Applying the Worksheet Approach to Assessing Combustible Dust Hazards

In today’s episode of the Dust Safety Science podcast, Dr Georg Suter, senior consultant with FireEx Limited, discusses the worksheet approach on combustible dust hazards and how to assess them. We’ll address why we created a worksheet approach, some practical examples, and some specific scenarios related to combustible dust, explosion isolation, and others.

Georg began his professional career in 1986 at the Sandoz testing laboratory. In 1994, he joined Clarion’s HSC team, where he played a key role in developing internal safety guidelines for all Clarion companies. This was also the first time he worked on creating worksheets on a larger scale. In 2000, he became a consultant at the Swiss Safety Institute, which was later acquired by Tissot. Since 2017, Georg has been retired, but he continues to support fireworks customers with explosion protection.

Why Was the Worksheet Approach Developed?

Georg had already been working with worksheets during his time at Clariant before joining FireEx. They were based on the observation that risk analyses in many companies consistently identify the same necessary protection measures for similar equipment. This means that instead of conducting a new risk analysis every time, you can rely on a list built from long-term experience. This approach saves a lot of time and is especially helpful when purchasing new equipment, as it helps define user requirements regarding explosion protection.

Georg mentioned that a worksheet is essentially a checklist of safety measures specific to certain equipment types. For example, in explosion protection, there’s a distinction between mixers without moving parts in the mixing chamber, mixers with slow-running stirring elements, and mixers with fast-running stirring elements—so-called high-speed mixers. These are different equipment types that are well known to those in explosion protection.

The presence of mechanical elements and their speed are critical factors. When it comes to dryers, there’s typically a distinction between contact dryers, such as tray dryers where no dust cloud is present, and convection dryers, like spray dryers, where a dust cloud is present. These equipment types need to be specified first, and when applying a checklist, you need to select the one that matches the equipment type. Beyond the equipment type, safety measures also depend on the process properties, such as the solid materials involved.

Determining safety data and characteristics is vital. You typically need data on combustibility, minimum ignition energy, minimum ignition temperature for dust deposits and dust clouds, thermal stability (such as DSC measurements or wire basket tests), electrical conductivity, and possibly the residual content of flammable solvents if any are present.

In multipurpose plants, it’s common for all this data not to be readily available. In such cases, you either determine the data, which takes time and money, or you start using the worksheet with conservative safety data assumptions, like assuming a very low minimum ignition energy or a high combustibility index, typically around five. So, for selecting the appropriate worksheet, you need to know the equipment type and have a set of determined safety data.

Nowadays, most equipment in Europe comes with an ATEX certificate for a specific intended use. As long as the user operates the equipment within the defined operating window, the safety measures are essentially built into the machine. However, additional safety measures related to the operation of the equipment fall under the full responsibility of the operator. Even with ATEX-certified equipment, operators still need a list of safety measures for which they are responsible. These can be easily found in the checklist or worksheet.

What Are the Outputs?

The output is a list of recommended or best practice controls. It’s then up to the user to take that list and figure out how to implement those controls. For example, with mixers, the worksheets typically indicate whether constructional protection is needed. If it is, you’ll have to do the calculation yourself according to the locally valid standard.

The worksheet isn’t a calculation tool like Reinvent; it doesn’t provide the exact specifications for the necessary constructional protection. Instead, it simply advises whether it’s strongly recommended to have constructional protection or not, or if you should apply it based on the situation.

Georg said he often finds equipment that isn’t certified. In these cases, you can either perform a risk analysis, like Pharaoh, or use a worksheet. For example, if you have an old high-speed mixer that was previously used to mix organic powder with low ignitability—something with a minimum ignition energy far above 1000 millijoules—but now the operator wants to introduce a process with two new organic solids, and the safety characteristics show a minimum ignition energy of around 30 millijoules and a minimum ignition temperature of 320°C, the situation changes significantly. The new powder is much more flammable, increasing the dust explosion risk.

Georg would then select the worksheet for high-speed mixers, which is the equipment type, and review the safety measures for the new, more ignitable mixture. Obviously, there are now several safety measures in the checklist that weren’t required before for the less sensitive powder. These might include:

Earthing all conductive equipment and parts, including mobile items like drums and trolleys
Limiting the circumferential velocity of the mixing arm, at least during the loading of the mixer
Avoiding the entrainment of foreign metal bodies by mounting a sieve in the loading channel
Limiting the mixing time to prevent heating due to mechanical energy input and;
Ensuring that no friction occurs at the bearing of the stirrer, which could lead to a hot spot, especially given the low minimum ignition temperature of 320°C.

