- Buyer's Guide
I visited a chemical manufacturing plant that used three three-cylinder reciprocating compressors. The machines had a long history (approximately 20 years) of intensive maintenance, but periodically a piston rod would break in one of the compressors. The machine would then be taken out of service and a standby machine put online.
The double-acting compressor cylinders were placed in a "W" arrangement; the outer cylinders of the "W" were first-stage cylinders and the center one was a second-stage cylinder (Figure 1). The pistons were double-acting with lubricated piston rings. The pistons and gland packings had traditionally been injection lubricated with a mineral compressor oil, but had recently been switched to lubrication with a poly alpha olefin (PAO) type of oil from a small central system lubricator skid.
The switch to the new oil was recommended by the local representative of the oil company in an attempt to reduce the carbonizing inside the machines. Unfortunately, the change had no effect at all. The process gas was both flammable and toxic. It came from the chemical process plant to the compressors through a drying tower before going back to the plant for further processing.
Figure 1. Compressor Cylinder with W Cylinder Formation (Compressor Products International Ltd., Hungerford, UK)
Upon my first visit to the plant and the compressor house, I was immediately struck by the amount of used or broken machinery lying around. One of the three machines was operating, one was on standby and one was stripped down for maintenance. Apparently, the plant had operated the machinery in this fashion for approximately 20 years and the compressor house was virtually the permanent place of employment for the service person!
I was advised by the service person and by a plant operator that the machines were regularly taken out of service due to carbon buildup inside the machine cylinders and valves. The end clearances inside the cylinders were becoming so reduced that the pistons touched the cylinder head and the piston rods were snapping underneath the piston attachment. Usually, the contact occurred on the downward stroke with the underside of the piston striking the head at bottom dead center (Figure 3).
When this happened, the failed piston rod surprisingly would push the piston back up the cylinder on the next stroke and the machine continued to run with one piston detached from its rod. The only clue to a piston rod breaking was pressure dropping off in the plant as the flow from the machine reduced. Although I was concerned about this, the plant operator and the service person were nonchalant about the situation because they saw it happen regularly. They simply took it as a sign that the machine was due for maintenance.
Figure 2. Compressor Cylinder
Examination of Machinery Parts
Inspection of a dismantled machine showed that heavy carbonization was characteristic of these machines for the last 20 years. All internal surfaces in the cylinders were contaminated with oily, black, hard carbon deposits. The gas passages of the inlet and outlet valves were also heavily choked (Figure 4).
The gas packings were a lubricated metallic Babbitt type and were heavily contaminated with the carbonized debris. The piston rods were worn by the carbon that made its way into the gas packings and wore away the rod surface. An examination of the broken piston rod determined that the rod had fractured immediately below its piston attachment. A change in material characteristic about three millimeters deep all around the periphery of the rod was visible.
After taking the rod to a metallurgy lab and cleaning up the specimen, it became clear that the discoloration seen at the job site was a thick layer of metallic flame-sprayed buildup from a metal coating process. This seemed bizarre, but upon talking to the plant supervisor, it became clear that new piston rods were not put in the machines because they didn't last long. Instead, the piston rods were sent to a local workshop where they were metal-sprayed back to the original diameter and sent back to the stores.
I spoke with the local repair workshop which prepared and flame-sprayed the rods. Upon our initial discussion, I discovered this workshop didn't know what the piston rods were or what they did. To them, the rods were merely a worn piece of round bar with a thread at each end. The workshop prepared them with a coarse-machined finish (perfect conditions for stress propagation to occur), sprayed them, then turned them to a given diameter to finish off.
The reclaimed rods were put back into stores as supposedly fully refurbished items. It is notable that neither the plant nor the reclamation workshop understood what was required in refurbishing a piston rod or what the parameters might be to scrap the rod. The compressor manufacturer's representative provided no advice on the condition or refurbishing of the rods; he simply used the parts he was given free by the client.
In fact, the broken rod had a flame-sprayed coating that was 3 mm thick on a rod that was only 47 mm in diameter. This reduced the effective diameter to about 41 mm. The workshop did not believe there was any limit to how thick the metal spray coating could be, as long as the final diameter was as specified.
Figure 3. Piston Rod
I was told by the plant manager and the plant chemical engineer that the contamination of the machinery resulted from unreacted carbon carrying over from the process, and that it had always been a problem. This theory went unchallenged in spite of the fact that there was another plant next door that experienced no problems with its (nonlubricated, oil-free cylinders and packings) reciprocating compressors.
At this point, the explanations left me pretty unconvinced. I knew there was a 98 percent sulfuric acid drying tower upstream of the compressors that the gas passed through on its way to the compressors. I also recalled being impressed as a 14-year-old high school student when the chemistry teacher turned sugar into a large piece of carbon by adding sulfuric acid.
I went to the site laboratory with a small jar of the PAO oil and asked if one of the chemists would add a few drops of 98 percent sulfuric acid to the oil. Immediately, the oil went black, and after a few more drops were added, free carbon was produced from the acid/oil mixture.
With this evidence, I persuaded a tribology lab to perform tests on the stability of the oil when mixed with concentrated sulfuric acid. During one of the tests, significant sulfur dioxide gas escaped from the resulting reaction. Large quantities of carbon were produced from the acid/oil mixtures. At this point, I felt confident in presenting the results of my chemistry experiments to plant management.
Both a mineral-based compressor oil and a premium-quality PAO oil were unsuitable for use in a lubricated compressor cylinder where the gas was contaminated with a mist of 98 percent sulfuric acid.
The theory of unreacted carbon contaminating the machines was proven false.
Neither the plant nor the workshop understood what was required when refurbishing compressor piston rods.
The manufacturer's service department had taken this work for granted for many years, and had made no attempt to rectify the long-term problems suffered by its client.
It was likely that the compressors would continue to run without the carbon contamination and the risk of broken compressor components based on two conditions:
a) the compressors were converted to dry-running piston rings/cylinders; and,
b) the machines and pipe work were cleaned of oil.
This increased the safety of the installation and reduced the plant's maintenance budget.
Table 1. Oil Stability Report1
Over the following months, the staff were re-educated about the importance of proper refurbishment procedures and a full procedure was put in place.
The manufacturer's service department was no longer used, and a third party was contracted to perform any compressor work.
The three old lubricated cylinder machines were replaced with two modern non-lubricated machines similar to the plant next door. This approach was preferable to converting the existing machines, although a conversion to non-lubricated pistons and packings was technically feasible.
The plant continues to operate profitably and the compressors now run year-round, requiring only annual service.
There are no longer problems with unreacted carbon clogging up the machines and the stories about the process gas affecting the machines are a thing of the past.
About the Author
Stephen Shakeshaft is a mechanical engineer and director of Stephen H. Shakeshaft Consulting Ltd. in the United Kingdom. He has more than 30 years experience of working in manufacturing industries with clients in the chemical, utility, industrial gases and pharmaceutical businesses. Stephen is a corporate member of both the Institution of Mechanical Engineers and the Institution of Incorporated Engineers.