Welding Laser Plasma Arc Processes Causes Major Problems

Welding laser plasma arc processes include welding fume capture, plasma and laser cutting tables, thermal spray operations. Each of these present unique and difficult issues that will cause a dust collection system to fail.

Welding

Two Stage Electrostatic Collectors; Venting welding fume operations poses some difficult application decisions. 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.

Efficiencies of general ventilation; The collection efficiency was variable depending on the velocity going through the collection plates. The lower the velocity through the collectors the higher was the collection efficiency. The same collector might have an 80% collection efficiency at 6,000 CFM and a 98.5 % efficiency at 1,000 CFM The same collector could be applied to different exhaust volumes that would vary as much as a ratio of 6:1. The higher the volume would produce the lowest efficiencies. But the same air would be re-circulated and an acceptable level could be maintained in a particular room or building. The cleaning of the precipitators were accomplished by a detergent wash system.
Loading for general ventilation; The loading for general ventilation units were from 0.1 to 0.5 grain per thousand cubic feet of volume. The washing frequency was typically once or twice a week. The presence of condensed hydrocarbons along with the fume was not a problem. Generally these would be oxidized into solids by the time the filter was washed. These collectors were generally the same ones that were applied as air filters in HVAC systems. The washing systems were designed for 1000 cycle life. This would translate to over ten years of life under these low loading conditions.

Hooded Systems; The trend was to hood the welding operations. The venting of hoods had some pronounced effects on the application of these precipitators. The load would vary from 5 to 20 grains per1000 CFM.

Effects of hooded systems; Usually the washing requirements were to wash the filter every shift or twice per shift because the load was so much higher. On a two shift operation, and washing twice per shift, the washing system had a life expectancy of less than 52 weeks.

Plating; It was necessary to operate particular precipitators at lower volumes with their associated higher efficiency, because of a phenomenon called “plating”. Referring to figure 1, the precipitator will ionize the gas and the welding laser plasma arc particles. As the dust passes through the precipitator it forms a bubble type shape, containing charged particles, which were not collected on the collection plates. The gas quickly loses it’s charge. However the dust that was not collected keeps it’s charge a little while longer and loses its charge as it leaves the boundary of thebubble marked “A” in figure 1. If the precipitator has a low efficiency the bubble is much bigger as marked by “B”. This low efficiency bubble is 2 to 20X as bigger in volume than the high efficiency bubble.
Under certain atmospheric ambient conditions, this low efficiency bubble starts to grow rapidly until the whole room atmosphere is ionized and the room and all the contents become collection plates for the dust. The dust s attracted to the walls, windows, machines, eyeglasses and every object that is grounded. All the surfaces turn blackened within seconds. There have been cases where this happened after the walls were painted white. After the atmospheric conditions go back to normal the plating stops.

Bad Inlet conditions. All precipitators either single or two stage need even velocity distribution across the plates. If we had a gas stream averaging 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%. This condition would cause phenomenon described above

Lower Efficiencies, caused by running at lower average velocities were common. In the example above the collector might be selected to run at 125 fpm and an average efficiency of 85%. Plating may be produced because of these lower velocities and lower collection efficiency. As a result, many collectors were purchased based on volume and the supplier’s guaranteed higher collection efficiencies. There were practically no way to specify the collectors except based on supplier claims. Velocity was not a good criterion. Some collectors at relatively higher velocity and longer sets of collection plates would achieve the same result as a collector with short plates and a slower velocity.

Electrical Controls; To further aggravate the problem improper electrical controls were offered. Some operators interpreted SIC controls to mean arcing was not allowed. To eliminate the arcing across the plates, they lowered the voltage controls to halt this sparking. Unfortunately the voltage was lowered so much that the particles were not charged. There were cases where the metal pre-filters were more efficient than the precipitators.

Insulator Coating; The main collection plates are at ground in a two stage electrostatic precipitators. The charged particles will be attracted to the lower voltage intermediate voltage level plates and to the grounded collection plates. The insulators were also at ground level and some of the dust (a very small percentage) stuck to the surface of the insulators. This was a very strong bond and the spray cleaning systems could not keep these insulators clean. Eventually they were coated badly enough that the power supply could not keep the charging electrodes to ionize the gas and the particles. To correct this problem required a major overhaul of the precipitator. The charging electrodes were made of very fine wires and would eventually break and require replacement. Most electrical maintenance men were not familiar with high voltage supplies and maintenance was neglected.

