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Preparation for unloading
Inert gas systems
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Cargo & Ballast pumps
Physical properties of various noxious liquid chemicals carried at sea
Specific Gravity (Density)
Tanks on a Chemical Tanker are normally designed to load cargoes of a higher specific gravity
than an oil tanker. Very often the design strength differs between groups of tanks on the
The information with regards to tank strengthening is normally found on the Certificate of
Class and Fitness, and the Master must be familiar with this lay-out and the restrictions that
may be imposed on loading high gravity cargoes. Especially important is the risk of slack
loading a tank as this can cause excessive sloshing in the tank that may cause damage to the
tank structure and/or its equipment. Equally important is the danger of exceeding the tank’s
design weight capacity.
The flash point of a liquid is the lowest temperature at which the liquid will give off sufficient
vapour to form a flammable gas mixture with air, near the surface of the liquid.
Auto Ignition Temperature
The auto ignition temperature of a solid, liquid or gas is the lowest temperature at which it
requires to be raised to support self combustion.
Flammable / Explosive Limits
The flammable (explosive limits) are the minimum and maximum concentrations of flammable
gas or vapour in air between which ignition can occur.
The Minimum vapour concentration is known as:
The Lower Flammable Limit LFL
The Lower Explosive Limit LEL
The maximum vapour concentration is known as:
The Upper Flammable Limit UFL
The Upper Explosive Limit UEL
Vapour pressure/Boiling point
Vapour of every liquid exerts a certain vapour pressure at any given temperature called the
vapour pressure. The liquid will boil when the vapour pressure equals the external
atmospheric pressure. In a closed ship tank, however, the liquid will boil when the vapour
pressure equals the atmosphere pressure plus the pressure setting of the P/V valve. The tanks
and vent systems are designed to withstand this pressure, plus the hydrostatic pressure of the
True vapour pressure (TVP)
The true vapour pressure of a liquid is the absolute pressure exerted by the gas produced by
evaporation from a liquid when gas and liquid are in equilibrium at the prevailing temperature.
Boiling Point The temperature at which the vapour pressure of a liquid equals that of the
atmosphere above its surface; this temperature varies with pressure.
Freezing point/Melting Point
Most liquids have a defined freezing point, sometimes described as the melting point.
Some products, like lube oil additives, vegetable and animal oils, polyoils etc do not have a
defined freezing point, but rather a freezing (melting) range or none at all. The product’s
viscosity is instead used as a measurement for the products liquidity or handling
characteristics. Products with a freezing point higher than the outside temperature in which
the ship is trading will need to be heated in order to remain liquid.
Ship’s structure and equipment normally have limitations on high heat. Exceeding this
limitation could damage the tanks or their structure. High heat will also reduce steel strength,
and the risk of cracking will increase. Caution should be exercised when carrying high heat
products as non-insulated lines and vents may freeze and clog the systems. Not insulated
cargo lines used for high heat products pose a safety hazard as they may cause severe burns
Adjacent tanks temperature limitation to be monitored. MARPOL Annex II requirements for
solidifying substances discharge temperature to be complied (in consultation with shippers).
Prewash may be required if discharge temperature as per Annex II cannot be complied with.
The cargo tank vapours pressure to be monitored carefully in freezing weather conditions to
monitor blockage of PV vent lines.
Solidifying Substance means a noxious liquid substance which:
1) In the case of a substance with a melting point of less than 15°C which is at a
temperature of less than 5°C above its melting point at the time of unloading; or
2) In the case of a substances with a melting point of equal to or greater than 15°C which is at
a temperature of less than 10°C above its melting point at the time of unloading.
Non-solidifying Substance means a noxious liquid substance, which is not a Solidifying
The pour point of a liquid is the lowest temperature at which the liquid will flow. It should be
noted that cargo with thixotropic properties (the properties of showing a temporary reduction
in viscosity when shaken or stirred) can be pumped at temperatures well below its pour point,
but at very restricted rates.
Viscosity is a measure of a liquid’s ability to flow and is usually determined by measuring the
time required for a fixed volume to flow under gravity through a thin tube at a fixed
temperature. As the temperature of the liquid increases its viscosity decreases and therefore
it flows more readily. It can also be described as a measure of the internal friction of a liquid.
The distinction between viscosity and pour point should be made clear. Oil ceases to flow
below its pour point temperature when the wax content solidifies.
A viscosity measurement of
a liquid depends upon the internal resistance of the liquid to flow. For a simple liquid this
internal resistance varies with the temperature in a predictable and regular way. However,
when oil approaches its pour point the rate at which viscosity increases as temperature falls
accelerates until sufficient wax has precipitated to solidify the product.
Viscosity is important as regards the pumpability of a product. Centrifugal and deepwell
pumps are acceptable for the majority of cargoes but high-viscosity products such as bitumen
or molasses are more suited for pumping with positive displacement pumps.
High-Viscosity Substance means a noxious liquid substance in Category X or Y
with a viscosity equal to or greater than 50 mPa.s at the unloading temperature.
Low-Viscosity Substance means a noxious liquid substance, which is not a
MARPOL Annex II requirement for high viscosity substances are to be complied with.
Certain cargoes are known as static accumulators, and become electrostatically charged when handled. They can accumulate
enough charge to release a spark that could ignite a flammable tank atmosphere.
All liquids will expand as temperature rises, or contract when temperature decreases.
Sufficient space must be allocated in the tank to facilitate any cubic expansion or contraction
expected during the voyage.
Vent line systems must be checked for operation at regular intervals, as malfunction could
cause structural damages because of changes in the liquid’s volume.
