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Cargo temperature monitoring equipments for Chemical tankers

Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

Types of thermometers

Liquid /vapour thermometers rely on the expansion or contraction of liquid in a very fine-bore calibrated tube or capillary. The liquids most commonly used are mercury, ethanol or xylene. it is important to ensure that the liquid column in the instrument is continuous, otherwise the reading will be inaccurate.

Liquid filled thermometers have a metal bulb containing a fluid which changes volume with temperature change. The changes are transmitted via capillary tubing to an indicator or recorder. The system is sealed under considerable pressure to overcome the effects of vapour pressure from the liquid. Mercury filled thermometers should not be used with aluminium and certain other materials.

Bi-metallic thermometers consist of two metals with different coefficients of expansion which are welded together to form a bi-metallic strip. When heated, the strip will bend because of the unequal expansion, and the flexing movement can be used to drive a pointer in a similar manner to the Bourdon tube. Bi-metallic thermometers are susceptible to vibration and should only be installed in positions free from this effect.

Thermocouples rely on heat applied to the junction of two dissimilar metals generating a very small voltage which can be measured. A change will indicate a change in temperature. Normally the voltage is sensed electronically and the read-out is remote.

Resistance thermometers use the fact that the electrical resistance of certain materials changes with temperature, and that if it is measured it will indicate temperature. The material normally used in resistance thermometers is fine platinum wire. Its resistance is measured by means of an electrical resistance bridge connected to an indicator or recorder, normally by electronic means, and the read-out is remote.

General precautions

The following precautions should be observed with all temperature indicating devices: the thermometers used should be suitable for the complete range of temperatures expected; the sensor should make good thermal contact with the material whose temperature is to be measured; if readings do not change when expected, the instrument should be checked; thermometers are easily damaged, especially those with capillary tubes.

They should be handled with care and protected from mechanical damage and extremes of temperature beyond their scales, otherwise they may become inaccurate; when a fixed thermometer is removed from its working location, care should be taken to avoid loosening or removing its pocket, especially if the system is pressurised; when a thermometer is replaced in a working location, care should be taken that it does not bottom in its pocket when screwed in as this could cause damage.

If the thermometer is slack in the pocket a material with high thermal conductivity (such as a suitable lubricating oil) can be used to ensure accurate readings; electrical connections should be clean, tight and correct. Care should be taken to see that intrinsically safe leads are not cross-connected with ordinary power sources.

Fixed cargo temperature indicating devices

Such equipment must be maintained in working order at all times and a calibration check utilising the UTI (Portable Gauging / Temperature Equipment) should be carried out at frequent intervals (see PMS). Records and results of the checks are to be maintained. Calibration of the fixed equipment by Third Parties will be carried out as required after replacement of or repairs to any of the existing units.

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How to determine chemical cargo temperatures at different level ?
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Recommended temperature monitoring equipments onboard
:Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

Cargo instruments
:In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, especially prior to tank entry, several different gas measuring instruments need to be available for use. Which one to use will depend upon the type of atmosphere being measured.

Liquid level gauges
:The accuracy required of chemical carrier level gauges is high because of the nature and value of the cargo. To limit personnel exposure to chemicals or their vapours while cargo is being handled, or during carriage at sea, the IBC Code specifies three methods of gauging the level of a liquid in a tank - open, restricted or closed

Overflow control
:Certain cargoes require the designated tank to be fitted with a separate high level alarm to give warning before the tank becomes full. The alarm may be activated by either a float operating a switch device, a capacitive pressure transmitter, or an ultrasonic or radioactive source. The activation point is usually pre-set at 95% of tank capacity.

Oxygen analysers
:Oxygen analysers are normally used to determine the oxygen level in the atmosphere of an enclosed space: for instance, to check that a cargo tank can be considered fully inerted, or whether a compartment is safe for entry.

Vapour detection
:Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.

Alarm circuit
:An important feature of many modern measurement and control instruments is the ability to signal a particular situation. This can be a main operational alarm that gives an indication of a pre-set situation such as liquid level in a tank, or a malfunction alarm indicating a failure within a sensor's own operating mechanism. The designs and purposes of alarm and shutdown circuits vary widely, and their operating system may be pneumatic, hydraulic, electrical or electronic. Safe operation of plant and systems depends on the correct operation of these circuits and a knowledgeable reaction to them.

Gas freeing
:Gas freeing onboard chemical tankers is required for entry into cargo tanks, for hot works or washing for clean ballast tanks. Gas Freeing is one of the most hazardous operations routinely undertaken onboard a Chemical Tanker and the additional risk created by cargo gases expelled from the tanks, which may be toxic, flammable and corrosive, cannot be over-emphasised.

Cargo tank damage during pigging operations
:Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.

Reference publications

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