Following on from the first three aeSolutions blogs on the subject of combustible dust concerns, this blog provides another deep dive into the topic. We previously addressed the basic concerns around combustible dusts, many of the standards that address dust hazard guidance, and the properties and testing for combustible dusts; and potential ignition sources. Pt1. Do You Know the Basics? Pt2. Dust Properties and Dust Hazard Signs Pt3. Dust Ignition Sources This article will build on those topics to address potential safeguards for dust fires and explosions for both internal and external dust clouds or layers. A later blog in this series will pull it all together and review commonly used dust hazard assessment (DHA) methods.
Dust Handling Safeguards
As previously described in this series, dust flash fires and explosions can have extremely serious safety, environmental, financial and reputational consequences. As you would expect for any potentially serious process safety consequences, there is a range of possible safeguards including both administrative and engineering controls.
Control of ignition sources is the first and most obvious family of safeguards and it includes both administrative and engineering techniques:
Proper grounding and bonding of equipment using both the NEC and NFPA 77 (Recommended Practice on Static Electricity) is a fundamental requirement. To provide assurance that the grounding and bonding system remains in good order, a routine ground inspection / assurance program, e.g., grounding system and piping/ducting strap inspection program, should be implemented in accordance with NFPA 654. Temporary grounding arrangements for loading or unloading of dusts require special attention to make sure of the integrity of frequently operated clamps and the operational discipline to use them every single time.
For some dusts with low minimum ignition energy, use of personnel grounding may be considered (e.g., static-dissipating shoes and special conductive flooring).
Continuing with the engineering controls, proper electrical area classification using the guidance in NFPA 499 (RP for Classification of Combustible Dusts and of Hazardous Locations for Electrical Installations). Proper area classification is important to minimize potential for sparks and to keep equipment surface temperatures below the ignition temperatures for a given dust. One of the most critical aspects of establishing Class II areas for dusts is selection of the temperature class for equipment. It is very important to have firm knowledge of the dust’s minimum auto-ignition temperature (MAIT), layer ignition temperature (LIT), and maximum rate reaction initiation temperature (if applicable) to correctly establish the required temperature class.
Minimizing the possibility for equipment to produce sparks due to mechanical malfunctions is another important aspect of ignition control. Mechanical spark sources include equipment parts rubbing together creating friction heat or sparks, bearing failures, lack of lubrication and similar issues. A strong mechanical integrity program in accordance with manufacturer recommendations is important. This includes routine tasks such as a routine lubrication program, power and/or vibration monitoring, and temperature monitoring in some cases. A written program is an important tool to initiate and maintain a strong mechanical integrity program.
Keeping stray metal (often called “tramp metal”) out of processing equipment is important due to the potential for mechanical sparking and frictional heating; exclusion of tramp metal may require a combination of administrative and engineering measures. Whenever equipment is opened, there should be a visual inspection just prior to closing to ensure no metal items, such as tools, filings, nuts, etc., are left behind. In some cases, where incursion of metal from upstream sources is credible, filtering prior to during steps is advisable.
Control of hot work is a very important administrative tool to minimize the potential for hot work to generate sparks or open flames when dust may be present. NFPA 51B Standard for Fire Prevention During Welding, Cutting, and Other Hot Work is a good resource for hot work program development.
An inherently safer method for dust explosion potential is to provide equipment designed for the maximum explosion overpressure (Pmax). Pmax is typically 8-9 times the initial absolute process pressure, so this can be a good solution for cases where the normal process pressure is near atmospheric. Care needs to be taken in consideration of “pressure piling” at interconnected ducting and equipment, since those pressures can be substantial, and it may be impractical to design for the higher pressures that may be expected at interconnected equipment. NFPA 69 Standard on Explosion Prevention Systems includes design information on this topic. A concept called deflagration isolation is sometimes used to prevent propagation of an explosion and the pressure piling at connected equipment that may go with it. Deflagration isolation systems may include rotary valves, flame arrestors, fast acting automatic valves which close on a rapid pressure increase, and others.
Reducing the oxygen concentration internal to dust-handling equipment to below the limiting oxygen concentration (LOC) for the dust is a great method to prevent ignition if it is feasible for your system. Many facilities use nitrogen for this purpose and manage the system as safety-critical. If selected, the oxygen concentration of the conveying gas should be specified at a safe margin below the LOC. Somewhat related to control of oxygen content, the concentration of dust may at times also be controlled to significantly below the minimum explosible concentration (MEC) for the dust. Concentration control may not always be practical from a commercial standpoint as it may limit production rates.
In cases where the conveying gas is not below the LOC, a mitigating safeguard to quench explosions in progress is sometimes specified. Nitrogen suppression systems will open very quickly based on fast-acting change of pressure switches. A rapid inflow of nitrogen may quench an imminent explosion internal to equipment. Proper sizing, numbers, and locations of the nitrogen cannisters is of crucial importance. The suppression system also has to act faster than the time for the development of the explosion. Specialist personnel should be engaged to handle the detailed design of suppression systems.
Deflagration vents are another potential mitigating system which act to reduce the explosion pressure by venting it, similar to a rupture disc. Deflagration vents may be installed on equipment and on buildings where the potential for dust explosions is present. Deflagration vents can be quite large, depending on the application and should be vented to a safe location. NFPA 68 Explosion Protection by Deflagration Venting includes design information on this topic.
A buildup of dust layers internal to equipment is a concern due to the potential for high surface temperature or a maximum rate reaction to ignite the layer. The primary control for this concern is an effective manual or automatic cleaning regime in place for equipment subject to internal layer buildup, including routine inspections to verify adequate cleaning. Supervisory signoffs and audits of this activity are also a good practice.
External leakage of dust is a concern that needs to be addressed, as it may result in either dust explosions (if leakage is above the minimum explosive concentration (MEC) or dust fires in the case of layer buildups. There is a two-pronged safeguarding approach to address leakage. First is a strong mechanical integrity program which addresses typical leakage points proactively. Second is a strong housekeeping program in which incipient leaks are rapidly addressed, which should be supplemented by a strong routine housecleaning culture which allows for prompt cleanup of leaked dust. Similar to the internal dust layer concern, Supervisory signoffs and audits of housekeeping are also a good practice. The routine review should include all flat surfaces in the facility, including those which may not be perfectly visible, e.g., tops of equipment and tops of structural members.
Building fire suppression systems are a sensible precaution to mitigate dust explosion consequences but as they are a post-explosion mitigating system, they are not typically regarded as a strong protection in these cases.
The Stakes
Do you handle potentially combustible dusts at your site? It is difficult to adequately control a hazard that is not well-understood, and no company wants to learn of dust explosion hazards the hard way.
How do you know if you have sufficient safeguards present for combustible dust hazards at your facility? A Dust Hazard Analysis (DHA) and careful review of the engineering and administrative safeguards in place is the clear answer.
So What?
If you have not previously taken a deep dive into the safeguards in place for your particular dust(s) ignition sources at your site, now would be a good time to do so. If you do not have the right technical expertise in your company to assess dust hazards, ignition sources and safeguards, consider selecting a process safety consultancy with deep experience and expertise to assist you. Their range of experience enables assessors to recommend reputable testing labs and to share the general and specific methods proven to minimize dust explosion hazards across industry. This independence from the site and company has the best probability of a careful assessment with fresh eyes on the relevant critical systems and leads to more efficient compliance with the necessary standards.
Stay tuned for more. A later blog in this series will address commonly used dust hazard assessment (DHA) methods.