Ask The Experts

We are looking for software that can assist us in sizing ducts/fans for new dust collectors or when modifying the ducting to an existing dust collection system. Is there something you can recommend?

jflorkowski — Tue, 27 Sep 2011 16:56:41 +0000

There are several software programs available. We do not have a direct recommendation, but one that comes highly recommended by colleagues may be found at http://www.industrialventilation.net/Heavent/hv_features.html

Can static electricity be caused by an exothermic reaction?

jflorkowski — Tue, 27 Sep 2011 16:52:32 +0000

An exothermic reaction may lead to electrostatic charge generation with certain chemicals if the reaction causes excessive turbulence or formation of fine liquid droplets.

Can I use the REMBE Q Rohr device for indoor venting on a dust collector with metal dust such as aluminum? The Kst = 90 and the Pmax = 5.61

jflorkowski — Thu, 15 Sep 2011 22:01:12 +0000

Metal dusts are very tricky when it comes to indoor venting systems such as the REMBE Q-Rohr-3. With your particular application, aluminum, the Kst is low but the immediate temperature increase in the event of an incident needs to be considered before determining that an indoor venting system will be effective for this situation. For this reason, generally, metal dusts are not included in ATEX approvals. You will need to have the dust tested by the manufacturer to verify that an indoor venting system is a viable option.

We have differing recommendations on spark detection systems for dry wood dust collection. One recommends only detecting and extinguishing black body radiation for specific ignition temperature. Another specifies to detect and extinguish any and all sparks prior to the collector. Given limited information available, which method is preferable?

jflorkowski — Tue, 30 Aug 2011 18:23:57 +0000

NFPA 664, Paragraphs A.8.6.2.2 states: “Provide a spark detection and extinguishing system on the main airflow duct between the dryer drum and cyclone. The spark extinguishing system should activate every time a single spark is detected [emphasis added]. It will reset after a few seconds (if no additional sparks have been detected), and the dryer can continue to operate. The spark counting features available in some approved spark extinguishing systems can be used to shut down dryers when an excessive number of sparks are encountered, but they should never be used as a measure of when to actuate the extinguishing spray.”
There is no discussion concerning “black-body radiation” in NFPA 664, and the only discussion of “black body” is in NFPA 1971 and concerns flammability of clothing. However, there is good discussion of detection methods – including a mention of “Planck’s Law” [regarding black-body radiation] – in NFPA 72, in Sections 3.3, 5.8, A.5.8, and B.5.1.4.
If small, low-energy sparks occur so frequently that spark extinguishment interferes with production AND a prolonged historical record indicates no significant explosion hazard from such sparks, then a “cut-off” based on thermal [black-body] radiation might be appropriate. However, for infrequent sparking, a large low-temperature spark or firebrand might have a temperature below an established black-body temperature criterion but might have more energy – to ignite a dust cloud – than a small particle having a temperature far above the black-body temperature criterion.
Thus, the “bottom-line” response to this question would be a suggestion to consider the above-quoted guidance from NFPA 664, and detect and extinguish every single spark.
Based on the results of a recent explosion investigation by Chilworth, it also would be prudent to count all sparks and document the counts, rather than document the number of extinguishment actions. An increasing number or frequency of sparks could indicate serious problems with upstream equipment.

Can you provide more information on compressed air attachments to dust control systems to act as a high vacuum inducer?

jflorkowski — Mon, 22 Aug 2011 22:04:31 +0000

A vacuum hose system with a pneumatic vacuum pump and floor sweep can be used to remove fugitive dust from around a duct collection/work area and draw it into the dust collector. Such an accessory provides an economical maintenance solution by adding a high-vacuum wand for clean-up to a low-vacuum dust collector.
The vacuum pump is powered by the same compressed air line that cleans the dust collector filters. The pump typically has venturi nozzles that create a high vacuum when the compressed air is applied. The dust is drawn into the inlet of the dust collector before the channel baffles so that large particles will not contact the filter media. A shut-off valve should be mounted in the vacuum hose to isolate it from the dust collector when not in operation. A second shut-off valve should be mounted to the vacuum pump to ensure that compressed air is only used when the system is in operation.

I have observed static electricity generated while sifting ibuprofen (the sulfamethoxazole material) through a vibratory sifter which leads to burning of the powder on the sifter mesh. Why does this occur?

jflorkowski — Tue, 26 Jul 2011 19:00:06 +0000

The burning of ibuprofen and sulfamethoxane powders in your vibratory sifter could be attributed to electrostatic “spark” discharges if the following conditions were simultaneously present in the sifter:
• A conductive (metal) section of the sifter that comes into contact with the powder such as the sieves/screens or the inlet or outlet conveying chutes is electrically isolated from ground
• The electrical capacitance of the ungrounded section of the sifter is sufficiently large to accumulate enough electrostatic energy to ignite/burn the dust particles, when released as a spark
• The movement (flow/vibration) of the powder particles over the ungrounded metal section gives rise to a sufficiently high level of electrostatic charge buildup on that section
• The isolated metal section is close (2-4 mm) to a grounded part of the sifter so that the spark can jump from the charged section to the grounded/uncharged section
• Dust particles are present within the spark gap
To prevent “spark” discharges I suggest that all the metal sections of the vibratory sifter be electrically bonded together and connected to ground so that the resistance to ground is less than 10 ohm. Please note that I have assumed that the vibratory sifter does not comprise of any plastic sections such as plastic pipes.
I also suggest that other sources of ignition such as frictional heating, frictional sparks, electrical arcs (from electric motors, etc), and self-heating be investigated and eliminated/ruled out.

