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Wet sand blasting – a technology prevails.

Wet sand blasting – the technology.

Besides the industry standard of dry blasting, wet sand blasting is establishing itself more and more in the preparation of industrial surfaces. With this technology one differentiates in principle between three different methods: Most devices are either classical dry blast cleaners where water is added in the jet nozzle or below the tank, or high pressure cleaners, where blast abrasive is added into the nozzle. In these cases the water forms a cone-shaped protective cover which should bind the dust and the blast abrasive.

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The third method already adds water to the blast abrasive in the tank and operates throughout the process with an abrasive-water-mixture. With this technology we speak about genuine wet sand blasting as pressurised water is used to produce pressure and to blast abrasive out of the tank. The formation of extremely fine dust particles, which occurs with conventional dry blasting, is reduced by 95% with our technology. All kinds of blast cleaning jobs, from gentle to powerful can be performed using just this one type of machine. The compressed air, which together with the blast abrasive makes up the aggressiveness of the jet, is regulated independent of the quantity of blast abrasive or its conveyance. Whether it is burnt-in old paint coats on thin aluminium shutters, coated steel girders or big differences in height of up 250 m that need to be dealt with, this technique can be used on almost every surface in every kind of weather (above 0°C) and is already in use in over 40 countries in completely different climatic zones.

Economic efficiency.

These patented wet sand blast systems offer elementary advantages where operating costs are concerned: the blasting abrasive does not need dry storage. At the same time we use 60% less abrasive and water with this method, which is not only environmental-friendly but also considerably more economic. A special dispensing valve makes these savings possible without any reduction in performance. This results in a 50 % reduction in the amount of waste generated, which especially in the case of polluted waste paint reduces the enormously increased disposal costs of up to € 200 per ton by half.

Unlike dry blasting machines, where the object that is to be blast-cleaned has to be hermetically sealed and equipped with an appropriate suction device, time-consuming and cost-intensive housing is not required with our technology. Due to the 95 % reduction in dust, these machines can be used even in critical situations such as in confined spaces, in the vicinity of highly sensitive technical devices or in areas open to the public.

Blasting abrasive.

In principle all blasting abrasives which are heavier than water can be used. These include for example conventional blasting slag, stone granulate as well as blast abrasives based on glass, chalk or soda. River bank or pit sand used in concrete repair should not be used for wet sandblasting of steel surfaces due to the proportion of quartz it contains.

Formation of flash rust.

One of the most frequently mentioned arguments against the use of wet sand blasting technology for blasting of steel surfaces is the so-called formation of flash rust. What is less well-known, on the other hand, is that it can be prevented by the use of environmentally harmless inhibitors and the application of a single-component polyurethane coating. I would therefore like to go into this subject in more detail.

The environmentally harmless rust retarders are manufactured on a water basis and enable abrasive blasting on steel with norm purity grade Sa 2.5. They are filled directly into the air flow together with the abrasive and water, thus preventing or delaying the formation of flash rust for up to 48 hours.

In addition, there are today special single-component polyurethane finishes which bind up to 99% of the surface humidity, thus sealing a surface permanently. The manufacturers guarantee protection against corrosion for five years. This protection is normally already achieved after a single coating. So the expense of multi-coatings no longer applies and the guarantee on the coating can be increased vis-à-vis competitive products.

The additional expense for the special coating and inhibitors is not only offset by our savings on abrasive but is even well compensated (35% less per hour of operation). The additional costs for moisture-binding polyurethane finishes amount to € 0.50 to € 1 per sqm. Rust retarding inhibitors lead to higher costs of an additional 6% per hour of operation. Requirements for the use of polyurethane coatings are dry or damp surfaces with a norm purity grade of Sa 2 to 2.5.

Below you will find three examples demonstrating the wide range of application open for the use of wet sand blasting machines on industrial surfaces.

American Navy submarines overhaul

torbo wet sand blasting machines took part successfully in the repairs to American Navy submarines last year. In the painting unit the hull of the submarine “US Michigan” was freed of lead paint in a reliable, environmentally friendly and time-saving manner. During the overhaul the unit worked in two 12-hour shifts round the clock to blast clean and coat the ship’s hull. By using our machines, as the Navy itself stated, they were able to complete the project in just 14 days and save a total of 10,000 man-hours and almost half a million dollars.

In order to be able to bind the lead components of the paint, a special additive was put into the abrasive blast mixture in the tank. An environmental-friendly inhibitor on a water basis prevented the freshly blast-cleaned metal from rusting.

The Royal Library in The Hague

A badly powder-coated aluminium façade of the Royal Library in The Hague had to be blast-cleaned and sealed over a surface of more than 35,000 sqm. The following assignment presented us with a special problem: the very hard coating had to be removed forcefully without however causing damage to the sensitive surface structure of the thin aluminium profile or denting it with the pressure of the jet. After extensive trial tests it became clear that this problem could only be solved by wet sand blasting machines. The blasting abrasive used was therefore fine slag with a grit size of 0.5-0.8 mm. The selected blast abrasive throughput was 2 litres per minute (80% blasting abrasive / 20% water). Finally a single component polyurethane finish was applied.

Both blast hoses as well as jet nozzles have been in error-free operation for 32 months. To date the costs for the wearing parts amount to € 150.00 per machine The low amount of blast abrasive used and the inexpensive waste disposal have resulted in lower operating costs on the whole. A further argument in respect of costs: a particularly pleasing factor for everyone was that the airtight housing for the entire library was no longer needed.

Electric power masts in Holland

The example of the surface treatment of high-voltage power masts in the Netherlands demonstrates the advantages of wet sand blasting on galvanised steel. Rust and old coating were removed over an area of 600 sqm per blast-cleaned mast and thus prepared for the new coating. The metering valve of the machines was set to gentle abrasive blasting so that the zinc layer was not damaged nor was there any oxidation caused. As a result of the 95% reduction in dust, hanging the area around the direct jet point with a sheet foil was all that was needed. So not only the cost-intensive, airtight housing of the 40 m high power masts was no longer required, but also the expensive encumbering protective clothing for the blast operators. They were able to move around freely which, at this working height, was particularly appreciated. The consumption of blasting abrasive was 2 litres per minute. The blasting abrasive used was slag with a grit size of 0-2 mm.

The bottom line.

Comparing dry and wet sand blasting technology, the wet sand blasting can lay claim to considerable advantages. With steel, dry blast surface cleaning is wide-spread but in most cases involves high environmental pollution. On the one hand, the dust formation causes problems so that everything has to be sealed airtight and undergoes extraction. On the other, the blast waste often has to be disposed of as hazardous waste. When removing old paint coats these are often heavy metal contaminated coatings which pollute the waste.

With genuine wet sand blasting, there are no longer any costs for the dry storage of blast abrasive, the consumption costs are lowered by 60%, the dust reduced by 95% and the use in critical environments possible without special housing. Environmentally friendly rust retarders and single component polyurethane coatings enable blast cleaning on steel with norm purity grade Sa 2.5 and yet still achieve savings of 35% in comparison with dry blast cleaning. Because of this, torbo wet sand blasting has already established itself as a standard throughout large sections of Holland’s petrochemical industry.

Josef Keizers jr., torbo ENGINEERING KEIZERS GmbH, Borken

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