FIXED FAN , PORTABLE
FANS , DEW POINT , TANK SWEAT, EPOXY TANKS VENTING AFTER SOLVENT CARGOES - CAPT AJIT VADAKAYIL
Tank cleaning and gas
freeing are the most hazardous aspects of chemical tanker operations. The risk
is greater if the vapours are toxic.
First the tank has to
be washed clean of residues.
The tank domes and
butterworth covers should be kept closed until the ventilation of the tank is about to commence.
Portable fans or
blowers should be so constructed that no hazard of incendiary sparking arises
if, for example, the impeller touches the inside of the casing. Dasic “Jet-Fan”
impellers are specially coated to prevent static between impeller/casing. The capacity
and penetration of portable fans should be such that the entire atmosphere of
the tank can be made gas free quickly.
Venting of toxic and
flammable gas during gas freeing should be through the vessel's approved gas
freeing outlets. No escape of cargo
vapours should be allowed in the proximity of accommodation spaces and the
velocity of venting should be sufficient to carry the vapours clear of the
deck.
If the ship does not
have fixed blower/de-humidifier, then portable fan must be used on a tank cleaning
hatch venting through VCS pipework and p/v valve. If gas
freeing is done through cargo pipelines , then the entire line system should be
thoroughly drained before venting.
When the gas level
within the tank has fallen below 30% LFL and below the relevant TLV, other deck
openings of the tank in question may be opened and THEN ONLY discharge of the
vapour mixture may continue at tank level.
Intakes of central air
conditioning or mechanical ventilating systems should be adjusted to prevent
vacuum inside the accommodation.
Ventilation outlets
should generally be as remote as possible from the portable fans.
Portable fans should
make an effective electrical bond to the deck.
Fixed gas freeing
equipment may be used to gas free more than one tank simultaneously but must
not be used for this purpose if the system is being used to ventilate another
tank in which washing is in progress.
When a tank appears to
have been completely gas freed, a period of about 30 minutes should elapse
before taking final gas measurements .
This allows relatively stable conditions to develop within the tank
space. Tests should be made at several
levels with the fan OFF at levels depending on the VD.
Gas vent riser drains
should be cleared of water, rust and sediment.
After ventilation is
completed, testing of the atmosphere should be carried out to determine its
flammability, as a percentage of LFL, and its toxicity relative to the
TLV. Samples should be drawn from the
inside of the ventilation pipe at its exit point.
Before venting ventable
cargoes reduce trim and list to increase the surface area
Air contains a certain
amount of moisture (vapor of water) depending on its temperature and
surroundings. If the air contains the maximum amount of moisture allowed at the
temperature, the
relative humidity is 100%. If the air contains no moisture, the relative
humidity is 0%.
Higher the temperature,
more the air can contain moisture, which means if air containing a certain
amount of moisture is cooled down, the relative humidity will increase. If air
contacts a cold surface and gets cooled down below the temperature of 100% of relative
humidity (so called "Dew Point") the amount of moisture contained in
the air in excess of 100% relative humidity will condensed and become dew
(sweat).
This is the reason why
dew is generated on a cold surface within a cargo tank containing air of higher
relative humidity. The cold surface can be a bulkhead between adjacent cargo
tanks containing cold temperature of cargo, a shell plate of which the other
side is immersed in the sea, etc.,
MEASURES IN CASE OF
OCCURRENCE OF SWEAT
The sweat in an empty
cargo tank can be generated when a cargo of lower temperature than atmospheric
temperature is loaded in an adjacent tank, or a cargo loaded at a cold area is
carried to a warm tropical area.
In the latter case, if
the destination is far from loading port and will take long time more than 4
days, the problem of sweat in surrounding cargo tank will not happen because
the temperature will be increased naturally. However in case of short voyage,
it is not enough to increase the temperature naturally.
If sweating is expected
to happen or has actually happened in the adjacent empty tanks, the followings
are the possible measures to prevent or cope with:
The best way is to
supply dry air into the empty tank, however this is not practicable because
such system to supply dry air is not equipped on board.
If sea temperature is
higher than cargo temperature, to ballast into concerned ballast tank in order
to increase the temperature of cargo tank in the adjacent cargo tank.
If the cargo is not a
polymerize cargo such as SM or MMA in surrounding tank, and not reactive at
higher temperature, to steam into empty cargo tank to be loaded (the tank which
is sweating). It is most effective method to increase indirectly the
temperature of cargo within the shortest period (4 or 5 hours).
If the cargo is a
polymerize cargo or reactive cargo for high temperature, Steaming into cargo
tank should be avoided. Vessel can increase indirectly the cargo temperature by
ventilation of surrounding empty cargo tanks with warm temperature, which
heated by heating coil. The temperature of empty cargo tank should not exceed
35C.
