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With lubrication acting as the lifeblood of today’s paper-making processes, paper machines are increasingly using oil circulation lubrication systems in both their drying sections and other heavily loaded, hot parts of the process. In addition to traditional oil-lubricated cylinder bearings, many of today’s machines have oil-lubricated felt rolls and gear reducers for the drying sections and the wire and press section rolls. They also run at faster operating speeds and higher temperatures which increases the demand for lubrication.
Despite these increased lubrication requirements, the basic construction of lubrication systems has remained unchanged. Even with some significant and important developments in oil filtration over recent years, there are many other important problems with oil lubrication systems for which effective technical solutions still need to be found, including:
With these problems in mind, a joint project between a leading industry supplier and the Technical University of Tampere in Finland was launched in 1995, aimed at simulating and studying the characteristics of oil flow in lubrication systems. The results of the project cast a new light on the subject of oil circulation and produced a fresh generation of equipment.
The principal aim of the study was to develop a lubrication system that would solve the main problems affecting existing lubrication systems, concentrating on:
The first stage of the study analyzed how traditional oil tanks operate. The resulting information was then used to develop a more efficient construction model that would improve the problem areas.
Oil Tank Efficiency
The dimensions of traditional rectangular oil tanks have been based on a 30-minute retention time, assuming that the oil moves smoothly from the return inlet to the pumping area, with the flow being controlled by intermediate walls of the tank.
To test this theory, a computer simulation based on the particle vectors (Fluent 3D) was created and tested. Surprisingly, the survey clearly showed that only 30 to 50 percent of the oil circulates smoothly, with the rest either moving slowly or simply remaining static in the tank.
Figure 1. Traditional Oil Tank
Figure 1 shows a return inlet on the left end of the tank, with the red and white colors indicating relatively fast-moving oil underneath it. Oil is flowing directly at high speed between the control walls to the suction side of the tank on the right-hand side, and then directly to the suction outlet.
Figure 2. Oil Speed Inside Tank
Figure 2 shows one cross-section of oil flow in the center of the tank. Other cross-sections toward the sides of the tank would show more areas of blue, indicating lower oil flow speeds. These results illustrate that the efficiency of the oil circulation in a traditional tank is extremely poor. Because not all the oil is flowing, the actual retention time is only between five and 10 minutes, instead of the hypothetical 30 minutes.
Water Contained in Oil
Many water problems in paper machines are caused either by the formation of condensation in return lines or leakage in the cylinder steam joints. Bearing manufacturers recommend a maximum water content for the oil of 200 ppm (0.02 percent). In reality, however, the percentage is often several times greater than this, and possibly even higher in extreme cases. Modern high-speed machines are prone to these water problems because of the greater temperature changes in return lines and the oil lubrication of the wire and press sections.
The water, either in microscopic drops or mixed in with the oil, will reduce lubrication efficiency by breaking the film of lubricant which separates and protects the rolling elements of the bearing. The higher the bearing loads, the more damaging this will be.
Water also reduces the useful lifetime of the oil by changing the chemical properties of component materials such as its corrosion-resistant and extreme pressure additives - a problem which is aggravated by high temperatures.
The impact of water contained in lubricant is illustrated in Figures 3 and 4, drawn from the results of tests on a paper machine in Finland.
Figure 3. Narrow Band Vibration Test
Fibure 4. Shock Pulse Measurement Vibration Test
Vibration trends during water shock are shown in Figures 3 and 4. They include the following:
Foam and Air in Oil
Foam and air in the oil cause lubrication problems as the microscopic air bubbles break the oil film between the bearing elements, thus shortening the life of the bearing. Air increases the oxidation of the oil, deteriorating its chemical and physical properties and shortening its lifetime. In addition, problems caused by air in an oil circulation system will increase if a tight or incorrect return filter is used.
