KANSAS CITY, MISSOURI, US — In an article in the April 2022 issue of World Grain, “Reviewing Maintenance Strategies,” the development and progression of maintenance strategies was presented. Implementation of those strategies vary by milling site and even within the same milling organization. The September 2022 World Grain article, “Where to Begin? A strategic maintenance journey for millers,” identified the importance of maintenance goal setting, incident analysis and identification of costs.
The purpose of this article is to consider equipment condition issues from design, installation to failure and discuss reliability and condition-based monitoring (CBM). Both maintenance and operations staff must be proactive in monitoring, reporting equipment condition to access the machines condition and deterioration rate over time.
Planning of preventive maintenance activities is taking less of a time-based approach, instead becoming more condition based. The condition-based approach may require more data handling but that has become easier to with the use of computers and development of Computerized Maintenance Management Systems (CMMS). Even greater amounts of information about plant equipment, drives and control systems are being documented and managed by CMMSs.
Advancing from time-based preventive maintenance to predictive maintenance requires data acquisition and analysis based on equipment condition. In addition to manual inspection and testing methods, measurements using sensors and perhaps devices connecting via the Internet of Things (IoT), allow for analysis of data to develop a predictive forecast. Collecting and analyzing historical data remains an essential component of predictive maintenance. The data collected must be accurate, reliable, and timely. Analysis and predictive forecast strategies must be flexible to meet changing needs allowing you to move from preventive to a predictive-based maintenance model or industry 4.0.
I-P-F curve
Figure 1 is a typical I-P-F curve from the point of installation. I and P represents potential failure, the point at which equipment is starting to deteriorate or wear moving toward failure. F is the point of functional failure where the useful limit of performance is reached, and the equipment is no longer functional. The goal of I-P-F curve development is to be prepared for equipment functional failure and maximize equipment I-P-F interval from both an economic (cost of ownership) and technical perspective.
The graph identifies the time interval between potential failure and functional failure or lifespan that can be assigned to the equipment as a whole or equipment components subject to wear. Lifespan will vary between equipment and given operating conditions. Examples of this include start-stop cycle or starting empty or under load.
The time intervals will vary between pieces of equipment and the work being accomplished by the equipment. Consider, for example, a wheat screw conveyor starting and stopping frequently under load delivering wheat to an undersized surge hopper or bin above B1 compared to a screw conveyor carrying filter dust (much lower load and product density) running constantly delivering filter stock to a pneumatic lift system for sifting. Within the milling system, time to failure differences may be observed between more heavily loaded B1 roll stands than less heavily loaded and B5 roll stands. In each of these two comparisons time to failure would be reasonably different.
Often ignored in the variation in equipment lifespan, maintenance, and the cost of ownership begins with equipment design and installation. With either a new or used piece of equipment, one must take care to ensure critical design parameters are clearly and accurately presented and accounted for during the design phase for either new or redeployed existing equipment. Overlooked or understated design parameters leading to design flaws lead to less-than-desirable performance or lifespan. Of course, overestimation may lead to installation of costly, inefficient equipment and higher than necessary operating expense.
Proper equipment installation
Equipment installation, just as design, must be accomplished with great care to achieve maximum equipment life and efficiency. Proper equipment installation on a wall floor or ceiling must provide a solid and level installation, particularly when installing high-speed rotating machines such as fans, positive displacement blowers, hammermills, roller mills and control sifters. The mounting surfaces may not be true planes, which can result in a twisted or torqued machine support frame inducing vibration. Level mounting of most machines is essential and often is overlooked or not confirmed. Misalignment of machine components, including troughs, casings, drive shafts, pulleys, belts, sprockets, and chain including drive tension or slack often result in premature wear and failure. Take the time to consider laser alignment and tension monitoring measurements to ensure proper installation.
