Problem prevention is the first line of maintenance defense, and repairs are the rear line reinforcements. This article explores the nature of service and maintenance, and offers some practical considerations for the defense-minded user.
The Economics of Being Prepared
In plastics processing it’s the exception when feeders do not play a vital role. By and large, when a feeder is down. so is the process. The same can be said for any critical equipment in the line, but that simply underscores the importance of applying the principles of the preven¬tive defense across the board.
There are three cost categories associated with pro¬cess problems and their resolution: cost of lost production, cost of repairs, and preventive maintenance investment. These cost areas interrelate to point to an optimal invest¬ment in preventive maintenance.
The cost of production losses decreases as preventive maintenance investments increase. Such a falling off of lost production cost stands to reason because if preventive measures are effective, they’ll work to reduce the in¬cidence and duration of surprise shutdowns. Preventive maintenance cannot eliminate equipment problems or production downtime, but it can minimize overall costs and facilitate prompt problem resolution.
In a similar way the cost of repairs dives with increasing preventive maintenance investment. The cost of replace¬ment of a worn weigh-belt, for example, replaced on schedule under a preventive maintenance program, is much less than the cost of replacement of that same belt if it should break unexpectedly and require emergency attention.
Regardless of the size of the plant or process the costs of an unscheduled production stoppage are staggering. Paying high stakes with each tick of the downtime clock makes one wonder whether any cost is too great to invest in problem prevention, or at least in the quickest resolution of prob¬lems that do occur. But in fact there is a point beyond which investment in problem prevention is not economical¬ly justified.
Total maintenance cost, the sum of the three cost categories, decreases rather sharply as initial preventive maintenance investments are made. But with further in¬creases in preventive maintenance investment total cost reaches a minimum beyond which each dollar invested in preventive maintenance more than offsets the combined costs of lost production and repair, and total maintenance cost begins to rise.
You must ask whether the cost of the preventive maintenance measure is less than the combined cost of repair and lost production that would be incurred if the measure were not taken. If it is, the measure is warranted; if not, the measure costs more than it saves and shouldn’t be adopted.
To this point all the different areas of preventive maintenance have been lumped under one category, but what areas are most relevant for feeders and feeding systems? Replacing a worn weigh-belt before it breaks and shuts down the line is a preventive defense, but other activities such as start-up service, operator training and spare parts stocking (in addition to scheduled inspection and cleaning) can also be classified, at least in part, as preventive in nature. A closer look at these areas will help you decide how to tailor your own preventive defense.
The Life Cycle of Service
The service needs of any process system change with time. and feeders are no exception. Different phases in a weigh-feeder’s operational life exhibit different service needs which must be met to combat the potential prob¬lems encountered at each stage.
The diagram below plots the incidence of trouble calls versus time, and helps to point out some interesting facts that could save the user time and trouble, not to mention scarce maintenance dollars.
Phase 1 begins with receiving the feeding system at the plant door and ends with the completion of installation. Problems experienced during this phase are usually minor and generally have to do with questions of wiring. misread blueprints and the identification of unfamiliar parts. Most can be quickly resolved over the phone.
As expected. the highest incidence of trouble calls occurs during Phase 2, the start-up stage. Since no two ap¬plications are identical in every respect, the nature of Phase 2 is one of adaptive problem solution. Because the manufacturer’s area of expertise is feeders and feeding, and the user’s expertise is his plant and process, neither one can fully minimize start-up problems by himself. But two heads are better than one, as is exemplified by the fact that the incidence of trouble calls drops drastically whenever start-up service is requisitioned.
The achievement of fully operational status marks the beginning of Phase 3, and with it comes a steep drop in actual problems. The user is getting daily hands-on ex¬perience with the feeding system, and his confidence is on the rise. Most calls to the factory during Phase 3 are to verify procedures.
Shortly, as trouble calls bottom out, we enter Phase 4. At what level trouble calls flatten out depends somewhat on the severity of the application, but most strongly hinges on the extent of operator training and the user’s prudent purchase of spare parts. As shown, both operator training and spare parts purchase have a real problem¬-preventing effect. Their benefit is additive and continuing.
Aside from its primary benefit of instructing customer personnel on how to use the feeding equipment, operator training is also an investment in preventive maintenance because without full familiarity of the feeding system (or any other process subsystem for that matter) misunder¬standings will emerge and almost inevitably lead to process inefficiencies, unwarranted shutdowns, and even equipment failure. Operator training sessions normally take only one day and cover the theory of operation, controller front panel indications, calibration techniques and troubleshooting guides for maintenance personnel.
