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Cargo control and measurement instruments -Liquid level gauges for chemical tankers

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 - according to the health hazard of the product. Many chemical cargoes may not be gauged by manual dipping because to do so requires an opening to the atmosphere during operation. The use of completely closed gauging systems is necessary, so that no vapour is emitted.

chemical tanker navigation at sea
Examples of closed systems are float gauges or radar systems. Indirect measuring methods such as flow metering may also be used.

Many more chemicals, although still hazardous, do not require quite such rigorous controls, and restricted gauging accepts that a very small amount of vapour may escape during gauging, An example is using a sounding pipe that reaches right into the liquid. Virtually all toxic cargoes require either restricted gauging or closed gauging.

However, other cargoes can be gauged through openings in the ullage space. This is called open gauging.

Float gauges

These are closed gauges, and consist of a float which rises vertically on the liquid. It is attached by a tape to an indicating device for local reading, with provision for a drive mechanism for remote read-out. Particular attention is drawn to the following:

i) floats should be secured when at sea, except briefly during measurement of tank contents. If the float remains unsecured at sea it will almost certainly be damaged due to sloshing of the cargo;

ii) remote and local readings should be compared frequently to determine discrepancies;

iii) readings may need to be corrected to allow for tape and tank expansion or contraction, and ship trim and heel. Tables are normally provided for this purpose;

iv) tapes should be checked regularly for free vertical movement of the float, and if damaged, should be replaced. Particular care is necessary with the rewind mechanisms which are carefully balanced: if obstructed, the gauge readings will be inaccurate;

v) when tapes are renewed, or a gauge reassembled after maintenance, allowance should be made for the level at which the float begins to lift. Manufacturer's instructions should be consulted;

vi) parts should be securely assembled: special care is necessary with tape-to-float and tape-to-reel attachments.

Chemical tanker liquid level gauges
Figure : Types of gauging

Chemical tanker radar ullage gauge

Radar, ultrasonic or microwave gauges

These are also closed gauges, and work on the principle used by a radar set or an echo sounder. Pulses are transmitted from the top of the tank and the time taken for them to be reflected back is measured, and displayed as an ullage or depth. Special arrangements may be made to reduce interference by internal tank structure.

Radar gauges are generally reliable, and most maintenance can be performed from outside, with the tank in the closed condition. Particular care is necessary to protect the delicate transmitters and receivers, and when calibrating this type of gauge.

Chemical tanker Pressure gauge system

Pressure gauges

Pressure gauges make use of the difference between atmospheric pressure and the pressure in the liquid near the tank bottom. Sometimes an additional sensor is mounted near mid-depth in a tank to improve accuracy and reliability. In some cases a system can adjust for cargo density by use of sensors at known levels, and in other cases a correction factor associated with specific gravity is necessary. A thermometer is usually included in the sensors, but a disadvantage of pressure gauges is that the cargo temperature may vary widely in a tank and, because correct density is dependent on temperature, an error in readings may develop. The delicate nature of sensors and their susceptibility to cargo leakage makes regular servicing essential.

Tape gauge systems

These systems use a tape to check the level of cargo in a tank. The measuring probe may be inserted into the tank through a small diameter pipe with a ball valve at the end, and portable units can be attached or released from this valve by a screwed adapter to minimise the amount of cargo released. They sometimes have an interface detector, and a temperature sensor built in, with appropriate indicators in the displays. A system employing this type of connection can also be used to take cargo samples.

Chemical tanker Tape gauge system

It should be borne in mind that individual ship has got own characteristics and limitations may involved handling various chemical cargoes . The master and all personnel in all cases must be aware of cargo/ship information that has been given and comply with relevant safety procedures.

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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