Dust and combustibility
Q: Do all dust collectors handling combustible dusts require isolation? Why or why not?
The answer to this question is yes, unless a propagated explosion won't do any harm or cause any great risk. However, this situation is rarely found in the industry.
In general, a dust collector containing a potentially explosive air (or gas) and dust mixture should be protected by pressure-reducing techniques based on venting the explosion overpressure or suppressing the explosion. Both techniques will reduce the maximum explosive pressure in the collector to a level that can be handled efficiently.
Most of the time the reduced pressure will vary from approximately 0.5 to 2 barg (7 to 30 psig), depending on the collector's design optimum. It's also possible, depending on your process, to build the collector to withstand the maximum explosion pressure, which is typically about 8 to 10 barg (120 to 145 psig). The maximum explosion pressure is based on the explosion's initial pressure. In this example, 8 to 10 barg is based on an initial pressure of 1 bar (atm or 14.50 psig).
We know that the overpressure and flames resulting from a dust explosion can propagate through a semiopen or open dust collector connection, such as at the dust-and-air inlet, dust discharge, or clean-air outlet with or without a fan. The connection can include the dust-and-air inlet, filter-product outlet, and the clean-air outlet with or without a ventilator.
An active system such as a quick shutoff valve or chemical suppression barrier activated by pressure or flash detectors inside the collector can prevent the explosion from moving from the collector through the dust-and-air inlet to upstream equipment, such as a bag-dump station or storage silo. A passive valve based on a floating-balanced-ball or check-valve principle can also be used. Another way of insulating and protecting the upstream area is by using an explosion diverter. Here, the explosion is deflected by a Y-valve arrangement and vented to a safe location.
In some cases, the dust discharge uses a single valve. This requires halting the filtering process prior to discharging any collected dust. If stopping the filtering system before discharging isn't an option, an airlock feeder or double-flap valve must be installed to provide some explosion isolation.
The same technologies, with exception of the check valve, can be used for the clean-air outlet. Even though an explosion is unlikely to pass through a filter element, it's possible that a fire or hot sparks resulting from an explosion will ignite any dust collected on the filter's clean-air side. This can result in a secondary explosion. However, it's also possible to release the clean air to a safe location outdoors without any insulating device.
It's widely accepted that, for pragmatic reasons, the design pressure of the dust collection ducts and the fan between the collector and isolation devices (or the clean-air outlet to the outdoors) must correspond to the collector's design pressure. If it’s not possible to design your ducts and fan in this way, consult an expert to determine the exact maximum pressure to be expected in the ducting and fan in the case of an explosion.
The type of isolation device to use depends on a wide range of factors, including your collector's duct sizes and location of the collector relative to the outdoors. Ask your local expert supplier for a detailed custom-designed solution for your system.
Think you’ve got what it takes to be the Expert?
We're looking for guest contributors for the Ask the Expert section—email email@example.com
for more info.