For calculating maximum intake, the density at 35`C is used for non heating cargoes and the
density at maximum discharge temperature for heated cargo is used. The volume at these
temperatures should not exceed 98% of cargo tank maximum volume. Allowances should also
be made for load density and IBC Code requirements.
Vapour density is expressed relative to density of air. Many chemical cargo vapours are
heavier than air, caution must be exercised during loading and any other cargo operation, as
vapour concentrations may accumulate and be trapped in certain deck areas. (If cargo tanks
are incorrectly cleaned vapour may remain in the bottom of the tank).
Solubility is expressed in many different ways; yes, no, slight, as a percentage or totally and in
this connection only with water. Most non-soluble chemicals are lighter than water and will
float on top; others like the chlorinated solvents are heavier and will sink to the bottom. This
latter condition may cause a safety risk in drip trays and even in cargo tanks where they may
be trapped under water in pump wells, and pose a danger even if the tank atmosphere is
tested safe for entry.
Colour is the comparison between a sample of product and standard colours measured under
closely controlled conditions. The colour of clean products is one of the more common causes
of cargo rejection or downgrading. This is generally caused by loading a light-coloured
product without adequate preparation into a tank that last carried a darker product. Most of
the lube oils and white-water white products show quite readily the traces of prior darker lube
oils or residual products, and because of this trait, it is most important that the tank cleaning
instructions are closely followed and proper line cleaning carried out.
What is inerting of cargo tanks ? How it prevents a flammable condition ?
- Toxicology and associated hazards onboard chemical tankers
Toxicity is the ability of a substance, when inhaled, ingested, or absorbed by the skin, to cause damage to living tissue, impairment of the central nervous system, severe illness or, in extreme cases, death. The amounts of exposure required to produce these results vary widely with the nature of the substance and the duration of exposure to it.
- Hazards of vapour given off by a flammable liquid while carrying at sea
Vapour given off by a flammable liquid will burn when ignited provided it is mixed with certain proportions of air, or more accurately with the oxygen in air. But if there is too little or too much vapour compared to the air, so that the vapour-and-air mixture is either too lean or too rich, it will not burn.
- Reactivity of various noxious liquid chemicals
Self-reaction: The most common form of self-reaction is polymerisation. Polymerisation generally results in the conversion of gases or liquids into viscous liquids or solids. It may be a slow, natural process which only degrades the product without posing any safety hazards to the ship or the crew, or it may be a rapid, exothermic reaction evolving large amounts of heat and gases.
- Most corrosive chemicals carried onboard chemical tankers
Acids, anhydrides and alkalis are among the most commonly carried corrosive substances. They can rapidly destroy human tissue and cause irreparable damage. They can also corrode normal ship construction materials, and create a safety hazard for a ship.....
- Posoning hazards & first aid treatment
The poison is a very toxic substance which when absorbed into the human body by ingestion, skin absorption, or inhalation produces a serious or fatal effect. Poison may enter the human body orally, by inhalation, or by skin contact. After being absorbed by the body it may affect certain organs or give a general poisonous effect. Lately the cancerogene effects of some industrial chemicals have been noticed. This has led to significant reductions of hereto accepted TLV- values in many countries.....
- Specific gravity,Vapour pressure and
boiling point,Electrostatic charging & measuring Viscosity
Tanks on a Chemical Tanker are normally designed to load cargoes of a higher specific gravity than an oil tanker. Very often the design strength differs between groups of tanks on the same ship.
- General precautions onboard chemical tankers
Additional precautions for specific cargoes are necessary and should also be incorporated in the ship’s cargo handling procedures....
- Mooring precautions onboard chemical tankers
The consequences of a chemical tanker ranging along a jetty or breaking away from a berth could be disastrous, especially during a cargo transfer involving multiple different chemicals. Correct and sufficient mooring is therefore of the utmost importance.
- Berth precautions onboard chemical tankers
If an unauthorised craft comes alongside or operates in an area which may create a danger, it should be reported to the port authority and, if necessary, cargo transfer operations should cease.
- Cold weather countermeasures, avoiding electric storms
During cold weather, precautions should be taken to prevent equipment and systems from freezing. Attention should be given to pneumatic valves and control systems, fire lines and hydrants, steam driven equipment, cargo heating systems, pressure/vacuum valves etc......
- Restriction on using radio equipments and other mobile devices in cargo
During medium and high frequency radio transmissions significant energy is radiated, which can create a danger of incendive sparking by inducing an electrical potential in unearthed steelwork.
- Securing cargo tank lids and required safety precautions
Improper closing and sealing of cargo tank hatches can be a major cause of cargo contamination. A properly closed and sealed tank hatch/opening will prevent sea water ingress and maintain a positive pressure Nitrogen blanket in the ullage space. ....
- Means of access (gangways or accommodation ladders) safety precautions
Emergency towing-off wires ( fire wires)
,Ship’s readiness to move
Deckhouses and superstructures safety precautions
- Precautions against static electricity
Static electricity is generated by friction that occurs between different materials during relative motion. Electrostatic charges can then accumulate in materials which are poor conductors of electricity or which are good conductors but are insulated.....
- Cargo tank entry safety precautions
On chemical tankers the entry of personnel into cargo tanks is a more common practice than on oil tankers as a result of the requirement for inspections between grades etc; despite this, it is essential that the necessary checks are conscientiously made and recorded prior to entry in order to ensure the safety of personnel, enclosed space rescue equipment must be made ready for immediate use.
What is reactivity of noxious liquid chemicals while carrying at sea ?
What are the most corrosive liquid chemicals transporting at sea ?
What is putrefaction process of liquid chemicals ?
Static electricity -How they generate & required safety precautions
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