Many of the powders that are used in the production of pharmaceutical oral solid dosage (OSD) formulations are combustible, and since most operations in pharmaceutical facilities can create explosible atmospheres, dust cloud explosion hazards should be expected.
Explosion hazards associated with powders will influence the specification of operating procedures, process equipment, HVAC systems, electrical area classification, and room construction. It is therefore necessary for OSD manufacturers to clearly understand the deflagration hazards presented by the powders utilized in their processes and be able to assess the level of deflagration risk that their operations may present.
Deflagration is a term describing subsonic combustion propagating through heat transfer (i.e., hot burning material heating the next layer of cold material and igniting it).
It is the expectation of the Occupational Safety and Health Administration (OSHA) that employers maintain a workplace free of hazards that are likely to cause injury or death to employees, including combustible dust deflagration hazards.
In addition, OSHA expects employers to assess dust explosion hazards and apply appropriate safeguards in keeping with current codes, standards, and best practices to prevent injuries. One new standard that is applicable to all powder handling/processing operations, including pharmaceutical solid formulation operations, is NFPA 652—Standard on Fundamentals of Combustible Dusts, 2016 Edition (Ref. 1), which states that the facility, processes, and equipment shall be designed, constructed, equipped, and maintained. And management systems shall be implemented to:
- Reasonably protect occupants not in the immediate proximity of the ignition from the effects of fire for the time needed to evacuate, relocate, or take refuge.
- Prevent serious injury from flash fires and explosions.
- Ensure the ongoing production/operating capability of the facility.
Dust Cloud Deflagration Hazards
When finely divided combustible powder particles are suspended in air in sufficient concentration and ignited by a sufficiently energetic ignition source, a combustion reaction can propagate through the dust cloud. If the speed of the flame-front through the unreacted medium is less than the speed of sound, the event is, by definition, a deflagration.
These rapid combustion processes create extreme temperatures (and possibly combustion gases) which cause a pressure wave to precede the flame-front. The pressure wave moves through the unburned dust cloud at the speed of sound. This pressure wave and the resulting fireball can cause significant damage and severe injury to personnel.
The new NFPA 652 requires the owner/operator of a facility with potentially combustible dusts to determine whether the materials that they are handling/processing are combustible or explosible and, if so, to characterize their ignition sensitivity and explosion severity properties, as required to support the dust hazard assessment (DHA).
NFPA 652 further states that the evaluation of the hazard of a combustible dust should be determined by the means of actual test data. Where dusts are determined to be combustible or explosible, additional testing shall be performed to acquire the data necessary to support the compliance requirements of this standard.
The testing requirements will vary depending upon the process operations, but relevant data typically includes the Deflagration Index (Kst), Minimum Ignition Energy (MIE), Minimum Ignition Temperature (MIT), Minimum Explosible Concentration (MEC), and Limiting Oxygen Concentration (LOC).
If the material is determined to be sensitive to ignition from electrostatic sources (i.e., typically having an MIE of less than 25 mJ), electrostatic properties, such as the Volume Resistivity and Chargeability (mass charge density) of the powder, should also be considered.
NFPA 652 allows the use of historical facility data or published data only if it is deemed to be representative of current materials and process conditions. However, as these properties are strongly dependent upon specific parameters, such as particle size, moisture content, oxidant concentration, and the presence of flammable vapors, caution should be exercised in its use.
Specific particle shape and size distribution will vary from operation to operation and can make a significant difference in explosion properties. If a deflagration hazard is suspected, it is recommended that a qualified laboratory determine these explosion properties via experiments, using site-specific samples.
Dust Deflagration Hazards in OSD Operations
When handling combustible solid materials, a deflagration hazard is present whenever fine powder particles (dust) become suspended in air. This may be intentional, for example during fluidized bed drying, or unintentional, such as powder transfer to vessels or powder blending. NFPA 562 requires that the safety of the occupants, facility, processes, and equipment shall be achieved by either a prescriptive approach or a performance-based design approach.
The prescriptive approach involves:
- Determination of the combustibility/explosibility characteristics of process dusts and powders.
- Performing a Dust Hazard Analysis (DHA).
- Conducting a documented risk assessment to determine the level of building design and protection features needed.
- Establishing a written management systems and procedures for operating the facility and equipment to prevent or mitigate fires, deflagrations, and explosions from combustible particulate solids.
