Read Part 1 here.
Most pump bearings fail long before their design life span. The American Petroleum Institute (API) typically requires a minimum bearing life (L10) of 25,000 hours, and ANSI B73.1 specification for horizontal ANSI pumps specifies a minimum L10 bearing life of 17,000 hours at maximum load and rated speed. Prudent end users frequently request bearings with more than 40,000 hoursL10, but most bearings do not reach that many hours of operation before failure.
More than half of pump bearings fail as a result of contamination, excess heat or both. Preventing this introduction of contaminates is easier and less expensive than removing them. Some studies suggest removing contaminates can be eight to 10 times more expensive than prevention.
This premature failure rate is typically the result of contamination of the oil not a fault of the bearing or pump manufacturer. Dirt, wear particles and other foreign debris as well as improper bearing installation procedures can contribute to contamination that leads to reduced bearing life.
Other forms of contamination include heat and air in the form of air entrainment and aeration. Increased levels of heat and air lead to increased oxidation rates.
One of the most common sources of contamination, however, is water, which is often introduced because of improper storage and handling. During pump operation, water can leak into the bearing housing from external sources such as area wash down, spray from failed mechanical seals (or packing) or leaks from equipment near or above the pump. Another common method of water introduction is condensation through machine aspiration (moisture laden air is drawn in due to pressure differentials).
For example, a pump running steady state at a given temperature above ambient for five days is shut down on Friday at 4 p.m. As the pump cools, the ambient air is drawn into the bearing housing where it cools and the moisture condenses, releasing the entrained water into the housing where it mixes with the oil.
According to sources at the SKF Bearing Company, 250 parts per million (PPM) water in the lube oil will reduce bearing life by a factor of four, and another source states that 0.002 percent water in the oil will reduce the bearing life by 48 percent. According to other sources, the reduction of oil contamination levels from the ISO 21/18 to the ISO 14/11 will increase bearing life by a factor of 7.
Because water in the oil is invisible at low levels, a lab should test the oil using the Karl Fisher method or the end user should conduct a simple "sizzle test" in which the oil is quickly subjected to a hot surface temperature of 250 to 300 degrees F. A hot plate is commonly used, but a metal spoon or aluminum foil with a butane lighter can also be used. If more than 800 to 1,000 PPM water is present in the oil, a sizzle sound can be heard when the oil temperature exceeds 212 to 220 F. If the oil sizzles, too much water is present in the oil. Because the sizzle test can have dangerous side effects, always check with plant safety procedures before conducting the test.
Companies that strive for longer mean time between failures (MTBF), mean time between repairs (MTBR) and improved plant reliability select their oil or grease based on equipment requirements and properly match them with the oil properties. They also store and allocate the oil using controlled and clean methods.
I have seen end users store oil drums upright, outside and unprotected with an open bung. I have also seen mechanics draw oil from drums into used paper coffee cups or soft-drink cans. When confronted, they reply, "That is the way we have always done it, and we are not having any bearing or oil problems," or "I washed out the container first."
If a pump's bearings are not lasting three to eight years, the plant's equipment lubrication practices should be questioned. Check with your oil supplier, or consult articles, books and other publications that discuss these subjects. The October 2006 issue of Pumps & Systems magazine explores this topic, and I highly recommend any books and technical papers on this subject by Heinz Bloch, Alan Budris or Rojean Thomas.
Bearing Lubrication Methods
Selection of a bearing design for a specific service will, to a large degree, determine how it is lubricated. Depending on the pump speed, type of service, horsepower (HP) range and size, different types of bearings are available.
A properly selected oil-lubricated ball or roller bearing will work for most applications less than 200 HP, 400 F (fluid temperature) and 3,600 revolutions per minute (rpm). For some smaller and lower temperature applications (less than 320 F), grease-lubricated bearings may also work well. Larger pumps at higher speeds and system temperatures will require line, sleeve or plain journal bearings for radial support (hydrodynamic journal bearings) and tilted shoe (pad) designs for thrust bearings.
Methods and designs that are acceptable for ANSI specification pumps may not be acceptable for API, process, marine and power generation applications where HP can often exceed 70,000 brake horsepower (BHP) with speeds in excess of 6,000 rpm.
Because most end users at the high end of the HP and speed spectrum are aware of oil types, best practice lubrication techniques and bearing selection, this article will examine the middle and lower range.