Abstract
Preventative bases of safety (BoS) (i.e. absence of flammable atmospheres
or avoidance of ignition sources) are the most economic to establish,
so there are clear drivers for using them where possible. However,
where they are used they must be robust and maintainable. For example
solvents can be used below flash point (with a safety margin) to
avoid flammable atmospheres or dusts can handled where possible electrostatic
discharges are below the MIE. To establish such a BoS the material
must be well characterised relative to the possible ignition sources
present.
Sprays and mists can be created deliberately in processing (e.g. cleaning
vessels; and spraying materials onto substrates) as well as from
leaks. A number of workers (e.g. Burgogne & Richardson, 1949) have
identified that mists can be flammable below the flash point. A figure
often quoted is that mists can be flammable as much as 60K below
flash point. However, results from Syngenta’s legacy organisation
shows sprays to be flammable as much as 125K (Maddison, 1983) below
the flash point; with no upper limit having been determined. There
may be an upper limit but it will not help us for many materials.
To use the BoS absence of viable ignition sources the sensitivity
of the mist to ignition must be established in an analogy to dust
MIE. Little work has been done in this field particularly for hazard
assessment.
A rig has been built to spray materials at ambient temperature and
perform ignition tests with pyrotechnic and electrostatic ignition
sources. Electrostatic ignition presents some challenges in making
reliable measurements: electrodes are wetted by the spray; the presence
of droplets between the electrodes can lead to early breakdown and
discharge below the desired voltage. Full scale nozzles use large
quantities of fluid and ignitions can contaminate this. Characterisation
of the spray will be necessary so that measurements are made at a
relevant droplet size possibly with a much smaller nozzle. Several
fluids have been tested.
Users
Please
log in to take part in the discussion (add own reviews or comments).