
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13517-7
10.1016/j.heliyon.2024.e37486
e37486
Research Article
Assessment of overall remaining useful life of lubricants by integrating oil quality and performance
Chokelarb Wasan a
Sriprom Pongsert Pongsert.sr@kmitl.ac.th
b⁎
Permana Lasuardi bc
Assawasaengrat Pornsawan a
a Department of Chemical Engineering, School of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok, 10520, Thailand
b School of Food Industry, King Mongkut's Institute of Technology Ladkrabang, Bangkok, 10520, Thailand
c Program of Food Technology, Institut Teknologi Sumatera, Lampung, 35365, Indonesia
⁎ Corresponding author. Pongsert.sr@kmitl.ac.th
05 9 2024
30 9 2024
05 9 2024
10 18 e3748613 4 2024
3 9 2024
4 9 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Effective lubricant health monitoring programs are essential for extending the lifespan of both the lubricant and machinery. An accurate and reliable remaining useful life (RUL) prediction is necessary for maintenance decision support. The degradation of used lubricating oil information trends evaluated using used oil analysis results is necessary. This study addresses the need for a comprehensive tool to consolidate oil analysis parameters and determine the remaining useful life (RUL) of used lubricating oil (ULO). A Performance Rating Index (PRI) was developed by integrating various oil degradation testing parameters, including membrane patch colorimetry for varnish and sludge potential, along with viscosity, foaming stability, water separability, air release properties, oxidation stability, rust prevention, and copper strip corrosion. The integration of routine and performance test results of a turbine oil at 25,480 operating hours exhibited a PRI of 48.5. Correlating with the ASTM degradation alarm limits of 50 %, the PRI results provide clear maintenance actions before the original equipment manufacturer's recommended overhaul. The PRI approach supports proactive maintenance by facilitating early oil replenishment, extending RUL, and reducing waste oil.

Graphical abstract

Image 1

Highlights

• Remaining useful life (RUL) prediction is essential for maintenance support.

• Performance rating index (PRI) is developed to evaluate the lubricant's RUL.

• The lubricating oil's quality and performance results are crucial for predicting RUL.

• Used lubricating oils (ULO) degradation parameters are required to evaluate its RUL.

Keywords

Remaining useful life
Used lubricating oil
Condition-based maintenance
Performance rating index
==== Body
pmc1 Introduction

Lubricating oils play a crucial role in various sectors, including industrial and automotive applications, power plants, mining, petrochemicals, pulp and paper mills, and food manufacturing [1]. The quality and performance requirements of lubricating oils vary significantly depending on their specific applications. These oils serve multiple functions: minimize friction and wear, keep the machine surface clean, prevent rust and corrosion, and ensure proper operation of the machine [2]. However, during machine operation, lubricating oil gradually deteriorates, producing sludge and varnish, acid substances, wear particles, and other contaminants [[3], [4], [5]]. Consequently, the oil must be replaced before it fails to meet the machine's quality and performance requirements. Traditionally, oil replacement has been based on the service provided by the original equipment manufacturer (OEM) or the lubricating oil manufacturer. However, this time-based approach can lead to premature or delayed maintenance actions, affecting the volume of hazardous oil waste and the carbon footprint associated with maintenance activities [6]. Condition-based maintenance leverages real-time oil analysis data to optimize oil drain intervals and adopts more environmentally friendly and cost-effective maintenance strategies. Extending the intervals between oil replenishments can significantly benefit the environment by reducing hazardous waste generation and minimizing the overall carbon footprint of the process. Additionally, researchers have explored alternative methods to replace traditional mineral oils with biolubricants that offer comparable or enhanced functionalities and incorporate green additives [3,[7], [8], [9], [10]]. Optimal oil replacement should balance environmental considerations with the lubricating oil's quality and performance. Early replacement wastes oil resources and increases economic and environmental costs, while delayed replacement can exacerbate mechanical wear and increase the risk of equipment damage. Therefore, determining the remaining useful life of used lubricating oil based on its actual quality and performance is crucial for predicting optimal replacement times or improving oil quality to extend drain intervals without risking machine failure.

Numerous degradation models have been used to predict the oil's RUL [[11], [12], [13]]. Inadequate data on oil degradation poses a challenge for accurate RUL prediction and limits the practicability of oil condition monitoring [14]. Several factors affect the lubricating oil degradation trends, including oxidation, thermal degradation, additive depletion, operating conditions, environmental variations, oil replenishments, and contamination [15]. Although many studies have examined the impact of lubricating oil degradation on its quality and performance, these studies often focus on isolated effects rather than offering a comprehensive assessment of overall performance. Interpreting oil analysis data requires advanced skills and extensive experience to effectively evaluate the real-time condition of oil based on multiple test parameters. This complexity makes accurately predicting the remaining useful life of used lubricating oil (ULO) challenging. In this study, we propose using routine quality and performance testing data, including varnish and sludge potential, viscosity, foaming stability, water separability, air release properties, oxidation stability, rust prevention, and copper strip corrosion, to enhance the precision of RUL prediction.

