This article spotlights the factors that operation and maintenance managers should consider to maximize the return on their investment in a natural gas engine driving a compressor.
Big savings in maintenance expenses can be obtained by consulting original equipment manufacturers’ (OEM) guidelines, promoting engine cleanliness, monitoring deposit formation and extending oil drain intervals.
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 meter 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 the engine lubricant. To help maintenance managers optimize engine performance, OEMs 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.
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.
Cleaner engines last longer. However, natural gas engines demand much from lubricants because of the oxidation and nitration caused in the oil by the combustion process. These products of combustion degrade the oil, producing sludge and varnish in the engine. These conditions increase oil consumption and 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 catalyze oxidation reaction. Oxidation is typically the main contributor to sludge and varnish formation in natural gas engines.
Nitration is another undesirable condition that indicates 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 are 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 neutralize acids from the 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%;
- Low ash: 0.2 to 0.6%;
- Medium ash: 0.7 to 1.2%, and;
- High ash: > 2.0%.
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 minimize 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 minimize downtime for unscheduled maintenance.
Oil Drain Intervals
Another great way to minimize expenses for your natural gas engine is to extend the lubricant drain intervals. This can help maintenance professionals optimize lubricant consumption, reduce labor, lower disposal costs and increase productivity. However, not all oils are suitable. Extending oil drain intervals with a low-quality oil can often lead to excessive deposits and other operational issues.
Before 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. The following steps should be integrated into a maintenance plan to achieve optimal results:
- Monitor your oil through oil analysis;
- Evaluate oil filters for abnormal deposits during scheduled filter changes;
- Examine crankcase for cleanliness during scheduled oil drains; and,
- Check valve decks during scheduled adjustments; and
- (Optional step) during the engine overhaul, inspect the components to validate drain interval and your proactive maintenance program.
It is recommended that maintenance managers participate in–and use the results of–a used oil analysis program. 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 extended drain intervals.
As an example, a natural gas producer and processor was able to reduce oil purchases by 45% by extending the lubricant drain intervals for a 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.
Today’s industrial gas engines are more sophisticated and specialized than their predecessors. While these design changes have resulted in more efficient systems, they often put added stress on maintenance professionals. It is critical for maintenance professionals to consult original equipment manufacturers’ guidelines, promote engine cleanliness, monitor deposit formation and extend oil drain life. For more information, visit http://www.mobilindustrial.com..
Based on a paper presented at the Gas Machinery Conference conducted by the Gas Machinery Research Council (GMRC), Oct. 2008, in Albuquerque, NM.
Kevin G. McKenna, PE, is an industrial products advisor with ExxonMobil Lubricants and Petroleum Specialties. He has more than 30 years of experience in industrial and commercial applications including 21 years of experience with lubricants, including sales, technical marketing support and product development with Mobil Oil. In addition, Kevin has more than 10 years of experience in the design, operation and maintenance of rotating equipment in gas compression with management experience in manufacturing and plant maintenance. Telephone: 918-299-4562 or by email care of email@example.com.