In summary, you as the owner or operator of a facility with potentially combustible dust are responsible for undertaking the following steps:
- Determining the combustibility and explosibility hazards of the materials processed at your facility
- Conducting a Dust Hazard Analysis (DHA) – Identifying and assessing fire, flash fire, and explosion hazards
- Managing identified fire, flash fire, and explosion hazards
- Establishing written Safety Management Systems
We keep getting requests from our clients for Dust Hazards Analysis (DHA); however, DHA is only one (albeit an important) component of the requirements of NFPA 652 and the authority having jurisdiction requires that you are in compliance with all the requirements of NFPA 652.
DHA is required if materials handled and processed have been identified as combustible and/or explosible. DHA is a systematic review to identify and evaluate potential fire, flash fire, and explosion hazards associated with the presence of combustible particulate solid(s) in a process or facility.
Please note that NFPA 652 requires that the DHA is conducted by an expert with the demonstrated ability and credentials to effectively identify, assess, and recommend practical measures for controlling the hazards related to processing and managing combustible particulate solids.
First off, NFPA 652 only requires us to determine if the dust cloud under the right conditions is “explosible” (Go/No Go)? In other words, would a dense cloud of dust, dispersed in air and subjected to an energetic ignition source, be capable of causing a flash fire or explosion?
NFPA 652 permits you to determine the combustibility or explosibility of your powders/dusts based on one of the following methods:
- Historical facility data or published data that is deemed to be representative of current materials and process conditions
- Laboratory analysis of representative samples
- You are also permitted to assume a material is explosible, forgoing the laboratory analysis
Note that the absence of previous incidents is not allowed to be used as basis for deeming a particulate non-combustible or non-explosible.
NFPA 652 does not specifically require any testing other than possibly Explosibility (Go/No Go). However, in order to perform any meaningful Dust Hazard Analysis (DHA) it is essential that one has applicable information/data on the ignition sensitivity, explosion severity, and electrostatic properties of dusts that are being handled. Typical tests that might be considered include:
- Minimum Ignition Energy
- Minimum Ignition Temperature (Cloud and Layer)
- Minimum Explosible Concentration
- Limiting Oxygen Concentration
- Maximum Explosion Pressure (Pmax) and Kst
- Electrostatic Chargeability and Volume Resistivity
Note that depending on the nature of your operations and processes you often do not need to perform all of these tests. Usually, you can utilize a step-by-step approach to testing whereby the result of the initial test(s) determines if any additional testing is required.
Although bonding and grounding is an essential component of electrostatic hazard control, often there are other very important measures that one must consider.
The control of static electricity requires specialist focus since it is not always apparent where charge can be generated and indeed if the generation of charge is likely to cause a discharge/ignition hazard or a processing/handling problem. In addition to metal plant one also needs to consider electrostatic hazards from people (facility employees), insulating (plastic) materials, non-conductive liquids, and powders.
Typical PHA methods are Checklists, HAZOP, What-ifs, Failure Modes and Effects Analysis, and Fault Tree Analysis (FTA). As a general rule, one should avoid more complicated methods particularly if the process is well defined and is covered by applicable codes and standards. Other factors that determine the PHA method include:
- Complexity of the process
- The inherent hazards of the chemicals and processes
- Existing operations vs. design concepts/stage
- Applicability of codes, standards, and recognized and generally accepted good engineering practices and to the process
- In-house process safety expertise and the experience of the company in process safety issues
Most powders handled in industry can form dust clouds which can explode if there is a strong enough ignition source present (e.g., flame, electrical spark, hot surface, friction spark etc.). There are a few powders that can “auto-ignite” – i.e., burst into flame and explode when dispersed in the air even if there is no external ignition source present. An example is freshly prepared metal dusts such as aluminum. There are also some powders that can self-heat and catch fire in a layer or bulk form, thus creating a source of ignition for any associated dispersed dust cloud.
Tests are available that will test powders for self-heating and also for auto-ignition risk. This is a specialist area. Speak to one of our consulting staff for additional information.
Some powders handled in industry can be stable at room temperature but have a tendency to self-heat and even spontaneously catch fire when warm or where cooling is restricted. Many fires/explosions have been caused in dryers and hoppers by powder self-heating, for example. Special laboratory tests can check if your powders will self-heat and additional tests can be used to evaluate safe drying and safe storage temperatures.
It is important to realize that a powder does not have a unique self-heating temperature. The temperature at which powder decomposition begins is dependent not only on the chemical nature of the powder in question but also on the volume of powder held, its geometric shape, thermal conduction, availability of air, contamination, and other factors. Examples of powders that can self-heat are detergents, milk powder, coffee, and many others.
No. Electrostatic sparks involve ionization of the air, which is the breaking apart of the air molecules. Impact sparks are glowing fragments of material resulting from impact. These two types of sparks have very different incendivity (different igniting capability).
The term electrostatic spark has two meanings. On the one hand it refers to a particular type of electrostatic discharge – from charged metal to grounded metal. Other types of electrostatic discharge are brush, cone (from surfaces of bulking powder), propagating-brush, and corona. In popular usage the term “spark” can also refer to any type of electrostatic discharge but this is potentially confusing and should be avoided when discussing electrostatic hazards. Different types of electrostatic discharge have different igniting powers.