Safety Issues with Venting Dust Producing Operations

Typical manufacturing, mining and material handling operations produce various types of dust and other contaminants. These contaminants may be quite toxic when they enter the workers’ lungs. Protection systems involve either suppressing the mostly solid particulate contaminants generated or the venting and collection of the contaminants.

Dust Suppression

The first approach is to prevent the generation of these toxic contaminants before they enter the work environment. This can be accomplished by dust suppression systems. An example of this would be a rock quarry or coal mine. As the gravel or coal is processed into smaller usable sizes by crushing and placed on belt conveyors to be delivered to trucks or railroad cars, and then delivered to further processing. All of these operations produce large quantities of dust if they are not controlled in these operations. Dust suppression is achieved by spraying liquid over the rocks and coal so that the operations do not produce dust. In dust suppression technology, compounds are added to the water that eliminates the surface tension of the water so that the liquid coating is spread over a much larger surface area. The dust then stays attached to the pieces and to adjoining particles to prevent dust generation. The spreading of presumably water based solution increases the rate of evaporation. In colder climates the dust and product does not allow the material to freeze so that loading and unloading are facilitated.

Venting of Dust Producing Operations
The most common contaminant in industrial manufacturing operations is solid particulate. First the contaminant is contained by putting enclosures or hoods around the dust generating machines that will allow access for the workers. The hood is then ventilated and the contaminated gas stream is cleaned by a dust collector. The dust is separated from the gas stream and the gas stream is vented to the work environment or outdoors. The process of collecting this dust, disposing of it, maintaining and servicing the dust collector equipment can expose the workers to serious hazards
a) The first hazard often present is accumulation of dust in the bottom portion of the horizontal duct runs. Most well designed vent ducts have cleanout doors every several feet. These ducts are near the ceiling usually 12 or more feet above the floor. The cleanout doors are located on the lowest underside of the duct and when the doors is opened dust will pour out towards the floor exposing the worker and environment to dust that might be inhaled. Special man hoists are recommended and breathing masks are indicated. A better approach is to install pneumatically actuated Duct Cleaner-Boosters in the system. These will momentarily increase the velocity in the ducts pushing the dust accumulation toward the dust collector. It makes the duct cleaning operation automatic and safe, with minimum exposure to the dust.
b) The next hazard is when flammable dusts are produced in the machine being vented to the dust collector. If dry dust is collected, sparks can be entrained from the hood(s), and carried into the collector. There is a coating of flammable dust on the filter elements. The velocities through the filters are much lower than in the duct and if a spark reaches the filter elements, the dust may reach the ignition temperature and start a fire. In well designed ductwork the flow is designed to be laminar. Sparks may be transported for more than a hundred feet. To guard against this occurrence, an in-line spark suppressor with a duct cleaner – booster should be installed. The suppressor device will induce extreme turbulent flow which cools the spark below the ignition temperature and protects against fires.
c) Explosions are another operating hazard. To have an explosion the concentration of dust in the housing or duct must be between the lower and upper explosive concentrations and a spark must be present. In mechanical cleaning (shaker) collectors, the flow is stopped in the filter compartment and the filter elements are agitated all at the same time. A potential for an explosion occurs since the concentration will likely pass through the explosive limits during this action. Protection consists of grounding the filter elements to prevent sparks generated during cleaning. Additional explosion rupture panels are installed and vented outdoors. In continuous cleaning pulse jet collectors, only small sections of the collector are reverse flushed. Around each bag in the cleaned section is a very small volume of air which can pass through the explosive limits. Even if a spark is present, an explosion would be dissipated without danger. If the collector filters are to be replaced the first procedure is to remove as much flammable or explosive dusts from the filters as possible. The exhaust fan’s direction is reversed to maintain a low flow and prevent dust from returning to the hood. The collector is cleaned one section at a time allowing time for the dust to settle into the collection hopper. After several complete cleaning cycles a large portion of the dust will be ejected. This lowers the exposure of the worker in handling the filter elements.
There are two general types of filter elements; those with smooth surfaces usually cylindrical or oval with smooth surfaces, and, pleated filter elements. There is a potential for pleated filter elements to bridge and have dust collected in the valleys of the pleats. Even if a reverse pulse collector is cleaned slowly with the fan reversed, considerable dust may be present in the valleys. Recent new technology provides for wider pleat spacing and stiffer filter media which allows off line cleaning as described above to be effective.
d) There are some contaminants either liquid or solids which are not suited to fabric media collectors. They will not form a filter cake or the dust is very unstable. Gunpowder and the propellant for inflating air bags in an automobile are two common examples. The most common approaches are gas washers that scrub the contaminant from the vent stream. Another approach is to mix inert dust into the vent system so the dry powder mixture is no longer flammable or explosive. Some operations will produce dust that is so wet that it will quickly turn the filter cake into a mud which will blind the filter elements.
e) Wet Dust Collectors have a variety of designs and must deal with the problem of surface tension of the water which is used to clean the gas. To get adequate collection efficiency, historically designers have resorted to higher pressure drop designs so that the solid and liquid contaminants might penetrate through the scrubbing surfaces. The same types of compounds, as in dust suppression systems, allow operating at very high efficiencies with minimal power consumption. It was necessary to design special multi-pass mist eliminators to collect the overspray from the scrubbing surfaces. Therefore gas leaving the collector is often below saturation from the heat regain as it passes through the exhaust fan.

