Transcript ANCILLARY ELECTRICAL SERVICES

(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
INTRODUCTION
Ships and offshore installations that
transport, process and store bulk quantities
of oil, gas and liquid chemicals are subject to
special codes of practice regarding their
electrical installations.
 The object of all such guidance is to prevent
the hazards of fire and explosion occurring
on board these tank ships.
 Spaces in tankers where explosive gas-air
mixtures may be expected to be present are
called dangerous or hazardous. All other
areas being regarded as safe.
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The best way to avoid explosions
caused by electrical equipment is simply
not to install such equipment in the
hazardous areas. However, special
electrical equipment is permitted and our
theme is the range and maintenance of
such explosion (Ex) protected
equipment.
. Hazardous Zones
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Are those which indicate the probability of an
explosive gas-air mixture being present and,
therefore, the likelihood of an explosion occurring.
Zone 0
In which an explosive gas-air mixture is
continuously present, or present for long
Zone 1
In which an explosive gas-air mixture is likely to
occur in normal operation.
Zone 2
In which an explosive gas-air mixture is not likely to
occur in normal operation and, if it occurs, will exist
for only a short time.
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An area which is not classified Zone 0, 1 or 2 is
assumed to be a non-hazardous or safe area.
Examples of this zoning applied to ships could be:
Zone 0
Interior spaces of oil cargo tanks, pipes, pumps,
etc.
Zone 1
Enclosed or semi-enclosed spaces on the deck of a
tanker, the boiler firing area on a gas carrier using
methane boil-off as a fuel and battery rooms.
Zone 2
Open spaces on the deck of a tanker.
The cargo pump rooms of tankers are, at present,
considered as falling somewhere between Zone 0
and Zone 1.
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In practice, three essential components must be
present to start a fire or cause an explosion:
A flammable gas or vapour (hazard)
Air or oxygen to support combustion (oxidiser)
Something to start the explosion (source of
ignition).
When all three of these components are
brought together ignition can take place, often
with devastating results.
The occurrence of a fire or ignition depends on
the probability of the simultaneous occurrence
of all three components
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Gases, when concentrated above the Lower
Flammable Limit (LFL), can be ignited by
heat generated from various electrical
sources e.g.:
Arcing between switch contacts.
Arcing between a live conductor and earth.
An internal arcing fault within an electrical
enclosure.
Overheating causing hot spots.
An electrostatic spark discharge between
charged bodies or between a charged body
and earth.
Chemical action.
Lightning strikes.
As might be expected, the flammability of a
gas/air mixture is dependent upon the ratio
of gas to air. A ratio of 100% gas I air
concentration will not burn and, as can be
expected, 0% will also not burn.
 Furthermore, each gas is quite different and
the flammability range depends on the gas
type.
 The terms used to describe these limits are
called: LFL, lower flammable limit, and UFL,
upper flammable limit (previously called LEL
and UEL, lower and upper explosive limits).
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There is also the amount of minimum ignition
energy required to ignite the gas, and the
temperature at which the gas automatically
ignites.
Apparatus Gas Groups
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The flammable gases in which explosion
protected electrical equipment may have to
operate are grouped according to the amount of
electrical energy, in the form of an arc, which is
needed to ignite the gas.
(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
Tanker
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Tanker je brod za prijevoz tekućih tereta
kojemu je čitav prostor za teret podjeljen
uzdužnim i poprečnim pregradama na
nepropusna odjeljnja, koja se nazivaju
tankovima. Za ukrcavanje i iskrcavanje
tekućeg tereta iz pojedinih tankova tankeri
imaju poseban sustav cjevovoda i pumpnih
uređaja. Prvi tankeri su se pojavili krajem 19.
stoljeća i prevozili su naftu iz Meksičkog
zaljeva za Englesku. Otada pa do danas,
tankeri su prisutni na svim morima. Tankere
možemo podijeliti po veličini i po vrsti tereta
koji prevoze.
Prema vrsti tekućeg tereta možemo ih
podijeliti na:
 tankere za prijevoz sirove nafte
 tankere za prijevoz naftnih derivata
 tankere za prijevoz ukapljenog prirodnog
plina
 tankere za prijevoz raznih kemikalija
 tankere za prijevoz pitke vode
 tankere za prijevoz posebnih tereta
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Tankeri, po pravilu imaju manje nadvođe
od ostalih teretnih brodova, ali pregrade i
stabilitet u slučaju prodora vode moraju
udovoljavati naročitim zahtjevima,
propisanim u Međunarodnoj konvenciji o
teretnim linijama, 1966, i u Međunarodnoj
konvenciji za zaštitu ljutskog života na
moru, 1974. Tankeri se grade za prijevoz
određene vrste tereta, najčešće sirove
nafte ili njezinih derivata. Postoje i tankeri
za druge vrste tekućih tereta, koji ponekad
iziskuju posebnu konstrukciju i opremu.
Međutim, tipičnim tankerom smatra se onaj
što prevozi sirovu naftu, koji i po tonaži i po
broju brodova nadmašuje sve ostale vrste
tankera.
Zaštita tankera od korozije
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Korozija tankova za teret, čeličnih cijevi i armatura u njima
vrlo je jaka, osobito u tankovima koji su se naizmjenično
punili naftom i balastnom vodom (što je danas zabranjeno) .
U tankovima za sirovu naftu korozija je najjača na uronjenoj
čeličnoj strukturi. U tankovima za lagane derivate od utjecaja
vlažnih para stradaju više gornji (izronjeni) dijelovi. Protiv
korozije primjenjuju se one iste elektrokemijske sile koje i
uzrokuju koroziju. Najčešće se primjenjuje zaštita
magnezijevim anodama , tzv. katodna zaštita. Ugradnjom
magnezijevih anoda postiže se da se cjelokupna čelična
struktura, koja je djelovala djelomično kao katoda a
dijelomično kao anoda, pretvara u katodu, pa više nije
napadnuta.
Magnezijeve anode, koje su do sada bile uobičajene na
tankerima, sve se više zamjenjuju anodama od cinka i
aluminija i to poslje eksplozije na jednom tankeru, koja se
pripisuje iskrenju nakon pada jedne magnezijeve anode na
dno tankera.