Sludge and varnish formation are the important parameters that shall be considered when determining the lubricating oil's remaining useful life. The formation of varnish and sludge that accumulate in machine parts [16] is potentially causing filter blockage, flow resistance, and excessive wear. Varnish and sludge formation, typically resulting from oxidation, increases viscosity and produces acidic substances that affect additive depletion and water separability in diesel engine lubricating oil due to decreased surface tension [6]. In severe cases, this can result in bearing failure or other essential component failure. For instance, Qiang et al. documented the emergency shutdown of a power-generation turbine due to varnish deposits on the main bearing and bearing bush [17]. To mitigate these issues, regular oil analysis and maintenance are necessary to monitor the condition of the lubricant oil and detect signs of degradation. This monitoring involves assessing various oil parameters, including oil viscosity, oxidation level, oil's quality and performance, and the presence of contaminants such as varnish and sludge [18]. Pashai and Dehghani observed that thermal degradation leads to a sudden decrease in surface tension, affecting water separability [19]. Phillips noted that high-temperature oxidation causes rapid darkening of hydraulic oils, varnish and sludge and affects air release properties [20]. Yano et al. examined turbine oil degradation using the dry turbine oil stability test (dry TOST), revealing a correlation between varnish and sludge formation and decreased oxidative stability [21]. The resistance to varnish and sludge varies among lubricating oils depending on the base oil and type of additive, as studied by Beran. Beran examined the variance in base oil degradation by testing its hydrolytic stability [22]. Varnish and sludge formation affect the chemical structure of oil, leading to an increase in the total acid number and resulting in higher copper strip corrosion. Various methods have been proposed for detecting varnish and sludge formation, including colorimetric patch analysis (CPA) and ASTM D7843-20 standard testing for insoluble varnish potential [23]. Soluble varnish potentials were measured alongside insoluble varnish potentials [24]. Additionally, Chokelarb et al. presented a case study on the synergistic monitoring of varnish and sludge in gas turbine lubricating oil [18]. Although extensive research has been conducted on lubricating oil degradation and sludge formation, a comprehensive analysis of the impact of varnish and sludge formation on lubricating oil quality and performance, considering factors such as foaming, air release, rust, and copper strip corrosion, remains lacking. This comprehensive approach is crucial for accurately predicting the remaining useful life of used lubricating oil and maximizing machine performance without compromising oil quality [25].

This study aims to evaluate the remaining useful life of used lubricating oil by introducing a novel and comprehensive approach, the Performance Rating Index (PRI), which consolidates all oil analysis results into a single metric. The PRI analysis assesses the impact of varnish and sludge buildup on the oil, correlating machine operating hours with its remaining useful life. Various parameters such as Fourier transform infrared spectroscopy (FTIR), antioxidant additive depletion via linear sweep voltammetry (LSV), membrane patch colorimetry (MPC), viscosity, total acid number, oxidation stability, foaming, water separability, rust prevention, and copper strip corrosion were evaluated to gauge the quality and performance of the used lubricating oil. This integrated approach facilitated a thorough assessment of how varnish and sludge affect the oil's performance, providing insights into its condition and potential operational impacts on lubricant quality. By consolidating all oil analysis results, this study offers data-driven interpretation and recommendations for the next maintenance activities aimed at synergistically extending the life of both the oil and equipment components.

2 Material and method

2.1 Chemicals and reagents

Analytical grade chemicals and reagents were used in all tests. Xylene (≥98.5 %, AR grade), toluene (≥99.5 %, AR grade), propan-2-ol (>99.8 %, AR grade), potassium hydroxide (>85.0 %, AR grade), hexane (>99.0 %, AR grade), acetone (≥99.5 %, AR grade), n-Heptane (>95 %), and petroleum ether (>85.0 %, AR grade) were obtained from RCI Labscan (Thailand) for use as dilution solvents and for cleaning purposes.

2.2 Lubricating oil samples

Used lubricating oil (ULO) samples were collected from a gas turbine in a power plant at specific intervals. The sampled gas turbine, with a power output of 62 MW, operated at approximately 6600 rpm with an exhaust temperature of approximately 600 °C. According to the original equipment manufacturer's (OEM) recommendation, major inspections and maintenance are performed every six years, or 48,000 h. As per lubricant manufacturer specifications, the lubricant is based on a viscosity grade of ISO 46 and utilizes Group II mineral-base stock with a rust and oxidative (R&O) additive package. ULO samples were collected to analysis every three months over three-years of operating time (25,480 h).

