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