OPTIMISE YOUR BLAST
Whilst the blast media and air mix defines the blasting process, optimising the air pressure, speed and abrasive content is vital if the blasting process is to be carried out efficiently.
Although the variables associated with the process are limited; compressor output, pot pressure, media valve setting, hose length, nozzle size, etc. - understanding how to optimise these variables is vitally important. Abrasive Blast Machine systems are designed to provide the optimum pressure and flow of both air and blast media through the blast nozzle. Ensuring that both the pressure & speed of the air flow and the abrasive media mix at the nozzle are set correctly will provide the ideal conditions for efficient blasting. To generate the ideal blast nozzle pressure requires a basic understanding of the effect of using a larger compressor capacity (cfm or m3/min) and the effect of increasing or decreasing the nozzle size.
As a general rule a higher compressor capacity (in cfm or m3/min) generates higher nozzle pressures and air speeds resulting in the opportunity to increase blasting efficiency. For example, if 100psi (6.8bar) is set as a nozzle pressure to generate optimum productivity, then a nozzle pressure of 80psi (5.5bar) will result in 66% blasting efficiency and a nozzle pressure of just 60psi (4.1bar) will result in 50% blasting efficiency. Such productivity reductions are quite significant and it is important to fully appreciate how the correct equipment choice can increase the effectiveness of your blasting. In the example above, increasing the nozzle pressure from 60psi to 100psi will result in the operator being able to double their efficiency and doubling their work speed. Using a higher compressor capacity also provides the opportunity to reduce grit consumption as explained below. It is inevitable that a larger capacity compressor will use more fuel. However when blast time can be halved, the additional fuel costs become insignificant. The above example shows how optimising the equipment settings is critical to performance. This is a function of both the compressor capacity and the blast machine settings. The nozzle pressure, system pressure, airflow and compressor capacity are all linked by a simple mathematical equation. Put simply, using a higher capacity compressor will generate higher system and nozzle pressures and also higher media speeds resulting in the opportunity to use bigger nozzle sizes and thus achieve more efficient blast area coverage.
Using larger nozzle orifice sizes produces a greater blast coverage area but requires a greater air capacity (cfm or m3/min) to ‘power’ the nozzle. A smaller nozzle size will typically produce a narrower blast pattern and consume less air. To maximise the potential benefits of using a larger nozzle diameter it is essential to ‘supply’ the nozzle with the correct air & media mix at sufficient speed and pressure. The choice of nozzle is therefore determined by the available air flow produced by the compressor. If a larger nozzle is chosen then to blast efficiently, a higher air capacity is required. The table opposite shows the correlation between volume of air, nozzle size and nozzle pressure and is often used in the industry to select nozzle size. Its real benefit is to select the optimum nozzle size for the nozzle pressure required to carry out the job. If the user requires a nozzle pressure of 100psi (6.8bar) for optimum blasting and is using a ½” #8 nozzle diameter then they would need a 340cfm (9.63m3/min) rated compressor. If a 222cfm compressor was used then the resultant nozzle pressure using the same #8 nozzle would be reduced from 100psi to 60psi (4.1bar), meaning the blast efficiency would be reduced by 50%.
The air capacity (in cfm or m3/min), nozzle size and nozzle pressure equation also suggests that blast efficiency will increase further if nozzle pressures above 100psi (6.8bar) are used. Increasing the air capacity from 340cfm to 430cfm then using the same #8 nozzle will generate 125psi (8.6bar) at the nozzle resulting in an increase in blast efficiency of 37% above that obtained from a 340cfm compressor. It is at this point where the pressure capacity of the abrasive blast machines now becomes increasingly important. Due to the system losses (blast hose leakage and flex, connections, internal friction efficiency, etc.), using an abrasive blast machine with a maximum pot capacity of as low as 120psi (8.2bar) or 150psi (10.3bar) will significantly restrict the nozzle pressure capability and may result in nozzle pressures being limited to significantly below 100psi (6.8bar). This can result in up to 50%, reductions in blast efficiency. It is for this reason that Elcometer offer two abrasive blast machine pressure ranges. The standard model has a pressure capacity of 180psi (12.4bar) which, when using the correct compressor, can significantly higher nozzle pressures and thus increase blast efficiency by as much as 50% above that possible when using a 150psi (10.3bar) rated abrasive blast machine.
Hose Length & Condition
Where long hose lengths are necessary, Elcometer also offer a range of high pressure (H) abrasive blast machines which have a maximum working pressure of 225psi (15.5bar). Due to the inherent pressure losses associated with long hose runs, these machines can continue to supply high nozzle pressures even when the hose length would normally result in low pressure at the nozzle. The Elcometer high pressure blast machines are ideal for use with large capacity compressors and long hose length applications such as those found in many construction sites and shipyards.
Media Valve Setting
Compressor capacity (in cfm or m3/min) and nozzle size generate the working pressure at the nozzle for optimum blasting. The media flow entering the air stream is controlled by the media valve setting. Whichever design is chosen the valve is designed to ensure the optimum air/media mix to enter the blast hose. Using an ‘open’ media valve will allow more media to enter the air stream in the hose. The weight of the excess media reduces the air speed which, whilst not negatively affecting the nozzle pressure, will result in lower media speed at the nozzle. Lower media speed results in less cutting action from the abrasive and thus less blasting efficiency. Furthermore, when the system pressure is not optimally set (use of insufficient cfm or use of oversize or worn nozzle) then media consumption will increase. As media costs are a significant part of the blast process then it is vital for the grit valve to be optimally set for the conditions.
Nozzle Pressure psi (bar)
|Nozzle Size & Orifice
|Volume of Air - cfm (m³/min)||50||60||70||80||90||100||125||150|
|(0.34)||(0.37)||(0.42)||(0.48)||(0.54)||(0.59)||(0.71)||(0.82)||3.2mm - 1/8"|
|(0.71)||(0.85)||(0.96)||(1.10)||(1.19)||(1.30)||(1.59)||(1.90)||4.8mm - 3/16"|
|(1.33)||(1.53)||(1.73)||(1.93)||(2.12)||(2.30)||(2.77)||(3.26)||6.35mm - 1/4"|
|(3.02)||(3.54)||(4.04)||(4.55)||(5.06)||(5.57)||(6.85)||(8.12)||9.5mm - 3/8"|
|(4.21)||(4.84)||(5.46)||(6.09)||(6.71)||(7.33)||(8.89)||(10.44)||11mm - 7/16"|
|(5.46)||(6.28)||(7.13)||(7.95)||(8.77)||(9.63)||(11.66)||(13.73)||12.5mm - 1/2"|
|(8.63)||(9.99)||(11.35)||(12.71)||(14.07)||(15.43)||(18.83)||(22.22)||16mm - 5/8"|
|(11.35)||(13.81)||(16.25)||(18.71)||(21.15)||(23.58)||(29.73)||(35.84)||19mm - 3/4"|