Engineering Controls Database
Field Evaluation of the NIOSH Mini-Baghouse Assembly Generation 3 for Control of Silica Dust on Sand Movers
NIOSH researchers collected personal breathing zone air samples for workers at 11 hydraulic fracturing sites in 2010 and 2011. Job classification was associated with exposures to silica. Sand Mover and Transfer Belt Operators had the highest exposures to RCS, due to their proximity to point sources of sand dust generation. Exposures of Sand Mover Operators were sometimes over ten times higher than occupational exposure limits, exceeding the assigned protection factor (APF) of 10 for half-face elastomeric or filtering-facepiece respirators. In that case, wearing a half-face elastomeric or filtering-facepiece respirator would provide insufficient protection.
Pneumatic transfer of sand enhances generation of silica dust aerosols. NIOSH researchers identified at least seven primary point sources of dust generation/release. They are:
• Thief hatches on top of the sand movers during filling
• Uncapped side fill ports on sand movers during filling
• Vehicular traffic on the site
• Transfer belt under the sand movers
• Sand being dropped or mixed in the belt or blender area
• Transfer belts between the sand movers and the blender
• The end of the sand mover conveyor belt
Several engineering controls were proposed to limit the generation of silica-containing dusts. These included a mini-baghouse assembly on the sand mover hatches, skirting and shrouding at the base of the sand mover and near the conveyor belt, and capping unused fill ports.
Although occupational exposure to respirable crystalline silica (RCS) is a well-established hazard in mining, sandblasting, foundry work, and construction, until recently it was not recognized as a hazard associated with oil and gas extraction. Hydraulic fracturing involves high pressure injection of large volumes of water (about 95%), mixed with 4 – 5% of “proppant” (typically silica sand) and approximately 1% of treatment chemicals, to enhance existing fissures and create new cracks in tight oil and gas formations. After release of hydraulic pressure, the proppant holds the cracks open, so that gas or oil can flow freely from the formation. In addition to silica sand, resin-coated sand or ceramic proppant may also be used, depending on the formation.
The American Petroleum Institute (API) has a standard for sand used as proppant. Proppant sand must be =99% silica, within a certain range of mesh sizes, smooth-edged and highly spherical. It must also pass tests of crush resistance, low acid solubility and low turbidity. Although hydraulic fracturing has been in use since the 1940s, its use for recovering oil and gas from tight formations has skyrocketed in the last 10 years due to the use of directional and horizontal drilling techniques coupled with high volume, high pressure hydraulic fracturing. U.S. production of dry natural gas alone increased by 39% from 2004 to 2014.
As part of a NIOSH research program to evaluate risks for chemical exposures to land-based oil and gas workers, NIOSH researchers were the first to systematically evaluate risks for occupational exposures on hydraulic fracturing sites across the U.S. Identified exposure risks include RCS in hydraulic fracturing and volatile organic compounds (VOCs) in flowback operations.
Silica, or silicon dioxide (SiO2), is found in a variety of crystalline and non-crystalline forms. The most common forms of crystalline silica are quartz, cristobalite and tridymite, with quartz being by far the most common. Inhalation of RCS is most closely identified with the disease silicosis, a scarring of the lungs that causes difficulty in breathing, and is progressive and fatal. Acute silicosis can develop in weeks to months following exposure to very high concentrations (tens of milligrams per cubic meter) of RCS. Long-term exposure to much lower concentrations of RCS can lead to accelerated or chronic silicosis years to decades later. Silicosis is a risk factor for developing tuberculosis, and silica exposure can also cause kidney and autoimmune disease. Workers exposed to RCS also have higher rates of other respiratory diseases, such as chronic bronchitis and emphysema.
Inhalation of quartz and cristobalite, two of the most common crystalline forms, can lead to cancer. In a study of diatomaceous earth workers published in 2001, the risk of mortality from lung cancer increased with increasing exposure to RCS dust (cristobalite). In a study of Vermont granite workers, increasing crystalline silica dust exposure was associated with lung cancer. A study of industrial sand workers in North America showed increasing risk of silicosis and lung cancer with increasing exposure to respirable silica. In Chinese workers at metal mines and pottery factories, long-term exposure to silica dust increased the mortality rate due to respiratory diseases, lung cancer and cardiovascular disease.
