Evaluation of Wet Blast Cleaning Units

Evaluation of Wet Blast Cleaning Units by Itis universally acknowledged that dry blasting cannot remove tight millscale, tight ~~~~~~d R. Appleman abrasive blasting is the most efficient and rust, and paint, and while some power tools are Senior Editor economical technique for cleaning structural available for the removal of tight residues, they and steel for painting in industrial applications. The are less eficient than blast cleaning. Other new Joseph A. Bruno, Jr. abrasive blasting unit delivers to the surface a techniques have been described, but have not Technical Editor high velocity stream of hard, angular abrasive, yet proven practical for large scale production which has the ability to rapidly remove existing cleaning of steel. paints, rust, and millscale to roughen the base Wet abrasive blasting offers the potential metal for improved adhesion. The equipment to reduce or eliminate many of the problems and techniques for dry blasting have become associated with dry blasting and at the same fairly well standardized and provide a high time offers relatively high production rates degree of reliability. and cleaning efficiency. Dry blasting has been restricted in recent There are several generic types of wet years because of health hazards from silica dust blasting equipment with large variabilities in inhalation; air quality concerns with visibility, operating parameters, reliability, cleaning rates suspended particulates, and fugitive or nuisance and effectiveness, cost, safety, and user satis- dust; and dust contamination of machinery or faction. This article describes the results of equipment. There has also been concern about field evaluations of several different types and the disposition of the spent abrasive, which may manufacturers of equipment for wet blasting. contain lead compounds or other toxic materi- The emphasis of this study, sponsored by als from the paint film. the National Shipbuilding Research Program, Alternatives to sand blasting include the Federal Highway Administration, and the silica-free or low-dusting abrasives, high US Army Corps of Engineers, was on field pressure water blasting, wet sandblasting, and demonstrations rather than literature values or power tool cleaning. Alternative abrasives such second-hand accounts. From a review of trade as mineral slags often eliminate the silica and technical literature, and public requests for hazard, but these abrasives may be more expen- information, ten different wet blast units were sive or more difficult to obtain than sand, and selected for field evaluation. These evaluations some have recently been under attack for trace were conducted on steel surfaces typically en- concentrations of toxic heavy metals. High countered in marine, highway, and water works pressure water blasting and hand and power maintenance, such as rusted and pitted steel, tool cleaning are suitable for removing loose millscale-covered steel, and painted steel. For Fig. 1. Conical water ring rust and paint. However, high pressure water lside view) each demonstration, the representative struc- Counesv of Clemtex. Inc. tures were cleaned using wet blast techniques and dry blast cleaning controls, with careful documentation of cleaning rates, cleanliness, and other factors required for the evaluation. DESCRIPTION OF UNITS AND TECHNOLOGY Wet blast units can be categorized into four major types as shown in Table 1. Two of these in- volve air abrasive blasting with water addition. The others are pressurized water blasting with and without abrasives. The basic principles and variations of these types of wet blasting Journal of Protective Coatings & Linings NOZZLE ADAPTOR / Fig. 2. Nozzle for ABRASIVE abrasive wet blast Courtesy of Clemc Industries Table 1 Fig. 3. Control unr Classification of Wet Blasting Units retrofit air abrasive blast Courtesy of Clemca Air Abrasive Wet Blasting Industries Air/Water/Abrasive Slurry Blasting Pressurized Water Abrasive Blasting High Pressure Water 6,000 psi to 15,000 psi Low Pressure Water 2,000 psi to 4.M0 psi Ultra High Pressure Water Jetting ZO,MM psi to 50.000 psi - - - - - - - will be reviewed briefly. The discussion will the abrasive is wet before it reaches the surface. also review the most important features and In the second type of air abrasive wet components of the various types of systems in- blasting, the water is added to the abrasive just vestigated. before it reaches the nozzle. In one version, a nozzle adaptor is mounted between the nozzle Air Abrasive Wet Blasting holder and nozzle (Fig. 2). Pressurized water The air abrasive wet blasting units vary with from an air-operated pump is controlled with a respect to the method and location of water needle valve. The water pressure is normally addition, the type of control system, the device on the order of 300 psi to 800 psi. For many of for adding and monitoring inhibitor, and the these units, the water and sand can be operated design of the nozzle and the overall