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.

Sound Engineering Basis for New Technology

Design flaw #1 for conventional designs:

Conventional designs with cylindrical bags propel the dust from the rows of bags in process of being cleaned toward the adjoining rows in the filter mode. This high speed jet (between  350 and 400 ft/sec) drives the dust through the filter and filter cake, partially blinding the bags and reducing dust holding capacity by 80-90 percent with dense dusts. To operate at reasonable pressure drops, the potential filtering capacity of the bag is reduced by up to 80%. This high velocity dust also raises outlet loading above 100x10^-4 grains per cubic foot.

The new technology design reduces the exit velocity from the bag to between 190 and 250 ft/sec depending on gas density. This keeps the permeability of the media plus filter cake to a few percentage points higher than a new bag. It typically holds several times more dust between cleanings, even at filter ratios of 15 to 20, compared to conventional designs.

Design flaw #2 for conventional designs:

The filtering capacity of the filter element is limited by the reverse air volume generated by the cleaning system. The reverse air volume is also based on the diameter of the venturi at the entrance of the bag. This, for a four inch by 1.875 diameter throat bag is only 20% of the area of the opening at the top of the bag.

The new technology removes the restrictive venturi used in conventional designs and opens up the opening by 4 to 5 times. This increases the cleaning volume while reducing the pulse jet speed by 3 to 3.5 times. Half of the bags are removed and replaced with new bags and cages with the venturi eliminated. The rest of the bag openings are plugged and no longer used.

Other considerations

When these changes are made, the fine dust which formerly bled to the outlet is collected on the bags and ejected to the hopper. Because it is so fine, the vertical flow entering to the bag compartment, from a hopper inlet, would prevent this dust from falling into the hopper. This is the effect of upward “can” velocity.

The retrofit design removes half the bags from the collector. The dusty air enters from the bottom and also through the opening in the center of the bag compartment. This reduces the upward can velocity coming from a hopper inlet to a level 70 - 80% less than before the modification. Now the fine dust falls into the hopper unimpeded. It is equivalent to putting a high inlet in the center of the collector.

95% of the time, the collector will pass the initial engineering review. A report will be issued for your approval, before any fabrication of components begins.

A normal compressed air requirement, for contemporary designs, is 0.9 to 1.2 SCFM of compressed air per 1000 CFM of filtered air. Predicted for advanced technology designs is only (0.328 x (0.9 to 1.2) = 0.3 to 0.4 SCFM per 1000 CFM of filtered air.

Based on an average system requirement of 10 inches water column, a two inch reduction in pressure drop across the dust collector would reduce power consumption in the exhaust fan by 20%. 

Read More... New Advanced Technology Dust Collectors

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

Welding Fumes

Venting welding fume operations poses some difficult application decisions. Welding is different now than it was even five years ago. Welding is welding, the difference primarily is the coating or lack of it on the metal to be deposited. Anti-spatter is putting the coating on the welding rod from the bottom.

In fact, many suppliers refuse to bid fabric or cartridge collectors for this application.

Electrostatic Precipitators:

Years ago, the preferred method of collecting weld fume was with two stage electrostatic precipitator dust collectors. These had several advantages, they were relatively compact and were generally very effective on general ventilation applications. They could handle relatively large gas volumes through the collectors and generally were located near the roofs of buildings. The collection efficiency was variable depending on the velocity going through the collection plates. The same unit could run at 98%, 90% or 80% collection efficiency at volumes that would vary as much as a ratio of 3:1. The higher the volume would produce the lowest efficiency. On general ventilation lower the efficiency did not cause any problems. The cleaning of the precipitators was accomplished by a detergent wash system. The loading for general ventilation units were from 0.5 to 1 grain per thousand cubic feet per minute. The washing frequency was typically once a week or once every two weeks. The presence of condensed hydrocarbons along with the fume was not a problem. Generally these would be oxidized into solids by time that they were collected. These collectors were generally the same ones that were applied in HVAC systems. The washing mechanisms were designed for 1000 cycle life which would translate to over ten years of life under these general ventilation loading conditions. As the welding operations generated heavier loads, especially in high production facilities, the trend was to hood the welding operations. This venting of the hoods had some pronounced effects on the application of these precipitators:

