Not everything that produces dust is safe – how to interpret Kst and Pmax indicators in practice?

If dust appears in your installation—whether it’s flour, aluminum, lactose, or coal—you need to know two key indicators: Kst and Pmax. These determine whether your material can explode, how violently it will do so, and how to protect yourself from the effects. Sounds technical? Rightly so. But in this post, we will guide you through the topic in a practical and accessible way, showing you how to interpret the data, what can change it, where mistakes most often occur, and how to prepare your plant in accordance with ATEX. 

How to interpret Kst and Pmax indicators before you start designing explosion protection?

Before you take any technical action, you need to know what Kst and Pmax indicators actually are and how to interpret them in the context of real dust hazards. These are not abstract laboratory data – they are values that directly affect the selection of safety measures, the placement of equipment, and the level of protection of your installation. Kst (deflagration index) determines the rate at which pressure builds up in a confined space during a dust explosion. The higher the Kst, the greater the rate of pressure increase and the greater the risk to people, machinery, and structures. In turn, Pmax (maximum explosion pressure) indicates the maximum pressure that can result from an explosion – i.e., the force with which it will act on the surrounding elements.

Kst is independent of the volume of the test vessel, making it a good comparative indicator between different materials. Its value is calculated using the formula Kst = (dP/dt)max × V^(1/3), where dP/dt is the maximum rate of pressure increase and V is the volume of the chamber. Pmax, on the other hand, always refers to a specific test volume and may vary depending on the type of dust, its granulation, or concentration. In engineering practice, this means that Kst allows materials to be classified in terms of explosiveness, and Pmax allows the appropriate strength of protective systems to be selected.

Do not ignore these values or treat them as mere “table data.” Understanding Kst and Pmax is the foundation of effective explosion protection.  They will help you determine how to design explosion ventilation, select isolation valves, and what type of HRD (High Rate Discharge) will be needed. If you do not take them into account at the risk analysis stage, subsequent modifications may cost much more than precise planning from the outset.

Not just numbers – what do EN 14034 and ASTM E1226 standards say about dust explosion testing?

If you use Kst and Pmax data, it is worth knowing where they actually come from. The EN 14034 and ASTM E1226 standards are the primary sources of knowledge on how to professionally conduct dust explosion tests. These are not documents to be filed away in a binder – they are guidelines that specify measurement conditions, sample preparation, ignition method, and data recording. Knowing them not only allows for better interpretation of the results, but also evaluates whether the data received from the manufacturer or laboratory is actually reliable.

In Europe, EN 14034-1+A1 defines how to determine the Pmax value, and EN 14034-2+A1 defines methods for measuring the maximum pressure rise rate and calculating the Kst index. In both cases, a 20-liter spherical explosion chamber is used, in which the dust sample is dispersed with compressed air and then ignited with two igniters with a total energy of 10 kJ. Piezoelectric sensors measure pressure with very high precision, allowing the explosion to be recorded in real time. Tests are performed for different dust concentrations to find the maximum value, i.e., the conditions under which the material behaves most dangerously.

In turn, ASTM E1226, popular in the US and Canada, differs in details of execution, but essentially leads to similar results – as long as the tests are performed reliably. One thing is important for you: it is not enough to know the numbers – you need to know their context. Always ask what standard the tests were performed according to, under what conditions, and on what sample. This is the only way to be sure that you are designing protection based on real, unbiased data.

Which dust is more dangerous than dynamite? Check the St classification and learn about high-risk materials.

Any dusty material that has the potential to ignite can be assigned to one of three explosion classes – St1, St2, and St3. This classification is based solely on the Kst value and is the basis for determining the level of safety measures in industrial installations. Although the names of the classes may sound innocuous, the transition from St1 to St3 represents a radical increase in risk, which requires completely different technical solutions.

Class St1 covers most organic dusts, such as flour (Kst = 112 bar·m/s), sugar (Kst = 90), and coal (Kst = 70). Does it seem safe? Not necessarily. Even materials with Kst below 200 bar·m/s can lead to very serious explosions if they are not properly secured. St2 is already a higher risk level – this includes aspirin (Kst = 217) and ascorbic acid (Kst = 250), i.e. dusts often found in the pharmaceutical industry. They require not only ATEX-compliant safety measures, but also compliance with GMP standards.

However, the most excitement is generated by class St3, where Kst exceeds 300 bar·m/s. An example? Aluminum – 620 bar·m/s. Magnesium – 508. Titanium – 300.  These are values at which we are already talking about extremely aggressive explosive reactions. In installations working with such materials, valves must close in less than 50 ms, and HRD systems must respond immediately to avoid damage. ATEX classification is not enough – advanced simulations and risk analysis are needed. And importantly, it must not be assumed that a given material “always” belongs to one class. Different batches, variable moisture content, and granulation can change the picture.

Humidity, particle size, oxygen—all of these factors influence the explosiveness of dust. But it is equally important whether the material itself is within the explosive limits. If you are unsure, read our post about these limits.

Ventilation, insulation, HRD – how do Kst and Pmax data affect the selection of safety devices?

Do you have a specific Kst value? Great – now it’s time to translate that into real action. These numbers determine how large the decompression panel should be, how quickly the isolation valve must act, and what class of suppressant will be needed in the HRD system. Simply put, without them, the design of the protection system will be guesswork. And there is simply no room for guesswork in explosion protection.