2.3 Lubricating oil degradation parameter

2.3.1 Varnish and sludge formation

Varnish and sludge formation in ULO was assessed by measuring viscosity, Membrane Patch Colorimetry (MPC), cleanliness by particle count, and sludge weight. The kinematic viscosity of ULO and the new lubricating oil was determined using a calibrated viscometer (CT100, Cannon Instrument, USA) in a precision constant temperature bath at 40 °C (±0.01 °C), following ASTM D445 [26], where the time taken for the oil sample to flow through the viscometer was recorded. The soluble and insoluble varnish contents in ULO was analyzed using membrane patch colorimetry (MPC) according to ASTM D7843-21 Standards [27]. The insoluble MPC (I-MPC) involved mixing ULO with petroleum ether and filtering it through a 0.45 μm membrane patch under vacuum conditions to capture colored insoluble varnish, while the soluble MPC (S-MPC) was determined by blotting ULO directly onto a membrane patch without filtration. Both were measured on the CIE L*a*b* color scale using a spectrophotometer (MPC Color, Fluitec, USA), and results were presented as ΔE, representing the color difference, calculated by Eq. (1).(1) ΔE=((ΔL*)2+(Δa*)2+(Δb*)2)1/2

The Varnish Potential Index (SI-MPC) was then derived by averaging the ΔE values for both soluble and insoluble varnish types, termed S-MPC and I-MPC, respectively, using Eq. (2). This index provides a comprehensive measure of the varnish potential in ULO by integrating the effects of both varnish types.(2) SI−MPC=(S−MPC+I−MPC)/2

Cleanliness was evaluated using a laser particle counter (SBBSS, PAMAS, Germany), employing the ISO 4406 classification to report the number of particles larger than 4, 6, and 14 μm per 1 mL of oil, further detailing the condition of ULO.

The sludge weight was determined by gravimetric analysis to quantify the mass of insoluble varnish. This method involves passing 100 mL of ULO mixed with 100 mL of petroleum ether through a membrane filter disk with a pore diameter of 0.45 μm (MF-Millipore, MERCK). The resultant increase in the mass of the filter indicates the amount of insoluble contamination. The fluid sample was drawn through the filter by vacuum, and the insoluble contaminants were collected on the surface of the filter. The sludge weight was expressed as mg/100 mL of ULO sample retained on the membrane filter with a pore diameter of 0.45 μm.

2.3.2 Additive depletion

Additive depletion in lubricating oil was determined using Linear Sweep Voltammetry (LSV) (Ruler, Fluitec, USA), Inductively Coupled Plasma (ICP) (Avio560, PerkinElmer, USA), and supported by Fourier transform infrared (FT-IR) spectroscopy (Spectrum100, PerkinElmer, USA). LSV was employed to measure the levels of hindered phenol and aromatic amine antioxidants based on ASTM D6971, calculating antioxidant levels as a percentage of the remaining antioxidants compared with new lubricating oils [28]. This method facilitates the monitoring of both aromatic amines and phenolic antioxidants as previously mentioned by Chokelarb et al. [29]. Additionally, oxidation by-products in used lubricating oil (ULO) were analyzed using FT-IR spectroscopy following modified ASTM D7214 and ASTM D7414 [30,31]. The analysis focused on comparing the carbonyl oxidation peak area (1820–1650 cm−1) of ULO with that of new lubricating oil, providing a comprehensive assessment of both additive depletion and oxidation by-products.

2.3.3 Oxidation stability

The oxidation stability of ULO was comprehensively analyzed using Fourier Transform Infrared (FT-IR) spectroscopy, the Rotating Pressure Vessel Oxidation Test (RPVOT), and Total Acid Number (TAN) assessments. FT-IR spectroscopy was employed to evaluate the formation of oxidation by-products, which correlated with changes observed in TAN and the oxidative stability remaining, as determined by RPVOT. For the RPVOT, a ULO sample (50 ± 0.5 g) was subjected to oxidation in a high-pressure vessel containing oxygen (90 psi, 6.2 bar), a copper wire catalyst (55.6 ± 0.3 g), and water (5 mL), maintained in a constant-temperature oil bath, and rotated axially at 100 rpm (Koehler Instrument, USA), as described by Selby et al. [32]. The oxidation stability was quantified by measuring the time required to reach a specific pressure drop (25.4 psi, 1.75 bar) below the maximum pressure. The percentage of oxidation stability remaining in ULO was calculated using Eq. (3). The acidity of the acidic oxidation by-products in ULO was assessed using TAN according to ASTM D974 by measuring the amount of KOH required to neutralize the acid in 1 g of the oil sample, expressed as mg KOH/g sample [33].(3) %Oxidationstabilityremining=(RPVOTofULO,mins)/(RPVOTofNewOil,mins)x100

