From JPCL January 2024
Photo: Courtesy of the Author
Wet blasting, the process of introducing water to a mixture of pressurized air and abrasive, is now a mainstream process in the surface preparation field. While it has been referred to as other names, including dustless blasting, mist blasting, slurry blasting or vapor blasting, wet blasting uses a mixture of water and abrasive media to create a surface profile prior to coating application—making it distinct from ultra-high-pressure (UHP) waterjetting, which does not include abrasive and does not create a surface profile.
The “why” and “what” of wet blasting is generally well understood. Simply put, only two words are needed: “dust mitigation.” While it is true that there are other ways to reduce dust (see box, below), adding water to the blast stream is often required to keep dust in check when abrasive blasting.
Less clear, however, is the “how” of wet blasting. This article will examine and compare the various technologies and equipment currently available for these applications in the industrial coatings market.
Wet abrasive blasting was introduced over 50 years ago, so it is not a new concept. Technology is evolving however, and options now exist that offer faster production as well as lower water consumption versus older technologies.
Before we examine the various methods of wet blasting, let’s look at two common scenarios where wet abrasive blasting equipment may be needed.
For these purposes, there are three common types of wet blasting equipment options on the market today. Let’s review and explore the pros and cons of each.
This is a category of blasting equipment where water and abrasive are mixed inside the blast pot. Some versions are designed for wet blasting only, while others can blast wet or dry.
With slurry blast equipment, because the water and media are mixed in the tank, no air dryer is required. As a result, using media that has gotten wet is OK, which can be beneficial.
However, this type of equipment cannot independently control the abrasive and water ratio; this must be carefully managed manually. For example, if production needs to be increased due to a particularly thick or tough coating, the operator will increase the flow of the abrasive/water mixture. While this will increase production speed due to the higher level of abrasive, the water consumption will also increase. If production needs to be decreased due to working on a sensitive substrate such as concrete, where the underlying aggregate may become exposed, turning the abrasive down will also turn down the water possibly reducing the dust mitigating effect of the water too much. Also, slurry blast equipment produces higher water consumption rates than hybrid units. Lastly, some slurry blast configurations do not have the ability to blast dry.
Photo: Courtesy of the Author
In this category, the abrasive is always dry in the blast pot. When wet blasting, water is injected at the metering valve or at the nozzle. Hybrid pots are quite versatile; they can blast dry, blast wet, blow down, or wash down. When combined with a bulk pot with multiple operators, each operator can even choose their desired function independently of the others. There are other considerations which will be reviewed later in this article.
Designed to do both wet and dry blasting efficiently, hybrid units are versatile, not a limited-purpose specialist machine. Overall, they have the lowest water consumption and highest productivity in this group.
On the con side, because the blast pot is dry, an air dryer is required, adding cost to the already pricy technology for companies who do not currently have one.
These versions use a separate water line, which connects at the nozzle. There are two main types: a Water Induction Nozzle (WIN), which is a nozzle designed specifically for wet abrasive blasting; and a Halo (water ring), which is a device that attaches to a standard nozzle.
These smaller, compact units carry a very low upfront cost. However, nozzle-mounted injectors have limitations which significantly impact their viability for industrial applications. The water-ring types use a large amount of water and the injection type negatively impacts production speed.
There are several manufacturers of wet abrasive blasting equipment and much to consider when selecting the right product for the right job. Table 1 features equipment specs and considerations for various wet blasting equipment types.
Table 1: Equipment Specs and Considerations for Generic Wet-Blasting Equipment Types
Author’s Note: Graco EcoQuip slurry blasters have been discontinued by the manufacturer and Farrow Systems has no models currently offered at the time of this writing.
Editor’s Note: The information presented in this table is solely a survey of the wet abrasive blasting equipment currently available on the market. It does not assess the performance or efficiency of any equipment type. JPCL does not endorse any specific products included in this table.
Now that you’re aware of the different types and sizes of wet blasting equipment available, let’s run through a list of questions to ask to make sure you’re selecting the most practical equipment setup for your wet blasting needs.
The metering valve is the heart of any blast machine. In the average lifespan of a blast pot, the value of the abrasive that will go through any pot can easily exceed $100,000. A valve that offers a high degree of fine metering control (allowing the operator to minimize consumption while maximizing production) may cost a few hundred dollars more up front, but can save many thousands in abrasive over the life of the pot.
Metering valves are not all created equal. Fine metering control is just one of the important aspects. Other items to consider about the metering valve include:
These questions are nearly impossible to answer without speaking with a knowledgeable sales rep, technician, or user who has used more than one type.
How much pressure drop is there from the inlet of the pot to the outlet? If the metering valve is the heart of any blast machine, PSI is the heart of productivity. It is simple to test and profoundly important to understand what (if any) pressure drop is caused by the piping and valve of the blast machine. It isn’t uncommon for machines to have a 10 PSI pressure drop and, in this example, the production speed will be 15% slower than one without any pressure drop. These pressure drops are insidious and difficult to identify without doing nozzle pressure checks or without doing timed comparisons between different equipment.
You can’t just look at the manufacturer’s website. You need to pick up the phone and call them. As far as the equipment is concerned, do they offer field support or just phone support? Qualified field support (both to optimize job setup as well as for equipment maintenance) can mean the difference between profit and loss on your projects. Possibly even more importantly, can they offer real-world assistance on best practices on the type of projects you want to pursue? Regardless of whether you are an old-timer in the blasting game or a greenhorn, there’s always something new to learn. Look for a company who can offer suggestions on abrasive comparisons, nozzle types, hose sizes, job safety, how to bid projects and any other questions you may have.
Does your blaster offer the ability to blast dry and if so, how well does it perform at this? Dry blasting is the preferred way to blast whenever possible and is much more commonly done. A limited purpose wet-only blaster (or one that isn’t efficient at blasting dry) will tend to gather dust in your warehouse, lowering the return on your investment substantially.
Does the manufacturer or distributor offer field demos and/or equipment rental? This is a great way to test and find out more about your chosen equipment (and your supplier). Actually using the equipment on a demo site or out in the field for a day will help you learn more about the equipment than you could doing a week of other research on it.
How many machines of your selected model are available on the used market? A disproportionately high number of a given model or manufacturer is a warning sign. Compare pricing for used versus new—how much depreciation is there in your top choices?
Photo: Courtesy of the Author
While this article can help supplement your research into specific wet blasting equipment, you should also seek out advice from real-world product experts. Ask your potential supplier for referrals, especially if they have any that have “converted” from other brands. Here are five questions to ask when you connect with a referral:
As with most products marketed and sold in the U.S., a variety of good options fortunately exist because of our free market economy. A wet abrasive blast machine purchase is a major investment and what type you want or need is dependent on your specific situation. But remember this: The initial cost is far less important than the ongoing operating cost and the support available from your supplier.
Jon Kenimer is a Product Manager in the U.S. for BlastOne International. Since joining BlastOne in 2014, he has held diverse roles, including Commercial Manager to Project Manager and Sales Representative, culminating in his current role. A seasoned veteran of the corrosion control industry, Kenimer has resided in Columbus, Ohio, since 1972, where three of his six children are also employed with BlastOne.
What else do I need to know? We asked Ben Fording, a field sales representative with over 10 years’ experience at BlastOne International, to share a few points and opinions gleaned from his experience in selling wet abrasive blast equipment into many different project situations.
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