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Design/methodology/approach – The model of an industrial pipeline has been assembled ranging more than 100 m away from the oil mist source, equipped with devices collecting oil deposited inside the pipes. Other tests were performed in stands constructed as parts of pipes coiled in helical form. Long time experiments with continuous oil mist flow enabled to achieve calculable results.
Findings – The quantitative results obtained in experimental investigation on the reduction of oil/air ratio in an oil mist header system show that considerable differences of the oil/air ratio may be observed in a typical long pipeline. Possible consequences of oil deficiency on lubrication of remote mechanisms are presented in the case study. Results of tests are shown in diagrams and tables. These results may be useful for correction of design calculations procedures.
Research limitations/implications – Tests have been made on the basis of one kind of the oil atomized in typical condition and conveyed with steady flow through the piping of rather simple geometry. However, there are other factors affecting oil droplets deposition and the most influencing are probably: the flow velocity/pipe diameter factor, oil atomization characteristics and the geometry of the oil mist piping.
Practical implications – The research has shown dramatic decrease of oil content in the long distance systems that may result in poor lubrication of remote mechanisms or over lubrication of those located close to the oil mist generator. It should be taken on account in calculation of oil mist demand to particular lubrication points.
Originality/value – Presented tests have been carried in the scale and flow parameters very close to those applied in industry. Thus, the results are reliable and could be very useful both for designers and the practitioners of centralized oil mist systems.
Introduction
The use of oil mist lubrication in the process and metallurgical industry dates back to the early 1960s when companies began to lubricate with this technique the motor and pump bearing, and somewhat later, reliable bearings of steel sheets rolling mills (four stand rolling mill, IHI Company, Japan, 1974). Since, that time, the oil mist lubrication has been proven as an efficient lubrication method, useful in particular for high revolution rolling bearings, mechanisms operating in vertical position or those situated in hard to reach places.
Oil mist technology has kept pace with advances made by industries and the mist systems being designed and installed today are far superior and more efficient than those installed 10 or 20 years before. Some of advances and new applications which are utilized in today's systems are:
- microprocessor controlled central oil mist generators compatible with central distributive control systems;
- more efficient and effective distribution and application of oil to lubrication points; and
- environment protective clean mist collection containers.
Principles of oil mist lubrication systems
Typical oil mist lubrication is realized as a centralized lubrication system that continuously produces, conveys and delivers oil to cooperating surfaces of mechanisms.
The principle of this airborne lubrication consists in air jet atomization of lubricating oil into micron-size particles, resulting in formation of an aerosol called “oil mist,” which could be conveyed through the distribution piping to lubrication points. Special reclassifiers measure and apply the mist in lubrication points in two ways:
- Pure mist application which allows to remain reclassifiers placed in the manifold; in this case, the oil reclassified into spray lubricant is blown immediately onto lubricated surfaces.
- Purge mist application; almost all the oil particles are coalesced into droplets that lubricate surfaces and/or create an oil bath for immersion of lubricated elements.
This lubrication method positively affects reliability and extends the life of mechanisms, mainly bearings, without high investment costs (Thomas and Ward, 1996; Bloch and Shamim, 2000).
As usual, the oil mist is delivered to many lubrication points by centralized lubrication systems, spread over a long distance, practically up to 100 m (110 yd).
Figure 1 shows the scheme of a centralized oil mist system applied for lubrication of bearings, gears and other mechanisms.
The following could be distinguished in the system:
- oil mist generator;
- main oil mist pipe and the header system to convey the mist to the reclassifiers in lubricating points; and
- controls and alarms of the lubricating system.
The system of headers should be properly designed and installed to convey the oil mist without any obstacles and to minimize losses of oil in the transportation paths due to its deposition in piping. Both hoses and rigid metal pipes are used with diameters proportioned to flow rate and reasonable flow velocity. The flow velocity within 3/5 m/s is optimal to minimize oil loss in piping. The ends of the piping headers are connected to the metering reclassifiers, located in the manifold or built immediately in the lubricated mechanism's housing.
