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How to determine presence of contaminants in cargo onboard chemical tankers ?

Most products shipped in chemical tankers are pure and well defined. The colour is known and any deviations indicate the presence of contaminants. Most chemicals, furthermore, are clear and translucent. A milky or turbid Appearance indicates that contaminants may be present.

A sample is best studied in a test tube against a black or white background in good daylight. Empirically a number of colour scales have been established. They are each particularly suited for certain kinds of products.

chemical tanker navigation at sea
A method called APHA (Hazen) is often used for very light products, defined in ASTM D-1209, viz aromatics, ketones. This colour scale is defined with an origin in 100 cc distilled water (value 0) to succesively higher values (max 500) by adding APHA-solution (a platinum-cobolt salt solution). The number of cc APHA-solution (min 5 cc) added to the distilled water gives the APHA-colour shade number. A comparison is made against a white background with a product sample in one test tube and a matching APHA-solution in another. This method can easily be carried out on board. APHA-solutions are best bought from a laboratory, already premixed for a number of shades.

The Saybolt colour scale uses coloured glasses against which the sample is compared in a defined apparatus. The colour scale goes from plus 30 (lightest) to minus 16 (darkest). The Saybolt scale is widely used in the petroleum industry. The method is defined in ASTM D156-64. The Saybolt method requires a special apparatus, is somewhat complicated and not particularly suited for use on board ships.

The Gardner-scale uses sealed reference tubes for comparison with samples in a tube of the same diameter. (The FAC-method resembles the Gardner method but has a different graduation). The Gardner scale goes from 1 - 18 and is defined in ASTM 1544- 63T.

The Lovibond-scale has several different colours (yellow, blue, red) and uses a test cell with a variable depth. This method is used for e.g vegetable oils.

ASTM D 1500, previously called NPA defines colour scale and an apparatus suitable for field work and application on board. A test sample is compared with coloured glasses in a revolving magazine . The scale goes from i (water) to 8 (extra dark red). The name "NPA grade . . . . " is still often used in shipping when a prospected cargo is circulated or when a vessel's cleanliness is described. A cargo may be accepted at NPA 2, with discharge permitted at NPA 2 1/2. A certain amount of degradation in such a case can be permitted.

With a colorimeter on board one can take samples and by experience get an idea of how much of the previous cargo residues can be left unattended without risk for cargo claims (refers to petroleum cargoes rather than to chemical products)

Dissolving contaminats from rust

As a rough check if cargo residues hidden in rust may discolour a cargo to be loadedthe following check can be made:

Break the rust into pieces, 0, 5 - 1, 0 cm in size, and place them in a clean bottle. Add some of the product to be loaded (or other similar available product) and leave for 5 min. If no discolouration has occurred by then, shake the bottle vigorously and leave for another 15 min. If still no discolouration appears the tank could be cleared for loading. If there is a discolouration: filter the contents through double filter papers in order to remove any possible suspended rust particles. A non-discoloured liquid after filtering should also clear the tank for loading. A discolouration means that the tank requires further cleaning.

Observe: this simple test does not necessarily clear a tank for loading of e g aromatics, but it may prevent you from trying.

Related Info:

Determining water contamination in chemical cargo

Varoius product contamination in chemical cargo - how to resolve?

Using acid wash method for a cargo of aromatics contaminated by a previous oil cargo

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