By Brad Buecker, Contributing Editor, and Dan Dixon, Project Engineer, Lincoln Electric System
Damage that can occur in steam generators due to in-leakage of contaminants during normal operation is a subject of much discussion. However, very serious damage is possible in systems that cycle on and off but are not shut down, laid-up or started up properly. Combined-cycle plants are particularly susceptible to these problems because of typical numerous startups and shutdowns. This article examines the most important issues with regard to off-line chemistry.
Oxygen is the Darth Vader of Poor Layups
Both conventional and heat recovery steam generators (HRSG) are a complex maze of waterwall piping, superheater and reheater tubing, boiler drums, and other equipment. When a unit is taken off-line due to reduced load requirements or other issues, the water inside the circuits contracts in volume. This volume reduction induces a slight vacuum within the system, which in turn draws in outside air. Now, a stagnant condition with oxygen saturation, at least at water-air interfaces, has been established.
Oxygen attack can be extremely serious for several reasons. The corrosion mechanism itself can induce severe metal loss in those areas of high oxygen concentration.
The attack often takes the form of pitting, where the concentrated corrosion can cause through-wall penetration and equipment failure in a short period of time. Also of major importance is that off-line oxygen attack will generate corrosion products that then carry over to the steam generator during startups. Deposition of iron oxides in the waterwall tubes leads to loss of thermal efficiency and, most importantly, establishes sites for under-deposit corrosion. These mechanisms may include the very insidious hydrogen damage,  acid phosphate corrosion in improperly treated units, and caustic gouging.
|Oxygen corrosion in a steam generator tube.
Another method by which oxygen can infiltrate steam generators is at startup when stored condensate or fresh demineralized water is needed for filling or boiler top-off. Quite often, high-purity water is stored in atmospherically-vented storage tanks. The water absorbs oxygen and carbon dioxide, and may even become saturated with these chemicals. When the makeup is injected into a cold steam generator, additional attack will occur.
Nitrogen Blanketing at Terry Bundy
At the Lincoln Electric System (LES) Terry Bundy combined-cycle plant, utility personnel have implemented several of the most effective techniques to prevent oxygen ingress and corrosion. We will examine these techniques plus some alternatives that can also be effective.
First and foremost is nitrogen blanketing during the last stages of shutdown and subsequent short-term layups. Experience has shown that introduction of nitrogen to key points in the system before the pressure has totally decayed will minimize ingress of air. Then, as the system continues to cool, only nitrogen enters, not oxygen-laden air. Key points for nitrogen protection in HRSGs include the evaporator, economizer, and feedwater circuits.
At Terry Bundy, primary power is produced by two GE LM 6000 combustion turbines and two Nooter-Eriksen dual pressure HRSGs (no reheat) feeding a 26 MW steam turbine. Feedwater conditioning is all-volatile treatment oxidizing [AVT(O)], with ammonium hydroxide injection to maintain feedwater pH within a range of 9.6 to 10. High-pressure evaporator chemistry is based on EPRI’s phosphate continuum guidelines, with tri-sodium phosphate as the only phosphate species and control within a 1 to 3 parts-per-million (ppm) range. The HP evaporator pH control range is 9.5 to 10. Free caustic concentrations are maintained at or below 1 ppm to minimize the risk of caustic gouging.
Following early operation of the combined-cycle units, plant personnel discovered oxygen pitting in one of the high-pressure evaporators. The first step to mitigate this issue was installation of a nitrogen blanketing system in 2005. One question that often arises is how best to store or generate nitrogen. Certainly it can be provided from nitrogen bottles provided by local gas-supply or welding firms, and liquid nitrogen is another possibility. LES personnel selected a different method, nitrogen generation via a pressure-swing adsorption (PSA) system.
The process utilizes a carbon molecular sieve (CMS), which, when compressed air is introduced at high pressure, adsorbs oxygen, carbon dioxide, and water vapor, but allows nitrogen to pass through. Obviously, the nitrogen can then be collected in receivers for use as needed. At a pre-selected interval, pressure is released from the unit allowing O2, CO2, and H2O to desorb from the material, at which time these gases are vented off to the atmosphere. The table below outlines nitrogen purity from this system as a function of production rate.