The International Agency for Research on Cancer (IARC) classifies crystalline silica dust, in the form of quartz or cristobalite, as carcinogenic to humans, based on studies such as the ones mentioned above, as well as animal experiments. The U.S. National Toxicology Program (NTP) also classifies RCS as a known human carcinogen.
The mini baghouse retrofit assembly controls sand dust emissions generated during pneumatic sand filling operations through the same principles used by commercial baghouses for air pollution control. The APPCO FS 30 sand mover is configured with four compartments (or bins) and two hatches for each bin. One mini baghouse assembly is attached to each of the eight 22-inch x 22-inch (56 cm x 56 cm) hatch openings. The high volume of dust-laden air used to move the proppant into the sand mover bin forms a dust cake on the inside of the bag, which traps particulate while allowing air to pass through the bag material. The dust cake collected on the filter fabric is shed when air flow is stopped, allowing the cake to drop back into the sand mover.
Each mini-baghouse assembly consists of a baseplate and clamping assembly connected to four: 8.75-inch-diameter, 82-inch long cylindrical sections of baghouse filter material. A band clamp attaches each filter bag to a pipe stub on the baseplate. Long screws that penetrate the baseplates tighten clamping arms inside the top of each bin, to hold the baseplate tight to the thief hatch opening.
The air-to-cloth ratio for the generation 3 mini-baghouse is nearly 3 times lower than the generation 2 unit, which reduces air velocity through the filter bag material, and helps to keep static pressure low inside the filter bags. The filter bags are made of a 15 ounce polypropylene felt lined with a polytetrafluoroethylene (PTFE) membrane to help shed the dust cake. The material of the base of each mini-baghouse unit has been changed from steel to aluminum, reducing the weight while improving corrosion resistance. A ¼ inch-thick (0.6 cm) sheet of polyurethane rubber is bonded to the bottom of each unit to help it seal to the thief hatch opening.
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Oil and Gas
Researchers from NIOSH evaluated generation 3 of the NIOSH-developed and fabricated mini-baghouse retrofit assembly for control of silica dust generation from sand moving machinery on May 19 – 21, 2015 at an SWN sand mine in North Little Rock, Arkansas. Air samples were collected at 12 different sampling locations. Six locations were on top of the sand mover (at each of the four corners and at two locations towards the middle of the FS-30), and six locations were on the ground at personal breathing zone height. A total of 168 air samples were collected during two days of sampling. Half of the air samples (84) were collected while using the mini-baghouse, and half were collected with the mini-baghouse absent. Temperature, relative humidity, and wind speed and direction were recorded continuously during each sample period using a Kestrel 4500 weather meter (Nielsen-Kellerman, Boothwyn, PA) located north of the sand mover.
To calculate the effectiveness of the mini-baghouse control, only data from the three locations with the highest measured concentrations of respirable dust and RCS were used; all of these locations were atop the sand mover. Impressive reductions in respirable dust and in RCS were achieved at these locations by use of the mini-baghouse. Reductions in respirable dust were all estimated to be 99+% (all controlled concentrations less than detectable limits), while reductions in RCS ranged from 98% to 99% at these sampling locations.
The performance of the generation 3 mini-baghouse retrofit assembly showed a clear improvement over the generation 2 units. Because of the interior porosity of the fabric used for the filter bags in the generation 2 mini-baghouse, filter bags had a tendency to blind. The calculated air-to-cloth ratio was also much higher. These factors necessitated stopping the flow of air periodically during sand transfer, and manually shaking each bag to release accumulated dust. Because of the improved fabric and the lower air-to-cloth ratio used with the generation 3 units, it was never necessary to interrupt sand transfer during the trials to shake the bags. Pressures inside the bags increased very gradually during runs, and returned to a level near zero at the beginning of the next run.