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Cargo instrumentation for Chemical tankers - use of 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.

There are several types of oxygen analysers. In each case it is of vital importance that they are carefully maintained and tested, and that correct checks are made before use. When oxygen detectors are calibrated it is essential to use clean and uncontaminated air. If used strictly in accordance with the manufacturer's instructions, these instruments can be regarded as reliable.

Level of oxygen in air

Most instrumentation in use on ships has a gauge or scale which reads to 21%. Strictly, however, the percentage of oxygen falls several hundredths of a percent below that figure, variously quoted between 20.85% and 20.95%.

Modem instrumentation with digital indicators can measure so accurately that the full 21% may be impossible to obtain. If an instrument capable of such accuracy is in use, the maker's instructions should be carefully read and understood, so that proper interpretation of the readings can be made. It may be appropriate for the ship operator's instructions to make reference to the level of accuracy obtainable.

Electrolytic sensors

Analysers of this type measure the output of an electrolytic cell that is exposed to a sample of the atmosphere being tested. The current flow is related to the oxygen concentration in the sample, and the scale is arranged to give a direct indication of the oxygen content.

The readings may be affected by the presence of certain chemical vapours. It is important that the manufacturer's advice is followed. An indicator which may be reliable for measuring the oxygen content of a space after thorough ventilation may not be suitable for checking the oxygen content in a mixture of air, inert gas and cargo vapour.


Paramagnetic sensors

Paramagnetic instruments measure the deflection of a magnet pivoted in a symmetrical non-uniform magnetic field. The magnet is suspended in a chamber into which the gas sample is introduced; the deflection is directly proportional to oxygen concentration. These instruments can be used for detecting oxygen in mixtures of other vapours.

It should be noted that some other gases, notably oxides of nitrogen, have comparable paramagnetic properties to oxygen. This technique cannot therefore be used if such other gases might be present in more than trace amounts.


Selective chemical liquid absorption sensors

In liquid absorption instruments a known volume of the atmosphere to be sampled is passed through a liquid which absorbs the oxygen, causing a volume change in the liquid. The final volume is measured on a scale which indicates the oxygen content of the original sample gas.


Personal oxygen monitors

Small instruments are available which are capable of continuously measuring the oxygen content of the atmosphere. They can be attached to clothing or sometimes are supplied with an armband. They should automatically provide an audible and visual alarm when the atmosphere becomes deficient in oxygen, so as to give the wearer adequate warning of unsafe conditions.





Other info pages

Draegar Chemical detector tubes use and reading correction guideline
These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture

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.


Reference publications








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