Maintenance — mixing and pelleting
October 1, 2010
Editor’s note: This is the second article in a three-part series that examines maintenance issues in feed mills. The first appeared in the September issue of World Grain and examined maintenance issues related to intake and grinding equipment. The third article in the series will appear in an upcoming issue of World Grain.
The mixing and pelleting sections of feed mills are perhaps the two areas that are the lightest and heaviest, respectively, in terms of expense in the maintenance department register.
Mixers can run for years without causing any problems. But when they do, they are often the most expensive items to replace in a mill, causing severe damage to the annual budget. Hence, observation and monitoring are key to effective mixer operations.
Checking that scrolls are not becoming too badly worn and ensuring that gearboxes are properly lubricated should be priorities when it comes to mixer inspections. Salt level checks to ensure mixing uniformity can be useful when looking for damage to scrolls, but there is no substitute for lifting the lid once a week and having a good look around. This will also identify where liquids are not being correctly applied. They will eventually coagulate on mixer walls and bomb door outlets if incorrectly applied or if spray nozzles are worn or lose.
You must not forget small pre-mixers for vitamins in the same maintenance scope as well as those used in fat-spraying applications. The correct blade angles in the Tatham-Forberg type machines, used for fat enrobing, are critical, and maintenance crews should know what they are looking for and what configuration should be expected when carrying out inspections.
Top and bottom hoppers in three-tier systems are often neglected when it comes to maintenance, but discharge mechanisms in these operate as many times in a year as the mechanism in the mixer.
Compressed air plays an important role in mixer bottom-door operations. With incorrect pressures or leaks, the effective door operation will not take place and either doors will not open fully or will leak during mixing. Both scenarios will affect product quality, thus a check on mixer outturn weights will tell you whether there is any discrepancy that might warrant further investigation.
While mixers rarely cause problems, the pelleting section of a feed mill is a completely different story when it comes to maintenance concerns. Heat, friction, stress, strain — you name it and you can find it in the pelleting section.
Let’s first examine the steam element. The steam you raise must be of good quality, dry and at the right pressure. Without good quality steam, you will not produce good quality pellets.
With the steam element in good order, you can examine the press itself. Obviously, dies and rollers need to be in good shape, not badly worn and properly aligned. Keeping the keys tight in the key ways at the back of the pellet press is, dare I say, a key element in avoiding excess wear within the pellet mill die drive train.
Tightening the shear pin assembly regularly throughout the shift and making sure there is minimal play between rollers and dies is paramount, and press operators should be trained to do this regularly. Similarly, where auto lubrication is not fitted (and I recommend it is always used), the operative needs to be trained correctly so he knows when and how to lubricate the press and rollers, and the operative should also be knowledgeable enough to spot when lubricant is not reaching where it needs to be applied. Many pellet mill operatives are quite happy to apply grease but have no idea whether it is working correctly and reaching its destination until a roll seizes … then they become all too aware of it.
Starting presses in the right sequential order and allowing time for individual elements to settle into the run mode are key to prevent excessive stress from being placed on the various parts of the press. Trying to start presses under choke conditions or taking risks with slacking rollers off and then repeatedly attempting to start a partially choked machine will either break, distort or put undue pressure on parts beyond their design criteria, and it will not be long before major damage is done. Treat presses with respect.
Stripping presses down and examining gearboxes and drives are an essential part of keeping presses operational. Regular heavy maintenance, changing the oil completely and making sure the keys and other torsion elements are not worn will pay dividends in the long run.
Using skimmed dies is not a cost saving element. Cash flow may dictate on occasion that dies need to be refurbished, but increased power consumption with skimmed dies can far outweigh the cost of using new dies. Using new rolls with a new die is a must if you are to avoid irregular wear patterns of dies and distort the face of the die. Pick the best dies available within your budget. Use good quality stainless steel or harder if you can afford to use it.
Feeders and turbulators are basically screw conveyors and should be treated as such with correct drive maintenance, lubrication and inspection. Turbulators should be cleaned internally on a regular basis, probably every month at least, and the beaters should be changed when barrel diameter decreases by more than a third.
Again, not replacing beaters when they are worn will increase power consumption as well as dramatically affect pellet quality through poor conditioning. If you let beaters wear too badly, then you are reducing the retention time within the turbulator and reducing the conditioning effect of the steam on the ground meal. This leads to poor pellet durability and increases processing of fines, which means running longer for the same output and a lot more on-farm complaints.
Coolers are much like mixers — they don’t often go wrong, but can be a major issue when they do. The older chain, flow and return coolers can wrap themselves up very slowly when you aren’t looking, and they are major items to unravel and get going again. Having the right amount of tension is important to avoid this, and they should be checked every week to make sure they are aligned correctly and that sprockets and chains are not worn badly enough to be a risk.
Cooler fans and cyclones are simplistic in their design. But to refer back to turbulators and short beaters, if these don’t work correctly and pellets break down in the coolers, the resultant granular break off grist will be conveyed through the cooler air stream and will quickly wear holes in cooler cyclones.
Automatic pellet mill controllers are practically a must when you have multiple press installations. They can be of great value if set correctly, but if they are not established correctly they can repeat the same mistakes very regularly. Pay close attention to the parameters used. Make sure that the settings are changed according to the diet being made, and make regular calibrations, especially temperature and time settings.
Exhaust vents to the atmosphere are now monitored at least annually, and in some cases there are emissions monitors installed on cyclones so emissions are checked throughout the periods of operation. Make sure these are calibrated correctly and regularly enough to give you peace of mind. You may also use low-level probes in cyclones to ensure that when the airlock trips out you are aware of it before the yard is full of meal.
Where filters are used, make sure the cleaning mechanisms are in good order and actually cleaning the sleeves. Filters may trap particles that would go to the atmosphere through a cyclone, but the heat and humidity will quickly take its toll on cloth filter sleeves and you need to be prepared to change sleeves regularly. Make sure to calculate the costs before installing.
Finally, it’s always important to remember that a daily walk around the mill to look for potential maintenance problems will pay dividends. The expense you save will handsomely recompense you for your loss of shoe leather, as you will see many things before they become items in your maintenance expenses ledger.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J.B. Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: email@example.com .