Rotary drum dryers are the workhorses of the industrial drying industry, but even workhorses can have maintenance problems. Fortunately, most drum maintenance problems center around a few core issues involving specific equipment:
- Trunnion wheels and shafts.
- Trunnion bearings.
- Drum tires.
- Drum shell and head plates.
Performing daily inspections goes a long way to benchmark what is normal in your plant. With the aid of daily inspections, the dryer operator is the first line of defense against problems. Those working with the equipment daily come to know what is normal and when something is just not right. It is much easier to spot something out of place — increased vibration, new or worsening leaks, or even new squeaks, creaks and groans — when performing daily inspections.
Perform a Walking Test on Your Dryer’s Shell and Tire
If there are shims between your drum’s shell and tire, you will want to test the tire for walking.
Walking means the tire rotates at a different rate than the drum. Over time, the outside of the shims and the inside of the tire can wear down, allowing the tire to walk.
You can perform this test while the drum is running by using a piece of chalk or spray paint. Mark the side of the tire and where it lines up on the shell/shim. Check on it throughout the day to see if the lines still match up. Do this again when shutting down and starting up to see if you get the same results.
If the marked line remains lined up, the tire is providing support to the shell and likely not the source of any problems you may be experiencing. If it does not line up, the tire likely is causing stress to the shell and shims. It may be causing other problems in the drum as well.
Trunnions are stout, steel wheels on which a rotary dryer drum rolls. Each must support anywhere from 10 to 20 tons. Trunnions must be smooth and round to reduce vibration, wear and friction. Common symptoms of trunnions issues include a rough wheel surface, crowned surface, tapered surface and shaft breakage.
A surface problem such as a rough surface, crowning or tapering must be reground or machined to correct it. Fortunately, the wheel does not need to be removed to perform this action. If the wheel surface becomes rough or gouged soon after resurfacing, however, something else is going on, and you will want to call in the dryer experts.
If a trunnion shaft breaks, shut the dryer down and call in the experts immediately. Something is going horribly wrong.
Proactive maintenance for dryer trunnions includes scheduled resurfacing of the trunnion wheels and drum tires followed by a precision alignment check. This maintenance should be completed every year or two as well as any time a trunnion wheel or bearing is replaced.
Optimizing Trunnion Bearings
Trunnion bearings support the trunnion wheels. Like trunnion wheels, trunnion bearings must be quite stout. Two bearings support each trunnion wheel.
To be effective and long lasting, a trunnion bearing must be able to handle some misalignment and thrust. The symptoms displayed at your plant will determine whether you need to focus on a self-adjusting bearing or a double-taper-lock bearing.
To identify trunnion bearing issues, look for these symptoms:
- Failed Bearings Internals. Signs of failed internals include powder formation, jerky movement, halted movement, rollers falling out, rusty-looking grease, increased bearing vibrations and bearings heating.
- Failed Bearings Externals. Signs of failed external bearing components include housing cracks at the setscrew, broken or cracked lock collars, and the shaft slipping inside the lock collar.
When these symptoms are present, immediate action should include changing the bearings.
The most common bearings-related problems in rotary dryer systems are bearings misalignment and bearings that do not properly handle thrust. If you are plagued with bearing problems, changing the bearing type may help. Another common fix is changing the greasing frequency.
Proactive activities include investigating the root cause of trunnion-bearing failure and acting accordingly. If your primary problem is thrust, which manifests as external failure, switch to bearings that use a concentric lock mechanism or double-taper lock. If your primary problem is misalignment, which usually manifests as an internal bearing failure, choose a self-adjusting bearing with more than 2° misalignment capability.
Trunnion bearings used with rotary drum dryers in the ethanol industry tend to fail regularly. Because bearing failures in this industry are common, some may assume that only a short operating life is possible. If you calculate the expected life of trunnion bearings using tools typically available on bearing manufacturers’ websites, you will discover that the trunnion bearings commonly used in the dryer industry should last 20 years or more. Instead of following the typical recommendation of replacing trunnion wheels and bearings every two years, the better solution is to find the root cause and deal with it. You can get an early warning sign by using an infrared camera, which will show heat buildup generated by friction.
Every time a wheel or bearing is replaced, the drum and trunnions must be precision aligned and resurfaced. A drum running out of alignment — for whatever cause — can do more damage than not replacing the trunnions and bearings every two years.
Correcting Dryer Drum Tire Issues
The tire provides a reinforced surface for the drum to roll on the trunnions. Without the tire, the whole shell would have to be much thicker to withstand this pressure.
The longest-lasting tires with least amount of maintenance necessary are rigid, round and aligned. A rigid tire will eliminate flexing and the problems caused by cyclic loading. A round tire is the first step in achieving a round drum. An aligned drum will improve the life of the trunnion wheel and tire surfaces.
Symptoms of drum tire issues include:
- Tire walking, either around the drum or horizontally.
- Tire shims falling out.
- Riding on one bumper wheel constantly.
- Tire-surface imperfections.
- Daylight between wheel and tire.
If drum tire symptoms are present on your dryer, immediate actions include replacing or redesigning the tire shims. Proactive action includes testing your drum for walking (see sidebar). Make your drum rigid, round and aligned. This includes shimming the tire such that it no longer walks, which likely involves a redesign of the shims. If they are falling out, then the drum and trunnions should be realigned and resurfaced.