Other Hazards and Considerations
Rotary Feeders located in the bottom of a collection hopper can pose a danger to maintenance personnel. There are instances where the feeder fails and dust builds up in the hopper. This dust can ignite and burn. The main approach is to shutdown the section of the collector and let it burn depriving it from getting oxygen. Normally, it will self extinguish. Spraying in water can create an explosion as the water displaces the dust with steam and will go through the explosive limits. On a power plant boiler, a maintenance man decided to pour water into the hopper. The dust was agitated and an explosion occurred.
A) It is preferable to install collectors where the filter elements can be changed from outside the collector housing. Collectors with removable roof doors are widely available. When a filter element extends 8 feet above the doors, even a moderate wind is a problem and workers must plan to protect themselves from these forces.
B) Another type of design is walk-in-plenums. There is a housing tall enough to change the filters, out of the weather elements. The entrance is usually a hinged access door. Rarely do these chambers have lighting. A particular hazard is when in the housing the door slams shut. It is hard to find the door in the dark and a wind can keep it closed. It is advisable to clamp the door with a cable and lock it. Remember the bags and cages must be lowered to the ground and hauled up again and fed to the collector through the access door. The pulse pipes must be removed and temporary storage space provided. These pulse pipes can fall through the holes in the filter mounting plates, forcing a man to enter the collector. Toxic gases can seep into the clean air plenum, so, the breathing zone should be monitored. When a collector is initially started, dust will seep into the plenum until a filter cake is formed. Personnel should not enter the plenum until this conditioning of the filters is completed.

For more information:
Gary Berwick, P.Eng.
Phone: (519) 746-2424
e-mail: gary@qamanage.com
www.qamanage.com

Spark Arresters and Coolers

Important Factors in Spark Arrester Selection
(1) Pressure drop across QUENCHER style of unit is a function of the Reynolds number which is proportional to the density for air. This means that a unit can be sized smaller if operating at a higher temperature. For instance a suppressor operating at 440 degrees F is 2/3 the size of the typical unit applied at 70 degrees F and the pressure drop will be designed the same. This lowers the cost of the suppressor. The density is also affected by the water vapor in the gas stream. It has little effect at temperatures below 125 degrees F but can be a major factor when operating at higher temperatures.
(2) If the gas steam has dust that might drop out in the duct at the velocities in the blender style or QUENCHER suppressor, a booster must be provided to periodically remove this accumulation. If this unit is not kept clean it might pose a threat by putting an extra load on the duct-work. Without an Automatic Booster System, the suppressor might require periodic manual cleaning.
(3) The booster design is also temperature sensitive and must be altered to accommodate changing gas steam conditions. Most suppliers do not have the capability to modify these booster designs.

There are several approaches to the issue of extinguishing sparks in a gas stream.

Cyclone Dust Collectors
Contrary to common belief Cyclones are not an effective spark arrestor. For a spark arrestor/cooler to work, there must be turbulence to be effective. If you have turbulence in a cyclone pressure drop is very high. They are designed to avoid turbulence. Many bag house fires occur in systems with cyclone pre-cleaners. Amazingly the inlet baffles on the bag-house are more effective as spark arrestors, however they are not foolproof.