Reducing Energy Consumption
In traditional pumping units, pressure regulation is controlled by a separate pressure valve that requires a certain overflow of the oil to maintain a steady pressure in the piping system. This means that normally some 15 to 25 percent of the pumping capacity is used just to maintain system pressure, representing a continuous inefficient use of energy.
In addition, all the other system components must be sized according to this maximum oil flow - for example, cooling of the 15 to 25 percent additional oil volume uses a tremendous amount of water in a year, causing more resources to be wasted.
The Flowline Tank
Based on the analysis of these problems and their relation to the traditional oil tank, a new design – known as the Flowline tank - has been developed. The Flowline’s most obvious feature is the radical circular footprint which replaces the traditional rectangular format. This new design allows more than 90 percent of the oil to circulate in the lubrication system and improves oil retention performance. This permits the total amount of oil in the system to be reduced by 30 to 50 percent, which dramatically cuts the size of the tank required and the associated oil costs. The usable lifetime of the oil remains largely unaffected, and in some cases it actually increases compared to a traditional lubrication system.
The Flowline tank installed on paper machine No. 5 at the UPM Kymmene Oyj Tervasaari mill in Finland has proved its worth in action, according to Heikki Kataja, the mill’s mecatronics supervisor for mechanical maintenance.
“We switched to a Flowline lubrication system featuring the new tank design in 1998,” he said. “The cellar of our building is low and narrow, so the reduction from the old 8 m3 tank to the 3 m3 Flowline tank was a major benefit, considering that the flow rates are equal in both tanks. The start-up went well and we’ve had no real problems to date.”
Removal of Air and Water
In the new Flowline tank, water removal has been improved (Figure 5).
Figure 5. Construction of Flowline Tank
The tank features numerous intermediate horizontal plates that behave in a similar manner to the intermediate walls of a traditional tank, helping to regulate the returning oil as it flows from the center of the tank to the sides. Because the plates are placed close together, any water droplets in the oil sink only a few centimeters before they meet the next plate which directs them down to the tank’s central drain tube.
The new Flowline design removes water more effectively than previous tanks, and oil need only be retained for a few minutes to separate out the majority of nondissolved water drops. This increases the efficiency of water removal and helps avoid water-related problems.
Water removal is another feature of the Flowline tank which has impressed Kataja.
“Under normal operating conditions the water content is very low – around 3.4 to 0.4 percent compared to the oil-specific water saturation point, as measured on the Vaisala scale. If water does get combined with the oil under exceptional conditions, we can remove it quickly and easily. This takes one day with the Flowline tank, whereas it used to take up to a week with our previous tank. Also, the normal water separation system is so efficient that we seldom use the separate removal system.”
The new Flowline tank design offers an additional benefit. As well as guiding water droplets downward, the intermediate plates also separate the air bubbles and guide them upward. The flow speed through the new tank design is shown in Figure 6.
Figure 6. Oil Flow Rate Through the New Flowline Tank
On-line Water Separation
Although the separating capacity of the Flowline tank is limited in the case of water shocks, the remaining water drops (as well the dissolved water) can be separated out by using an on-line vacuum dehydrator. This equipment automatically starts when the circulating oil reaches its operating temperature, using a separate pump to create a vacuum in which the water evaporates out from the oil.
The system automatically controls the vacuum pressure and temperature to prevent any interruption to the pumping or lubrication. To prevent cavitation in the pump, the vacuum dryer is positioned above the main unit, typically on a higher floor of the building, so that a positive suction head is retained (Figure 7).
|Figure 7. Installation of a Vacuum Dehydrator|
Reducing Energy and Cooling Water Consumption
The oil lubrication unit energy consumption can be reduced by using components with a higher efficiency ratio, such as screw pumps and plate heat exchangers rather than gear pumps and tube shell coolers. However, the biggest savings can be achieved by reducing unnecessary pumping capacity.
If the oil pressure of the system is regulated by adjusting the rotation speed of the pump with a frequency converter (variable speed AC-drive), the pumping unit will operate at its optimum energy consumption, providing the exact amount of oil required by the lubrication points.
