Process operations where an explosion hazard may exist require a basis of safety to
ensure that the hazard is being controlled such that the overall risk is within an acceptable
range. A basis of safety usually comprises of a primary and secondary basis of safety. The
primary is usually based on prevention methods reducing likelihood of an explosion. The
secondary is based on protection methods, mitigating the consequences of an explosion.
In conjunction, risk is clearly reduced. Clearly, avoidance of an explosion is desirable as
even with well designed protection systems an inherent reliability is present, so they may
not work when required.
Powder coatings Raw materials are usually supplied to the powder coating facility in preweighed bags of 25Kg directly from the manufacturers or storage area. Therefore further
pre-weigh shouldn’t be required provided check weighing is carried out in multiples of the
bag weights. All pigments and small amounts are weighed from storage into polythene
bags, although these should be normalised to the maximum number of full bag weights in
the formulation. The possibility of explosion prevention during pre-weigh is achieved
primarily by control of dust concentration and ignition sources. Therefore reliance is placed
on handling and use of efficient dust extraction to prevent dust emissions.
Good housekeeping in the pre-weigh area is necessary to prevent accumulation of dust
layers and risk of secondary dust clouds. Operators should wear anti-static footwear, the
floor should be suitably conductive, or a stainless steel earthed plate should be provided
for the operator to stand on during powder raw material handling operations. The weigh
scales themselves should be protected to IP65 as a minimum, and the use of any nonenclosed electrical equipment such as 240/120 V sockets, light fittings etc., should be
avoided near to any area where fine powder coating is being handled.
In terms of pre-weigh container specification for powder raw materials and rework, the
containers should be earthed during weighing. If only small quantities are being weighed
into plastic bags they should be as small as possible and of a heavy-duty type.
Weigh scoops should be made from stainless steel and should also be earthed. A way of
achieving this would be to attach the scoop to a flexible wire, connected to earth. This way
the portability of the scoop is maintained.
When loading powder into the mixer or mixer container, care must be taken to avoid
generation of dust clouds. This is achieved by avoiding spillage onto the floor and by use
of efficient dust extraction. Dust extraction is easier to provide for the batch size containers
from a fixed platform with extraction to remove the dust produced from loading bags of
material into the top of the container. It is more difficult to provide extraction from a fixed
mixer with opening lid and obtain good dust source extraction.
Operators must be earthed during the filling process by provision of anti-static footwear.
The loading platform should be manufactured from steel, so should ensure adequate earth
conductivity. Since the loading platform is likely to be mild steel, it will require painting.
It would be recommended to either use a conductive paint or to provide an independently
earthed stainless steel plate on the floor dimensioned and positioned so that the operator
has to stand on the plate before loading powder. Spills should be cleaned up and soon as
possible and not allowed to accumulate.
The mixing platform should be designed for ergonomic handling of the raw materials to
minimise the possibility of lower back or repetitive strain injury from lifting and loading.
This is easier to design for loading similar sized batch loads into container or mixer with
the use of scissors lifts or bag lifting devices. Safe disposal of waste plastic and paper
bags can be designed with waste compactors incorporated into the loading platform.