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Monday, 14 October 2013

Correct lubrication cuts centrifugal pump bearing failure.

Correct lubrication cuts centrifugal pump bearing failure, downtime and costs.

Downtime to repair failed equipment is costly, that’s why it’s important to understand that correct lubrication cuts centrifugal pump bearing failure and saves your company money. Bearings that are properly lubricated with minimal contamination will operate at lower temperatures with extended life.

There have been various studies done on why rolling element bearings fail prematurely, and consistently, the No. 1 cause can be attributed to poor lubrication. One particular study stated that 50 percent of damage is caused by defective lubrication. Poor or defective lubrication can be classified as:

  • Incorrect lubricant 
  • Incorrect quantity of lubricant 
  • Contaminated lubricant 
  • Lubricant degradation

How to correctly lubricate centrifugal pump bearings to prevent failures.

Many companies require life ratings of 40,000 hours (5.7 years) when purchasing new pumps, thereby making correct lubrication practices vital.

The bearing lubricant in centrifugal pumps can be grease, mineral oil or synthetic oil. The primary purpose of oil, or the oil constituent of grease, is to separate the roller elements and raceway contact surfaces, lubricate the sliding surfaces within the bearings, and ensure corrosion protection and cooling.

In order to correctly lubricate centrifugal pump bearings to prevent failures it’s critical to use of the correct viscosity oil for the speed, operating temperature and load. This guarantees that a full oil film between rotating parts is maintained. Viscosity can be negatively influenced by load, temperature, water, contaminants and chemical change. It’s important to refer to the owners manual for the recommended viscosity, but it’s also important to measure the oil sump operating temperature since viscosity decreases as temperature increases.

The viscosity of synthetic oil is less sensitive to temperature changes and more widely used when temperature fluctuations exist. If operating temperatures exceed 100 degrees Celsius, a synthetic lube is recommended as the oxidation rate of mineral oil accelerates faster at higher temperatures.

Determine the correct quantity of lubricant to prevent centrifugal pump bearing failures.

There are several forms of bearing lubrication used:

  • Grease is primarily used on lower horsepower pumps where the parameters are in the size and speed range of rolling element bearings. 
  • Oil splash lubrication is the most common method utilized. Oil splash designs include direct contact, oil rings, flinger discs or combinations of each.  
  • Oil rings and flinger discs are used where speed or loads are factors and the oil is not in direct contact with the bearing. These are dependent on shaft speed relative to the depth of submersion, and the rule of thumb is to use 10mm at the deepest point. 
  • Oil mist lubrication takes place when air atomizes the oil into particle sizes of one to three microns. Airflow transports these small oil particles through a piping system into the pump housing which flows through bearings. It is a centralized type of low-pressure centrifugal pump lubrication system. In pure mist lubrication, the oil/air mist is fed under pressure to the housing. There is no reservoir of oil in the housing and oil rings are not used. Purge mist lubrication utilizes the same principles of pure mist, but a reservoir of oil in the housing exists. 

To correctly lubricate centrifugal pump bearings to prevent failures it’s critical that levels are correctly maintained.

If the level is too low the bearing doesn’t receive enough lubricant to maintain proper film strength – a precursor to surface contact, skidding and possible catastrophic failure. Without enough oil to prevent friction, thermal runaway can happen quickly to a steel bearing. As the temperature of the bearing increases, the ball and race both expand, which creates an even tighter fit. This increases the temperature even more, and the cycle continues to a rapid, catastrophic failure.

If there’s too much lubricant churning will occur, accelerating the oxidation rate due to excessive air and elevated temperatures. It is a common mistake to believe that more is better – especially when it comes to oil sump lubrication. Another consequence of high lubricant levels is leaking seals.

Contaminated Lubricant will lead to centrifugal pump bearing failures.

The most common contaminants are particulate, moisture, incompatible fluids and air entrainment. Particles impede lubricant performance and further localize pressure on components causing denting, fatigue, spalling and abrasion to the surface of mating surfaces. Water will affect the lubricant’s ability to provide a proper fluid film, causing excessive wear and premature failure. Corrosion, cavitation, and premature oxidation and filter plugging of the oil are other symptoms of water contamination. Air contamination affects oil compressibility and causes poor heat transfer, film strength loss, oxidation and cavitation.

Lubricants degrade over time making the correct lubrication selection vital in reducing centrifugal pump bearing failures.

All lubricants will degrade over time, requiring the oil to be changed. The frequency of these changes can be extended by maintaining the quality of the lubricant.

Elevated operating temperatures are a major contributor of oil oxidation. Combined with air, particulate and water contamination, the chain reaction of oil oxidation begins. In mineral oils, for every 8 degrees C increase in oil operating temperature, the oxidation rate doubles. This can be significant when considering pump operating temperatures are frequently near, or above, 60 C By simply lowering the operating temperature of the oil to 50 C, a 50 percent reduction in the rate of oxidation would be realized – doubling the effective life of the oil.

The most basic methods to extend oil life are:
  • Use the correct viscosity oil
  • Use quality oil. 
  • Use the right amount of oil. 
  • Keep the oil clean. 
  • Use synthetic oil 
The importance of proper lubrication in centrifugal pumps is well known, but achieving it is not always easy.

For all your synthetic lubricants requirements call Habot Synthetic Lubricants: We can supply you the correct lubricants to cut centrifugal pump bearing failures.

5 comments:

Elina Maria said...

It is important to have information about the quality of grease or other lubricant before using it. Some times it causes problem for auto parts. This blog gives some good information about such conditions.
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Search2 said...

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Ambica Machine Tools said...

More and more plants have goals focused towards extending the mean time between repairs for their rotating equipment which includes centrifugal pumps.

Aadit said...

Thank you for using my Guide and if it work for you that makes me happy

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