Methodology for diagnosing hydraulic oil in production machines with the additional use of microfiltration

1 Introduction

In hydraulic systems, hydraulic oil plays a very important role. Its tasks include transferring energy, lubricating mechanical elements, cooling the hydraulic system and protecting against corrosion. The quality of hydraulic oil has a direct impact on the performance, durability and efficiency of the hydraulic system. Lack of oil or its improper condition in relation to its contamination will cause the most advanced hydraulic pumps and gears to operate improperly. Contaminants in hydraulic oil, such as dust, metal chips, or water, can significantly reduce the efficiency of hydraulic pumps. These contaminants can cause blockage of filters, damage to working surfaces and increase friction, which leads to faster wear of pump elements. In extreme cases, it will lead to seizure and destruction of the pump [1].

However, poor quality hydraulic oil can also lead to corrosion and mechanical wear. Corrosion is particularly problematic in systems that are exposed to moisture. Mechanical wear caused by improper lubrication leads to a shorter pump life and increased repair costs. In addition to the costs associated with restoring the technical condition of the hydraulic pump, losses associated with taking the construction or industrial machine out of operation must be taken into account. The sources of oil contamination are different and are shown in Figure 1.

Figure 1

Sources of contamination in hydraulic oil: 1 – worn seals, 2 – washing the system with water, 3 – evaporation and condensation, 4 – wearing mechanical elements, 5 – damaged seals, 6 – damaged hydraulic distributor, 7 – contaminated oil, 8 – lack of periodic filtration, 9 – worn filters, 10 – contaminated cooler, and 11 – sludge at the bottom of the tank [2].

According to Figure 1, contamination enters the hydraulic system through worn and damaged seals on hydraulic cylinders, by washing machines with a water jet, and through the products of wear of mechanical elements of the system such as hydraulic pumps or distributors. Other sources of water in the system include evaporation and condensation of water in the hydraulic tank, untreated oil before flooding the hydraulic system, or water ingress from a leaky radiator.

In order to maintain high efficiency of the hydraulic system, it is necessary to ensure high quality hydraulic oil. Methods to ensure that the oil is kept at the required level in terms of its condition free from metallic and other impurities and free from water are:

• Hydraulic oil filtration;

• Regular hydraulic oil testing; and

• Periodic hydraulic oil replacement.

Due to the growing awareness of hydraulic system users about the need for periodic filtration, this method is now used more and more often than periodic hydraulic oil replacement. Previously, it was believed that oil replacement in accordance with the machine’s technical documentation was sufficient to maintain the technical condition of the hydraulic system. It should be noted that oil replacement in accordance with the study by Liu et al. [3] allows for the removal of approximately 95% of impurities from the system. Approximately 5% remains on the internal walls of the pump and other hydraulic system components. Regular filtration or microfiltration allows for the removal of impurities and water from the hydraulic system without changing the oil. The cleaned oil returns to the hydraulic system through an external system with bypass filters. The combination of filtration or microfiltration with hydraulic oil testing allows for the control of the oil condition in terms of compliance with the cleanliness class according to current regulations.

In the literature, the concept of microfiltration of hydraulic systems is related to construction vehicles, road vehicles, industrial and mining machines, power equipment, and machines used in maritime transport. According to Chang et al. [4], contaminated oil in hydraulic systems causes damage in 80% of cases. Contaminated oil is one in which insoluble materials have been collected, such as metals, dust particles, sand, water, rubber, and varnish. The smallest particles below 2 µm are a common cause of damage to the system equipment. Microfiltration, according to Filipponi et al. [5], is a low-pressure separation process using membranes with very open pore structures.

Microfiltration filters can be made of both organic materials, such as polymeric membranes, and inorganic materials, such as ceramics or stainless steel [6]. Work in the field of microfiltration is mainly related to the selection of the appropriate membrane as a microfiltration filter. Microfiltration membranes have the most open pore sizes of all polymeric membranes. The range of pore sizes is from 0.1 to 10 μm. Microfiltration membranes are able to separate large suspended solids, such as colloids, particulates, fats, and bacteria, while allowing sugars, proteins, salts, and low molecular weight molecules to pass through the membrane. According to Poli et al. [7], microfiltration membranes are characterized by an asymmetric pore structure, with tighter surface pores to control rejection and more open macrovoids in the membrane cross-section to optimize flow flux [8].

