 is the air+gas mixture speed at the outlet opening (m/s);

A2 is the outlet opening effective area (m2).

2s

2s

b

max0 rar

vC

)dt/dG(A

vCa

)dt/dG(r

b

max2s

3s

3sz rbrV je osnovni princip proporcionalnosti

za zamašni mlaz ispuštanja.

3s

3sz rbrV is the basic principle of

proportionality for a momentum jet.



gdje je: where: a, b faktori proporcionalnosti rs polumjer prividnoga izvora ispuštanja (m).

a, b are the factors of proportionality; rs is the pseudo source radius of release in (m).

b

Vr z3s

vCa

)dt/dG(

b

Vr

b

max3/2

z2s

2/3

b

maxz vCa

)dt/dG(bV

)m(vC

)dt/dG(V 3

2/3

b

maxz

Ako zanemarimo koeficijent k, konačna jednadžba ista je kao (1.4). Daljnjom transformacijom ta jednadžba poprima mnogo uporabljiviji oblik:

The final equation is actually the same as (1.4). With further transformation the equation gets a more usable form:

)m()A/A(vC

)dt/dG(V 3

2/3

022b

maxz

Na slici 6. prikazano je skalarno polje koncentracijaProsječna, pozadinska koncentracija u prostoriji je Xb,Cb, dok su koncentracije oko izvora ispuštanja više, tj.100% na samu izvoru.

The illustration of the scalar field of concentrations is given in Figure 6. The average, background concentration in the room is Xb, Cb while the concentrations around the source of the release are higher, being 100% at the very source.

A1

A2

v2

v1

V0, Xb, Cb

L

fresh air

air gas mixture

gas release

A1

A2

v2

v1

V0, Xb, Cb

L

fresh air

air gas mixture

gas release

Slika 6. Polje koncentracija plina u provjetravanoj prostoriji Figure 6 Field of gas concentrations in a ventilated room

Važno je napomenuti da oba prethodno opisanapristupa predstavljaju čist oblik neometana ispuštanjaplina. Ovdje nisu uzeti u razmatranje mlazovi ispuštanjakoji udaraju o prepreke kao ni različiti oblici ponovnauključivanja ispuštena plina. Očigledno je da se u obaslučaja konačne jednadžbe pojavljuju s određenimkoeficijentima (Ф, α). Vrijednost tih koeficijenata možese odrediti samo empirijskim metodama, obično nekomvrstom laboratorijskih ispitivanja i/ili računalnim (CFD)simulacijama. Međutim, potvrđivanje pretpostavki u

It is important to point out that both approaches set forth represent pure form of unobstructed gas release. Impinging jets and various forms of gas reentrainment have not been taken into consideration. It is obvious that in both cases the resultant equations emerge with certain coefficients (Ф, α). The value of those coefficients can only be estimated by empirrical methods, usually by some type of laboratory testing and/or CFD simulations. However, validation in the context of explosion protection requires more than



kontekstu protueksplozijske zaštite zahtijeva više odtoga. Pokusi u stvarnim pogonskim uvjetima ispuštanjajedina su metoda koja može pokazati što će se dogoditi ustvarnim životnim situacijama. Takvi su pokusi skupi iteško ih je pripremiti, tj. nije uvijek moguće postićistupanj preciznosti, točnosti i ponovljivostikarakterističan za laboratorijska ispitivanja u smanjenumjerilu.

II. POTVRĐIVANJE

A. Teorija ispuštanja

Eksperimentalno potvrđivanje rezultata za Vz nijenimalo lak zadatak. Ono što možemo snimiti i izmjeriti,jest mjerljiv/vidljiv mlaz ili oblak plina. Međutim, tajmlaz ili oblak ne odgovara onome što nazivamo Vz.Njegova je prosječna koncentracija normalno viša odDGE, a to podrazumijeva da bi Vz trebao biti veći.Drugim riječima, Vz bi se trebao protezati iza onoga štoje mjerljivo i/ili vidljivo. Međutim, nije jednostavnoutvrditi gdje su granice Vz. O tome možemo samonagađati s manjom ili većom točnošću, koristećimjerljive/vidljive granice kao uporište. Tek tadamožemo ekstrapolirati Vz s manjom ili većom točnošću.

Međutim, moramo imati na umu da ovo nije problemračunalnoga (CFD) modeliranja. Ono što stvarno želimopostići, jest jednostavna jednadžba za brzu orijentacijukoja će davati sigurne rezultate za velik broj neutralnoteških plinova (u odnosu na zrak). Ilustracija ovogaproblema prikazana je na slici 7. kao grafičkopojednostavljenje mlaza ispuštanja.

that. Large scale field tests are the only method that could show what will happen in real life situations.

Such tests are expensive and difficult to arrange, i.e. it is not always possible to achieve the degree of precision, accuracy and repeatability which is characteristic for small scale laboratory testing.

II. VALIDATION

A. Theory of release

Experimental validation of Vz results is not an easy task. What we can screen and measure is the detectable/visible gas jet or cloud. However, this jet or cloud does not correspond to Vz. Its average concentration is normally higher than LEL and that implies that Vz should be larger. In another words, Vz should extend beyond what is detectable or visible. However, it is not easy to establish where the boundaries of Vz are. We can just guess with more or less accuracy using the detectable/visible boundaries as a "foothold". Then, we can extrapolate Vz with more or less accuracy.

However, we should keep in mind that this is not a CFD modelling issue. What we really want to achieve is a simple equation for quick reference that would deliver safe results for a large number of neutrally buoyant gases. The illustration of this problem is given in Figure 7.

Slika 7. Obujam slobodnoga mlaza ispuštanja u odnosu na Vz

Figure 7 Free release jet volume vs. Vz

Dijagram prikazuje pad prosječne koncentracije dužosi mlaza. To je tipična krivulja za bilo koje ispuštanjepod tlakom. Prosječna je koncentracija čitavamjerljivoga mlaza viša od DGE. Da bismo procijenilidoseg Vz, trebamo ekstrapolirati krivulju koncentracije

The diagram shows average gas concentration decrease along a jet axis. This is a typical curve for any pressurized release. The average concentration of the whole detectable jet is higher than LEL. To estimate the extent of Vz we should extrapolate the



tako da isprugana površina iznad koncentracije DGEpostane jednaka ispruganoj površini ispod nje.

concentration curve so that the striped area above LEL concentration becomes equal to the striped area below.