The Flowline system increases the useable life of the oil, and this has been another significant aspect of the installation at Tervasaari. Kataja confirms that the oil remains in good condition due to its low water content, and acknowledges that this has enabled the machine to run on a lengthy change interval of between five and 10 years, as well as extending bearing life and increasing the runnability of the paper machine. The resulting economic benefits have prompted the Tervasaari mill to invest in an additional Flowline tank for the press section of its paper machine No. 6.
Advanced System Control
Oil circulation systems have traditionally2 been controlled by a combination of manual monitoring and alarm-based automatic operations. Yet as the size of the systems increase, the task of optical monitoring with manual controls has become increasingly complicated.
As a result, an accompanying range of Flowline system control equipment has been developed to complement the new tank design. This equipment can be used to form an independent system and can be retrofitted to existing paper machines when the lubrication system is rebuilt. It is also suited for use with a Flowline system. By connecting to the control mechanisms of the paper machine, it can provide a comprehensive range of facilities which include the following:
One key benefit of a Flowline control system is the modern display technology used to provide maintenance engineers with the same versatile monitoring and control possibilities normally found in a mill’s process control system (Figures 8 and 9). These displays identify potential malfunctions to be identified quickly and efficiently, and allow the operator to take preventive action far more rapidly than might otherwise be possible.
Figure 8. Lay-out
|Figure 9. PI Schema-based Display|
A Flowline control system allows full automatic start-up of the oil lubrication system. This avoids the optical monitoring and manual adjustments normally needed to cold-start a lubrication system (particularly the larger ones) without cold, high-viscosity oil overflowing through the bearing housings. Automatic field control valves in the Flowline system allow the procedure to be done step-by-step.
Improved Oil Flow Monitoring
Monitoring has traditionally used optical methods such as sight glasses or oil rotameters. Over the last 10 years, oil rotameters have developed into reliable, easily operated devices. However, they cannot accurately pinpoint an individual fault and provide the type of detailed information necessary to prevent a shutdown.
Figure 10. Complete Turbine Wheel Monitoring Unit with Flow Display
In comparison, today’s digital oil flow monitors - which can be based on gear sensors or turbine wheels (Figure 10) - offer advantages in terms of accuracy and the ability to feed digital information about the lubrication system back to the operator and into the mill’s main control systems. Features such as these can boost the reliability and efficiency of paper machine lubrication, and this technology is now more affordable.
The digital oil flow meters featured in a typical Flowline system use separate monitoring software to provide accurate and reliable monitoring of the lubrication points. However, some local field monitoring is still required to make oil flow adjustments and to resolve problem and alarm situations. The Flowline system enables this field monitoring to be performed by a separate monitoring unit serving each flow meter group. This unit is placed in the same block as the sensors and adjusting valves in order to simplify operation and adjustment. A complete monitoring unit with sensors and its own local displays is the ideal solution.
Flowline oil flow meters used in a lubrication system can connect to a Flowline Hub – a central monitoring system which provides coordinated control and monitoring facilities that transmits alarm and complete monitoring information to other mill monitoring systems (Figure 11).
The monitoring software is configured to suit each individual system, and can be included as part of the paper machine condition monitoring software or as separate PC-based software. In the field, Flowline monitors can be serviced and configured by temporarily connecting a laptop PC.
The Flowline’s digital oil flow meters measure the temperature of the oil supply to the bearing, automatically compensating the flow display if needed, based on any oil viscosity changes detected. They provide accurate, reliable and automated control which can help avoid potential problems before they arise.
Based on the features referenced above, North American paper mills have been utilizing this technology for more than five years.
“North American mills recognize the need to be more competitive in the global paper market. The state-of-the-art oil circulation and monitoring technology provides mills the opportunity to increase their production while significantly reducing their downtime - thus dramatically improving their competitive position,” said Marcus Pillion, John Crane Safematic North American director.