When it comes to plant maintenance there has traditionally been 5 types of maintenance strategies. Each of these are differentiated by the nature of the tasks and how the business itself chooses to operate. These 5 techniques include:
- Corrective maintenance: Machinery that needs to be corrected is passed onto the maintenance department who are specialized in quickly replacing the machinery so it can become operational as quickly as possible.
- Preventive Maintenance: In an effort to maximize operations parts are replaced systematically at times where production is not lessened even if there are no signs of imminent failure. That is, the equipment is inspected and replaced even if it has not shown any symptoms of having a problem.
- Predictive Maintenance: Analysis is done (vibration, oil analysis etc.) to predict when a part is likely to fail. The part is then replaced just prior to its expected failure point. To apply this maintenance, it is necessary to identify physical variables (temperature, vibration, power consumption, etc.) of each piece of equipment and there failure points. This maintenance it is the most technical since it requires advanced technical resources, and at times strong mathematical, physical and / or technical knowledge.
- Zero Hours Maintenance (Overhaul): The goal of this system is to review the equipment at scheduled intervals and track their signs of failure/loss of production. Once the equipment has decreased output considerably to the point that it is difficult to produce forecasts of production capacity the entire machine is overhauled. One the piece of machinery is overhauled it should be equivalent to a new piece of machinery (i.e. With zero hours of production).
- Periodic maintenance (Time Based Maintenance TBM): The basic maintenance of equipment specified by scheduled services (like a car service every 10’000 miles). It consists of a series of elementary tasks (data collections, visual inspections, cleaning, lubrication, re tightening screws etc.) for which no extensive training is necessary, but perhaps only a brief training. This type of maintenance is the based on TPM (Total Productive Maintenance). The majority of scheduled service times are provided by the manufacturers of plant equipment.
The Difficulty Of Choosing A Type Of Maintenance
The difficulty of finding a maintenance system that suits your business is many types of equipment needs a mix of each of these maintenance types so you can’t just apply one theory to your equipment.
Take for example a car engine, we will take care of lubrication every 10’000 miles (periodic preventive maintenance); when it is required, we measure the vibrations or temperature while in use (predictive maintenance); If something fails it may need a complete engine rebuild (overhaul) and we will repair any faults that come up along the way (corrective maintenance).
The most suitable mixture of these types of maintenance will dictate to us the best options to avoid production losses and reduce equipment failure, repair costs, environmental impact, safety and quality of a product or a service, among others.
The Use Of Maintenance Models
To answer this question it is convenient to define the concept of “Maintenance Models”. A Maintenance Model is a mixture of the previous types of maintenance in certain proportions, and it responds appropriately to the needs of a particular piece of equipment. You need to understand that every piece of equipment will need a different mix of maintenance types, a particular mix of tasks, so that maintenance models will be varied throughout different pieces of equipment. As a general rule 4 of these methods can be clearly identified, and they can be supplemented with two additional methods when applicable.
Each of the models below include several of the maintenance styles listed above. Moreover, all of them include two activities: visual inspections and regular services. This is because it is demonstrated that these tasks reduce the chance of plant failure. Even in the simplest model (Corrective Model), in which virtually the equipment is left on its own and you do not deal with it until a fault occurs it is advisable to observe it at least once a month, lubricate it with suitable products etc.
Visual inspections bring virtually no additional cost so additional resources do not need to be allocated to the task. This inspection allows us to detect faults in an early stage and its resolution will generally be cheaper the sooner its detected. Servicing equipment is almost always a profitable endeavor. Although it does represent a cost (lubricant and labor), it is generally so low that it is more than warranted, since a malfunction due to a lack of lubrication will always involve a greater expense than the corresponding cost to lubricant a piece of machinery.
- CORRECTIVE MODEL: This is the most basic model, and includes, in addition to visual inspections and lubrication mentioned previously, the repair of broken equipment. It is applied to equipment which is not critical to manufacturing, so a breakdown does not stop operations.
- CONDITIONAL MODEL: It includes all the activities of the corrective model and also includes a series of tests on more important equipment. If after testing we discovered an anomaly, a preemptive repair is carried out. If everything is correct then business as usual is carried out.
- SYSTEMATIC MODEL: This model includes a set of tasks that are performed no matter the condition of the equipment. On top of scheduled maintenance, you also perform measurements and tests to decide whether the equipment is acceptable to continue running at current operations. If any faults arise, they are repaired as necessary. This model is widely used in equipment of medium importance. These pieces of equipment are of some importance in the production system and failures cause some disruption, but they are not critical. It is important to note that equipment subjected to a systematic maintenance model does not have to have all its tasks done on a fixed schedule. The main difference with the previous two models is that in this one a maintenance task should be some sign of failure.
- HIGH AVAILABILITY MAINTENANCE MODEL:It is the most demanding and exhaustive model of them. It is applied to equipment that cannot breakdown or malfunction without causing significant stoppages to production.
To best maintain this equipment it is necessary to use predictive maintenance techniques that allow us to know the status of the equipment while it is still operating. As parts move closer to failure rates then a full overhaul is done at a team that least effects productivity.
Since in this model corrective maintenance is not used the aim should be zero breakdowns on the equipment; parts should be replaced before they fail. Quick provisional repairs that will maintain the equipment working until the next overhaul are acceptable.
Examples of equipment when this model of maintenance should be used:
- Turbines for power production
- high temperature furnaces, where an intervention means cooling and re-heating the furnace, resulting in energy expense and production losses associated with it.
- Reactor deposits or reaction tanks that are not duplicated and need to be kept in operation as many hours as possible.
When designing a Maintenance Plan two important considerations should be taken into account. Firstly, some equipment is subjected to legal rules that regulate their maintenance, forcing you to perform certain activities with an established frequency.
Secondly, some of the maintenance activities cannot be performed with regular maintenance equipment (either their own or hired) because it requires knowledge and / or specific resources that are only up to the manufacturer, distributor or a specialist team.
These two aspects should be assessed when trying to determine the maintenance model that you should apply to a piece of equipment.
Some equipment is subjected to rules or regulations by Government regulations. These are often hazardous to people or the environment if something foes wrong. These often require the completion of a series of tasks, tests and inspections performed by individuals or companies authorized to carry them out. If you plant has any machinery where this is necessary it needs to be incorporated into your maintenance plan.
Some equipment subjected to this type of regulations are:
- Equipment and devices that operate under high pressure
- Installation of High and Medium Voltage cables
- Cooling Towers
- Certain lifts: service or people
- Fire Prevention Facilities
- Storage tanks of certain chemicals
Specialist we refer to a company specialized in maintenance of a particular piece of equipment. The specialist may be the equipment manufacturer, importer’s technical service, or a company that has specialized in a particular type of intervention. You turn to an outside party when:
- You do not have sufficient knowledge to maintain the system accurately.
- You do not have the necessary resources to do the maintenance yourself.
The use of subcontracted maintenance to a specialist is usually the most expensive alternative. There are often very little competition so the fees are not market prices but monopoly prices. You should try to avoid outsourcing unless it is absolutely necessary.