Innovations in Design; In the late 70’s, two stage precipitators with pressurized insulators and more rugged washing systems were introduced. The insulators were subject to a gas stream that entered the collection compartment at a higher velocity than the collection velocity across the plates. This protected the insulators from charged particles. The charging electrodes were made heavier to give much longer charging electrode life.

These innovations increased the collection costs with electrostatic collectors to the point where cartridge dust collectors introduced at the same time were more economical to purchase and operate.

The advantages of the electrostatic collectors were:

1. The pressure drop was constant and usually low.
2. They could collect liquid droplets.
3. They had the potential of long periods of service without maintenance.

Cartridge Dust Collectors

With the development of cartridge collectors, another method of collecting fume dusts became available. The standard design pulse jet fabric collectors with cylindrical bags did not work because the cleaning systems propelled dust through the cake of adjoining rows of bags during the cleaning cycles. In the late 70’s and early 80’s thousands of cartridge collectors were applied to both hooded and non-hooded ventilation systems.

Problems developed in many systems after the mid eighties. High-pressure drops and short cartridge life developed in many systems. The causes were one or more of the following:

The presence of thin films of oil on the surface of the parts that were welded. When electrostatic powder coating finishing systems were widely applied to reduce or eliminate hydrocarbon generation from paint systems, the faces of steel parts required protection from oxides on the surfaces. During the welding process condensing hydrocarbons were liberated and swept up into the ventilating systems and their associated collectors. One of several results followed:

a) The solids to liquid ratio was so high that the dust blotted the liquids and the collection system was not affected by the liquid droplets.

b) The solids to liquids ratio was in range where the powders and hydrocarbon mixture formed a paint and the collection media was gradually plugged. This could take days, weeks or months, but the net effect was the cartridges had to be replaced or laundered pre-maturely.

c) The solids to liquid ratio was so low that liquid wetted the cartridges and they were plugged as in (b) above. Even in cases where the coating was barely discernible, this could occur.

A case in point was in a plant making stainless steel mufflers. The metal was washed after forming and the load in solids was 0.02 grains per 1000 CFM, the pressure drop rose in a six month time period. The re-enforced cellulose media would be air-dried and the pressure drop would be reduced from six inches to 0.3 inches after the elements were installed. After 4 months the pressure drop went up to six inches. After washing the pressure drop went down to 0.8 inches. The next washing cycle came two months later and the pressure drop returned to 1.2 inches. It was less expensive to replace the cartridges than to wash them in such a short interval.

Washing cellulose media cartridge elements: After each washing, the media is wetted, the permeability of the media diminishes, even if no dust remains at or below the surface of the media. The wetting causes the media to matt. If oil wets the media it is a good blotter and the fibers may grow. This causes the pressure drop and base permeability to decrease.

Other media are available that can be washed and are not wetted by oils. These are referred to as oleophobic media. This is a coating on the fibers that does not change the permeability. Otherwise they will be called washable. Often they can collect a mixture of fumes and hydrocarbons because the fibers do not swell.

Treated Spun bond medias are widely applied. Some of these are excellent choices but have limitations. For instance, with tight pleats, the top of the pleat may squeeze so the media in that portion of the pleat may make contact on the clean side when the pressure drop rises. On some applications, over 80% of the pleat of the media may not be effective. The remedy is one of the following.

A) Provide pleats with wider spacing and make them shorter in depth. This will allow full use of the media available in the filter element.

B) Provide a media that has stiffness and will not collapse on itself.

C) Provide a laminated media with the clean side backing very open so that if the pleat squeezes there will be flow through the media.

De-agglomerating dust; Normally we would run a properly designed dust collector at 1 to 1.5 inch water column pressure drop. Sometimes a system will only stabilize at a higher reading (E.G. 3 to 4 inches). One possibility is that it takes 3 to 4 inches to cause the dust to agglomerate and fall to the hopper. It may be de-agglomerating when you pulse at lower pressure drops. In that case off-line cleaning should drop out the de-agglomerating dusts. Some dusts are more susceptible to this phenomenon than others. Often, they put an anti-rust wipe on the material being cut. If it contains ceramics then we will have this problem.

