Updated: Dec 1, 2020
Sanya Mathura of Strategic Reliability Solutions Ltd
Client was experiencing high levels of oxidation on the header return line in the power plant which has steam and gas turbine technologies.
Facility is a combined cycle power plant with four gas turbines and one steam turbine (all GE frame 7s) responsible for generating power to be supplied to the grid. They have noticed that the oil is turning dark very quickly and through their onsite lab testing facilities, they have seen evidence of high sediment build up. This sediment build up has caused the differential pressures to rapidly increase and the filters have been changed more frequently than in the past.
Temperatures for the various elements in the system including the storage tank, header, gas and steam turbines and bearing temperatures were recorded to note any discrepancies. It was noted that the temperatures varied from 69°C (at the header) to 125°C (at the bearing drain from the gas turbine). RPVOT, QSA, viscosity and TAN tests were done on the oil in service. The most alarming result was the QSA which stated the varnish potential at 100 while the Phenol levels were non-existent.
The Amine levels were still at 35% but the RPVOT value had dropped to 15%! The oil needed to be changed and quickly to avoid any damage on the inside of the system.
Luckily, the wear particles did not show any alarming rates but the low values did start to show an upward trend over the course of one month. Since the system could not be shutdown and was due for an overhaul with two new turbines being commissioned later in the year, the plant needed to stay operational until those were commissioned.
After performing a quick inspection of the lines which experienced the highest temperatures, it was realized that there were gas leaks that vented on the lines causing rapidly increasing temperatures of the oil. However, the gas leaks could not be stopped until the plant was shut down. Lead barriers were placed between the areas of the gas leaks and the lube lines to shield the lines from the drastic increases in temperature as a temporary solution until the plant could be shut down.
A running sweetening process was done for the oil that remained in the system with an initial sweeten ratio of 40%. After this new oil was placed into the system, a kidney loop filtration system was installed to help clean up the oil that now occupied the system. Weekly checks utilizing the onsite lab ensured that the ISO ratings decreased while a monthly sample was sent for RPVOT and QSA in an external lab.
It was found that after the temporary temperature blockages were installed that the overall temperatures of the system decreased. Coupled with the partial new charge of oil and the kidney loop filtration system, the oil was saved from being further degraded at such an accelerated rate.
Additionally, this allowed the plant to continue operation and meet it required output levels until the new turbines were commissioned.