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Oil contaminated by water is a common problem in the recirculating oil systems used on paper machines. This article looks at how contamination may occur and discusses methods for preventing and removing water contamination.
Experience tells us that no bearing lubricant exists that completely protects a bearing against the effects of moisture. Water-contaminated lubricants suffer a range of problems, the most serious being corrosion and lubrication breakdown. Relatively small amounts of water in lubricants can have drastic effects on bearing life.
Experience suggests that contamination levels as low as 0.02 percent (200 parts per million) can have an effect on bearing life. The relationship between contamination and life reduction is, however, non-linear and complex. Factors such as oil type/solubility, additive packages, degree of degradation, etc., all affect the susceptibility of a lubricant to this problem.
Bearings used in the paper industry are particularly prone to such problems due to the nature of the application. There are many possible sources of water contamination, including:
Oil systems associated with the wet end of a paper machine are particularly vulnerable. The nature of the paper forming process means that the air in this area is inevitably moisture-laden. Bearing housings are generally warmer than the surrounding atmosphere and frequently employ sealing arrangements that afford little protection from spraying water.
Vents in oil drainage headers intended to facilitate oil flow back to the reservoir have been found in some cases to contribute to moisture contamination of the oil.
Transportation, handling and storage problems can result in moisture contamination of “new” oil, perhaps even as much as 1,000 ppm.
Numerous studies have demonstrated the detrimental effect that water has on the wear characteristics of rotating and hydraulic equipment. Oil, when mixed with water, loses some of its lubricating properties. The oil film between the rotating elements in a bearing is a critical factor in the bearing life. Presence of water in the oil will reduce film thickness.
Figure 1. Left: new grease. Right: milky grease as the result of water inclusion.
There are several elements of water-in-oil contamination. These include:
Free water: Free water typically settles toward the bottom of the oil reservoir. This factor usually is allowed for when designing the reservoirs used in oil circulating systems for paper machines. Typically, the reservoir will be sized so as to ensure that the oil is retained for at least 30 minutes in the reservoir; this allows free water to settle before the oil is recirculated. This usually means that the reservoir capacity will equate to 10 times the rated pump capacity. Indeed, for wet-end oil circulating systems, some manufacturers recommend a reservoir sized for 60 minutes of retention time.
Emulsified water: Emulsified water exists as very fine bubbles suspended in the oil. It is characterized by the whitish or hazy portion that is often seen in contaminated systems. The bubbles are usually about five to 10 microns in diameter. Emulsification of water can result from a number of courses:
• Use of high-shear pumps
• High-velocity pipelines
• Fine filtration medium
• Additive packages designed to emulsify water
Dissolved water: Oil, like air, can hold water in solution, up to its saturation point, which will depend upon the oil type and temperature. Oil containing dissolved water may still appear clear and bright. Once the saturation point is exceeded, the oil usually appears cloudy or “milky”.
Several methods exist for detecting and measuring water contamination of oil. These range from largely subjective methods to sophisticated laboratory techniques offering a high level of accuracy.
Fluorescing dyes are available that allow the user to judge the amount of water that is present by the degree of fluorescence, but this method is really only suited to confirm a suspect condition. Experience shows it to be limited in terms of repeatability and accuracy.
A more accurate method involves driving off the water in the form of steam and then quantifying the amount. At least one supplier of oil systems offers this technology to mills for their own in-house use. The method offers accuracies to 0.01 percent.
Lab services are now commonly available from oil and filter companies. However, experience suggests that sampling is often not undertaken frequently enough to prevent formation of damage on bearing races. Damage will, in time, lead to failure.
Undoubtedly, the best way to avoid these moisture-related problems is to prevent the contamination from occurring. Effort expended in this area can have significant effects on mill reliability and availability.
Steam joints: Leaking steam joints are a major source of water contamination. Typically, escaping steam is blown against bearing housings on the back side of dryer sections. The resulting condensate contaminates the oil system. Some mills employ flingers to prevent this. The best solution is to avoid having steam leaks at all.
Seals: Most bearing housings are fitted with a labyrinth-type seal that allows water to pass through into the bearing housing. Flingers or stationary add-on shields improve the protection afforded to the bearing.
Inspections: Check lubrication drainage systems for holes or openings that allow water or water vapor to get into the system. Such problems are commonly found in vents. Rigorously check piping for holes.
Oil/water heat exchangers: The purpose of these units is to cool the oil as it returns to the reservoir. The design of modern systems is usually such that the oil pressure is higher than the pressure of the cooling water so that any leaks should result in oil contaminating the water, rather than the other way around. Severe leaks of this kind can be a serious problem, but it’s usually of a different kind (e.g., environmental).
Some older-type reservoirs also use steam coils to heat oil in the reservoir before it goes to the machine. Leaks here can cause water to enter the tank, and shutting off the heaters may be a short-term solution. However, sending cold oil to the bearings results in other lubrication problems, and so this is not a permanent solution. Modern systems tend to employ electric immersion heaters to avoid this problem.
Sweep air: Oil returns to the reservoir at a higher temperature than its environment. Moisture in the returning oil, or in the air surrounding it, may condense in the free air space above the oil in the reservoir. If the tank design doesn’t allow sufficient retention to allow settling to occur, this moisture will contaminate the oil in the reservoir. One solution is to pass filtered air across this air space by means of an exhaust fan. Some system suppliers use a more elaborate air dryer to condition the air in the air space.
There are several methods with which to remove contamination. These include:
Vacuum dehydrators: The contaminated oil is heated in a vacuum. This lowers the boiling point of the water. The water contained in the oil (both free and emulsified) is thus released as water vapor, which is then condensed and removed.
Unlike other methods, this process doesn’t also remove oil additives, and air and other non-condensable gases are removed through the vacuum pump. Commercial systems are available to implement this process, and some of these also feature separate filters for particulate removal. Such systems are usually rated in terms of throughput capacity. However, water removal efficiency is dependent upon the level of vacuum obtained. As a result, it can vary widely from system to system.
Centrifugal cleaners: Centrifuges are effective in removing free water but are ineffective for removing emulsified or dissolved water, which will not separate by gravity. Centrifuges also are limited in their application because of their inability to remove entrained gases and air that are present in lubricating systems. Also, centrifuges are often perceived to be high-maintenance pieces of equipment.
Other methods: Coalescence-type separators speed the process that oil and water do naturally (i.e. separate). This is achieved by use of filters (known as coalescence media) made of materials that are hydrophobic (water repellent) and oleophilic (oil attracting). These cause the oil to form into droplets that float to the surface of the separation chamber. This effectively forms two zones of liquid in the chamber. The separated oil flows into a collection chamber for removal, while the clear water underflows the oil and is discharged on a continuous basis. Solids also settle out through the filter media and are collected in a sludge tank.
Other filter-type approaches involve use of absorbent filter media to remove free and emulsified water as it passes through the cartridges. Such methods are considered by some to be cumbersome because of the need to periodically renew the filters and because of the uncertainty of operation.
Presence of water in lubricating oil is detrimental to bearing life. The best way to prevent this is to avoid water contamination of the oil, rather than use techniques for removing water from the oil. However, if resources aren’t available for preventing the problem at the source, then money must be spent on removal processes. Removal systems vary in cost and effectiveness, and choice is dictated by the specific requirement and the resources available. Expenditure incurred in keeping water out of lubrication systems will have a payback in terms of increased machine uptime. Avoiding the need for maintenance staff to undertake bearing replacements on an emergency basis also avoids compromising planned maintenance activities and the problems that can arise as a result of such activities not being timely undertaken.