Al prossimo Congresso Internazionale sulla Tribologia (WTC 2013) Mecoil presenterà, in collaborazione con l’Ing. Bassanini, la propria relazione sul controllo della contaminazione nei combustibili vegetali che alimentano i motori diesel stazionari, intitolata “Technical Specifications and PM procedures merging, for a more robust reliability approach”. La presentazione avverrà nella Room Perugia durante la Prima Sessione di Giovedì 12 Settembre.
1. Abstract
Predictive maintenance is a powerful tool to avoid catastrophic failures, when applied to heavy duty stationary Diesel engines, ran on biofuels on a 24/7 basis.
The challenge is combining fuels’ price and genset manufacturer’s specifications, to obtain the proper mix of reasonable cost energy and durability in operation: being competitive, without forgetting dependability [1].
Mecoil and Vipetrol have been since long involved in condition monitoring of several fleets of stationary engines fuelled by mix of non-esterified vegetable oils: the most important are Rapeseed oil and other seeds sources available through the “green economy” market. The quality of such fuels is very seldom totally controlled at the source, and it often degrades during the transportation from the production sites (tropical countries) to the plant site.
So far, minimal amounts of alkaline substances like Sodium and Potassium, may lead to important degeneration of the upper “hot side” of the engines, including valves and piston heads. Lubricating oil is responsible of cooling and removal of by-products that may settle on this critical components of the system.
Mecoil analysis have been able to detect very few ppm of Alkali coming from the biofuel and ingested during combustion. We found that a steadily-increasing trend of Na and especially K was corresponding to an abnormal wear rate in several MAN D2842 engines.
Although the absolute level of such contaminants could be considered acceptable for practical purposes, the quantity of these exogenous and very reactive substances slowly accumulates in the lube oil and has been proven to cause chemical aggression, with corrosion (first) and (secondary) fatigue wear through all the valves and cylinder heads.
2. Introduction
In-service lube oil analysis has been proved able to support equipment reliability since long time. In the fast-growing scenario of Pure Plant Oils (PPO) used as fuels for stationary Diesel gensets, in-service oil analysis is often used as a tool to monitor the operating conditions of the engine, including wear rate and undesired contaminations found in the lube oil [2, 4].
Experience demonstrated that fuel quality is about 50% responsible for engine stops [3]. Since the global demand for PPO fuel is drastically increasing, and so is PPO price, fuel quality monitoring is becoming crucial. In order to help regulate this fast-growing market, national and international standards have been developed, the most popular being DIN 51605:2010 for pure rapeseed oil used as fuel [5]. More recently, the European Norms and Standards Organization has established a workshop (CEN WS 56) which developed a wider-applicability draft standard (CWA 16379:2011) for PPO fuels [3]. These standards include specifications for physical and chemical properties, combustion qualities and contamination levels. The most critical contaminants in PPO fuels have been found to be Ca, Mg and P, that should always remain below limit values of few ppm. The presence of these contaminants in the fuel can result in ash deposition and severe corrosion. Na and K are typical contaminants in plant oils, but did not undergo limitations by these specifications, so that their concentrations are often out of control, for PPO-fuelled gensets end users.
The situation is further complicated by the fact that many OEMs do not approve PPO as a fuel for their engines (while biodiesel is normally accepted), but the same engines are often installed in 3rd-party “turnkey” gensets to be fuelled with pure plant oil [6].
This study reports a case in which in-service oil analysis gave precious informations on equipment health and also on fuel quality.
3. Materials and Methods
Pure Plant Oil-fuelled Diesel Engines – A number of 8 stationary Diesel engines for power generation (each approximately 420 kWe) were monitored by in-service oil analysis, in order to select lubricant change intervals and perform condition monitoring. Engines were MAN D2842 LE211 adapted for being fuelled with PPO. Oil samples were collected concomitantly with the oil change, approximately every 400 hours of operation.