Effects of hooded systems

1. The load to the precipitators was increased to levels that varied from 5 to 20 grains per 1000 cfm. This was 10 to 40 times as high as the general ventilation requirements.
2. The distribution of air across the intake of the precipitators became critical.
3. Lower efficiencies; The precipitators, because of competitive pressures, were sized for the higher volumes and lower efficiencies

The increased loading meant that the life of the washer systems became 1/10 to 1/40 of that of the general ventilation units. The translated into washer units that had less than six months service life. These HVAC service designs also had problems with coatings building up on the high voltage insulators so they required replacing at about the same frequency as the washers.

The gas distribution across the collection plates radically affected the collection efficiency. If we had a gas stream that averaged 100 fpm that would be designed to operate at 95% collection efficiency and the real velocities entering the plates varied from 50 to 150 fpm, the section at 50 fpm might have a collection efficiency of 98% and the section at 150 fpm would be running at 80%. This would mean that the overall efficiency might operate at close to 85%. Lower efficiency caused by running at higher average velocities were common. In the example above, the collector might be selected to run at 125 fpm and an average efficiency of 85%.

The electrostatic precipitator ionizes the gas molecules and in the process places a charge on all of the dust particles. The charge attracts the dust to the grounded plates. Later these collecting plates are cleaned to remove the dust from the unit.

Running the collectors at these higher loads and decreased efficiency allowed some dust to pass through the precipitator while the dust still retains its charge. As the exhaust gas mixes with the ambient air the dust usually loses its charge and the gas stream loses it’s ionization. The length of time required to lose its charge depends on the dust loading in the exit gas stream as well as the humidity of the ambient air. Dry air will allow the charge to be retained longer. Under certain conditions of low humidity and higher dust concentrations, the dust will hold the charge indefinitely. Suddenly the whole room will be ionized and the charged dust will be attracted to all the neutral surfaces in the room including walls, eyeglasses worn by personnel, windows and even light bulbs. This phenomenon is known as “plating”. There have been occasions where a newly painted plant was coated with black soot within minutes. It is because of these circumstances that welding fume collection was changed to pulse cleaned dust collectors.

Cartridge Dust Collectors:

There are thousands of cartridge collectors in service on these applications

However, there were big differences in power requirements, cartridge life and penetration of dust through the collectors: These were due to the following :

1. Cleaning controls: The much used pressure drop actuated cleaning control has some serious flaws. It is impossible to predict the pressure drop at which the most effective filter cake is formed. It is likely, that this pressure drop may be just slightly higher than the initial pressure drop. If the cartridges are not cleaned early enough the result is that the dust will bridge across the pleats and a large percentage of the filter media in the cartridge (that which is below the bridge) will be unclean able. Initially, the collector should be cleaned by a timer control until the pressure drop stabilizes. The control can be extended until the pressure drop starts to rise and then the timing speeded up slightly. Alternatively, the pressure control can be set slightly above the stabilization point. The best operation occurs when the collector runs at the lowest possible pressure drop. Penetration of dust, cleaning frequency and cartridge replacement time, increase with pressure drop, usually related to the square of the increase in pressure drop.
2. The filter area of the cartridge can be and often is excessive. The media that can be cleaned by the cleaning system is the only media that is useful. Often, suppliers cram more media into a given size cartridge by inserting closely spaced pleats. This cramming of media provides useless media and causes operational and structural defects. If pleats are close together, the adhesive that holds the cartridge together will not wet the media and a strong bond cannot be developed between the end caps and the filter media. A poor bond may allow leakage between the media and the adhesive. If the filter cake is deeper than the open distance between the pleats the cartridge will bridge. Typically cartridge filtering ratios should be no less than 3 FPM, and generally will operate at 4-7 FPM. Most systems operate at lower filter ratios where pressure actuated controls do initiate frequently enough to clean the cartridges effectively. Conventional cleaning systems may not allow for recommended filter ratios of 4-7 FPM. Advanced technology collectors will run at even higher filter ratios and clean the media more effectively, promoting higher efficiency and longer cartridge life.
3. The cartridge shape should be flat and cylindrical. Truncated cone bottoms, formed into the closed end cap, causes damage to the filter cake allowing excessive penetration through the dust collector. The cone develops more cleaning pressure near the bottom than in the rest of the cartridge. Premium construction cartridges are often necessary to develop maximum cartridge life and improved collection efficiency.

