Maintaining proper abrasive levels in your blast equipment is crucial to keeping operating costs low and productivity high. Often times, blast equipment operators believe they can save money by not keeping the blast machine hoppers full of abrasive. What drives this thinking? A false impression of cost savings is created when, by not continually introducing abrasive into the blast system, the operator believes they are getting more cycles of abrasive for their money. One customer recently told me, “I get more bang for my buck by not adding abrasive every time I run this machine”. Actually the opposite is true. Adding constant and measured abrasive to your blast machine will SAVE MONEY on the surface preparation process. The reason for the savings is explained by examining three areas; abrasive consumption, faster and more effective blasting and wear on the blast equipment.
Let’s look at these areas in more detail:
Abrasive is consumed over time as it cycles through the blast machine and abrades the work. Cast steel shot and grit, perhaps the most common abrasive media used in centrifugal blast machines, becomes smaller and more fractured each time it impacts the work and recycles through the blast machine reclaim system and abrasive hopper. A low abrasive level in the storage hopper means the shot does not have to wait in line. In other words, the shot will cycle through the equipment more frequently as compared to a high or properly filled abrasive hopper. The media simply breaks down faster when proper abrasive levels are not maintained. So you get “less bang for your buck”. More cycles per hour means the abrasive that is doing the work breaks down faster. In a full hopper situation the abrasive recycles less and lasts longer.
Proper abrasive levels mean that the size distribution of abrasive cleans the work faster and more effectively. The proper size distribution of shot means there is a mixture of new larger media with smaller media particles. This mixture allows for the best coverage or scattering of impacts on the work. When a proper distribution of media is thrown at the work, the cleaning is much more effective which in turn allows for faster line speeds and consequently higher productivity. When abrasive levels are low the distribution of particle sizes is tilted to the smaller end of particle sizes. The absence of larger particles effects the size distribution resulting in slower line speed or decreased productivity. A tell tale sign that abrasive levels are not proper is when the operator notices it is taking longer to clean the work. Simply put; running the machine faster and therefore for less time decreases overall machine operating cost.
An operating mix that is heavily weighted to fine particles and abrasive dust is more abrasive to the blast equipment than a properly maintained operating mix associated with full media hoppers. Fine particles and dust act less like high velocity media impacting the work and more like sandpaper working to wear the blast wheels and reclaim system in the equipment. In a low hopper condition it will take longer to clean thereby allowing more time for the abrasive dust to work on the machines wear components, particularly with the blast wheel.
The following images, supplied by W Abrasives, demonstrate the effects of proper abrasive levels in high, medium and low abrasive hopper levels. The significant one is the low hopper level; it clearly shows how the abrasive has broken down resulting in a high percentage of fines in the operating mix. These images explain the reason for possible longer cycle times, a lower profile, and more importantly higher abrasive consumption (the abrasive circulates through the machine quicker).
Ways to maintain proper shot levels.
For direct drive blast wheels, (electric motor directly attached to the blast wheel), the best way to determine if the proper amount of abrasive is flowing into the blast wheel is to check the electrical load on the motor or amp draw. Abrasive puts load demand on the electrical motor thus increasing the amp reading. Too little abrasive puts less demand on the electrical motor and lowers the amp reading. Ideally the motor should operate at the “Full Load Amp” rating of the motor to maximize the productive capability of the blast system. Amp reading should be checked periodically during blast machine operation to determine if the motors are working properly and indicate the need for the addition of more abrasive.
Some blasting equipment will display a warning that alerts the operator to add abrasive. With PLC and HMI screens these alerts are easy to program on a touch screen user interface and provide a real convenience for the operator. A better and more convenient method is the addition of an Automatic Abrasive Adder (see photo). The automatic abrasive adder is an external hopper with sensors and controls that provides incremental additions of shot without the need for operator tending. The operator needs only to maintain a level of shot in the easy to access and monitor external hopper
In conclusion, the best way to save money on your abrasive costs is to maintain full and proper levels of abrasive. Allowing your blast machine to run without adding abrasive to maintain the recommended level does not save money, in contrast, it will increase your costs.