There is nothing mystical about the operation of a feeding system, but it is a simple necessity that those who operate and maintain it also understand it. Training is the quickest and surest way to eliminate ignorance as a downtime cause.
While training helps to prevent problems caused or aggravated by people, spare parts help prevent problems caused by equipment failure. The case for spares is a simple one, but also an often overlooked one in the hope that nothing will ever go wrong. When the user places his bet on Lady Luck, sooner or later she will desert him. As said earlier, the common reality is that when a feeder is down, the process is down. That fact must dictate the user’s strategy for spare parts pur¬chase. A frequent first-time user mistake is to buy no spares, and then see what he needs when something final¬ly wears out, breaks down or blows. But he need not ride the razor’s edge of fate; feeder manufacturers already know which parts are innately most likely to wear out, break down or fail with age, misuse or abuse.
Part of engineering the feeding system includes the manufacturer’s appraisal of the spare parts requirements for the particular application. Feeder makers are as in¬terested as you are in avoiding shutdowns, and they are in the best position to know which spare parts you should stock to reasonably assure you won’t be caught with your process down. Consider their recommendations carefully.
Solving Problems That Need Not Be
It’s valuable and instructive to delve into the economics of the preventive defense and the problem-solving nature of start-up service, operator training and spare parts purchase, but what can you do in the common meaning of preventive maintenance to avoid feeding system malfunction and the associated high costs of an idled process?
The two most common types of weigh-feeders are weigh-belt and loss-in-weight feeders. Consider each for some practical pointers on problem prevention.
Belt Replacement - Sooner or later all weigh-belts have to be replaced, either because of wear or breakage. Replacing the belt can be a five minute job or a five hour terror depending upon whether enough elbowroom was provided at installation. Most manufacturers are careful to recommend at least three feet of clearance around the feeder to facilitate cleaning and maintenance; if that ad¬vice is not followed, the user is in for a real headache someday.
Belt Tracking - Whenever poor housekeeping is evi¬dent, chances are the belt tracking system is not working at its best. Consider what happens when the belt rides to one side of the feeder. It will hit the side of the feeder, lift up, cause a falsely low weight signal, and the controller will sense the bogus condition and obediently increase belt speed accordingly causing, in reality, a higher flow than desired. Again the problem is material build-up, either on the pulleys over which the belt travels, or on the tracking mechanism, if the feeder has one. Better housekeeping and possible adjustment of pulley alignment will clear up the condition.
Side Skirts - To prevent material from falling off the side edge of the belt, adjustable plates called side skirts are brought to within a small distance of the belt surface.
However, the skirts may be misadjusted and come into contact with the weighed portion of the belt. Materials such as grain or pellets may also become lodged be¬tween the skirt and the belt. When either condition occurs, a false weight signal is generated and the controller again adjusts belt speed, this time downward to offset the ap¬parently increased weight of material on the belt. The readout will show that flow rate is as desired, but actual flow will be substantially lower than setpoint. Side skirts should be adjusted so there is a gradual increase of gap from the inlet to the discharge end, from less than particle size at the inlet to about two times particle size at the discharge end.
Weighing System - One of the most easily prevented problems common to all weigh feeders is weigh system damage due to abuse. All operating and maintenance personnel should be aware that a sensitive weighing sur¬face is not a good place to stand on or drop a wrench.
Weigh System - In loss-in-weight feeding the entire feeder, bin and material are weighed. Any coats, coffee cups or lunch pails placed on the feeder will understandably cause flow rate disruption. Forewarned is forearmed, so forewarn your personnel and back it up with a sign to remind them a loss-in-weight feeder is not a coat rack.
Flexible Connections -- Since loss-in-weight feeding depends on a reliable weight signal to control material flow, im¬properly fitted inlet and discharge connections will give rise to an erroneous weight signal. Be on the lookout for this problem after anyone has cleaned or adjusted either connection.
Refill - If material is entering the feeder’s hopper at any time other than refilling, errors will result. Check to see that the refill device is not leaking.
Isolation - A contaminated weight signal will result if there is any rigid physical connection between the feeder and its surroundings, so don’t allow any part of the feeder to become a point of attachment for electrical conduit or anything else in the plant.
Service Before the Sale?
Because service is so often thought of as crisis in¬tervention and not crisis prevention, no one relishes the prospect of needing it. But maintenance and service are facts of process life, and until someone invents a perfect widget, service will be part of every machine.
