Power plant alters way it determines moisture in lube oils

R.C.J. Wilson, Ferrybridge C Power Station
Tags: maintenance and reliability, energy management

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:

  1. The turbine gland steam pressure is manually adjusted if the unit is to operate at a lower than normal load.
  2. A mechanical device that separates water from oil is used to remove moisture from the turbine main oil tank.

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.

  • FTIR analysis of water in oil is rapid.
    • The FTIR analyzer takes three to five minutes to measure the water in oil, from sample introduction to final results.
    • With the FTIR system, the level of moisture in the sample does not affect the analysis time. With KF, a low moisture (less than 0.05 percent) sample can be measured in about the same time as the FTIR; however on mid- to high-moisture samples (greater than 0.05 to 0.5 percent), the measurements can take five to 30 minutes.
  • No reagents are required for the FTIR analysis of water in oil.
    • A single drop of neat, used oil is analyzed – no reagents are required.
    • The KF method is a titration which requires test chemicals and reagents, which are expensive and must be reordered.
    • The KF method uses reagents that contain iodine and sulfur dioxide in the presence of methanol and an organic base such as pyridine or imidazole. These are potentially toxic reagents and care must be taken with regard to exposure.
  • The FTIR analysis of water in oil is easy to carry out.
    • The FTIR method is quite simple and the procedure is programmed into the system so that less-skilled personnel can make accurate measurements independent of the level of water present.
    • The KF method requires a skilled technician to carry out the analysis, and very wet oils can be challenging.
    • After several wet samples are measured, the KF titrator has to be taken out of service, cleaned and reagents replenished. This requires that we use multiple KF titrators to keep up with the sample demand.
  • It is easy to train personnel to use the iPAL FTIR system.
    • With the standard method that is already programmed into the iPAL analyzer, it takes no more than a few minutes to train a technician.
    • The KF takes at least half a day since operators need to be trained on how to safely use the toxic reagents, to determine when the reagents require changing, how to clean and dry the titrators, how to replenish reagents, and where to order the relatively expensive reagents required for the KF measurement.
  • The FTIR analysis is as analytically accurate as the KF measurement and, in some cases, more so.
    • Whenever a method is easy to use and doesn’t require multiple steps and reagents, it has the potential to be analytically more accurate than a more complicated test. Our experience in using the KF and FTIR methods indicate that the FTIR is more accurate when the moisture level in the oil is very high.
    • With no pre-treatment of the sample, the iPAL system can accurately detect moisture in oil to 200 ppm.
  • In addition to the determination of moisture in the oil, the iPAL FTIR can measure other important oil specifications, all on the same sample using pre-calibrated, on-board methods. These include:
    • additive depletion of the oil
    • overall condition/oxidation of the oil
    • oil in water for discharge purpose
  • The FTIR system affords real-time analysis, on site. This allows us to immediately and accurately know the condition of the lubricating fluid.
    • If oil is found to be out of specification, on-site testing allows corrective action to be taken and the effectiveness of our actions can be determined virtually in real time. All this can be accomplished well before the initial results from an offsite testing lab have been reported. By the time the sample comes back from an offsite testing lab, the results are usually irrelevant, as competent operators cannot wait a couple of weeks before they take corrective action.
  • The FTIR system increases our level of confidence in results we obtain from off-site testing labs.
    • We have found that if lubricants are not sampled, packaged and sealed correctly for shipment, there can be a significant difference in moisture test results.
    • Frequently in the past, we obtained results from outside testing labs that we knew were (at best) suspect or (at worst) completely inaccurate. By carrying out on-site testing with the FTIR analyzer, which is capable of measuring multiple important analytes, we are far more confident of the results, and this provides a check on off-site testing lab results.

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

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