6.5.2. Blast-cleaning.
Blast-cleaning is the common term for all methods using abrasives. Different kinds of abrasives are propelled either by centrifugal force or by high-velocity stream of fluid (i.e. air or water) to remove rust, millscale, existing coatings and other contaminants from the substrate. The different methods of blast-cleaning are:
Centrifugal abrasive blast-cleaning.
Compressed-air abrasive blast-cleaning.
Vacuum or suction-head abrasive blast-cleaning.
Moisture-injection abrasive blast-cleaning
Wet abrasive blast-cleaning
Water blasting-water jetting
Figure 6.2.
Illustration of various blasting methods
6.5.2.1.Centrifugal abrasive blast-cleaning
Centrifugal abrasive blast-cleaning is carried out in fixed installation or mobile units in which the abrasive is fed to rotating wheels or impellers positioned to throw the abrasive evenly and at high velocity on to the surface to be cleaned. The abrasives used are steel grit and shot (very often a mixture) and cut steel wire. The abrasive is recirculated. This method is suitable for continuous treatment of plates and profiles, and have a very high-volume through-put. Normally, in connection with the centrifugal blasting machine there is an application unit which automatically apply a shopprimer or prefabrication primer on the steel. Major yards have their own plant, while smaller yards buy their steel ready primed from specialist companies or steel manufacturers.
The blast-cleaning standard should be Sa 2 ½ (according to ISO 8501) before the shopprimer is applied. The shop-primer’s paint inspector should check the plant and ensure that this degree of cleanliness is maintained before the shop-primer is applied. However, in most situations, the steel is supplied shotblasted and shop-primed allowing little opportunity to check the quality of the shot blasting.
The shopprimer, which is intended to provide temporary protection, is applied to a specified film thickness by an automated process. The thickness is usually 15 - 20 microns for iron oxide shop-primer and 10 - 15 microns for zinc shopprimer. It is very important to keep inside the specification, because excessive shopprimer thickness will reduce the cutting and welding speed and thereby prolong the production time at the yard.
6.5.2.2.Compressed-air abrasive blast-cleaning.
Compressed-air abrasive blast-cleaning is the most important and most widely used method of surface preparation. The abrasive (generally a non-reusable, non-metallic type) is carried by a jet of compressed air through a hand-held nozzle. Through the nozzle the air/abrasive mixture is directed at high velocity on to the surfaces to be cleaned. The abrasive may be injected into the air stream from a pressurised container or be drawn into the air stream by suction from an unpressurized container.
The compressed air supply used for abrasive blasting should be free from water and oil. Adequate moisture and oil traps should be provided in the lines to make certain that the air does not contain oil or water. This can be checked by taking a white rag (“blotter test”) to determine if the blast air is free from moisture and oil when leaving the nozzle. NB! No abrasive in the air stream during this test.
The air pressure should be approximately 7 kg/cm² (100 p.s.i.) and should be measured with a needle pressure gauge inserted into the blast hose close to the nozzle. It is very important to measure close to the nozzle as there will always be a loss of pressure in the hoses between the compressor and the nozzle. The pressure drop depend on the diameter and the length of the hoses. If the air pressure at the nozzle is only 4,2 kg/cm² the production rate will be reduced with 50%. As a rule of thumb we say that the blast hose shall have a light opening which is 3-4 times the “light opening” of the nozzle.