Ask The Experts

We are trying to convey a potash dust using a chain conveyer. The dust is coming from a dust filter and it is very fine – less than 50 micron. Do you think we will be able to convey it in an inclination of 20 degrees?

jflorkowski — Wed, 25 Apr 2012 23:16:50 +0000

A chain conveyor is a very efficient method for conveying fine potash dust from a dust filter and up a 20-degree incline. The chain conveyor will lose some efficiency as some of the fine dust falls back down the incline. The chain speed may need to be increased to overcome the inefficiency of conveyor. The chain conveyor will be more efficient as you reduce the angle of incline below 20 degrees.

We have a 100 tn capacity silo for sugar packaging. Two vent panels have been installed on the top shell and have an adequate vent area but they are close to each other (not diagonally). Would it not damage the silo during an explosion as the pressure force will be acting only on one side of the silo?

jflorkowski — Wed, 25 Apr 2012 22:43:11 +0000

Your concerns are well founded. The ideal installation in this situation is to have the panels located such that recoil forces will not have a damaging impact on the silo or other enclosure – in other words, across from each other. If, for any reason, the vent panels have to be installed on one side, then the enclosure must be sufficiently strong to withstand the potential recoil force.

We wish to manually charge powders from bags into a vessel containing aqueous medium via the manway. The powder has a MIE

jflorkowski — Tue, 24 Apr 2012 17:19:15 +0000

There will of course be a significant ignition risk if a dust cloud with a concentration within the explosible range is subjected to an ignition source with an energy greater than the dust cloud’s Minimum Ignition Energy (MIE). For manual operations such as manual transfer of powders to receiving vessels the basis of safety is often the avoidance of all potential ignition sources such as:
• Using the correct type of bag (super sack) and liner (if there is one present) to reduce electrostatic discharge from the bag. [Ref: IEC 61340-4-4 Edition 2.0].
• Bonding and grounding of all metal objects in the vicinity that could become electrostatically charged
• Grounding of the operator(s)
• Grounding of the container (drum, if powder is being poured from a drum) and the receiving chute and vessel.
• Transferring (pouring) the powder from the bag to the vessel as gently as possible to avoid generating a dust cloud.
• Avoidance of all mechanical (friction sparks and heating) ignition sources through a proper maintenance program.
• Use of approved electrical equipment in the bag unloading area.

Additionally it is recommended to prevent the spread of the dust cloud by taking measures such as:
• Use of a properly designed and operated local exhaust ventilation
• Transferring (pouring) the powder from the bag to the vessel as gently as possible
• Immediate cleaning of any powder spillage using an approved vacuum cleaner

We wish to obtain a simple design layout and/or criteria for an effective integral dust collection system on our various hand dump stations for packaged or pre-weighed ingredients for our mineral premix operations. The key deliverables are: (1) control of dust at the point of mineral introduction and (2) self contained dust recapture at point of introduction and use within the batch being produced vs. central collection and loss of material to recycle, rework and/or downgrade. Any thoughts?

jflorkowski — Thu, 19 Apr 2012 00:11:52 +0000

We wish to obtain simple design layout and/or criteria for an effective integral dust collection system on our various hand dump stations for packaged or pre-weighed ingredients for our mineral premix operations. The key deliverables are: (1) control of dust at the point of mineral introduction and (2) self contained dust recapture at point of introduction and use within the batch being produced vs. central collection and loss of material to recycle, rework and/or downgrade. Any thoughts?