0

0

0

0

0

LELv

LELv LELv

Line that borders average LEL concentrations

Average

Average

Average

rd

Concen tration X (%)

Concentration curve

x (m)

X 0

xs

r(x)

0

LLEL

u(x)

u0

Lcrit > LLEL

0

0

0

0

0

LELv

LELv LELv

Line that borders average LEL concentrations

Average

Average

Average

rd

Concen tration X (%)

Concentration curve

x (m)

X 0

xs

r(x)

0

LLEL

u(x)

u0

Lcrit > LLEL

Slika 8. Profil slobodnoga mlaza ispuštanja u odnosu na Vz

Figure 8 Profile of a free release jet vs Vz

Drugi pogled na ovaj problem prikazan je na slici 8.Ovdje vidimo tipičan profil ispuštanja pod tlakom spadajućim koncentracijama. Profili duž osi ispuštanjaimaju tipičan oblik "šešira" s koncentracijama koje teže0 u rubnoj zoni turbulencije. S tim profilima preklapajuse prosječne koncentracije na odgovarajućim presjecimamlaza. Ako pretpostavimo da je na udaljenom krajumlaza prosječna koncentracija presjeka DGEv, tada jeprosječna koncentracija čitava mlaza značajno viša odDGEv. Tako se taj mlaz ne podudara sa Vz. Da dobijemoobujam koji bi odgovarao Vz, trebamo ekstrapoliratiprosječnu koncentraciju na svakom presjeku niz strujudo razine DGEv (ili k x DGEv ovisno o tome kojukoncentraciju smatramo kritičnom). Učinivši to,dobivamo graničnu crvenu crtkanu liniju koja označavavaljak koji omata obujam s prosječnom koncentracijomDGEv. Očigledno je da je taj obujam veći od onogamjerljivog mlaza/oblaka:

Another perspective is given in Figure 8. Here we see a typical pressurized release profile with decreasing concentrations. The profiles along the axis have typical "top hat" form with the concentrations tending to zero at the boundary turbulence zone. Overlapping those profiles are the average concentrations at the cross sections. If we assume that at the far end of the jet the average cross section concentration is equal to LELv, then the average concentration of the whole jet is significantly higher than LELv. So, this jet does not correspond to Vz. To obtain the volume that would correspond to Vz we should extrapolate average concentrations at each downstream cross section to LELv (or to k x LELv

depending on which concentration we consider critical). By doing that, we get the boundary red dashed line which depicts the cylinder that envelopes a volume of average concentration being LELv. It is obvious that this volume is larger than the one of the detectable jet/cloud:

cz VV

cz VV

gdje je: where: Vc is the volume of a detectable jet/cloud (m3). Vc obujam mjerljivoga mlaza/oblaka (m3). 1) Krivulja koncentracije i doseg mlaza

Da bismo dobili stvaran prikaz mlaza, važno jeodrediti zakon koji slijede krivulje koncentracijaprikazane na dijagramima. Polazište treba ponovno bitijednadžba kontinuiteta:

1) Concentration curve and the jet extent

In order to get the real picture of a jet, it is important to determine the law which follow the concentration curves displayed on the drawings. The starting point should again be Continuity Equation:



)x(A)x(u)x(Cdt

dG

max

(2.1)

C(x) je prosječna koncentracija duž osi mlaza(kg/m3);

u(x) je prosječna brzina istjecanja plina duž osi mlaza (m/s);

A(x) je površina presjeka okomita na smjer istjecanja duž osi mlaza (m2).

C(x) is the average concentration along the jet axis (kg/m3);

u(x) is the average gas speed along the jet axis (m/s);

A(x) is the area of the perpendicular cross section of the jet along the axis (m2).

)x(A)x(u

dt

dG

)x(C max

x

xu)x(u s

s

22 )2/tg(x)x(A

gdje je: where:

us brzina istjecanja plina na prividnome izvoru (m/s);

xs aksijalna udaljenost prividnoga izvora od otvora ispuštanja (m);

x aksijalna udaljenost od otvora niz struju (m); x xs

φ prostorni kut radijalnoga širenja mlaza ispuštanja (0).

us is the gas release speed at pseudo source (m/s);

xs is the axial distance of the pseudo source from the release opening (m);

x is the axial downstream distance from the opening (m); x xs

φ is the spatial angle of radial expansion of the release jet (0).

2/tg

rrx 0s

s

2/1

a0s p

pr71,0r

(vidi poglavlje "Procjena Vz konceptom zamašnoga mlaza")

gdje je: where: r0 efektivni polumjer otvora (m) r0 is the effective radius of the opening (m)

2/tg

rp

pr71,0

x0

2/1

a0

s

2/tgx

1p

p71,0ru

)x(u

2/1

a0s

1

p

p71,0ru)2/tg(x)x(A)x(u

2/1

a0s

1p

p71,0ru)2/tg(x

dt

dG

)x(C2/1

a0s

max



1p

p71,0ru)2/tg(x

ru)x(C

2/1

a0s

s2

ss

Tada, uvrštavanjem prethodne formule za rs dobivamosljedeću jednadžbu:

Then, by interpolating the formula for rs above, we obtain the following equation:

)m/kg(

p

p21)2/tg(x

p

pr71,0

)x(C 32/1

a

2/1

as0

Pod pretpostavkom da je kut φ približno 300, tada je za sve slučajeve prigušenoga ispuštanja:

Under the assumption that the angle φ is approximately 300 in all cases of sonic pressure release:

)m/kg(

p

p21x

p

pr65,2

)x(C 32/1

a

2/1

as0

(2.3)

Uzimajući u obzir da je: Taking into account that:

MV

)x(XM)x(C

s

ss TR

pM

gdje je: where:

M molarna masa (kg/kmol); X(x) prosječna vol./vol. koncentracija; VM volumetrijska konstanta 22,41 m3/kmol;

as pp tlak na prividnome izvoru (Pa); R opća plinska konstanta 8315 J/kmol K; Ts apsolutna temperatura plina na prividnome

izvoru (K).