In-service oil analysis – In-service oil samples were collected by the field technicians from the monitored engines and sent to the laboratory for analysis. The test slate included Elemental Spectrometry, Kinematic Viscosity at 100°C and IR Spectroscopy. All the sample-related information (such as sampling date, oil and engine operation hours, etc.) were managed via the Mecoil’s proprietary Permantenere web platform, which allows historical results trending and password-restricted web access to the machine condition reports.
Elemental Spectrometry was carried out by ASTM D6595, using a Spectroil Q100 Rotating Disc Electrode Atomic Emission Spectrometer.
Kinematic Viscosity was carried out by ASTM D7042, using an Anton Paar SVM3000 Stabinger Viscometer.
IR Spectrometry was carried out using a Digilab FTS-2000 FT-IR Spectrometer with a PIKE MIRacle ZnSe crystal (for PPO dilution, calibrated with spiked samples between 2 and 20% by weight) and a Spectro Fluidscan Q1000 (for soot, nitration, remaining antiwear additive percentage, glycol and water contamination).
4. Results
Table 1: Monitored Engines
| Engine No. | Plant Code | No. of Oil Samples |
| 3 | SR | 1 – excluded |
| 6 | SI | 15 |
| 7 | SI | 15 |
| 8 | GF | 13 |
| 9 | GF | 15 |
| 11 | SO | 6 |
| 13 | DF | 3 – excluded |
| 17 | IT | 2 – excluded |
Of the 8 monitored engines, 3 were excluded from this study, due to the low number of received oil samples (hence the low data amount). Thus, historical trend evaluation was performed on 5 engines (N°6, 7, 8, 9, 11) belonging to 3 different plants (coded SI, GF and SO, see Table 1).
Some of the monitored engines showed very high amounts of Potassium (K) in the lube oil, peaking up to 750 mg/kg. The presence of Potassium (K) in used oil from Diesel engines has always been considered a sign of coolant leak, together with Boron, Sodium and Molybdenum, depending on the type of anti-corrosion formulation. In this case, the presence of K did not match with any other symptom of coolant leak, therefore we ascribed it to K contamination in the rapeseed oil used as fuel, which was found to be 10-11 mg/kg. In all the considered instances, the increased K level in lube oil was accompanied or shortly followed by an increase in wear and corrosion metals, particularly Iron.
We did not find any significant correlation between the increase in Iron concentration and other important chemical parameters in used oils, such as PPO dilution, viscosity variation and water contamination.
Therefore, the Mecoil laboratory issued alert reports regarding the engines’ health conditions. Consequently, the plant management reported to have found severe signs of corrosion and fatigue wear on valves and cylinder heads, particularly on engines 6, 7, 8, 9.
The damage found was coincident with the use of a single supply (30 tons) of high K vegetable oil in those plants.
5. Discussion
This case history confirms that in-service oil analysis is a powerful tool for plant reliability, not only in detecting wear debris and contamination in the lube oil, but also giving some important indications on PPO quality.
This work also demonstrates that K is an important contaminating element, when it comes to fuel quality in Pure Plant Oil-fuelled Cogeneration, and maybe worth considering to be limited in the forthcoming visions of vegetable fuel specifications.
6. References
[1] Chiaramonti D., Prussi M. Pure vegetable oil for energy and transports. Int. J. Oil, Gas and Coal Technology, Vol. 2, No. 2, 2009.
[2] Prussi M., Chiaramonti D., Riccio G. Theoretical and Practical Experiences on a 30 kW MGT fed with Pure Vegetable Oil. Proceedings of the 17th European Biomass Conference & Exhibition, 29 June – 3 July 2009, Hamburg, Germany.
[3] Bachner A.-F., Demonstration of 2nd Generation Vegetable Oil Fuels in Advanced Engines. The 2ndVegOil Consortium, 2012.
[4] STLE Alberta Section, Basic Handbook of Lubrication, Second Edition, 2003.
[5] DIN 51605:2010 – Fuels for vegetable oil compatible combustion engines – Fuel from rapeseed oil – Requirements and test methods.
MAN Service Information 180911