Perseverance, Surprises and Unintended Consequences
EP Editorial Staff | July 1, 2007
By the time I joined Rentech in 2000, the three Clark TCV 16 engine/compressors compressing hydrogen and nitrogen at our East Dubuque Ammonia Plant had begun experiencing shortened packing life. Packing life on the 4600 psi 3rd and 4th stages—that historically had reached 18 to 24 months—suddenly fell to just three to six months. Excessive ring wear also was observed.
Maintenance personnel at the plant had noticed that the major brand lubricant we were using on these units looked somewhat different than in the past. When questioned about this, the supplier assured us that no changes had been made. As we continued to pursue the problem, the manufacturer later admitted that the formulation had indeed been changed and the diminished performance we were experiencing was unintended and unexpected. Consequently, we began to look for an alternative lubricant.
The first alternative we selected, an independent brand cylinder lubricant, appeared to work well for about a year. Then we encountered another unintended consequence. The oil carried downstream of the compressor was plugging the chillers in the synthesis loop. At that point, we determined that an oil with a lower flock point (temperature at which wax crystals form) was what we really needed.
We opted to go with Royal Purple’s NGL-NS synthetic lubricant as it had no flock point and no sulfur that could poison the process catalyst. After just three weeks, however, we experienced a packing failure on a synthesis loop compressor cylinder. A repetitive failure occurred just four weeks later. In all, we had six packing failures over a twoto- three-month period. Then, the failures stopped—just as suddenly as they had begun.
Interestingly, only the packings on the recirculation cylinders of compressors C & B were failing. We sent samples of the failed packings, along with foreign material found in them, to Royal Purple for analysis. Because the oil travels downstream of the compressor, the supplier attributed the failures to the NGL-NS cleaning out 37 years of “stuff” that had accumulated in the loop. We were advised to persevere with the clean-up and to expect that the failures would eventually stop. They did. Our packing life increased to two years. Subsequently, we also changed packing materials, increasing our current life to four years with expectations of six years life.
Each of these engines/compressors has two turbochargers. In summer months, these units would experience pre-ignition problems due to low intake air volume. We would have to reduce the load on the engines in order to operate. At some point in the past, as part of a lubricant consolidation program, our plant had elected to lubricate this equipment with the SAE 40 weight gas engine oils used in the compressors. We were looking for a better bearing lubricant for the turbochargers and elected to try Royal Purple’s Parafilm 68. Again, we found ourselves experiencing unintended consequences.
The first unintended consequence was related to the engines being started with compressed air. The air to the engine is automatically shut-off when the turbocharger speed indicates that the engine is running. After changing lubricants, the engines would not start. We determined that the new bearing lubricants had reduced friction in the turbochargers’ bearings so significantly that they were spinning up just on the compressed air, and that they no longer needed engine exhaust gas to do so. We had to reset the air cut-off parameters in order to start the engines.
The second unintended consequence was that the preignition problems in the compressors completely went away. The engines could now be run fully loaded yearround while still maintaining a slight open position of the waste gate valves.
Identifying more opportunities
We operate a 2500 hp Elliott turbo expander in our Nitric Acid Plant that is one of only four ever made and the last still in operation. It operates at 16,000 rpm. This unit has always experienced problems with short bearing life and high vibrations. We thought this resulted from the equipment’s fabricated case coupled with too much overhung mass on the rotor. At times, the unit would shake the floor grating so violently that it hurt your feet to stand on it. Replacement bearings cost $8000 and would have to be replaced twice a year. The bearing alarms were normallyset based on whatever vibration levels existed after a rebuild. In an effort to reduce vibrations and extend bearing life, we elected to change out the ISO 32 turbine oil to Royal Purple Synfilm 32. We expected to see improvement from this oil change due to the much higher film strength of the Royal Purple lubricant and we were not surprised. Immediately after the oil change bearing vibrations went down from 4 mils to 2.7 mils. Bearing life went from six months to 20 months. Recently we had the rotor balanced, which further reduced the vibrations to 1.2 mils—and is expected to extend bearing life even longer.
Based on these successes, we began to look for even more opportunities to improve machine performance with lubricant upgrades.
Single screw compressor…
We elected to change the oil on a 400 hp Vilter single screw compressor at our two 20,000-ton ammonia storage tanks. This compressor had a number of operational issues. It continually tripped on high temperatures. Valves failed to operate because of oil deposits gumming up the valves. We had excessive make-up oil due to the oil’s inability to readily separate from ammonia. In this application, we elected to change out the factory oil for Royal Purple Uni-Temp 300 refrigeration oil. Based on assurances from Royal Purple that we would also achieve substantial energy savings, we installed data loggers on the compressor to record volts and amps. All of the operational issues with the compressor disappeared and we documented a 9% reduction in power consumption.
