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 same ship.
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 cargo.
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 if touched.
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 Substance.
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 High-Viscosity Substance. 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 ?
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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