Fume Generating Processes Similar to Arc Welding and Gas cutting

Thermal Deposition Processes

Spray Coating The first type was a flame spray coating machine. These fed a material into a high heat gas torch. The temperature achieved was so high that feed material would produce a material in gaseous/liquid form that started to condense into molten droplets. Though the process is not understood, it is presumed that some of the adhesion was from a nuclear bonding, in addition to the cooling of the molten droplets on the piece to be coated. There were some materials that were too porous and there was limits to the thickness of the coating. The over-spray that did not adhere varied from about 5 - 20% of the material fed into the coating generating gun. The over spray was generally collected by medium pressure air washer scrubbers at a 99% collection efficiency.

Plasma Arc Spray To get smoother surfaces and better adhesion to the target surfaces, an electric arc was added to gas flame. This produced much higher temperatures in the gun at the point where the powder or wire feed entered. It generally produced more over spray (10%-40%). This over spray was much finer and would lose its ability to stick and adhere to surfaces. This over spray was too fine to be collected efficiently with air washer wet scrubbers. Fabric or pleated cartridge collectors were necessary.

One serious problem was encountered. This involved residence time of the dust between the gun and the media collection surfaces. In a system installed in 1975, on a plasma arc spraying machine for coating electrical capacitors. The process was coating plastic surfaces with metal. The cartridge collector filter elements, venting the over spray, plugged up in less than ten minutes. The six cartridges each with 50 square feet of filter, (300 sq. ft. total) received less than 250 grains of dust. The dust collector was connected within 20 inches of the gun. The over spray dust adhered to the media surface and blocked the pores.

Through experimentation and field experience it was determined that if the dust stayed in the gas stream for relatively long periods of time, it would lose its ability to coat the media. Depending on various factors such as the feed rates of gas, solids and the arc current, this time varied. It varied from 0.5 to 1.0 seconds. Referring to the next figure, the residence time will be analyzed.

The part to be coated is placed in a hood with the gun at the front of the hood. The hood is 6 foot long and is rectangular with a 4 x 4 opening. The face velocity of the hood is 350 feet per minute. The duct is sized at a 2500 feet per minute duct velocity and the duct is 15 feet long. We will assume the back of the hood has a transition 2 foot long, designed like an evasé to have uniform velocity distribution.

1) Time to travel through the hood 6 ft / 350 FPM = 0.017 seconds

2) Time to traverse duct to the collector 15 ft / 2500 FPM = 0.006 seconds

Residence time = 0.017 + 0.006 = 0.023 seconds.

The flow through the system is 350FPM x 16 sq. ft. = 5600 CFM

To re-design collection for longer residence time the length of travel in components are altered and the velocity can be modified. The hood is the first to be looked at.

3) The hood would be made 10 foot wide with the same 4 foot by 4 foot opening for the gun and part. The velocity in the wide part of the hood would be 5600 CFM / 100 sq ft = 56 FPM

The residence time in this portion of the hood would be 18 feet divided by 56 FPM = 0.32 seconds.

4) The duct could be extended to 200 ft by putting in ductwork in a “serpentine fashion” and enlarged to drop duct velocity to 1,000 FPM. The residence time in duct would be 200 feet/ 1000 FPM = .0.20 seconds

The residence time of the system would be 0.32 + 0.20 = 0.52 seconds.

High Temperature Cutting Processes

This high temperature flame coming from the gas gun proved to be an excellent improvement in flame cutting. Instead of jagged edges near the cut, it became much smoother and for most applications it did not require smoothing the edge or the operation was very quick. With digital cutting machines the precision rivaled other cutting processes.

Plasma cutting and laser-enhanced cutting are in common use. The type of dust produced runs the gamut from arc welding to that of metalizing operations. Most dust is more similar to venting systems for arc welding operations, but to get some cutting characteristics the temperature and flow in the gun are adjusted. This may produce a dust that is prone to coat surfaces and media. When this happens the residence time requirements may be in the same range as the electro deposition processes. Laser cutters work well with 1 second residence time. Some flame cutters have been applied to non-metallic pieces such as wood and plastics. These dusts can contain tars, and oils from non-metallic parts and the collector media can get plugged easily, within a few seconds. With metallic parts, the oils can be an imperceptible film on the metal or originate from the compressed air compressor. In that case, a low-pressure scrubber may be a good choice. Roll filters with replaceable media or a self-feeding pre-coat material system have been employed.

It is crucial to have the correct airflow at initial start-up. Too much airflow will reduce residence time and cause the painting effect. Install a control damper in the main duct and use an approved method to accurately measure the exact airflow. Use the damper to choke the system if needed.