system. independently. Thus, for example, by closing the Fig. 4. Control unit i needle valve, one can dry sandblast in areas abrasive wet blast (cc Water can be added downstream of the nozzle, plete system) just before the sand enters the nozzle, or at the where wet blasting may not be needed. Also, by Courtesy of Clemtex. source of the abrasive (air/water/abrasive slurry releasing the nozzle handle, one can use low blasting). One of the earliest methods was the pressure water to wash off the sand from the water envelopment process or "water curtain surface. method," which projects a cone of water around These units may be designed as retro- the stream of air and abrasive as it leaves the fits (Fig. 3) for existing abrasive blasting units nozzle. A simple water ring adaptor fits around or sold as complete units, including abrasive the blasting hoze nozzle. This technique is blast machine, air powered pump, and a mix- reported to reduce the airborne dust by about ing tank (Fig. 4). These types of units are also 50 percent to 75 percent. It has a minimal effect very effective in reducing the amount on the cleaning rate because the water does not of dust. mix with the abrasive. It does make the unit slightly more unwieldy and could affect clean- AiriWa ter/A hrasive ing rate in that manner. Slurry Blasting The water stream could also be sprayed Another technique is addition of water to the into the abrasive stream beyond the nozzle abrasive stream at the control unit upstream (Fig. 1). This gives a greater degree of dust con- of the nozzle. In these systems, the mixture of trol than the water envelope method because air, water, and sand is propelled through the hose to the nozzle without any additional coup- a specially designed small orifice nozzle. This ling at the nozzle. In several of these units the jet has some erosive force and has been utilized air, water, and sand can be independently con- for removing paints and corrosion products from trolled by the operator, either by microswitches structural steel. The principal focus of this at his control, or remotely, by another operator, evaluation is on water blasting with abrasives who may be in audio contact with the blaster. rather than on pure water blasting. However, As with the previous types of systems, these for comparison purposes, several of the high units allow the operator to rinse off the wet sand pressure units were operated without abrasives. from the surface with pure water, often contain- In addition, one unit was observed that was ing an inhibitor. Certain units can be used to designed to be operated without sand because feather back paint by reducing the air pressure, of the extremely high pressures attained. resulting in a less erosive slurry stream. The major components of a water blasting Because the sand is intimately mixed with the unit are water, these units are extremely effective in positive displacement pump and appro- reducing the amount of dust (Fig. 5). priate power unit, high pressure hydraulic delivery hose, High Pressure Water BIasting high pressure nozzle, and Fig. 5. Control unit for High pressure water blasting is a technique control valve system. aidwatedabrasive slurry which produces a high velocity stream of water Other components include water filter, I~last by passing a flow of pressurized water through pressure gauge, flow meter, inhibitor, and Courtesv of Williams Con- tracting Group, Inc. metering and monitoring attachments. High pressure water blasting without sand has not shown the capability of removing tight rust or intact millscale from steel except at exceedingly slow rates or at ultra high pressures (> 30,000 psi). In addition, it cannot produce a profile (surface roughening) of the steel itself. In order to introduce additional erosive force into water blasting, abrasives must be incorporated into the water jet. Pressurized Water Abrasive Blasting High pressure units use water pressures from 6,000 psi to 20,000 psi. The flow rates are normally 5 gallons to 15 gallons of water per minute. These units require a different type of nozzle than used for straight high pressure water jetting. The nozzle orifice must be large enough (typically 318 in.) to permit the abrasive to pass through. Also observed were several units which operated at substantially lower pressures and thrust rates than those given above (Fig. 6). Water blasters with pressures of 3,000 psi to 4,000 psi would be expected to provide much greater ease of handling and safety than the high pressure units. A few of these were sim- ply high pressure units operated at reduced J~~urnaiof Pmtecrivr Coating8 d Linings Fig. 6. Low pressure sand hdasting (patentc nozzle design); note width. pressures. Others were designed for use at lower alternate design is claimed to make it possible pressures. for the water to maintain the maximum velocity, minimize the loss of energy, and deliver Operator Back Thrust more abrasive at higher impact (Fig.

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