Pump Bearings 4

[Rob100]

Excuse my ignorance but I have a pump question. I have two suppliers of pumps requesting opposite bearings in the pumps. One is suggesting sleeve bearings and the other roller bearings. The pumps are split case horizontal pumps and will run at approximately 900 rpm with 350 hp motors. The services are plant water and wash water for a municipality. Are there any rules of thumb on this that I could use? Which one will provide the longer life?

 

[Heckler]

What type of pump? I've used DU Bushings in gerotor pumps that spin 9K rpm which pumps lub oil. And roller bearings in gear pumps that pumped sea water. Both very reliable in their respective applications.

Heckler
Sr. Mechanical Engineer
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[Rob100]

They're centrifugal horizonal pumps.

 

[JJPellin]

There are advantages and disadvantages to both options. Rolling element bearings are no tolerant of water or dirt in the lube oil. Even tiny amounts of water will corrode the surfaces of the bearing and drastically shorten bearing life. Sine there is rolling contact between rollers and races, solid particles cause immediate damage. But, a rolling element bearing (especially a roller bearing) can handle very high loads over a wide range of speeds. With the proper lubricant, the bearing can handle quite high temperatures.

The sleeve bearing is much more tolerant of small quantities of water. Since the bearing is probably babbet on bronze, nothing will rust. Under some conditions, I have seen these bearings run on a 50/50 mix of water and oil with no damage at all (although, I wouldn't recommend it). The sleeve bearing is also somewhat tolerant of debris. A small particle of dirt can pass right through the bearing. Even if it leaves behind a small scratch in the babbet or shaft, the bearing will probably survive. The main weaknesses of a sleeve bearing are speed and heat. The sleeve bearing is designed to operate within a tighter range of speeds. If you run it too fast or too slow, the oil wedge may not full develop (slow) or the bearing may overheat (fast). If there is a loss of cooling, even at ideal speed, the bearing cannot handle much heat. The babbet metal melts at something over 350 °F. Before it melts, it gets soft and can't handle as much load.

Rolling element bearings are very easy to monitor with case mounted vibration instruments. The vibration transmits out through the housings very clearly, in most cases. Bearing failures are easy to detect, since all of the components of the bearing have characteristic vibration frequencies that are known and easily recognized.

The sleeve bearing does not transmit vibration data out through the housing very well. Other then oil whirl, the bearing does not have characteristic vibration frequencies that are detectable for most failure modes. The only way to get good vibration data is using non-contacting proximity probes which are much more expensive and complex.

The rolling element bearing tends to have a finite life. Based on load, speed, lubricant, etc. they rate the bearings with an L-10 life meaning that 90 percent of a given population of that bearing would last for that long. But the bearing will eventually wear out and fail. A sleeve bearing with proper lubricant has infinite life. There is no metal to metal contact and with proper lubricant, there should be no wear.

It is because of these differences that most pumps use rolling element bearings and most turbines and large compressors use sleeve bearings. For an unspared compressor, we are going to install a lube system with coolers and filters to provide ideal lubricant to the sleeve bearings. We need infinite life because a shut down of the compressor will shut down the unit at a cost that could exceed US$1 Million per day. For the thousands of pumps in the plant, we could not afford to build the elaborate oil systems and vibration monitoring systems. For those, finite life is acceptable since we have a spare pump and can easily detect a pending bearing failure many weeks in advance.

For the very low speed machines you describe, I would normally expect rolling element bearings. If you will have no continuous vibration monitoring, no circulating lube oil system with filters and coolers rolling element bearing seem like a better choice. Since they are in water service, this might make me question this choice. With roller bearings, you will need to be sure to keep water from getting into the bearing housings. But with mechanical seals, bearing isolators (or magnetic shaft seals), this should be easy to do. You should take vibration data on the pumps at least once per months and change the oil at least twice per year (more often if the environment is very dirty).


Johnny Pellin