Answer: The easy answer to give here is “less than 10 ohms”. This value is a practical value that is often used as a base case when undertaking electrostatic hazards resistance checks. In fact, much higher resistances – hundreds or even thousands of ohms – can still be safe in many situations since currents generated by static electricity are generally tiny and ohms law tells us that the voltage that will be generated on a component is a product of the electrostatic charging current and the component’s resistance to ground. A current of 1mA to a nozzle handle, for example, with a resistance of 1k Ohm would only generate 1 volt on the nozzle; not an ignition hazard.

If we look at static dissipative footwear, for example, we see that an acceptable resistance range for footwear is usually between 106 to 108 ohms. It is not zero since such footwear would provide no protection for the wearer who might be unfortunate enough to encounter mains electricity. In fact, footwear resistance-to-ground can be as high as 108 ohms because however fast and hard an operator works, he is only ever going to be able to generate limited charging currents through his actions.

[For interest, your works electrician is unlikely to be happy with a ground connection giving a reading of 1000 ohms, nor with footwear that is not highly insulating. But that’s another story!]

NFPA 77, Recommended Practice on Static Electricity, provides examples of acceptable resistance values in a variety of different plant situations.

Answer: Conduct an electrostatics/ resistance measurement audit on plant.

Why do it: Build-up of electrostatic charge on ungrounded metal plant and equipment can give rise to hazardous sparks, fires and explosions.

Performing resistance-to-ground measurements on plant is not necessarily a straightforward task when your purpose is concerned with protection against static electricity. If you have fixed metal plant that is interconnected to other fixed metal plant, then there is usually no reason why resistance-to-ground measured will be anything other than zero ohms. An appropriately trained electrician would be able to confirm this for you. The problem is more likely to come from moveable plant and equipment and from plant components that by design do not make metal-to-metal contact. Many of our clients invite us to undertake such specialist resistance checking and electrostatics audit work, and here are a few examples that reveal why they might do this.

How do you address:

  • A vessel stirrer with bearing and coupling design that prevents metal to metal contact,
  • A drum or flask (or even an operator) standing on an insulating floor,
  • The supporting spiral wire inside a plastic dust extraction or liquid conveying hose,
  • The indispensable vacuum cleaner on nylon wheels with no ground connection,

With all the above examples, your electrician will never establish a resistance to ground at ‘less than 10 ohms’, but even so, there may not be an electrostatic hazard. There are situations where resistance values much higher than 10 ohms may be permitted, for example, 106 to 108 ohms for static dissipative flooring, 106 ohms for rollers, less than 103 ohms/m for certain hoses….. and so on.

There are also practical issues to address, such as when/if jumpers should be used across the flanges or couplings at metal pipeline joints. These jumpers are usually not necessary; but occasionally they are essential to prevent sparks forming or for ease of confirming electrical bonding across the connections! They are also expensive to maintain.

….and by the way, electrostatic test equipment must be appropriate selected and used safely in a classified area, to ensure the test equipment cannot cause ignition of flammable atmospheres during the testing!

A good question, but best begin by understanding the actual lab test conditions and comparing them to plant conditions.

The same test equipment design – used to measure maximum dust explosion pressure (Pmax) and Kst value – is the 20-liter sphere (see Stonehouse Explainer article on Kst here). It is essentially a pressure vessel in which we create a dust cloud, ignite it, and measure pressure rise over time. The dust cloud is created by first evacuating the pressure vessel, then opening a full port valve to introduce the dust sample. A timer sequence then activates a pair of chemical ignitors (detonators) according to a set delay time. At time of ignition, the dust cloud has a degree of turbulence. Actually, most dust clouds will be somewhat turbulent – otherwise there would not be a dust cloud!

When the original research on dust explosion test methods was done in Switzerland and Germany, the delay time and turbulence level in the 20 litre sphere were a focus of attention because turbulence does increase explosion rates of pressure rise – and Kst value – and the maximum explosion pressure, Pmax. In practice, the actual test turbulence level is a little arbitrary – but levels were deemed at the time to be reasonably representative of process plant conditions (e.g. turbulence in a silo during pneumatic filling or perhaps in a reverse jet bag house). Tests of explosions in 1m3 vessels and eventually in full sized process plant showed this to be reasonable. Of course, the observant of you will appreciate that in truth, although turbulence level is a little bit arbitrary in the test in as much as it is the same for ALL POWDERS tested in the test equipment – and therefor Kst value is a very useful comparator to use, showing which powders are more strongly explosible than others. There has been academic work that has been critical of this (for example, ref. 1) but we would observe that investigations of real industrial incidents – explosions – have not yet, at any rate, shown the standard explosion relief design methods to be unsound.

