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

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

Static Electricity and Dust Collector Systems

General Considerations
The effects of static electricity on the collection of dry particulate in fabric collectors is rather simple but misunderstood. For the most part, cartridge dust collectors experience the same issues.
First we must consider the cause of static charge build up in a collector. It occurs because the dust being collected is akin to a capacitor in an electronic circuit. In this day of computer chips the designer may not be familiar with this phenomenon. The capacity has two conductive plates separated by a layer of insulating material that has high enough insulation values that the static charge remains for relatively long periods. The charge can be removed by grounding one side of the capacitor. The charges then drain.
In dust collectors where the dust forms in the filter cake, the static charges may enter the collector on the surface of dry particulate dust. If the dust has high dielectric resistance properties, it can accumulate and build up in the filter cake. It can be viewed as many particles each carrying a static charge and acting like a miniature capacitor. The static charge will then build up on the surfaces and may reach a high enough level where a spark can be produced. This spark can trigger the explosion of explosive dusts.

Mechanical Cleaning (Shaker) Dust Collectors
In a fabric collector with a mechanical shaking mechanism to remove the dust, the collector is most vulnerable during the cleaning process. The dust is shaken from the filter bags in the process of shaking the cake, sparks sometimes are produced. Invariably, the dust/ gas mixture passes between the upper and lower explosive limits. A serious explosion may occur.
Usually these collectors will have explosion vents which relieve the high pressures that are generated in an explosion, presumably keeping the housing from being damaged and protecting the operating personnel near the dust collector.
In an attempt to keep this static charge from building to threatening levels, measures are included in an attempt to bleed this charge to ground. These include one or more of the following:
1) Sewing in grounding wires into the filter media.
2) Impregnating carbon or other conductive coating into the filter cloth.
These often give the designer a false sense of security in applying these to dust collectors. As explained above the dust, itself, insulates the charge and it remains in the cake until it reaches a point where a spark is generated. If the dust concentration is above the lower explosive limit and below the upper explosive limit, an explosion can occur. Fortunately, generation of the spark may not occur if the timing of the spark and dust concentration level do not coincide. An explosion does not occur in these cases.

Continuous Cleaning Reverse Jet Pulsed Dust Collectors
When dust, with the same properties described above, is vented in the same operations, using a reverse pulse jet cleaning system, the danger is considerably diminished unless the pulsing is applied in “off line” cleaning mode where the fan is stopped.
These collectors clean the bags by injecting air from the clean air plenum backwards through individual bags as the flow continues through the collector. This cleaning agitates the filter cake so the static charges are dissipated.
The danger of explosion occurs when the dust concentration coming into the collector reaches a level between the lower and upper explosive limit concentrations. This is highly unlikely but we recommend that properly sized explosion vents are installed which normally coincides with the requirements of insurance underwriting firms.
The explosions can occur when there is dust build up in ducts especially when long horizontal runs are encountered. The spark can be generated in ducts and the explosion front can travel down the duct into the dust collector, igniting a secondary explosion as the concentration in the collector housing is driven above the lower explosive limit for that dust. Even with no build up in the ductwork, an upset can occur in the process which generates sufficient dust concentrations.
One method of nullifying the possibilities of danger due to duct build up is to install an automatic booster / duct cleaner device. This booster can serve to automatically clean out any drop out in long horizontal duct runs.
Another phenomenon can affect of dust collector systems, is where the dust has high dielectric properties and the dust, because of static charges, will build up on the outside bend of an elbow. This dust can trigger an explosion if this dust is also flammable and explosive. Some examples of dust where this problem is often a factor are toners for copy machines and electrolyte powder used in alkaline batteries. The solution is to insert a pulsed air jet that agitates the built up dust that dissipates the charge. Some dry powder coating compounds are also subject to static charge build up in powder coating systems.

Read more about ... Booster / Duct Cleaner, and Quencher spark arrestor