To answer this question properly we need to understand the components that consume air.

Blast nozzle
The blast nozzle orifice (bore) is what we use to determine the compressor size as this is the main air consumer. Nozzles do wear as they are used and so a reserve of up to 50% extra air should be supplied per nozzle.

Breathing air Filter
This unit is fitted between the compressor and the blast helmet. If a cool air tube is fitted, this part of the system will then consume 20-25 cfm of air.

Air leaks
Although this is not part of the blast system, air leaks do consume air and need to be rectified. Do not ignore air leaks as these cost money.
 

When using GMA Garnet, the optimum air pressure is 95-100psi at the nozzle for speed of cleaning and for recyclability.

Note: If other equipments are being driven by air from the same compressor, these will then need to be factored into the compressor size calculation.

The above table is to be used as a guide only.

Example: No 6 nozzle at 100psi + 25cfm (breathing air filter) + 50% (100psi) for wear etc. = 325cfm minimum compressor size per nozzle.

For further explanation on the above, please contact us.

Air pressure along with abrasive selection can be the most important factor in an abrasive blast cleaning system. For many blasters, nozzle air pressure is not so important  as long as the job gets done.

Let's look at how the nozzle air pressure affects the productivity of the blasting system.

Rule of thumb: For every 1psi under 100psi = 1.5% loss of possible target production.

If you are performing abrasive blasting at 70psi (at the nozzle), you are blasting at 55% of your possible production rate - a loss of 45% of productivity, or it will take 45% longer to blast the same surface area than if the nozzle pressure was at 100psi.
It is very important when performing abrasive blasting with GMA Garnet to maintain a nozzle air pressure (test with the air and abrasive running through the blast hose) around the 95-100psi range as it has been found to be the optimum pressure for both blast cleaning rate and recycling.

The complete abrasive blasting system needs to be checked all the way from the compressor to the blast hose nozzle. Any air leak in any part of the system must be repaired it can reduce the nozzle pressure which, in turn will reduce production.

Check the pressure at different locations as air hose and blast pot fittings can reduce the nozzle pressure. If pressure drops are found (i.e. reduced fitting, air leaks, supply hoses, etc), modify the equipment to increase the nozzle pressure.

A simple nozzle pressure gauge can be used to determine the nozzle pressure at different points in the abrasive blasting system.

For further explanation on the above, please contact us.

Generally, not many changes need to be made to your existing equipment to use GMA Garnet. Normally, it is only the abrasive valve that needs to be changed to reduce the flow of GMA Garnet into the blasting hose.

Other areas to check are the compressor for a # 7 nozzle. The cfm requirement is 270 cfm per minute. Allowing for nozzle wear, possible friction loss in the air supply and blasting hoses, breathing air filters and blasting helmets, we recommend using a compressor with a minimum of 50% of the nozzle consumption i.e. 270 cfm + 50% of the nozzle consumption (135 cfm) = 405cfm per # 7 nozzle. For optimum efficiency, the nozzle pressure must be maintained between 95 - 100psi. This is critical in order to maximise the square meters cleaned per hour.

The air from the compressor must be free of moisture as this can reduce cleaning efficiency by up to 15% per hour. Use a non-restrictive, high volume dryer that does not cause a pressure drop through the system.

The blast pot supply hose from the compressor must have a minimum internal diameter of 40-50mm. This will allow for unrestricted airflow to the blast machine. Do not use restrictive fittings on the hose.

All blast hoses must have a minimum internal diametre of 32mm as this size reduces the friction loss in the blast hose. Use light wall blast hose for the whip end and not 25mm as this reduces the air abrasive flow through the hose to the nozzle. Also, use a good quality rubber blast hose as cheap hose does not allow static electricity to be dissipated and can lead to hose failures and possible injury.

The blasting nozzle is what determines the air compressor consumption. For efficient operation, it is critical that the nozzle is changed as it wears out to 1/16" larger than its original orifice size i.e. # 7 (7/16") and when this nozzle reaches ½", it needs to be changed. The venturi is what wears in the nozzle and as this wears, the abrasive velocity is reduced and air consumption is increased resulting to fewer square metres cleaned per hour due to reduced nozzle pressure and more fuel needed to run the compressor, etc. Note: CHANGE YOUR NOZZLES AS THEY WEAR.

As mentioned previously, we recommend that the abrasive metering system be changed to a GMA Garnet abrasive metering valve. This valve ensures that a constant, lean flow of abrasive is metered into the blast hose. It is very important to ensure the correct air/abrasive mix in the blast hose as too much abrasive reduces velocity - more energy is needed to propel the abrasive out of the nozzle and too little abrasive will not clean the steel at an acceptable rate per hour.

Some different blasting techniques need to be adopted to efficiently and effectively blast with GMA Garnet. Where practical, the blaster should blast from a distance of approximately 18-24" (450-600mm) from the surface being blasted. Angle the nozzle between 45 and 60 degrees depending on what is being removed from the surface and avoid blasting at 90 degrees to the surface. Blasters need to be trained to move their arm more quickly across the surface to achieve a higher production rate. If they blast as normal, over-blasting will occur, which is wasteful of both time and abrasive. They need to be informed that they will see less abrasive in the air stream and perfectly normal as GMA Garnet is a faster cutting abrasive.

GMA Garnet is very tough and hard and produces very little breakdown of the grains during blasting (approximately 10-15%). Therefore, the abrasive can be recycled a number of times provided it can be recovered. Before every re-use, the abrasive should be re-processed i.e. all of the fines, dust, paint particles, rust etc. should be removed. We can provide appropriate systems for the efficient and economical recycling of GMA Garnet. Recycling enables great reductions in abrasive consumption and disposal costs.

