When an ignition source meets a combustible dust cloud within an enclosed space, such as in a dust collector, silo or bucket elevator, a devastating explosion may occur.
Explosion vents are often installed on these vessels to open and safely relieve pressure and flames. When these applications are located indoors, “free venting” isn't possible and flameless venting devices may often be used instead. These devices include a flame arrester constructed of several layers of wire mesh to absorb the heat and keep flames from escaping.
How effectively a vent relieves pressure is referred to as its “venting efficiency,” which helps system designers determine the proper size and quantity of vents that are required to safely protect the process. However, with flameless venting, a number of particles may clog the filter, which greatly reduces its venting efficiency.
This is why product clogging and several other variables must be included in the design of a flameless venting system. If this efficiency rating isn’t properly calculated for an application in which flameless vents are installed, they may not safely relieve the deflagration when called upon.
Essential Elements of Flameless Vent System Design
The performance of every flameless vent filter depends heavily on the unique characteristics of the burning dust cloud. Therefore, when designing a flameless venting system, an explosion protection provider should know the following about the application and its handled dust:
- PARTICLE SIZE: Various particle sizes may clog the filter slower or faster.
- PARTICLE MORPHOLOGY: Affects the rate of clogging as well as the size of aggregates that get blocked on the surface of the filter. For example, fibrous dusts may clog the filter in a different way versus melting dusts (such as sugar) or granular dusts.
- DUST CONCENTRATION: 250g/m3 in a certain volume will challenge a filter differently in comparison to 2000g/m3. In the second case, the filter may clog faster and it can even fail due to the increase in pressure.
- VESSEL GEOMETRY & IGNITION: In a long vessel, where ignition may occur at the bottom and the flameless venting system is installed at the top, flame will push the maximum amount of dust from the entire volume through the filter.
- VESSEL VOLUME: Each flameless venting size has a maximum process equipment volume where dust is dispersed and can be successfully quenched. Above that limit, flame quenching is not guaranteed due to the high thermal load.
Why is Efficiency Reduced in Flameless Venting Devices?
Flameless venting performance cannot be described by one efficiency number but rather must be disclosed through a complex model that considers all of the factors of the process and its handled dust
For example, there is a possibility that a flameless venting device of a certain size performs very well under 500g/m3 of cornstarch, showing an efficiency of 70 percent, but clogs very quickly under 500g/m3 of wood flour, showing an efficiency of just 40 percent or lower.
If multiple factors are ignored and severe clogging occurs, there is a chance of the device failing and the vessel rupturing.
Why It Matters
Purchasers of flameless venting systems must understand that within these devices is a hidden, complex problem of fluid mechanics, including fluid- particle flow, the transferring heat and the releasing of pressure, all within milliseconds.
The application of such a device to mitigate an explosion requires more information than traditional venting.
Users need to be aware of the complexity and ask the correct questions with the aim to define the real performance of these devices to ensure that when they are called upon, they work as intended.
