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Is Your Combustible Dust Risk Greater Than You Think? What You Need to Know About Kst Values

Posted on March 27, 2019

When we talk to people about combustible dust, we consistently find that they underestimate their risk. This happens across industries and in all kinds of facilities.

As we’ve dug into why this is the case, we’ve uncovered several reasons people have a false sense of security. Here are some of the most common ones:

  • Combustible dust isn’t a concern in our industry.
  • Our facility has never had a combustible dust problem.
  • We have dust, but the particles are too big to be combustible.
  • We’ve had our dust tested, and the Kst is low, so we’re not concerned.

We’ve addressed the first three issues in previous articles (check them out here and here). Let’s dig into #4.

What is Kst?

Kst, aka the dust deflagration index, is one of the parameters used to quantify the severity of a dust explosion. Basically, it tells you how severe an explosion is likely to be.

OSHA classifies dusts into four explosion classes based on their Kst value.

Dust explosion class Kst Characteristic Typical material
St 0 0 No explosion Silica
St 1 >0 and = 200 Weak explosion Powdered milk, charcoal, sulfur, sugar, zinc
St 2 >200 and = 300 Strong explosion Cellulose, wood flour, polymethyl acrylate
St 3 >300 Very strong explosion Anthraquinone, aluminum, magnesium

Why Kst can be misleading

Measuring Kst is a crucial component of a dust test. This value helps you determine both your risk and the actions you need to take to minimize the chances of an explosion.

However, it can be misleading. In particular, people often mistakenly assume that a low Kst value means they’re in the clear. Here are two things you need to know about Kst values.

Kst is both a relative number and an estimate

OSHA defines Kst  as follows:

Kst, the dust deflagration index, measures the relative explosion severity compared to other dusts. The larger the value for Kst, the more severe the explosion. Kst provides the best “single number” estimate of the anticipated behavior of a dust deflagration.

We’ve emphasized in bold two parts of the definition that tend to get overlooked. First, Kst is a relative number. Second, Kst is just an estimate. Under the right circumstances, low-Kst dusts can cause very severe explosions.

In fact,  low-Kst dusts have caused some of the worst combustible dust incidents in history. The 2008 Imperial Sugar explosion, which killed 14 people, injured 38 more, and led to OSHA fines of nearly $9 million, was caused by sugar, which has a Kst of 35 and is classified as St 1.

How severe an explosion will be is related to how much dust has been allowed to accumulate. Wayne Labs of Food Engineering asked Gary Q. Johnson, owner of Workplace Exposure Solutions, if the fact that cornmeal has a lower Kst than cornstarch (47 compared to 163) meant that food processors could be less careful with cornmeal spills. His response: “I would take no comfort if I only have a Kst of 50 vs. 250. All that means is that if I have five times as much of the Kst 50 dust floating around, I have just as much risk as I do with the [dust rated at] 250.”

Bottom line: The only risk-free materials are those rated St 0.

The Kst for a particular material may vary

Kst values aren’t set in stone. Typical cornmeal may have a Kst value of 47, but that doesn’t mean that all cornmeal is 47. Physical characteristics of the dust, including particle size, shape, and moisture content, can impact the Kst.

And these physical characteristics can change during processing or manufacturing. This is a big reason you need to have your dust tested rather than just relying on the Safety Data Sheets (SDS), which only tell you the Kst when the material shipped from the supplier.

Also, note that to ensure compliance with NFPA 652: Standard on the Fundamentals of Combustible Dust, you need to retest your dust every time you change your process.

Kst value isn’t the only measurement that’s important for determining whether a dust is dangerous. But it is the most misunderstood. For more on dust testing, consult these resources:

At Nilfisk, we don’t provide dust testing services, but we collaborate with experts that do. Once you have your results, we can perform site assessments, demonstrate cleaning equipment, and help you select the right products to mitigate combustible dust risks at your facility.