How to Manage Combustible Dust

The October 2011 explosion of a Kansas grain elevator that killed six people tragically highlights one of the most serious hazards feed and grain processing companies manage every day — combustible dust. The loss of an employee and community-member is certainly the ultimate price paid for ineffective dust control, but loss of equipment and product, as well as nonfatal injuries, can also occur.

Nearly 280 dust fires and explosions occurred in U.S. industrial facilities from 1980 to 2005, resulting in approximately 119 fatalities and over 700 injuries, according to the Chemical Safety Hazard Investigation Board (CSB).

Protecting your facility from a potentially lethal situation starts with understanding the cause of an explosion and assessing the hazards present in your processes. Then, following up with an effective dust control and explosion prevention plan is integral to ensuring a safe environment.

Explosion pentagon

我们都知道自发燃烧比现实更城市传奇，那么爆炸发生什么？马萨诸塞州阿什兰市芬瓦尔的区域销售经理戴维·格兰德（David Grandaw）说：“经验法则是，如果它可以燃烧为固体，它会以灰尘形式爆炸。它要做的就是实现正确的因素。”

通常被称为“爆炸五角大楼”，必须融合造成灰尘爆炸的五种情况是：

1. Fuel - dust or fine particles
2. Dispersion – dust suspension in air
3. Oxygen
4. Confinement – inside a vessel or room
5. Ignition Source

Dust collection

Managing dust — an explosion’s fuel source — is one of the most important steps an employer can take toward reducing combustible dust hazards. An effectively designed dust mitigation approach combines exhaust fans, dust collectors and housekeeping.

The National Fire Protection Agency (NFPA) is a national safety standard setting organization that has developed a standard for combustible dust hazards in industrial facilities. The standard — NFPA 654 — is what OSHA relies on to inspect facilities since the agency still has not submitted its own much anticipated final ruling on combustible dust.

NFPA 654 says to minimize the release of dust companies should not employ open loading processes.

However, many loading, unloading and processing applications in a feed and grain facility cannot be done in a closed environment. Colonna says when that’s the case, the standard’s alternate option is to use a hood or exhaust system, so rather than dust releasing into the air, it’s captured and taken to either a main, central dust collector or a smaller, dedicated collector.

明尼苏达州明尼阿波利斯的唐纳森·托里特（Donaldson Torit）的工业空气过滤计划经理凯特·波士顿（Kirt Boston）说，现在比以往任何时候都更加设计了尘埃收集器。

“As recently as 10 years ago, it was commonplace for all processes to be connected through ductwork leading to one central dust collector,” explains Boston. “The danger is if a collector explosion occurs, fire goes back into the ductwork, involving other systems that otherwise might not have had contact with the explosion.”

As a result, Boston has seen a downsizing trend in dust collectors take shape. Smaller, dedicated collectors designed specifically for installation near a piece of equipment and runs concurrently with the process can improve dust control efforts and reduce the chance of an explosion traveling throughout the facility.

投资于更高的执行尘埃收集器also helps reduce the housekeeping load a facility has to manage, a main provision in NFPA 654. “If you do a better job of containing dust to begin with, it isn’t in the housekeeping environment; it’s kept inside the process,” adds Boston.

Housekeeping measures

Dispersion — dust suspension in the atmosphere — the second factor in the explosion pentagon, can be controlled with various housekeeping measures. If an explosion has already begun, in a collector for example, dust accumulated on the floor and in the beams and rafters can be knocked loose and become suspended in the air to provide fuel for a secondary, and often more powerful blast.

NFPA 654’s stance on housekeeping is that a hazard is present when accumulation is anywhere from 1/8 to 1/32 of an inch, depending on the bulk density of the dust, which proper laboratory testing reveals.

One-thirty-second of an inch is about the thickness of a mechanical pencil lead or a metal office staple. The NFPA’s Guy Colonna, staff liaison, admits that 1/32 of an inch sounds miniscule, but if that amount covers the entire working surface of a facility, it could create a dust cloud with the ideal concentration to form a combustible dust explosion.