Membrane microfiltraction technology is also found in the food industry and includes the removal of fats and microbiology in whey protein concentrate or isolate, fractionation of casein or whey, clarification of fermentation broths, removal of microorganisms, clarification of plant extracts, and purification of industrial wastewater [9]. A wide range of materials are used to produce membranes for the purification of aqueous wastewater using membrane technology. Polymeric membranes tend to foul when used for oil-water wastewater, which reduces the membrane flux [10]. The 0.22 µm pore-size polyvinylidene fluoride (PVDF) microfiltration membrane modified with polydopamine has a hydrophilic surface, with higher pure water flux and reduced fouling during emulsion filtration, compared to the commercial PVDF microfiltration membrane. The modification, creating a multilayer membrane, increases the pure water flux up to four times after the membrane cleaning procedure [11].

Modern technology includes many methods of product purification, such as large-scale chromatography, precipitation, crystallization, centrifugation, distillation, and sedimentation. Membrane techniques for separating mixtures have long been treated as auxiliary separation methods on a laboratory scale [12]. Recent years have made it possible to use membrane techniques on a large scale. This is related to the development of plastic chemistry, and especially synthetic polymers, from which most permeable and selective membranes are built [13].

Microfiltration uses membranes with pores of the order of 0.l–5.0 µm. This process removes fine suspensions, bacterial cells, some viruses, plant material particles, fat particles in emulsions (e.g., milk) from the solution. The visual effect of this process may be a change in the color of the filtrate, a decrease in turbidity, or a decrease in the intensity of light scattering. The main application of this technique is the clarification of solutions, the separation of cell biomasses, and the sterilization of media, i.e., substances on which bacteria are grown [14].

The microfiltration process is based on the difference in hydrostatic pressure on both sides of the partition in the range of 0.05–0.5 MPa [15]. Microfiltration is often carried out in a tangential system, which largely prevents the deposition of sediment on the membrane surface. Filters used in membrane processes are usually made of ceramic materials or synthetic polymers, such as polysulfone, Teflon, or cellulose acetate. These materials are not cytotoxic, nor do they affect the filtered solution in any other way. The walls of the filter membranes are characterized by an anisotropic structure, i.e., the pore channels expand from the membrane surface into its structure. Thanks to this, the molecules that are retained by a given membrane, are retained on its surface and do not clog the lumen of the capillaries inside it. In industry, flat, spiral, capillary, and tubular filters are distinguished [16].

The microfiltration method has also found wide application in biotechnology for cold sterilization in the beverage production process and in the pharmaceutical industry. Where the milk subjected to the microfiltration process is microbiologically clean, typical pasteurization, clarification of juices, wine and beer are not required. In wine production, the use of microfiltration and ultrafiltration, e.g., of finished wine, ensures, on the one hand, the removal of undesirable microflora, and on the other hand, deprives the wine of compounds causing its turbidity. In this way, the production process is eliminated from the troublesome stages of filtration using clarifiers and sulfurization as a factor destroying microflora [17].

Separation of bacteria from water (biological water purification) is performed in cases where particles larger than 0.1 mm must be removed from the liquid. Microfiltration is also performed in the purification of wastewater, separation of oil-water emulsions, pre-purification of water before nanofiltration and reverse osmosis processes, and separation of liquid-solid substances in the pharmaceutical and food industries [18].

Biotechnological progress has revolutionized the commercial production of many biochemical products, especially industrial enzymes. Traditional methods of collecting cells and purifying enzymes produced using fermentation, genetically modified organisms are centrifugation and filtration using an auxiliary filter layer. Cross-flow membrane filtration is increasingly replacing the above separation steps due to lower investment costs and increased enzyme production, as well as simplification of further processing methods of biochemical products [19].

The literature review on microfiltration shows that it is a very broad concept, and this technology is used not only in mechanical or civil engineering but also in chemical and process engineering and agri-food engineering. The literature is dominated by works on materials for filtration membranes and the efficiency of these membranes [20]. There are also works on filtration techniques or microfiltration [21,22]. However, in relation to mechanical and civil engineering, there are few works [23] on the impact of microfiltration on the mechanical wear of hydraulic system units and works related to hydraulic oil tests or the impact of oil waste on the natural environment.