You bring up some good questions on industrial ventilation. It is difficult to answer with simple answers because there are many variables that can affect the hood design. Some of the variables that could affect the hood design are distance from the source, weight off material, cross drafts, size of hood, accessibility around hood, bag disposal, etc. It seems to be that you have two different application dump stations and weighing hoods. Both applications will require the hoods to be located behind the station, in front of the worker. This will draw the fresh air around the worker while dirty air is pulled away from the worker.
The dump station can have two different styles of dust collectors, bag dump with integral dust collector or source capture with ductwork going to a central dust collector. If there are only a few dump stations in the plant, then most companies will use a bag dump with integral dust collector. A bag dump with integral dust collector is a self-contained dust collector mounted to a dump station and the dust will be pulsed off the filter back into the process. As a general rule the hoods should be enclosed (having top and side walls) with a design of 200-500 FPM (feet per minute). When there are multiple stations, a central dust collector is used. Usually, a major source of dust will be generated from the bag (garbage) after the material is dumped out. Extra source capture could be required.
The weighing hood stations will have ductwork going to a central dust collector, because there is not a place for the dust to drop back into the process. A general number to use is 50 cu ft/min/sq ft (cubic feet per minute/square foot) based on opening of the hood. It would be best to have a local dust collector vendor to examine our process to suggest the correct type of dust collector.

Can you confirm that it is not acceptable to use PVC piping (particularly with 90° bends in it) to vent a rupture disk for a process with an ST-2 powder having Pmax of 8.7 bar and deflagration index of 210 bar-m/sec?

jflorkowski — Thu, 19 Apr 2012 00:06:35 +0000

PVC piping could be used so long as the strength (Pred) is high enough and it is sufficiently temperature resistant. The larger concern would be the 90 degree bends. Although NFPA standards do not definitively say you cannot have such bends, we don’t recommend it.

In a fuel -air environment, at atmospheric pressure, what is the minimum oxygen percentage required to create an explosive hazard?

jflorkowski — Wed, 04 Apr 2012 00:02:30 +0000

Generally–and I say this with some hesitation–if your fuel is gas of some kind, the minimum could be approximately 5%. If your fuel is dust of some kind, the minimum could be approximately 10-12%. For smoldering dust, it could be as low as 2%. Note that I say could be. These generalizations really don’t mean anything for a specific application. The answer always depends on the specific fuel and a LOC (Lowest Oxygen Concentration) must be determined for that specific fuel.

Is there a high risk of explosion for polyethylene that has been ground from pellet form to a 35 mesh powder which is an industry standard before using the material in the rotational molding process?

jflorkowski — Tue, 27 Mar 2012 21:24:12 +0000

Of course a fine polyethylene dust cloud would present a dust cloud explosion hazard. The risk of an explosion will however be the product of the likelihood of an explosion event and the severity of such an event (R = L*S). Polyethylene pellets are generally considered to be too coarse to present a dust explosion risk. However, particles passing through a 35 mesh US screen would have diameters less than 500 microns and would usually be considered fine enough to pose an explosion risk if present in a dense enough cloud in the presence of a credible ignition source. To assign a risk value; however, more information on the process would be required. Particle size distribution and explosion severity testing (Kst) along with Minimum Ignition Energy and Minimum Ignition Temperature (cloud) tests, for a representative sample of what is coming out of the mill, would provide the information generally needed to estimate the severity and likelihood of an explosion event for such an operation and thus the relative risk.

Is limestone powder explosive? If a combustible material (carbohydrate) such as starch or maltodextrine is combined with the limestone, at what level will that material make the whole mixture combustible?

jflorkowski — Tue, 27 Mar 2012 21:16:35 +0000

Limestone, or calcium carbonate, has not been shown to be an explosible dust. However, when mixed with other explosible materials, a combustible dust cloud can be formed. For example, calcium carbonate/graphite (50:40) can produce an St 1 (Kst >0 and <200) explosion. Glycerol monostearate plamitate/calcium carbonate/tricalcium phosphate (70:20:10) can produce an St 2 (Kst >200 and <300). In order to determine the concentration of starch or maltodextrine in limestone that would give rise to an explosible dust cloud one needs to conduct “Explosibility Screening (go/no go)” tests on limestone dust samples at various starch concentrations.

