Respirable Crystalline Silica and the Hierarchy of (New) Controls


Air sampling results describing worker exposure risks to respirable crystalline silica (RCS) during hydraulic fracturing (using quartz sand as proppant) were published by National Institute for Occupational Safety and Health (NIOSH) researchers in 2013. The results were surprising; exposures were found to exceed OSHA and NIOSH limits, sometimes by a factor of 10 or more. NIOSH developed recommendations to control RCS emissions from the seven-point sources identified during their field research with industry partners:   

  1. Dust ejected from thief hatches on sand movers during filling.
  2. Dust released from the sand mover conveyance belt.
  3. Dust created from the momentum of proppant falling into the blender hopper.
  4. Dust released from transfer belts when proppant is deposited onto the belt and conveyed to the blender.
  5. Dust generated as proppant leaves the end of the transfer belt (i.e., “the dragon tail”.)
  6. Dust ejected from fill ports on the sides of sand movers during refilling operations.
  7. Dust generated by well site traffic.

Since 2013, new engineering controls have been developed and implemented to minimize or prevent aerosolization of silica-containing dust from many of these point sources. Companies now specialize in wellsite RCS dust control to aid compliance with the new OSHA Silica Standard, including the 8-hour permissible exposure limit (PEL) of 50 micrograms per cubic meter (μg/m³) and the action level of 25 μg/m³. The Standard requires implementation of engineering controls by June 23, 2021.    

In March 2018, professionals from industry, government, and the American Industrial Hygiene Association and American Society of Safety Professionals gathered at the Silica in the Oilfield conference in Denver, Colorado to discuss advances in silica control technologies. Presenters described and explained new controls to minimize worker exposures to RCS during hydraulic fracturing. This article briefly summarizes some presentations made by industry professionals, organized by type of control as described in the hierarchy of controls (HOCs) [see figure ]. Developed in the 1950s, the HOCs are a framework to prioritize implementation of effective control solutions including eliminating hazards, substituting less hazardous materials, engineering out the hazard, use of administrative controls (e.g., policy and procedures, worker training, hazard signage), and lastly, use of personal protective equipment (PPE).

Elimination: use of alternative proppants 

Engineered ceramic proppants are used for certain formation applications as a replacement for quartz sand. Ceramic proppants contain kaolin clay and bauxite ore. While kaolin contains crystalline silica, sintering during manufacturing integrates the silica into a pellet of uniform dimensions harder than quartz, preventing disintegration to respirable-size particles. CARBO, one company that manufacturers ceramic proppants, reported on air monitoring results collected for RCS during proppant handling operations at their manufacturing plants.  Reportedly, air samples collected during mechanical handling of their sintered lightweight ceramic proppant did not generate RCS concentrations greater than the current OSHA action level.

Substitution: use of treated quartz sand

Quartz sand can be treated using proprietary processes and ingredients to make the sand less dusty during handling operations. A Covia Corporation representative described their development of a quartz sand treatment formulation and application process. They reported on air sampling measurements comparing treated vs. non-treated quartz sand transported from a sand silo onto a transfer belt and pneumatically transferred into a mock-up of a sand mover bin. Reductions of 87–98% were reported for total dust when comparing treated to non-treated sand. Using real-time instrumentation and size-selective samplers at sand transfer points, area sampling for respirable dusts reportedly ranged up to 400 mg/m3 using non-treated sand but reduced by 97–99% using treated sand. 

Engineering Controls: non-pneumatic sand transfer  

Pneumatic transfer of quartz sand is a legacy process involving dry-bulk delivery trucks, sand movers and open belt transfer into the blender hopper. A PropX representative described how sand can now be delivered near the blender in stackable, modular bins and gravity-fed onto enclosed transport belts to supply the blender. Minimizing proppant drop distance from the bin to the belt helps prevent generation of aerosolized RCS. Non-pneumatic transfer reduces sand disintegration; proppant stored on-site reduces the numbers of sand truck deliveries and RCS re-aerosolization from site traffic. PropX reported to have achieved significant RCS dust reduction based on evaluations performed by third party industrial hygienists and AIHA-accredited laboratory sample analyses.