Oil-Smoke:

The above applies to welding of clean parts. Recently, there has been a trend to vent from welding operations where the parts are coated with a light film of oil.

A mixture oil and powder can produce oil based paint. This oil/powder mixture can progressively blind the cartridges and produce premature cartridge failure. When you collect oil on the cellulose cartridge, it acts like a blotter and the media swells, permeability and available collection media is reduced. This latter phenomenon is in at edition to the painting action. With some hydro/oleo phobic media, that is specially coated, the oil will not wet the fibers and the media will not swell. Some of these cartridges can be recovered by washing them in detergent, but over a period of time the oil starts to dry and a solid is formed which cannot be washed. Typically after two or three washings the cartridges must be replaced, as there will a creeping higher initial pressure drop.

Usually, the welding fume load is extremely light and with the special fibers, the cartridges can last for several months before they need to be washed or replaced. Actually even cellulose based fibers can be washed successfully two or three times, but the time between washing goes down for each wash as the residual permeability decreases after each washing.

There is some promise in using a precoat (filter aid) of some dust on the media to blot some of the oil so a paint will not form. There are some commercially available coatings that are promising. The choice will be empirical.

Stainless Steel

This is one of the most deceptive applications we can deal with. Most of the time, you will believe that the material has not been pickled with oil or another preservative. The metal looks perfectly dry. Some manufacturing processes will prewash the stainless, thinking they are removing any residue that may be there. When you well donned the material, to everyone's surprise, a residue appears at the dust collector filter elements. A way to determine this is to soak the filter elements in a tank and observe a film of oil form on the surface of the water. Another test would be to take a folded piece of tissue paper and squeeze it between the pleats of the filter element for 24 hours. Automatic pre-coating, or seeding, of the filter elements is a solution to this problem.

Weld-aid (anti-spatter)

Welders often use “weld aid” type of substance to prevent spatter from adhering to the work piece. It is mostly water with a 15% fatty substance. It is usually sprayed on the work piece before welding. This is a very greasy substance that is difficult to remove from the cartridge media and fills in the pores. You should have a washable media, such as polyester spun bond, preferably with a non-stick surface. All paper, cellulose formulations are unacceptable media selections because water gets absorbed and the media swells. Do not use exotic medias that have low permeability. It is recommended to install an automatic filter-aid feeder, such as the one designed and sold by QAM. You also should consult a welding expert to change the welding process, which will eliminate the necessity of using weld-aids or change to a powdered version.

Automatic seeding system

With a special design and a seeding system, it can last indefinitely. A layer of inert absorbent dust is fed into the collector. The collectors not pulsed except twice a day. The inert layer blots up the oil fume and the oxides from the welding are also mixed on the surface. There is an airlock, usually the smallest kind available which feeds the dust back into the bags after the collector is restarted or even pulsing is done while the collector is operating. The ULTRA-FLOW bag-house can run at 18:1 ratio and is probably less costly than the cartridge collector for the application.

Read more on ... Ultra-Flow Baghouse dust collectors

SPARKS & FIRES

Sparks are a very common occurrence in the welding process. These sparks are swept up into the collection hood or device and are transported to the collector. Fires can occur in exhaust ducts as well as inside dust collectors. Requirements of fires or any combustion process are: a) Fuel, in gas, liquid or solid form. b) Oxygen (Atmosphere consists of 20 per cent oxygen) c) Fuel must be raised to the ignition temperature to start burning.