Static Baffle-Box Spark Arrestor
Many dust collector suppliers offer this type. It consists of air entering at one end of a baffle box running over a baffle plate which drops out the sparks and much of the dust collected. The air exits at the other end, and then travels to the dust collector. The big drawback is that a hopper and flexible or solid hose connection to a collection barrel is required. Also, these devices do not eliminate all of the sparks. There is not enough turbulence generated to ensure hundred percent spark arrestance. Sparks may ignite the contents of the collection bin

Mesh Filters
This is a common stop-gap measure where the filter is placed at the exhaust duct of hoods or installed in the duct-work. When clean, the mesh filter will stop at best 80% of sparks. These filters do not produce enough pressure drop to be fully effective. It only takes one spark to ignite dust in the duct or set a dust collector on fire. The only thing these filters do is clog up and add to your maintenance.

Blender Type Air Mixers
A number of these air blender/mixers have been applied successfully as spark coolers and suppressors. Over the last 5-6 years standard air mixers have been adapted and applied between the spark generating process and dust collector. They were applied in processes where fires in the dust collectors had previously occurred. One supplier hired a consultant to develop a market for these air blender/mixers as a spark arrestor/cooler. This blender design was an outgrowth of mixing two gas streams of different temperatures to insure a uniform temperature after the static mixer. It was deduced that the gas stream produced turbulent flow as it passed through the blades and this was the reason it could be adapted to spark cooling. However, these are air mixers first and spark arrestors second. There are performance limitations because not enough turbulence is imparted to the spark ember.

Improved Spark Arrestors
QAM developed the QUENCHER, which is a variation of the blender/mixer design. Employing a 60 year old spin vane mist eliminator technology developed by Sly Manufacturing in the early 1960’s, led QAM to vary the blade designs to have the most effective performance, inducing maximum turbulence to the gas stream, and lowering the cost. Maximum turbulence is the key to spark arrestance. After several tests it was found that the air blender/mixer design did not impart enough turbulence and some sparks got through, especially at low gas stream velocities. Eventually, there was a specific design which imparted the most effective swirling and turbulence thereby extinguishing the sparks quickly and most effectively. In fact, during testing of the QUENCHER, the arrestor cell would light up as a ball of fire, however, one inch past the cell nothing was left in the gas stream. These designs were incorporated into the QUENCHER. QAM has developed special application data in which the blade angles are adjusted to produce minimum pressure drop for different temperatures and gas densities. To our knowledge, no one else accounts for the gas density effects on spark arrestors. In truth, due to the advanced design, even applying the incorrect parameters to a QUENCHER may not result in a failure to put out sparks. Since the pressure drop across the blender and mixer are a function of the velocity through the device, the development of a pneumatically operated booster was introduced to prevent dust dropout accumulating in the static blender/mixer. It also blows out accumulations on the blades.
Read more: Quencher Spark Arrestor


Liquid Spray Systems.
For many years these systems were the only available systems to prevent fires caused by sparks. The system consists of electronic detectors that detect sparks and react to their presence. When a spark is detected liquid sprays are actuated and water sprayed into the duct. The sprays actually cool the gas stream below the dew point. However, in dust collection systems, the water then wets the filter bags or cartridges. This prevents fires but the gas flow is interrupted and the bags must be either replaced or dried out before the process can resume. The detector sensitivity can be lowered to prevent excessive actuations, but, this reduces the reliability of the systems. The detector missing a spark is an ever present danger and a fire may occur. Bag or cartridge replacement is definitely required.

Static Blade Spark Suppressor (Tri Pass)
These were developed in Japan to replace multiple cyclones in Coal fired boilers. They found that the multiple cyclones did not stop sparks from entering the dust collectors. The first ones were installed in the early 70’s. They ran at 1.5 inches of pressure drop and were fabricated from structure angles to resist the wear of the abrasive ashes in the coal that they fired. There are several of these applications installed in the USA designed by one of our colleagues.

We trust that the above information will enable you to evaluate and select the most suitable method and supplier for your application. Buying our QUENCHER/BOOSTER combination will give you a risk free unit, fine tuned for each application. 