There is a wide diversity in control and alarm systems available. As a rule, controls are provided to maintain oil and air temperatures and oil level at the correct settings. Other controls include an air pressure regulator and an oil/air density adjusting screw. The air pressure regulator adjust pressure supplied to the generator, so that the mist header pressure is held at a constant level.
Alarms are typically provided for air temperature, oil temperature, oil level, and mist header pressure. Other points sometimes included are supply oil pressure, and regulated air pressure. Each parameter has a design operating set point plus low and high alarm set points. Alarms can be either local, in a remote control room or both.
These information is sufficient to know the actual operating status of the system, however, nothing says on the oil density (oil/air ratio) in the mist.
Oil density (oil/air ratio) in the oil mist
As a matter of fact, the oil/air ratio is the parameter, one of a few that most seriously decides on the effectiveness and economic efficiency of lubrication (Bednarek and Szydło, 1988).
Oil mist density at the outlet of the generator and further in the system depends on the following general factors:
- the design and operating parameters of the oil atomizer;
- oil properties and particularly its viscosity;
- the rate of oil flow to the atomizer;
- flow rate of compressed air for atomization;
- compressed air temperature; and
- the structure of the header system and the actual distance from the oil mist generator.
The size spectrum of oil particles in the oil mist outlet is 0.5 till 10 μm with the mean value of the distribution about 2 μm. The oil particles in the oil mist stream are in continuous turbulent motion resulting in their coalescence and deposition of bigger droplets inside the pipes. These processes make the size spectrum gets closer to an optimum, the oil mist is more stable but at the same time the oil/air ratio is gradually reduced.
The oil mist density is a gray area for alarms and controls. A number of monitors have been marketed over the years, but none have performed accurately and reliably for extended periods. Practically, up to now the best assurance that the system works is the routine check for mist presence at the most remote lubricating point in a system.
Anyway it is clear that a minimum oil/air ratio of the oil mist must be presumed in calculations of the oil mist required for sufficient lubrication of mechanisms. The simple empirical relations are used in these calculations, mainly proposed by the oil mist equipment manufacturers (Alemite, 1992; Delimon Fluhme & Co., 1976).
Oil mist necessary flow is calculated regarding on the size and the type of lubricated mechanism wherever it is situated providing the oil mist is the aerosol of a stable minimal oil density in the whole system. As a rule, the minimal value used in calculation is 4 g/m3 (0.24 oz/h/cfm) (Alemite, 1992).
It is not true, and further will be presented experiments and data, that proved the significant reduction of the oil/air ratio, below the recommended values, could be expected in long header system. This fact should be taken on account in calculations of the amount of the oil mist delivered to the lubrication points, particularly those situated far away from the oil mist generator.
Experiment
The problem of oil mist density decrement, outlined in the previous chapter have been under our interest since we could observe bad results of oil deficiency on the reliability of remote bearings in an industrial oil mist systems we monitored a time ago.
We decided to clarify quantitatively these matters by experiment, therefore, a practical and reliable method was developed to determinate the oil/air ratio in the oil mist pipeline, at any distance from the generator outlet.
The method
The simple method based on the mass measurements of oil quantities at the beginning of the test and after finishing the test has been successfully applied in all experiments.
The following masses were measured:
- mass of oil in the generator's tank (at the start and after finish of the test);
- mass of oil deposited in different parts of piping system; and
- mass of oil reclassified at the outlet of the pipe system.
The amount of air consumed in the whole test was measured, hence the oil/air ratio in any point of interest could be calculated as the relation between measured or calculated mass of oil in that point and the amount of air consumed in the process.
This method has been successfully examined in laboratory conditions but there are no major obstacles to apply it in industrial conditions if an examination of design presumptions is required.
The structure of experimental set up
The first objective of the experiment was to determine the decrement of oil/air ratio along the pipeline that conveyed the oil mist produced from a standard industrial oil by the mist generator, rated similarly as those applied in practice. Second objective was to investigate what effect on the oil particles deposition could be exerted (if any) by curved flow of the oil mist stream like is in bent pipes of lubricating system. Three stands (of two kinds) were assembled to realize these objectives.
