KANSAS CITY, MISSOURI, US — The topic of explosion safety often is underrated by plant operators and machine manufacturers when it comes to handling or transporting combustible dusts. In comparison to society’s assumption that an increased risk of explosion only exists for gases, enormous forces and temperatures also can be released by combustible dusts.
To minimize the risk of explosions when handling combustible particulate and dusts, it helps to first understand the requirements for an explosion and the respective dust characteristics. To create an explosion inside a production facility or vessel, the following parameters must be fulfilled:
- Fuel (dust)
- Sufficient oxygen for combustion
- Suitable mixing of the fuel oxygen (dust cloud)
- Source of ignition capable of igniting the dust cloud
- Confined space containing the event.
If one of these conditions are eliminated, the explosion hazard has been neutralized (although other hazards such as flashfire may still exist). Often it is not possible during operation to control or eliminate parameters such as oxygen or containment, and the risk of explosion remains.
Typically, the common approach to explosion safety is to apply explosion protection such as venting and explosion isolation to protect the equipment and interconnections to save lives, prevent injuries and limit damage. In principle, this thought is similar when compared with the safety approach to operating motor vehicles.
Accidents cannot be 100% avoided. As such, it is mandatory to wear seatbelts and rely on safety systems such as airbags to reduce the effects of a crash. This form of hazard management is referred to as protective safety where the primary hazard event is expected to occur, and the protection solution serves to manage the effects or consequences. However, if we take a step back in the example above, we realize that accidents can be avoided with the help of driver-assistance systems such as braking assist, lane departure warnings and traction control. As a result, the accident is avoided by recognizing a hazardous situation and preventing the event from occurring. This form of hazard management is preventive safety where the primary hazard event is prevented from occurring by controlling initiating factors.
In the combustible dust explosion world, we rely on explosion protection systems to reduce the consequences of an event. But with some thinking outside the box, we can identify ways to prevent an explosion from occurring. In many industrial processes where powder and dust handling occur, four of the five elements of the explosion pentagon are normally present and only a source of ignition separates a normal day from a catastrophic event. There are 13 categorized ignition sources that could initiate a dust explosion:
1) Hot surfaces
2) Flames and hot gases (including hot particles)
3) Mechanically generated sparks
4) Electrical apparatus
5) Stray electrical currents, cathodic corrosion protection
6) Static electricity
8) Electromagnetic waves, radio frequency (RF) from 104 to 3 x 1012 Hz
9) Electromagnetic waves from 3 x 1011 to 3 x 1015 Hz
10) Ionizing radiation
12) Adiabatic compression and shock waves
13) Exothermic reactions, including self-ignition of powders.
While some of the above listed ignition sources are quite rare, others are more common and often can be detected and prevented. According to National Fire Protection Association (NFPA) 69, chapter 9, ignition source control can be used to reduce the chance of a dust cloud ignition. But considering the wide array of possible ignition sources and circumstances, ignition source control cannot be relied upon as the exclusive basis of explosion safety. Analogous to our motor vehicle safety systems example, we don’t rely only on collision avoidance systems, we also ensure the seat belts and air bags are in working order and ready to do their job if necessary.
Ignition hazard examples
Let’s examine several ignition hazards that are regularly present in grain handling facilities and strategies to prevent ignition. Spark discharges, propagating brush discharges, and cone discharges are capable of igniting dust/air mixtures. To prevent these, sufficient grounding and bonding of the various vessels and machines must always be ensured in accordance with NFPA 77 – Recommended Practice on Static Electricity.
During pneumatic unloading of tankers and bulk delivery vehicles, high energy electrostatic potential can occur in the material due to the high-speed friction on the conveying pipes. When charged material finds an electrical path to ground, an electrostatic spark can occur. This risk can be eliminated by grounding all components of the material handling system, including the silo, intake station, piping and vehicle.
As a permanent grounding connection is not possible on the vehicle, smart ground-monitoring systems can be used to ensure ground connections are in place prior to conveying. This eliminates the risk of human error to ensure the ground connection is secure. These systems ensure that the grounding clamp is connected to the vehicle by measuring electrical resistance to ground. Additionally, the system distinguishes between a vehicle and an isolated piece of metal by measuring the electrical capacitance of the connection. When both parameters are validated, the onboard relays close to allow the pneumatic system to start.
Another ignition hazard that can be identified and controlled at an early stage is a temperature increase in the conveyed or processed material.
Mechanical friction often causes a slow temperature increase, which could eventually ignite dust clouds or produce glowing embers of accumulated dust layers. Depending on the material properties, Maillard reactions can occur within the material layers, which can lead to self-ignition or smouldering embers.
Such temperature increases in product layers cannot be reliably detected by spark detectors or conventional slow response ambient temperature sensors. Smart monitoring using long wave infrared thermography cameras/detectors may be used to monitor for temperature anomalies though opaque dust clouds between a typical range of 0-212°F (0-200°C). In comparison, flame and daylight spark detectors are only able to detect temperatures (or very bright light sources) beginning around 662°F (350°C), which can be too late to react proactively.
A suitable application example for smart temperature monitoring is the inside of pellet coolers and bed dryers.
In an environment with expected elevated temperatures, smart long wave infrared thermography can identify overheating of a localized area at the onset of overheating before reaching ember or fire.
Considering the chemical process of combustion, before smoke develops or a fire occurs, combustible material usually experiences a “roasting process,” which releases various fire gas precursors. The phase from warming through to roasting can be lengthy, presenting an opportunity to detect overheating by the pre-combustion gases. Most commonly, carbon monoxide detection is used for organic drying fire prevention monitoring.
To be reliable with only monitoring one gas, a high-performance analyzer is needed. With the understanding that additional gases such as hydrogen, nitrogen oxides and hydrocarbon compounds also are produced during the roasting stage, a semiconductor-based multi-gas detector can provide additional detection reliability.
Gas monitoring in a central duct such as dryer exhaust duct or aspiration filter exhaust duct adds extended detection coverage beyond the primary equipment to the connected upstream machines and work areas.
What happens after detection?
Each of the above-mentioned situations requires a tailored solution to manage the insipient ignition source from developing into a hazardous situation. This may include a combination of water deluge, system shutdown, bypass or operator intervention for example.
If a critical situation is detected, a predefined communication and action is needed to inform and react on the next steps.
Equally important, the selection, placement and calibration of early detection explosion prevention systems must be coordinated and configured for optimum reliability. This is important to prevent nuisance false alarms and complacency toward detection events as the moments after a real occurrence are vital to neutralizing the ignition as part of the preventive explosion safety solution.
Alexander Kemmling is a sales executive in explosion prevention for Rembe. He may be reached at Alexander.Kemmling@rembe.de. Jeramy Slaunwhite is senior explosion safety engineer for Rembe. He may be reached at Jeramy.Slaunwhite@rembe.us.