More on... Dust Collection and Spark Arrestors

Spark Arresters Prevent Fires

Transport of sparks through ducts; Referring to the sketch below, there is a glowing ember (red particle) surrounded by some hot air (yellow envelop)) which gives the spark buoyancy. This spark (at approx 1400degF) and the hot gas (at approx 800degF) associated with it can travel hundreds of feet in a duct. The ductwork is designed to give laminar (smooth ) flow. This is illustrated on the left of the QUENCHER spark arrestor. Spark suppressors are placed in the duct to change the flow to turbulent (coarse) flow, as shown on the right of the QUENCHER spark arrestor. This agitation or turbulence strips the air from around the ember thereby removing the fuel (oxygen) and breaking up the envelop of hot air, therefore extinguishing and cooling the spark below ignition temperature (pink particle).
Prevention depends on eliminating the causes of ignition. Spark traps can change laminar to turbulent flow and extinguish any sparks in a duct. Design duct systems for dust transport velocities. Install a pneumatic actuated duct booster to flush dust into the dust collector. Use air jets to remove electrostatic charges on duct surfaces. 
Read more about... QUENCHER Spark Arrestors
Read more about... Booster - Duct Cleaner

View a video on the QUENCHER

Furnace and Dust Collector Fire Hazards

Fires in brass furnaces have always been a danger.

First let us review the process; as the material is fed into the furnace it has many metals including zinc, tin etc. Some of these actually go to vapor and then condense and turn into solids.

The key is that these metals are very fine with very large area to weight ratios. The exhaust is generally cooled by mixing with ambient air so the metals are not appreciably oxidized. The dusts collect, with the other dusts, in the dust filter cake.

When the collector is shut down the metals start to oxidize and the effect is like catalytic combustion. The oxidation produces heat. The dust is usually a good heat insulator and "hot spots" occur. Sometimes the temperature is high enough to start a fire when the flow was stopped. More often, when the collector is turned on, the initial flow fans the sparks and when conditions are optimum for combustion, a fire will start. Most of the time small holes or scorching can be noticed on the bags before a fire.

Sparks may occur as scrap is added to the molten metal. This is common when the scrap is oily.  The usual time to add scrap into the furnace is at the end of the shift when the collector is especially vulnerable to fires.

The approach to prevent fires is to extinguish sparks if they are present and to cool the hot spots when air is not flowing through the system.

To extinguish sparks the flow before the collector must be changed from laminar to turbulent flow. This is accomplished by installing a QUENCHER spark arrestor in the air conduit to the dust collector.

To keep the "hot spots" cool, my suggestion is to pulse the collector off line every thirty minutes or so for one complete cycle to cool the "hot spots". If the off-line cleaning is too frequent, the cake will be destroyed or damaged, so, the cleaning must be controlled.

When selecting a fabric pulse jet collector, high-ratio technology designs can operate at filter ratios of 16:1.  Cartridge collectors are not a good selection as the pleats may promote formation of the hotspots described above.

We first used this technique at St Joe Mineral, which was near Pittsburgh, 30 years ago, on their zinc oxide furnaces. We were informed that they were venting through an AAF pulse jet collectors. AAF has managed to put out some of the worst pulse jet collector designs in the Industry. From the description it sounds like a AAF FabriPulse. That collector if it is top access design has these venturies that wedge in the top of the bag. Using the American vernacular, it sucks. The purpose of the venturi is to seal the top of the bag with the cleaning jet. There are openings around the top of the bag below the wide part of the venturi. This, in effect, allows the jet to grow until the growth is stopped by the walls of the bag. That is an over simplification of the process, but it is a fact that it sucks. The net result is that the collector cleans poorly and there is a lot of dust that is forced into the surface and subsurface filter cake.

On any kind of brass furnace it is best to keep the dust cake porous and thin. As I explained previously, in a brass and other process, the zinc goes from vapor to liquid to solid and forms zinc fume. This zinc fume because of its large surface area to weight ratio can burn or explode quite easily.

We were involved in a legal action where the customer hired a man to change bags on a MikroPul collector venting a zinc dipping operation where they were coating pipes. The young man, after he was half finished (inside removal) sat on the temporary grate and decided to light up a cigarette. The collector exploded and then burned down. He was blown out the access door with the explosion and the sprinkler heads went on after the fire started and water poured over him as he was lying on the ground.

Since we were told that the fires started when the process flow continued we need to look at the source of ignition. If the ignition is caused by sparks, the best way to suppress sparks is by going from laminar to turbulent flow in the dust before reaching the collector, with a good in-line spark arrestor. The next source of ignition might be through the cleaning jet. The cleaning jet can supply oxygen from the compressed air and when it reaches the cake maybe sufficient to cause some sparks similar to small explosions to occur in the cake. This may ignite the rest of the fine fume fuel to start a fire. This would be very pronounced, if the collector was running at a high pressure drop with a dense thick cake and frequent pulsing.