2.3.4 Foaming, air release and water separability

The tendency of ULO to foam was assessed via the Foaming Characteristics per ASTM D892 Standards [34]. Foaming tendency and stability were evaluated at controlled temperatures (24.0 °C). The foam volume generated at the end of each period was measured and reported in milliliters (mL). (Koehler Instrument, USA)

The air release properties of ULO were measured using the air release property test following the ASTM D3427 Standards [35]. The ULO sample was subjected to compressed air heated to 50 °C. Upon stopping the air flow, the time taken for the entrained air in the ULO to decrease in volume to 0.2 % was recorded as the air release time in minute. (Koehler Instrument, USA)

The ULO's capacity to separate from water is referred to as water separability, and good water separability properties were required of the ULO. The testing comprised a bath (Koehler Instrument, USA) in which 40 mL of the ULO and 40 mL of distilled water were stirred for 5 min in a graduated cylinder at 54 °C, in accordance with ASTM D1401 Standards [36]. The oil/water emulsion was observed for separation every 5 min. The volumes of ULO, water, and emulsion remaining at each observation time were reported, along with the total separation time in minutes.

2.3.5 Rust preventive and copper strip corrosion

The copper corrosive resistance of ULO was evaluated through a standardized test conducted according to ASTM D130 Standard [37]. This test involved immersing a polished copper strip (12.5 mm × 75.0 mm x 1.5 mm, 99.9 % purity) in a constant temperature bath (Koehler Instrument, USA) at 100 °C for 3 h. The resulting staining was observed and rated from very little to no staining (1a) to very dark stains (4c).

The rust-preventive properties were assessed following ASTM D665 [38]. The test involved mixing ULO samples (300 mL) with distilled water (30 mL) and stirring them with a cylindrical steel test rod in a constant temperature bath at 60 °C for 4 h (Standhope-Seta, USA). The steel rod was then inspected for rust formation, with results recorded as a pass or failure.

2.3.6 PerformanceRatingIndex(PRI)

The performance rating index (PRI) was introduced to evaluate the impact of the varnish and sludge on various lubricating oil parameters and calculate the rating from the new oil to the used oil. The PRI scales range from 0 to 100, with 100 corresponding to a new oil, and 0 indicating an abnormal or unacceptable value. The PRI evaluation is shown in Fig. 4 and was calculated using Eq. (4).(4) PRI=Abs[(NewOil−UsedOil)/(Unacceptable)−NewOil)]x100

Unacceptable or abnormal values are determined based on the ASTM 4738 and OEM [39] standards for used lubrication oil. The overall performance index was calculated as the average of each performance parameter.

3 Results and discussion

3.1 Used lubricating oil degradation parameters

3.1.1 Varnish and sludge formation

As the machine operates and the lubricating oil performs its function, it undergoes oxidation and thermal degradation, potentially resulting in sludge and varnish formation. These substances can alter the chemical and physical properties of the lubricating oil, thereby impacting its quality and performance. The evaluation of varnish and sludge potentials involved the measurement of insoluble MPC (I-MPC), soluble MPC (S-MPC), laser particle count, and sludge weight (mg/100 mL). Table 1 presents a summary of the observed sludge and varnish formation in the ULO over an operating period of 0–25,480 operating hours.Table 1 Varnish and sludge formation in ULO and their effect on particle count and sludge weight during operating hours.

Table 1(a) Warning limits are based on ASTM D4378.

During the initial phase from 0 to 7096 operating hours, an increasing S-MPC (ΔE 1.20 to 33.30) and a slight increase in I-MPC (ΔE 1.40 to 5.81) were observed. This period represents the early stage of varnish formation, where varnish particles can still dissolve in the oil, known as the soluble varnish stage. These observations suggest ongoing oxidation and saturation of varnish particles in the oil. At 9640 operating hours, a significant increase in I-MPC (ΔE 12.23) was observed, reaching its peak at 25,480 operating hours (ΔE 60.04). This indicates the onset of the 'insoluble varnish stage,' a critical progression in the varnish formation process. This progression was further confirmed by an increase in the laser particle count (34,817 particles, >4 μm) and sludge weight (10.1 mg/100 mL).

By 19,264 operating hours, the insoluble varnish (ΔE 52.79) exceeded the acceptable limit (ΔE 30) [40]. The OEM limit is a standard set by the equipment manufacturer, and exceeding this limit can lead to equipment failure or reduced performance. To address the varnish and sludge formation in the ULO system, the mechanical filter was replaced, resulting in a reduced I-MPC (ΔE 28.14), while S-MPC remained stable (ΔE 41.00). This indicates that the soluble varnish persisted, which was inadequately addressed by the standard mechanical filter [41].

