By David Steil, Camfil Air Pollution Control
In the previous month’s introduction, we set out to examine the dangers of combustible dust generated in chemical processing facilities, applicable regulatory guidelines, and how to use a high-efficiency dust collector to keep workers safe and facilities in compliance. Below, we will discuss the regulations governing combustible dust as well as look over techniques for mitigating combustible dust and its exposure.
REGULATIONS GOVERNING COMBUSTIBLE DUST
In the United States, there are three key entities involved in combustible dust issues, each with its own particular area of responsibility:
- NFPA sets safety standards regarding combustible dust, amending and updating them on a regular basis. NPFA’s standards aim to prevent an explosion, vent it safely, and ensure that it will not travel back inside a building. Most insurance agencies and local fire codes state that NFPA standards shall be followed as code. Exceptions would be where the authority having jurisdiction (AHJ), such as local fire marshals, specifies an alternative safety approach, which might be even more stringent.
- OSHA, together with local authorities, enforces NFPA standards. OSHA’s Combustible Dust National Emphasis Program (NEP) outlines policies and procedures for inspecting workplaces that create or handle combustible dusts. OSHA began rulemaking in October 2009 for a general industry standard for combustible dust; however, the agency has yet to issue a proposed rule.
- U.S. Chemical Safety Board (CSB) is an independent federal agency responsible for investigating industrial chemical accidents. Staff members include chemical and mechanical engineers, safety experts and other specialists with chemical industry and/or investigative experience. The CSB investigates combustible dust explosions, sifting through evidence to determine root causes and then publishing findings and recommendations. For example, one CSB study identified 281 combustible dust incidents between 1980 and 2005 that killed 119 workers, injured 718, and extensively damaged industrial facilities. Since that time, the CSB continues to identify serious dust-related incidents on a regular basis.
RELEVANT NFPA STANDARDS
In trying to sort through the list of combustible dust standards, a good starting point for every chemical processing facility engineer or manager is NFPA 652, the Standard on the Fundamentals of Combustible Dust. This covers the requirements for managing combustible dust fires and explosions across industries, processes and dust types. This standard applies to all facilities and operations that deal with combustible dust, not just hazardous or classified locations. For chemical processors, this means they are required to perform a dust hazard analysis (DHA). The owner or operator of any facility where combustible dust exists is responsible for conducting a DHA to identify the hazards, create a plan for managing the hazards and providing training for anyone affected by the hazards.
- NFPA 654 (Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids) is an all-encompassing standard on how to design a safe dust collection system. It is the most general on the topic, and it will lead you to other relevant documents. Depending on the nature and severity of the hazard, NFPA 654 will guide you to the appropriate standard(s) for explosion venting and/or explosion prevention.
- NFPA 68 (Standard on Explosion Protection by Deflagration Venting) focuses on explosion venting on devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure, for the purpose of minimizing structural and mechanical damage.
- NFPA 69(Standard on Explosion Prevention Systems) covers explosion protection of dust collectors when venting is not possible. It includes the following methods for prevention of deflagration explosions: control of oxidant concentration, control of combustible concentration, explosion suppression, deflagration pressure containment and spark extinguishing systems.
- NFPA 70 (National Electrical Code) covers everything related to the installation of electrical equipment across all industries and all types of buildings. This code is enforced in all fifty states. Chemical processors need to be aware of two main sections of NFPA 70 because they apply to housekeeping: Combustible dust definition and hazardous locations. NFPA 70 defines combustible dust as “dust particles that are 500 microns or smaller and present a fire or explosion hazard when dispersed and ignited in air.” The NEC defines different classes of hazardous (classified) and non-hazardous locations. These classes determine the wiring of buildings and also the equipment and housekeeping procedures that can be used in different areas of facilities.
MITIGATING COMBUSTIBLE DUST—DUST HAZARD ANALYSIS
In chemical processing it is critical to know the explosive potential of the dusts, gases, and dust/gas mixtures emitted during operations. NFPA states that a dust hazard analysis is needed to assess risk and determine the required level of fire and explosion protection from combustible dust. The analysis can be conducted internally or by an independent consultant, but either way the authority having jurisdiction will ultimately review and approve the findings.
The first step in a hazard analysis is determining whether your dust is explosive. NFPA classifies dusts according to their explosibility, that is, their “Kst” value. Kst is the normalized maximum rate of explosion pressure rise. A bar is a metric unit of pressure, which is slightly less than the average atmospheric pressure on earth at sea level.