M is the molar mass (kg/kmol); X(x) is the average vol/vol concentration; VM is the volumetric constant 22,41m3/kmol;

as pp is the pressure at pseudo source (Pa);

R is the universal gas constant 8315 J/kmol K; Ts is the absolute temperature of the gas at

pseudo source (K). Tada je vol./vol. koncentracija duž osi mlaza: Then the concentration in vol/vol along the jet axis is:

2/1

as

2/1

aaM0

p

p21MTRx

p

ppMVr65,2

)x(X

2/1

as

2/1

a0

p

p21Tx

p

pr714

)x(X



2/1

as

2/1

a0

p

p21T)x(X

p

pr714

x

Stoga je doseg mlaza: Hence, the extent of the jet is:

)m(

p

p21TX

p

pr714

L2/1

ascrit

2/1

a0

crit

(2.4)

gdje je: where:

vcrit LELkX kritična vol./vol. koncentracija / is the critical vol/vol concentration

Ova kritična koncentracija može biti odabranaproizvoljno. U pravilu se izražava određenim postotkomDGEv kod kojeg je detektor plina podešen da alarmira(npr. tvornička postavna vrijednost detektora).

Određivanje točne vrijednosti Ts zahtijevalo biiteracije u skladu s Joule-Thomsonovim efektom zasvaki pojedini plin. Da izbjegnemo taj složeni postupak,Ts možemo aproksimirati s 293 K. Ovo naravno nijeidealan pristup, ali razumno služi svrsi koju smoprethodno naveli.

This critical concentration may be chosen at will. Typically, it is a percentage of LELv at which a gas detector is set to trigger (e.g. factory setting of the detector).

The accurate estimation of Ts would require iterations according to the Joule-Thomson effect for each particular gas. In order to avoid this complex procedure, Ts can be approximated with 293 K. This, of course is not an ideal approach but serves the purpose reasonably.

Stoga bi pojednostavljena jednadžba bila: Therefore, the simplified equation would be:

)m(

p

p21X

p

pr5,2

L2/1

acrit

2/1

a0

crit

(2.5)

U svrhu grafičkoga prikaza možemo postaviti

sljedeću pretpostavku: For the purpose of graphical display, we may set the following premise:

Xcrit = 0,01 Ona podrazumijeva da je unaprijed odabrana neka

jedinična kritična koncentracija od 1%. Tada jednadžba2.5 poprima praktičniji oblik koji je prikladan zauniverzalni grafički prikaz (vidi sliku 9.):

This implies that we have chosen in advance certain unit critical concentration of 1%. Then, the equation 2.5 gets a more practical form for the universal graphical display (see Figure 9):

)m(

p

p21

p

pr250

)L(2/1

a

2/1

a0

1crit

(2.6)



Slika 9. Kritični doseg u odnosu na polumjer otvora ispuštanja Figure 9 Critical Extent vs. Radius of the Release Opening

Tada za svaki pojedini slučaj Xcrit možemo pomnožitirezultat iz dijagrama s korekcijskim faktorom:

Then, for each individual case of Xcrit we may multiply the result from the diagram by a correction facto

critc X

01,0f

2) Doseg Vz kao aksijalno produženje mlaza 2) Extent of Vz as the axial extension of the jet

Da bismo odredili doseg kod kojeg će prosječnakoncentracija mlaza biti Xcrit, postavljamo sljedećujednadžbu:

In order to obtain the extent at which the average concentration of a jet will be Xcrit:

XL

dxxX

X

L

xs )(

prosječna vol./vol. koncentracija mlaza dosega L (m): is the average vol/vol concentration of a jet with an extent of L (m):

dxx

1

p

p21TL

p

pr714

XL

x2/1

as

2/1

a0

s

dxx

1

p

p21TL

p

pr714

Xh

s

L

x2/1

ash

2/1

a0

crit

gdje je: where: Lh doseg (m) koji bi odgovarao obujmu

mlaza/oblaka veličine Vz, odnosno ono štonazivamo rizičnom udaljenošću (vidi sliku 7.).

Lh is the extent of a jet in (m) that would correspond to the jet/cloud volume of Vz, i.e. hazardous distance (see Figure 7).



ss

h2/1

ash

2/1

a0

crit xx

Lln

p

p21TL

p

pr714

X

h

ss

h

s0crit L

xx

Lln

)T,p,r(fX

crit

s0

ss

h

h

X

)T,p,r(f

xx

Lln

L

crit

s0h X

)T,p,r(f)L(f (2.7)

Rješenjem jednadžbe po Lh možemo pokazati da jeaksijalni doseg koji odgovara obujmu Vz, mnogo veći odLcrit te da se Vz proteže niz struju daleko izakoncentracija od k x DGEv. Možemo razumnopretpostaviti da takav doseg ulazi u prostor u kojem semožda nalazi svega nekoliko molekula plina, ako itoliko. Ovakav koncept nema mnogo praktična smislatako da je pristup prikazan na slici 8. znatno prikladniji.

By solving this equation for Lh we can show that the axial extent corresponding to Vz is much longer than Lcrit and thus Vz spreads downstream far beyond the concentrations of k x LELv. We may reasonably assume that such extent reaches the area where maybe few molecules of gas are present if any. Such a concept does not make much sense in practical terms, so an approach displayed in Figure 8 is more appropriate.

