CARGO TANK VENTILATION ON CHEMICAL TANKERS - CAPT AJIT VADAKAYIL

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 )