Figure 2 shows the scheme of the system used for measuring the oil/air ratio in the oil mist flowing past the straight pipeline. The pipeline was divided into 15 measuring sections, in this number 3 initial, 2 m (2.2 yd) long and 12 sections of 8 m (8.6 yd) length. The total length of the measuring pipeline was 102 m (111.6 yd).
Shorter measuring sections in the initial part of the pipeline were installed owing to the large amount of oil deposited in the pipeline in the vicinity of the oil mist generator outlet.
Owing to limited area in the laboratory, it was not possible to arrange the total length as the straight line so the segment of the pipe, of radius R=0.5 m (0.56 yd) was used to connect two straight parts. As it will be discussed further it was not of greater effect on the test results.
Figure 3 shows two stands used for measuring the oil/air ratio in the oil mist in pipes assembled as the helical lines of diameters: 1.0 m (1.1 yd) and 0.5 m (0.56 yd). Total lengths of the pipelines, were the same in both stands and namely 22 m (24.1 yd). Helical parts were divided into two measuring sections, each of 8 m (8.6 yd) length. Three initial measuring sections, each of 2 m (2.2 yd) straight pipe were added similarly and for the same reasons as in the experimental set up shown in Figure 2.
The helical pipelines were made of a rubber hose.
Figure 4 shows the simple apparatus, which has been designed to collect and measure the quantity of oil, deposited in pipes. It consists of an element with circumferential groove made inside, and detachable container for collection of oil. The element with groove connects two parts of the pipe, and in this way the subsequent collecting apparatus divide the whole pipeline into measuring sections. The oil deposited inside the pipe in the section above a collecting apparatus, could be drained off through the circumferential groove and the hole to the container. The pipes both straight and arranged in the helical form were sloped by 1:20 toward the mist generator thus facilitating drainage of oil deposited inside the measuring section into appropriate collecting apparatus.
It should be noted that industrial installations have also sloped parts to facilitate return of deposited oil to the generator and to avoid eventual oil traps in piping. As usual, the slope is 1:50/1:12 toward the oil mist generator, the exact figures dependent on the oil viscosity grade and the ambient temperature.
The oil mist generator was equipped with the air heater with thermometer and adjustable thermostat enabling control of the compressed air temperature within 288 till 408 K (15-135°C or 60-275°F), non-adjustable heater of the oil in the generator's tank and the oil mist pressure gauge of the range up to 0.05 MPa (72.5 psi).
Compressed air was supplied to the generator from the non-lubricated compressor to prevent the test system against the compressor's lubricating oil penetration that could disturb the measurement results.
The pressure reducing valve, the pressure gauge and the flow meter were installed at the inlet of compressed air into the generator.
At the outlet of oil mist pipes was installed the special reclassifying device – “impactor” of high operational efficiency. The impactor maintained required pressure in the oil mist system (similar as in industrial systems) and trapped as much as possible of the oil contained in the mist to avoid environmental pollution by stray mist outgoing the system.
Procedure and calculations
The generator filled with tested oil, empty containers for oil deposited in piping and the end impactor's container were weighed separately. After assembly of all detachable elements and adjustment of control parameters, the test was started to continuous operation for the time (some ten hours) necessary to obtain measurable quantities of atomized and deposited portions of oil. After finishing the test, the oil deposited have been draining in a defined steady period, then the weight measurements and calculations were performed.
The mass of oil atomized in the test was determined as the difference between the mass of the whole generator before and after the test. Quantities of oil deposited in particular sections were obtained in the same way, by weighing the detachable containers of collecting apparatus before and after the test.
Calculation of the oil/air ratio at the generator's outlet was very simple as it demanded only to divide the mass of oil atomized in the generator by the total air volume consumed in the test.
Determination of the oil/air ratio in subsequent measuring points was a more complex task. It should be noted that the oil content in the mist in any measurement point is decreased by the mass of oil collected in all containers installed before that point. To obtain the reduction values of the oil/air ratio in particular sections, the values of mass of oil collected in containers were divided by the total amount of air consumed in the test. Then the actual oil/air ratio in a point of interest was obtained by deduction: the value of the oil/air ratio at the generator's outlet was deduced by the sum of the oil/air ratio reduction values in all sections preceding that point.