If the drum tire and shims have worn out to the point that the tire is moving back and forth on the shell, it is time to call in the experts. You also may want to call in the experts if you have long grooves in the tire or if the surface gets rough soon after resurfacing. Daylight between the wheel and tire may mean there are surface imperfections such as a crown on the trunnions and tire. Check this with your laser alignment kit.
Schedule trunnion wheels and drum tire resurfacing at the same time as an alignment check — about every year or two. Also, perform resurfacing directly after any trunnion wheel or bearing replacements.
Adjusting the Dryer Drum
The drum is the most important part of a rotary dryer system. This is where the wet material showers through the hot drying gases to evaporate water.
Drums that are rigid tend to last longer than those that are not. A rigid drum reduces flexing and the problems caused by cyclic loading. Typical problems with dryer drums include head plate cracks, shell cracks, cracks in welds, seal leakage, and drums that break in two and collapse on the floor.
Cracks anywhere in your drum should be fixed immediately. Use the test described in the “Why is My Drum Cracking?” sidebar to determine if your drum is flexing. However, if cracks keep coming back in the same locations repeatedly, you have something else going on that could be caused by drum flexing. In such cases, have a dryer expert investigate the cause of repeated cracking in the drum.
To be proactive, make your drum rigid and prevent the tire from walking. A rigid drum will eliminate flexing and the problems caused by cyclic loading. In addition, you should schedule regular inspections for cracks by plant personnel and dryer experts. It is recommended to have a structural drum inspection by a dryer expert once every year or two, depending on the history of your drum.
Why Is My Drum Cracking?
Most cracks develop over time as the drum flexes, which can only rarely be seen by the naked eye. But, for a visual image of what is going on, think about what a water balloon looks like if you fill it up and then roll it down the sidewalk. It will always be wider than tall, but the exact points that are furthest and nearest are always changing as it rolls.
To determine whether your drum is flexing, begin by taking internal measurements while the dryer is down. Mark off four spots — two each on the horizontal and vertical axes. Measure from 1 to 3, the vertical axis, and then 2 to 4, the horizontal axis, as shown in the figure. Next, rotate the drum 90° and measure from 1 to 3, which is now on the horizontal axis, and 2 to 4, which is now on the vertical axis. If the first measurements of 1 to 3 vs. the second measurement of 1 to 3 differ, the drum is flexing. Likewise, if the first measurements of 2 to 4 vs. the second measurement of 2 to 4 differ, the drum is flexing in that direction. Any flexing will reduce the life of your dryer, but if the measurements differ more than 0.125”, you will have problems with cracks sooner than later — if you do not already.
Flexing causes cyclic loading — two cycles at every point for every rotation. At this rate, fatigue limit is reached within the first five years of a stainless steel drum’s life. Carbon steel acts differently under cyclic loading. Stainless steel has a significantly lower fatigue limit than carbon steel. Special care should be taken when inspecting for cracks and flexing when you have a stainless steel dryer drum.
Sealing Up Industrial Dryer Seals
Because most rotary dryer systems are set up with negative pressure, the seals are responsible for keeping transient air out of the system. Excess transient air can make fires or explosions more likely as well as cool the ductwork and lower the wet bulb temperature. A low wet bulb temperature can worsen buildup inside of the ductwork. Excess transient air also can decrease the effectiveness of cyclones, baghouses, thermal oxidizers, etc., because of the higher gases volume. In short, good seals enhance personnel safety and dryer efficiency, and they should be maintained accordingly.
Seals that are leaking, tearing, falling off, breaking, burning up or wearing out quickly signify a problem. In these cases, you should immediately replace your seals and resurface the sealing surfaces. The sealing surfaces get rough like trunnions and tires. If you already use an outside company to resurface your tire and trunnion wheels, you usually can schedule this activity at the same time.
Changing your seal style altogether and adding a heat shield would be a proactive action, as would be making your drum rigid, round and aligned. When the drum is not round, there will be intermittent gaps between the seal and the sealing surfaces that allow in transient air. If the drum is not rigid, there is no hope of getting the sealing surfaces round. And, if the drum is not aligned, the seals could be getting damaged by excess wear against the constantly moving sealing surfaces.
Seal material needs to be flexible, airtight or almost airtight, rated for high temperature and durable. Some dryer manufacturers recommend a high temperature rubber belting; however, a high temperature belting still should not be used in temperatures higher than 400°F (204°C). The gases at the front of the drum can be in excess of 1000°F (537°C), which will burn up a seal in a matter of hours after installation without a heat shield. For these areas, there is a sealing material made of Kevlar that is commonly used in dryer applications. It does not burn up as fast, but it still must be replaced quite often. A heat shield will prevent hot gases and radiant heat from destroying the seal. With a well-designed heat shield, high quality rubber belting could last a year or two before needing to be replaced.
In conclusion, dryer personnel should be performing daily inspections as a way to benchmark what is normal in your plant. Most problems occurring in industrial rotary dryers can be solved by making your drum rigid, round and aligned. Among other benefits, a rigid drum will eliminate flexing and the problems caused by cyclic loading. A round drum will improve the effectiveness of the seals. An aligned drum will improve the life of the trunnion wheel and tire surfaces as well as the health and well-being of the trunnion bearings.
If you are in the middle of an emergency, take the immediate action to get up and running as quickly as possible. But, to prevent emergencies in the future, take a look at the proactive actions and try incorporate viable solutions into your preventive maintenance program.
Editor's Note: This article was published in the April 2016 issue with the headline "Diagnosing Rotary Drum Malfunctions and Maintenance Tips to Avoid Them."
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