A predictive maintenance department at a chemical customer was alarmed by a sudden increase of overall vibration values on their Wi-care™ system, installed on a fan on their production units.
The spectrum below is the spectrum at the moment the highest amplitudes was reached
The high peak is at 8 orders or 380 Hz with an amplitude of 7.6 mm/sec
What is the reason for this high 8 order? This is a fan with 8 blades. 8 orders is the blade pass frequency.
This is also a fan that needs cleaning on a regular base. The cleaning is done in the weekend with water under pressure. During the cleaning procedure, maintenance people have to open a fan at the bottom of the fan house to allow the cleaning water to leave the installation.
In this particular case, they forgot to open the fan, and the fan house was full with water with the fan was started. The fan turned on and acted as a pump.
After opening the valve and removing the water, the fan was running again in smouth conditions.
The cascade plot below shows the spectrum before, during and after.