Macawber Design Philosophy
Dense Phase Pneumatic Transporters

Dense phase pneumatic conveying pressure vessels were introduced to the U.S. markets in the early 1960s. Pressure vessels were used because the pressures required for dense phase conveying normally exceeded 15psi - the point at which ASME coded vessels are mandatory.

The valving available at that time for use as infeed to the vessel was limited basically to butterfly valves although slide gates were also tried. There are several problems associated with butterfly valves when used as pressure vessel infeed valves:

• Butterfly and slide gate valves are not able to cut through a falling or static column of material since the material gets trapped between the valve disc and the seat preventing effective sealing when the vessel is under pressure.
  
  Two valves must be used one above the other. The upper valve cuts off material flow but does not have to seal. The lower valve can close properly and hence seal effectively. (A level probe is also necessary in the vessel to sense when the upper valve must close)
  
  
• When the valves are open the valve discs are directly in the flow of the material. This not only inhibits flow into the vessel but makes the valve susceptible to wear - even with mildly abrasive materials. The valve deteriorates at an increasing rate and sealing again becomes ineffective. Often, the valve fails altogether.
  
  
• To avoid poor sealing, the valves must be continuously maintained and often completely replaced - even as frequently as every 2-3 months.

To reduce the above effects on the valve, the pressure vessels are designed to cycle infrequently - usually 10 - 12 times per hour. This reduces the load. This means, however, that quite large vessels are used. For instance, a vessel of 15-25 cu. ft. operating on a 4" line is quite normal.

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