At 25,450 operating hours, I-MPC reached its peak (ΔE 60.04), impacting the quality and performance of lubricating oil and leading to the formation of sticky black or dark brown residues. The insoluble varnish potential deposits on the bearing pad underscore the need to address early soluble varnish formation using appropriate removal technology, such as ion exchange resin, electrostatic discharge, and depth media filters [42,43]. The increasing presence of varnish and sludge in the lubricant correlates with an increase in the particle count, emphasizing the significance of determining the varnish potential alongside monitoring the particle count in the system.

Over time, the ULO sample's color deepened, indicating progressive varnish and sludge formation. The color transitioned from that of the new lubricating oil color to light yellow, yellow, red brown. As shown in Table 1, the color of the varnish formation darkened with prolonged operating time. The ΔE (CIE L*, a*, b*) color scale serves as an effective technical method to capture colored soluble and insoluble varnish bodies. Particularly, L* measures the lightness and darkness of the ULO sample.

3.1.2 Additive depletion

As the lubricant is used in the equipment, it is exposed to metals, air, temperature stress, and moisture, making it more susceptible to a spontaneous oxidation reaction. On a molecular level, the base stock degrades and transforms through different chemical reactions, including oxidation, cracking, hydrolysis, and others, depending on the factors present and the molecular characteristics oil base stock. Varnish buildup, a contaminant resulting from oil degradation and possibly depleted additive molecules, was investigated in relation to additive components and varnish and sludge formation in lubricating oil. LSV assessed the levels of amine and phenol antioxidant additives remaining in the ULO, while ester-base additive levels were measured by FT-IR at a peak of 1740 cm−1 following ASTM D7214 [30]. Metallic additive elements, such as phosphorus, were detected in the ULO using inductively coupled plasma (ICP). Fig. 1 shows the LSV spectra of aminic and phenolic antioxidant additives in ULO over operating hours, revealing spectra times of 6.25–10.60 and 11.6–15.7 s, respectively. A decrease in antioxidant additives was correlated with increasing sludge and varnish formation during operating hours, with phenolic antioxidants depleting faster than aminic antioxidants, as shown in Table 2. This phenomenon, known as premature phenolic depletion, occurs because the phenolic additive is used to preserve the amine antioxidant and enhance the oxidation resistance of the turbine oil [44].Fig. 1 Linear Sweep Voltammetry (LSV) Spectra showing depletion of aminic and phenolic antioxidant additives in ULO across operating hours.

Fig. 1

Table 2 Oil condition analysis results with remaining additives.

Table 2Sequence #	Limits*	1	2	3	4	5	6	7	8	9	10	11	12	
Oil Hours		0	3208	5080	7096	9640	11,752	14,320	16,408	19,264	21,328	23,296	25,480	
Oil Condition	
Viscosity at 40 °C, cSt	41.1–45.6	43.4	42.90	43.40	43.21	42.67	42.89	42.92	42.69	42.76	43.15	43.00	43.07	
Oxidation Index			10.2	15	15.6	16.8	17.8	18	18.6	19.6	20	20.2	21.4	
TAN (mg KOH/g)	0.17	0.070	0.053	0.070	0.073	0.095	0.096	0.110	0.111	0.125	0.146	0.147	0.165	
RULER Amine, %	25	100	94.2	91.6	90.3	88.3	77.3	77	76.9	76.5	69.4	57.1	43.3	
RULER Phenol, %	25	100	59.8	48.8	45.8	44.4	36.8	16.5	16	14.5	2.6	2.3	1.1	
RPVOT (minutes)		1659	1205		1191		1177		1020		976		795	
% Oxidation Stability	25	100	72.6		71.8		70.9		61.5		58.8		47.9	
Additive Element, ppm	
Boron		0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.27	1.48	
Magnesium		0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.08	0.00	0.00	
Calcium		0	0.00	0.00	0.00	0.00	0.00	0.00	0.06	0.00	0.00	0.00	0.00	
Barium		0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.52	0.00	
Phosphorus		1352.0	1261.6	1081.6	1223.5	1123.0	1300.8	1147.2	1172.0	1152.3	1169.0	1233.4	1220.4	
Zinc		0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	

Fig. 2 shows the FT-IR spectra of ester additive depletion in ULO, characterized by a peak at 1740 cm−1, attributed to the –C=O stretching vibration of the ester additive compound. Notably, elemental analysis for phosphorus cannot distinguish between fresh and spent phosphorus additives due to structural changes that occur between fresh and spent stages while maintaining a similar phosphorus elemental concentration.Fig. 2 FT-IR spectra of new lubricating oil and ULO samples across operating hours, illustrating oxidative oxidation change.