Recently, it has come to our attention that some plasma cutting processes are throwing out the 1 second residence time rule of thumb. Either the process temperature is being cranked up so high that the molten metal atoms still don’t have enough time to form molecules or the dust concentrations are so low that the atoms never get a chance to collide with one another in the laminar flow of the duct system. In these cases, finding the correct residence time

is almost a trial and error process. A new product has come on the market, called a Quencher, which is inserted in the ductwork as close to the source of dust as possible and no less than 10 duct diameters upstream from the collector. This device imparts a high energy multi-directional swirl to the air stream which cools the metallic atoms, accelerate their oxidization, and forces them to collide together and form molecules which can safely be collected without the painting effect.

Use the links below to obtain more information on welding laser plasma arc applications.

More... Quencher spark arrestor

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MINI-QUENCHER Spark Arrestor for Small Vacuum lines

QAM is pleased to announce the MINI-QUENCHER spark arrestor for use in 1", 2", 3" and 4" dust collection vacuum lines. This is the latest extension of of our incredible QUENCHER in-line spark arrester line.

The Q-1, Q-2, Q-3 and Q-4 spark arrestors answer a demand for spark protection in these smaller applications. Until now nothing has been available on the market for vacuum dust collection applications.

The MINI-QUENCHER:

• needed for welding, grinding, cutting operations
• extinguishes and cools sparks and embers that set fire to dust collectors and duct work
• in-line device, that is easily inserted in the duct work or vacuum tubing
• no maintenance, no additional drop out collection point required
• no moving parts, static device, no power required

More on... The Mini-Quencher

Determine Dust Loading in Dust Collectors

Knowing how much dust will be collected in a dust collection system is very important. The method below is what is as good as any:

Let’s use the example of an 18-bag dust collector on typical woodworking dust from two table saws. We expect a large portion of the dust to be sawdust.

Each table saw requires about 660 CFM, therefore we need 1320 CFM at the dust collector.

Assume a loading of 10 grains per cu.ft. (from experience; woodworking is 10-20 grains, Material handling transfer points is 3-5 grains, fumes is 10grains per 1000 cu.ft.)

1320 CFM x 10 grains = 13200 grains/min

13200 / 7000 grains/lb = 1.89 lbs/min x 60 min = 113 lbs/hour

A 55 (45 imperial) gallon drum is 9 cu.ft. volume.

From a dust collection selection chart, density of wood is 10-30 lbs/cu.ft.

Therefore, a drum will hold 20 lbs/cu.ft. x 9 cu.ft = 180 lbs/drum

We will fill 113 lbs/hour / 180 lbs/drum = 5 drums per 8 hour shift.

For assistance with: dust collection applications and troubleshooting 

Plasma Cutting & Cartridges

This is the result from using our cartridge and fabric filter element inspection service.

1) It was not the cartridge that was normally supplied to clients.
2) It had an inverted cone reinforcement in the bottom closed end cap. We always recommend a flat closed end cap for maximum life. The pleat spacing was optimum for this kind of application.
3) The seals (gasket) were resilient which insures effective sealing between the clean air and dirty air compartments. The cartridge exhibited no evidence of improper installation or handling.
4) You could expect indefinite life on this filter element on all suitable applications with an advanced technology pulse jet cleaning system like ULTRA-FLOW.

This plasma cutting operation is quite common and we see many cartridges with problems from these operations. The dust generated is extremely fine and problems with seals and installation account for a big majority of problems. The other problem, that we see is that the settings on the cutting head are such that the dust can be prone to plasma coat the filter elements. The solution of the coating problem is to give the dust time to lose its reactivity before it reaches the filter media. None of these usual problems were evident on this filter element.

A) My first observation was the color of the coating on the filter. In cutting ferrous metal with a plasma arc cutter, the dust is black and coated with fine easily removed powder. In handling of these filter elements, we usually wear a mask because of the fine dust generated in the inspection process. In this case there was no dust generated in the procedure. The color of the coating was brown.

B) My second observation was that there was hard inflexible crust covering the dirty side of the pleats as if the element had been painted. It had the same strength as baked on auto finish.

C) The only time I previously observed this kind of coating on a cutting operation was when the plates were covered with some kind of coolant, cutting oil or pickled. On some cutters they use compressed air at the head. I would suggest checking the air line lubricant as a possible source of the binder that is creating this paint like coating.

