Extending the life of an ageing fan

Fans that have been in service for some time are, today, likely to be working harder than ever, handling hotter gases, meeting higher flow rate demands, as well as being expected to work longer between maintenance intervals. For many of these older installations it is just a matter of time before damage or even catastrophic failure occurs. Whether the fan is centrifugal, axial or tangential, factors such as high speed, heavy loads, shaft misalignment and vibration will all exacerbate the problem.

Getting the best possible performance from more demanding fan applications requires an integrated approach to the selection of bearings, seals, housings and lubrication systems. This is particularly important when varying operating conditions lead to thermal expansion of shafts and other fan components, which can greatly increase friction and vibration levels, leading to early failures.

Excess heat generation can be addressed by replacing components with those that solve some of the underlying problems. One solution is to use a self-aligning bearing – such as SKF’s CARB toroidal roller bearing – in the non-locating position and a spherical roller bearing in the locating bearing position. This arrangement avoids the problem of induced axial loads created by thermal expansion of the shaft, as well as reducing friction and vibration.

Insufficient, excessive, contaminated or incompatible lubricants will accelerate bearing failure. Such lubrication issues can be addressed with sealed-for-life, re-lubrication-free bearings, like the SKF ConCentra ball and roller bearing systems. For grease lubricated applications, the SKF total fan solution approach favours the use of a contact seal design that can handle speeds of up to 13m/s compared with the 4-8m/s limits of conventional seal designs.

Once the bearing arrangement has been determined and associated lubrication requirements met, the next step is to consider maintenance and condition monitoring – the latter using either hand-held instruments as part of a routine maintenance inspection, or by data logging in real time via an embedded, fully networked system.

Problem solving

A good example of how such choices are made is provided by a recent project carried out by SKF for a UK-based scientific establishment.

Three air handling unit (AHU) fans (two axial; one centrifugal) serving certain areas of this establishment were deemed important candidates for condition monitoring. The client's initial plan was to use hand-held instruments - in this case an SKF Machine Condition Advisor CMAS 100-SL temperature and vibration measurement pen - and the fitting of low-cost CMSS-200 vibration sensor/indicators to the fan bearing housings.

Under the establishment’s operating license, the three AHUs are deemed critical assets, and as there is limited access to the fans for maintenance purposes, the SKF team were able to demonstrate the benefits of a more robust approach to condition monitoring – one that used fixed sensors networked to a central monitoring location, which would provide a more timely indication that maintenance was due and thus limit the number of unnecessary maintenance visits.

The resulting installation includes strategically located vibration and temperature sensors and transmitters from an SKF Copperhead fault detection system. The 4-20mA vibration and temperature measurement signals from the Copperhead system are transmitted via a hard-wired network back to the establishment’s Building Management System where they are monitored in real time to provide trending and alarms.

In summary, choosing appropriate bearings, ensuring that they are correctly and adequately lubricated, and that their running condition is frequently monitored for maintenance purposes will all help to prolong the life – and possibly even improve the performance – of an existing fan installation.