Combustible dust can pose a hidden hazard when accumulation occurs in unseen locations such as in mechanical spaces, above false ceiling, ventilation systems and dust collection systems. Such hazards may be particularly well hidden in certain pharmaceutical manufacturing facilities where use of clean rooms with surrounding mechanical areas are common and the scale of the equipment and facility is relatively modest. Even small quantities of combustible dust may result in a dust cloud flash fire or an explosion capable of significant damage in a plant environment. Although events of this magnitude may not make headline news, the potential impact on an individual present during a flash fire could be life changing.
So, here, I share an example that occurred at Eli Lilly and Company to show how combustible dust may become “hidden” within a dust collection system, and to describe a methodology for safe combustible-dust removal, as well as actions that can prevent future problems.
This example comes from a pharmaceutical blending operation located in a typical clean room. Technicians are preparing to blend 110 kg of dried pharmaceutical powder. All surfaces within the room are dust free and the polished stainless steel blender has just been cleaned. The technicians connect a small 2-in. ventilation trunk between the blender and a port on the clean room wall labeled “to dust collector.” The technicians then open the access cover of the blender and press a button to start the dust collector, which is located elsewhere. Seven bags, each containing 16 kg of dried powder, are charged to the blender through the opening. The technicians are wearing personal protective equipment (PPE) to prevent inhalation of the dust but no dust is observed outside the opening. When the product charge is completed, technicians turn off the dust collector and disconnect the 2-in. ventilation trunk. The trunk is visually clean. The self-contained blending operation completes normally. All equipment and the room itself then are cleaned in preparation for the next batch. Lastly, the technicians leave the clean room to check for accumulation of material in a small drum under the dust collector; the drum is empty as always. The technicians know the routine well; they have completed these tasks at least once a week for the last ten years.
By their training, the technicians understand the powder they are handling is a combustible dust. They know the minimum ignition energy (MIE) has been tested at approximately 200 mJ with an average particle size of 27 microns, which means the risk of ignition from an electrostatic discharge from personnel is greatly reduced, and personnel grounding isn’t required . The electrical outlets and switches in the clean room look different from others in the area, and signs hang on the doors indicating the room is electrically classified as Class II, Division II for combustible dust. If technicians observe a dust cloud for any reason (e.g., a dropped product bag), they are to immediately leave the area until the cloud settles. In general, technicians believe little if any dusting occurs during loading of product to the blender — a belief supported by the lack of dusting seen during blender loading and emptying the dust collector discharge drum.
The technicians and technical support personnel assumed that because no dust is coming out of the dust collector, no dust is going in. The assumption was widely believed to be true and even documented in a previously completed formal hazard review. The idea that dust accumulation might be possible simply did not occur to those supporting the blending operation.
A TELLING INSPECTION
In 2012, the facility initiated a hazard review process for all solids handled at the site. This included looking specifically at the dust accumulation risk for each operation. One recommendation stemming from this activity was for engineering to perform an internal inspection of the blending operation dust collector.
Prior to the inspection, the team reviewed available design information for the dust collector and field-verified all ductwork. The system was designed for an airflow of 500 ft3/min to ensure sufficient capture velocity at the blender opening during loading. The ductwork in the field begins at the clean room wall, where the duct diameter increases from 2 in. to 4 in. and then transitions to a diameter of 6 in. immediately prior to a 15-ft vertical riser. The duct then travels horizontally several hundred feet through multiple mechanical rooms before reaching the dust collector inlet plenum. Portions of this ductwork run above false ceilings. At the inlet plenum, the 6-in. duct expands to a 1-ft × 3-ft rectangle at which point it enters the dust collector. That unit, which is 1 ft in diameter and 3 ft in length, contains four cartridge filters. The dust collector is equipped with a differential-pressure pulsation system to clear the filters under conditions of high pressure drop. At the bottom of the dust collector, a manual slide gate valve leads to the aforementioned drum for dust disposal.
During the engineering inspection, the four cartridge filters were removed and found to be heavily loaded with dust. Internal inspection of the dust collector revealed ½-in.+ layers of dust settled on all horizontal surfaces including the inlet plenum (Figure 1). Samples were taken and submitted for particle-size and MIE testing. The average particle size of the material in the dust collector was 12 microns, half the size of the bulk powder loaded to the blender. That in itself isn’t surprising because the dust collector air stream primarily captures fines churned up during blender loading. The MIE for the material in the dust collector was approximately 25 mJ — an order of magnitude less than that of the bulk powder loaded into the blender! With an MIE as low as 25 mJ, the risk of ignition from electrostatic discharges becomes a greater hazard, necessitating enhanced safeguards including personnel grounding .