Significant points to consider in equipment installation are very simple: read and follow the manufacturer’s installation instructions before and during installation. If given a specific pattern for tightening fasteners or fastener torque values, carry out the instructions completely. If installation instructions identify specific equipment checks following a specific run-in time, schedule and complete the checks as described. Check the motor rotation before drive installation to see if there is any rick of equipment damage due to improper rotation. Finally, be sure equipment is properly lubricated and oil levels filled with recommended lubricant.
Frequent repairs and shortened equipment life are often the result of design and installation shortcomings. One should always work to identify the root cause of failure. For example, the angle, instantaneous speed, and location which material is introduced into equipment may lead to excessive machine surface wear or torque on the drive system. Repair cost decisions also may lead to more frequent failures. For example, selecting a low-cost, marginally capable roller mill bearing for $500 replaced annually over a $2,000 bearing that has a 6- to 8-year lifespan increases the cost of ownership for the roller mill. When accounting for the total repair cost and potential for collateral damage, the decision is pennywise and pound foolish.
Upon installation, as shown by the vertical line “I” in the Traditional Failure Curve shown in Figure 1, equipment has its greatest functional lifetime potential. Once the machine is engaged, the inevitable wear begins, and lifetime is shortened. Faults such as bearing race delamination, pitting, or bearing defects create noise because of metal-on-metal contact. However, its pitch is beyond human hearing range, such as a dog whistle. Ultrasound measurement techniques identified at P1 can identify such a fault in the very early stages.
The fault, however, may progress quickly, causing machine vibration, which also can be monitored and measured. Vibration magnitude and frequency measurements at P2 may be analyzed to identify specific faults. But left unchecked, it may result in additional equipment collateral damage such as framework and drive damage. Along the failure path, many pieces of equipment have oil baths and reservoirs for gear reducers, differential drives on roller mills or positive displacement blower drives. Often the suggestion is to change the oil after a fixed number of operation hours. While hours of operation are not often well known, the time is set to an arbitrary number of months or years.
Given lubricant volume utilized in many pieces of equipment, oil acquisition, and disposal expense as well as maintenance service time, it is appropriate to monitor oil condition before draining and replacing the oil. Oil analysis cost is minimal, perhaps $35 to $75, with the potential of saving hundreds or even thousands of dollars changing oil prematurely or allowing expensive equipment or components to be destroyed. The Basics of Oil Analysis for Equipment Maintenance suggests assessing lubrication oil physical, microscopic, and spectrographic properties over time.
Utilizing infrared technology
Finally, in reliability or condition-based maintenance, the utilization of Infrared technology to check or monitor heat generation in equipment at P4 can be helpful. Heat is energy being released due to friction in mechanical system or resistance in electrical systems. In either case, excessive heat is bad and leads to equipment and perhaps catastrophic failure, including loss of life. Infrared measurement technologies allow maintenance to gather information from locations they cannot reach or would otherwise be unsafe.
By the time humans can hear the noise at P5 or feel the heat at P6, failure is imminent and likely not confined to the initial fault component. Humans do not all share the same sensitivity to sound or feel. As a result, when we do hear something using a stethoscope or by resting our head on a screwdriver touching a piece of equipment, the repair cannot happen soon enough. Hands calloused from repair work may not be as sensitive as hands checking reduction stocks under roller mills or working in the laboratory. Thermometers placed on surface measure the heat that has dissipated to the surface while the internal temperature where damage is occurring may be even higher. While hearing and touch are important senses, they are inadequate to identify maintenance issues in the early stages for many pieces of equipment where both collateral damage and total cost of repair can be minimized.
Regarding equipment failure and repair costs encountered in your plant, what quality issues, downtime, and maintenance expenses could have been avoided if you had identified faults earlier? To stay ahead using time-based preventive maintenance practices, what materials were underutilized, with unnecessary expenses incurred and potential ancillary damage created moving in and out of equipment for an unnecessary repair? Keeping plant systems available for operation on demand is a challenge and methods to accomplish the task are ever evolving. Keep up!
Jeff Gwirtz, a milling industry consultant, is president of JAG Services, Inc. He may be reached at [email protected].
