Powder Handling: Deter A Dust-Up

This Month’s Puzzler

I am a new graduate now working as a production engineer at an extrusion plastic manufacturer and would like to make a good start. I manage a process to produce paint pigments and would welcome some tips on optimizing the operation (Figure online).

From what I’ve gathered in a few weeks, we have bouts of clogging in the paint spray nozzles at the extruder. We also see far more fines in the screen than corporate engineering considers appropriate. We change bags every couple of weeks, which seems too often; the bags are tossed in drums as toxic waste.

Laboratory results show a lot of dust collected on the coarse product. Dust sometimes appears to get through the baghouse to the blower. I see dust on the duct connections to the blower and at the outlet to the atmosphere. One operator complained that dust clogs the product container nozzle of the grind collected at the screen.

I talked to several managers and engineers. The production manager said not to worry about it because the plant always has run this way. Someone in corporate engineering suggested digging through the files, hinting this wasn’t the first design for the system. The plant safety manager isn’t worried about the dust but the corporate manager is concerned because exposure to this pigment is a problem in Europe.

Can you suggest anything I should look at to reduce downtime and improve product quality?

Dust Off Old Files

This certainly is a problem that requires attention and a great way to make an impact at your new job. Let’s see: a dust problem in the product, an undertow of politics (corporate engineering is concerned but plant production isn’t), a potential safety and environmental problem, and a reliability issue.

Someone in corporate engineering hinting that this wasn’t “the first design for the system” is a big clue. However, with the production manager unconvinced there’s a problem, you won’t make any headway if you can’t justify at a least a 25% rate of return (four-year payback).

Let’s consider routine things to eliminate: divide the system into before and after the hammer mill. The first step is to review maintenance logs and perform sampling.

Samples are crucial, so you will need funding for sample analysis. If you can’t get that, give up — the politics are against you. Get samples from the feeder, the top (after the air lock) and bottom of the hammer mill, the cyclone discharge, the fines and coarse product, and the dust in the dust collector. You should, at least, have the product dust samples. You’re looking for particle size distributions — where they change and how. Keep in mind that round particles are an anomaly in nature and uniformity invariably means you missed something. Sample at regular intervals for a few months; ideally take winter and summer samples. Also, identify the dryness of the compressed air from utility records; review more than a year’s worth.

Now, consider maintenance. What was the system shut down for? Has anything changed since corporate engineering commissioned it? Was the process ever fine-tuned? Too often, after equipment is installed, nobody checks the product particle size against the details in the equipment vendor’s bid package. Dig into the files; compare the bids in the folders against the actual equipment. Look into what typically goes wrong with screening operations, such as tears in the screen, corrosion in the mill, motor failures, etc.

We’ve focused so far on the back end of the process. Now, let’s consider the front: the hammer mill. Maintenance records may reveal that its motor has given a great deal of trouble, e.g., burnouts, clogging or jamming. This could suggest reducing the tip speed of the mill. Seek help from corporate engineering about electrical issues with the motor. In addition, contact your local winding shop to ask how many visits they’ve made. Because the optimum rotation speed may change with what’s in the feeder hopper, you may want to look into a variable speed drive so you can optimize the speed.
Dirk Willard, consultant
Wooster, Ohio

August’s Puzzler

We’ve run our nitric acid purification process (Figure 1) for about ten years. Recently, we shut it down for minor repairs. Less than an hour into the subsequent startup, an explosion occurred. The building was filled with acrid orange-yellow gas. This prompted a unit evacuation. We barely had time to complete the emergency shutdown procedures before the general evacuation. Fortunately, nobody was hurt.