The UK’s Health & Safety Executive, in collaboration with the Chemical Business Association (CBA) and Solvent Industry Association (SIA) has issued general guidance outlining the type of assessments that should be carried out to manage the risks associated with IBCs storing flammable and combustible material.
Of particular interest is the assessment for managing the risk of electrostatic ignitions. The HSE refers to the SIA’s notice No.51a which provides guidance on minimising the risk of incendive electrostatic spark discharges when storing solvents in IBCs.
Electrostatic hazards and IBCs
The risk of electrostatic discharges in potentially flammable or combustible atmospheres is well documented in best practice standards like Cenelec’s CLC/TR:50404 and NFPA 77. Although identifying static as a hazard is difficult to visualise, as it is not readily tangible or easily detectable, the underlying theory and safe practices that can be put into place are relatively straightforward.
The flow of any material in pipes, filters and fittings, whether the material is conductive or non-conductive, results in the separation of charges. The separation of 1 electron in half a million is all that is required to provide the right conditions for an incendive spark discharge to occur. Much the same way a spark plug works in the engine of a car, electrostatic discharges result from the existence of a spark gap. The spark gap only needs to be momentary and if a flammable or combustible atmosphere is present in the spark gap, the energy released can exceed the minimum ignition energy of the surrounding atmosphere. Uncontrolled spark discharges have enough energy to ignite the majority of flammable atmospheres.
When liquid entering an IBC has surplus charges attached to it, it creates an electric field which induces opposite charges on the inner wall of the IBC. If the IBC is not properly grounded, it will act like a capacitor plate in an electric circuit, accumulating charges on the outer surface of the IBC.
The accumulation of charges is now a potential ignition hazard as surplus charges are available to discharge to objects in the vicinity of the IBC in an uncontrolled manner. The commonest form of object charged IBCs will discharge to are grounded conductors like surrounding plant equipment, dip tubes, forklift trucks and, most commonly, the operator handling the IBC. What is of critical importance is that the IBC is conductive and has a low resistance static dissipative connection to earth. This will enable any surplus charges to flow immediately to ground from the hazardous area in a controlled manner. The standards, including the guidance issued by the SIA, categorically state this resistance must be less than 10 ohms and regularly checked to ensure the IBC is always capable of dissipating charges.
A connection resistance of 10 ohms or less ensures there is no doubt that the rate of charge dissipation exceeds the rate of charge generation and charge accumulation, allowing the static charges to be dissipated safely from the IBC.
It follows that the first thing an operator must do before filling or dispensing from an IBC is to ensure the IBC has a positive static dissipative ground connection.
There are also a number of additional factors that must be borne in mind when using IBCs. Filling flow rates and the conductivity of the liquid are especially important factors to consider. When the IBC is filled initially, a potential spark gap will be present between the end of the filling pipe and the surface of the liquid. The SIA guidance recommends 1 m/s until the fill pipe is covered by the liquid and a limit of 2 m/s thereafter. Splash filling must be completely avoided as this will encourage the separation of charges.
If the liquid is conductive charges can dissipate through the conductive wall of the ground connected IBC. If the liquid is low conductivity (<50 pS), the appropriate charge relaxation times should be incorporated into the handling process. NFPA 77 provides a comprehensive list of flammable liquid conductivities and their corresponding charge relaxation time periods.