3) Doseg Vz kao radijalno proširenje mlaza

Pristup prikazan na slici 8. zasniva se na sljedećimpremisama:

3) Extent of Vz as the radial extension of the jet

The approach set in Figure 8 is based upon the folowing premises:

crith LL

crit2

dz LrV

gdje je: where: rd polumjer (m) presjeka mlaza na udaljenosti

Lcrit od otvora ispuštanja na kojem je prosječna koncentracija pala do Xcrit

rd the radius (m) of the jet cross section at the distance Lcrit from the opening where the average concentration decreases to Xcrit

2/tgLr critd

23critz )2/tg(LV

)m(L226,0V 33critz (2.8)

Međutim, postoji također i drugi izraz za Vz (1.4) prethodno izveden iz jednadžbe kontinuiteta:

However, there is also another expression for Vz (1.4) obtained prevously by Continuity Equation:

)m(vLELk

)dt/dG(V 3

2/3

m

maxz

(1.4)

Izjednačavanjem (2.8) i (1.4): By equalizing (2.8) and (1.4):

)m(vLELk

)dt/dG(L226,0 3

2/3

m

max3crit

2/3

m

max3crit vLELk

)dt/dG(L226,0



2/3

crit

M002

03crit vXM

VurL226,0

2/3

crits

002

03crit

4

vXTR

purL1083,3

2/3

00

crits3

0

crit

pu

vXT

r

L290

(2.9 )

gdje je: where: u0 brzina istjecanja plina na otvoru (m/s); p0 tlak plina na otvoru (Pa);

p > p0 > ps pa

u0 is the gas speed at the opening (m/s); p0 is the gas pressure at the opening (Pa); p > p0 > ps pa

Tlak plina na otvoru je dinamički i stoga niži od tlakau spremniku ili cjevovodu. S obzirom da se radi onestalnoj vrijednosti, pouzdaniji je način za određivanjekoeficijenta Ф korištenje jednadžbe u poglavlju 1.2.1:

The gas pressure at the opening is the dynamic pressure and hence lower than the pressure in the containment. Since it is an erratic value, the more reliable way to estimate Ф is by using the equation set under clause 1.2.1:

2sin

2cos1

3

2

3

Pretpostavljajući da je φ = 300: Assuming that φ = 300:

74,0

B. Uvod u pokus

Da bismo ustanovili metodologiju eksperimentalnapotvrđivanja jednadžbi u stvarnim pogonskim uvjetima,moramo poći od fizikalnih osnova slobodnog ispuštanjaplina pod tlakom. Vremenski uprosječen (stacionarni)mlaz nastao takvim ispuštanjem, ima tipičan stožastioblik koji neznatno varira kod malih do umjerenih brzinavjetra. Tipičan mlaz prikazan je na slici 10. Prostorni kutradijalnoga širenja negdje je između 250 i 300 ovisno otlaku pod kojim se plin ispušta, brzini i smjeru vjetra. Usvrhu daljnjeg postupka potvrđivanja, uzimamo da je tajkut približno 300.

B. Introduction to the experiment

In order to establish the methodology of a large scale experimental validation of the equations, we have to begin from the essentials of a free pressurized gas release. The time averaged jet generated by such a release has a typical conical shape which varies slightly within the small to moderate wind speeds. Typical jet is given in Figure 10. The spreading angle of radial expansion is somewhere between 25 and 300 depending on the pressure of the gas being released and the wind speed and direction. For the purpose of further validation procedure, we take it as being approximately 300.



Slika 10. Tipično slobodno ispuštanje pod tlakom Figure 10 Typical pressurized free release

U principu postoje tri manje ili više izražene zone mlaza:

- Konzistentni mlaz (područje u kojem mlaz ima čvrsto organiziran/strukturiran oblik koji je jasno raspoznatljiv u odnosu na okolinu i nije podložan utjecaju atmosferskih nestabilnosti),

- Prijelazno područje (područje u kojem mlaz gubi svoj organizirani oblik, a turbulencije duž granične zone postaju vidljive dok je područje mlaza podložno utjecaju atmosferskih nestabilnosti najvišega stupnja),

- Dim (područje u kojem se mlaz transformira u pasivni oblak sasvim podložan utjecaju atmosferskih nestabilnosti).

Prikaz na slici 10. karakterističan je za situaciju snultom brzinu vjetra ili umjerenom brzinom u smjeruispuštanja. Vjetar nasuprot ispuštanju može deformiratipočetni oblik mlaza u smislu da prostorni kut radijalnogaširenja postane veći. U situacijama s jakim vjetromnasuprot ispuštanja, udaljeni dijelovi mlaza mogupromijeniti smjer gibanja i biti ponovno uvučeni umatricu ispuštanja što će mlazu dati valjkasto obilježje. Sobzirom da situacije s jakim vjetrom ne predstavljajunajlošiji scenarij u smislu protueksplozijske zaštite, samoće nulta ili umjerena brzina vjetra predstavljatiatmosferski okvir za bilo koje ispitivanje u pogonskimuvjetima.

There are in principle three more or less distinguishable zones of the jet:

- Consistent jet (the area in which a jet has a well organized, consistent form which is clearly distinguishable from the surrounding area and not susceptible to atmospheric instabilities),

- Transitional area (the area in which a jet looses its organized form and the turbulences along the boundary zone become visible while it is susceptible only to the atmospheric instabilities of the highest degree),

- Plume (the area in which a jet transforms in an advective plume and is highly susceptible to atmospheric instabilities).

The illustration given in Figure 10 is characteristic for a zero wind or a moderate downwind situation. An upwind situation may distort the initial form of jet in a sense that the spreading angle of radial expansion becomes larger. In upwind situations characterized by high wind speeds, the distant part of the jet may reverse the direction and be reentrained thus giving the release a cylindrical form. Since high wind situations do not represent the worst case scenario in terms of explosion protection, only zero to moderate wind speed situations should be the atmospheric frame for any field testing.

1) Opis ispitivanja u pogonskim uvjetima

Cilj eksperimenta u pogonskim uvjetima trebao bi bitiodređivanje obujma mlaza što je dalje moguće niz struju.Što je veći dio mlaza snimljen, to će bolja biti procjenaraspršivanja. Od posebna interesa moraju biti rubne zonegdje koncentracija plina teži 0. Pod takvim premisamaizvedena je serija pokusa na naftnom polju STRUŽEC,Hrvatska. Plin korišten za snimanje ispuštanja bio jevlažni prirodni plin iz sustava za plinsko podizanje nafte.Pokusi su izvođeni s različitim veličinama otvora podstalnim tlakom od 30 bara. Ispuštanje je bilježeno

1) Description of the field testing

The aim of any field testing should be to estimate the volume of a jet as far as possible downstream. The more of the jet has been screened, the better the evaluation of the gas dispersion. Of special intrest must be the boundary zones where the gas concentration tends to zero. Under such premise a serie of field tests had been performed on STRUZEC oil field; Croatia. The gas used for the release screening was wet natural gas from the gas lift system of the field. The tests were performed with different



infracrvenom (IC) kamerom, uz kontrolno mjerenje dvadetektora plina tvornički podešena tako da alarmiraju kodkoncentracije od 20% DGE.

