flammability test data in risk assessment

THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS“ OF GALAŢI
FASCICLE VIII, 2008 (XIV), ISSN 1221-4590
TRIBOLOGY
75
In a full risk assessment the evaluator has to do
a detailed examination of both technical system and
its environment and to perform a “full” inventory
(many accidents are the result of a “forgotten” aspect)
of potential ignition sources. This will include the
location and temperature of any hot surface, the
presence of lagged pipes or the existence of unsealed
electrical equipment etc. In normal functioning, if the
surface temperatures the fluid might contact, were
below the hot manifold ignition temperature (that
means for ISO 20823 the fluid is in N category), then
the source may be discounted in all but fault
conditions. Tribology could help specialists to
evaluate temperatures of different surfaces in relative
motion, in critical scenarios (leak of lubricant, plastic
deformations etc., total or partial removing of friction
coatings etc.). The evaluator may develop the
following scheme for ignition on hot surfaces:
- if normal functioning temperatures exceed the
test hot manifold temperature T c , the ignition
probability is 1,
- for functioning temperatures below T c , the
evaluator has to designate an ignition probability on a
scale that could be zero for some fraction from T c
(some specialists give 0.5T c or 0.75T c [27]),
- additional rising of ignition probability when
fault conditions are possible to happen.
In some cases the evaluator could apply risk
index methods, calculating a “risk value” or a risk
index, based on the relationship:
where
a j
= ∑ n
risk j ⋅ j
j=
1
I a r (1)
is the attribute j related to risk evaluation
(for instance, ignition temperature on hot surface,
smoke production, electrical ignition sources etc),
j=1...n, and is a value associated to probability of
r j
occurrence and consequences. Both
a j
and
r
j
have
to be introduced in a normalised scales. r j
may have
the following values: 0 – the occurrence is not
credible, 1 – unlikely, 2 – medium probability, 3 –
highly likely. For the attribute of ignition on hot
surfaces, the associated value could be related to
the ignition temperature of the fluid involved, but in
an indirect proportionality. For instance, if the
engineer had to select an industrial fluid among
several with different hot surface ignition
temperatures, T1〈 T 2...T
〈 n, after testing under the
procedure of ISO 20823, the fluid j has the
normalised attribute
a = T /T j (2)
j
It is obviously that a lower value of this
attribute is desired for safety functioning and for a
low probability of hazardous events. The problem to
be solved is the compromise between initial costs and
performances of the fluid to be selected. Several
decades ago the ratio between high security fluids and
n
r j
hazardous fluids was as great as 5...3 to 1. For
instance, a fluid-power system will be more
expensive when using water-based fluids due to the
materials involved in designing (especially corrosion
resistant steels, sealings etc.) as compared to a system
with similar performances but using mineral oils.
Recently, specialists give design solutions that
overpass only with 30...50% the classical ones that
use more hazardous fluids [20].
In mining industry potential sources of ignition
such as sparks, flames, electric arcs, high surface
temperatures, acoustic energy, optical radiation and
electro-magnetic waves have been identified in Directive
92/104/EEC [7] as being potentially present in underground
mines (the discharge of static electricity, stray
electric currents or discharges from malfunctioning
electricity supply equipment that could produce overheating
of surfaces or sparks capable of causing
ignition, friction between moving surfaces or the
entrapment of foreign bodies between moving
surfaces caused, for example, by failures of
mechanical plant, causing localised overheating, high
surface temperatures present in internal combustion
engines, braking systems, transmissions or exhausts,
the use of smoking or other materials that may be
contraband in some mines, existing fires caused by
ignition of other flammable materials in the mine).
Industrial development depends also on hydraulic
fluids, so their risk to fire has to be reduced by
different means: modifying their chemical
composition in order to shift temperatures related to
fire risk, mixing fluids in order to enlarge the
temperature range with no flammability or to reduce
fire risk, design solution for avoiding situations with
fire risk, a continuous training of operators, efficient
fire suppression systems.
The fire resistance of some hydraulic fluids may
change with time or with operational service. Fireresistant
fluids rely on their water content or their
chemical composition and physical properties to
provide fire resistance. Circumstances that could
result in the reduction of the water content below its
original value or chemical or physical changes in the
fluid could produce hard-to-estimate fire resistance.
Such situations could arise through persistent high
temperatures, fluid spillage where evaporation or
separation could occur, or breakdown of fluid
chemical properties during use. No specific test has
been designated to cater for these situations, which
should be addressed through regular fluid monitoring
and good maintenance procedures. The risk assessment
should assess the likelihood of the fire
resistance of a product being reduced in the application
for which it is intended. Other aspects such as
environmental requirements may have to be considered
and these may require additional or alternative
safety precautions to be employed [9, 32, 40, 43].
For reducing potential risks it is very important to
train employees on the risks associated with any
chemicals used at the operating site, the federal Right