Martin Engineering
Most conveyors handling bulk materials have some amount of material loss from spillage, leakage, dust and carryback emissions, collectively called fugitive materials. The root causes are often obvious but rarely addressed. Rather, the standard approach is to treat the symptoms. The consequences of failing to control fugitive materials include unplanned downtime, excessive cleaning costs, regulatory actions, poor public relations and safety incidents. Addressing the issues with workable longterm solutions improves availability, housekeeping and safety, ultimately enhancing the company’s cash flow.
Perhaps the most insidious of all forms of fugitive material is dust. Some of the issues are obvious – accumulations on floors and structures, premature bearing failures and even explosion risk – but some are less noticeable, such as the long term physical and mental health issues caused by respirable and nuisance dust. A particle is considered respirable if it’s 10 μm or smaller, which can penetrate into the gas exchange regions of the lungs and cause serious lung diseases.
Nuisance dust is generally understood as inhalable particles from 10 to 100 μm that are known to irritate the airways and cause inflammation, which can affect breathing and lead to chronic conditions such as COPD. It’s estimated that for every accidental death in industry there are 20 deaths due to the long-term effects of dust exposure. These deaths are not as sudden — but just as traumatic — and often don’t show up as fatalities in the company injury statistics because a worker can be on disability for years before passing.
Reducing visible dust emissions usually defined as ≥ 40 μm from conveyors is typically a primary goal, partly because it attracts the attention of workers, neighbors and inspectors. OSHA provides a list of hazardous dusts in 29 CFR 1910.1000 Z and recommends wet or vacuum cleaning, while prohibiting the use of compressed air, dry sweeping or brushing in most situations. See 1910.1053(h). For dust management, OSHA requires engineering controls such as enclosures and dust collectors be considered first and if not practical or effective, work practices or respirators are needed to limit exposure. Quite often the use of respirators is seen as an acceptable alternative, but a closer evaluation will show respirators reduce productivity by as much as 19%, and prolonged use can affect cognitive and sensory abilities significantly. These decreases in productivity alone can justify improvements to conveyor dust containment to reduce emissions.
Skirtboards
The skirtboard enclosure is essentially a low-efficiency settling chamber. The basic concept is that a dust particle will settle out of a laminar air stream based on the speed of the air flow, (Vair), and the terminal velocity, (Vt), of the dust particle. [Figure 1]
Figure 1. Theoretical dust particle settling distance “L” in skirtboard enclosure
There are many rules of thumb, along with traditional and industry-based practices for skirtboard sizing and dust curtain placement in an attempt to contain the dust in the skirtboard enclosure. Most of these practices are without proof of performance other than, “That’s the way we’ve always done it.”
Current practice for conveyor skirtboard enclosures is to design for Vair ≤ 1.0 m/s by increasing the height of the enclosure. Two common rules of thumb for the enclosure length are two times belt width or 0.6 m for every 1.0 m/s in belt speed. It is interesting to note that if H is increased,
the distance L that the average dust particle must travel also increases. A detailed design study of air flow and particulate settling was performed using SolidWorks 2019 Flow Simulation software.
A “standard conveyor” was established as the baseline for the study. The standard conveyor is a 1200 mm wide belt with a 35-degree trough angle, traveling at 2.0 m/s. A generic material was used to produce the baseline data, with a bulk density of 1442 kg/m3 and a nominal 50 mm minus particle size distribution with a 20-degree surcharge. The discharge chute was sized based on the rule of thumb: material volume equal to or less than 40% of the chute cross section. A drop height of 3 m, an open area of 0.9 m2, an average particle size of 25 mm and bulk flow of 1680 mtph were used to calculate the induced air volume.
Figure 1. Theoretical dust particle settling distance “L” in skirtboard enclosure
Several variables were investigated to simplify the analysis. The complete conveyor with discharge and receiving belts was modeled and, while there were significant regions of recirculation in the upper discharge section, the air flow in the chute was reasonably consistent So the chute was simplified to that shown in Figure 3, with the air volume and dust particles injected into the last 2 m of the chute.
The combination of variables studied are given in Table 1. Both external and internal analyses were conducted, with complete moving discharge and receiving conveyors. The bulk material surface was set to absorb particles and the walls set to reflect particles. The effectiveness of the enclosure variations was determined by counting the number of each size particles that escaped the end of the enclosure compared to the number injected.
Figure 3. The standard conveyor used for baseline internal analysis
Table 1. Variables used in the particle study
The results of the external analysis indicated that escaped dust particles increased in speed, as the air current is affected by traveling around the belt and the discharge pulley. This phenomenon is known as the Magnus Effect and emphasizes the need for effective belt cleaning as close to the discharge as possible. A space of 1 mm between the bottom of the skirtboard and the belt was used to simulate leakage.