Are there any specific filtration and/or maintenance concerns for dust collectors handling combustible metal dusts that may be tinged with pickling compound? The facility in question blasts fabricated carbon steel parts (the carbon steel is pickled to prevent corrosion) with steel shot or grit. The dust collection system for this operation is a pulse-jet dust collector using fire retardant pleated cartridge filters, with a spark arrestor located at the inlet to the unit. Despite regular “routine” maintenance and approximately annual cartridge changes, the dust collector has caught fire about once every 2-3 years. Is more frequent maintenance/cartridge changeout necessary? Or should a custom cartridge designed for both oil resistance and flame retardance be considered for this application?

jflorkowski — Thu, 02 Jun 2011 21:23:17 +0000

Occasional fires in iron dust can be attributed to a low rate of heat transfer [resulting from iron-particle oxidation] to the external environment, resulting from occasional thick layers of dust and prolonged infiltration of air into the “center” of the accumulated dust. To prevent fires in iron dust, one may consider avoiding the formation of thick accumulations of dust by frequent [or continuous] removal of dust from trays, hoppers, or drums, or discharge iron dust into an “excess” of water [since "moist" iron dust could generate hydrogen gas].

If you have an existing dust collector with no provisions for handling explosive/combustible dust and you test your dust and find out it is combustible, what are the issues to consider in determining if the system can be modified to handle explosive dust or if it needs to be replaced with a new system?

jflorkowski — Tue, 24 May 2011 19:22:16 +0000

Under typical circumstances where you have complete information about your dust collector, such as the strength of the collector, retrofitting should be no problem. In that situation, in accordance with NFPA standards 654, 68, 69, and perhaps other standards that specifically address your industry, a dust collector must be vented/suppressed and isolated ( the inlet always needs to be isolated; the clean air side must be isolated if it is a return-air installation). If the dust collector is inside, the dust collector might be vented through a duct to the outside, an indoor flameless vent can be installed or chemical suppression might be used. If the dust collector is located outside, explosion panels or flameless vents can be used depending on the proximity to other structures and people. NFPA standard 68 provides the method by which to calculate the required vent areas.
If you are in a situation where you don’t know the strength of the dust collector and you have no way of finding out the strength, you will either need to have an engineering analysis done on the dust collector or replace the collector to be absolutely sure you are properly calculating the vent area. There is no way to calculate the vent area if you don’t know the strength of the dust collector, and effective vent area is the critical component to minimizing damage to people and structures should there be a combustible dust explosion in the dust collector. In that case, you may decide you are better served by replacing the dust collector but you will still need to equip the collector with the appropriate venting/suppression and isolation equipment as indicated above.
Note: Even if you have all the information about your dust collector, you may find that the strength of the dust collector is such that it is more cost effective to increase the strength of the collector to reduce the costs of equipping with explosion protection equipment or to buy a new collector and properly equip it to protect it.

If an existing older dust collector is collecting dust that can explode, is it better to upgrade the existing dust collector with explosion protection or buy a new one that already has the explosion protection built in?

jflorkowski — Tue, 24 May 2011 18:05:44 +0000

Per NFPA regulations, each enclosure containing a combustible particulate has to be vented or suppressed, no matter how old the enclosure is. In the case of a dust collector, if the reduced pressure (Pred) for the existing dust collector can be determined either by the manufacturer’s specifications or through a structural analysis, the old dust collector can be upgraded and still be used. In some cases, the cost to determine the Pred could be quite high, and the dust collector may need too much “strengthening” or other redesign—retooling doors that will not withstand explosions, for example, so that purchasing a new dust collector could be the more economic solution. Whether you retrofit an old dust collector or install a brand new one: either one of them has to include explosion protection and explosion isolation.

This question pertains to dust collectors that are 8 ft in diam. and 11 ft high. Currently our dust collector bags are at the same elevation as the explosion vents. We are considering a modification - increasing the air to cloth ratio by increasing the length of the bags, hence increasing the height of the dust collector housing. Are there any regulations that require the explosion vents to be mounted lower than the bottom of the dust collector bags?

jflorkowski — Tue, 24 May 2011 17:58:15 +0000

NFPA Standard 68 gives you precise guidelines for how explosion vent panels should be installed. The goal of NFPA Standard 68, 8.7.1 is to prevent the bags from blocking the vent such that the bags might be “sucked through” the vent if there is an incident.
Explosion vent panels should be installed underneath the bottom of the filter bags as described in NFPA Standard 68, 8.7.1. If, for any reason, there is not enough space to install the panels underneath the filter bags, you may, per NFPA Standard 8.7.1 (2), install the panels along the dirty air wall if “… bags are either completely removed or shortened so that they do not extend below the top of the vent for a distance of one vent diameter from the vent. In addition, bags immediately adjacent to the vent shall be removed and the remaining bags shall be restrained from passing through the vent.” NFPA Standard 68, 8.7.1(2), 2007 edition.
Another acceptable explosion vent panel placement option, per NFPA Standard 8.7.1 (3): “…the bottom of the vent(s) is at or above the bottom of the bags,…and the row of bags closest to the vent are restrained from passing through the vent… For this case, the volume used to calculate the vent area shall be the entire volume (clean and dirty) below the tube sheet.”