NFPA Class ST1 dusts are rated below 200 Kst, Class ST2 dusts range from 200 to 300 Kst, and Class ST3 dusts are rated above 300 Kst. As a rule of thumb, when dusts approach 600 Kst, they are so explosive that wet collection methods are recommended. However, any dust above 0 Kst is considered to be explosive, and the majority of dusts fall into this category. If OSHA determines that even a very low Kst dust is present in a facility with no explosion protection in place, a citation will result, per OSHA’s NEP policy.
In addition to Kst, it is important to know other combustible dust properties such as Pmax (the maximum explosion pressure of a dust cloud, measured in bar) and Pred (the maximum pressure developed in a vented enclosure during a vented deflagration). These can be determined using ASTM E 1226-10, Standard Test Method for Explosibility of Dust Clouds.
Your dust collection equipment supplier will need the Kst and Pmax values in order to correctly size explosion venting or suppression systems. Failure to provide this information may increase your costs, since the supplier will have to use worst-case estimates of the Kst and Pmax values or may even refuse to provide the equipment.
In addition to conducting explosibility testing to determine whether a dust is combustible, it is important to analyze other dust characteristics to determine the best dust collection system and filters for your chemical processing operation. Other key dust properties to know include particle size, dust shape, gravity, moisture level, and abrasiveness. Understanding these components lend to the optimal design of dust-control equipment. Quality equipment suppliers can conduct this type of dust testing and work with you to specify the best system for your application.
DUST COLLECTORS AND EXPLOSION PROTECTION
Combustible dust explosions are a risk in many areas of a chemical processing plant, but a critical location is the dust collection system itself. There are many types of explosion protection devices and systems used to help dust collection systems comply with NFPA standards. They fall into two general categories: passive and active.
Passive systems react to the event, while active systems detect and react prior to or during the event. The goal of a passive system is to control an explosion so as to keep employees safe and minimize equipment damage in the plant. For example, designed to install over a standard explosion vent, a flameless vent is one type of passive device that extinguishes the flame front exiting the vented area, not allowing it to exit the device. This allows conventional venting to be accomplished indoors where it could otherwise endanger personnel and/or ignite secondary explosions. A safe area around the flameless vent still needs to be established due to the release of pressure and dust/gases.
An active system, by contrast, can prevent an explosion from occurring. An active system involves more expensive technology and typically requires recertification on a regular basis. These systems can include chemical isolation or suppression, or mechanical options such as a valved designed to close within milliseconds of detecting an explosion installed in either inlet and/or outlet ducting.
ADDITIONAL PRECAUTIONS
When planning and designing explosion protection, don’t overlook additional devices and materials that can help reduce fire risk within the dust collection system. For spark-generating applications, a range of features and technologies are available, from flame-retardant and carbon anti-conductive filter media to spark arrestors in the form of drop-out boxes, perforated screens, or cyclone devices installed at the collector inlet. Fire sprinklers and active fire extinguishing systems may also be required with some installations.
In high dust loading applications, a dust collector that uses vertically-mounted filter cartridges can also reduce fire and explosion risks. This type of arrangement uses gravity along with the pulse cleaning system to effectively and efficiently remove dust from the filters and the collector housing. With some horizontally mounted cartridges, high loading dust becomes trapped in the pleats in the upper third of the filters. This trapped dust can burn even if the filter media is fire retardant.
SUMMARY
Effectively controlling the dusts generated in chemical processing facilities is an essential life-saving and legal obligation. Dust can cause serious harm to employee health, reduce product quality, and cause devastating explosions that can hurt or kill workers and bring irreparable damage to your operation.
A high-efficiency dust collector designed specifically for your operation is an accepted and proven engineering control that will filter hazardous contaminants and combustible dusts to make indoor environments safer. With the help of engineering consultants and reputable and experienced equipment suppliers, chemical processing facilities can minimize risk factors and maximize combustible dust safety.
FOR MORE INFORMATION
Camfil Air Pollution Control is one of the world’s largest manufacturers of industrial dust, mist, and fume collection equipment. As part of the global Camfil Group, we’re represented on six continents and draw upon over five decades of experience and expertise to ensure that our customers are kept safe and productive. Every manufacturing process creates some kind of dust, mist, or fume. These byproducts can be harmful to workers, and they can cause machinery to become less efficient. Camfil APC’s product range is designed to keep your workforce safe and healthy, and ensure that your operations and processes remain as efficient as possible. For more information, visit www.camfilapc.com.
MODERN PUMPING TODAY, June 2021
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