We can attack the symptoms or the causes. One way to attack the symptoms is to limit the thickness of the cake. This can be accomplished by installing PTFE membrane laminated bags. Another way to attack the symptoms is to clean the collector with compressed nitrogen instead of compressed air.

One cause may be because of the atrocious design of the cleaning system. The way to remedy the poor design is to modify the cleaning system design. To implement the change we need to throw away the venturies and modify the pulse pipes so they can run without venturies. We can get the pulse pipes modified so they will induce more cleaning air per unit of compressed air, possibly lowering the formation of sparks on the bag surface. It would allow the collector to run at a lower pressure drop with less frequent pulsing.

I always like to look at how the operation of the collector interacts in the process of venting the furnace. 

Read More...  About assistance with dust collection applications.

Booster - Duct Cleaner

It is very common to come across airflow problems, in industrial ducting systems;

• Dust drop-out in the ductwork, causing high maintenance
• Blockages in pneumatic conveying systems
• Low air conveying speeds in ducting systems
• Fire and explosion hazards caused by debris igniting in the duct
• Dust accumulations in spark arrestors and other devices in the air stream

A very simple solution exists for remedying these common problems. It is called an Auto-Booster / Duct Cleaner.

   

The duct booster is a pneumatically propelled jet generating system using the same jet pump design and components as are found in advanced technology pulse jet dust collectors. It is like having a booster fan in the duct system, with no moving parts. It will increase air speed in ducts by 3000-5000 feet per minute for short bursts of time. This will pick up the dust lying on the bottom of the duct and push it along to the dust collector. The air jets also remove electrostatic charges on the duct surfaces which are a source of ignition. When averaged over a day’s operation the cleaner need not be actuated except once in every one to four hours, and therefore air consumption is negligible. The Booster is usually powered by shop air at 85 PSI. Optional supersonic nozzles can be added to the blow pipes for more efficient pressure-to-velocity conversion.

It can also be designed for various low air pressures from 7-20 PSI, thereby allowing operation where shop air is not readily available.

The duct cleaner can be actuated by a manual push button or using the output from one of the positions on a pulse sequencer controlling the cleaning cycle of a dust collector.

Read more about... the BOOSTER - Duct Cleaner

Fires - Smelting Process

First we must be careful to find out if the fires are a result of ignitions by a spark. All these acids mixed with carbon can spontaneously ignite, especially if the precious metals include catalysts, such as vanadium, like they use in catalytic converters. Or, they get fires when they are in the ten hour mode? My calculations indicate that during the ten hour precious metal mode they are basically diluting the exhaust to meet discharge requirements into the atmosphere.  To discharge from the hood at 125 degrees they either need a lot of dilution air or water vapor. For 30 hours a week, they use it as a trash burner. Here again they seem to be trying to hide the true nature of the operation. In the trash burning 30 hours they probably have wild swings in emissions which might ignite poorly combusted components and would be ignited by sparks in the baghouse. I would hate to live downwind during either operation. You need to find out the following.

1.  Do the fires occur while operating?
2. Do fires occur when they are shut down?
3. What are the temperatures in the ducts over a 60 hour period?
4. If they run 8 hours a day do they run 2 hours on smelting mode and six hours trash mode?
5. Do they insulate the duct?
6. What is the charge to the smelter? Coins? scrap gold, scrap silver? perhaps catalytic converters?.
7. It seems like they would have a severe corrosion problem and preheating of ducts should be considered.

We can put out sparks with Quencher^tm spark arrester but we cannot remedy poor control.

The ten hour mode is the ten hours per week when they are smelting. The 30 hour mode is when they are trash burning. Get a piece of bag to examine. It will be obvious if it has been chemically attacked. You should be able to tear or burst a piece with pliers and a vise. 

