Blog | Fauske and Associates, LLC

Kst and Pmax Tests For Combustible Dust: Who or What Are They?

Written by AnnMarie Fauske | Jul 12, 2013 5:15:00 PM

What are KSt and Pmax?

While KSt and Pmax may sound like a rap group from the '90's, they are actually the explosive properties measured in the laboratory to quanitify the severity of a dust explosion. Basically, the tests tell you how much pressure an explosion will generate and how fast the explosion will travel.  

The “Explosion Severity Test”  is a standard dust test used to quantity the maximum pressure of a dust cloud explosion (Pmax) and the speed of the pressure rise (KSt). 

The Explosion Severity Test is generally conducted in a 20L sphere because it is directly scalable to the 1m3 sphere, which is the original instrument used to test combustible dust. Considered to be the “gold standard” for dust testing, the 1m3 sphere (shown here) is useful for providing data whenever there are questionable results from the 20L.

Why is it important?

Testing your dust to determine the KSt value & Pmax is essential for any type of equipment design. In particular, these values are used by manufacturers to validate the design of protection systems (such as spark detection, explosion venting, explosion suppression and explosion containment).

Example test results

Sample

KSt value
(bar m/sec)

Pmax
(bar)

ST class 

Grain dust

89

9.3 

1

Coal dust

85

6.4

1

Flour

63

9.7 

1

Sugar

138

8.5 

1

Wood dust

224

10.3 

Aluminium dust

515

11.2 

Sewage sludge

102

8.1 

1

GRP dust

216

7.6

2

The Explosion Severity test will tell you what “Class” your sample falls within, which is helpful in guiding service providers as they analyze your equipment and your facility. The ST class is based on the KSt value as follows:

ST class 0 - KSt value = 0

ST class 1 - KSt value less than 200 bar m/sec and greater than 0

ST class 2 - KSt value between 200 and 300 bar m/sec

ST class 3 - KSt value greater than 300 bar m/sec

Why add a 1mchamber to the arsenal? 

The 20-L chamber has become the modern workhorse of dust cloud explosibility testing. At Fauske & Associates, LLC (FAI), we have four operating 20-L chambers. Explosibility testing in these chambers are performed per ASTM E1226, E1515 and E2931 as well as the EN 14034 methods. Units worldwide provide valuable data to help create dust explosion hazard mitigation strategies in various process industries ranging from agriculture, wood working, pharmaceutical, plastics, fine chemical as well as metal working.

However, due to their small size they do have two limitations. The first is “overdriving”. Overdriving occurs when the powerful ignition source used to conduct experiments in the 20-L chamber preheats the test material and burns the dust cloud under study without really generating a propagating flame. The second limitation is “underdriving” – where the walls of the 20-L chamber abstract heat from the dust cloud explosion and thereby partially quenching the intensity of the deflagration. Both of these phenomena degrade the test data and thus impede the establishment of adequate explosion hazard mitigation. The vast majority of dusts and powders are not affected by these phenomena but there are a few that are.

The solution to both these issues is to perform the test in a large vessel such as the 1m3. The 1m3 vessel is not susceptible to overdriving or underdriving – go back to the “gold standard”! This summer FAI will be commissioning our very own one cubic meter chamber; the second unit of its kind in all of North America.  We are very excited with the added testing capability this chamber brings and the benefits it will bring to our clients in helping establish dust cloud mitigation strategies.

If you have more questions about combustible dust tests, please call Jeff Griffin at 630-887-5278, or email griffin@fauske.com.  www.fauske.com