The Terry Bundy N2-generator applies nitrogen, at a pressure of 5 psig, to the LP and HP drums during wet layups, and the nitrogen is utilized to “push” water from an HRSG during dry layup draining. A nitrogen pressure of 5 psig is maintained during the dry layup, provided no major tube work is required. An obvious major concern with nitrogen blanketing, and the reasoning behind its rejection at some plants, involves safety. Of course, elemental nitrogen is not poisonous, as it constitutes 78 percent by volume of our atmosphere. However, an individual who enters a confined space where nitrogen has not been purged may pass out nearly instantaneously due to lack of oxygen. Death can occur within minutes.
An alternative to pressure swing adsorption is gas separation by membrane technology. In these systems, compressed air flows along specialty hollow fiber membranes. The material allows oxygen and water to pass through each membrane, but N2 does not penetrate and can be collected at an outlet port. The literature indicates that this process can produce 99.5 percent pure nitrogen.
Another important point with regard to wet layup chemistry is periodic water circulation. This minimizes stagnant conditions that can concentrate oxygen in localized areas to cause pitting.
Both Terry Bundy HRSGs have circulating systems installed on the high-pressure and low-pressure circuits for use during wet layups. Each circuit utilizes one of two redundant pre-heater recirculation pumps, which normally are in service during HRSG operation to mitigate acid dew point corrosion of external circuits. Each pump provides approximately 100 gpm of flow per circuit. Valves and piping have been added to provide to allow for seamless transition from layup circulation to normal operation. Sample/injection systems are available to allow operators to test the layup chemistry for pH and dissolved oxygen (using colorimetric ampules), and to inject ammonium hydroxide if the pH needs to be raised. Also, modifications made in each boiler drum allow the layup water to bypass the drum baffle, promoting circulation and minimizing short-circuiting via the downcomers. The pumps are typically started once drum pressure is less than 50 psig, and remain in service for the duration of the layup.
|A PSA nitrogen generator.
Dissolved Oxygen Removal from Condensate and Makeup Water
Very often, demineralized water is stored in atmospherically-vented storage tanks. Thus, oxygen-laden water enters the steam generator during normal operation and even more critically during boiler filling operation. In the latter case, the influx of cold, oxygen-saturated water can cause severe difficulties. A possible method to minimize this problem is to limit oxygen ingress to storage tanks, but this is typically a difficult proposition. Terry Bundy personnel selected another gas transfer membrane technology to treat condensate return and makeup water.
The process is similar to the gas-gas transfer membrane process outlined above, but in this case the carrier is water. As the liquid flows along the hollow fiber membranes in the vessel, gases pass through the membrane walls but the water is rejected. The technology is capable of reducing dissolved oxygen concentrations to less than 10 parts-per-billion (ppb). Most importantly, the system eliminates introduction of air-saturated makeup (where the oxygen concentration may be 7.5 ppm, which is 75 times the recommended limit) during boiler fills.
Don’t Forget the Steam Turbine
During my (author Buecker) 30-plus years in or affiliated with the power industry, I have seen many instances where the condenser hotwell was allowed to remain moist, or even contain standing water, during outages where the condenser vacuum was broken and air entered the condenser and LP turbine. The combination of a moist-laden atmosphere and the salt deposits that collect on LP turbine blades during routine operation can be quite damaging. Pitting and stress corrosion cracking (SCC), two very harmful mechanisms, are two of the potential outcomes.
A very practical method to combat this corrosion, and one that has been adopted at Terry Bundy, is desiccated air injection to the condenser during all but short-term layups (<72 hours).
This system is capable of providing 700 standard cubic feet per minute (SCFM) of 100oF air at 10 percent humidity to the condenser and low-pressure turbine. This flow can lower the relative humidity from nearly 100 percent to less than 30 percent in just a few hours.
In 2005, drum inspections showed significant pitting. It was after this inspection that the changes outlined above were implemented. A repeat inspection in 2008 showed no new pitting.
Iron level monitoring, via particulate collection on 0.45-micron filters showed a significant decrease in samples from the condensate pump discharge (CPD) and both LP and HP drums. Quicker startups are now commonplace. Main steam cation conductivity drops to within the recommended guideline (0.2 µS/cm) up to 1.5 hours sooner, and CPD cation conductivity consistently remains below 0.1 µS/cm, whereas in the past it might climb as high as 0.45 µS/cm. Of significant importance is that the units can be left in wet layup for extended periods, which saves the plant six hours (over dry layup) to reach full load per a dispatchers request.
1. Cycle Chemistry Guidelines for Shutdown, Layup and Startup of Combined Cycle Units with Heat Recovery Steam Generators, EPRI, Palo Alto, CA: 2006, 1010437.