These additional safety measures appear because of the new safety data for the same mixer. However, sometimes these measures can’t be implemented in older equipment. In such cases, the worksheet clearly advises applying inert gas blanketing, as eliminating ignition sources may not be reliably achieved. This is an example of how changing the substance properties for the same type of equipment can lead to new safety measures being required.

What is the Worksheet Approach for Interconnected Vessels?

“Usually, you’re not dealing with just one isolated piece of equipment,” Georg explained. “You have to assess an entire process, which involves a chain of machines.”

For example, in a milling process, you might have the loading of the inlet container, the dosing screw conveyor, the mill itself, a sieve to select the right particle size, and finally, the product being filled into a silo. Each of these is a different type of equipment.

In such cases, Georg finds the worksheet approach particularly useful. You can set up the checklist in a table format where the columns represent the different equipment types, and the rows list the various safety measures. For each piece of equipment, you mark the measures that need to be implemented. It’s straightforward—if you need to apply earthing to all conductive parts, you mark that for every piece of equipment. Mechanical ignition sources might require you to limit circumferential velocities, but only in specific equipment like mills and screw conveyors. This approach gives you a clear overview in a short amount of time.

It also makes it easier to follow up on the implementation of the safety measures, which is the most important part of the work. The assessment is just the beginning; the real task is ensuring that the safety measures are actually put into practice.

How Do You Make Sure You’ve Covered Everything?

This is a very important question. When you’re doing a risk analysis, it’s challenging to claim that everything has been covered 100% or that nothing has been overlooked. Many incidents show just how difficult and demanding it is to ensure complete coverage. However, when it comes to the worksheets, Georg can rely on a long and positive experience in using them. These worksheets are based on years of experience from many different consultants and experts who are familiar with applicable standards globally. While no one can honestly say they are 100% safe, using these worksheets provides a strong sense of security.

Georg mentioned the earlier example of limiting the mixing time in a mixer. When he discussed limiting mixing time, it was based on an incident—a small fire—that highlighted the need to consider mechanical energy input. In the first versions of the worksheets, this safety measure wasn’t included, but the incident taught them that it should be. So, just like standards, the worksheets continue to evolve over time based on new experiences and lessons learned.

Can the Worksheet Overcome Common Challenges?

You need a basic understanding of the important terms in explosion protection. When Georg introduces a worksheet in a company, it’s usually combined with some training. In smaller companies, this is often done through conventional training courses lasting one or two days, where participants work through examples. However, in multinational companies, a distributed form of training is necessary. For these situations, Georg uses e-learning courses for the specialists who need to implement the worksheets on site. It’s clear that if someone unfamiliar with the basics of explosions starts using the worksheets, it would be a mistake. The terms used in the worksheets are important and must be understood to ensure safety.

Is the Worksheet Approach Recommended in all Situations?

There are some applications where worksheets are a better fit, and others where they’re not as well suited. Typically, the worksheet approach works well for batch processes, like those in pharmaceutical production or specialty chemicals. However, for continuous large-scale processes, especially in dedicated mono plants—such as in the milk powder industry, where the same product is produced continuously over long periods—Georg would still recommend defining safety measures based on a specific risk analysis. This approach leads to a more tailored safety concept.

If you rely on worksheets just to be on the safe side, you might end up being too conservative, which can be costly over time, especially in a mono plant. In such cases, it’s more practical to develop a custom safety concept based on an individual risk analysis rather than using a general checklist that might include unnecessary precautions.

There are also some batch processes where the requirements in the worksheets might not be feasible. In these cases, a specific risk analysis is recommended. For example, if you’re dealing with highly sensitive milling processes, these are so specific that they can’t be fully addressed by a worksheet. That’s why each worksheet includes an exclusion clause at the beginning, specifying substances not covered. If a substance isn’t covered by the worksheet, you need to take further steps—typically, this means conducting a specific risk analysis.

Substances like self-reactive materials or explosives aren’t covered by worksheets, and in such cases, using a worksheet wouldn’t be the right approach.

Conclusion

By using worksheets, companies can leverage years of accumulated knowledge and experience, making safety measures more efficient and accessible. However, Georg also emphasized that they aren’t always the perfect solution for every situation. Continuous large-scale processes and highly specific operations may require a more tailored safety analysis to avoid overly conservative or inadequate measures.

As safety standards evolve and new challenges arise, so too do the worksheets, ensuring that they remain a reliable resource for explosion protection. By staying informed and applying the right tools for the job, you can help ensure that your workplace remains safe and compliant with the latest safety guidelines.

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Dr. Georg Suter directly:

Website: https://www.fireex.ch/

LinkedIn: https://www.linkedin.com/in/georg-suter-56b23b162/?originalSubdomain=ch

Email: [email protected]

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.