Managing Cleanliness Of Natural Gas Engines Cuts Costs

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.

Engine Cleanliness

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.