Five Ways New Explosion Venting Requirements
For Dust Collectors Affect You (continued)

1. NFPA 68 has changed from a "guideline" to a "standard."

NFPA 68 now includes mandatory requirements for dust collection applications with combustible dusts. This change from a "guideline" to a "standard," which incorporates much more stringent requirements than past editions, is echoed by OSHA's recent launch of its National Emphasis Program (NEP) on safely handling combustible dusts. Simply stated, it's NFPA's role to set the standard and the role of OSHA and local authorities to enforce it.

Most insurance agencies and local fire codes state that NFPA standards shall be followed as code, so in nearly every US town and county, NFPA 68 is to be treated as legal code. The only exceptions are where the authority having jurisdiction (AHJ) specifies another safety approach, such as Factory Mutual.

OSHA defines combustible dusts as "organic or metal dusts that are finely ground into very small particles, fibers, chips, and/or flakes. These dusts can come from metal, wood, plastic and organic materials such as grain, flour, sugar, paper, soap and dried blood. Dusts can also come from textile materials. Some of the industries in which combustible dusts are particularly prevalent include agriculture, chemical, textile, forest [and] furniture...." The OSHA NEP directive on safely handling combustible dusts is available on the organization's Web site.

What does all this mean to you and other bulk solids plant managers and engineers? In many cases, you will now have to install updated dust collection and explosion venting equipment to ensure that your plant complies with the new requirements.

2. You need to determine whether your dust is explosive.

In a closed vessel such as a dust collector, an explosion usually begins when a suspended cloud of combustible dust is present in high concentration inside the collector. As the fan draws in large volumes of air, an outside spark or ember can be sucked into the collector and collide with the dust cloud under pressure, triggering an explosion. The spark's source may be a production process, a cigarette butt thrown into a dust capture hood (believe it or not, this really happens), or a static electricity discharge from improperly grounded nearby equipment.

In describing the standard's scope in Chapter 1, NFPA 68 says that it "applies to the design, location, installation, maintenance, and use of devices and systems that vent the combustion gases and pressures resulting from a deflagration within an enclosure so that structural and mechanical damage is minimized" (1.1) and "the standard applies where the need for deflagration venting has been established" (1.3).

The important point here is to determine whether your application requires deflagration venting. And that, in turn, requires you to determine whether your dust is considered explosive. A dust's explosive power is the dust cloud's deflagration index (that is, the rate of pressure rise), denoted as "KSt." Both NFPA and Factory Mutual use this value in formulas to calculate the amount of explosion vent area required for a dust collector.

NFPA classifies dusts according to their explosibility -- that is, their KSt values. Class 1 dusts are rated below 200 KSt, Class 2 dusts range from 200 to 300 KSt, and Class 3 dusts are rated above 300 KSt. As a rule of thumb, when dusts approach 600 KSt, they're so explosive that wet collection methods are recommended. In addition to KSt, other important measurements that factor into the standard include "Pmax," the maximum pressure developed in a contained explosion (or in a contained deflagration of an optimum mixture), and "Pred," the maximum pressure developed in a vented enclosure during a vented deflagration.

The need for explosion venting can be dismissed only when a dust is known to be inert (that is, it has a 0 KSt value). Limestone, fume silica, and rock dust are examples. Even Class 1 dusts with relatively low KSt values (such as 50) are considered explosive. Therefore, if your dust's KSt value is greater than zero, you must follow NFPA 68 requirements and use explosion venting in your dust collector.

The KSt values of several common dusts are listed in Table I. For a much more comprehensive compendium, go to www.hvbg.de/e/bia/gestis/expl/index.html. This Web site contains a European database known as GESTIS-DUST-EX that lists the combustion and explosion characteristics of more than 4,000 dusts from virtually all industry sectors.

As you prepare to update existing or install new dust collection equipment, your equipment supplier may ask you to do one of two things: 1) supply in writing the KSt value you know your dust doesn't exceed, or 2) supply a report of the dust's KSt value based on your dust tests. Pharmaceutical applications are an example of the first option. Pharmaceutical manufacturers sometimes can't determine the KSt of a new product because it hasn't been manufactured yet, so they'll go with a conservative estimate of 300 KSt or even higher to be on the safe side. On the other hand, if you have a dust sample but aren't completely certain of its KSt, you should go with the second option and test the dust. In fact, NFPA 68 advises that all dusts be tested, and the standard clearly dictates that you, the end user, and not the equipment supplier, is responsible for establishing the dust's KSt value. Your dust collector supplier can provide a list of dust testing facilities for this purpose.

Figure 1 - Explosion vent designed according to NFPA 68

A standard explosion vent that has been sized for 200 KSt dust and manufactured according to NFPA standards is shown on a cartridge dust collector in Figure 1. For highly combustible dusts, vent sizing and vent discharge ducting requirements become more complex and may require special calculations and equipment modifications to achieve compliance. (For more information, see the later section "How NFPA 68 will affect your explosion venting design.")

3. You need to commission a hazard anlysis of your dust collection system.

Chapter 4 in NFPA 68 introduces a new hazard analysis requirement that states "the design basis deflagration hazard scenario shall be identified and documented" (4.2.3.1) and "a documented risk evaluation acceptable to the AHJ shall be permitted to be conducted to determine the level of protection to be provided" (4.2.3.2).

This means that you'll have to commission a hazard analysis (also called risk evaluation) of your plant's dust collection system and keep the report on file to show to the local fire marshal or other AHJ at a moment's notice. It's possible that some equipment suppliers or independent sales reps may start to offer these analyses. Given how time-intensive a hazard analysis is, however, we anticipate that a whole cottage industry of consultants will spring up to meet the newly created demand.

4. You need to maintain extensive documentation.

A hazard analysis isn't the only documentation required by NPFA 68. In Chapter 11, the standard lists 19 documents that you must now maintain on file to satisfy the local fire marshal or other AHJ. Some of these - such as suppliers' equipment data sheets and drawings, instruction manuals, and specifications - are readily obtained. Obtaining others - including a combustible material (dust) properties test report, user documentation of conformity with applicable standards, and employee training requirements - will present a more substantial challenge to you or the plant or safety engineer in charge.

5. You need to schedule an annual inspection of your dust collection system's explosion venting equipment.

Chapter 11 also stipulates that your explosion venting equipment must be inspected at least annually and possibly more often, based on your documented operating experience. This inspection's objective is simply to determine that all of the system's components are operating correctly. The chapter outlines a 16-point vent inspection with this objective in mind (11.4.4). Your plant owner or operator must also verify in writing that your process material hasn't changed since the last inspection (11.4.5). You must file the inspection reports with your other required documentation (11.2).

For inspections and documentation, responsibility again rests with you, the end user. Although the new requirements are challenging for everyone, they will be especially taxing for smaller bulk solids plants that don't have a dedicated safety engineer.