Dust Hazard Analysis (DHA) is a systematic review to identify and evaluate the potential fire, flash fire, and explosion hazards associated with the presence of combustible particulate solids in a process or facility, and to determine what safeguards could be implemented to prevent or mitigate the hazards. DHA must be conducted by someone who, by education, certificate, professional standing, or skill and who, by knowledge, training, and experience, has demonstrated the ability to deal with problems related to processing and managing combustible particulate solids.
A DHA does not need to comply with the Process Hazards Analysis (PHA) requirements of the OSHA regulation, 29 CFR 1910.119 (e). NFPA 652 requires that for existing processes that are not undergoing material modification, exceeding 25 percent of the original cost, the owner/operator shall schedule and complete dust hazards analyses (DHA) within a three-year period of the effective date of the standard.
Additionally, the DHA should be periodically reviewed and updated to reflect changes to the process, the equipment, and process materials, as well as any new learnings related to operational safety to ensure that the DHA continues to be an accurate and up-to-date reference for continued safe operation of the process. The review and update could be conducted on a repeat frequency of five to seven years.
If the prescriptive method proves to be impractical for some reason, NFPA 652 permits the use of a performance-based alternative design approach for a process, or part of a process, specific material, or piece of equipment in lieu of the prescriptive requirements of the standard. The performance-based design shall be based on the appropriate combustibility/explosibility characteristics of the dusts and be prepared by a person with qualifications acceptable to the owner/operator.
Additionally, all calculations, references, assumptions, and sources from which material characteristics and other data have been obtained, or on which the designer has relied for some material aspect of the design, shall be documented in accordance with the requirements of this standard.
Operations where dust deflagration hazards must be considered during the design of an OSD facility will typically include:
- Powder transfer operations
- Blending
- Granulation
- Drying
- Milling
- Sieving
- Compression/coating
- Packaging
- Dust collection
During the design (or review) of the process(s), Basis of Safety needs to be defined and documented for each unit operation. A basis of safety is the logic, systems, and procedures (safeguards) in place to ensure that an ignitable atmosphere does not occur, or if it cannot be avoided, that an effective ignition source is not present.
Beyond prevention, protection features may also need to be considered to mitigate the effects of ignition. The selected basis of safe operation will be a function of the type of operation, potential ignition sources, the flammability properties of the materials being handled, and the tolerable risk level. Typical bases of safety used in the pharmaceutical industry are shown in Table 1.
When selecting a Basis of Safety, consideration should be given to the effectiveness of each method, reliability of the system, and the personnel hazards inherent in each method. Further guidance on the applicability and design of such systems can be found in Industry or commodity-specific NFPA standards and guidelines such as:
- NFPA 68 Guide for Venting of Deflagrations
- NFPA 69 Standard on Explosion Prevention Systems
- NFPA 70 National Electric Code
- NFPA 77 Recommended Practice on Static Electricity
- NFPA 499 Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
- NFPA 652 Standard on Fundamentals of Combustible Dusts
- NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids
Other Facility Design Considerations
The processing of combustible materials can also require modifications to the design of the facility. In areas of the facility where dust deflagration hazards can exist, external to processing equipment, the NFPA standards require the process room to be provided with explosion venting and damage-limiting wall construction, which could prove to be costly and difficult to achieve. Recognizing the potential for deflagration hazards early and improving process containment during the design may avoid this situation.
In summary, key measures for deflagration hazard control associated with operations and processes where potentially combustible/explosible powders are present include:
- Investigation of the validity of any available laboratory data to support the compliance requirements of NFPA 652.
- Identification of any gaps that might exist in the required laboratory data.
- Identification of the dust sample(s) that need to be tested and performance of the necessary laboratory tests.
- Conducting a Dust Hazard Analysis (DHA).
- Identification of the gaps in the dust fire, explosion prevention and protection requirements.
- Identification of the gaps in the dust hazard management systems.
- Implementing effective measures for ensuring safety of personnel, operations, facility and the community, and assurance of compliance with the requirements of applicable codes and standards.
By applying the above measures, the deflagration hazard associated with the manufacture of pharmaceutical OSD forms can be effectively controlled and risk minimized.
Reference
- National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471
Dr. Vahid Ebadat Ph.D., M.Inst.P, MIEE, C.Eng., C.Phys. is the Chief Technical Officer of Chilworth and CEO of Chilworth Asia Pacific. He has worked extensively as a process and operational hazards consultant for the chemical, pharmaceutical and food industries. Dr. Ebadat is a member of NFPA 77 Technical Committee on Static Electricity, NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particular Solids and ASTM E27 Committee on Hazard Potential of Chemicals.
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