Figure 2. This modification enables venting if a valve lacks a vent port.

Look for frictional differences between products. Sometimes after conveying one material and either making a change to the ingredients or trying to convey a different material we forget that the particles may not behave the same. While basic physical properties can be helpful in predicting a problem you can’t go wrong with a few tests. Frictional changes can be subtle. Even the same product can have different shear rates and pickup velocities. In addition, don’t forget to check the characteristics of an existing material after conveying a new material. A fine coating or change in surface may alter the pressure drop in the system.

 

Running a convey test

One of the most important parts of putting in a new conveying line is correct testing, especially on a new product or long layout. Most equipment manufacturers will perform a test for a nominal fee that seldom covers its real cost. Remember vendors are not clairvoyant and usually don’t understand your overall process as well as you do. Some engineers arrive at a test expecting the manufacturer to guide them through the testing process and to point out what the buyer needs to do. That approach doesn’t sell a lot of pneumatic conveyors, which after all is the vendor’s objective. Most manufacturers’ test setups are not research systems but demonstration devices. So, it is best to have a test plan that you and the vendor agreed upon beforehand. This may mean bringing in additional testing resources at your cost to supplement what is provided.

Pointers

1. Look at particle attrition. Conduct particle sizing tests and examine the particles under a microscope.
2. Attempt to find the choking and saltation velocities. Compare these to calculated values. Design the system at 1.5 times the maximum value but plan to reduce this velocity after installation (by reducing the blower flow rate).
3. Run at both twice and half of the expected design velocity.
4. Ensure the solids-to-air ratio is constant during the test.
5. Calculate velocity along the line. This requires data on pressure and temperature.
6. Roughly determine the particle-to-gas velocity along a straight section of pipe.
7. Get the layout close to what is expected in the plant. Try to match elbows and lift location.
8. Make some of your own measurements. This keeps the vendor honest.
9. Inspect the piping. Pay attention to the type of joints, construction material, elbow R/D, feeder and collector details.
10. Run a “power failure” test. Deliberately stop the gas flow and let the line sit overnight, if possible. Failures happen. The best place to find out their consequences is in the lab.

While following the test and designing the conveyor, enjoy the process of understanding your material. Imagine yourself running around inside the pipe. Pneumatic conveyors are powerful and the solids convey considerable force. I was reminded of that one time when every few hours we heard a clanking noise in a vertical section on a recirculation pneumatic conveying blender. We shut down the conveyor and removed the bottom elbow to find a 6-in.-long, 1-in.-diameter bolt that had made several trips through the system. Oh well!

Thomas R. Blackwood is director of technology for Healthsite Associates, Ballwin, Mo. E-mail him at trblac@att.net.

References

1. Haraburda, S.S., “7 steps to efficient conveying,” <italic>Chemical Processing<end italic>, Vol. 63 (2), p. 57 (February 2000).
2. Duckworth, R.A. and H.E. Rose, “Transport of Solid Particles in Liquid and Gases,” <italic>Engineer<end italic>, Vol. 227 (5,905), pp. 392-396, 430-433 and 478-483, London (1969).
3. Jotaki, Y. and Y. Tomita, “Flow in the Acceleration Region of Pneumatic Transport of Granular Solids in Vertical Pipes,” <italic>Proceedings of Pneumotransport 3<end italic>, pp. 99-108, Bath, U.K. (April 1976).