The objectives of condition monitoring of machinery include: a) control of the machinery, especially for high power and dangerous machines; b) optimizing the availability of machines by avoiding unexpected shutdowns, especially for critical machines in a continuous production process, and c) implementation of condition-based maintenance, for which the operations are planned according to various constraints (cost, production, failure condition, etc.).
11.2.1 General considerations
The ultimate goal of machine condition monitoring is to get useful information on the condition of equipment to the people who need it in a timely manner. The personnel include operators, maintenance engineers and technicians, managers, vendors, and suppliers. These groups will need different information at different times. The task of the person or group in charge of condition monitoring is to ensure that useful data is collected, that data is changed into information in a form required by and useful to others, and that this information is provided to the people who need it and when they need it. Useful references on this subject are the books [11.2] to [11.16].
The focus of this chapter is on vibration-based data, but there are several different types of data that can be useful in assessing the machine condition. These include lubrication oil/grease analysis, wear particle monitoring and analysis, noise, temperature, force, output (machine performance), product quality, odor, and visual inspections.
11.2.2 Maintenance strategies
Maintenance strategies can be divided into three main types: a) run-to-failure, b) preventive, and c) predictive maintenance. Each of these different strategies has distinct advantages and disadvantages. Specific situations within any facility require the application of a different strategy. Therefore, no one strategy should be considered as always superior or inferior to another.
11.2.2.1 Run-to-failure maintenance
Run-to-failure, or breakdown maintenance, is a strategy where repair work or replacement is only performed when machinery has failed. In general, run-to-failure maintenance is appropriate when the following situations exist: a) the equipment is redundant, b) low cost spares are available, c) the process is interruptible or there is stockpiled product, d) all known failure modes are safe, e) there is a long mean time to failure (MTTF) or a long mean time between failure (MTBF), f) there is a low cost associated with secondary damage, and g) quick repair or replacement is possible [11.17].
Fig. 11.2 (from [11.17])
Figure 11.2 shows an illustration of the relationship between the machine time in service, the load (or duty) placed on the machine, and the estimated remaining capacity of the machine. Whenever the estimated capacity curve intersects with (or drops below) the load curve, a failure will occur. At these times, repair work must be carried out. If the situation that exists fits within the seven rules outlined above, all related costs (repair work and downtime) will be minimized when using run-to-failure maintenance.
11.2.2.2 Preventive maintenance
When specific maintenance tasks are performed at set time intervals (or duty cycles) in order to maintain a significant margin between machine capacity
and actual duty, the type of maintenance is called preventive (or scheduled) maintenance.
Preventive maintenance is most effective under the following circumstances: a) data describing the statistical failure rate for the machinery is available, b) the failure distribution is narrow, meaning that the MTBF is accurately predictable, c) maintenance restores close to full integrity of the machine, d) a single, known failure mode dominates, e) there is low cost associated with regular overhaul/replacement of the equipment, f) unexpected interruptions to production are expensive, g) low cost spares are available, and h) costly secondary damage from failure is likely to occur [11.17].
Fig. 11.3 (from [11.17])
Figure 11.3 shows an illustration of the relationship between the machine time in service, the load (or duty) placed on the machine, and the estimated remaining capacity of the machine when preventive maintenance is being practiced. Maintenance activities are scheduled at regular intervals in order to restore machine capacity before a failure occurs. In this way, there is always a margin between the estimated capacity and the actual load on the machine. If this margin is always present, there should theoretically never be an unexpected failure, which is the ultimate goal of the preventive maintenance.
11.2.2.3 Predictive maintenance
Predictive (on-condition) maintenance requires that some means of assessing the actual condition of the machine is used in order to optimally schedule maintenance, in order to achieve maximum production, and still avoid unexpected catastrophic failures.
Condition based maintenance should be employed when the following conditions apply: a) the machine is expensive or critical, b) a long lead-time is necessary for replacement parts (no spares are readily available), c) the process is uninterruptible, d) equipment overhaul is expensive and requires highly trained
personnel, e) failures may be dangerous, f) secondary damage may be costly, and g) failures are not indicated by degeneration of normal operating response [11.17].
Figure 11.4 shows an illustration of the relationship between the machine time in service, the load (or duty) placed on the machine, and the estimated remaining capacity of the machine when predictive maintenance is being practiced.
Note that the margin between duty and capacity is allowed to become quite small, but the two lines never touch. This results in a longer time between maintenance activities than for preventive maintenance. Maintenance tasks are scheduled just before a failure is expected to occur. This requires the existence of a set of accurate measures that can be used to assess the machine integrity.
Fig. 11.4 (from [11.17])
There are instances where a given machine will require different maintenance strategies during its operational life, e.g. scheduling the maximum time between overhauls during the early stages of the machine life, and increased frequency of monitoring as the age of the machine increases, looking only for unexpected failures.
11.2.3 Factors influencing maintenance strategies
While there are some general guidelines for choosing the most appropriate maintenance strategy, each case must be evaluated individually.
Principal considerations will always be defined in economic terms. Sometimes, a specific company policy, such as safety, will outweigh all other considerations.
The following eight factors should be taken into account when deciding the best maintenance strategy for a given machine: a) classification (size, type) of the machine, b) critical nature of the machine relative to production, c) cost of replacement of the entire machine, d) lead-time for the replacement of the entire machine, e) manufacturer’s recommendations, f) failure data (history), MTTF,
MTBF, failure modes, g) redundancy, and h) safety (plant personnel, community, environment) [11.17].