Uzimajući u obzir karakteristike detektora plina,metodologija mjerenja bila je podešena na odgovarajućinačin (za detalje vidi sliku 11.).

release openings and under constant pressure of 30 barg. Screening of the release was recorded by IR camera and two industrial gas detectors factory set to trigger at 20% of LEL gas concentration.

Taking into account the characteristics of the gas detectors, the methodology of the detection had been adjusted appropriately (for the details see Figure 11).

Slika 11. Položaj detektora plina za vrijeme eksperimenta Figure 11 Position of the gas detectors in the course of the experiment

Eksperiment je organiziran tako da je ispuštanje plinaistovremeno snimano specijalnom, vrlo osjetljivominfracrvenom (IC) kamerom za prepoznavanjepropuštanja plina i običnom digitalnom kamerom. Dvijeosobe s ručnim detektorima plina bile su pozicionirane uzoni ispuštanja, jedna niz struju u osi mlaza, a druga postrani (vidi sliku 11.).

Tako je mlaz/oblak bio istovremeno definiran s trirazličita sredstva prepoznavanja. U tom smislu riječima"mjerljiv" i "vidljiv" dana su sljedeća značenja: mjerljiv -misli se detektorom plina, a vidljiv - misli se kroz okularIC kamere. S obzirom da su detektori plina tvorničkipodešeni da alarmiraju kod 20% DGE, osjetljivostmjerenja je ograničena. Slijedom toga eksperimentalna jemetoda prilagođena ograničavajućim okolnostima.

Nakon što se mlaz ispuštanja stabilizirao, osoba 1pozicionirana u osi mlaza počela se kretati nasuprotmlazu s određene početne točke na sigurnoj udaljenosti.Trenutak u kojem je detektor plina alarmirao, bio jesignal da se prestane približavati izvoru ispuštanja.Nakon što se koncentracija koliko toliko stabilizirala nazaslonu detektora, osoba se počela kretati u suprotnomsmjeru, tj. niz struju, sve dok detektor ne bi prestaoprikazivati postotke DGE. Na taj način bio je približnoodređen mjerljivi doseg mlaza.

Istovremenim IC snimanjem mogli smo usporediti tzv.mjerljivi i tzv. vidljivi doseg. Čini se da je IC kameraosjetljivija od detektora plina. Moguće je vidjetizanemarive koncentracije plina niz struju. Vizualni jeaspekt takav da se plin pojavljuje kao crni do bijeli dim

The experiment was organized in the way that the gas release was screened by special, very sensitive Infra Red camera for detection of gas leaks and a regular digital camera. Two persons with hand held gas detectors were positioned in the area of release, one downstream in the jet axes and the other sideways (see Figure 11).

Thus the jet/cloud was defined simultaneously by three different means of detection. In that sense the words "detectable" and "visible" are given the following meaning: detectable-means by gas detector and visible-means by IR camera. Since the industrial gas detectors are factory set to trigger at 20% of LEL, the sensitivity of the measuring is limited. So, the experimantal method was adjusted to the limiting circumstances. After the release jet had stabilized, the person 1 positioned in the jet axis started to move upstream from the certain initial point at the safe distance. The moment the gas detector alarmed was the signal to stop moving closer towards the source of the release. After the concentration had stabilized at the gas detector display, the person started moving in the opposite direction, i.e. downstream until the gas detector stopped displaying percentages of LEL. In this way the detectable extent of the jet was approximately established. By simultaneous IR screening we could compare the so called detectable and so called visible extent. It appeared that the IR camera is more sensitive than the gas detectors. It is possible to see negligible concentration of the gas downstream. The visual effect



poprimajući sve nijanse sivog dok se giba od izvoraispuštanja (vidi slike 19., 21., 23., 25. i 27.). Što je dimtamniji, to je koncentracija viša i obrnuto. U stvari,kamera registrira područja nižih temperatura kao tamna, ata su upravo ona blizu izvora ispuštanja zbogpothlađivanja plina uslijed Joule-Thomsonova efekta.Valja reći da je ispuštanje plina kroz velike otvore, kaonpr. 2,0″, u svakom slučaju vidljivo i golim okom uslijedobilne kondenzacije i stvaranja aerosola tijekomistjecanja. Naravno, IC kamera "vidi " znatno dalje nizstruju i stoga registrira razvoj oblaka vrlo precizno. Možese također pretpostaviti da su molekule plina prisutne čakizvan polja osjetljivosti kamere zbog postepenaizjednačavanja temperature plina s ambijentalnomtemperaturom. To je posebno važno jer je udaljeni krajoblaka, koji je inače nevidljiv golim okom, podložanatmosferskoj nestabilnosti i giba se u svim smjerovimaovisno o trenutnu smjeru vjetra.

Nakon što je doseg utvrđen, osoba 1 kretala se polakoprema izvoru ispuštanja da ustanovi brzinu kojomkoncentracija raste. U svim pokusima koncentracija jerasla naglo, gotovo trenutačno nakon što bi detektorpočeo alarmirati prvi put. Detektori plina imaju postotnukalibraciju s maksimalnim prikazom od 100%. To nijestvarna koncentracija od 100% nego DGE. S ovim tipomdetektora plina ne možemo registrirati stvarnukoncentraciju jednom kad je postignuta DGE. To jesvakako bilo ograničenje za eksperiment. Možemo samopretpostaviti da su područja "crnoga dima " ujednopodručja s koncentracijom 100% ili sasvim blizu tevrijednosti.