Fig. 2

Fig. 3 depicts the increasing level of soluble and insoluble MPC alongside decreasing additive concentrations. Between 0 and 16,408 operating hours, soluble MPC surpasses insoluble MPC, suggesting ongoing dissolution of varnish and sludge in the oil. However, from 19,264 to 25,480 operating hours, insoluble varnish exceeds soluble MPC, indicating saturation of the oil with the varnish, as shown in Fig. 3. Depletion of the antioxidant additives in ULO serves as a leading indicator for varnish and sludge buildup. Antioxidants, being the most reactive species in ULO, deplete first, signaling a decrease in oil solubility and performance. Addressing the decrease in antioxidant additive levels is crucial for improving ULO solubility in sludge and varnish. It is recommended to partially drain the ULO and replenish the additive by adding fresh oil.Fig. 3 Correlation between antioxidant additive depletion and soluble and insoluble MPC levels.

Fig. 3

Fig. 4 Spider plot of performance analysis with (a) operating hours; (b) at 25480 operating hours.

Fig. 4

3.1.3 Oxidation stability

The oxidation stability of ULO was analyzed using FT-IR, RPVOT, and TAN, as shown in Table 2. FT-IR spectroscopy evaluated oxidation by-product formation, correlating with the increasing total acid number and the decreasing oxidative remaining, as determined by RPVOT. The total acid number measures the acidic organic components in the lubricant, with a rise indicating decreased oxidative stability. Oil acidity greatly impacts ULO additive depletion. Elevated TAN and reduced RPVOT are outcomes of the oxidation during lubricating oil operation, as shown in Table 2. FT-IR spectra of new lubricating oil and ULO samples at various operating hours are shown in Fig. 2. Intensities of bands in the ULO samples at 1730–1683 cm−1 indicate increased unsaturated carbonyl functional groups (-C=O bond), including acid ketone, aldehyde, ester, and lactone [45]. Free radical (R•) chain reactions involving chain initiation, propagation, and termination produce high-molecular-weight hydrocarbon equations (5), (6), (7), (8) [46].(5) R-H + O2 → R• + HOO•

(6) R–R + Energy → R• + R•

(7) R• + O2 → ROO•

(8) ROO• + RH → ROOH + R•

The oxidation stability of ULO was analyzed using FT-IR, RPVOT, and TAN. FT-IR spectroscopy evaluated oxidation by-product formation, aldehyde formation, or ketone formation (9)–(10) [42]. A secondary alkoxy radical (RR'-CO•) may decompose to form an aldehyde (R-CHO), and a tertiary alkoxy radical (RR'R″CO•) may decompose to produce a ketone (R-C=O-R′). During chain termination processes, carbonyl compounds (-C=O) and alcohols may also form via peroxyl radicals (R'OO•).(9) RR'-CO• → R-CHO + R′•

(10) RR'R″CO• → R-C=O-R′ + R″•

For this ULO, at 23,296 h and 25,480 h, the FTIR spectra showed a step on the left side of the asymmetrical peak at 1698 cm−1. The formation of carbonyl by-products is expected as one of the changes a fresh base stock of oil will undergo throughout its useful service life. Peaks at 1683 and 1730 cm−1 are attributed to the stretching of ketone carbonyl groups (C=O) and carboxylic acid groups, respectively, which come from high-temperature by-products [43]. The intensities of the bands near 1698 cm−1 for the 21328 and 25480 operating hours (C=O bond from carboxy functional groups such as acid ketone, aldehyde, ester, lactone, and α- and β-unsaturated ketone) are increased. This demonstrates that the oil undergoes degradation and results in changes to its molecular integrity. In conjunction with the change in FTIR oxidation peaks, varnish and sludge will form. These by-products in oil, such as alcohols, aldehydes, ketones, and carboxylic acids, can generate insoluble polymers by condensation reaction [44], also called insoluble varnish. An observable increase in the varnish and sludge potentials was observed to have an insoluble MPC value of 56.61 and 60.04, respectively. The onset of oxidation can also be confirmed through the interpretation of the LSV test. Lower additive levels, in this case, at 50 % remaining, decrease the ability of the oil to dissolve varnish and resist oxidation, thus increasing MPC levels and changing FTIR spectra.

The lower-molecular-weight alcohols, aldehydes, and ketones immediately impact lubricant physical properties, leading to a decrease in oil viscosity, increased oil volatility, and heightened polarity. Under high-temperature oxidation conditions, aldehydes and ketones can further react to produce acids and high-molecular-weight species known as varnish and sludge [46]. The FT-IR, RPVOT, and TAN were monitored to assess the recent condition of the oil lubricant, as shown in Table 2. However, analyses such as FTIR, RPVOT, and TAN solely monitor the oil degradation by-products, not the quantity of varnish in the lubrication oil. Thus, these analyses assist in identifying potential precursors to varnish and sludge formation in the system.