Finally, a continuous coating of filter aid could be maintained on the surface of the filter media. The dust load is usually quite light and the coating might be about a 64th thick. The surface coating would be painted instead of the media. When the collector pulses the inert filter aid, coated with the paint, would be ejected into the collection hopper. How often to clean would be a judgment call. I would expect the cleaning and re coating every 2- 4 hours would be appropriate.

Read more ... Retrofit Service for Cartridge Dust Collectors
Find out about ... Most Advanced Technology Dust Collectors

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”.

Read more ... Explosion Venting

Plasma/Laser Cutters Cause Fires

Some people have used Quenchers, and other style spark arrestors in plasma and laser cutting applications but still experienced fires in their dust collectors. Sparks are only one issue to deal with these applications. A good spark arrestor is definitely needed to stop sparks and embers, but, it is no guarantee against fires in the dust collector.

The problem:

1. The operator may have to reset the heat setting of the plasma head. It could be generating too much atomic static particles. This causes a "painting" effect on the cartridge media, eventually clogging it.

2. Large heavy particles of molten metal can be generated in the process.

3. You should use spun bond wide pleat cartridges, to ensure proper clean out of the cartridges. That way the dust will spread over a large surface of media, instead of on the outer surface only.

4. Current cartridges that are clogging over time (can vary from hours to weeks, depending on loading). When clogging occurs, the air flow drops and sparks can slip through any spark arrestor (not just the Quencher). This sets fire to the combustible dust accumulated on the surface of the cartridges.

Normally, plasma cutters have different characteristics depending on the settings of the cutter torch. The quantity of dust produced is relatively small. At some torch settings the dust is reactive by initiating an atomic bond between the dust and the surface of the cartridge, forming a hard durable impervious coating which totally or partially plugs the filter media. This mechanism is an inherent part of the plasma coating process to put wear resistant coatings on shafts, turbine blades etc. that allow the parts to receive very long lives. In the plasma coating machinery, the key to collecting the overspray in cartridge or fabric collectors is to allow the atomic bond to dissipate. This is accomplished by extending the time that particles travel from the torch to the filter media elements. In plasma coating systems at this time, depending on torch settings will vary from 0.5 to 0.8 seconds depending on the metals being sprayed.

In plasma cutting applications often the dust being emitted from the torch does not require any special considerations. In fact, collectors can operate for many months quite well with moderate pressure drops. Then the torch settings are changed because of various factors such as the composition or thickness of the pieces that are cut. As the settings of the gun or the speed of the cut is changed, the dust can act as a plasma coating torch and the cartridges start plugging. Sparks are often produced. If the dust is combustible the sparks may ignite the coating on the cartridges. Normally the fuel on the cartridge surface is not very heavy so the fires do not damage the housing of the collector. The cartridges are then usually replaced. The QUENCHER spark arresters are sometimes applied to limit the risk of fires and extend cartridge life. In the tandem horizontal type collectors, the cartridges are usually tight spaced, so, as the pressure drop rises, the pleats are pinched in the valleys so the pressure drop goes up. Combustible dusts can put pounds of dust to be stored in the cartridges to fuel a fire in the collectors. However, the squeezing of the pleats also causes pressure drop to increase and slow the flow through the dust collector. This often allows dust to be released into the work area.


Although spark arrestors will protect the system from sparks, pieces of molten metal go through the spark arrestor unaffected. These heavy, hot particles lodge on the surface of the cartridge and ignite the combustible dust coating. The heavy molten particles need to be dropped out of the system prior to the spark arrestor and collected safely, so as not to cause a fire in that collection device. Cyclones and drop out boxes are sometimes used for this. However, be aware that these devices have little effect on sparks / embers which are light buoyant particles and slip through to the dust collector.

An excellent example of these effects was the experience of the Day division of Donaldson who supplies this design. In cutting the filter mounting plates for their design they plasma cut holes in a 1/4 inch thick plate. They found that the filters plugged quickly in the after filters. They added distance in the filters venting the operations. This experience occurred 20 years ago and we do not know how this operation is now performing.

Our recommendation is to replace the current cartridges with a wide spaced stiffened spun bond media carried and precoat the cartridges with a 1/64 inch thick coating of inert pre-coat material.

We suggest you send each job application data (layouts & pictures) to QAM technical support at gary@qamanage.com and/or call him at (519) 746-2424. We’ve dealt with plasma cutting applications for decades and feel that yours would be a common problem. If you contact us, we'll be happy to work with you on this.

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