Arrows Up LLC also developed containerized, gravity driven sand handling technology using a riser supporting three sand bins with capacity of 43–50,000 pounds. Bins are positioned to minimize transport and handling and proppant drop distance. Gate valves and delivery chutes connect to the blender hopper; sand is not pneumatically or belt conveyed. Arrows Up reported evaluating their technology in numerous basins under representative conditions. Eighteen personal and seven area samples were collected for RCS; average concentrations for all samples were reportedly below the OSHA action level for 12-hr shifts. The action level was reportedly exceeded once when spilled sand required shoveling and respiratory protection was required.  

Calfrac Well Services developed and implemented an engineering control system for sand transfer operations from bulk sand delivery trucks. Calfrac’s system offloads trucks through a belly dump onto a conveyor belt that gradually fills vertically oriented bins. The bins transfer sand onto an enclosed horizontal conveyor system that feeds the blender hopper, eliminating pneumatic transfer. Except for discharge into the blender hopper, almost all conveyance is enclosed, reportedly mostly eliminating RCS aerosol emissions. Emissions from the blender hopper are addressed with engineering controls including shrouding or vacuum dust capture. Area and personal air monitoring were used to develop worker exclusion zones around the offloading zone for the sand trucks and the blender hopper. Administrative controls include development of an exposure control plan, air monitoring using in-house staff, worker training, annual pulmonary function testing and respirator fit testing, and medical monitoring for potentially exposed personnel. 

Engineering Controls: vacuum collection 

Airis Wellsite Services designs and implements local exhaust ventilation systems to capture RCS emissions from sand mover hatches, sand silos, and boxes. The system includes a 45,000 cubic feet per minute vacuum, ductwork, and manifold system. Ductwork connects the vacuum unit to sand mover hatches with patented hoods to collect RCS emissions. Shrouding is used around transfer belts and the blender hopper to contain RCS emissions. The vacuum collection system reportedly includes 60 filters that are air-purged every 10 sec. A screw augur transports the collected RCS dust to a Super Sack® for containment and disposal.  Connections for up to six sand movers can be configured with the system.  Airis reported their systems can control RCS emissions so that approximately 75% of air samples collected are less than the new action level. 

New Technologies Provide Better Control of RCS 

New controls now provide numerous options for consideration when implementing controls for RCS.  However, it should not be assumed that exposure risks are completely controlled simply because new technology is implemented. Controls have limitations and the use of a single control for RCS is unlikely to be effective because multiple point source emissions exist on completion sites. Confirming the effectiveness of controls is as important as implementation.  Despite new developments in controls, rigorous research confirming the effectiveness of controls is still needed to inform and educate well servicing companies and health and safety professionals within the industry. While the information about new controls presented at the Silica in the Oilfield conference was a step in the right direction, additional steps are also needed.

Companies planning to purchase, or contract commercially developed controls should contract out or conduct their own evaluations to carefully evaluate the effectiveness of controls. Confirming effectiveness of new controls under the dynamic conditions of completions, where numerous point source RCS emissions are present, is a challenging endeavor requiring industrial hygiene/safety, engineering and managerial expertise and support. 

When evaluating the effectiveness of controls, a well-conceived, systematic, and ongoing strategy is required; a “one and done” assessment is likely insufficient. Confirming control effectiveness as process operations, proppant type/use, work practices, equipment and site configurations change needs to part of every company’s exposure control plan.  

Anticipating and preventing acute, safety-related risks has traditionally been a focus for policy and procedures, worker training, tail gate talks, and OSHA compliance efforts. It is equally important to focus efforts on understanding exposure risk factors, implementing controls and then confirming their effectiveness to prevent the chronic adverse health outcomes that can occur from overexposure to RCS. If we effectively control occupational exposures today, we will have prevented occupational diseases decades away.  

NIOSH is actively soliciting operators and well-servicing companies to partner with NIOSH in evaluations of the effectiveness of exposure controls used during completions. NIOSH has begun Controls and Interventions for Hazardous Exposures in Oil and Gas Extraction, a four-year project to study the effectiveness of controls for silica exposures in the oil and gas industry and advance knowledge about worker health protection, through effective use of controls.  For more information contact John Snawder, Ph.D., DABT, at NIOSH, email or (513) 533-8496.

Disclaimer. The findings and conclusions in this article are solely those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health, Whiting Petroleum, the American Industrial Hygiene Association, or the American Society of Safety Professionals. Mention of company or product names does not constitute an endorsement by these parties. 

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