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Chemical tankers cargo instruments - overfill detection systems - High level alarms

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.

The high level alarm must be independent of the normal gauging system. High level alarms must be maintained according to maker's instructions and tested as required before cargo operations begin.

Tank overflow control systems

A better name for this would be a tank overflow prevention system. It is required for particular cargoes designated in the IBC Code. The system should come into operation when the normal operational procedures fail to stop the tank liquid level exceeding the normal full condition. The activation point is usually set at 98% of tank capacity.

Operation of the system is required to be independent of the high level alarm described above. On activation, the system should give a visual and audible alarm, and a signal in sufficient time to permit sequential shutdown of onshore pumps and/or valves, and the ship's valves. The shutdown can be dependent or independent of the intervention of operators.

Particular care must be exercised with regard to automatic shutdown systems that are independent of operator intervention. Express approval for their use is required from both the flag administration and the port state authorities. Automatic shutdown systems are normally designed to shut the main cargo tank filling valve if the liquid level rises above the maximum level permitted.

Great care should be taken to ensure that the activation point is set accurately, and that the operation of the device is checked by simulation whenever the system is recommissioned. If the ship and shore shutdown circuits are to be linked their operation should be checked before cargo transfer begins; if not, the terminal should be informed of the closing rate of the ship's valves.


Pressure indicating devices

Pressure gauges are fitted at various points in the cargo system, on pumps, in pipelines and in tanks, some of which are specified in the IMO Codes. They may be used to indicate pressure in a liquid being pumped into or out of a tank, or static pressure such as inert gas overpressure. They can indicate negative as well as positive pressure, and can be linked to shutdown or alarm systems.

It is important that procedures exist for ensuring that pressure gauges are checked and calibrated in accordance with manufacturer's instructions.


Bourdon tubes

These instruments measure pressure by the movement of a coiled or helical tube, the amount being directly proportional to the applied pressure. The movement is used to drive a pointer for local readings, or to control a gas pressure valve or to alter a variable resistance that will serve indirect readings. Indirect readings may be necessary to avoid direct connection between safe and dangerous areas.

The following precautions should be observed:

the indicator should be periodically checked for zero calibration;

the gauge should not be used to consistently indicate pressures beyond 75% of its maximum reading if the expected pressure is steady, or 60% if it is fluctuating;

Bourdon tubes may be damaged by vibration or by excessive pressure pulsations; the latter can be eliminated by the use of a flow restrictor.

Chemical tanker capacitive pressure transmitter

Fig: Capacitive pressure transmitters

Pressure in vapour spaces of cargo tanks (and elsewhere) can be monitored by measuring the effect of the existing pressure on sealed units that have a known internal pressure. By establishing the reference atmosphere in the sealed units at a low pressure, for example 0.8 bar absolute or 800 millibars, it is possibie to continue to measure modest underpressure within a tank as well as overpressure. Deflection of the sealed unit is measured by an internal capacitor, which sends an electronic signal to a remote display. An external measurement of atmospheric pressure is necessary for the display to show gauge pressure. Alarm levels can be set as desired.

The physical size of the sensors is quite small, and when used for inert gas monitors the devices are often incorporated into housings of other sensors, such as radar ullage gauges. Similar units can be used to indicate higher pressures in liquids.


General precautions

The following precautions apply to all pressure sensing equipment:

materials of construction should be compatible with the cargo. For example, brass must not be used for pressure gauge internals for amine cargoes such as ethylenediamine. The IBC Code gives guidance on cargoes where special attention must be paid to materials of construction;

before measurements are taken, all valves in the direct line should be opened and all cross-connections shut;

no pressure gauge should be subjected to violent pressure change;

in ships carrying cargoes which can solidify or form polymers (e.g. phenol or styrene respectively) it may be necessary to flush gauge lines and sensor chambers;

if sensor lines are temporarily disconnected during maintenance they should be blanked.





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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