3.1.4 Foaming, air release, and water separability

The presence of polar aging products, impurities, and specific additives affects the duration for ascended bubbles to burst [47]. This underscores the importance of effective foam resistance and entrained air release for ULO performance. These lubrication systems are specifically designed to minimize foaming, and ULO contains defoamants to reduce foaming tendency and enhance foam stability. However, it is crucial to note that oxidation can enhance foaming tendency and foam stability, highlighting the significance of good oxidation stability in maintaining foam resistance during service. This is where our study becomes crucial in ensuring optimal performance.

Table 3 illustrates the progression of foaming, water separability, and air release. An increase in foaming tendency is observed with prolonged oil operations. Over 0–25,480 operating hours, foaming tendency increased from 40 mL to 500 mL, air release time increased from 2 to 3.18 min, and water separability time escalated from 4.0 to 15.4 min. These foam levels exceed the acceptable standard limit (450 mL) as specified by ASTM D4378 [40]. Foaming and air release correlate with varnish and sludge formation. As lubricating oil undergoes oxidation, it transforms into a progressively polar-oxidized product. Ketones, acids, and alcohols produced during oxidation are highly oxidizable radicals, contributing to the formation of heavy compounds, varnish, and sludge.Table 3 Changes in air release time, rust prevention, copper strip corrosion, and foaming properties of the ULO.

Table 3

3.1.5 Rust preventive and copper strip corrosion

As the TAN of ULO increases, the oil becomes acidic and therefore has higher corrosive properties to metals. The copper strip test evaluates the corrosive potential of the oil, which is influenced by its acidity.

Table 3 summarizes the results for rust prevention and copper strip corrosion. Rust preventives remain acceptable, however copper strip corrosion exhibits an increased corrosive rating level, transitioning from 1a to 1b. This escalation correlates with increased total acid number and sludge and varnish formation, highlighting the potential corrosive effect of oxidation-derived acids like carboxylic acid on copper surfaces. The presence of acidic compounds, as indicated by higher TAN, can accelerate the corrosion process. This is because acidic components formed during oil oxidation can react with copper surfaces, leading to corrosion and the degradation of copper-containing components [48]. In conjunction with TAN measurement, the copper strip corrosion test provides valuable insight into the oil's potential to cause corrosion and overall health of lubricating system.

The detection of increasing varnish and sludge in the lubricating oil necessitates an evaluation of copper strip corrosion, particularly concerning yellow metal machine parts such as those made of copper, bronze, or brass. Assessing the corrosiveness of the lubrication oil and potential varnish buildup can aid in identifying active sulfur compounds in the system.

3.2 Remaining useful life of lubricating oil based on performance rating index (PRI)

The remaining useful life of lubricating oil is a critical factor in maintenance strategies based on oil conditions. To address the complexity of interpreting multiple oil analysis parameters, we use the Performance Rating Index (PRI) as a comprehensive measure to determine the RUL of the oil. This index integrates various data points, including sludge and varnish potential, viscosity change, foaming, air release, water separability, oxidation stability, rust preventive, and copper strip corrosion, on each of the operating hours.

Individual interpretation of all the aforementioned parameters provides a direct conclusion regarding the remaining useful life of the oil. However, determining which specific parameter should most be prioritized in the maintenance decision-making process present a challenge. For instance, consider the scenario where ULO exhibits failed results in term of additive levels and copper strip corrosion, yet the varnish test result remains within acceptable limits. Analysis such complex data requires extensive industry experience and comprehensive training.

Additionally, setting limits is crucial for monitoring ULO performance and quality. Table 4 presents the control limit and abnormal limit based on ASTM D4738 and OEM, essential for assessing lubricant property criticality [40]. Each parameter has its own cautionary and critical limits and shall be judged accordingly. The new oil analysis establishes the baseline physical and chemical properties of the ULO, where the new oil results represent a PRI of 100 in, as shown in Table 4. This approach aligns with the recommendation of Zhu et al. [49], who emphasize the importance of comprehensive oil analysis in predictive maintenance strategies.Table 4 Performance Rating Index of lubricant oil in different operating hours.

