Key points
How Flame Filters Affect Venting Efficiency & Safety Distance
When an ignition source meets a dust cloud in a confined space, such as within a dust collector or silo, a devastating explosion may occur. Explosion vents are often installed on these vessels to openand safely relieve the pressure.
However, when these applications are located indoors, this “free venting” explosion protection method isn’t possible and flameless venting may be required instead.
A flameless venting system consists of an explosion vent panel covered with a casing and attached flame filter. During an explosion, the vent panel opens, and the filter absorbs the heat so flame doesn’t extend past the device.
Owners of flameless venting systems must understand that within these devices is a hidden, complex problem of fluid mechanics, including fluid-particle flow, the transferring of heat and the releasing of pressure, all within milliseconds.
These variables must be included within the design of the safety system because they have a drastic effect on the number and size of required flameless vents and on the safety distance around them.
Effect on Venting Efficiency
An essential performance indicator for an explosion venting system is its venting efficiency, which describes how effectively it relieves pressure in a vessel.
Depending on the type of filter used in the flameless venting device, some of the unburnt dust particles will be retained by the filter to reduce the chance of external re-ignition; however, when the flame filter becomes clogged by variable amounts of burnt and unburnt particles, its venting efficiency is reduced.
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 this critical factor is ignored during system design and severe clogging occurs, there is a chance of the device failing and the vessel rupturing.
Typical variables which may affect venting efficiency and must be considered during the design of a flameless venting system include:
- Particle Size & Morphology: Affects the amount of aggregates that get blocked on the surface of the filter; for example, fibrous dusts may block the filter surface differently versus melting dusts such as sugar.
- Dust Concentration: 250 g/cu m in a certain volume will challenge a filter differently in comparison to 2000 g/cu m. In the second case, the filter may clog faster, and it can even fail due to the increase in pressure.
- Vessel Volume & Geometry: If a deflagration were to occur within the vessel on the opposite side from where the flameless vent is installed, more dust throughout the volume will be pushed into the filter. Also, larger volumes contain more dust particles and may reduce the venting efficiency, unless the flameless venting device size is adjusted accordingly.
These are all important reasons why flameless venting performance cannot be described by one efficiency number but rather must be disclosed through a complex model that considers all of the aforementioned factors of the process and its handled dust’s properties.
Without an accurate venting efficiency number, the primary risk is that the resulting pressure might become higher than the expected maximum reduced explosion pressure; otherwise, the process may not be as reliably protected as owners may believe.
Effect on Safety Distance
During a deflagration, varying amounts of particles and gases will be ejected from the device depending on the effectiveness of particle retention within the flame filter. Therefore, every flameless vent should include a recommended safety distance to ensure workers aren’t hit by debris or nearby combustible materials aren’t ignited into a secondary explosion. Four factors must be taken into account to determine the required safety distance:
- Unburnt Particles: If not enough unburnt dust particles are retained within the Flame Filter, a sometimes large Zone 22 hazardous area needs to be defined external from the device, within which (electrical) ignition sources during normal operation are not allowed.
- Temperature: The released gases including some of the dust particles, can sometimes be hot. For that reason, there is a certain safety zone where no personnel are allowed to pass.
- Noise: An explosion and the accompanying opening vent may produce an intense sound that can cause eardrum damage. As a result, a zone for ear protection is recommended.
- Blast Wave: The wind load of expanding gases out of the venting opening may lead to a fall-over risk for nearby personnel.
Why it Matters
The application of flameless venting to mitigate an explosion requires more information than traditional venting.
Users need to be aware and ask the correct questions to ensure the system is accurately designed to their unique process and are also considering the factors which affect nearby zones in the path of the flameless vent.
For more information about this phenomenon or if you need help ensuring your flameless venting device is being used reliably, please contact us at fike.co.uk or on 01622 677081.