The article sets two goals. The first is to present the methodology of testing hydraulic oil in accordance with ISO, SAE, and NAS standards using a portable oil condition analyzer. The second goal of the article is to present the results of hydraulic oil tests on an industrial machine, i.e., a plastic injection molding machine with additional use of hydraulic oil microfiltration.

Currently, microfiltration in industrial conditions is performed without oil testing. The condition of the oil and the filtration time are determined based on the contamination of the filter inserts. In rare cases, oil samples are sent to laboratories for testing. The novelty in the authors’ article is the presentation of the possibilities of oil testing on portable analyzers, which, according to the authors, should be used in the process of oil microfiltration in industrial conditions.

2 Regulations for oil purity classes

In terms of regulations governing the cleanliness of hydraulic and lubricating oils, the following standards should be indicated:

• ISO 4406;

• SAE AS 4059;

• NAS 1638;

• GOST 17216.

  The ISO 4406:99 standard was published by the Turkish Standards Institute (TSE) under the title: Hydraulic Fluid Power - Fluids - Method for Coding the Level of Solid Particle Contamination. The SAE Aerospace standard, the full name of this standard is: SAE AS 4059 Aviation Fluids - Cleanliness Classification for Hydraulic Fluids. This standard specifies cleanliness classes for particulate contamination of hydraulic fluids and includes methods for reporting the relevant data. The NAS 1638 standard is an American aviation standard, and the GOST 17216 standard is a Russian oil cleanliness standard. The NAS 1638 standard was replaced by SAE AS 4059 in 2001. Both ISO and SAE standards refer to the number of particles recognized as contamination larger than 4, 6, 14, and 21 μm. To assess the condition of the oil in terms of its contamination, the first three contamination values (4, 6, and 14 μm) found in 100 mL of the tested oil are selected. For example, hydraulic oil, which after testing (according to ISO 4406) received a cleanliness class result of 21/19/16, which means that it contained the following number of contaminants [24]:

• From 10,000 to 20,000 with a size above 4 μm;

• From 2,500 to 5,000 with a size above 6 μm;

• From 320 to 640 with a size above 14 μm.

The cleanliness classes according to the ISO 4406 standard have been correlated with the average cleanliness classes according to the NAS 1638 standard, which is presented in Table 1. The table also presents the requirements for various hydraulic system units in terms of oil cleanliness. The cleanliness of oil 21/19/16 according to ISO is equal to cleanliness class 10 according to NAS standard. This oil is already out of class and requires filtration.

In the case of finding that the hydraulic oil purity class is too high for the requirements set by the hydraulic system units and its intended use, it is possible to reduce the oil class using microfiltration. According to Li et al. [25], microfiltration allows for reducing the oil purity by up to 6 classes, as is shown in Figure 2. This depends on the filtration time and the number of used filter inserts.

Figure 2

Dependence of the NAS oil class on the oil age [24].

3 Methodology and research object

The object of the research in the field of assessing the condition of hydraulic oil was the Billion H3500/550 injection molding machine. The injection molding machine is designed for high-performance applications related to plastic molding. With a clamping force of 3,500 tons and a maximum injection weight of 550 kg, the injection molding machine enables precise and efficient handling of the production of plastic products on an industrial scale. The high injection speed and pressure of the machine allow for quick filling of molds and precise control of the plastic flow [22]. The machine is equipped with a hydraulic system for both the working and control systems. A view of one of the injection molding machines is shown in Figure 3, while the basic technical data are included in Table 2.

Figure 3

(a) View of the Billion H3500/550 injection molding machine and (b) view of the control panel.

The injection molding machine had the hydraulic pump replaced, which involved draining the oil from the hydraulic system. After replacing the pump, the microfiltration process was carried out for several hours every day and oil tests were carried out. Hydraulic oil condition tests were carried out on the portable analyzer OPComII Portable Oil Lab PPCO 300–1,000 by ArgoHytos. The general construction of the analyzer and its view are shown in Figure 4. The principle of the analyzer operation consists in shining a laser beam through the flowing oil through the solid particle monitor. Contaminants that are in the oil block the beam of light falling from the source to the detector. Then, a signal is generated proportionally to the size of the particles in the oil. The electronic system signals to assign the particle size in μm and the number of particles in the oil. Table 3 shows the parameters and values measured by the oil analyzer.

