Summer peaks are still with us, and every unit on your system must be prepared to operate at a moment’s notice. Spot power prices are so high that you expect phone calls asking for a few more megawatts from your units. Then your plant chemistry lab calls to report a condenser tube leak. Your options are few: Shut down immediately and get charged with a forced outage, ignore the leak and keeping running until fall, or schedule a maintenance outage next weekend and hope the leak can be found and fixed. In Part I, we examine what you need to know in order to make an informed decision. In Part II, we’ll explore the actual damage mechanisms.
Condenser tube failures continue to be the most common source of plant boiler and steam contamination. They are also unpredictable in size and location and can be difficult to detect. Improvements in water treatment equipment, such as reverse osmosis membranes, have reduced demineralizer regeneration problems, the next-most-frequent major cause of boiler water contamination. At some plants, the combination of reverse osmosis and a continuous electro-deionization unit has essentially eliminated contamination caused by the water treatment system. Some guidelines for selecting new water treatment equipment were presented in an earlier article (“Avoid These 10 Mistakes When Selecting Your New Water Treatment System,” September 2009).
Although condenser tube material is an important factor in the durability of your condenser, you can’t always “alloy” your way out of condenser tube leaks. Stainless steel is subject to cracking on the steam side and microbiologically influenced corrosion on the water side. Even titanium tubes have been known to leak.
Deciding between a forced shutdown to repair a potential condenser leak and pushing your luck by continuing to run your unit requires an understanding of the common modes of condenser failure. When a leak is confirmed, your next move will determine if—or, in most cases, how much—damage is done to the boiler and turbine.
Protect Your Steam Turbine
For the majority of plants that use condenser cooling water, a condenser tube leak means a drop in boiler pH. For many units, control of the boiler pH is the primary determinant of whether or not the contamination is severe enough to take the unit off line. Obviously, if the boiler pH cannot be controlled, and particularly if the pH drops below 8, the unit must come off line immediately. However, the converse is not always true. Just because the boiler pH can be maintained above 8 by adding additional phosphate and caustic and increasing the rate of boiler blowdown doesn’t mean that contamination is not damaging the boiler or steam turbine.
Steam turbines are particularly vulnerable to even minute amounts of contamination accumulating on the steam turbine blades. It is critical that steam purity guidelines be constantly maintained to minimize steam turbine corrosion. Corrosion in a steam turbine typically occurs in the low pressure (LP) section of the turbine where the steam has lost most of its superheat and is approaching saturation temperature. For this reason, steam purity guidelines are not significantly different for different boiler operating pressures or temperatures.
During a contamination event such as a condenser tube leak, significant damage can occur to the LP turbine in relatively few hours of operation with high levels of sodium hydroxide, chlorides, or sulfates in the steam. These chemicals, precipitating in the LP turbine, can result in stress corrosion cracking or corrosion fatigue failures.
Steam passing through the turbine (preferably sampled at the reheat steam sample station after attemperation) should contain less than 2 ppb of sodium and have a cation conductivity of less than 0.2 microsiemens/cm (Figure 1). These two critical parameters need to be continuously monitored and displayed and alarmed in the control room. The limit of 0.2 µS/cm of cation conductivity can be affected by the presence of organic acids and carbon dioxide in some units (see sidebar).
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| 1. Measuring sodium. Measuring the sodium content of water and steam in modern power plants is critical; however, measurements are problematic. Modern analyzers, such as the Hach Model 9245, can detect levels of sodium down to 0.01 ppb. Courtesy: Hach USA |
Recall that cation conductivity is not a measurement of any specific contaminant but a “composite” parameter—the sum of the acid form of all anions in the sample (see “Cation Conductivity Monitoring: A Reality Check,” May 2008). The actual corrosive species in steam that are inferred by cation conductivity are chloride and sulfate. The actual limit of chloride and sulfate in steam for normal operation should be less than or equal to 2 ppb. However, it is rare that a utility will install the dedicated instrumentation and personnel to analyze these parameters directly. Normally, if the cation conductivity is within limits, chloride and sulfate will also be well below 2 ppb. However, there is a potential bypass route for contamination that is often overlooked. When feedwater is used for attemperation, it can be a source of steam contamination during condenser tube leaks.
If chloride and [Ed: correction] sulfate in the steam cannot be maintained below 8 ppm (less than four times the normal operating limit of 2 ppb), the unit should come off line as soon as possible and definitely within 24 hours, regardless of boiler water chemistry readings. When contamination is suspected, the direct measurement of chloride and sulfate may be required to ensure that the turbine is not being harmed.
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