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Reducing the Total Cost of Ownership
Shankar Karnik, VAsia Pacific Mobil SHC Brand Advisor, ExxonMobil Lubricants and Specialties It is no secret that energy producers rely heavily on the performance of their natural gas engines. Simply stated, if these paramount machines are not running, the profit margins can be quickly drained. It’s imperative for energy producers to develop and implement lean manufacturing strategies to meet their production targets and remain competitive. The article details the factors that operation and maintenance managers should consider to maximise the return on their investment in a natural gas engine.

By consulting Original Equipment Manufacturers’ (OEM) guidelines, promoting engine cleanliness, monitoring deposit formation and extending oil drain intervals, maintenance professional can help their companies generate big gains in operational efficiency and bottom-line savings.

The natural gas industry embraces ideas that will increase efficiency to help produce the lowest possible average cost per kilowatt-hour of electricity generated or cubic metre of natural gas compressed. To support this goal, OEMs are continuing the trend of designing engines to be more fuel efficient, to operate at faster speeds and to produce higher outputs.

Increased sump capacities, added spinner-type filters and improved engine controls through advancements in microprocessors are just a few of the modifications that have been made over the last few years.

As engine technology becomes increasingly more sophisticated, these units require different performance characteristics from a lubricant to address these new operating environments. To help maintenance managers optimise engine performance, OEMs will provide a list of oil specifications that complement their engine designs.

This list typically will include optimal viscosities and a range of sulfated ash content. Most of today’s recommended natural gas engine oils are SAE 30 or 40 grade. As for the sulfated ash content, this value ranges depending on engine design and will be discussed in more detail later in the article.

One of the best ways to demonstrate lubricant performance is to review results from actual in-service performance tests. It is common for lubricant manufacturers to partner with customers or OEMs in conducting field trials on their products. Typically, these tests last from 4,000 to 10,000 hours. During this time, the lubricant manufacturer will closely monitor the oil and document characteristics of the lubricant’s performance. These results are then submitted to the OEM to be considered for addition to the lubricant list featured in the engine's manual.

It goes without saying that cleaner engines last longer. However, natural gas engines are extremely challenging to lubricants because of the oxidation and nitration caused in the oil by the combustion process. These products of combustion are oil – degrading, producing sludge and varnish within the engine. These conditions increase oil consumption, shorten filter life and lead to extended periods of downtime – all factors that increase the total cost of ownership. While oxidation and nitration are typically mentioned together, they are actually very different.

Oxidation is the reaction of oxygen with the hydrocarbon molecules in the engine oil. The rate of oxidation increases exponentially as temperature rises and with the presence of metallic contaminants. An increase of 10 degrees Celsius in the temperature of the oil effectively doubles the rate of oxidation. Copper, bronze, brass and iron contaminants are typical materials that catalyse oxidation reaction. Oxidation is typically the main contributor to sludge and varnish formation in natural gas engines.

Nitration is another undesirable condition that indicates an oil is reacting with nitrogen oxide compounds produced in the combustion process. Field tests have revealed the rate of nitration increases when ambient air temperature rises and/or loads become higher in stoichiometric engines. Additionally, mechanical conditions such as a lower oil makeup, poor ring sealing and poor crankcase ventilation also speed nitration. Sludge and varnish from nitration is usually found in four-cycle gas engines. The ability of a natural gas engine to promote engine cleanliness is strongly affected by the lubricant formulation. Today’s natural gas engine oils have mixed results in field performance, thus, it is imperative to select a lubricant that is engineered with a balanced formulation of the base stocks and additives to maintain a clean engine.

Sulfated ash is a characteristic of natural gas engine oils that gives an indication of the oil’s ability to neutralise acids from the engine combustion process. Lubricant manufacturers identify the percentage of ash weight (wt%) in an oil by performing the ASTM D874 test and placing the product into one of the following categories:

Ashless: < 0.1 per cent
• Low ash: 0.2 to 0.6 per cent
• Medium ash: 0.7 to 1.2 per cent and;
• High ash: > 2.0 per cent

The proper level of sulfated ash is important and depends on the specific engine design. Medium and high-speed (greater than 450 rpm) four-stroke engines typically require either a low or medium ash oil to properly lubricate the engine components, like the exhaust valves and seats, and to control valve recession. Slow-speed (less than 450 rpm) four-stroke and two-stroke engines require either an ashless or low ash oil to properly lubricate engine components and minimise concerns from higher sulfated ash oils in the engine. OEMs will identify the level of sulfated ash that best suits the engine’s design in the owner’s manual.

When natural gas engine oils are burned, sulfated ash creates deposits that contain metal sulfates, including barium, calcium, magnesium, zinc, potassium, sodium and tin. The elements, sulfur, phosphorus and chlorine, can also be present in combined form. Large quantities of this remnant can result in reduced heat transfer, detonation, valve burning and ring sticking or breaking. The amount of ash deposits that form in an engine is related to a lubricant’s formulation and oil consumption of the engine. Working closely with your lubricant provider and OEM to achieve the proper ash levels will help promote optimal engine performance and minimise downtime for unscheduled maintenance.

Another great way to minimise expenses for your natural gas engine is to extend the drain intervals of your lubricant. Extending lubricant drain intervals can help maintenance professionals optimise lubricant consumption, reduce labor, lower disposal costs and increase productivity, however not all oils are suitable for this purpose. Extending oil drain intervals with a low-quality oil can often lead to excessive deposits and other operational issues. For extending lubricant drain intervals, maintenance managers first should establish baseline data for their engines. Information such as engine loads, jacketwater and oil temperatures, types of filters, and changing intervals and preventive maintenance protocol should all be documented. Routine maintenance also plays a significant role in extending drain intervals.

Monitoring your oil through oil analysis; evaluating oil filters for abnormal deposits during scheduled filter changes; examining crankcase for cleanliness during scheduled oil drains; and checking valve decks during scheduled adjustment of valves are the steps which should be integrated into a maintenance plan to achieve optimal results. As an optional step, during the engine overhaul, the components to validate drain interval and your proactive maintenance programme need to be inspected.

It is recommended that maintenance managers participate in and use the results of a used oil analysis programme. By closely monitoring viscosity, oxidation, nitration and total base number of your lubricant, you can readily determine whether the selected oil is maintaining performance during the For example, a California hospital was able to save more than USD 65,000 per month for its ebullient-cooled cogeneration engines with a long drain interval, while using its used oil analysis programme to show that oil properties remained within acceptable limits. They achieved a doubling of the time between oil drains and also dramatically reduced oil nitration and oxidation to enhance equipment durability.

In another example, a natural gas producer and processor was able to reduce oil purchases by 45-per cent by extending the lubricant drain intervals for a standard designed Caterpillar G3516 model engine operating at a field gathering compressor station. Viscosity, oxidation, nitration, oil consumption and deposit control were monitored to determine the optimal drain interval. Deposit control was monitored through periodic borescopic engine inspections and component inspections. The photographs below illustrate the excellent condition of the exhaust valves and piston after 19,000 hours while being lubricated with Mobil® Pegasus 1005.

Through advances in technology, today's industrial gas engines are more sophisticated and specialised than their predecessors. While these design changes have resulted in more efficient systems, they often put added stress on maintenance professionals to maintain profitable levels of production. It is critical for maintenance professionals to consult original equipment manufacturers’ guidelines, promote engine cleanliness, monitor deposit formation and extend oil drain life.

By implementing the tips detailed in this article, companies can generate big gains in operational efficiency, and reduce total cost of operation.