STATIC fixers

Companies that store or process flammable materials run the risk of an explosion caused by electrostatic discharge. This underestimated industrial hazard occurs wherever there is relative motion between two surfaces.

Static is invisible to the naked eye, so it can persist undetected until it is too late. Yet an undischarged build-up of static charge can take a long time to relax back into equilibrium.

Such issues needed to be faced up to by coating and laminating chemicals company Rexam Custom,based in Wrexham. The company had always engaged in an active policy of dealing with the threat of static-related incidents, but the issue took on more serious proportions after a fire at a customers' plant.

The Wrexham site specialises in custom roll-to-roll coating and laminating of flexible materials for the electronics, graphics and medical industries. Consequently, several areas on site are classified as flammable atmospheres.

As part of a site-wide evaluation, Dave Atherton, Rexam's assistant plant engineer, looked at the traditional methods of static control; earth clips for grounding portable items of plant and the 'hard wiring' of fixed items of plant to earth. Both prevent a piece of equipment becoming an 'isolated conductor', a typical source of static sparks.

Table 1 (below) gives an idea of the sort of minimum ignition energies (MIE) required to ignite some common liquid vapours in air. MIE is defined as the minimum spark energy needed to ignite an flammable material under ideal conditions. If this is compared with the level of energy that it is possible to store on some common objects which may be regarded as 'isolated conductors' if not connected to earth (table 2), it can be seen that static control precautions are absolutely necessary within flammable atmospheres.

Atherton considered the possibility of an object of plant becoming isolated from earth, perhaps in the case of an earthing cable becoming broken or working loose during operation. In these cases there would be no inherent warning of the possible danger.

The potential problems were eventually overcome by fitting Cenelectrex Earth-Rite systems to all of the main safety critical operations: mixing and drum filling (see above). These systems are employed in a range of industrial applications to control the risk of static build up by not only providing an reliable earth connection to the piece of equipment, but also by monitoring the earth down to the high integrity connection, with a minimum acceptable resistance to earth of 10 ohms.

The Earth-Rite then provides a switch contact which interfaces with the control circuit of the piece of equipment or alarm system. The operator has a local earth status indicator to provide a visual check of the earth condition.

The benefit to the user is that the system will not allow the operation to commence before a satisfactory earth is achieved. Should that connection become faulty during operation, the system will shut down the process or raise the alarm.

Atherton says he was 'particularly pleased' that the installation of the earthing system created no additional procedural difficulties for operators. 'Staff do exactly as they did beforehand. They attach a clamp to the vessel or drum before proceeding. Only now we have the peace of mind of knowing that this is constantly being verified by the system.'

Rexam Custom rewarded Atherton with the 'Rexam Spirit of Innovation' award, in recognition of his work. His innovation will now be adopted by all Rexam sites where flammable coatings chemicals are used.

The Cenelectrex range of static grounding equipment is manufactured by Newson Gale, of Nottingham.