Transport of sparks through ducts.

There is a glowing ember surrounded by some hot air which gives the sparks buoyancy. This spark and the hot gas associated with the spark can travel hundreds of feet in a duct. The ductwork is designed to give laminar (smooth) flow. Spark suppressors are placed in the duct to change the flow to turbulent (coarse) flow. This agitation or turbulence strips the air from around the ember removes the fuel (oxygen), therefore extinguishing and cooling the spark below ignition temperature.

Read more about ... Spark Arresters and Coolers

Prevention depends on eliminating the causes of ignition. Spark traps can change laminar to turbulent flow and extinguish any sparks in a duct. Design for proper dust transport velocities. Install pneumatic actuated duct booster to flush dust into dust collector. Use air jets to remove electrostatic charges on the duct surfaces.

View and print... fires and explosions

Cartridge Collector on ARC SPRAY

The first suspicion for pressure drop problems on arc spray is that the spray is coating the cartridge, since the compounds have not lost there coating characteristics before they reach the surface of the media and seal or partially seal the surfaces. This is referred to as “painting effect”.

The symptom of this is that the pressure drop goes up quickly within 20 minutes. It does not recover from on line cleaning. The pressure drop for off-line cleaning quickly ratchets until the you get to the top of the hump in the fan curve.

We ran some tests on an operation in Atlanta about five years ago where we installed a pilot unit with varying lengths of pipe and found on that operation that it required 0.8 seconds of transit time from the spray gun to the surface of the cartridge filter elements, to prevent “painting”. Other reports that we received from Torit installations reinforced our conclusions. They sprayed aluminum, titanium, nickel, and ceramics. They were coating turbine blades for jet airplanes.

The observation of very fine dust floating in the collector, when the fan was off, can be very significant. It would be important to observe whether the same was evident in the clean air plenum. If it was, I would want to question the efficacy of the filter seals. The next possibility that this observation raises is whether the dust agglomerates on the media surface and/or stays agglomerated during the cleaning cycle. If it does not stay agglomerated during the cleaning cycle the coarser agglomerates will fall into the hopper and the finer fractions will return to the surface of the filter media. This will cause a ratcheting of the pressure drop at each cleaning cycle.

It was also observed a gradual increase in pressure drop. This can be caused by the lack of agglomeration as described above. However if this is the cause, the collector should return to initial pressure drop after an extended cycle of off-line cleaning. We would say that you need to put fifteen minutes off time and pulse the collector four times.

There are other processes that might have some of the symptoms. Among them:

Oil in the compressed air. If they have a screw compressor there is a filter that comes with the compressor. Sometimes the installer forgets to put it in. With pneumatic cylinders, the air line cleaners are sufficient to allow suitable operation.

Moisture in the compressed air line. Here again they should have a refrigerant or a desiccant dryer.

Condensation on the media surfaces because of the cooling of the cleaning air jet. If the latter is the case, the pressure drop increase will generally be apparent more likely in the morning when the difference between the dry and wet bulb temperatures are most likely to occur.

Replacing a third of the cartridges is generally not a good idea. It is also important which ones you replace. If they are tandem cartridges I would replace one on each tandem set. Personally, if I were to replace cartridges to run a test I would replace the ones furthest from the pulse-jet orifice/nozzle. When you do that, you do not even need to initially clean off-line. I figure you are wasting most of your media in a tandem configuration so when you replace cartridges on a trouble job, it is smart to replace only the outer one.

We need to determine if the cartridge is plugging from the dirty side or the clean side. We assumed it was from the dirty side but the clean side is a possibility. We usually check to see if there is any dust in the clean air plenum. If there is, the problem may be the seals. You may need special seals for this fine submicron dust.

You can always send us a used cartridge for a lab test. Make sure you seal the opening(s) to the clean air side. I usually use good ol' duct tape.

For assistance with a trouble job... Technical assistance with dust collectors

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