For further explanation on the above, please contact us.

Abrasive size
Abrasive selection is one area where price alone seems to dictate the product used, while the size and shape of the abrasive particles are generally not considered. Now, more than ever, there is irrefutable proof that the size and shape of the abrasive particles affect the speed of cleaning, surface cleanliness and the shape of the anchor pattern that is left on the surface.

Large abrasives (i.e. slags) generally leave a coarse, uneven and unclean surface that is undesirable to paint over. The smaller, sub-angular and harder garnet particles leave the surface with a smoother, more even profile that is cleaner (Sa2-3) and able to accept over-coating using suitable products.
Currently, most coating manufacturers are recommending a surface profile of between 50-75 microns (2-3 mils) and extensive pull-off adhesion tests have shown that this profile height is acceptable if the substrate has an even profile across the surface.

How do smaller grains work?
It has been found that smaller garnet grains work because of their size, specific gravity and hardness. There are approximately 11 million grains of (GMA Garnet 30/60) garnet per kg as opposed to 1 million grains of (20 mesh) slag per kg. As garnet has 11 times as many impacts per square centimetre, it stands to reason that the surface will be cleaned much faster than with a larger grain abrasive. We have now determined that smaller grains clean faster because there are more of them. The abrasive valve must be closed down because the abrasive mixing with the air needs to be reduced to achieve the desired abrasive velocity. In closing this valve down, abrasive consumption is reduced (a cost-saving) and the blast pot time before refill is extended 2-3 times that of a slag pot, resulting in less stopping and higher productivity.

Consumption
We have now determined that reducing the abrasive flow reduces consumption from (approximately) 1000kg/hour with slags to 250kg/hour with garnet (figures based on 30/60 GMA Garnet) and the paint usage will be reduced due to the more even profile on the substrate.

Breakdown rate
It is true that both slags and garnet breakdown on impact. Due to its greater hardness and smaller particle size, GMA Garnet breaks down at a far slower rate than the single use slags. GMA Garnet breaks down at the rate of 10-15% per blast, making it a very recyclable product. It has been found that a range of sizes through an abrasive will clean more efficiently than if all particles are of the same size. GMA Garnet can be recycled between 5-7 times through a purpose built recycling machine and adding 10-15% of new abrasive each recycle will maintain an even surface profile. Recycling will also lower the initial cost of the abrasive.

Conclusion
Smaller, harder sub-angular abrasive particles will clean at a faster, cleaner and more efficient rate than larger, softer, sharper abrasives. The more even profile of garnet will pass on paint savings and reduce coating failure due to poor surface preparation. Higher productivity will be achieved because of reduced abrasive flow and less refills of the blasting equipment. Compressor fuel savings are also achieved as the surface can be cleaned 2-3 faster than with slags etc. In addition, cost reductions from recycling of the abrasive and disposal costs will be reduced as less abrasive is used to clean the project as well as reduced freight costs to and from the projects.

For further explanation on the above, please contact us.

Industrial environments both on and offshore are experiencing high maintenance costs from the high cost of labour and materials needed to rejuvenate or replace the corroded areas of the structure.

The use of protective coatings through a properly planned maintenance programme reduces long-term maintenance costs. There are many new coatings and application methods available that can assist in extending the time between maintenance. However, no matter how good the coating, if it is not applied over a sound surface, failure will occur and maintenance will be required prematurely. The facility owner or purchasing department no longer ask "What is the price per kg or litre?", but instead are required to determine "What is the cost per square metre per year". Here are some thoughts that will determine a targeted maintenance programme.

Maintenance protection of steel substrates:
Given that most industrial metallic substrates are steel, this article assumes steel as the substrate. Steel is blasted and coated primarily for substrate protection from corrosion and to a lesser extent for decoration. The coating system is selected on the basis of the environment that the structure must withstand, e.g. offshore marine.

In new structures, the main reason for surface preparation is to remove the surface rust and millscale prior to coating. In an older structure, surface preparation becomes more difficult as there are other added contaminants such as salt, thick rust scale and deteriorated coatings as well as access problems, working plant, etc.

Painting over any surface contaminant no matter how good the coating will result to coating failure. Experience and scientific research have shown that for the best results to be achieved for all of the above named contaminants MUST be completely removed before coating and the coating MUST be applied over a clean and dry substrate that has been slightly roughened.

Various methods of surface preparation are available providing varying levels of surface preparation that in turn result to different coating life expectancies. Abrasive blast cleaning is the preferred method of surface preparation as the high velocity abrasives will work with a scouring action to completely clean the affected area, leaving it perfectly clean for coating. The required level of cleanliness is determined by the specification and coating used. Please refer to ISO for various standards on surface cleanliness.

Conclusion: The higher the level of surface cleanliness, the longer the coating life. Far too often, a lot of money is spent on the coating with the abrasive being purchased on cost alone and this practice should cease. The coating life is determined by the level of surface preparation and the abrasive used should also come with an ISO standard 11126-10 2000E. This standard determines the silica and chloride levels for chlorides need to be removed from the surface and not added by the abrasive.

GMA Garnet 30/60 mesh Premium Blast Grade garnet is a very high efficiency abrasive capable of producing superior performance in terms of lowest abrasive consumption and very high production rates while meeting all standards, environmental and health legislations.

For further explanation on the above, please contact us.