Natural Gas Engine Crankcase Rust Case Study

During the time I’ve been involved in predictive maintenance, I have often heard medical analogies used to define and describe predictive technologies. I have even used these analogies in training sessions and executive meetings to aid in the understanding of basic fundamental principles of somewhat complicated concepts.

I often describe our technical service team members as specialists - doctors who are called for advance diagnosis or for routine consultation. In one of our company’s cross-training initiatives, I described oil analysis to a group of IT/HR personnel, using the analogy of a doctor sampling blood to determine health or disease.

The basic principle was clearly understood and I felt satisfied that I had described my craft effectively. Unknown to me at the time, I would receive a field service request from an equipment analyst with the plea, “Help! One of these engines has cancer and crumble-ization has set in.”

Diagnosis

In February 1993, a fuel valve broke and punctured the cylinder of a Cooper Bessemer GMWA8 natural gas engine with integral compressors. This flooded the crankcase with the entire supply of the unit’s coolant, a glycol/water mixture. Although an unusual failure, the unit was repaired properly, cleaned and put back into service.

Three years later, an oil analysis report on this unit was flagged for an increase in sodium and potassium, which suggested a coolant leak. The unit was thoroughly inspected to reveal no sources of contamination. Two months later, the company entered into an agreement with an oil vendor to establish a one-source supplier.

Due to flagged test results on the oil analysis report, a wholesale oil change-out was performed on this unit to take advantage of the new agreement. During the oil change-out, additional inspections were performed, one of which revealed the first signs of cancer. The inspection uncovered an usual amount of rust in the compressor doghouse area (Figure 1).

Determining Origin

The doghouse of the compressor transitions the area of horsepower production to the area of horsepower consumption. In any transition area between two systems there is a risk of cross-contamination from one system to the other.

With that in mind, my first focus was to look for any corrosive contaminants in the gas stream and determine how and where they were entering the system. At this point, the rust was localized to this doghouse area and to this unit. I asked myself why had this area, this unit only, developed this condition out of all of the 35 units spread over 1,000 miles of pipeline?

This particular compressor station had seven of these units that were located in the same building and shared the same suction and discharge headers. All gas analysis reports showed no corrosive contaminants and none of the compressor packing seals in the doghouse transition area were leaking.

Other higher priority issues surfaced so the project was left unresolved. The next field service request, which indirectly reprioritized the rust project, was a bearing failure on the blower drive assembly. Routine vibration analysis performed by the equipment analyst detected a bad bearing.

The unit was taken out of service and the bearing was replaced. Why is this significant? The unit in which the bearing failure occurred resides next to the compressor doghouse area with the rust problem. A visual inspection of the damaged bearing revealed rust on the balls and races.

This bearing failure lead to further inspections of the second unit. Traces of rust were located inside the crankcase including the crankshaft and rod pins (Figures 2 and 3).

I focused on the other commonality of both units, the oil supply. My investigation uncovered deficiencies in the oil holding tanks and the oil recovery system. Traces of water were found in both systems.

Forty years of age and service left the holding tanks in an undesirable condition, so the systems were cleaned and the tanks were reconditioned. Water, of course, typically causes rust. Once the ingression of water was terminated, one skirmish in the war against rust was won.

The Second Opinion

Although the source of water contamination had been eliminated, the rust continued to spread. I recommended contracting an outside engineering firm for additional support and scheduled an advanced oil analysis training seminar for myself. This was the beginning of the real education gained from this experience.

The contracted firm was merged into the project and a recommendation for further testing was identified. With more education and understanding about lubricants and oil analysis, I was able to identify several company-wide deficiencies in our lubrication program. The first deficiency was lack of education about lubrication - across the board, from managers to millwrights.

The second deficiency, and probably most significant was how lubricants were selected. To sum it up, lubricants should be selected primarily by application and performance expectation. The secondary selection criterion should be price, not the other way around.

The education acquired through this project came at the best possible time. The one-source oil agreement on the contractual obligation was coming to an end, and the contract was renewed.

A company-wide consensus prompted a study that would allow past, present and new oil vendors to compete in several carefully selected lubricant performance tests. An outside firm, using third-party lab and total anonymity between candidates, conducted the study.

Two findings were received as a result of this study. One was the result for the best performance lubricants for our application. Second was the missing link on our rust project.

The contracted consulting firm’s evaluation revealed a residual corrosive agent of sodium was present in the unit. The sodium could have come from any coolant contamination from years past or present.

However, even when the water ingression was eliminated and a corrosive agent was identified, the rust was getting worse, not better. The missing link was identified by one of the performance tests in the new lubricant study. An ASTM D665 rust test was performed on all candidates in the study.

One lubricant in particular was the brand and type of oil we were using in the units with the rust problem. This lubricant failed the test.

The D665 is a simple test to evaluate how effectively a lubricant inhibits rust on ferrous material once water is introduced into the oil. A brief description of the test can be found in the sidebar.

Prognosis

The current oil was selected years earlier when the original one-source oil agreement was negotiated. At the time, a good business decision was made using the information at hand; this oil was a lesser grade lubricant and was slightly less expensive.

This particular oil was used in the same application all over the world with great success. We even had 35 units running on this particular lubricant. Some units at the same compressor station operated alongside the units with the infectious rust.

The results of the D665 test indicated that as long as the ingression of water and corrosive contaminants was held to minimum, the oil’s antirust inhibitors performed satisfactorily. If the contaminant level increased, the oil’s antirust inhibitors were depleted, allowing the rust to spread.

The lubricant in the GMW unit was replaced with another mineral-based oil. The replacement lubricant was the top performer in the conducted oil study. Steel rods, which matched the specifications of the D665 test, were installed in the areas where the rust was the heaviest.

The unit was sealed for 10 months and operated under normal conditions. Upon inspection, the rods were observed to be in the same conditions as the day they were first installed (Figures 4 and 5).

Figure 4. Newly Installed Steel Rods in Doghouse Compressor Area

Figure 5. Ten months after steel rods were installed, visual inspection revealed that the condition of the doghouse area remained unchanged.

Anyone who has dealt with rust knows how hard it is to stop, but the means of fighting this rust-cancer can be summarized in one word - prevention. Everyday you hear a medical report that there is something you can do, or stop doing, to prevent cancer.

The key in determining which report best suits your needs is in the education you receive with the finding. A higher awareness of oil and oil analysis has advanced not only in the predictive maintenance group but also with the field personnel, to the point where oil is never taken for granted.

Sidebar

• Take 300 ml of oil and mix 30 ml of distilled water.
• Submerge a steel rod into the mixture.
• Heat to 140°F for four hours.
• After the four hours, remove the rod and visually inspect for signs of rust.

Examples of an ASTM D665 Test Results

Lubricant with sufficient rust inhibitors.

Lubricant without sufficient rust inhibitors.