We were recently conducting an ISO Vibration Category II certification training when an interesting technical debate started between the trainees (who were mostly 20+ years experienced mechanical maintenance engineers) regarding the best method for protecting fin fan coolers against vibration. The debate was about the usefulness and reliability of the traditional Mechanical Vibration Switch that was provided by the fan manufacturer, and how this switch is not protecting the fan against high vibrations. There were stories about fans failing while switch did not act at all.
To start with, it’s wrong and unfair to expect the mechanical (spring loaded) vibration switch alone to protect your fans against vibration. It was simply not designed for this function! Surprise!
The mechanical vibration switches are designed to protect the integrity of the cooling tower “structure”, so that when one or more fans vibrate excessively they would not cause the fans structure itself to collapse. In doing this function, the mechanical switch may actually discover some fan problems and protect the fan itself from damage before it happens; but remember it’s a byproduct of its main function which is protecting the structure. Some people even call it an “Earthquake Switch”, you may call it “Fan structure vibration switch”, but nobody should call it a “Fan Vibration Switch”.
There a few facts to consider when working with mechanical vibrations switches:
- The fans will vibrate, and may vibrate excessively, before the switch act. Remember, the switch protects the fan structure (tower) not the fan itself.
- Fan OEMs install one mechanical vibration switch close to each fan in order to capture structure vibrations closer to their main source. Good idea.
- If you remove the mechanical vibration switch then your fan structure will not be protected.
So how shall we protect the fan itself?
The answer is simple you should install permanent sensors on the fan, gearbox (if any), and motor bearings. The readings from these sensors can be collected by a portable data collector, or a wired or wireless online system. Only then you will be able to read and diagnose fan mechanical problems, and depending on the data collection frequency, you may provide of level of protection also.
Back to the fan structure vibration protection
You can replace the mechanical switch with an electronic switch, or you can even install an online system to measure the structure vibration. But these choices will cost you more, and for many people a completely unjustified cost because the mechanical switch is more than adequate for the job. The mechanical vibration switch is:
- Easy to set, and test
- Mechanically robust
- Mechanical vibration switches do not require power supply, and can be installed in hazardous areas easily.
- They are the usually the least expensive option when all vibration system components are considered.
Here is our expert recommendation
You should maintain your mechanical vibration switch, and ensure that it’s set correctly! In our experience we have seen switches set at four times the designed allowable vibration limit. We have also seen switches installed upside down or at least in the wrong location. If you use a hammer to test your mechanical vibration switch, then I can guarantee there is a 75% chance your switch is not set correctly, and your fan structure is not protected. Additionally, you also need to monitor your fan bearings with any of the available techniques (portable, wired, or wireless).
Make sure that you are covered
Don’t forget that mechanical switches may fail after operation for many years depending on the plant environment. So if your plant is in a hot, humid, and dusty area, then it may be a good idea to think about replacing the old switches with new ones.
The team at Proact is willing to support any customer on site or remotely in checking and setting their mechanical switches. We provide a one day training that helps customers realize the full function of this simple and useful tool.