Maintaining pneumatic dust control systems

by Teresa Acklin
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   By Wayne Sanders, sales representative, George A. Rolfes Co., Boone, Iowa, U.S. This article was first presented as a paper before the 1992 annual meeting of the Grain Elevator and Processing Society (GEAPS).

   An adequate dust control system is essential for the safe operation of a grain elevator or grain processing facility. Such a system can provide a better working environment for the employes, with cleaner air, better visibility and cleaner and safer walkways. Less time is required for the menial and redundant jobs of sweeping and cleaning. All of these factors contribute to a safer workplace and better employe morale.

   In order to keep a safer workplace, your dust control systems must be maintained in good working condition. Even the best and most expensive systems require maintenance. Without proper attention, an excellent system eventually can become ineffective.

   The complete dust control system is composed of various pieces of equipment. Sheet metal components include (hoods, pipe, elbows, wyes, etc.), filter, fan, airlock, maybe a pneumatic pump, sometimes a compressor and an air dryer.

   Each piece of equipment has its own maintenance requirements prescribed in the owner's operational manual. Each maintenance task is to be done on a daily, weekly, monthly or annual schedule. The best way to keep track of these requirements is to combine all of the information on a good preventive maintenance software program. Then, your computer can give you a printout of the required maintenance items on a daily or weekly basis. If your maintenance program is not computerized, a calendar will serve the same purpose. Whichever method you use, there must be an effort, by someone, to organize a maintenance schedule for your dust control systems.

   These maintenance tasks, when done on time, will help prevent breakdowns and equipment failure. But, some items are not covered in the manuals, and breakdowns will occur, no matter what you do. And, they most often occur at the most inconvenient moment.

   In addition to a regular maintenance program, having an operator or maintenance worker with common sense and a working knowledge of the system is a real asset. Knowing what it takes to capture and convey the dust, without it settling on the pipes, will go a long way in keeping your dust control system functioning properly.

   Because my job is selling dust control systems, I have had the opportunity to talk with many people about dust control problems. I also have seen many mistakes made in installing this type of equipment. Usually, when someone is complaining about an existing system, they are not complaining about motor or bearing failures. They don't complain about loose belts or compressor problems. They can understand these mechanical problems. What they want to know is: “Why do these pipes keep getting plugged?''; or “Why doesn't it pick up the dust at the far end of the belt?''; or maybe “Why does it try to suck all of the grain out of the leg, when the floor sweeps won't even pick up a few hulls?''

   These people, and many others, do not understand the basic principles of air movement. In fact, they don't understand “capture velocity'' or “transport velocity.'' They don't understand the principles used in hood sizing or hood placement. The owners and operators of these facilities are not supposed to know everything about designing a dust control system. The problem is, some of the contractors they hire to install this equipment don't know anything about air movement. I know this is true, because I see the sheet metal work that they are installing. Just because you hook up some pipes to a fan, doesn't mean you have installed a dust control system.

   Let's look at a few principles to see how they affect the performance of your dust control system.

   Hood placement: The hood should be placed at the point where all of the airborne dust can be easily separated from the product. This may or may not be the point where the dust is being generated.

   Too often, hoods are placed too close to the point where dust is generated. This makes it hard to separate the dust from the product. As a result, some of the product is conveyed into the system, and some of the dust is lost to the atmosphere.

   If hoods are not capturing the dust because of poor placement, it may be necessary to move the hood or replace it with a better designed hood.

   Face velocity: The actual velocity of the air that is entering the hood is termed face velocity. This velocity is usually expressed in feet per minute (f.p.m.).

   Capture velocity: The velocity of the air that is necessary to capture the dust and cause it to flow into the hood is termed capture velocity. This velocity is measured at the face of the hood and may change because of the type of dust and the opposing air currents. This velocity also is usually expressed in f.p.m.

   In designing a dust system, it is necessary to design the hood so that face velocity is equal to the capture velocity of the dust. If you make a hood too large, the face velocity will be too slow to capture the dust. If you make a hood too small, it may try to capture the product and the dust. Generally, in grain dust applications, the face of the hood should have an area four to 10 times greater than the area of the pipe that feeds the hood.

   The face velocity of a hood can be changed by adjusting the blast gate above the hood. Capture velocity of a product cannot be changed without changing the opposing air currents.

   Duct velocity: The actual velocity of the air within the duct is termed duct velocity. This velocity is usually expressed in f.p.m.

   Transport velocity: The minimum duct velocity required to convey the dust through the duct system without settling of the dust is termed transport velocity. This is sometimes referred to as conveying velocity or design-duct velocity and is expressed in f.p.m.

   The design-duct velocity for most dust control systems in the grain industry is 3,600 to 4,000 f.p.m. (1,090 to 1,212 meters per minute). If you can maintain this velocity, you should not have any problems with pipes becoming plugged with dust.

   A duct velocity that is too low can cause a settling problem, but a velocity that is too high can cause excessive wear and wasted energy.