U isto vrijeme, osoba 2 s drugim detektorom plinakretala se okomito prema osi mlaza. Početni je položajutvrđen na aksijalnoj udaljenosti od približno 1000promjera otvora i na približno 450 u odnosu na os mlaza(vidi sliku 11.). U pravilu, detektor plina nije alarmiraona tom početnom položaju, čak ni nakon duljeg vremena.Tek nakon što bi se osoba primakla na približno 200,detektor bi počeo alarmirati. Tada bi koncentracijaporasla na 100% DGE gotovo trenutačno. Prilikomkretanja paralelno s osi mlaza od početna položaja nizstruju detektor je reagirao različito, ovisno oambijentalnim uvjetima. Što je detektor bio udaljeniji odizvora ispuštanja, to je njegova reakcija bila manjepredvidljiva.

Općenito, na udaljenom kraju mlaza, koncentracijefluktuiraju ovisno o brzini i jakosti vjetra, pa je prosječnukoncentraciju preko čitavog presjeka gotovo nemogućeutvrditi.

is such that the gas appears as a black to white smoke taking all the shades of gray while flowing downstream (see Figures 19, 21, 23, 25 & 27). The darker the smoke the higher the concentration. Actually, the camera registers the areas with lower temperatures as dark areas which are exactly those near the source of the release because the gas cools down due to Joule-Thomson effect. It has to be said that the gas release through the large openings like e.g. 2,0″, is visible anyway due to abundant condensation and aerosol formation in the course of release. Of course, the IR camera „sees” much further downstream and thus registers the development of the cloud very precisely. It may also be assumed that molecules of the gas are present even beyond the sensitivity field of the camera due to the gradual equalizing with the ambient temperature. It is especially important because the far end of the cloud, which is otherwise invisible, appears highly susceptible to atmospheric instability and moves in all directions depending on the momentaneous wind direction. After the extent had been established, the person 1 moved slowly closer to the source of the release to establish the rate at which the concentration had been increasing. In all the experiments the concentration increased sharply, almost instantaneously once the detector had alarmed for the first time. The gas detectors have the percentage calibration with the display of max 100%. This is not the 100% concentration but actually LEL. With this type of gas detectors we cannot register the real concentration once it has reached LEL. This was certainly a limitation to the experiments. We can just assume that the "black smoke" areas of the jet are the areas of 100% to near 100% concentarations.

At the same time, the person 2 with the second gas detector moved perpendicular towards the jet axis. The initial position was established at the axial distance of approximately 1000 diameters of the release opening and at the angle of approximately 450 to the jet axis (see Figure 11). As a rule, the gas detector did not alarm at this initial position even after considerable time. Only after the person moved closer, at approximately 200, the detector started alarming. Then the concentration increased to 100% of LEL almost instantaneously. When moving parallel downstream from the initial position the gas detector reacted differently depending on the ambient conditions. The more distant the gas detector was positioned downstream the more unpredictable was it's reaction.

Generally, at the far end of the jet, the concentrations fluctuate depending on the wind speed and direction and an average concentration across the cross section is almost impossible to establish.



GASFINDIR IR CAMERA

Slika 12. IC kamera za raspoznavanje propuštanja plina Figure 12 IR camera for detection of gas leaks

Slika 13. Pogonski atmosferski ispust za rasterećivanje sustava plinskoga podizanja (otvor 2″)

Figure 13 Blowdown field vent for relief of the gas lift system (2 in opening)

Slika 14. Podupiranje ispusta zbog svladavanja reaktivne sile mlaza

Figure 14 Supporting of the vent to sustain the reactive force of the jet

Slika 15. Priprema otvora ispuštanja kao navojnog T-komada ispod pogonskog manometra

Figure 15 Preparation of the release opening as T-pipe threaded fitting below field pressure indicator

Slika 16. Priprema mlaznica različite veličine koje će biti primijenjene na otvoru ispuštanja ispod manometra

Figure 16 Preparation of various size orifices to be applied at the opening below the pressure indicator



TABLICA I. - SNIMANJE KONTROLIRANOG ISPUŠTANJA PLINA IZ SUSTAVA PLINSKOG PODIZANJA NA NAFTNOM POLJU "STRUŽEC“, HRVATSKA

TABLE I - SCREENING OF CONTROLLED GAS RELEASE FROM THE GAS LIFT SYSTEM OF OIL FIELD "STRUŽEC", CROATIA

Ambient conditions Flammable gas Release conditions

(1)

Test No.

(2)

Date and hour

(3)

Wea ther description

(4)

Tempe rature

(0C)

(5)

Press ure

(hPa)

(6)

Humi dity

(%)

(7)

Wind speed/ direc

tion

(8)

Flammable gas

composition

(9)

Press ure

(barg)

(10)

Tempe rature

(0C)

(11)

Orifice

dia.

(mm)

(12)

Orifice heigth above ground

(m)

(13)

Release direc tion/ type of

release

(14)

Dura tion

(s)

(15)

Detect able* extent

(m)

(16)

Visi ble** extent

(m)