“It is a very small amount of dust, yet it represents that marginal threshold above which you can be certain there is enough dust present that if it gets thrown into the air and becomes suspended and concentrated, you have a significant and legitimate hazard,” says Colonna.

根据Colonna的说法，NFPA 654的一项规定将于明年采用，包括正式的安全管家措施的层次结构：

1. Vacuum dust with a properly designed vacuum that is intrinsically safe, bonded and grounded, so it doesn’t become an ignition source. It may be vented or ducted to a dust collector.
2. 如果真空无法到达角落或狭窄的空间，请用扫帚进行水洗或扫荡
3. Finally, cleaning with compressed air is permissible in small areas if the previous two measures are not effective in removing the dust.

火花检测

即使有有效的灰尘收集器和勤奋的家政服务，如果未正确处理点火源，爆炸的威胁仍然存在。

一般而言，五角大楼爆炸的前四个因素可能会共存而不会引起爆炸，但最终因素（点火）是引发爆炸所需的所有催化剂。

Common ignition sources include sparks generated by milling processes that are grinding or abrasive, or heat from friction-causing equipment. Colonna notes that upkeep and maintenance of equipment plays an important role in minimizing ignition sources.

Colonna说：“在谷物处理中，您有轴承和齿轮推动的桶电梯，输送机和其他设备。”“当维护失败时，设备可以抓住或堵塞，从而导致过热或摩擦，从而产生火花并可以成为点火源。”

火花检测systems were introduced to the U.S. market in the late 1970s, according to Allen Wagoner, president of Greensboro, NC-based Flamex.

火花探测器是高速红外传感设备，通常安装在材料处理管道上，在材料处理管道上进行了气动输送的灰尘。

“If a spark is detected in the ductwork or a collector, the sensor redirects the airflow or causes suppression by water or another extinguishing agent to eliminate the ignition source,” says Wagoner.

Bob Barnum, vice president of sales for Grecon, Alfeld, Germany, notes that many agricultural companies choose diversion or chemical suppression over water suppression.

Barnum说：“典型的铣削应用程序结束了将产品发送到存储筒仓中。”“使用优质的水熄灭火花可能会损害储存的谷物，并形成霉菌或霉菌毒素，因此化学抑制或转移技术通常用于农业中。”

Supression, isolation and venting

In addition to managing dust and eliminating ignition sources, precautions including suppression, isolation and venting are required to ensure a safe outcome if an ignition source is generated and an explosion occurs.

With suppression, when sensitive heat or pressure sensors detect a hazard, they release a chemical, usually sodium bicarbonate, to quench the fireball and reduce the build up of pressure before an explosion can happen.

同时，爆炸隔离（NFPA 654中的一项规定）需要在整个加工系统中置于障碍，以防止爆炸下游行驶。

Chemical suppressants may be released at one process to quell the initial blast, or can be installed throughout the ductwork to act as an isolation method.

“If you invest in a sensor and the control panels, it’s a very small incremental step to do both the suppression and isolation chemically, rather than try to do them independently through mechanical means,” says Boston.

允许机械隔离设备，例如迅速关闭以切断附近区域的幻灯片门，但芬瓦尔的Grandaw指出，机械隔离有一些局限性。

格兰道说：“这些大门必须非常快地关闭才能有效，而且在大管道上，通常不可行，因为它们太重和昂贵。”“机械溶液更适合小管道。”

NFPA also recognizes venting as a method for isolation. If there’s an explosion in a dust collector, sensors detect the excess pressure and open a relief valve, thus protecting the dust collector from explosion.

Flamex的Wagoner解释了背景阻尼器在爆炸发泄中的作用。

“If there is an explosion in a dust collector or a bin, the explosion will travel in all directions to the weakest point and take the path of least resistance,” says Wagoner. “In many cases it will go up the ductwork towards the production area so the explosion backdraft dampers can be used to prevent a deflagration from going back into the duct and, venting the collector at its intentionally most vulnerable spot.”

—Elise Schafer