Static is elusive and can appear anywhere we have materials flowing through a conductor. The obvious answers are when transporting materials through a metal conductor you can just ground the system and you are pretty close to having the problem solved. Not all conductors are metal however. So the question becomes when blowing materials through a nonconductive pipe how do you keep static from building up? I have a few products that are very useful for transporting palletized materials. However the question becomes how do we protect ourselves from static? That is the general gist of what I’m trying to understand. I also deal with very large silos. In those large silos during filling it is also possible to get a static discharge. If there is a lot of grain dust you can get one large explosion.

jflorkowski — Fri, 23 Mar 2012 17:42:25 +0000

1. Pneumatic conveying pipes constructed from conductive (metal) materials: All metal parts of the pneumatic conveying system, including conveying pipes must be electrically bonded together and grounded with a resistance to ground not exceeding 10 ohms. This would ensure that there is no electrostatic charge buildup on these metal pipes/items. However, the use of electrically grounded metal pipes does not necessarily prevent electrostatic charge generation and buildup on the powder particles that are being conveyed through the pipe (please also refer to No. 4 below)
2. Pneumatic conveying pipes constructed from non-conductive (plastic) materials: Grounding of non-conductive conveying pipes would not be effective in reducing the buildup of electrostatic charges on the pipe work. Therefore, the pneumatic transfer of powder particles would result in the generation and accumulation of very high levels of electrostatic charge on the inside surfaces of the pipes that could lead to very high energy “propagating brush” discharges (PBD)”. PBD’s are reported to have equivalent energies of up to about 2,000 mJ and hence are capable of igniting most combustible dust clouds. Even in the absence of a flammable/combustible atmosphere, propagating brush discharges could cause injury to people. Generally speaking, it is suggested that only metal or conductive plastic pipes be used for the pneumatic conveying of powders. There are, however, situations when non-conductive pipes may be used – for example, if the chargeability of the powder (when tested with the intended non-conductive pipe and conveying conditions) is sufficiently low.
3. Plastic hoses and pipes that have a grounded spiral wire embedded within their walls: Such hoses and pipes could still give rise to propagating brush discharges from their inside surfaces (see No. 2 above). The presence of the spiral wire, when connected to ground, would only limit the reach of the propagating brush discharges to the inside surfaces of the hose/pipe. If the wire becomes broken it can become charged by induction, another hazardous condition.
4. Is it possible to get a static discharge when filling large silos that is capable of igniting the dust cloud?
Yes, cone discharges could occur and be incendive to the atmosphere. In order to assess the electrostatic ignition hazard associated with the transfer of powder into a receiving vessel one should determine the “Minimum Ignition Energy (MIE)” of the dust cloud as well as the “Chargeability” and “Volume Resistivity” of the powder that is being conveyed. Generally speaking, the lower the MIE and the higher the chargeability and volume resistivity values the higher would be the possibility of an electrostatic ignition hazard.

We load very fine, light, fluffy clay powders. We have tried various ways to detect when the vessel is filled to near capacity. Nothing has been successful at reliably preventing overfill. Do you have any recommendations?

jflorkowski — Tue, 20 Mar 2012 19:54:43 +0000

There are three methods of overfill detection I would recommend you investigate for your application. Though there may be more options on the market, for me, the choices would be between capacitance, vibrating fork (or rod), and laser level. All three have been mounted on solids loading arms so that the level detection device is always ready when loading is taking place. Each has its requirements, advantages, and drawbacks, so the application will determine the correct choice.
Capacitance will require the powder to have a minimum dielectric value. This is not likely to be a problem depending on the probe selected. Clay has a fairly high dielectric, around 12 depending on the moisture content. Most manufacturers make a sensitive capacitance switch for bulk solids applications. Buildup and bridging can be an issue with this technology if the powder is sticky.
Vibrating forks or rod switches require a minimum bulk density for switching and that density will affect the depth required for switching to take place. Again, the bulk density of clay is fairly high, so this technology would also be a good choice. The forks should be installed vertically for best performance and to reduce the chance of bridging.
Laser is a non-contact solution to this application. The advantage of laser is that it will measure the level no matter the angle of repose of your fill. You also don’t have to be concerned about the operators damaging the device when they are positioning the fill arm since it will be above the vessel and likely also in a nozzle. The two concerns you would need to address would be the unmeasurable distance right at the sensor and heavy dust in the atmosphere. Device settings can address some dust, but if the cloud is too thick the light will not penetrate to the level.