Keeping check of cooling towers

In the 1990s, vibration and accelerated wear caused a fan blade to shear off a cooling tower in Pennsylvania, resulting in significant imbalance. Fan, motor and gearbox were ripped from their support and the entire assembly fell into a basin below the tower deck, taking the walkway that supported the fan system with it.

That’s an extreme example of catastrophic failure but, nevertheless, an instructive one, as it shows the tremendous power of the forces at work within cooling towers.  It also illustrates why monitoring the efficiency of such critical but potentially vulnerable systems is essential.  Cooling towers are prone to the typical causes of rotating machine failure such as misalignment, but also face a range of specific environmental challenges, including high or gusting wind speeds, dust, sand and, in coastal locations, high concentrations of salt. This means that vibration monitoring equipment must not only be supplied but correctly specified to resist corrosion. Inspectors of the catastrophic failure in Pennsylvania identified that although vibration switches were used they had failed to function due to ingress of moisture. Today, however, suppliers such as Hansford Sensors can prescribe and supply robust industrial accelerometers with high IP (ingress protection) ratings to minimise such failure. With knowledge of the best sensor and detection technologies available today and how these can best be used, maintenance engineers can not only prevent failure but significantly maximise machine uptime.

Inside the fan system

Within each cooling tower is a large fan system, providing forced air to the heat exchangers.

The fans within are typically several metres wide and, along with the drive motors, gearboxes and associated linkages and control mechanisms, all of these are prone to vibration if alignment is not performed correctly.  A typical vibration monitoring solution is to monitor the motor at the drive end (DE) and non-drive end (NDE), with the sensors mounted radially to examine motor bearing condition.  Sensors on the gearbox input and output shafts are also mounted radially to monitor bearing condition and fan imbalance (velocity), while the addition of an axial accelerometer on the input shaft gives a good indication of the thrust on the shaft. 

One of the key decisions to make during the specification stage is between online or offline monitoring. Online vibration monitoring, in which the outputs are connected to protection/monitoring or PLC systems, is the preferred solution but if budgets are tight it is also possible to maintain an effective vibration monitoring programme using offline monitoring techniques.  In an online system, the vibration sensor is connected to a local junction enclosure and then connected to the PLC via multicore (screened twisted pair) cable.  If more detailed analysis is required, vibration modules can be installed with specific filters to suit the characteristics of the cooling towers.  For offline monitoring, the local junction enclosure described above is routed not to a PLC but to a switch/connection enclosure.  The engineer then conducts monitoring by connecting a data collector to the switch enclosure.

Using vibration monitoring as part of a predictive and preventative maintenance programme has proven to be an extremely cost-effective and reliable solution for cooling tower operators. Early detection of faults, and accurate pinpointing of problems, enables maintenance to be scheduled efficiently, cutting repair costs and increasing the return on investment, as well as providing insurance against catastrophic failure.