During the above
ventilation method, sweating in empty cargo tank is inevitable. After raising
temperature of cargo to the necessary degree, Heating of empty cargo tank
should be stopped and normal ventilation to be carried out after mopping up the
sweat.
Prior to arrival
loading port, the condition of the concerned empty tanks is to be checked to
avoid any last minute surprises.
PREVENTION OF
OCCURRENCE OF SWEAT
If vessel loads cargo
in a tropical area at atmospheric temperature and carries it to a cold area. In
this case, the sweat will not be generated in empty cargo tanks surrounding the
loaded cargo tank. However some cargoes such as SM (styrene monomer) or MMA
(methyl methacrylate), which react at high temperature, are loaded to cold
temperature lower than atmospheric temperature at Tropical area. In case of
parcel cargo, if vessel loads such cargo first, the sweat in cargo tanks which
surrounded by such cargo tank will be generated.
Vessel should load
cargoes of normal temperature first and then load cold temperature cargo last.
It could be discussed with agent/charterer in advance.
For avoiding generation
of sweat in empty cargo tank, it is necessary to find dew point. The sweat is
generated when temperature in certain area is lower than dew point.
Therefore, if dew point
of atmosphere is lower than that of empty cargo tank, ventilation into cargo
tank is very effective, however dew point of atmosphere temperature is higher
than that of empty cargo tank, ventilation should be stopped.
A tank which is having
100% relative humidity at 30c will have zero RH at -40 C dew point
Dew point= wet bulb
reading on X axid/ diff between dry and wet on Y axis.
Ship sweat happens when
ship travels from warm to cold place when tank temp is greater than dew point
of outside air—ventilate and get rid of the sweat. Wait for temperature of air
to be > 3 deg cooler than air inside. When tank temp is less than dew point
of outside air—do not ventilate.
Warmer air can hold a
lot more water than cooler air.
The rule
of thumb is that raising the air temperature 10°C doubles its moisture
capacity. If we cool air without changing its moisture content, eventually
we'll reach a temperature at which the air can no longer hold the moisture it
contains. Then water will have to condense out of the air, forming dew or fog.
The dewpoint is this critical temperature at which condensation occurs
Ordinarily the dewpoint doesn't vary much during a 24-hour period. Unlike
temperature and unlike relative humidity, the dewpoint is usually the same at
night as it is the daytime.
A high relative humidity indicates that the dew
point is closer to the current air temperature. If the relative humidity is
100%, the dew point will be equal to the current temperature The dewpoint
temperature depends on the air temperature, since hotter air can hold more
water vapor per unit volume than can colder air. The higher the dewpoint
temperature, the higher the moisture content of the air.
Dew point is a better
absolute measure of the air's moisture content. Here's a good example of why:
Proforma: Temp/ DP/ RH
90/ 80/ 67%
100/ 80/ 47%
110/ 80/ 33%
RH=actual vapor
density/ saturation VD x 100%
While relative humidity
is a relative measure of how humid it is, the dewpoint temperature is an
absolute measure of how much water vapor is in the air (how humid it is). In
very warm, humid conditions, the dewpoint temperature can reach 75 to 77
degrees F, but rarely exceeds 80 degrees.
Dew-point does not
change with temperature of the air; very much different from relative humidity.
It is a measure of atmospheric moisture. It is the temperature to which air
must be cooled in order to reach saturation
The closer the
temperature and the dew point, the more humid the air is.
Accommodation entrances
and air inlets to ER should not face cargo area.
Gas freeing fan 9000
m3/ hr which gives the purge pipes of 10 cm dia
an exit velocity of>30 Mtrs/ sec.
Cargoes such a styrene
monomer and methyl methacrylate are loaded cold. So if MEG and styrene have to
be loaded in adjacent compartments—load MEG first and styrene last.
Almost all chemical
vapours are heavier than air and so remain at the bottom of tank.
Start gas freeing only
after removing water from the bottom suction pit, or it takes too long.
Displacement method (
1.5 air changes reqd )
entry velocity must be
low to prevent short circuit.
entry from top via
butterworth port using portable inlet blower. at 114 cum per minute
exit from bottom
entering via drop line mouth and eject
via manifold..
Dilution method ( 4
airchanges with high entry velocity )
Method 1:
entry from top
butterworth port
exit from top
butterworth port.
Method 2 :
entry from bottom pump
stack.
outlet from top
butterworth port.
this will remove few litres water or volatile cargo from
suction pit.
use blower fixed at manifold to put in air at 200
cum “per minute..”
All modern chemical
tanker pumprooms must have 45 airchanges “per hour” as perCDI regulations.