Mycom ammonia compressor…
Shortly thereafter we changed to Royal Purple oil in the Mycom ammonia compressor serving one of our Nitric Acid Plants. All went well for about a year until we got a water leak in the shell and tube heat exchanger, and unintended consequences recurred. The internals in the compressor rusted and took out the bearings. We also found a black residue in the compressor we believe came from the oil. After a second compressor failure using the new oil, we elected to return to the previous oil as it appeared to have a superior ability to handle wet ammonia.
Rotary screw compressors…
Next we elected to change the oil in our three Sullair rotary screw air compressors to Royal Purple Synfilm. For whatever reason, the larger 400 hp compressor was being lubricated by a major brand multi-purpose mineral oil requiring eight oil changes per year. The two 100 hp compressors were lubricated with a factory synthetic oil with annual oil drains. Being a polyalkaline glycol type oil, the factory oil was incompatible with most other lubricants. Thus, it was necessary for Royal Purple to supply its Royal Flush product to flush the old oil from the compressor before adding new oil. In addition to its price advantages, the new oil has reduced discharge temperatures by 12 F degrees and has extended oil drain intervals via oil analysis to 12,000 hours.
4-stage urea plant compressors…
In April of 2002, we elected to address issues we were having with our two Clark CMB 4-stage compressors in the urea plant. These units, which compress CO2 to 3000 psi, were experiencing excessive cylinder ring wear, packing sealing problems and plugging of the downstream separators. We elected to change the major brand cylinder lubricant to Royal Purple CAP701W ISO 220. This proved to the solution to each of these issues. So again, we looked for other areas where we could improve the performance and reliability of our equipment with lubricant upgrades.
Steam-driven centrifugal compressor…
We also looked at a steam-driven 5000 hp Allis Chalmers centrifugal compressor in Ammonia Chiller Service. The speed increaser gear box (4900 rpm input/12,800 rpm output) was in high vibration alarm. The turbine and gearbox share a common lube sump containing 1200 gallons of ISO 32 turbine oil, so we could not drain and replace the oil because shutting the turbine down meant shutting the plant down. We decided to drain the turbine oil to the lowest level we thought was safe and then we added six drums of Synfilm 32 to the existing oil (27 ½%) hoping to get enough film strength into the oil to bring the vibrations down. It worked. Vibrations were reduced from 0.2 IPS to 0.17 IPS (a 15% reduction), which was below the alarm limits. We ran the turbine until our next scheduled turnaround. We drained and replaced the major brand turbine oil with Parafilm 32 and have run the turbine and gear box without incident for the past five years.
This compressor has a separate oil system for the trapped bushing seal that seals the ammonia into the compressor. Feeling good about our successes, we decided to tackle what we believed was a lubricant related seal problem. The major brand ISO 32 R&O oil we had successfully used for years suddenly became unavailable. We were assured that the supplier’s new offering was equally good, but then some of those unintended consequences reared their ugly heads again. What used to be minor carbon deposits found in the seal at turnaround became significant carbon deposits on the seal—which caused premature seal failure and plant shutdown. We consulted with Royal Purple about a replacement oil and began a new round of unintended consequences.
First we tried Synfilm which didn’t separate well enough from ammonia. To overcome this we changed to Uni-Temp, which we knew separated rapidly from ammonia, but its high solvency began to aggressively clean up deposits from the seal system that were carried to the seals—again causing premature seal failure. Finally, we changed to Royal Purple Barrier Fluid FDA 56, which solved the problem once and for all. That’s because this product does not have the cleaning abilities of the Unitemp. In this case, we felt it simply was better to leave whatever “stuff” was in the system alone.
We operate over 150 pumps at the East Dubuque Ammonia plant, all of which had been lubricated by an ATF fluid. A little over a year after changing these pumps over to Synfilm, it occurred to us that we had not experienced a single bearing failure since the move. Even now, we seldom see bearing failures on these pumps—the few problems we do have are usually seal failures.
The road to success
Today our plant is running better than at any time in its 40+ year history. Budget trends are down and equipment availability is up. We are currently at two-year turnaround intervals and feel we could easily go to three. Though it may have started as an accidental journey, we have learned that good lubrication is key to reliability and good lubrication begins with optimum lubricant selection. We also learned that solutions are not always obvious and that patience and perseverance are required in order to stay the course to some solutions.
Sometimes we wound up having to take two steps back before being able to take three forward. We also learned that there is no such thing as a magic bullet—or magic oil. For many applications we see no advantage in using anything but traditional lubricants. For many others, the benefits vastly exceed the cost of premium performing lubricants.
One thing is certain, however. We no longer look at lubricants as an expense. Instead we look at lubricant selection as an opportunity to maximize productivity and profits.
Lance Wilkinson is maintenance superintendent/technical manager with Rentech Energy Midwest, in East Dubuque, IL. He has extensive experience in the Petro-Chemical Industry, including 21 years in engineering, seven years as a craftsman and supervisor and three years as a machinist. Wilkinson holds a BSME from the University of Texas at Austin. E-mail:firstname.lastname@example.org