In modern, conventional power stations, the overall condition of the fluids that lubricate large, high-value machinery is critical to the successful and economic operation of the plant. In particular, the amount of moisture present in the oil can affect the lubricating fluid’s performance since water can wash out critical antioxidative compounds, and contribute to lubricant oxidation and subsequent loss of lubricant performance. For years, Karl Fischer (KF) titrations have been used to measure the degree of water in oil, but this analytical method has a number of limitations. Three years ago, we replaced our KF method and now use Fourier transform infrared (FTIR) analysis to measure and control the level of water contamination in lubricating fluids. We have found that the FTIR analyzer provides accurate data in less time and with less complication than the “gold standard” Karl Fischer titration.
Lubrication monitoring at Ferrybridge
Ferrybridge C Power Station is a 2,000-megawatt coal and biomass co-firing power station situated in West Yorkshire in England. The plant’s four immense steam turbines and main feed pumps produce enough power for 2 million homes, or 4 percent of the United Kingdom’s daily electricity requirements. The power from one steam turbine would be sufficient to power six Queen Mary 2 cruise liners at full speed ahead. Each turbine shaft is more than 170 feet long and exceedingly heavy; with 12 support bearings all lubricated by mineral oil. This lubricating oil serves more than one purpose since it is also the control oil for operating the turbine governor valves and steam admission valves. Therefore, it is imperative that the condition of the oil is monitored and kept within the required specification. Since the level of moisture in the lubricating oil changes over time as a function of environmental and operating conditions, it is imperative to rapidly obtain accurate analytical information.
The measurement of water in lubricating fluids via FTIR analysis
At Ferrybridge, we are using A2 Technologies’ iPAL FTIR analyzer equipped with TumblIR transmission cell system (Figure 1).
Figure 1. A2 Technologies’ iPAL FTIR analyser is being used at the Ferrybridge plant for oil analysis.
To analyze a sample, the operator places a drop of neat, used oil on the lower TumblIR window, which is mounted in the surface of the analyzer, and then rotates a second, gimbal-mounted window into place, thereby creating a reproducible 100-micron gap that holds the oil. The system comes equipped with a pre-calibrated, automated method for analyzing water in oil, and a simple command initiates the transmission IR method. The FTIR analyzer then collects, analyzes and reports the data. The iPAL system is capable of accurately analyzing water as low as 200 parts per million (ppm) with no sample preparation, so detection limits are not at issue. A2 Technologies has developed a method using surfactant that allows quantitative detection of water in lubricating oil down to 65 ppm.
We tested the iPAL analyser method vs. our Karl Fischer titration method, and it showed good correlation between the methods. The trend in the amount of water present is monitored, and thus absolute values are not necessary. Even with KF measurements, absolute values are not measured since the result may be biased by the amount of sample used and the inherent immiscibility of oil and water. Therefore, repeat measurements are made with both the FTIR and the KF (many times with the KF) analyses. Since the FTIR measurements are so rapid, repetitive measurements are much faster and easier to carry out. The small discrepancies between the two methods are not significantly different than those obtained by carrying out two KF tests on the same sample.
After gaining confidence in the accuracy and reliability of the FTIR method, we have largely replaced our KF measurements. An example showed that the iPAL system tracked the level of moisture in both the turbine oil and the main feed pump oil.
When the moisture in the lubricating fluid is greater than the allowable specification, corrective action is taken to remove the water in the oil. There are two methods to adjust the moisture content of the turbine oil:
In addition to monitoring the level of water in oil and alerting us to take corrective action when necessary, the iPAL FTIR analyzer is used to track the effectiveness of our methods to eliminate water and return the oil to acceptable moisture limits.
The value of utilizing FTIR analysis for lubrication monitoring
There are numerous reasons why we have adopted the iPAL FTIR analyzer at Ferrybridge and have eliminated much of our Karl Fischer titration analyses.
The Ferrybridge Power Station has a proactive, on-site lubrication monitoring program in place. We have found that the iPAL FTIR analyser is an important part of that program as it allows us to measure the level of moisture in lubricating fluid virtually in real time. This enables us to take corrective action to adjust the moisture level when it exceeds prescribed limits. The FTIR analyzer is as analytically accurate as the “gold standard” Karl Fischer method and quite a bit easier to use since it doesn’t require expensive, toxic reagents or extensive training of operators. The iPAL FTIR analyzer has become an important part of our on-site testing protocol at Ferrybridge and we are in the process of extending its use in other applications.
Figure 2. Ferrybridge C Power Station