For more on spark arresters, see  QUENCHER spark arrestor

Wood Fired Boilers

There are many conditions that lead to fires in wood fired boilers. The engineering solution requires recognizing the factors that contribute or cause these problems. Most are related to the combustion process and that the wood composition varies widely:

1. The emissions from the boiler consist of dust and gases that are not completely burned. These can be ignited by sparks coming from the boiler. The loading of these pollutants can vary widely. Gases may continue burning with a flame, and, sparks may be in the process gas stream. 
2. If we install a spark arrestor, such as a QUENCHER, before the dust collector, it will cause the gas pollutants to burn producing heat, carbon dioxide and some small fraction of water vapor. The QUENCHER will also prevent any sparks from entering the dust collector as the hot ciders will be immediately cooled to the gas temperature, as the air goes from laminar flow to turbulent flow and back to laminar flow. We now have cooled dust and ash entering the dust collector. The pulsing action of the cleaning system will fan any red-hot cinders if they are present. For reasons beyond the scope of this report, the dust cake will be dense and subject to ignition. Fortunately, conditions must be in a narrow LEL/UEL range to start a fire. With conventional pulse cleaning systems only a fraction of the filter cake on the bags is functional and the rest of the bags are plugged. 
3. We would modify the cleaning system with modifications that would enable the full surface of the filter media to be active. This would mean that the inventory of flammable dust would be reduced by over 95% so there would be no combustible dust between cleaning pulses. Surprisingly, the cleaning frequency would also be reduced. To accomplish this, we would need information regarding the design of the existing dust collector.

When completed this would be the best design, and, barring unforeseen circumstances such as power failures at inopportune times, the system should perform flawlessly for years. The only risk is that the filter bags could be attacked chemically. 

More information on... retrofitting existing dust collectors 

Combustible Dusts

(Aluminum, Magnesium, Niobium, Tantalum, Titanium, Zirconium)

These dusts are highly combustible and present a very significant explosion hazard. There are some stringent fire codes dealing with these dusts which draw their regulations, for the most part, from NFPA 484, Standard for Combustible Metals.

Unfortunately, most end-users are not aware of these standards or safe methods of dust collection for these dusts. Worse, the dust collection industry is very negligent in guiding these people with proper and safe applications engineering. This document is an attempt to provide some of this valuable information.

First of all, we strongly encourage readers to obtain a copy of NFPA 484 and comply with it. There are far too many stipulations which go beyond the scope of this document. A copy of excerpts pertaining to each type of dust can be requested from Quality Air Management.

We will analyze the use of different dust collection methods to these dusts:

1. Dry Dust Collectors; include baghouse (both mechanical cleaning and pulse-jet self-cleaning), cartridge or pleated filter collectors, disposable media filters, electrostatic precipitators, cyclones.
2. Wet Dust Collectors; there are many styles of wet collectors available. The pro’s and con’s of each type is beyond the scope of this document.

In all cases the blower for drawing the dust-laden air into the collector shall be located on the clean air side of the collector. The dust producing equipment and dust collector must be

Mixing of Metals is not permitted, unless the entire system is disassembled and thoroughly cleaned prior to and after its use. A placard must indicate, for example, “Aluminum Metal Only - fire or explosion can result with other metals”.

Wet collectors are designed specifically to be used for all these dusts. These collectors are designed for collection of metal dust only, not for powder, smoke or fumes. The use of additional dry filter medium either downstream or combined with a wet collector is not permitted. Contact QAM technical support for a safe method to handle these contaminants which the wet collector can not handle. The cleaned air can be recycled to the work area if the collector is efficient enough to ensure safety of personnel. A provision for an unimpeded vent, when the machine is shut down, must be provided. Magnesium dust requires a powered positive venting of the sludge tank at all times during shutdown of the collector.

Dry collectors are allowable for aluminum, niobium  dusts, but are prohibited for all the other dusts. They must be located outside buildings. Filter media must dissipate static electric charge (be aware that grounded conductive media gives a false sense of security). You must avoid accumulation or condensation of water at all costs which could cause a hydrogen gas explosion. Explosion vents must be provided. Recycling of air into the building is prohibited.

Mechanical shaker style collectors; are highly susceptible to static electricity charges, and explosions.

Baghouse collectors; there are conventional designs, sold by 95% of dust collector suppliers, and new advanced technology designs.
Conventional; Due to the inefficient clean systems, only 10-20% of the filter media ever gets clean. This allows dust to accumulate in the collector beyond what is permissible by NFPA 484.
Advanced technology (i.e. Ultra-Flow); These are designed to clean 100% of the media on a regular basis. The cleaning frequency can be set to maintain a cleanliness that meets NFPA standards.