When designing explosion ventilation, the Kst value is used to calculate the area of openings in accordance with the NFPA 68 or VDI 3673 standards. The formula looks inconspicuous: A = (Kst × V^(2/3)) /(Pstat × C), but its consequences are enormous. Too small an area means too much pressure, which can damage the structure. In HRD systems, on the other hand, not only the response time and opening pressure are selected, but also the amount of suppressant – St3 class dust requires significantly more material than St1.

Quick-closing valves must be synchronized with the flame propagation time, which means that precise calculations based on test data are necessary. For dusts with high Kst values, the response time must be less than 50 ms, which requires the use of high-end equipment. Remember that any error in interpreting Kst and Pmax data may result in underestimating the safety measures, which translates into a real risk of explosion. When designing protection systems, it is not enough to know the values – you need to be able to translate them into engineering practice.

Moisture, oxygen, and turbulence – when can Kst and Pmax data surprise you?

The Kst and Pmax values, even if specified by the manufacturer, are not absolute and unchanging. In industrial practice, these data can vary significantly depending on many physical and environmental factors that are often underestimated. If you assume that once measured, the material will always behave the same way, you risk insufficiently precise safeguards. This is one of the most common mistakes in explosion protection design.

The moisture content of dust is the first factor that can completely change its characteristics. Increasing the water content of the material by 5% can reduce the Kst value by up to half, which in practice means significantly lower explosiveness. The problem is that this change also works the other way around – if the material dries out, its explosive potential increases rapidly. Oxygen concentration works in a similar way – in environments with low concentrations (e.g., after inerting with nitrogen), explosivity indices decrease. But in an oxygen-rich atmosphere? The risk of explosion increases many times over.

The turbulence in dust flow cannot be ignored either. Under controlled laboratory conditions, the material may appear relatively safe, but in dynamic industrial processes—e.g., in silos, feeders, cyclones—turbulence can increase the intensity of an explosion up to tenfold. Added to this is particle size — dusts with a diameter of 10–40 micrometers have the highest explosive potential. Larger fractions (above 500 µm) are generally safer, but a single production step — such as grinding — is enough to change everything. The variability of materials is something that cannot be ignored. Even different batches of the same dust can have different properties. If you want to act responsibly, you must treat the Kst and Pmax data as a starting point, not a ready-made solution.

How much does a dust test cost, and how much does it cost to ignore it? A comparison of risks and investments

The decision to perform professional dust explosion tests often comes down to cost. The price of a single sample in an accredited laboratory is approximately $5,000, and a comprehensive DHA (Dust Hazard Analysis) costs between $8,000 and $25,000, depending on the size of the plant. For many companies, this is an expense they want to put off “until later.” The problem is that in the event of an explosion, there is no later. There are only consequences – costly, irreversible, and very often… impossible to predict and estimate.

For comparison: the average cost of production downtime after an explosion is $100,000 per day. Added to this are equipment losses, reconstruction costs, potential casualties, and—very importantly—regulatory penalties. In the United States alone, OSHA imposes an average of $70,000 for a single serious violation of explosion protection regulations. And history shows that these figures are not unrealistic. Imperial Sugar (2008) – over $200 million in losses. West Pharmaceutical (2003) – 6 casualties, $150 million. CTA Acoustics – 7 casualties, $50 million. All because of dust that was not tested or properly secured.

On the other hand, investing in data and security brings tangible benefits. Reduced insurance premiums by up to 30%, reduced downtime, increased process reliability – these are real savings that, over time, exceed the cost of testing several times over. If you run a plant that works with dust, don’t ask if it’s worth testing. Ask if you can afford not to.

Not only tests, but also mistakes cost money. It is worth knowing how much an explosion risk assessment costs – and why it is not worth looking for savings in this area.

Where does it really explode? In which industries are the Kst and Pmax indicators most important?

The risk of dust explosions does not only apply to “big chemicals” or ATEX-certified plants. It occurs where no one expects it—in bakeries, silos, pharmaceutical laboratories, or during metalworking. This is where Kst and Pmax values are crucial and should be the basis of any explosion protection design.

In the food industry, risks are posed by, among other things, flour, sugar, and starch—all of which are classified as St1, but have a very high ignition capacity in the presence of a spark source. Mills, bakeries, sugar refineries, potato processing plants – these are places where a moment of inattention can lead to an explosion. Next is pharmaceuticals, i.e., St2 class materials: aspirin, ascorbic acid, lactose. Here, there are additional requirements – a combination of explosion protection with GMP conditions and technological cleanliness.

However, the metallurgical and energy industries require the most attention. Aluminum, magnesium, and titanium are classified as St3, with the highest deflagration energy. Used in foundries, aviation plants, the aerospace industry, and battery production, they require safety responses in milliseconds. In turn, in the energy sector, coal dust and fly ash, although often neglected, are one of the most common causes of explosions in power plants and heavy industry facilities.

If you work in any of these industries, you cannot rely on general assumptions. You need specific Kst and Pmax values for your material, your technology, and your working conditions. It is not a matter of documentation—it is a matter of real safety.

Are you wondering which rooms in your facility should be protected against explosions? Check out our post, in which we explain how to identify explosion hazard zones – both inside and outside buildings.