September Process Puzzler: Uncovering cause of solid build-up

Readers offer suggestions to improve reliability of a vibrating screen in Chemical Processing's monthly Process Puzzler feature.

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Install an enlarged funnel-type spool between the vibrating screen and the hopper. Care must be taken because of the large mouth, which could tend to leak. Reduce the gap as much as possible and check the tramp air loading on the vacuum system. Feed from the hopper should be verified. Confirm that the feed screw below the hopper is a variable pitch design.

I recommend two final improvements. Install vibrators (not an air cannon) at the bottom and side of the hopper and at the bottom and side of the pulverizer hopper. Avoid using these flow aid devices except during start-up. Since no clean-out ports were designed, it would be wise to install pairs (180° apart) on either side of RVs and at the bottom of all hoppers. Check the air loads on the pneumatic conveyor: ports may leak.

Prabin Shrestha, energy engineer
ROQUETTE America, Keokuk, Iowa

Look at the material balance

Based on the problem description the system is under-designed. Combination of the following is possible cause of the problem: 1) coal feed rate is too high for the size of vibrating screen; 2) as the coal moves, further size reduction is generating additional dust; 3) not enough air is being used to remove dust from the system; 4) air impingement nozzles aren’t properly placed; and 5) dust-bridging is leading to plugging in the hopper.

Coal build up on the screen may suggest that the there is insufficient slope for the coal to move forward.

If the figure is correct, the orientation of the dust intake may be wrong. Pick-up pipe should be straight up so that all the dust can be directed to the dust collector without any accumulation or bridging in the pipe. Air has to impinge at high enough velocity to dislodge the dust and carry it to the pick-up pipe. In addition, it might be necessary to have air flow countercurrent to the coal flow along the vibrating screen bed so that all the dust goes to the dust collector with a minimum of dust entering the hopper.

The system should be redesigned with a good mass balance. I believe a proper design with above considerations can solve the problem.

Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

Add vent lines

I’m not sure how to resolve the problem with the dust but have some thoughts on avoiding plugging in the coal. I believe the symptoms can be solved by adding some additional lines. The following problem being faced in the system: the feed line to vibrating screen is getting choked; the material isn’t sufficiently flowing to screw conveyor; and, sometimes the line to screw conveyor is getting plugged.

Figure 3 describes the changes that I recommend. There are several solutions that should be considered separately.

Figure 3. Flow is blocked at discharge of vibrating screen.
Figure 3. Flow is blocked at discharge of vibrating screen.

To avoid choking of feed line to vibrating screen, the gap between line and screen should be increased. If there is a cloth bellow at the end of the pipe, its length should be reduced so that there will be sufficient gap between it and the screen.

The counter weights of the motor of the screen should be adjusted to increase the vibration in horizontal and/or vertical direction. It may be that the vibration motion is inadequate for the task.

To avoid air-locking of the rotary valve, a vent line should be installed connecting the rotary valve vent port with the top of the feed hopper. This line is shown in the drawing. Take care with this line that there is sufficient slope to avoid build-up of solids. Piping should be flanged, not welded, for ease of disassembly. Adding view-ports to such lines might be useful. Size the vent line based on the volume discharge rate for the rotary air lock.

Watch the process after making these improvements. If the problems remain, an additional vent line can be provided with a removable screen. This line will vent only air and solid won’t plug the line. Piping should be designed for disassembly. Use the leakage rate for the rotary valve for sizing. To avoid plugging of the feed line to screw conveyor, add a vent line to the screw conveyor connecting it to the feed hopper; the cross-sectional area of the vent line for screw conveyor should be the same as the area for the screened line.

If these pipe changes don’t effect an improvement, consider re-evaluating your equipment choices.

Kamal Parikh, general manager, technical service
Reliance Industries Ltd., Surat, India

Fine-tune the screen

With this type of equipment it is difficult to develop an understanding of the solid flow problem without seeing the equipment in motion. I would look for ways to impart energy perpendicular to the screen to dislodge particles and prevent blinding. One idea might be to increase the angle of the screen. This could reduce residence time and prevent backup into the feeder. Another idea is to increase the screen mesh size. This may reduce blinding, depending on the particle size distribution. The drawback, obviously, would be increased particle loss — to the dust collection system. After the change to the material balance is established, look at the cost — both for disposal cost (of the dust) and the loss of product.

In our plant, we use a vendor’s product that includes rubber balls in its vibration screen. The balls bounce up against the underside of the screen to keep it from blinding off. The balls come in different materials that impart varying amount of force. They need to be inspected for wear periodically and replaced as needed. If adjusting the screen doesn’t work out, consider this type of screen, if temperatures allow.

Tim Bonner, quality engineer
Celanese, Calvert City, Ky.

It’s a moisture problem

Based on your info and the fact that upstream and downstream conveying also is plugging up; the moisture content of the coal is probably too high for reliable handling. Obviously then a drying step is needed upstream.

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