by Alex Kice, KICE Industries, USA

 

Dust explosions in the grain and milling industries usually start inside process equipment such as mills, dryers, mixers, classifiers, conveyors, storage silos and hoppers. Dust explosions can cause catastrophic loss of life, injuries, and destruction of facilities and assets.

Maintaining the solution

An important item that affects the continued performance of a dust control system is proper maintenance. When it comes to maintenance, dust control systems are often overlooked and ignored. If a piece of machinery (such as a conveyor) goes down, the whole operation is interrupted.

Such a disruption cannot be ignored and requires immediate attention. However, if a dust control duct gets plugged with material, it is often not even noticed since the suction will often progressively get worse. Soon, the dust control system is rendered ineffective due to a lack of attention. The solution is to include the dust control system in your planned maintenance program.

Troubleshooting the system

Regularly inspect the system, just as you do with other equipment, to make sure it is operating properly. Check not only the obvious things, such as fan drive belts and bearings, but also check for a plugged cyclone, a worn-out elbow, a plugged-up duct or hood. Many of these problems will affect the amount of suction at the hood.

Therefore, by taking an air reading (static pressure) at the hood and comparing it with the reading recorded when the system was originally installed, it can be determined if the system is still functioning properly. This type of reading is simple to take, and the equipment required to do it is inexpensive and easy to use.

Some of the items you should think about when troubleshooting a problematic system could include:

Fan function

One of the items you need to double-check is the rotation of your fan in your dust collection system. Is the fan rotation correct and match the manufacturer's requirement? This is often associated with motor change out. Fans that are turning the incorrect direction will still move some air but will not overcome nearly as much static as it is intended to.

Cleaning systems

Carefully examine your baghouse cleaning system. There are several parts working together for the baghouse filter to function properly. Any faulty item can affect the efficiency or operation of your baghouse. A magnehelic gauge has low pressure and high-pressure port connection to the filter housing to measure internal pressure differential.

This gauge will read 0' – 10' w.c.. when the pressure drop reaches 6' w.c. new bags should be ordered, then changed when the baghouse reaches 8' w.c. As pressure drop increases, the fan damper should be opened more to maintain airflow.

Filter discharge

If a single sheet of paper is held up by suction on the airlock discharge, then the airlock may be worn out. If no product is discharging from your baghouse, there is a plug somewhere that needs to be addressed. If dust is allowed to build up inside the baghouse, the overall performance of the system could be greatly affected.

Lack of suction

If there is a noticeable decrease in suction at your pickup points, considering addressing the following items:

  • Someone changed the dampers
  • Fan running backward
  • Filter bags blinded over
  • Line plugged with dust
  • Leaks in ducting
  • Hood design

Suction fan discharge

Bags will get worn out and tear over time. But you can also have bags that are not installed correctly. It is relatively common that filter bags are not properly installed. Bag installation is just as important as any other component.

If broken bags are an issue or the location of the broken bag is in a specific part on the filter, you need to look at ways to protect against wear. For example, consider using a material handling inlet which enters the baghouse housing below the bags and has a shroud to prevent material from impacting the bags. Too frequent pulse cleaning can lead to premature bag failure, as well.

Dust change

If excessive moisture is allowed into a dust collection system many issues may arise, including:

  • Wet dust indicates leaks in ducting, housing, etc. during rain or snow conditions
  • Dust gets heavier and lays in line
  • Agglomerates on the surface of the bag

Bridge location

Is it bridged above or below the airlock? Worn airlock clearance allows air to leak into the filter. As with any type of product, dust must be allowed to flow freely and easily. Improper duct or hopper angles and chokepoints can lead to bridging issues.

Timing

It is important to understand the overall system. Knowing how it operates in optimal and upset conditions can help you identify issues before they become problematic. Does it happen at certain times, such as light product collection or spring/fall?

Dirty filter bags

Are the filter bags dirty and need changing? Any baghouse filter should have a magnehelic gauge installed. This will measure the pressure drop across the baghouse and allow the operator to have knowledge of how dirty the bags are becoming over time.

Noises

Although most facilities are noisy during operation, air leaks and equipment faults can often be heard or detected, such as screeches, squalls, bearings, belts, metal-on-metal.

Dampers

Even small changes to a dust control system will affect the overall system. Once a system is balanced and functioning properly, it's important to mark the location of dampers and free air inlets and, if possible, lock them into place to prevent accidental or unnecessary adjustments.

Pulling good product over

It's possible for a dust control system to pull too much air. This may lead to the loss of 'good' product. In this instance, it's important to allow free air into the system or adjust your fan if necessary.

Filter bags have been changed too often

Do you have to put in new bags more often than expected? Why is that? Well, it's possible your filter is undersized. As we know, air to cloth ratio is a very important part of sizing a baghouse filter. If you have a poor design to begin with, perhaps it's undersized to begin with. Or if this filter was purchased 25 years ago for one application and has been re-allocated for another, perhaps it's undersized for that process.

Over time, we modify systems to get them to a point to operate and sometimes that includes adding more air or more concentration of dust into a dust collection line. Ask yourself what has changed? Has anything changed that is going to contribute to that higher pressure drop? Also, don't forget about checking your timer board settings on the reverse air cleaning system.

Bag cleaning pump

Some baghouse systems use a PD pump to provide the cleaning air. It's important to monitor and maintain the PD pump as part of the overall dust collection system. Is there too little CFM and too high of pressure with the PD blower? Is the PD blower speed too high? Try decreasing the off time to allow the tank to get to higher set pressure. Typical cleaning pressure on systems that utilize a PD pump is 10 – 12 PSIG. If the blower is not able to build up to that pressure, the bags will not be cleaned as well as they should be.

Cleaning valves

Any pulse jet style baghouse should contain an electronic timer board. This board will allow you to adjust the time between pulses as well as the duration of each pulse. Too frequent pulsing can lead to premature bag wear. Infrequent pulsing can lead to more frequent bag changes. It's important to find the right balance and set accordingly.

Explosion vent

If explosion vents fail prematurely, it's possible that there is unnecessary stress on the panel. The system shut down sequence is important so you don't add stress to the panel. Several styles of panels are available for unusual or problematic systems.

Working with an experienced and reputable system designer will help ensure you are installing a safe and cost-effective solution. Kice Industries has been designing, analysing, and building dust control systems and equipment for over 70 years. When designing a dust control system, one must consider many factors. Kice engineers understand the principles of handling air and apply their experience to every situation.

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