The amount of oil reclassified in the impactor at the outlet of pipes was the element of the balance calculation of oil quantity utilized in the experiment, useful for examination of the experiment correctness. These calculations are not included to the paper.
Parameters
These were test parameters, the same in all runs:
- Oil used in tests. Mobil Mist 34, viscosity 210 cSt in temperature 323 K (50°C or 122°F), special for oil mist application.
- Temperature of oil in the generator's tank. 313-321 K (40-48°C or 105-120°F).
- Flow rate of compressed air for atomization. 6 m3/h (3.53 cfm).
- Compressed air pressure. 0.24 MPa (34.8 psi).
- Compressed air temperature. 348 K (75°C or 149°F).
- Flow velocity of oil mist in pipes. 3.4 m/s (3.72 yd/s).
- Oil mist pressure. 0.015 MPa (2.18 psi).
- Temperature of oil mist in pipes. Not less then 291 K (18°C or 65°F).
- Quantity of oil atomized in one test. 3.2-4.1 kg (7,1-9,0 lb).
- Time interval of particular tests. 86-96 h.
Results and discussion
Tests were repeated three times for each arrangement, both straight pipes and that assembled in the helical form. Results of tests are presented in the form of tables and diagrams. In tables are included numerical results of all three repetitions and the arithmetic mean of three values for every measuring point. There are also given the numerical values of the oil/air ratio reduction rates in subsequent sections of piping. These data immediately show high differentiation in oil deposition depending on the distance from the oil mist generator.
Diagrams are plotted on the base of mean values obtained in tests.
In Table I are listed results of the measurements of oil/air ratio in straight pipes. Figure 5 shows these results in the diagram form.
In Table II are listed results of measurements of oil/air ratio in the flow of oil mist through the pipe arranged in the helical form of diameter 1.0 m. Figure 5 shows these results in the diagram.
In Table III are listed results of measurements of oil/air ratio in the flow of oil mist through the pipe arranged in the helical form of diameter 0.5 m. Figure 6 shows these results in the diagram (Figure 7).
Experiments showed the considerable reduction of oil/air ratio in the oil mist conveyed by a long distance piping system. Almost three times reduction of oil content in the mist was proved in tests with straight pipes at the distance 102 m (111.6 yd).
Secondly, the reduction rate of oil/air ratio is strongly diversified in the pipeline. Within 30 m (33 yd) distance from the oil mist generator more than 50 percent the oil atomized in the generator was deposited in pipes. The oil/air ratio has been falling down along the pipeline on the whole distance, however, the reduction rate in remote measurement points was significantly lower then that in the initial part of piping. Anyway, it could not be said the conditions of a stable aerosol were obtained within the distance of the experimental set up.
Results of experiments with pipes arranged in helical form compared with the straight pipe experiment in the same flow condition, show only slightly higher reduction of the oil/air ratio that could be caused by the stream curve.
Numerical results of experiments will be used further in the case study analysis. This study will show the effect of the reduction of oil content in the mist on the efficiency of lubrication in a long distance centralized oil mist systems proportioned according to commonly applied relations (Alemite, 1992; Delimon Fluhme & Co., 1976).
Case study
Given
The oil mist centralized lubrication system is used in a factory, to lubricate a number of mechanisms of moderate service located at different distance from the oil mist generator: 6 m (6.56 yd), 30 m (33 yd), 62 m (68 yd) and 94 m (102,836 yd). Between others there are single row, I.D. 10 in. ball bearings, lubricated by the oil mist system, which are critical for considered mechanisms' reliability. The oil mist centralized system consists of typical industrial devices and its operation parameters are similar as parameters of the experimental set up described in the paper (see part: Experiment – Parameters).
Find
- Determine the quantity of oil mist required for lubrication the 10 in. ball bearings of mechanism.
- Determine the quantity of oil the calculated oil mist can deliver to 10 in. ball bearings in mechanisms at different locations.
- Determine the quantity of oil required for lubrication the 10 in. ball bearings by an alternative lubrication method.
- Compare two different lubrication methods.
Analysis
- Quantity of oil mist required for lubrication of bearings. Volume of oil mist required for lubrication I.D. 10 in. ball bearing according, the same for any bearing's location in the system is (Alemite, 1992): Equation 1 where: D – shaft diameter in inches; R – number of rows of rolling elements.
