And nine (9) important tips to observe when “air blasting” …. but firstly the principles!

Introduction 2

Operating principles of a pressure blast system

The blast cabinet itself remains almost the same as the Venturie type; it is mainly the abrasive delivery system that is more highly engineered.

Function

The media hopper under the cabinet is used as a collection area, slowing the spent media almost to a stop before being drawn from the hopper bottom to be further processed.  This could be through a cyclone, airwash separator or even a magnetic separator, to take out fine contaminant and media fines (undersized dust, broken media).

Under the media hopper a pressure vessel is placed, inside which there is a “pop up valve”.

Through auto controls and before pressurisation by compressed air, the pop up valve sits on its internal yoke under the abrasive entrance to the pressure vessel and upon which sits the base of the media hopper. (As shown above).

[By providing a double chamber (not shown above) this would allow a continuous process to evolve].

• The charge of media is allowed into the pressure vessel.
• The machine can now be charged and ready for work.
• Upon activation the air pressure closes the pop up valve and pressurises the vessel with the media inside, equal to the line pressure.
• A separate air line is provided for and runs externally under the pressure vessel, through the bottom of an abrasive pinch contol valve.  At this point two controlled elements meet.
• The air line at a controlled volume and pressure.
• The Abrasive media Pinch Valve Volume control.
• Once these have been set for air to media ratio these are left or locked for integrity and secured process.  Each time the cabinet is activated the process is limited or controlled to the set parameters; performance is then repeatable time after time.

Depending on the volume of air available, higher volumes of compressed air supplied at generally higher pressures will provide more process speed (coverage); and accordingly, an appropriate abrasive delivery nozzle size can be decided upon.  However, when compared with a venturie/suction fed system it is not necessary to work at say 80 psi (5.5bar) and in most cases 45 to 60 psi (or 3 to 4 bar) is sufficient. The difference in this case is likely the increased volume of air at a lower pressure (i.e. using a larger nozzle to deliver the media).

The advantage of a “pressure system” abrasive delivery is that it will provide:

• Greater media velocity.  A pressure blast machine produces greater abrasive particle velocity than a suction blast machine, which means faster cleaning of tough contaminants.
• Moves more media.  More abrasive particles are propelled against the work surface per second, for a given nozzle size.  Again, this translates into greater productivity particularly on difficult applications.
• Greater stand-off distance.  Because the abrasive particles are moving faster to begin with, you can increase the stand-off distance of the blast nozzle from the work surface, producing a larger blast pattern while still achieving velocities great enough to clean the work.
• More productive.  As a general rule of thumb, pressure abrasive blast machines are four times as productive as suction abrasive blast machines.

Higher Ra/Rz values.  Pressure systems enable the keying of the surface to a deeper  degree (e.g. higher Ra or Rz for bonding/adhesion). This system of delivery allows larger media and with higher densities to be more sucessfully delivered as a process.

• Peening/stress relieving.  A greater compressive strength and depth of peening is capable of being established.
• Dedicated productivity.  Pressure systems will provide greater overall productivity than a suction blast system.

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Post written by
Haydn Kitchen
Shot Blasting Technical Manager