Table 4Operating hours	Limit of performance	New oil	3208 h.	7096 h.	11,752 h.	16,408 h.	21,328 h.	25,480 h.	
Control Limit	Abnormal Limit	Test result	PRI	Test result	PRI	Test result	PRI	Test result	PRI	Test result	PRI	Test result	PRI	Test result	PRI	
Varnish Potential (MPC,ΔE)	30	60	1.4	100	4.12	95.4	5.81	92.5	21.33	66	45.51	24.7	28.14	54.4	60	0	
Viscosity, (cSt)	45.6	47.7	43.4	100	42.9	88.5	43.21	95.6	42.89	88.2	42.69	83.6	43.15	94.2	43.07	92.4	
Foaming, (mL)	450	500	40	100	300	43.5	420	17.4	460	8.7	470	6.4	480	4.3	500	0	
Air release (min)	7	8	2	100	2.05	99	2.25	95	2.51	90	2.82	84	3	80	3.18	76	
Water separability (min)	25	30	4	100	8.2	84	9.7	78	12.6	67	14	62	14.6	59	15.4	56	
Oxidative stability (%)	50	25	100	100	72.6	63.5	71.8	62.4	70.9	61.2	61.5	48.7	58.8	45.1	47.9	30.5	
Rust Preventive (scale, rating)	50	25	100 (pass)	100	100 (pass)	100	100 (pass)	100	100 (pass)	100	100 (pass)	100	100 (pass)	100	100 (pass)	100	
Copper corrosion (rating)	1	2	1	100	2	66.7	2	66.7	2	66.7	2	66.7	3	33.3	3	33.3	
Overall PRI				100		80.1		76		68.5		59.5		58.8		48.5	

To have a holistic and single-metric approach to the evaluation of the remaining useful life of the ULO, the Performance Rating Index equation shall be used. This approach consolidates the contributions of each test parameter to provide comprehensive insight into the overall condition of the ULO. The new oil analysis establishes a baseline as the standard physical and chemical properties of the ULO, as depicted in Table 4, where the new oil results represent PRI of 100. The result indicates a declining PRI with increasing operating hours: PRI 80.1 at 3208 h, PRI 76.0 at 7096 h, PRI 68.5 at 11752 h, PRI 59.5 at 16408 h, PRI 58.8 at 21328 h, and PRI 48.5 at 25480 h. This downward trend suggests that varnish and sludge presence greatly impact ULO performance and quality. These findings align with the comprehensive review by Hong and Jang, which highlights the detrimental effects and removal of varnish contamination in lubrication systems [41].

Fig. 4a illustrates how varnish and sludge increase during normal operation from 3208 to 25,480 h, affects oxidative stability, foaming tendency, and air release time. The dotted red line denotes the unacceptable zone, while the green line represents the new oil value. At 25480 h, air release, oxidative stability, and MPC fall into the unacceptable zone.

Fig. 4b displays the ULO performance rating after 25,480 h, indicating varnish and sludge accumulation (PRI 7.80) adversely affect oxidative potential (PRI 30.5) and foaming tendency (PRI 0.00). In such cases, oil changes and proper maintenance are required. However, other parameters such as copper corrosion (PRI 33.3), rust prevention (PRI 100), water separability (PRI 56.0), viscosity (PRI 92.4), and air release (PRI 76.0) remain within acceptable ranges. The graph underscores the correlation between various parameters and the presence of varnish and sludge, emphasizing the need for proper monitoring to enhance performance rating and forestall accelerated lubricant oxidation, which could prematurely impact machine operation. This contributes to the overall PRI of the oil at 25,480 h, which is found to be 48.5. This indicates that the oil has already surpassed the midpoint of its serviceable life.

Fig. 5 presents the overall performance rating index (PRI) of the ULO, illustrating an expected decrease in PRI during prolonged operations. At 25,480 h, the overall performance rating notably declines to a PRI of 48.5, falling below the cautionary threshold of 50 %. This decline, which correlates with the ASTM 4378 degradation alarm limits of 50 %, necessitates careful maintenance planning and monitoring to prevent potential downtime. The PRI serves as a valuable tool for proper maintenance planning, aiding in mitigating unplanned outages associated with varnish deposits and costly downtime.Fig. 5 Overall performance rating with operating hours.

Fig. 5

4 Conclusions

The development and validation of the Performance Rating Index (PRI) demonstrate its effectiveness as a tool for assessing the remaining useful life of lubricants by correlating with the ASTM degradation alarm limits of 50 %. The PRI evaluation addresses data management challenges for a successful oil analysis program, facilitating the assessment of overall oil performance. The integration of routine and performance test results of a turbine oil at 25,480 h provided a PRI of 48.5, indicating clear maintenance actions before the OEM-recommended overhaul at 48,000 h. This approach enhances maintenance scheduling and machine longevity. The PRI consolidates various parameters into a single metric, simplifying analysis and supporting proactive decision-making. Future work should refine this model and explore its application across different lubricants and machinery.

Data and code availability

Data will be made available on request.

CRediT authorship contribution statement

Wasan Chokelarb: Writing – original draft, Validation, Methodology, Investigation, Formal analysis. Pongsert Sriprom: Writing – review & editing, Validation, Supervision, Resources, Methodology, Conceptualization. Lasuardi Permana: Validation, Formal analysis. Pornsawan Assawasaengrat: Validation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors extend their gratitude to Andy Sitton and Thanant Sirisithichote of Focuslab Ltd. for their valuable advice and assistance with oil analysis testing and instrumentation. We also thank Thea Marie Viaje for her thorough review and editing.
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