Electrostatic Precipitators are prohibited because they filter the air by applying an electrostatic charge across the air stream. That is a source of ignition and the dust will accumulate in the unit and coat the collection plates. This is a prescription of a very large and very load BOOM (explosion).

Cyclones; high efficiency models can be used for these dusts but must be located outside the building. Explosion vents are permitted. Recycling of air into the building is prohibited.

Read more on ... Wet dust collectors

Wet Collector Underperformance

Equipment:
C5-2500, orifice scrubber style wet dust collector, rated for 2500 CFM, purchased to handle explosive aluminum dust particles.

Problem:
The dust was going right through the collector and packing into the fan / outlet compartment. Very little dust was collected in the dust collector sump.

Investigation and observations:
We requested pictures and system layout drawings (sketches were actually provided). From these we observed that the client did not describe the application accurately at the time of purchase.
1. The dust was produced from a spray coating operation. Therefore, it was fine powder type aluminum dust. Wet collectors are designed for metal dust 5 microns and larger, as generated from grinding and cutting operations.
2. The inlet was connected to a properly sized 8” duct but was over 20 feet long with three elbows. These units are designed for maximum 10 feet of duct directly to the collection point.
3. The client also decided to exhaust the discharge of the collector to the outdoors with another 20-25 feet of duct, and three more elbows. These collectors are designed for an open, unrestricted discharge on the top of the unit.
The result of this was a questionable capability of collecting the powder type dust. The biggest problem was that the collector performance was choked by far too much resistance to airflow in the installation. By doing this the air entered the collector with far too little volume to cause the necessary turbulent energy in the “omega” style baffles. The necessary wetting action of the dust particles was not taking place and filtering action was non-existent.

Solution:
1. We asked the client to place some of the collected dust in a closed jar with water. Then shake it and let it sit for a few minutes. If the dust settles, it can be collected. If it doesn’t, a different dust collection solution must be found.
2. A wet dust collector is very particular about the airflow through it. You need to be in a range of +/_ 10% of the rated CFM for that model dust collector. In this case, the minimum flow that could be tolerated is 2250 CFM. Conversely, with more than flow than the 10%, water gets drawn up too much and discharges out the unit. We recommended bringing the collector closer to the application and remove the duct on the outlet. Alternatively, add a booster fan to overcome the restriction.

Other Considerations:
A. If the excess resitriction is minor (within the 10% range) but dust is discharging at the top, You would add more water to the collector, in small increments, until the dust/water stops coming out the top. The added water compensates for the higher restriction. Then reset the float control to maintain this new water level.
B. In some cases the discharge is required to be exhausted by code. An example is beryllium. In such a case, do not attach a duct on the outlet. Instead build a capture hood over the top of the wet collector outlet, approximately 4-6 above the top, and duct that to the outside. Install a fan to give you 5-10% more air flow than what is running through the collector, to ensure no contaminents escape back into the room.
C. If you oversized the wet collector, Do not restrict the flow with a damper more than the 10% tolerance. Install a bleed-in on the inlet duct with an adjustable shut-off damper. Open the damper to the point where the collector performs properly. This is often a scenario when you size the collector for multiple collection points but don’t have them all installed until later.

Need help? ... System trouble-shooting

Explosion Vents

This is a touchy topic and greatly misunderstood. Today, plant operators recognize that there is more danger from lawyers, on this issue, than from actual explosions in the dust collector. In fact, the accumulation of dust in the plant itself is a greater danger for conflagration than a properly engineered dust collection system. Go to our website and view the videos on “Combustible dust in the workplace” by 60 Minutes on CBS and the U.S. Chemical Safety and Hazard Investigation Board.

Go to view .... Combustible dust videos

Explosions in Dust Collectors

Explosions in pulse jet collectors invariably are when cleaning off-line and can be prevented by sound practice. We run into a risky process when we write the safe procedure because it is application dependent and relies on common sense of the operators. If your design is good engineering, there will be no explosion. All explosions in pulse jet dust collectors, we have investigated (about 100 or so), have been clear cut stupidity. Among the typical ones; horseplay, ignoring warnings posted on equipment and disgruntled employee sabotage.

The most risky application is a mechanical cleaning (shaker style) dust collector when cleaned off-line, and, you can only clean them off-line. A dangerous spark is generated by static charges produced during the shaker action. If I were there, I would tell someone else to turn off the collector or put a long delay on the shaker actuator while I go to the restroom. There is no particular reason to warrant us to observe an explosion first hand. I am cowardly since I heal from injuries slowly.

