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Introduction
A number of changes in the industry have led to the development of new abrasive blasting tools and methods, driving the widespread acceptance of wet abrasive blasting to replace dry blasting for surface preparation and cleaning. (For an introduction to surface preparation, see Substrate Surface Preparation for Corrosion Prevention.)
New Rules Aim to Reduce Silica Dust
A new rule from the United States Department of Labor’s Occupational Safety and Health Administration (OSHA) took effect on June 23, 2016 and is aimed specifically at crystalline silica, which is used as an abrasive material. The rule defines a permissible exposure limit (PEL) for respirable crystalline silica that is 50% lower for general industry and maritime and 80% lower for the construction industry than the previous limits.
Crystalline silica is dangerous because its physical structure has sharp points, and when inhaled can become lodged in the lungs, leading to increased risks of lung cancer, silicosis and various non-malignant respiratory diseases.
The rule is forcing the abrasive blasting industry to explore new methods that reduce their worker’s exposure to abrasive dust. A promising trend is a movement towards wet abrasive blasting (also known as vapor abrasive blasting) as an alternative to dry abrasive blasting.
Dry Abrasive Blasting (Air Blasting)
Dry abrasive blasting is a surface cleaning and surface preparation technique where loose abrasive media (e.g., silica sand) and compressed air are propelled against a work surface. This process has a wide range of applications in surface preparation, including:
Dry abrasive blasting uses an air compressor's energy to deliver an air/media mixture at high speeds and volumes to impact the surface or parts being processed. A pressure regulator is used to control the air speed of the air compressor.
The downside to dry blasting is a lot of dust is created when the abrasive media strikes the substrate’s surface and breaks into finer pieces. Due to this, job sites require a containment facility or tenting to prevent the dust from escaping the worksite and going into public or residential areas. While this makes clean up easier, it also reduces the worker's visibility to the surface being blasted.
The worst effects from dry abrasive blasting are the human health hazards associated with the large quantities of dust if inhaled by an operator not wearing proper personal protective equipment (PPE). If dust escapes containment in a public place, people nearby could also be exposed to the dust. This makes compliance with the silica rule increasingly difficult if using dry blasting methods.
The Benefits of Shifting Towards Wet Abrasive Blasting Technology
Wet abrasive blasting uses a similar technique to dry blasting, but moisture is added to the blast media in the blasting process. The water tends to encapsulate the fine particles that are produced when the abrasive shatters upon striking the surface, so the particles are less likely to disperse into the air. This provides an advantage over dry blasting because the amount of dust that tends to bounce off the surface is reduced, resulting in a workspace that's easier to contain and clean up.
There are four wet abrasive blasting methods, each with their own advantages and disadvantages, designed to accomplish the same goal of less dust during surface preparation.
- Water injection nozzles and halo nozzles. With abrasive air blasting, the abrasive is accelerated by the air from an air compressor through a hose and then ejected out of a nozzle. A system with a water injection nozzle (WIN) has another hose and nozzle running parallel to the blasting hose and nozzle (think of a double-barreled shotgun) that squirts water alongside the abrasive media. A halo nozzle takes the concept one-step further by enclosing the abrasive blast in a ring of water. These methods are relatively inexpensive and suppress 50-80% of the dust, but they consume large quantities of water.
- Cabinets and blast rooms. In this alternative approach, the water and abrasives are mixed and sprayed inside a containment enclosure. This method has the advantage that the abrasive can be recovered and reused, but the items to be blasted are limited to the size of the enclosure.
- Wet abrasive blasters. The abrasive and water is stored in an air pressurized supply tank. The mixture is drawn into the hose using the Venturi effect. While this method suppresses dust, it is difficult to precisely control the abrasive flow and it consumes large amounts of water and abrasive.
- Vapor abrasive blasters. The abrasive and water are stored in a supply tank under water pressure. The mixture is then precisely metered into the airflow from the air compressor. These systems are very effective at dust suppression (up to 95% over dry blasting) and provide fine control over both the air pressure and the water/abrasive mixture. The fine control permits the blasting of a wider range of surfaces (including steel, aluminum, concrete, wood, and softer materials) and reduces the amount of water and abrasive media needed to prepare a surface. Less mixture means less toxic runoff, thus saving on cleanup and disposal costs. This is an important consideration for maritime applications (either in dry dock or when sailing) where runoff cannot be allowed to spill into the water.
Conclusion
Concern for worker safety and the environment, along with new regulations has led to the development of new abrasive blasting tools and methods. Due to technological innovations, wet abrasive blasting technologies are being adopted and replacing dry blasting for surface preparation and cleaning operations. Before you invest in wet abrasive blasting equipment, make sure you research what method the company uses as well as the claims they are making. You will want to make sure the company you buy from is reputable and their claims are supported by 3rd party testing.
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