Preparation Of A Cargo Tank Atmosphere In Chemical Tanker

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  • For some cargoes the IBC Code requires vapour spaces within cargo tanks to have specially controlled atmospheres.
  • Visual inspection can only be carried out when tank is gas free by entering the tanks.
  • Any area that appears to have crystalline deposits should definitely be tested.

Ship checks prior to loading

For some cargoes the IBC Code requires vapour spaces within cargo tanks to have specially controlled atmospheres, principally when the cargo is either air reactive resulting in a hazardous situation, or has a low auto-ignition temperature, or has a wide flammability range.

The correct atmosphere in a tank, can be established either inerting to prevent the formation of flammable mixtures of cargo vapour and air, or padding to prevent chemical reaction between oxygen and the cargo. It may also be necessary to reduce the humidity (dewpoint) of the atmosphere within the cargo system.

The extent of atmosphere control to protect the quality of the cargo will normally be specified by the cargo shippers. 

Some cargoes are extremely sensitive to commercial contamination or discoloration, and for quality control reasons are carried under a blanket of nitrogen that is very pure and which must often be obtained from shore.

Visual inspection

Visual inspection can only be carried out when tank is gas free by entering the tanks. If the tank is to be entered particular attention has to be paid to the risks of lack of oxygen, toxic and/or explosive tank atmosphere. 

Always consider the tank ‘dangerous’ and act according to theEnclosed Space Entry procedures . Condition reports of any visual inspections of cargo tanks prior to loading must be maintained on file for subsequent inspection by Third Parties.

Odour check

Some cargoes require being loaded in odour free conditions. It is important that tanks are cleaned accordingly and presented free of odour.

Wall wash tests

Depending on the product to be loaded and the previous product, contamination tests will be carried out with indicators. There are usually tests as follows:

  • Chlorides
  • Hydrocarbons
  • Inhibitor residues
  • Lead compounds
  • Permanganate time test (PTT)
  • pH–tests
  • Suspended matter

The results of all Wall Wash Tests are to be recorded in the Wall Wash Test Reports are to be maintained on file.

Wall wash Procedure

This describes an approved method for collecting and analysing wall wash samples to determine the presence of contaminants on the bulkheads.

The procedure involves contacting a constant area of the bulkhead with a given amount of specification grade methanol, collecting the liquid and analysing it for the presence of chlorides, hydrocarbons, colour and particulate matter, or whatever might be required by the Charterer.

Precautions

  1. a) Safety Considerations – eye protection is required when collecting the samples to prevent the inadvertent contact of methanol with the eyes during the sample collection process. Gloves should be worn to prevent the absorption of methanol into the skin.
  2. b) Disposable plastic gloves are also worn to prevent contamination of the samples during the collection process. (A sufficient amount of chlorides can be absorbed from the skin to cause the sample to fail the chloride analysis.)
  3. c) Chlorides are abundant in the marine environment. All sampling equipment including bottles, funnels and other apparatus must be thoroughly rinsed with methanol (of less than 0.2 ppm chlorides content) and stored in plastic containers. Bottles are to be capped prior to sample collection.
  4. d) Personnel collecting the samples must be certain that no perspiration or bare skin contacts the sample or sampling equipment while the wall washes are being collected.

Choice of test sites

As a minimum, four sites of approximately 1.2 square feet each must be chosen in each tank. (If additional sites are chosen, 100mls of methanol should be applied to each location and collected in a separate container.)

Any area that appears to have crystalline deposits should definitely be tested.

Separate test of non-typical areas greater than 2 square feet (discoloured patches etc.) should be conducted. The sample collected should be labelled with a description of the nontypical area. (These areas should be analysed separately.)

Sample collection procedures

Choose four surfaces to test.

  1. Using the plastic wash bottle, squirt methanol on the test section at the highest practical point (normally 1.5 up to 2 metres) above the tank bottom in a stream of about 10cm wide.
  2. Allow the methanol to run down the wall approximately 15 cm and begin collecting it with the funnel, squirting additional liquid as necessary to rinse the flushing into the sample funnel.
  3. Continue this process until approximately an area of 10 by 120 cm has been rinsed with 100 mls of methanol.
  4. After the washings from the four sites are collected, submit a portion of the sample for analysis of chlorides, colour, suspended matter and hydrocarbons, whatever is applicable. The accuracy of this test depends upon consistency.
  5. Consistent number of sites tested.
  6. Consistent area tested at each site.
  7. Consistent amount of methanol applied to each site.
  8. Consistent amount of methanol recovered from each site.

For the purpose of standardising methods and maintaining the desired consistency; the following criteria have been established:

Four wall wash sites

  1. i) An area of 10 by 120 cm for each wall wash.
  2. ii) 100 mls of methanol applied to each site.

iii) 250 mls total minimum recovery of methanol (approximately 60% of each of the four 100 ml washings).

Equipments and reagents

  1. Polyethylene washing bottles, 500 ml capacity
  2. Bottles, glass with screw cap and polyethylene lined, of sufficient capacity to hold the washings.
  3. Plastic disposable gloves.
  4. Specification grade methanol (laboratory pure methanol) that has been tested to be less than 0.1ppm chloride by ion chromatography. (High quality methanol is vital to the accuracy of this test.)
  5. Sample funnel, plastic or stainless steel with one flat side that can be held flush with the bulkheads.

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Source: chemical tanker guide 

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