Water Filtration Snuffs Dust Hazards

Wet dust filters not only ensure regulatory compliance, but also can improve workers' comfort and production flow.

• By Ed Sullivan
• Oct 01, 2011

The potentially explosive dust resulting from metal finishing operations is a major concern among fabricators today, not only due to safety hazards, but also because recent changes in NFPA regulations have toughened compliance.

The grinding and sanding of metal work pieces produces fine metallic dust that even after filtration can be exposed to sparks and result in smoldering, catching fire, or even a dust explosion in circumstances when ordinary dry dust collection methods are used. The situation becomes still more hazardous when the dust from certain metals is "mixed" in a dry collection system. Mixing aluminum and steel dust, for example, is an accident waiting to happen.

Combustible metals that are common in manufacturing and machining operations include aluminum, lithium, magnesium, niobium, tantalum, titanium, zirconium, and cold rolled steel.

By switching to wet collectors, many companies are upgrading their protection against dust fires and also improving air quality for workers at the same time. Some wet collection systems also can be customized to improve ergonomics for workers by positioning work pieces at more comfortable levels or facilitating access of hoisting equipment that eliminates the need of workers to lift heavy items. Such improvements may lead to improved production flow.

Another advantage of many wet dust collection units is that they filter explosive dust directly into water in-plant, thus eliminating another potential hazard site at the duct. These devices comply with federal regulations and NFPA without interfering with manufacturing processes.

"We switched most of our dry dust collectors to wet ones," said Terry Graham, equipment engineering specialist at Bell Helicopter in Fort Worth, Texas. "Because we work with a variety of materials, including metals like steel, titanium, magnesium and aluminum, the wet collectors will improve our protection against any smoldering material or fire."

Graham said his plant has two major machine centers; one has six stations and the other has eight. Most of them have now been outfitted with downdraft wet collectors from a company that specializes in off-the-shelf and custom dust collection systems.

"Although the NFPA doesn't specify the installation of a wet collector, there is a higher risk of fire with a dry filter," said Mike Sweezy of Filter 1 Clean Air Consultants. "The old-school approach would be to replace the cartridges and put a fire suppression device on the dry collector if it catches on fire. But why take the risk when a wet collector can eliminate the problem altogether? Plus, if the equipment can be customized to fit the operation, then the ergonomics and productivity at the workstation can be improved at the same time."

Wet Collector Protection
To ensure compliance and cut excessive upkeep requirements, Midwest Products and Engineering (MPE), a Milwaukee-based designer and fabricator of enclosures, carts, and consoles used by the medical and electronics industries, decided to take a new look at its dust collection system requirements.

The main dust concern at MPE was handling that generated fine dust during the metal finishing (grinding and sanding) of regular cold rolled steel. Due to the more hazardous situation of combining dust from aluminum grinding with that from steel, the aluminum metal finishing area is located in a completely separate part of the shop. "The steel that we're grinding turns into a form much like steel wool lint," explained Teresa Stortz, MPE process improvement engineer. "The hazard occurs when that lint is hit by grinder sparks -- it could smolder and ignite. Of course, that is a situation that we absolutely must prevent. In addition, we want remove as much of the very small dust particulate from the air as is possible, and these units seem more than capable of helping us on both fronts.

The dust collection system manufacturer's president and chief applications specialist visited MPE to discuss its needs and then loaned it wet collectors of the design that the engineers felt would probably suffice for the application. "We wanted to replace existing dust collection systems because of shop-wide air quality," Stortz said. "But we didn't have any preconceived notions or specifications about what kind of system we should get. So I was surprised and reassured when the president of this company, who is a filtration engineer, visited our company to see our application firsthand.

"That has helped us tremendously, and no other supplier offered to give us trial equipment," she said. "The units they suggested we try, downdraft table designs, worked very well. The operators loved them."

MPE decided to go with the seven wet downdraft tables. This system purifies air through a combination of centrifugal force and violent mixing of water and contaminated air. As the air stream passes the fixed baffles, particulate is separated by a heavy, turbulent curtain of water created by high-velocity air. The centrifugal force caused by the rapid changes in airflow direction forces the dust particles to penetrate the water droplets and become entrapped. Contaminated water is then removed from the air stream by special mist filters. Dust, as sludge, settles to the collector bottom, and the water is reused.

Customizing for Comfort and Productivity
Many industrial applications are better served by a customized dust collection system than by an off-the-shelf model. Popular custom design elements include making systems fit into tight spaces and integrating special features, such as a crane slot, adjustable up-and-down tables, multiple hoppers, and wet spark traps. A choice of fan designs also may be important for optimizing performance and providing high-energy efficiency to applications with high-pressure requirements.

At MPE, a downdraft table 4 feet deep and 8 feet wide was customized by lowering the surface into the tank so that it was only 2 feet 2 inches off the ground, allowing workers to place large parts on it. "We also had them raise the blowers upward in the back of the system," said Stortz. "This is important to the operators because it will keep their breathing space cleaner. It will also keep them much cooler, since it gets quite warm in the area."

The new dust collection system allows MPE to build in lift assist equipment at each workstation, increasing the safety of operators and technicians who work on products that may be large and weigh more than 100 pounds.

Graham, of Bell Helicopter, said the customized wet collectors at his plant also improved the ergonomics of the machine stations. "There have been ergonomic improvements with the wet new collection systems. The units are configured so that the work surfaces are more in the worker's power zone. Where there are stations that work on heavy parts, the filters have slots so that the crane can move a part inside the booth and load the part on the table. The improved ergonomics benefit seems to speed up production and also help employee morale, as well as making it less likely that workers will be off due to an injury," he said.

Mixed Dust Applications
David Creaser at Elite Manufacturing Technologies, Inc. (Bloomingdale, Ill.), a sheet metal fabricator, said his company recently installed a 30-foot-long wet "control booth" to facilitate the safe collection of metal dust from the company's grinding operation. "We work in a variety of metals, including steel, galvanized stainless, and aluminum, for example," he said. "We decided on the wet type of collection booth for our grinding operation primarily because of the hazards that can come from the dust from dissimilar metals, particularly aluminum and stainless steel."

In the process of researching the most effective form of dust collector, Creaser said he realized that only a wet booth would enable them to work with a variety of metals without having a separate booth for each type of material. The one they chose "provides us with a cleaner work environment because it is arresting all this particulate matter in the air and then blows back over the workstations, which also provides a cooling effect," he said.

This article originally appeared in the October 2011 issue of Occupational Health & Safety.