1 reply to this topic

[samrat]

I am posting this real case study as I need lot of feed backs to question,contest,challenge and arrive at an equitable solution which is bothering me for some time. The details are furnished as below.I am furnishing a brief report.
1. The application is compressing natural gas(CNG) to 3625PSI from suction pressure of 228PSI in three stages.
2. The first stage has 4.75" bore with 3.00 stroke: The second stage has 3.375" bore with the same stroke:
and the 3rd stage has 1.5" bore with 3.00stroke.
3. The gas composition mainly has a methane,ethane and propane and has an average molecular weight of 16 to 17
4. The inlet gas is saturated at 35deg C or 95deg F.
5. The process simulation has been done and in the final stages, there is some knock off of water content, though not significant.
6. The process runs till the cascade is filled up to 250barg or 3625PSI and then shuts off.
7. In general based on the density of Trucks,Buses,4 wheelers and 3 wheelers, the compressor runs on an average of 15 to 18 hrs a day.
8. Pistons of the first two stages are nodular iron with filled teflon rings(the composition is not known) and the last stage has steel piston with filled teflon(composition not known)
9. The compressor has run for about 4500hrs approximately.
10. The first two stages have 4 piston rings and the last one has 5piston rings.
11. What has been observed lately during the periodic check up is that the piston ring grooves are widening.
12. This has led to blow across the piston ring grooves and a development of high discharge temperatures
13. This has also resulted in reduction of aspiration of the compressor. The design capacity is 1200SCMH(1200 standard cubic meters per hour, OR 706SCFM)
14. The surprise part is that the rings are not wearing out, which I suspected before inspection by our rep.
15. The grooves are widening and creating the steps inside the piston ring groove. This you will see in zipped file containing photographs attached.
16. The design clearances between the piston ring and the grooves would be between 0.005" to 0.008"..
17. The cross section of the rings 0.375" square.
18. If any body needs additional information, I welcome in this forum. I will post additional information called for by anybody within my capacity as I am also bound by integrity.
I request all professionals,experienced individuals and even students to contribute. More the feed backs the better it would be to come to some tangible solution.
With Best Regards,
samrat

Attached Files

•  Desktop.zip   1.31MB   9 downloads

[kkala]

My familiarity with reciprocating compressors is limited to compressed air Operations, having no relation to the size indicated in photos. In http://www.tpub.com/engine2/en2-58.htm, near Fig 3.20, there is an explanation of mentioned groove widening (called groove shouldering) as below.
"Give particular attention to the ring grooves, especially if the pistons have been in service for a long period of time. A certain amount of enlargement of the width of the grooves is normal, and SHOULDERING of the groove may occur. Shouldering, as illustrated in figure 3-20, results from the “hammering out” motion of the rings. The radial depth of thickness of the ring is much less than the groove depth, and while the ring wears away an amount of metal corresponding to its own width, the metal at the bottom of the groove remains unchanged. Shouldering usually requires replacement of the piston since the shoulders prevent the proper fitting of new rings. After determining that a piston is serviceable, inspect the rings carefully to determine whether they can be reused. If they do not meet specifications, you must install new rings".
I think a specialized mechanical engineer can understand and look better into this matter.