Is a ST Class 1 Dust Dangerous?

By Dr. Ashok Dastidar, MBA, VP of Combustible Dust and Flammability Testing and Consulting, Fauske & Associates, LLC, Ron Allen, PE, Senior Consultant, Fauske & Associates, LLC and Jeff Griffin, MBA, Director of Business Development, Fauske & Associates, LLC

One of the questions we commonly encounter is: if my (combustible) dust has a low Kst (or Pmax) is it dangerous? The short answer to this question is a resounding YES.

To explain a bit about the properties of class 1 dusts, and some of the potential dangers associated with those dusts, we’ve invited Dr. Ashok Dastidar and Ron Allen (the ‘Dust Buster’) to share their thoughts.

Jeff: So what exactly is a Class 1 dust, and why is it dangerous?

Ashok: Combustible dusts are ranked into one of four classes; ST0, ST1, ST2, ST3.

The level of explosion violence increases with the class number. The only dust that has zero risk of explosion is an ST0 dust. All other dusts have an explosion hazard. Even a St-1 dust generates sufficient power to cause a flash fire, compromise containment on a piece of equipment, or blow out the walls of a building.

Jeff: What are some common Class 1 dusts?

Ashok: Grains, Sugar, Coal, PVC, Flour, etc. are all considered St Class 1 dusts. In fact, some of the most famous dust explosions case studies come from class 1 dusts: such as the Imperial Sugar explosion in 2008 in Georgia, and (historically) the Washburn A Mill explosion in 1876. It’s interesting to note that only one of the ten combustible dust investigations completed by the Chemical Safety Board reported fuel sources above St-1.

Jeff: Ron, what would you say are best practices for dealing with St Class 1 dusts?

Ron: Best practices for dust are much the same regarding of what the class is, that is to say, good housekeeping, explosion protection and isolation, and employee education. All of these factors are outlined in NFPA 652. That said – I think the most challenging aspect of safety is keeping workers educated and actively involved in mitigating their risks. This takes daily discipline.

Jeff: What are…other concerns, best practices, etc.?

Ron: Five things immediately come to mind:

• Understanding the characteristics of the dust is first. It is difficult to develop effective combustible dust prevention and protection strategies without understanding the ignition sensitivity and explosion severity properties of the dust that may be present. Laboratory testing provides the foundation needed to develop those strategies.
• Controlling ignition sources comes next. Some ignition sources may be obvious (e.g., open flames, heated surfaces) but others are more subtle (e.g., static electricity, friction caused by misalignment, inadequate maintenance).
• Fuel deprivation – a cousin to housekeeping -- is an often-overlooked need. For the most part, housekeeping is performed after combustible/hazardous dust has been released. Fuel deprivation is focused on upstream identification and control of combustible dust sources before the release occurs. Examples of fuel deprivation “opportunities” include: enclosing transfer points; installing local exhaust ventilation, and; redesigning/replacing leaking seals.
• I don’t think that the risk of flash fire can be underestimated. Providing properly rated fire resistant clothing (FRC) for “at risk” workers can be critical.
• Conducting a Dust Hazards Analysis (DHA) to gauge the extent to which the hazard is present in the plant and formulate a “to-do” list on how to reduce the risk of an accident.

In our next post, we will discuss: How does the Hazard class related to the hazardous area classification? If you have any questions about your dust, please feel free to contact us at griffin@fauske.com or +1 630 470 7168. www.fauske.com