Terry Stemler summarises the most common problems with feeders and provides ways to potentially solve the problems, or at least alleviate them
It is quite common to encounter flow problems in various types of equipment for feeding dry solids. These problems can be constant and unpredictable, and so they can also be costly.
As an example, if a no flow condition occurs on a feeder, the following action has to take place.
Someone has to recognise that the no flow condition has occurred. This could be acknowledging an alarm on an automation system, or recognising that the scale is not gaining weight or the product doesn’t look right.
The person then has to take corrective action. This may require someone to pound on the hopper supplying the material, restart the feeder, or poke the bin with a stick or pole to investigate. Then, they must also restart the process.
This may only take a few minutes, but if it happens several times a shift, over the course of a year it can really add up to some lost production.
For example, if this takes five minutes and happens once a shift on a three shift operation running 250 days a year , then that is 62.5 hours a year of lost production.
If it happens once an hour, then that means 500 hours of lost production.
Obviously, it is better to be proactive and address the problem rather than fighting an ongoing battle with problematic ingredients.
I would first like to stress the importance of working with the ingredient supplier and feeder supplier to evaluate and test the material in order to select the correct type and size of feeder.
Quite often, the supplier or manufacturer of the ingredient can draw on their own experience in handling the material and the experience of the numerous other manufacturers that are using the material.
They can almost always make recommendations on the best type of system for metering the product. It is to their advantage if you are comfortable with the ingredient that they are supplying and you are using it at the correct addition levels.
The same holds true for the feeder supplier. Chances are they have seen your ingredient before, or at least one that is similar. Most are happy to do testing because it is to their advantage if the feeder operates in a trouble free fashion.
It is important that you supply the feeder manufacturer with as much information as possible about the ingredient. In addition to the usual parameters such as bulk density, flow rate, inclusion amounts, corrosive nature of the product, and product particle size, this should also include the environment where the ingredient is being dispensed.
It may not do much good to test the product in a lab at 70F with a humidity of 20%, if the product is going to be added to a process in a wet environment at 120˚F. After testing has been completed your supplier should be able to recommend the right hopper and feeder for your application.
There are a couple of quick tests that you can try with the material that will give you an idea of how well it flows. This can help you decide how it will behave inside a feeder. If the product is dropped out of a funnel on to a flat surface, the angle of the pile measured from the middle of the pile is the angle of repose.
If the angle is flat then the product will have a tendency to flush or flood, if the angle is steep, then the product may need to have flow aids put in place to enhance the flow.
You can also try squeezing a sample of the product in your hand. If the product has a tendency to flow out of your fingers and does not form a ball, then it will be very free flowing. If it forms a ball very easily, then flow aids will probably be needed to keep the product moving.
Unfortunately, it is not always possible to accomplish this kind of testing beforehand.
You may have a feeder that you inherited from another person’s decision or you may have recently changed the ingredient or supplier of that ingredient and the feeder is not performing correctly. The most common problems with feeders are
Bridging of material above the feeder, feeder is plugged, motor is undersized so feeder kicks out and obstruction above the feeder due to a foreign object dropped in the bin
Feeder is undersized, hopper is bridging or ratholing, a buildup of material on the feeder
Feeder has product build up on it, rat holing of hopper above the feeder, obstruction above the feeder due to a foreign object
Product is packing in hopper, bridging and rat holing above feeder, incorrect control loop
Flushing or flooding of material due to low friction material with flat angle of repose, inconsistent free fall due to compaction
Product is abrasive, product is under pressure
Product is shear sensitive, and breakable
Not enough turn down
Feeder drive is wrong, may require two feeders, or a smaller feeder
If you can evaluate the specific cause of the problem, then it makes it much easier to apply specific solutions to the problem.
This may mean that instead of the operator pounding on the bin when a no flow condition occurs, someone opens the hopper to observe what is happening inside. This may disrupt production temporarily, but it is better than suffering the long term loss is production that an upset condition can cause.
Another troubleshooting method is to insert a rod or pole into the material to see if you can feel voids where bridging over the feeder may be occurring.
If the product is very dense in one location and looser in another, then there may be effects from compaction or rat holing. It may be that you will actually have to empty the hopper in order to find a problem with buildup of material on the screw or on the sides of the hopper.
These observations should of course be made with the proper safety interlocks in place.
Once you have determined the nature of the problem, here are a few solutions that you may be able to use.
A mechanical agitator or vibrator may be added to the hopper. This can be set on a timer to run at intervals or may run continuously.
The mechanical agitator can have fingers to break up the product or can be paddles or ribbons that condition the product before it enters the feeder.
It is better if the mechanical agitator has it’s own drive, so the speed can be adjusted based on the material characteristics.
Air pads are also used to break up the bridge and also to aerate the product to get it to flow. If air pads are used then the manufacturers recommendation for on and off duration should be followed to prevent the product from becoming too aerated or becoming air born.
If a vibrator is used, then the time duration should be adjustable to prevent excessive packing. It is also a good idea to get recommendations from the vibrator supplier for proper mounting to prevent premature fatigue of the hopper that the vibrator is mounted on.
If there are no obvious obstructions above the feeder that would reduce the flow then the feeder may be under sized for the application.
If the feeder is undersized and does not have a VFD on it already, a speed control can allow you to run the feeder faster than 60htz.
In some cases you could go up to 120 hz and double the speed. It may be also possible to change the reducer on the drive to get more RPM to the feeder. Care should be taken to make sure that the max rpm for the feeder is not exceeded.
If build up is happening on the feeder, then it may be enough to attach the feeder frame to ground the feeder to keep the static electricity from starting the initial material build up.
There are also treatments and coatings that can be put on the feeder and hopper that can prevent buildup. These include electro polishing the feeder or having Teflon applied.
You can address this through mechanical or another form of agitator to break up bridging or rat holing.
Removal of obstructions above the feeder is necessary and you need to check the control loop to eliminate oscillation.
If you find that material is flushing or flooding after the feeder shuts off, this can be due to how the product is delivered to the bin.
If it comes from a pneumatic system, or a mixer it could be aerated or under pressure. A vented hopper above to isolate the condition and allow the product to de-aerate can help. A slide gate or butterfly valve at the discharge point can also stop this from happening.
This is usually because the feeder and or hopper is softer then the material that you are trying to meter. Special coatings and liners can be used to slow down wear. A larger feeder that would run slower can also help the problem.
This is usually caused by using a feeder that is not the correct type for the product.
A belt feeder or vibratory feeder would be preferred over an auger type feeder for friable products. It can also help to decrease the tolerance between the auger and the trough to eliminate the pinch point that can cause product breakage. This works if the tolerance can be made smaller than the particle size.
If the auger is too large, then an auger with smaller flights and a larger tube can be used to decrease the output from the feeder (see picture). It may also be possible to use two feeders to get the necessary high and low speed range to increase the accuracy.
In conclusion, it is always in your best interest to be proactive when dealing with ingredients that are known to be sluggish. It is important to talk to experts and take their experience into account when choosing a feeder. If you find yourself in a situation where you are dealing with an existing feeder and also a problematic material, there are options to help lessen downtime caused by issues with flow.
Terry Stemler is President, Automated Process Equipment Corporation.