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They're often the forgotten link in the supply chain. But when cement tankers are late, plant operators start to watch their silo indicators just a little more closely. The last thing they need on a busy day is to shut down because there's no cement.

Recently, Keith Solimar, president of Solimar Pneumatics, a Minneapolis-based manufacturer of pneumatic equipment, gave a presentation with suggestions of ways producers can decrease unloading time from their bulk haulers.

Here are four tips Solimar offered as starting points for decreasing cement-unloading time.

  • Increase your plant's pipeline diameter. The tanker's unloading rate is directly proportional to the cross-sectional area of the pipe.
  • Shorten your plant's unloading-line length. Often, due to either plant expansion or changes in truck queuing procedures, the length of the unloading line increases.
  • Plumb all unloading pipes to vertical. Any change from vertical reduces the fluidized cement's density. Thus, to maintain acceptable product flow rates, inclined pipes require blowers to output higher velocities and keep the cement moving smoothly.
  • Select the optimal air volume for your product. In the 1960s Solimar conducted 130 full-scale tests with various operating pressures, pipe lengths, pipe diameters, products, and air volumes. "To my amazement, I found air volumes could have a profound effect on unloading rates, especially when using short lengths," says Solimar.

By graphing the relationship between the flow capacity (pounds per minute) and the air volume, Solimar found that there is a "smooth" range where unloading is trouble-free and the risk of plugging is negligible. For longer lines, the plots of optimal air volume ranges were not as pronounced. Thus, to achieve optimal unloading rates, producers should choose higher air volumes.

Pipe length also affects air volume, since the compressed air expands as the pressure decreases, thus increasing the flow volume. Under 1 bar of tank pressure, the air velocity doubles by the time the product reaches the system's end.

For short pipelines, Solimar recommends an optimal air volume of about 225 standard cubic feet per minute (SCFM) for cement in a 4-inch pipe using 1 bar (15 psig) of pressure. "For a long line, the optimum air volume increases by about 70% to 380 SCFM," says Solimar.