Figure 4

(a) General structure of the oil condition analyzer: 1 – Engine with pump and electric gear, 2 – battery, 3 – control electronics, 4 – top side with control panel, and 5 – particulate monitor [2], and (b) view of the portable oil condition analyzer OPComII Portable Oil Lab PPCO 300-1000 by ArgoHytos.

The basic parameters of the device are [26]:

• Operating pressure range from 2.5 to 350 bar (35–5,000 psi);

• Operating viscosity range from 1 to 300 cSt;

• Operating temperature from –30°C to + 80°C;

• Operating temperature for oil from +5°C to +80°C;

• Operating temperature for fuel from –20°C to + 70°C;

• Relative humidity in the range of 0–100% RH.

Based on the basic parameters of the oil condition analyzer, in particular, the wide range of viscosity, it is stated that the analyzer can be used not only for testing hydraulic oil. The device can be used in testing engine oils and gear oils in automotive industry or fuels (petrol and diesel oil).

In the tests of the condition of hydraulic oil, oil with the HV46 viscosity class designation was used, which was used to fill the installations of plastic injection molding machines. This is a hydraulic oil with a high viscosity index, used in control systems and hydraulic systems operating at variable temperatures.

The research was an active experiment, where oil purity class and relative humidity were measured on purpose at specific times. The block diagram of the research methodology is shown in Figure 5.

Figure 5

Hydraulic oil test diagram and plan with microfiltration stages.

Two parameters were measured in the tests, i.e., the oil cleanliness class according to ISO and NAS and relative humidity RH. According to Tummons et al. [27], above 70% RH, the water contained in the oil occurs in a dissolved form. For hydraulic oils, the permissible relative humidity of the oil is exceeded and oil microfiltration is recommended. Based on the oil cleanliness class and relative humidity, a decision was made during the tests to microfiltrate the hydraulic oil. Figure 6 shows a view of the hydraulic oil microfiltration operation on the Kleenoil MS2 + MM5 device.

Figure 6

View of (a) the Billion H3500/550 injection molding machine during oil microfiltration with the Kleenoil MS2 + MM5 device from the hydraulic power supply side, (b) view of the MS2 + MM5 filter machine from the working side of the station.

For each test, three measurements were performed for oil cleanliness according to ISO and NAS and for relative humidity.

4 Research results

Table 4 presents the results of the tests of the cleanliness class of hydraulic oil according to ISO and NAS and relative humidity. Figure 7 presents the average results of the change in the cleanliness class of hydraulic oil according to NAS and relative humidity in the subsequent stages of microfiltration on the portable device MS2 + MM5.

Figure 7

Dependence of (a) average oil cleanliness class according to NAS and (b) relative humidity RH, in subsequent microfiltration operations for the Billion H3500/550 injection molding machine.

On analyzing Figure 7 in terms of the cleanliness class of the hydraulic oil, it was found that the permissible cleanliness class of the hydraulic oil was exceeded. It was noticed that despite draining the old oil and filling the hydraulic system of the injection molding machine with new oil, it was mixed with the remnants of the old oil remaining on the internal walls of the system. A small amount of old oil mixed with the new oil caused the oil to obtain cleanliness class 13 after testing, with the permissible and maximum cleanliness class 9. The first filtration after 6 h of microfiltration on the Kleenoil device allowed for obtaining the nineth cleanliness class of the oil in three measurement tests. This indicates that the system was contaminated with the remnants of old hydraulic oil. The second, third, and fourth microfiltration allowed for reducing the cleanliness class of the oil to class 8. On the other hand, the fifth microfiltration led to obtaining the seventh cleanliness class of the oil, which is recommended in hydraulic systems with proportional valves.

In the case of testing with an oil condition analyzer after filling the hydraulic system with new oil, a very high value of relative humidity RH was found, close to the maximum value, i.e., 70 RH. At such a high value of relative humidity, water in the hydraulic oil is already in a dissolved form and microfiltration is necessary to drain water from the oil. Subsequent filtrations reduced the water content and after the fourth microfiltration, the relative humidity dropped from about 65 to 37%.

