By Brad Buecker, Contributing Editor
Steam generating power plant personnel know that many fluids are utilized throughout the process. These fluids must be of proper chemical and physical makeup and must be conditioned correctly to minimize or prevent corrosion, deposition, mechanical failures and other difficulties within the steam generator. Part I of this article, which was published in September 2009, considered water. Here we will consider lubricating oils and hydraulic fluids, including broad-brush reminders of the technologies and processes to keep fluids in pristine condition.
At the risk of sounding trite, a power plant has much rotating machinery. Poor control of lubricating oil conditions can lead to equipment failure, which, just like a water-related problem, can easily take a unit off-line.
Turbine Lube Oil
The turbine and generator represent many tons of machinery spinning at 3,600 rpm. What a stressful environment! Turbine lube oil is highly engineered, most commonly from alpha olefins with wear, oxidation and other additives blended, to provide precise characteristics for the demanding application. Two principal impurities that greatly affect oil performance are water and particulates.
Water is introduced from a number of sources including turbine steam seals and condensation in the lube oil storage tank. While water buildup influences the lubricating properties of the oil, corrosion caused by water introduces particulates to the oil. Also, microbes will grow in the water and add solids.
Particulate contamination is a self-perpetuating mechanism. Particulates that lodge within the close tolerances of spinning machinery will cause wear and eventual component degradation that generates more particulates. These newly formed particles are of course metallic pieces and are very hard. They in turn will cause wear and additional particle formation in other locations.
Proven and reliable equipment exists to remove contaminants from turbine oil. Particulate filters (with filter media designed not to produce its own particles) can be installed on a kidney loop of the main lube oil tank or may even be installed in-line. With regard to water, the old gravity filters and centrifuges were once popular, but they only remove free water from the oil. Coalescers are also good at removing free water, but mass transfer vacuum dehydration will remove free water plus 80 percent or more of the dissolved water in oil. The latter is of particular benefit because water that is dissolved at higher temperatures within the lube oil system will condense in cooler areas like the lube oil reservoir. There, the free water can initiate corrosion and fouling.
Another major influence on lube oil performance is varnish formation. This topic has already been discussed in Power Engineering, but is worth additional comment because a revitalized technology is emerging for varnish removal. It is an adsorption process utilizing specially designed filter elements that collect varnish but not clean oil or its additives. The process has been shown to reduce varnish levels well below the standards for a “clean” system.
Electrohydraulic (EHC) Fluid
Steam turbine valves operate using hydraulic fluid, but the valves control steam that can be 1,000 F or higher. Thus, fire would be a serious danger if turbine lube oil was the fluid. The majority of EHC fluids in the power industry are composed of some form of triphenyl phosphate.
While each benzene group may have a substituent compound attached to establish the desired properties for the application, the fundamental features of triphenyl phosphates include good fluid characteristics and fire resistance.
Ingress of water into the EHC fluid from steam seal leakage, condensation and other sources will hydrolyze the phosphates to acids and phenols. These products degrade the fluid properties and introduce corrosive species to the system. A standard term used in the industry is acid number (AN), which refers to the amount of potassium hydroxide needed to neutralize the acid content. (See Table 1.) As acid begins to form in the fluid, it initiates a self-sustaining process of acid formation followed by increased fluid decomposition, producing more acid.
A popular method for filtering EHC fluid is to pass a kidney-loop stream through material such as fuller’s earth or activated alumina. These compounds neutralize acids by reaction with alkaline ions such as calcium and magnesium. However, this process introduces hardness ions to the fluid, which in turn can react with degraded EHC to produce tenacious deposits such as calcium phosphate. Similar to varnish, these hardness deposits can impede valve operation and cause operational upsets.
An alternative technology to remove acids without hardness-based deposit formation is to install an ion exchange column on the slipstream, where the exchange media removes the hardness ions. Use of ion exchange for phosphate ester treatment allows the operator to selectively target both acidity and resistivity of the fluid by combining different concentrations of anionic and cationic resins. The flow rate required for these systems is relatively small, resulting in minimal resin volume requirements, where the resin may last for several months before a change-out is needed.
Other Lubricating and Insulating Oils
We have examined issues related to turbine lube oil, but other oils, and the equipment they lubricate or insulate, can benefit from conscientious monitoring and treatment. The list includes the following:
- Pulverizer lube oil
- Hydrogen seal oil (new filtration equipment is available that improves the safety of the filtering process)
- Boiler feed pump lube oil
- Forced draft (FD) and induced draft (ID) fan lube oil
- Transformer oil (on-line degassing is now possible).
Author: Brad Buecker is a contributing editor for Power Engineering. He currently serves as technical support specialist for AEC PowerFlow, Kansas City, Mo. His experience includes serving as an air quality control specialist and plant chemist for Kansas City Power & Light Co.’s La Cygne, Kan., power station. He also served as a chemist, flue gas desulfurization engineer and results engineer at City Water, Light & Power, Springfield, Ill.