The key issue is how to guarantee good service support from the manufacturer, and that issue is best addressed before pur¬chase, during the process of supplier evaluation.
Just as you would evaluate a feeder on its ability to meet the criteria of your application, a manufacturer’s ser¬vice organization should be evaluated on its ability to meet your specific needs.
Here’s a brief checklist of points to consider when evaluating a manufacturer’s service force:
• What is the size of the field service force?
• Is emergency service available whenever needed?
• What is the expected response time for a service call? (If the service force is dispatched from only one or two locations across the country, production downtime and travel expenses can skyrocket. And if your installation is in a remote locale to begin with, a long response time can also force you into purchasing more spare parts than you otherwise would have to.)
• What is the service rate structure?
• Who will perform service on your system? What is his ex¬perience?
• Can the manufacturer supply customer references regarding service performance?
• Are operation and maintenance manuals thorough and clear?
• Is the feeding system well designed and constructed? (Higher quality equipment generally requires a lower fre¬quency of service.)
• What is the normal response time for the shipment of spare parts?
• Does the manufacturer offer operator and maintenance training programs? What are the details?
• Is a service contract available if desired?
• What is included in start-up service?
Evaluating the manufacturer’s service force before equipment purchase isn’t the only homework you can do to avoid trouble down the road. By anticipating the com¬plete equipment needs of your application, you’ll often be able to avoid the expense and bother of adding to or altering your feeding system after it’s installed.
Here’s another checklist of some frequently overlooked after¬market afterthoughts. Weigh each point carefully in the light of your specific needs.
• Will you want your feeding system to be controlled from your central computer? (Although individual feeder con¬trollers will still be required, interfacing a feeding system with a computer can provide centralized formula set¬point generation as well as centralized and automatic reporting of data such as mass flow, flow totalization, etc. Feeder manufacturers offer various computer inter¬face modules for input and output computer com¬munication. If your process is not computer controlled, you may choose to input formulations via a card reader. It’s quick, easy and eliminates the errors associated with constant readjustment of thumbwheel settings. Also, you may decide that a printer is needed to provide feeder data in hard copy and avoid errors of manual recordkeeping.)
• Do you need to proportion several ingredients? (If you do, it’s best to specify beforehand that the feeders be tied together in master/slave fashion instead of being provided as stand-alone units which your operating per¬sonnel would have to control independently.)
• Is the feeding system to control or to be controlled by other process equipment? (If feeders are tied into other process equipment, all input and output signals must be compatible. For feeders with frequency-based output signals of belt speed or mass flow, for example, a frequency-to-current converter is required to control analog equipment. If that same digital feeding system is to take its commands from analog process equipment, a voltage-to-frequency converter and a frequency scaler will be needed.)
• When you only want to totalize material throughput and not control flow rate, make sure that’s all you want to do. If it is, then you need a weigh meter, not a weigh feeder. (A weigh feeder controls flow rate, a weigh meter just totalizes material throughput.) But if you find it may be desirable to control flow rate - even occa¬sionally - the small additional cost of a feeder will be more than offset by the increased versatility it will pro¬vide. A feeder can perform all the functions of a meter, but a meter can’t do what a feeder does.
• Should the feeder stop when the process line shuts down? (A simple interlock is all that’s needed. Interlocks work both ways, so the feeding system can either shut down interlocked equipment, or be shut down by it.)
• What are your remote display requirements for mass flow, belt speed, setpoint and throughput totalization? (A handy add-on, remote displays are often a good invest¬ment. Even though a feeder’s controller may be located in an isolated control room, a display positioned near the feeder can be very convenient.)
• In what units do you want your display to read? Pounds per hour? Kilograms per minute? (Display units can easily be changed from English to Metric or vice versa, and the time base on the display of rates can also be easily altered. Avoid reprogramming costs and decide early.)
• What mechanical options does your application de¬mand? (A host of mechanical options fall into the category of aftermarket afterthoughts - everything from inlet slide gates and sampling valves to flexible connections and dust collector chutes. Carefully con¬sider the need for devices such as these. They are not required in all cases, but when they are, there is no substitute for them.)
By broadening the definition of preventive maintenance to include all the factors that affect feeding system operation, users can minimize process costs and in¬efficiencies. A preventive defense recognizes that prob¬lems can and will periodically occur in any process system, feeders included. The real-world key to combating unscheduled shutdowns is anticipation and preparation combined with user knowledge and a manufacturer’s capable and cooperative service force.