Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
–
Page 12
Poor visibility affects dispersants
‟ action only indirectly through impeding spraying operations.
6.
PHYSICAL CHARACTERISTICS OF DISPERSANTS
Some physical properties of dispersants may have practical consequences on the use of these
products (application, fire hazard, conservation). For this reason some countries include in their
approval procedure some requirements concerning the viscosity and/or pour point, flash point,
and stability/shelf life.
6.1
Viscosity
The viscosity of a liquid is defined as its resistance to flow. The units most commonly used in
the Mediterranean region for quantifying viscosity are
the dynamic viscosity in ”centipoise” (cP)
and the kinematic viscosity in "centistoke" (cSt).
Note: in this context, as dispersant density is not far from 1, especially for the concentrates, the
units centipoise and centistoke are roughly equivalent.
The viscosity of dispersants depends of the temperature. Typical viscosity range are indicated
in the table below:
Table 5: Dipersant typical viscosity range
Dispersant typical viscosity ranges cP/ temperature °C
0 °C
20 °C
Conventionals
10
–
50
5
–
25
Concentrates
60
–
250
30
–
100
Viscosity has an effect on the dispersant droplets size. In this respect, some countries may
require some limitations in the dispersant viscosity (e.g. France dispersant viscosity must be
below 80 cP at 20 °C).
6.2
Specific gravity
The ratio of the weight of a solid or a liquid to the weight of an equal volume of water, at some
specified temperature.
Conventional dispersants have generally lower specific gravities (0.80
–
0.90) than concentrates
(0.90 - 1.05).
6.3
Pour point
The temperature below which this liquid will not flow.
Pour point of most dispersants is well below 0
C (-40 to -10
C) and in the conditions
prevailing in the Mediterranean these should never solidify.
6.4
Flash point
The lowest temperature at which vapours above the volatile substance will ignite in air when
exposed to a flame.
Most dispersants have flash point above 60
C and should be considered as non-flammable.
For practical safety reasons some countries may limit the flash point (e.g. in France dispersant
flash point must be higher than 60 °C).
Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
–
Page 13
6.5
Stability / Shelf-life
During the period declared by the manufacturer as the shelf-life of the product, its properties
should not change. Most manufacturers claim a shelf-life of 5 years or more for their product.
6.6
Others
Some dispersants‟
components may cause the corrosion of the packages (drums or
containers) in which the product is stored over the prolonged periods. Accordingly, regulations
concerning dispersants in some countries require that the product does not contain such
components.
Figure 9: Samples of dispersant during quality control in the laboratory
7.
ENVIRONMENTAL EFFECTS
Environmental effects of dispersants' use are mainly related to: (a) the toxicity of dispersants or
oil/dispersant mixtures; (b) their influence on microbial degradation of spilled oil; and (c) their
effects on seabirds and marine mammals populations.
7.1
Toxicity
Toxicity can be defined as the negative effects on organisms caused by exposure to a
chemical or substance.
These negative effects may be lethal (causing death) or sub-lethal (causing negative effects
that damage the organism in some way, but do not cause death). Exposure depends on the
concentration of the substance and the period of time for which the organism is exposed to.
Toxicity is usually expressed as an effect concentration at a specific time, or as an effect time
at a specific concentration. Most often, effect concentrations are expressed by ratios, as parts
per million (ppm) or parts per billion (ppb), sometimes replaced by the mg/L and µg/L.
Toxicity of dispersants should be ideally tested on organisms in situ. However, the
impracticability of such field tests has led to the development of numerous laboratory testing
procedures. Results of such tests should be interpreted very cautiously since the tests are not
intended to be ecologically realistic or to predict effects of using dispersants in the field. Most
tests use concentrations and exposure duration which substantially exceed expected field
exposures. In addition, organisms are exposed to fixed concentrations for several days, while
in the sea initial concentrations of dispersant and/or dispersed oil would be diluted
progressively and generally rapidly. Moreover, major errors in interpreting laboratory test
results may also originate from the fact that thresholds are most often reported as nominal