For completeness, it is probably worth pointing out that at high levels of turbulence, the tried and tested formulae used for explosion vent design may not always work. Indeed, in pressure piling situations (e.g. explosions in long pipes), even relatively weak dust deflagrations can transition to detonations – with much more devasting potential than the simple explosion protection design can accommodate.

As one of the US’s major dust explosion contract test laboratories, this is a question that we hear regularly and sometimes the sentiment expressed here can inhibit actioning a dust testing program. “What can of worms are we opening here?”

There is good news. In all likelihood, there will only be a need for limited testing; but ‘what to test?’ and ‘which tests to perform?’

Most times when we get this question, we explore the status of any Dust Hazards Analysis (DHA) on our client’s facility – according to the NFPA 652 standard on combustible dusts.  This has to be completed by September 2020 this year anyway, so it makes sense as a logical first step. This approach would allow for the evaluation of each hazard situation and selection, not only of representative sample(s), but also the applicable test(s).  It should be noted that which test(s) to perform depends – amongst other factors – on the likely ignition sources that are identified during the DHA as well as the prevention and/or protection measures that will need to be taken to ensure safety of people and the facility.

In our view, the Dust Hazard Analysis should include the entire list of the client’s materials:

  • to see if various materials could be grouped together so that a small number from each group would need to be tested,
  • to study the materials’ SDS’s for information on explosibility/combustibility that might help us exclude some materials from testing,
  • to determine average quantity of each material used – we could perhaps initially focus on materials that are used in larger quantities and higher frequencies,
  • to determine if the testing of some samples from the dust collectors and any central dust removal system alone would be sufficient in obtaining the necessary data for the DHA – instead of testing all possible powders.

Sometimes, to narrow the list of samples for testing, one could perform ‘screening’ tests which would obviously be quicker and cheaper to perform and would then allow us to perform ‘full’ tests on a smaller number of materials.

Example:  You have a dust collector serving multiple points and collecting different powders and you want to check that the explosion relief vent area is large enough. To check this, we will be looking for the maximum credible explosion severity (Kst) value powder (mix) that can occur in the dust collector. We’d begin by looking at your list of powders. Are they all explosible? Do we have data on the same materials? Any information in the SDS?

This review may identify a few ‘red flag’ materials. Next, we’d review powder usage data. Are any powders used in such small quantities that it is not credible that they could significantly contribute to the overall worst case Kst in the dust collector. Are powders used in sequence or will there always be mixtures in the collector?

Eventually we come down to a short list of most commonly used powders with the highest Kst values. Depending on our initial findings, we would then advocate testing a few worst-case powders – or even sampling at certain times from the dust collector when worst case conditions are likely to be prevalent.

We would contend that there are 4 critical material properties that contribute to evaluation of electrostatic hazards in a powder handling processes. Three of them are independent of each other. These properties are:

  • powder Minimum Ignition Energy (MIE)
  • powder Volume (or bulk) Resistivity (related to charge relaxation time)
  • powder Chargeability

The dust cloud Minimum Ignition Energy (MIE) tells us how easily (or indeed, if at all) the dust can be ignited by electrostatic sparks.  It is a measure of the lowest (capacitive) spark energy capable of igniting an optimal dust cloud concentration.

The powder Volume Resistivity tells us if charge that builds up on a powder in a grounded, conductive container will lose much charge by conduction. It is related to charge relaxation time (or charge decay time) which is a direct measure of rate of charge loss.

Powder Chargeability will indicate if the powder will charge, and to what level, when moving against various plant surfaces (plastics, stainless steel …., pneumatic conveying, mixing, sieving …)

It’s important to note that the chargeability of a powder is completely independent of the Volume Resistivity/Charge Relaxation Rate.  A very conductive powder may become highly charged (high chargeability) and a highly insulating powder may or may not charge much, for example.

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)
  • Self-Heating
  • 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.

Ask A Question