Each year, millions of workers, primarily in construction, sandblasting, and mining, are at risk of silicosis (and lung cancer) from exposure to respirable crystalline silica dust

Investigators Design Experimental Engineering Control for Silica Dust

Silicosis is an irreversible, but preventable, occupational lung disease caused by inhalation of respirable crystalline silica dust (RCS). Each year, millions of workers, primarily in construction, sandblasting, and mining, are at risk of silicosis (and lung cancer) from exposure to RCS. National Institute for Occupational Safety and Health (NIOSH) investigators study how to prevent workers’ exposure to this serious health risk. Recently, they developed a novel engineering control, called the NIOSH mini-baghouse retrofit assembly, to help control RCS released from sand moving machinery on oil and gas extraction sites. Below, study lead author Barbara M. Alexander, Ph.D., P.E., C.I.H., NIOSH engineer and industrial hygienist, explains how the experimental tool works.

Clouds of dust are visible as sand trucks are unloaded at a hydraulic fracturing site. Photo courtesy of Michael Breitenstein, NIOSH.

Q: What is the risk to workers near sand movers?
During the process known as hydraulic fracturing, workers use special equipment that injects fluid under high pressure into a drilled well to break apart the rock formation and release the oil and gas. Typically, the fluid primarily contains water, sand or a similar substance, and treatment chemicals. Using pressurized air to transfer sand from delivery trucks into sand movers generates RCS, which then disperses into the air.
Workers face a significant health risk from inhalation of RCS during hydraulic fracturing and other oil and gas extraction tasks. At hydraulic fracturing sites, NIOSH investigators identified concentrations of respirable silica that were 10 to 50 times over the permissible limits set by the Occupational Safety and Health Administration. NIOSH research has identified at least seven sources of airborne RCS release at oil and gas extraction sites during hydraulic fracturing.
Q: What is the NIOSH mini-baghouse and how does it work?
To protect workers from RCS, engineering controls must be used whenever possible to limit the RCS released. The NIOSH mini-baghouse is a new engineering control that we developed for this purpose. Basically, the NIOSH mini-baghouse is exactly what it sounds like: a smaller version of the large cloth bags used in commercial equipment to trap particles and reduce air pollution. The NIOSH mini-baghouse consists of filter material and ductwork connected to a baseplate that clamps to the openings, called thief hatches, on top of a sand mover. The NIOSH mini-baghouse reduces sand dust emissions coming out of the thief hatches by trapping RCS before it disperses into the air where it would pose a health risk to workers.
The initial design concept incorporates high-efficiency filtration. It exploits energy used to blow the sand into the sand mover, which creates the dust, and so uses no additional power such as a diesel engine. It is a low-cost, bolt-on retrofit for existing machinery, making it ideal for the thousands of machines operating on hydraulic fracturing sites.

NIOSH mini-baghouse assemblies installed on eight thief hatches atop a sand mover during filling operations. Photo courtesy of Mike Gressel and Jerry Kratzer, NIOSH.

We tested the effectiveness of the NIOSH mini-baghouse in the field at an Arkansas sand mine on November 18-21, 2013. During the trial, we collected area air samples at 12 locations on and around a sand mover with and without the NIOSH mini-baghouse control installed. Our results indicate that the NIOSH mini-baghouse effectively reduced both respirable dust and RCS downwind of the thief hatches. The reduction of airborne respirable dust ranged from 85% to 98%, and the reduction in airborne RCS ranged from 79% to 99%. In another significant finding, a sample of dust collected by the NIOSCH mini-baghouse showed the presence of freshly fractured quartz, which is a particularly hazardous form of RCS.

Q: What are the next steps?
Next, we plan to enhance the performance of the NIOSH mini-baghouse with several engineering improvements to the clamping mechanism, filter fabrics, sealing surface, and material measurements. After that, we plan to evaluate the new and improved NIOSH mini baghouse in the field and then commercialize and license the technology to industry.
More information is available:

• The Development and Testing of a Prototype Mini Baghouse to Control the Release of Respirable Crystalline Silica from Sand Movers
• Silica