Containing a Site for Vapour Abrasive Blasting
Uncontained emissions and debris from surface preparation processes, such as paint, coatings, paint chips and primers, can be toxic and lethal to aquatic life. It is also possible for them to seep into the soil and be absorbed by plants, bioaccumulate up the food chain and ultimately affect humans. The goal of containment is to prevent blast emissions and debris from leaving the work site, where they can be effectively collected for disposal.
One of the chief advantages of wet abrasive blasting over dry blasting is the reduced requirement for containment. Because the emissions and debris are encapsulated and weighed down by water, they can be more effectively contained with fewer preventative measures.
Ground containment is the simplest form, consisting of overlapping, anchored tarps overlaid with geotextile material. Geotextile material is especially valuable to the wet abrasive blaster: this light-weight material, with a texture similar to cheesecloth, allows water to pass through while filtering abrasive and debris. At the end of the job, the geotextile fabric is simply wrapped up and deposited in drums or other suitable containers for disposal.
When hazardous run-off is a consideration, the blaster can construct a berm to catch spills from heavy machinery, storage tanks and drums. Depending on the type of contaminants to be contained, the berm can be constructed at heights ranging from inches to several feet, with metal, wood or foam and an outer layer of rubberised or polyurea coating, and lined with geotextile fabric. A submersible pump can be placed inside the berm to transfer waste water into a collection container or drum barrel.
When the blast site is located near open water, residences, urban environments and other high-traffic areas, stationary structured containment is required. In these settings, traditional dry blasting requires full containment – or ‘hoarding’ – where the blast area is hermetically sealed to prevent non-filtered air, emissions and debris from escaping. For vapour abrasive blasting, partial containment is usually sufficient and can be managed by enclosing the area with tarps supported by a rigid or flexible structure, or a plywood barricade. Due to the inherent dust-suppression of vapour abrasive blasting, ventilation and exhaust filtration are not required in the majority of cases.
The Society for Protective Coatings (SSPC) defines four classes of containment for surface preparation, with Class 1 having the most demanding requirements. There are separate standards for wet blasting. For instance, where Class 2 dry blasting requires negative air pressure, filtered exhaust and a resealable entryway, wet blasting does not require any ventilation and an overlapping tarp doorway could be considered sufficient.
On large projects, often the most cost-effective containment is a portable structure. For example, blasting operations on an overpass can be contained with tarps suspended from trusses by cables overground containment. As work progresses, the rigging can be relocated to contain the next section. A bridge over an open body of water may require a sophisticated, engineered structure that not only contains and collects emissions and debris but provides a platform for workers to stand on.
The containment requirements for projects submitted for tender will usually be specified in the request for proposal, however, they take several forms: a formal SSPC containment class; a National Ambient Air Quality Standard (NAAQS) for airborne particulate matter set by the EPA; a variable standard with different levels of containment reflecting the environmental sensitivity of diverse areas; or a simple requirement that all emissions and debris will be contained, collected and disposed of.
In most cases, it is up to the blaster to recommend, design and construct containment that satisfies the project requirements.
“Containment for dry blasting can easily take up to forty per cent of the budget, most of which is labour hours for erecting and disassembling containment,” says Tim Cook, Operations Manager for Wetblast.ca. “What takes dry blasting operations a week to do, we can be in and out in two to three days. Project managers are starting to realise the cost savings of reduced containment and the advantages of wet abrasive blasting as the best economical and environmentally responsible choice.”
When constructing containment, be sure to consider wind. The structure should be able to withstand winds of up to 40 km/hr (or more, depending on the environment.)
Tarps will not last indefinitely; wind and abrasive take their toll. It often pays to invest in high-quality tarps (woven polypropylene, translucent for better visibility). Mechanical damage from wind is a common cause of tarp damage, so be sure to minimise flapping seams.
With structured, elevated containment, try to achieve a minimum of 1 m of work area between the blast surface and the containment walls to avoid blasting the containment structure. Where space is a luxury, a 20-foot perimeter of ground containment should suffice.
Clean up inside and outside of the containment daily.
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