Advanced Technology vs Poorly Designed Dust Collectors

An overwhelming number of explosions have occurred on badly designed collectors with bottom inlets in which the fine dust has difficulty in making its way to the hopper until the fan is shut down. Advanced technology dust collectors (such as ULTRA-FLOW), with their high side inlet and high ratio cleaning system, have some marked advantages that further reduce the risks involved in explosions. One major advantage is the extremely high efficiency of these designs which prevents dust being returned to the plant, thereby reducing the hazard of accumulated dust referred to the introductory paragraph of this bulletin. The cleaning system more thoroughly cleans the bags and the inventory of dust on the bags is very low, and usually not sufficient to cause the dust concentration to go above the lower explosive limit in the event of an explosion front traveling into the collector from the inlet ducts. With these collectors, the fire and explosion generally occurs outside the collector, in the ductwork, and is drawn into the collector. In normal operation there is only a very small part of the collector that passes through the lower explosive limit. This consists of a narrow band about 1/8 to 1/4 inches thick that surrounds the bag when it is cleaned. Eggshell or singed finishes on the filter bag is recommended to further reduce dust inventory on the bags.

Placing the explosion vent below the filters (i.e. in the hopper) is a bad idea. It allows the pressure to build up in the housing before it can be released by an undersized vent in the hopper. This is especially true with conventional dust collector designs that have venturis restricting the neck of the filter cage at the tube-sheet. With advanced technology designs (having no venturi), we have 12-15 times more open area to the outlet which in itself is a natural explosion vent. We place the vent in the housing side where it offers the most protection by venting the explosion immediately where it occurs.

Woodworking

Because of the NFPA rules do not directly apply to dust collectors, there is much latitude in their interpretation. The solution is to apply sound engineering to assess the risk and to provide equipment suitable for a particular service. Venting woodworking applications is probably the largest number of installations in the dust collector industry. Explosions have occurred and the venting has been quite effective in controlling them.

The norm in the industry, for the last 30 or so years, has been to provide a 60:1 vent ratio. This has been sufficient for this service. ULTRA-FLOW uses a standard 20:1 vent ratio for its explosion vents, which further protects against the harmful effects of an explosion.

Vent Ratio

This was developed by UL labs. It is the ratio of the volume of the dirty air compartment of a dust collector to the area of the explosion vent. For example; a cylindrical bag dust collector with (24) 6 inch by 6 foot bags, dirty air housing size of 6ft x 4ft x 6ft, hopper which is 1/3 x ( 6 x 4 x6 ). The gross volume of the collector = 192 cu.ft. The volume of the bags is 24 x 1.2 cu.ft./ bag = 28.2. The volume of the collector = 192 – 28.2 = 164 cu.ft. Therefore, if we want a vent ratio of 20:1; 164/20 = 8 sq.ft. of explosion vent.

Ultra-Flow dust collectors use a vent ratio of 20:1. In general the insurance companies determine the specification that they want and we supply it accordingly. In the end, good engineering is the key.

Kst Ratings

This issue is very complex and not as easy as just meeting a “Kst” deflagration rating. It is an NFPA 68 test requirement for ideal lab conditions. “Kst” refers to the rate of pressure rise in an explosion. Unfortunately defining of the number is difficult since NFPA never really measure it except when they use a sealed globe enclosure, stir the dust in it and then try to ignite it with a sparkplug. This is not the real world of dust collectors.

A more accurate test was performed by AAF specifically on dust collectors. See the “Combustible Dusts” chart at the end of this bulletin. That chart shows the “Explosion Pressure” or burst pressure where theoretically a dust collector will blow apart in an explosion. If a dust collector is built of 12 gage steel to withstand +/- 20 SP (inWG). The burst pressure is usually a factor of 4 times that or 80 psi. As an example, for wood dust it was determined that a vent ratio of only 180:1 was safe in a dust collector. The chart says the burst pressure would be 35psi which is less than the 80psi allowed. Ultra-Flow uses a 20:1 vent ratio, therefore it is 9 times that value, so, you are as safe as you can get. No matter what you do, there will always be some risk. All we can do is make it inconsequential. As mentioned above, there is a far greater risk from dust in the plant than you will find in the dust collector itself. Look on the home page of our website for the news reports on “Combustible Dusts in the workplace”.

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