Figure 8 shows the dependence of the hydraulic oil cleanliness class NAS and relative humidity RH on the hydraulic oil microfiltration time expressed in hours. Additionally, an attempt was made to model the change in cleanliness class and humidity from the microfiltration time and a Pearson correlation analysis was performed. In terms of modeling the change in NAS and RH from the microfiltration hours, it was found that it is difficult and does not bring benefits because both the NAS class and relative humidity decrease very slowly in subsequent microfiltrations. Aiming for a horizontal asymptote, which indicates that further microfiltration does not bring a decrease in value but only stabilization at one level. Both the linear and quadratic function do not give satisfactory results. The decrease in the cleanliness class or relative humidity value should be modeled and recognized as an exponential course a(1 + e^−(t/b)), where a is the value of the horizontal asymptote that the NAS cleanliness class or relative humidity RH strives for, b is the exponential function constant, t is the microfiltration time.

Figure 8

Dependence of: (a) average oil cleanliness class according to NAS and (b) relative humidity RH, on microfiltration time expressed in hours.

Figure 9(a) shows a view of the slide (piston) of the hydraulic distributor from a machine in which microfiltration was regularly carried out for a year. Figure 9(b) shows a view of the slide of the distributor without periodic oil filtration. Scratches and dulling of the cylindrical surface of the slide are visible.

Figure 9

View of the hydraulic distributor slide after 1 year of operation of the hydraulic system with (a) regular microfiltration and (b) without microfiltration.

Based on the conducted research using a portable hydraulic oil analyzer and a portable hydraulic oil microfiltration device, at this stage, the authors can already indicate the advantages and disadvantages of working with an oil analyzer and a filtering machine. The advantages and disadvantages are presented in Table 5.

In terms of the environment, the simultaneous use of a portable oil analyzer and a filter machine allows for a quick assessment of the oil condition. Microfiltration with continuous oil analysis can be carried out until a satisfactory cleanliness class and no water in the oil are obtained. This allows the oil service life to be extended without changing it, even though the technical documentation of the machine or vehicle requires changing the oil to a new one.

Based on previous works [2,14], regular use of microfiltration allows, first of all, no machine downtime during production. Microfiltration is performed in parallel with the production process. During operation, a more thorough cleaning of the hydraulic system is achieved. Second, regular microfiltration reduces the wear of mechanical elements of the system, such as the slide in the housing of the hydraulic distributor or the piston rod of the hydraulic cylinder. Moreover, by controlling the cleanliness class of the oil, it is possible to extend the life cycle of the hydraulic oil even twice. This depends on the working conditions, the influence of the environment and the load and the associated increase in oil temperature.

The next stage of the authors’ work in the field of oil testing and its microfiltration is the purchase of a portable oil viscometer for a mobile test stand. Then, it would be possible to carry out the work in the following order:

1. Oil condition test,

2. Oil microfiltration,

3. Oil viscosity test,

4. Oil condition test.

Carrying out tests using a viscometer will allow obtaining a relationship between the number of microfiltrations performed for the same oil and the viscosity of the oil. Both time (oil age) and multiple oil filtration can cause a change in the oil base, which will allow the measurement of oil viscosity to make a decision on further use of the oil in the machine.

5 Conclusion

The literature review in the field of microfiltration and the authors’ research allow us to draw the following conclusions:

• The use of a portable hydraulic oil condition analyzer enables ongoing diagnostics of hydraulic oil in all conditions, whether laboratory, industrial or field;

• The simultaneous use of a portable oil microfiltration device with an oil condition analyzer allows the hydraulic oil cleaning process to be carried out to the required cleanliness class resulting from the hydraulic units used. The microfiltration time will depend on the required cleanliness class of the oil and the components used in the hydraulic system;

• A single microfiltration of hydraulic oil lasting about 8 h usually allows to reduce the cleanliness class of hydraulic oil by one class;

• The operation of machines and devices with hydraulic systems in a wide temperature range, i.e., before start-up at an ambient temperature of about 0–15°C (depending on the season) and during operation up to a maximum temperature of 55°C (cooler setting in the hydraulic system), affects the increase in relative humidity. In hydraulic tanks, water evaporates and then condenses, which mixes with hydraulic oil. This phenomenon has already been observed by other researchers [1,2,24].