1

16.06. 2011 10.00

am

Sunny 28,8 1004,7 45,3

0,85 to 2,05 m/s

parallel to release

dwstream

wet natural gas, ~80%

CH4 M = 20 kg/

kmol

30 23 50,8 1,6 horizon tal

/free jet ~66 ~40 ~50

2

16.06. 2011 11.00

am

Sunny 29,3 1004,7 45,0

0,85 to 2,05 m/s

parallel to release down stream

M = 20 kg/ kmol

30 23 50,8 1,6

horizon tal /impinged on a 5 m distant

vertical plate***

~70 ~20 ~25

3

16.06. 2011 0.30 pm

Sunny 27,4 1004,7 45,3

1,30 to 2,95 m/s

600 to release down stream

M = 20 kg/ kmol

30 23 12,7 1,1 Horizon tal

/free jet ~330 ~12 ~17

4

16.06. 2011 1.00 pm

Sunny 27,4 1004,7 45,3

1,30 to 2,95 m/s


M = 20 kg/ kmol

30 23 4,0 1,1 Horizon tal

/free jet ~420 ~6,5 ~11

5

21.06. 2011 9.30 am

Sunny 20,9 1007,4 49,3

0,74 to 1,82 m/s


M = 20 kg/ kmol

30 23 12,7 1,1 Horizon tal

/free jet ~540 ~12 ~17

6

21.06. 2011 10.00

am

Sunny 21,2 1007,4 49,3

0,74 to 1,82 m/s


M = 20 kg/ kmol


/free jet ~480 ~6,0 ~11

7

21.06. 2011 10.30

am

Sunny 21,6 1007,4 49,0

0,74 to 1,82 m/s


M = 20 kg/ kmol


/free jet ~420 ~2,0 ~3,0

Notes: The different colors applied for the rows refer to the different groups of tests, The gas had been released under constant pressure from the vents of the gas lift system, Duration of the release was enough for the gas cloud to stabilize, The terrain at the vents is slightly sloped down the release direction (appr. 5%), * Detectable: means detectable by the gas detector at 20% LEL, ** Visible: means visible through the lens of the camera, *** Vertical plate is an artificial vertical wooden panel of dimensions B x H = 0,8 x 3,0 m supported to withstand the pressure blast.

Instruments used: GasFindIR camera with screen filter for hydrocarbons, Regular digital camera, Digital hand held anemometer with sensors for wind speed & ambient temperature, pressure and humidity, Field hand held gas detectors for methane set to alarm at 20% LEL and displaying from 0 to 100% LEL (pcs 2).

Personnel engaged: Certified IR camera operator (1), Field test coordinators (2), Instrumentalist (1), Mechanics (2), Firemen (1).



TABLICA II. - POTVRĐIVANJE REZULTATA DOBIVENIH IZRAČUNOM

TABLE II - VALIDATION OF THE RESULTS OBTAINED BY CALCULATION

Slika 17. Specifičan kritični doseg i Vz u odnosu na polumjer otvora ispuštanja Figure 17 Critical Extent and Vz vs. Radius of the Release Opening

)m(

p

p21X

p

pr5,2

L2/1

acrit

2/1

a0

crit

(2.5)

)m(L226,0V 33critz (2.8)

)m(vLELk

)dt/dG(V 3

2/3

m

maxz

(1.4)

Group 1 Group 2 Group 3

No. of experiment 1 2 3 4 5 6 7

r0 (mm) orifice radius

25,4 25,4 6,35 2,0 6,35 2,0 1,0

S (mm2) orifice cross section

2026 2026 127 12,6 127 12,6 3,14

rs (mm) pseudo source radius 100 100 25 7,9 25 7,9 3,9

v (m/s) average wind speed

1,45 1,45 2,12 2,12 1,28 1,28 1,28

wind direction parallel to the

release downstream

parallel to the release

downstream

~600 to the release

downstream

~600 to the release

downstream

~600 to the release

downstream

~600 to the release

downstream

~600 to the release

downstream v (m/s)

average wind speed axial 1,45 1,45 1,06 1,06 0,74 0,74 0,74

Ta (K) ambient temperature

301,8 302,3 300,4 300,4 293,9 294,2 294,6

(dG/dt)max (kg/s) 8,78 8,78 0,56 0,055 0,56 0,055 0,014

Lcrit (m) calculated by equation 2.5

47,4 NA 11,8 3,73 11,8 3,73 1,87

Lcrit (m) measured (field experiment)

40,0 NA 12,0 6,5 12,0 6,0 2,0

Vz (m3)

calculated by equation 1.4 14322 NA 369 11,4 633 19,5 2,5

Vz (m3)

calc. by equations 2.5 & 2.8 24068 NA 371 11,7 371 11,7 1,48

Vz (m3)

calc. upon measured Lcrit & 2.8 12401 NA 391 62 391 49 1,81

Vz (m3)

IEC 60079-10-1: Ed 1.0 7247 NA 460 45 450 44,3 11,3



Slika 18. Ispuštanje plina zabilježeno običnom digitalnom kamerom (početni stupanj razvoja mlaza)

Figure 18 Release of the gas through lens of the ordinary camera (initial stage of the jet development)

Slika 19. Ispuštanje plina zabilježeno IC kamerom (početni stupanj razvoja mlaz

Figure 19 Release of the gas through lens of the IR camera (initial stage of the jet development)

Slika 20. Ispuštanje plina zabilježeno običnom digitalnom kamerom (potpuno razvijen slobodni mlaz)

Figure 20 Release of the gas through lens of the ordinary camera (fully developed free jet)

Slika 21. Ispuštanje plina zabilježeno IC kamerom (potpuno razvijen slobodni mlaz)

Figure 21 Release of the gas through lens of the IR camera (fully developed free jet)

Slika 22. Ispuštanje plina zabilježeno običnom digitalnom kamerom (potpuno razvijen odbijeni mlaz)

Figure 22 Release of the gas through lens of the ordinary camera (fully developed impinged jet)

Slika 23. Ispuštanje plina zabilježeno IC kamerom (potpuno razvijen odbijeni mlaz)

Figure 23 Release of the gas through lens of the ordinary camera (fully developed impinged jet)


Ex-Bilten 2012. Vol. 40, br. 1-24

Slika 24. Ispuštanje plina zabilježeno običnom digitalnom kamerom (mlaz kroz mlaznicu od ½″)

Figure 24 Release of the gas through lens of the ordinary camera (jet at the ½“ orifice)

Slika 25. Ispuštanje plina zabilježeno IC kamerom (potpuno razvijen mlaz kroz mlaznicu od ½")

Figure 25 Release of the gas through lens of the IR camera (fully developed through the ½“ orifice)

Slika 26. Ispuštanje plina zabilježeno običnom digitalnom kamerom (mlaz kroz mlaznicu od 4,0 mm)

Figure 26 Release of the gas through lens of the ordinary camera jet at the 4,0 mm orifice)

Slika 27. Ispuštanje plina zabilježeno IC kamerom (mlaz kroz mlaznicu od 4,0 mm)

Figure 27 Release of the gas through lens of the IR camera (fully developed through the 4,0 mm orifice)

ZAKLJUČCI:

Slike ispitivanja u pogonskim uvjetima i rezultatiprikazani u tablicama I. i II. upućuju na sljedećezaključke:

a) Vidljivost velikih ispuštanja (kroz otvor od 2″) gotovo je ista s običnom kao i s IC kamerom jer učinci pothlađivanja i formiranje aerosola čine plin vidljivim.