Figure 4. Typical external analysis results – dust particle trajectories
Curtain Designs
Several experienced maintenance technicians were surveyed and their preferred curtain arrangements modeled. In addition, multiple curtain designs and placement schemes were studied, including staggered, slit, curved, angled, with and without slits, with holes and no curtains. Several unconventional skirtboard enclosures were modeled in an attempt to create recirculation in the enclosure and improve dust settling. The optimum design for the standard conveyor was determined to be a conventional enclosure with a height of 600 mm a length of 3.6 m and three dust curtains placed in defined locations. [See Figure 6.]
Figure 5. Example of one of the unconventional chute designs analyzed
Worn exit curtains were also modeled, and as the spacing above the load increased, the dust settling performance deteriorated. The use of a single curtain right at the exit proved problematic in all cases, acting to speed up the exit air flow even further when close to the belt and reentraining dust in the exiting air stream, while being ineffective in creating recirculation within the enclosure. When the curtain placed at the exit was worn too much, it was as if there was no curtain at all. A curtain placed right at the exit and adjusted close to the load creates another fugitive material problem, sometimes called the popcorn effect, where the curtain causes spillage by knocking material off the belt.
Figure 6. Typical recirculation air flow results with 3 curtains
Results
Particle Density
Solid density had little effect on the settling of nuisance dust particles from 100 to 25 μm. In every case 100% of the 100 and 40 μm particles settled almost immediately. As the bulk density increased, there was a moderate reduction in respirable dust emissions.
Discharge Chute and Tailbox
The junction between the discharge chute and the skirtboards was found to be an important design detail for creating recirculation. Making the width of the discharge
chute narrower than the width of the skirtboard helps to fold the air flow going into the first curtain, and that encourages distribution of the air flow toward the top of the enclosure, rather than along the surface of the material. The retrofit and mitered junctions were significantly more effective than a simple butt connection and 300 mm height as shown in the standard conveyor Figure 3.
The tail box had little effect on dust emissions out of the exit end of the skirtboards. In most configurations, the height of the tail box was set at 300 mm. The tail box length was set at 600 mm to match the typical 600 mm idler spacing used in the load zone by most conveyor manufacturers and engineers.
Length of Skirtboard
It was found that for most situations a 3600 mm long skirtboard produced the best results. Increasing the length to 4800 mm and height to 900 mm had some marginal effect, but may not be worth the extra investment.
Length of Skirtboard
An enclosure height beyond 600 mm for the standard conveyor with a single exit curtain did reduce nuisance emissions but tended to increase respirable dust discharge, because the average settling path was greater with the higher enclosure.
Air Flow
As would be expected, the average air velocity through the skirtboards was directly proportional to the induced air flow and cross-sectional area. Average velocities in the skirtboards due to induced air ranged from 0.8 to 2.8 m/s. Belt speed has a minor effect on the average velocities. The maximum air velocities were almost always found where the air flows under the skirtboard curtains. These high air speeds kept the respirable dust suspended, so reducing induced air into the chute is also important in improving performance.
Curtains
The best results were obtained with three or more curtains. The design of the slits in the curtains is important to allow air to pass through, allowing the airflow paths to fill the entire chamber and not just flow at high speeds under the curtains. It was found that the individual flaps should be
about 50 mm wide, with slits at least 5 mm wide and the curtains extending the full width of the enclosure.
Figure 7. Summary of results – Percent of dust particles exiting the enclosures
Preferred Embodiments
The best value for the cost of the skirtboard enclosure and its effectiveness is judged as skirtboards 600 mm high and 3600 mm long and 3 full width slit curtains using either the retrofit or mitered discharge chute-to-skirtboard connection.
Figure 8. Recommended mitered skirtboard enclosure for new construction
Figure 9. Recommended skirtboard enclosure for retrofit or angle transfer
Design Recommendations
- Discharge chute width across skirtboards 200 mm < width between skirtboards.
- Skirtboard outside width based on horizontal dimension of free belt edge for sealing and belt wander edge allowance ≥ 115 mm per side (Foundations™ method).
- Skirtboard height ≥ 600 mm.
- Inlet to skirtboards air volume flow ≤ 0.50 m3/s.
- Length of skirtboards for material loading turbulence ≥ 1000 mm when required.
- Length of skirtboards for dust settlement ≥ 3600 mm plus extra allowance for loading turbulence if necessary.
- Skirtboard dust curtains:
- Entrance (1st) curtain 300 mm past end of extra allowance for material turbulence and distributing
air flow. - 2nd (middle) curtain centered between entrance and exit curtains.
- Exit (3rd) curtain 300 mm from end of skirtboards.
- Curtain clearance above the bulk material: 25 mm preferred, 50 mm max.
- Curtain flaps: ~ 50 mm wide strips separated by slots ≥ 5mm.
Conclusion
While some improvement is seen with increased skirtboard height and length, it is doubtful that it can be economically justified on the reduction of respirable dust alone. A return on investment for control of nuisance dust for new and retrofit designs can be based on reducing cleaning labor, increased equipment life and/or elimination of dust collection. If the improvements reduce the TWA of respirable dust emissions to the point where engineering or administrative controls could be less stringent, then a financial case could also be made based on improvements in labor productivity.
