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How to determine cargo temperature onboard chemical tankers ?

In a modern chemical tanker in order to maintain product quality, to minimise the potential for discoloration, and to facilitate cargo transport in a safe manner cargo temperatures need to determined at times and when required.

The temperature difference between the top and bottom of a tank may be considerable. This applies particularly to shore tanks, where the ship’s officers are sometimes requested to attend when readings are being taken. It is advisable that temperature readings are to be taken at three levels when the tank depth exceeds 4, 5 m. Ordinary thermometers should be kept immersed at least 5 minutes at each level.

As regards skin-contact dangerous products such as phenol, acrylonitrile etc one should, if possible, avoid handling contaminated thermometers. Readings are much better taken by means of thermometers pocketed in the discharge manifold.

Glycerine or other suitable contact medium should be filled in the pocket in order to give a good thermal contact. Simple, portable, remote reading thermometers which can be lowered into the tank would be a welcome development.


Measuring Temperature

The temperature required can be established by using:
  1. Remote thermometer
  2. UTI
  3. Manual thermometer lowered into the cargo through the tank opening, where open gauging is allowed.
The resulting temperature measurements as found at the differing levels will be averaged for the purpose of calculation.

The thermometer used should be regularly checked by means of a calibrated thermometer. The delta correction per °C. should be recorded and applied to if necessary on the temperature read.

When remote thermometers are used, the number of measuring points is restricted to the number of built-in sensors and the level of liquid in the tank. Since differences of temperature may occur within the cargo several measurements should be carried out if possible, in particular for heated cargoes.






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Requirements of various grade chemical cargo heating
: The voyage orders will contain heating information, if heating is required. As a rule the final heating instructions are given by the Shipper in writing to the Master / Chief Officer in the port of loading. If those written instructions are not given, the master should request them and issue a Letter of Protest if they are not received at departure. In the latter case the management office should be immediately informed.


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.




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