b) Prednost IC kamere dolazi do izražaja kod srednjih i malih ispuštanja kada je plin gotovo nevidljiv golim okom osim u neposrednoj blizini izvora ispuštanja.

c) Izmjerene se vrijednosti općenito dobro podudaraju s teoretskim rezultatima dobivenim prethodnim jednadžbama. Čak i tamo gdje podudaranje nije potpuno, ono u najmanju ruku

CONCLUSIONS:

The images of the field testing and the results displayed in TABLES I & II lead to the following conclusions:

a) The visibility of large releases (through 2″ orifice) is almost the same with the ordinary as with the IR camera because the cooling effects and formation of aerosol makes the gas visible.

b) The advantage of IR camera comes into play at medium to small releases when the gas is virtually invisible except at the very proximity of the release orifice.

c) The values generally comply well with the theoretical results obtained by the equations set forth. Even where the compliance is not perfect, it is at least maintained at the level of order of



ostaje unutar reda veličine. d) Stupanj podudaranja gotovo je potpun u nekim

slučajevima, a u nekima baš i ne. Pretpostavlja se da je to zbog utjecaja vjetra na udaljenu kraju mlaza zbog čega tamo detektori prikazuju nestalne brojčane vrijednosti. Treba uzeti u obzir da vjetar rijetko ima stalnu brzinu, pa prosječne brzine vjetra u tabelama valja uzeti s velikim oprezom.

e) Priroda dvije jednadžbe za Vz je različita. Dok jednadžba (1.4) ima brzinu vjetra kao varijablu, jednadžba (2.8) izvedena iz (2.5) nije uvjetovana brzinom vjetra. To također može doprinositi odstupanju rezultata, posebno u slučajevima umjerenih do visokih brzina.

f) Postoji također moguć slučaj nulte aksijalne brzine vjetra kada bi rezultati za jednadžbu (1.4) postali beskonačni. Očigledno je da se ta jednadžba mora koristiti pri barem nekakvoj atmosferskoj nestabilnosti, inače jednadžba kontinuiteta ne bi funkcionirala. Zbog toga je poželjno koristiti računalno modeliranje da bi se dobila točnija približenja.

g) Rezultati za Vz dobiveni jednadžbama u normi su sasvim očekivano preveliki za mala ispuštanja, a premali za velika ispuštanja.

h) Jednadžba (1.4) najprikladnije je sredstvo za procjenu Vz. Međutim, poželjno je da se koeficijent Ф odredi računalnim modeliranjem (CFD) pretpostavljajući da on može poprimiti različite vrijednosti za različite scenarije ispuštanja. U svakom slučaju, predložena vrijednost 0,74 dobro je približenje za prvu ruku.

i) Ovim se pristupom dobivaju kvalitetniji rezultati u odnosu na pristup iz sadašnjega izdanja norme. No, za to su potrebni i kvalitetniji ulazni podaci, npr. o svojstvima zapaljivih tvari koje se ispuštaju. To ponekad kod nekih realnih zapaljivih tvari, primjerice u rafinerijama, nije nimalo jednostavno. Ove se analize uglavnom odnose na zemni plin ili eventualno druge plinove koji se slično ponašaju (približno slične relativne gustoće i koji se ponašaju kao idealni plinovi) te su i ograničenja veća u odnosu na dosadašnji pristup.

magnitude. d) The level of compliance is almost perfect in

some cases and in some cases not quite so. It may be due to influence of wind at the far end of the jet what makes the detectors to display erratic numbers. It has to be taken into account that wind rarely has a constant speed, so the average speeds quoted in the tables must be taken with great care.

e) The nature of the two equations for Vz is different. While the equation 1.4 has wind speed as a variable, the equation 2.8 derived from 2.5 is not influenced by wind speed. That may also contribute to the variation of the results, especially in the cases of moderate to high wind speeds.

f) There is also the possible case of axial wind speed being zero and then the results for the equation 1.4 would appear as infinite. It is obvious that this equation must operate with at least some kind of atmospheric instability otherwise Continuity Equation would not work. This is why CFD modelling should come to help to deliver more accurate approximations.

g) The results for Vz obtained by current equations set in the standard are quite expectedly overlarge for small releases and too small for large releases.

h) The equation 1.4 is the most appropriate means to estimate Vz. However, coefficient Ф should preferably be determined tgrough CFD modelling assuming that it may take different values for different release scenarios. Anyway, the value of 0,74 proposed in this paper is a good, first hand approximation.

i) This approach delivers more realistic results in comparison with current approach in the standard. However, more reliable input data are needed, e.g. regarding properties of the released flammable substances. This is sometimes not so simple by some real flamable substances, e.g. those in raphinery processes. The analysis in this text are mainly related to natural gas and some other gases that behave similarly (gases with approximately same relative density which behave like ideal gases), so that the limitations are greater than with the current approach in the standard.


Ex-Bilten 2012. Vol. 40, br. 1-24

LITERATURA / BIBLIOGRAPHY: [1] Norma IEC 60079-10-1 [2] N. J. J. Marinović, Protueksplozijska zaštita za eksplozivnu

atmosferu, 2. izmijenjeno i prošireno izdanje (2005.) [3] P. Peršić, Classification of hazardous areas and determination of the

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[4] R. P. Cleaver, R E Britter, A workbook approach to estimating the flammable volume produced by a gas release, FABIG Newslette, Issue 30, R416 (2001.)

[5] M. G. Cooper, A model for jet dispersion in congested environment, Contract research report for HSE, RR 396 (2001.)

[6] P. Peršić, Classification of hazardous areas-An approach to open air situations, Presentation at the Congress on Electrical Installations in Explosive Atmospheres, Sao Paulo, 2008.

[7] M. J. Ivings, S. Clarke, S. E. Gant, B. Fletcher, A. Heather, D. J. Pocock, D. K. Pritchard, R. Santon, C. J. Saunders, Area classification for secondary releases from low presure natural gas systems, Prepared by Health and Safety Laboratory, RR 630 (2008.)

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