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Overcoming slow, horizontal powder runs

Q: We have a customer blowing our barite with SG 4.3 and ground to -75 micron, with around 16% -6 micron. While this material blows more quickly from truck to silo than Moroccan Barite with an SG of 4.18, ground to -75 micron, but with 25-30% -6 micron, on long horizontal runs it blows much slower on a horizontal run of approximately 100 feet from the silo to the boat. Is it possible that the higher SG and larger paticle size of our barite lets it settle out? What is normally the solution? Will using a smaller transport pipe raise the pipe velocity and overcome this problem, or can one just use a bigger blower?

A: It sounds like there is little horizontal line from the truck to the silo and the same blower is being used to transport the barite from the silo to the boat. Also, it sounds like this is intended to be a dilute-phase transport line. I suspect that the solids to air ratio is very high for both lines. While a solids-to-air ratio of more than 5 for this dense material would be acceptable in a vertical line, the dense material would have a higher saltation velocity than its choking velocity and could result in slug flow in the horizontal line. This system may require a larger line with more air flow, especially on the line from the silo to the boat to reduce the solids to air ratio and maintain the higher solids flow rate. As an alternate, the line from the silo to the boat could be converted to dense phase with the same line size, but you will need a high pressure blower. Calculate the solids to air ratio to see where you are. (Note: don't use blower capacity since in a pressurized system the leakage back into the silo and the lower blower output at the higher pressure result in much high solids to air ratios.) The 100-ft line does not appear to be long enough for this -75 micron material to settle. However, you may want to check the de-aeration rate. Take a large coffee can, fill it about 2/3 full, cover the can and shake it. Set the can on a flat non-vibrating surface and quickly remove the lid and throw a quarter on top of the material. If the quarter disappears, this material de-aerates too fast and will need to be conveyed in dilute phase at a solids to air ratio of less than 5.

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