   You can sometimes increase the duct velocity in a particular line if you need to eliminate a settling problem. This normally would be done by adjusting the blast gate to reduce the resistance and increase air flow. When you increase the duct velocity, you can also increase the face velocity. If the hood is marginal, this increase in velocity may cause the system to pick up the product, resulting in one problem solved and another created. In this case, it may be necessary to add an air bleed-in to the line. This normally is installed directly above the blast gate and hood. This device allows you to bleed in additional air to increase the duct velocity without increasing the face velocity. See Figure 1, page 11.

   There are many dust-collection systems in operation that share the same problems. Some of these problems are designed into the system and little can be done to improve it. Other problems are minor, and small modifications can greatly improve the system.

   Poor collection: If you have a hood that is not catching the dust, check to see if the pipe is open and carrying its designed velocity. This can be done with a velocity meter, a Pitot tube, or by reading the static loss at the hood with a manometer. These readings should be compared with the recorded readings taken when the system was first balanced by the contractor.

   If the designed velocity is present, a hood modification or replacement may be necessary to solve the problem. The hood may need to be moved, made more confining, expanded, or replaced with a different style.

   Pipe or elbow damage: When pipe, elbows, and other pipe fittings develop holes and begin to wear out, it usually means high velocity and loss of product. To correct this problem, you need to readjust the blast gate and check the velocity. It may be necessary to replace the blast gate. It may be worn too badly to adequately restrict the line.

   If elbow wear continues to be a problem, it will be necessary to replace the elbow with a flat-back elbow or an elbow of a heavier gauge metal. See Figure 2, page 11.

   Dirty filter bags: In a normal grain handling operation, filter bags are replaced or cleaned once every 12 months. This may vary from filter to filter because of the loading, the air to cloth ratio, the type of product, and hours of use per year.

   The best way to tell if the filter bags need replacing is by reading the magnehelic gauge. Most systems are designed for a pressure drop of 4 inches (10.2 cm) across the filter bag. If your magnehelic gauge is reading higher than 4 inches (or the designed resistance), you should run the cleaning mechanism for an extended period of time. If this doesn't solve the problem, you should change the bags.

   Many things can happen to your dust system to shorten the life of filter bags. If your filter bags need to be replaced more often, something has probably happened to the dust system. Condensation is a big problem during the winter. Condensation may form on the filter housing and duct work on a cold day. To minimize this problem, try running the system 30 minutes to an hour before you start the grain moving equipment.

   Check the bag-cleaning device. If you are cleaning the bags with compressed air, check the solenoid valves and hoses. Check your compressed air for water, ice and oil. Winter condensation is a big problem with high-pressure filters. If you want to minimize your bag problems, you need a good air dryer, not just a cheap catch bowl and cartridge-type filter.

   If your filter is a “reverse air'' type filter, you need to check all mechanical parts of the cleaning system — the rotation of the cleaning fan, any chains, drives, sprockets, gear reducers or trip devices. You may have to open the door to observe the operation. If you do open the door, be careful of the rotating machinery. Don't go inside the filter while it's running.

   Plugged filter hoppers: A plugged filter can shut down your entire dust system and possibly your entire operation. Of all the maintenance jobs that are required on a dust system, cleaning a plugged filter is probably the dirtiest and most dreaded. If the collected dust stops feeding out of the bottom of the filter, you will soon have a filter full of dust.

   Several things can cause this problem, and some types of filters are more susceptible. Cone-bottom filters without live bottoms are probably the most likely to plug. Several manufacturers make round filters with cone hoppers. Some of the models have a small opening at the bottom of the hopper for the collected dust to exit. This type gives the most problems.

   If you are feeding the dust through an airlock into a pneumatic conveying system, the airlock must be tight. If there is any air blowing through the airlock, it will interfere with the feeding and cause the filter to plug. The airlock must be installed with the shaft perpendicular to the flow of the product. If the shaft of the airlock is installed parallel to the flow of the pneumatic pipe, it will not feed correctly, and the filter is likely to plug.

   If you have a cone-bottom filter with a plugging problem, the best thing you could do is to replace the lower portion of the hopper with a wide-mouth opening, complete with a live-screw conveyor in a flare-top trough (see Figure 3, page 11). This type of feeding hopper will eliminate your plugging problems.

   Dusty bucket elevator legs: The bucket elevator leg is one of the worst producers of dust in a grain-handling facility. It is also the most dangerous place for concentrations of dust to exist. Many elevator legs are equipped with inadequate dust collection. In years past, it was an accepted practice to install a hood on the top of the boot section. This type of collection may be suitable for slow moving, low-volume (less than 1,000 bushels per hour, or 27.2 tonnes per hour) elevator legs, but it is totally inadequate for the large volume legs that are common in modern grain handling facilities. These small boot hoods should be replaced with two “laminar flow'' hoods. These should be installed on the upside of the leg casing, at least 10 feet (3 meters) above the boot. This type of hood will aspirate the leg better, and reduce the dust concentrations within the leg (see Figure 4, page 11).

   We all know that the grain industry has had some hard times in the past few years; many facilities are being operated with “skeleton crews,'' and it is hard to do all of the daily operational requirements with limited help. Because of this, maintenance sometimes gets put on the “back burner.'' I want to encourage you to make sure that your dust control systems are kept in a good working condition. I also encourage you to make the changes that are necessary to improve systems to an acceptable level of performance. You owe it to your employes, your customers and your communities to make your facility a clean and safe place to work and do business.