Do not do too much of
ventilation on a wall wash earmarked
tank to prevent salt in sea wind / dust / oil getting in. The compressed air exhaust of the portable
vent must be blown/ led away (using a
short pipe) from the fan blades to
prevent compressor carry over oil from getting into tank.
Products with low vapor
pressure, or products which leave residues or contain inhibitors should never
be cleaned by evaporation (ventilation). The non-volatile matter (NVM) content
in the specification of the product is a good indication to determine, whether
a product will leave residues
Gas freeing is most
effective if 2 blowers are used in a single tank—with one on supply and the
other on exhaust with a long duct leading to the bottom.
Ethanol which is not
static can be removed by bubbling through the sump. Epoxy Paint resistance
lists and Tank coating manufacturers give clear instructions on long term
ventilation before water is introduced into the tank. Usually a water wash is not necessary after a prolonged
ventilation period unless there is NVM at the bottom or verticals/ sloping
surfaces.
Venting after
aggressive cargoes:
Epoxy absorbs solvents
. The problems are—damage to coating and
contamination of next cargo. The cargo must be desorbed by ventilation.The
release is faster if the steel temp is high
This steel temperature increase
must be from the adjacent compartment.. The ventilation for desorbsion must be
devoid of moisture—the moisture takes the place of vapours which the air should
have accepted and carried away. The ventilation must go to bottom of tank and
exit from top from the other end,with the steam coils cracked open.
Don’t bubble through
sump if the cargo is a static accumulator. Do not butterworth or even steam the
tank without ventilation if the cargo is water mixable or water reactive.
A check is
mandatory—will the next cargo be contaminated by the previous cargo. Say
styrene monomer after benzene. Empty tanks must be ventilated during the voyage
to ensure that the index is outside the flammability composition cup diagram.
Some of the water
mixable/ reactive cargoes which are bad for epoxy coating and better off in SS
or Zinc tanks are—
Acetone, Acrylonitrile,
Ethanol, Methanol, IPA, MEK, Methyl acrylate, VAM, Butyl Acetate, Ekta Solve,
Adiponitrile etc.
For many cargoes
ventilation is the only acceptable means of removing the last traces from epoxy
tanks. For such cargoes bottom flush with water is not a good idea. Read the
paint manufacturers instructions for the next cargo. It may say—only warm
organic cargoes. Be very sure to read
the footnotes.
The test is to shut off
the tank and open the steam coils. Open the tank again and check for odour of
last cargo. If the odour is still there then ventilation has to be done again.
Caution should be
exercised when carry high MP heated products as the non-insulated ( from cold
wind ) PV stacks can clog or the PV valve may stick.
CAPT AJIT VADAKAYIL
( 30 YEARS IN COMMAND )
Hi Sir,
ReplyDeleteBelated ONAM to you and your family..
conveying the wishes of Narendra Modi as well :)
http://www.youtube.com/watch?v=-zh9Knez580&noredirect=1
Prakash
hi jr,
Deletei guess modi should have taken some mallu help in speaking malayalam.
it was NOT music to the mallu ear.
capt ajit vadakayil
..
Hello Capt,
ReplyDeleteCan you tell us about U. R. Anantamurthy of Karnataka, who threatened to leave India if Modi become PM? I never heard of him.
Thanks,
Dr. Raman
hi r,
Deletenever trust a foreign award winner for human rights , environment etc.
these are like the indians who got SIR title during british rule.
in this case UR Anantamurty is a COMMONWEALTH SCHOLARSHIP winner.
punch into google secrh-
INDIA MUST QUIT THE COMMONWEALTH VADAKAYIL
capt ajit vadakayil
..
Hi Captain,
DeleteHe is from our state 'Karnataka. He is a Jnanapith award winner.
He has already revoked strong responses and replies in all of the Kannada dailies, for his comment on leaving the country.
We don't care and we have stopped taking him seriously. If he wishes to leave. It is his choice.
Regards,
Venkat
Annamuthy has passed away some months ago, I think,.
Deletejust as Modi become PM.
BTW , i don't support Modi.
Dear Captain,
ReplyDeleteWhat is your opinion on fedel castro? as you said cuba is one nation with no Rothschild contol, and castro is white so how come its not still under Rothschild rule.
Jayan
Dear Captain,
ReplyDeleteYou wrote down as "When a tank appears to have been completely gas freed, a period of about 30 minutes should elapse before taking final gas measurements . This allows relatively stable conditions to develop within the tank space. Tests should be made at several levels with the fan OFF at levels depending on the VD."
Shouldn't it be 10 minutes as per Tanker Safety Guide Chemicals? Or is it what you experienced?
Thank you.
Hello, This blog is too informative and the pictures are giving a good decsription about Pneumatic Jet Fan. Thank you
ReplyDelete