Maintenance is an essential aspect of any machine or equipment’s life cycle. The two most commonly used maintenance strategies are “Preventative’ Time-Based Maintenance (TBM) and “Predictive” Condition-Based Maintenance (CBM). In this article, we will compare and contrast these two maintenance strategies, specifically in the context of rotary screw air compressors.
Brief history of maintenance practices
Before diving into the comparison of TBM and CBM, it is essential to understand the history of maintenance practices. Back in the 1950’s, when rotary screw air compressors were first introduced, maintenance tools and practices were relatively primitive and most compressor maintenance was corrective or reactive. This means that maintenance was performed only when needed after a piece of equipment had already broken down, or was showing obvious signs of distress like noise, vibration, or leaks.
Some years later as maintenance practices evolved, regularly scheduled preventive maintenance or TBM was introduced. TBM relied on arbitrary time intervals to determine when to perform certain maintenance tasks, with no consideration given to the machine or its components’ actual condition. These TBM tasks most often included regular replacement of machine components, like oil and filters, whether they needed to be replaced or not.
By the 1980’s, with the advent of semiconductors, microprocessors and PCs, new maintenance tools and practices emerged with machine data taking a lead role. These technologies led to today’s Predictive or Condition-based maintenance (CBM), where actual machine data informs maintenance personnel on what specific tasks actually need to be performed, and when. Unlike TBM, with CBM the machine or its components’ actual in-service condition are a central factor in determining how and when maintenance is performed.
Comparison of TBM and CBM for rotary screw air compressors
In the context of rotary screw air compressors, outdated TBM requires the compressor to be regularly shutdown to replace its oil, filters, separators, and other items the manufacturer deems to be “consumables”. These tasks are performed on a fixed schedule, regardless of whether they actually need to be replaced. This often results in expensive compressor oils, separators and filters being needlessly replaced with plenty of useful life still remaining. While TBM may be good for those who supply these “consumables”, for the compressor users TBM is inherently wasteful resulting in excessive oil and parts consumption, and unnecessary compressor downtime, maintenance labor, and waste disposal.
On the other hand, predictive CBM uses real-time data from the compressor’s control panel or other sources like oil analysis and vibration analysis to monitor key metrics and determine in advance precisely what type of maintenance is needed and when. Rather than blindly replacing expensive components at predetermined intervals, CBM enables users to use data like pressure drop, temperature, oil analysis results and vibration levels, to optimize and conserve their maintenance resources. This data not only alerts compressor users to the “symptoms” that need to be addressed, but it also can provide valuable information that can help treat and cure the underlying “disease” responsible the problem.
For example, routine oil analysis reveals a compressor’s oil has a high acid number, which normally would trigger a costly oil change. However, data from the same oil analysis also indicates that the oil’s other properties were perfectly normal, except that its additive levels were trending low. Based on this data, instead of an expensive oil change maintenance simply replenished the oil’s additives and avoided all the cost, labor, downtime, waste oil disposal, and environmental impact associated with changing their oil.
Benefits of CBM versus TBM for rotary screw air compressors
In summary, today’s data driven predictive CBM has several distinct advantages over the outdated preventative TBM practices from a half-century ago. These include:
- Reduced maintenance costs from:
- Reduced oil and parts consumption, purchases, and waste disposal
- Reduced maintenance frequency and labor man-hours
- Reduced compressor downtime and production interruptions
- Increased compressor reliability with:
- Increased attention to component condition and key metrics (less attention to meaningless time in-service)
- Improved monitoring of key parameters that warn of pending problems and help avoid major repairs
- Increased ability to analyze data, determine root causes of failures, and utilize proactive reliability-centered maintenance best practices
- Increased sustainability through:
- Reduced oil and parts consumption and hazardous waste disposal
- Reduced environmental impact
- Improved safety by:
- Reducing risk by minimizing maintenance frequency and exposure of maintenance personnel to hazardous machinery (high voltage, high pressure, high temperature)
- Increased awareness of critical parameters that can lead to catastrophic failures and result in high pressure and high temperature oil leaks and fires
Both TBM and CBM have their place in rotary screw compressor maintenance, and the choice between the two depends on a variety of factors, not the least of which is the criticality of your compressors, the size of your maintenance budget, and your company’s environmental directives. However, for most rotary screw compressor users who are looking to optimize their maintenance resources and improve reliability, CBM offers significant advantages over TBM.