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Wednesday, 13 June 2012

What causes centrifugal pump bearing failure?

Centrifugal pump bearing failure can cost a lot of money!

On face value it seems silly to ask what causes centrifugal pump bearing failure. A bearings life is determined by the number of hours it will take for the metal to "fatigue" and that is a function of the load on the bearing, the number of rotations, and the amount of lubrication it receives. Companies predict pump bearing life measured in years.

What factors cause centrifugal pump bearing failure.

There are two main causes of centrifugal pump bearing failure:
  • Contamination of the bearing lubricant (Mostly by water or moisture.) 
  • High heat often caused by too much bearing lubrication. 
 As little as 0.002% water in the bearing lubrication will reduce bearing life by 48%. The water often enters from leaking packing, wash down hoses and moisture caused by the temperature cooling down in the bearing casing after shutdown, with moisture laden air entering the bearings case. A 6% water content in the oil will reduce bearing life by as much as 83%.

This water or moisture contamination comes from three main sources:
  • Packing leakage. 
  • Water hoses used to wash down the base plate area because of packing leakage. 
  • Aspiration or moisture in the air entering the case especially when the pump is stopped. 

The following sources of heat lead to centrifugal pump bearing failure:

The major source of excessive heat is over-lubrication of the bearings.

But surely if a little lubrication is good, wouldn't a lot be better? Not really! Here’s an analogy: Walking along the beach on a hot day you decide to go into the ocean; at first you go into the water up to your ankles, and this cools you down. But as you go deeper and the water gets up to your waist you have to exert more energy – which generates heat. It takes a lot of energy to get through the same temperature water and this would make you hot and fatigued instead of cool and refreshed.

It's the same scenario that leads to bearing failure. Too high a lubrication level and the bearings will consume energy as they plough through the lubricant. This energy will show up as heat added to the lubricant causing it to first lose its viscosity, and then the lubricant will begin to form varnish and coke as it gets hotter. Varnish and coke are another name for solids... and this cycle will rapidly cause centrifugal pump bearing failure.

The problem with grease and oil lubricants is their low specific heat and their poor conductivity. Synthetic bearing lubrication is better, but still has a temperature limit that is still too low for many pumping applications.

 The SKF company claims that uncontaminated (Mineral) grease and oil has a useful life of thirty years at 30°C. They further state that the life of grease and oil is cut in half for each 10°C rise in temperature. That means that at 100°C oil and grease have a useful life of only 90 days.

What form of lubrication will reduce centrifugal pump bearing failure?

Grease packed
  • Grease is hard to change because the usual method is to pump grease into a grease fitting and let the new grease push out the old grease. This method guarantees over-lubrication, resulting in premature centrifugal pump bearing failure. 
  • The only proper way to grease a bearing is to hand pack it full, but not the cavity where it is located. As the bearing heats up some of the grease will leak into the cavity reducing the amount of lubrication. 

Oil is easy to install and change, but beware of the following.
  • Be sure you have an oil level indicator on your pump. 
  • Be sure the pump is level. Many pumps have been aligned without checking to see if they were level.
  • The oil level should be half way through the bottom ball when the pump is at rest. 
  • Unfortunately you cannot use oil lubrication on a vertical installation. 
  • Some mechanical applications use bearings of different diameters. This makes it impossible to maintain a correct oil level. Vertical applications have the same problem.
Oil mist is the preferred method if you can solve the fugitive emissions problem.
  • Oil mist can provide a positive pressure inside the bearings to keep out contaminants. 
  • It takes 5000 to 6000 psi (340 to 405 bar) to mist 30-weight oil and that pressure is not available in your pump. 
  • Mixing the oil with air presents a problem because of venting hydrocarbons to the atmosphere.
  •  If you find the bearing lubricant is getting too hot, most pumps have a facility for cooling the oil in the bearing case. Never attempt to cool bearings by cooling the outer case. Steel will expand or contract at the rate of 0.001 mm/mm/ 50°C. In other words if you cool the casing it will contract or shrink and increase the load. 
The rule is "cool the oil, never the bearing".
So what’s the best oil to use to minimise centrifugal pump bearing failure:
These are the choices:
  • Mineral oils, pure and refined. 
  • Synthetic oils for higher temperatures. 
  • Animal and vegetable oils that are not normally used for bearing lubrication because of the risk of acid formation after a short operating period. 
For many the preferred route to reduce centrifugal pump bearing failures is to use synthetic oils:
  • Diesters that are usable to 120°C. 
  • Silicone oils that are usable to 200°C. 
  • Fluorinated oils have good oxidation stability but are so expensive most lubricating companies do not use them. 
  • Polyglycols are good for bearings over 90°C Their oxidation stability is good and they have recorded service lives ten times longer than those of corresponding mineral oils. Their specific gravity is more than 1.0 so water floats on top of them. 
  • Synthetic hydrocarbons have the advantage of a viscosity that is reasonably independent of temperature. They can be used to 200°C 
Equipment failure costs can often exceed the original cost of the system, and when the major contributor to this is centrifugal pump bearing failure, then it’s important to understand what leads to these failures, as well as knowing how to lubricate a bearing.

Switching to a Habot Synthetic Lubricants product can dramatically reduce the occurrence of centrifugal pump bearing failure. Contact us for professional advice.


centrifugal pumps said...

Thank you
The information you shared is very informative.

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Fritz Clark said...

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gareth batty said...

Like most pumps, a Centrifugal pumps converts mechanical energy from a motor to energy of a moving fluid. A portion of the energy goes into kinetic energy of the fluid motion, and some into potential energy, represented by fluid pressure (Hydraulic head) or by lifting the fluid, against gravity, to a higher altitude.

Thanks a lot for sharing this wonderful informative post..

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