The preventive maintenance approach is to schedule maintenance and service work based on a somewhat rigid schedule whether or not the field device actually requires it immediately. This practice has the advantage of increasing uptime, but the disadvantages of potentially wasted labor and supervision time, spare parts usage and sub-maximum uptime. The predictive maintenance approach is to develop a service and maintenance schedule customized to actual analyzer system performance and to perform the activities just before one anticipates system failure.
Predictive maintenance is an important improvement over reactive and preventive maintenance because it avoids the secondary costs of reactive maintenance and the waste of predictive maintenance. Bob Call (See Analyzing The Relationship Of Preventive Maintenance To Corrective Maintenance) suggests a 6:1 ratio of preventive (predictive?) maintenance to reactive maintenance, because predictive maintenance often reveals some type of reactive work to be done on an asset.
Analyzer maintenance can incur unnecessary costs because, for simplification, it treats “identical” analyzer systems identically. For example, one process chromatograph may measure hydrogen in a clean gaseous stream and another apparently identical chromatograph may perform a similar measurement in a liquid stream laden with residual tars. Therefore, the analyzer system on the gaseous stream, under preventive maintenance, may incorrectly be assumed to have the same maintenance requirement as the analyzer system that can be contaminated by tars. This is an example of preventive maintenance that can result in reactive maintenance, when predictive maintenance would be superior because the latter anticipates actual failure.
Extending the example of the two chromatographs from the previous paragraph, predictive maintenance would distinguish the two analyzers with the chromatograph on the gaseous streams as requiring predictive maintenance perhaps monthly and the chromatograph on the tarry stream requiring similar service as frequently as weekly.
Continuing the example of preparing for lifetime preventive maintenance for a new chromatograph system, according to the Ramon Vorne article referenced in the sidebar, we must identify key performance indicators, or KPIs, that quantify waste, provide an early warning system for analyzer system failure, and provide information as to where improvements should be made. KPIs should be current or forward-looking metrics. The referenced article by Bob Vavra add that the key is to extract useful information from the KPIs to improve business and operating profitability.