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Under the following circumstances the cargo tank or any enclosed space on-board the ship has to be evaluated to ensure that the particular space is gas-free and has ample amount of oxygen for personnel to work there if required. The circumstances may be:
- Prior to entry,
- Prior to any repair work (which may be at dry-dock or even at the shipyard)
- As quality control before loading (A person called the vetting inspector usually inspects the cargo tank.)
Tank evaluation is done to ensure that the atmosphere inside the tank is safe enough for personnel to make an entry. There are different equipments available on-board for the evaluation of tank atmosphere. Some of them are:
- Combustible gas indicators or explosimeters
- Tankscope or non-combustible gas indicators
- Multi-gas analyzers
- Oxygen analyzers
These are some of the widely used equipment onboard tankers. In this article we will discuss in detail the working principle of the above equipment.
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1. Combustible Gas Indicators or Explosimeters
An explosimeter is a device used to detect the amount of combustible gases present in a sample of the given atmosphere. This gives the reading in terms of percentage of the LFL (lower flammable limit).
“Resistance proportional to heat" is the working principle.
The equipment consists of a Wheatstone bridge in which one of the resistances is variable. The circuit is shown below:
It consists of four resistances in which one varies according to the amount of the gas present. A hand pump is used to draw the gas or the atmosphere containing the gas inside the device. A filter and flash back arrestor is used to filter the gas and also acts as a flame arrestor.
The device is switched on. As the hand pump is operated to suck a sample of gas from the cargo tank, simultaneously the filament gets heated. Any combustibles in the sample will land on the filament in the sample chamber. The combustibles will burn as the filament is already hot causing an increase in resistance which disturbs the Wheatstone bridge. The reading can be read from the indicator. The instrument gives the reading in percentage of the Lower Flammable Limit or Lower Explosive Limit which is 1%.
This type of gas meter can only be used if the gas content is very low (i.e.) this instrument should not be used if the atmosphere contains:
- H/C + inert gas – then the gas will not burn as there is no oxygen
- H/C + oxy-acetylene – then the burning will be too violent
- H/C + oxy-hydrogen – Same as above
- Lead petroleum vapors – Lead oxide deposits on the filament cause a reduction in sensitivity
All meters require calibration. This meter requires the following before using:
- Zero check
- Span check
- Battery check
Proper working of the equipment can be achieved by regular maintenance. Clean the filters regularly and it is advisable to have it serviced by the manufacturer once every six months.
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2. Tankscope or Non-combustible Gas Indicator
A Tankscope® is a device used for measurement of hydrocarbon gas content in a sample of given atmosphere. This instrument is meant for measuring the hydrocarbon vapor in inerted atmospheres. This instrument is not as sensitive as the explosimeter. The reading is only in percentage of the volume of the hydrocarbon vapor and hence used only during the gassing up operations and during inerting. This is purely meant for measuring the volume of the hydrocarbon vapors present inside any enclosed space, and hence it is not meant for measuring during a man-entry.
It works on the same principle as that of an explosimeter except that the gas does not burn inside the sample chamber; there is an alteration in the temperature of the heated filament which enhances the change in resistance.
It is always advisable to flush the sample tube with fresh air after every use. The following checks are done to ensure the proper working of the instrument:
- Zero check
- Span check
- Voltages check (battery check)
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3. Multi-Gas Analyzers
Multi-gas analyzers are used to detect only targeted gases and vapors. It is very specific to that type of gas only, so care has to be taken to ensure that correct tubes are used for the particular type of gas.
The multi-gas analyzer consists of a portable bellows pump and detector tubes. The detector tube is like a vial filled with reagent that will react with the specific chemical. Both the ends of the tube are closed. In order to use it we have to break the two ends of the tube and insert it into the pump according to the directions mentioned on the tube. Now start pumping 3-4 times (or as specified by the manufacturer) to suck in the particular gas from the atmosphere. If the atmosphere contains that particular gas or vapor, then the color of the tube changes. The length of the color change can be read from the tube and compared to obtain the level of that particular gas or vapor. Some of the gases include carbon monoxide, chlorine, hydrogen sulphide, organic arsenic compounds, arsine, and phosphoric acid esters.
An extension hose is provided to measure the concentration of vapor present at a different height. In a situation like this, we have to insert the hose with the pump and the tube connected to the other end of the hose.
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4. Oxygen Analyzers
The oxygen analyzer is a device used to measure the concentration of oxygen in a given atmosphere. This device plays a vital role since with the reading provided, only man-entry is done. The important checks that are done on the oxygen analyzers are:
- Calibration with fresh air (which contains 21% of oxygen)
- Battery check
Never change the batteries in a gas dangerous zone.
Further references can be found in the Bright Hub article "Oxygen Analyzer used On-board Ships."
EHS Expert Services - Tankscope®
Navsea Naval Sea Systems Command - Multi-gas Detectors