In 2003, the National Asphalt Pavement Association (NAPA) led the formation of a Silica/Milling Machine Partnership that was formed to evaluate milling machines and silica exposure hazards to protect workers surrounding these operations. This partnership included the International Union of Operating Engineers, the Laborers’ International Union of North America, equipment manufacturers Roadtec Inc., Volvo Construction Equipment, Wirtgen America Inc., Terex Roadbuilding and Caterpillar Inc., the Association of Equipment Manufacturers (AEM) and NIOSH.
“Prior to the partnership being formed in 2003 there were some observations done by OSHA or NIOSH people where they saw a milling machine working on the East Coast and observed this plume of dust,” says Tony Bodway, operations with Payne & Dolan. “Prior to that, no one studied the silica or dust issue on milling machines.”
Soon, the team came together to begin research on silica dust and the use of milling machines.
“The first stages from 2003 through 2006 was trying to understand exposures, and in that process trying to understand where dust might be generated internal to these machines,” says Gary Fore, a retired NAPA Vice President for Environment, Health and Safety. “That was a technical exercise. We would bring engineering diagrams and turn these machines upside down, inside out. From an engineering point of view, we would also look at where dust can get generated.”
That led the team to look at the redesign of the water system in the machines. Some time was spent trying to figure out how to redesign the nozzle systems internal to the machines to get at the source of the dust. In 2008, the team organized for a field trial in Marquette, MI using prototypes focusing on the changes to the water systems.
A third field test took place in 2010 in Shawano, WI. Each manufacturer brought their own set of nozzle system designs. The idea was to find and optimize the best system for capturing any dust that was generated during asphalt milling.
Terex Roadbuilding incorporated an evacuation system with several spray nozzle designs.“We initially tested an evacuation system on our mill during the second field test and noticed a significant, consistent reduction in the particulate numbers,” says Joe Musil, senior engineering fellow at Terex Roadbuilding. “During the third field test, we added a more robust evacuation system to the mill.”
With the information gathered over the course of the research conducted by the Silica/Milling Machine Partnership contractors can expect to see updates to equipment involving the water spray systems and mechanical evacuation systems. The optimized dust control systems set by the team will help contractors provide a safe environment for their employees.
“It’s driven by safety, naturally, and the desire to limit the exposure of milling crews to any silica dust,” says Jeff Richmond, president of Roadtec Inc. “As manufacturers, we’re looking at what the most economical and effective way to eliminate silica dust emissions around milling machines.”
Evaluating water system controls
One key component of milling machines is the water spray system used for cooling cutter bits on the cutter drum, which can offer some dust suppression capabilities. To maximize the effectiveness of water spray systems, several alterations have been made to optimize the flow, pressure and nozzle locations.
According to Scott Lyons, engineering manager at Wirtgen America Inc., the Silica/Milling Machine Partnership recognized the importance of properly maintaining and operating milling machine water systems to ensure dust control. This focus led to the creation of a basic best practices guide for water spray systems to assist contractors when milling. This was created in 2008 when the team gathered in Marquette, MI for Mill Fest to test each unit’s water spray system.