- Quantity of oil delivered by the oil mist. Oil quantities delivered to bearings located in different positions will be determined on the base of data obtained in experiments with the straight pipeline, with exact figures taken from Table I. The oil quantities are calculated from the per hour flow of oil mist to bearings (it is actually 0.42 m3/h, for all locations) multiplied by the oil/air ratios (data from Table I) different depending on a distance the considered bearing is located:
- at the distance 6 m (6.56 yd), the oil/air ratio is 5.60 g/m3 (0.33 oz/h/cfm), then the supply of oil per hour is: Equation 2
- at the distance 30 m (39 yd), the oil/air ratio is 4.03 g/m3 (0.24 oz/h/cfm), then the supply of oil per hour is: Equation 3
- at the distance 62 m (68 yd), the oil/air ratio is 3.01 g/m3 (0.18 oz/h/cfm), then the supply of oil per hour is: Equation 4
- at the distance 94 m (102,836 yd), the oil/air ratio is 2.45 g/m3 (0.15 oz/h/cfm), then the supply of oil per hour is: Equation 5
- Quantity of oil required by an alternative lubrication method. As the alternative lubrication method, we propose an oil terminating system, e.g. progressive lubrication line. Oil lubricant requirement for ball bearings lubricated with that technique depends on the shaft diameter (I.D. of the bearing), the number of rows of rolling elements and the lubricating film thickness replacement rate (Lubriquip, Inc., 1999).For I.D. 10 in., single row, ball bearing and 0.001 in./h film thickness replacement rate, the volume of oil film, terminating oil system is: Equation 6 where: D – shaft diameter, inches; R – number of rows of rolling elements; and T – film thickness replacement rate, in./h. Assuming mass density of lubricating oil is 0.95 g/cm3 (0.55 oz/in3), the required mass quantity of oil for lubrication of a single considered bearing is: Equation 7
- Comparison of oil mist lubrication and progressive lubrication calculations. The analysis shows the considerably different oil quantities will be supplied by oil mist depending on the location of the bearing in the oil mist lubrication systems. This is because the calculation methods are based on the steady mist volume demand despite the fact that oil/air ratio varies in different locations along the header system. Hence, the oil supply to the bearing in the distance 6 m (6.56 yd) from the mist generator (the mechanism located close to the generator) is 2.35 g/h (0.083 oz/h), when in the distance 94 m (102,836 yd) from the mist generator (the mechanism most remote from the generator), it is only 1.03 g/h (0.036 oz/h).
The analysis of the mist lubrication system in this study compared with data obtained in the experiment, show the flow 1.68 g/h will be achieved slightly above 30 m (39 yd) from the mist generator, and the flow 1.56 g/h (0.036 oz/h) we can expect in the distance about 50 m from the generator.
Conclusions
- Considerable reduction of oil/air ratio is observed in the oil mist conveyed by lubrication header system.
- Reduction rate of oil/air ratio is highly diversified in the pipeline. More than half the total oil deposition is collected by the initial 30 m (33 yd) of the pipeline.
- Curve flow of oil mist do not cause excessive fall out of oil droplets in conditions of moderate flow velocity and not too small radius of the mist stream curve. In experiments with flow velocity 3.4 m/s and the radius of the pipe 0.25 m (0.27 yd) the reduction of oil/air ratio in the mist was still comparable with that obtained in the straight pipe of the same length.
- It is proposed to make correction in the calculation of oil mist demand for mechanisms in remote locations to avoid results of insufficient lubrication that may reduce mechanisms' lifetime. Providing the operational conditions are similar to those in presented experiments, the volume of oil mist supply to lubrication points at the distance greater then 50 m (55 yd) should be ca. 50 percent higher then calculated from recommended relations.
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Zbigniew A. Szydlo did research and education in the scope: lubrication, sealing technology, magnetic and magnetorheological fluids application in machine design. Working as a Faculty of Mechanical Engineering and Robotics in AGH-University of Science and Technology, Krakow, Poland. Zbigniew A. Szydlo can be contacted at: zbszydlo@uci.agh.edu.pl |