There are many reasons to consider an online vibration monitoring system.

1. Criticality: This is the most common reason why online monitoring systems are installed. Air compressors,  main mill drives,  chillers, or any potential production bottlenecks are the primary candidates.
2. Safety: Rotating equipment can be dangerous to be around. Excessive noise, heat, or moving product can be a treacherous environment in which to work. Often guards, interlocks, safety devices, and other safety-related obstructions prevent us from getting good data. This is especially true in containment areas, hazardous locations, and in confined spaces.
3. Accessibility: Just because the machine is visible does not mean that it is accessible. The area may be restricted, moving equipment may block access to it, or the critical asset itself may move (such as a locomotive or mining equipment that is not readily accessible). Complicating considerations are commonly related to cost, logistics or manpower.
4. Remote Locations: Distant pumping stations, offshore platforms, or shipboard machinery at sea may make onsite data collection and analysis very difficult and expensive to justify. An online monitoring system can be the ideal solution for these situations.
5. Time & Cost: Often an asset runs intermittently or is only used in certain cycles. Sometimes the cost of assigning a person to collect the data and the time it takes to travel to and from an asset is greater than the value of the data it offsets. If this cost and time, including waiting time for the machine to run a cycle, exceeds the perceived value of the manually collected data, an online system might be considered.
6. New Equipment: An online system may be useful for new rotating assets, especially if they embody a new design for which little or no historical data exists, involve high criticality, or just need to be monitored to ensure they function correctly through the new equipment break-in period or warranty period.
7. Older Equipment: If an asset needs to be nursed through late life cycle stages and monitored for continued proper operation even though it is entering the wear-out zone of its components, an online system will give the information needed to make those judgment calls or to buy enough time to get through a production run.
8. Unique Machinery: For machines that are one-of-a-kind, unusual, or might contain a great deal of foreign content whose replacement parts have a longer delivery lead time, an online system will give the advance warning one needs to avoid service disruptions.

Thanks to Jay Gensheimer with Solute LLC for this valuable post.