A: Eric Finley, Rembe, says:
This is a great question because isolation protection is a significant component that can be overlooked in combustible dust protection systems. Deflagration isolation protection is imperative in preventing the spread of a combustible dust explosion throughout a facility. The pressure wave and fireball from a nonisolated dust explosion can rapidly travel through mechanical conveying or pneumatic conveying arteries, causing dust layers within a plant or system to become suspended and ignite. The effect from this pressure wave can, in turn, create a secondary deflagration causing catastrophic destruction throughout the entire facility. This can all occur even if each piece of equipment is properly vented since the explosion can still propagate through any pathway such as a conveying inlet, a material discharge chute, a feed line, or an air outlet. This is why all NFPA standards focused on combustible dust (NFPA standards 61, 68, 69, 652, and 654, just to name a few) require isolation protection to ensure your facility is completely safe.
First and foremost, it’s important to note that a dust hazards analysis (DHA) is required to be performed in accordance with NFPA 652: Standard on the Fundamentals of Combustible Dust, 2019 Edition before proceeding with any explosion protection projects. The DHA should be completed throughout the entire facility by a capable professional who’s experienced in combustible dust hazards. A DHA will provide you with a report of what actions are required to ensure the facility complies with the applicable NFPA standards. The analysis results may shed light on other connecting processes that could adjust how you handle the overall protection plan. The system layout may need to be altered due to other processes in the facility to create a safe working environment. This is why it’s always best to examine the complete facility through a DHA before tackling individual systems.
After a DHA has been completed and isolation protection is deemed necessary, there are a few options that can be considered depending on the application type and what equipment or area you’re protecting.
Dust collector: The dust collector has three possible propagation points. These are the material inlet, material discharge, and clean-air exhaust. The most common way to isolate the material inlet is with a backdraft damper or flow-actuated flap valve. These valves operate in a similar manner to a check valve as they allow the nuisance dust to flow toward the collector but will then be closed by the pressure wave of an explosion. The flap will then lock in place and inhibit any flames from continuing upstream of the valve.
The next point to isolate is the material discharge, and this can be done in several ways depending on the situation. The two most common approaches are via a rotary valve or a collection bin under the discharge. The common rotary valve design for isolation ensures there’s a constant close clearance between the vanes and the valve body to ensure that flames can’t pass through. The collection bin design essentially becomes a part of the volume being protected, so the bin and all connections must be strong enough to contain the explosion. The final point of concern is the clean-air discharge, which also has several options for isolation. The simplest approach, in this case, is to discharge the clean air safely outdoors. This will ensure that any explosion propagation is released to safe area outdoors. If the clean air is being discharged back into the building, then a flow-actuated float valve, flameless vent without an explosion panel, or even a suitably designed clean-air plenum can be possible isolation devices.
Filter-receivers: There are three main points of concern for the dust collectors; however, the main difference is for the material inlet isolation. Backdraft dampers aren’t the best isolation valve for the material loading typically seen with a filter-receiver. The backdraft damper can clog quickly and create a constant maintenance issue to remove the built-up material. Instead, active isolation devices such as a pinch valve, knife gate, or chemical barrier should be used. These isolation techniques use electronic signals such as spark or pressure detectors to trigger the fast-acting isolation devices. They respond to this signal within milliseconds to become a barrier and stop the propagation of the explosion. The pinch valves and knife gates are pneumatically driven to pinch inward or slam down, creating a barrier that stops the propagation. Chemical isolation consists of pressurized canisters that inject a suppressant (for example, sodium bicarbonate) into the ductwork thus creating a chemical barrier that prevents the deflagration from combusting further. These options don’t inhibit the material flow, which makes them well-suited for a high material loading application.
Silo or bin: There can be many different configurations for silos and bins that can affect the isolation protection required. The isolation strategy of the material inlet is similar to the filter-receiver due to the material loading. The material discharge isolation can be accomplished via a rotary valve as described above or possibly through screw conveyors, depending on the application. Material inside a round screw conveyor can be compacted by the pressure wave such that it halts the propagation of the explosion. This is dependent on a few variables such as the flowability factor and the material’s minimum ignition energy (MIE), but material blockage in screw conveyors can be an effective isolation technique. The air exhaust can generally be released safely outdoors; however, there are times when there’s a dust collection system connected to multiple silos. Using a two-way isolation device such as a pinch valve, knife gate, or chemical barrier is important to prevent the propagation between the interconnected silos and the dust collectors.
In any isolation application, it’s vital to ensure that all ductwork, pipelines, connections, or equipment are of sufficient strength to handle the vented explosion. If they aren’t, the connections could blow open and allow the explosion to enter the facility, thus rendering the isolation devices useless. Additionally, almost all isolation valves have specific installation guidelines that must be met. These requirements are critical to the device’s function and must be followed to ensure the system is safe.
There’s a lot to consider in every combustible dust application, and it’s important to create a completely safe working environment. Isolation protection is a significant question to examine since it’s a critical safety component that can stop a catastrophic explosion. Overall, it’s always vital to have an expert look through the system to determine the optimal protection system. This will ensure the facility remains safe for its workers, complies with the latest NFPA standards, and is completed with the most economical approach possible.
Rembe, Charlotte, NC, manufactures and supplies rupture discs as well as explosion safety systems and accessories.