With more and more steam boilers being asked to provide intermittent service, there is renewed focus on the proper treatment of boiler systems while they are offline.
In the Power industry it is not uncommon for a combined cycle plant to operate less than 30% of the time, meaning the vast majority of the time the boiler is idle. The rise of renewable power assets that have intermittent and sometimes unpredictable availability (wind, and solar) coupled with variable demand for electricity makes plant “cycling” between operation and standby a business necessity. While plant cycling was formerly reserved primarily for natural gas plants, more and more coal plants are being asked to run intermittently as well. To further complicate things, most power purchase agreements are written such that the power plant must be at “full load” in a matter of hours, so additional attention must be given to ensure that this time hurdle can be met. Since most boiler make-up systems do not have the capacity to fill the boiler system quickly enough, most cycling plants must idle while the boiler is full of water. This presents some technical challenges as plant asset protection can conflict with business realities that often results in some compromises. This article will explore some of the options that are available to the boiler operator and weigh the risk factors associated with each.
Having removed heat from the system and lowered the scaling tendency of the boiler cycle water, the primary concern with offline boilers is metal loss, or corrosion. This can occur in several ways. The first and most predominant is dissolved oxygen corrosion. As the water temperature decreases, the dissolved oxygen content in the water will increase provided the water is exposed to oxygen or ambient air. Once oxygenated water contacts mild steel, corrosion will initiate. The other common corrosion mechanism is acidic corrosion caused by stratification or concentration cells that develop due to stagnant conditions. Corrosion of the boiler internal surfaces not only weakens the integrity of the system, but also releases iron oxides into the boiler water. While not a concern when the boiler system is offline, iron scaling can be a concern after start-up as these iron oxides produced during lay-up can deposit on steam generating tubes.
The preferred choice for boiler lay-up is dry lay-up. Removing boiler water/moisture and preventing new sources of moisture from forming are essential to good dry boiler lay-up and is the best way to prevent offline corrosion. This accomplished by draining the water from the boiler while it is still hot and under 25 – 50 psi of system pressure. This is done so that the residual heat can dry the internal surfaces and ensure that all “dead legs” are drained.
Unlike the relatively clean waterside areas, a boiler's fireside can contain deposits of sulfur or other fuel containing contaminants that, when wet, can be very corrosive. Even if the waterside lay-up choice is wet lay-up, dry lay-up is needed for the fireside surfaces. In geographic areas that commonly have relative humidity readings of >30%, a desiccant may be used to prevent dew from collecting inside the boiler, or dehumidified air can be circulated throughout the waterside and fireside areas of the boiler. If a desiccant is used, care must be taken so that the desiccant is completely removed prior to start-up. General rule of thumb for silica based desiccants is to use 10 pounds of silica gel per 1,000 gallons of boiler capacity; however each boiler and system operating dynamics are unique and you should consult your water treatment provider before making any changes to your lay-up program.
Once the water is removed from the system, the primary concern is to keep boiler surface temperatures above the ambient dew point to prevent new sources of moisture from forming and initiating corrosion. Most times this requires some level of isolation from ambient air, so stack dampers or balloons are employed to minimize the influx of rain and humid air on the fireside. One scenario that can develop involves the formation of condensation from cool tube surfaces coming in contact with warm air that can sometimes occur during spring and fall months. This condensation provides the moisture needed to initiate corrosion. In practice, however, dry lay-up, while the most resistant to offline damage, is rarely employed on the waterside of the boiler due to the start-up time it requires.
The goals of wet lay-up are to prevent oxygen and acidic corrosion and have the boiler internal chemistry ready to operate the boiler within operating control limits when called upon for service. To prevent acidic corrosion, it is recommended that the internal pH be elevated to between 10.0 and 11.0 prior to shut down to allow the water to be well mixed. Once offline, the boiler water should be re-circulated to prevent pH stratification and acidic concentration cells from developing. pH elevation can be accomplished with a suitable chemical compatible with your cycle chemistry and metallurgy.
To prevent oxygen ingress, there are two basic methods; hot lay-up and cold lay-up. Hot lay-up is preferred when practical because it reduces the thermal stresses on the system metallurgy but, of course, requires a source of heat. The idea is to keep the system under positive pressure so that oxygen is unable to enter the boiler. If the boiler will only be out of service for several hours, the residual system heat may be sufficient to keep the system under pressure. If an extended lay-up is anticipated, other methods should be considered.
One common method for hot lay-up is what’s commonly known as “cascading blowdown” in which the blowdown from an adjacent, operational or auxiliary boiler is sent to the offline boiler whereas the entry point is in the lower header (or mud drum) so that natural circulation can be achieved. The use of an adjacent boiler as a heat source is dependent on the size of the boiler; as is sometimes the case, there may too little blowdown to supply enough heat to the offline boiler in order to maintain positive pressure on the system during the lay-up process. This, as well as a few other items, will need to be addressed prior to implementation. Other things you will want consider prior to implementation include adequate monitor and control capabilities of both the pressure differential between the two boilers as well the water levels in the offline boiler during the lay-up process.
Another method to achieve hot lay-up is the installation of a heating coil in the lower header (or mud drum). Either steam or electric coils can be used here as well. This is a very effective method of hot lay-up and alleviates some of the concerns with cascading blowdowns. If enough heat can be supplied to the boiler, positive pressure and natural circulation can be achieved.
One last method of hot lay-up that has been used is the injection of steam (called steam sparging) into the offline boiler. While this may be simpler than installing a heating coil, there are several disadvantages of direct steam sparging. First, as with cascading blowdown, water levels in the boiler must be managed as the injected steam will condense and add liquid to the boiler. Second, the steam will dilute the boiler chemistry so chemistry monitoring and chemical additions will be needed in order to maintain the system pH. Finally, selection of the injection point must be taken into consideration as direct steam injection can cause some damage from steam impingement.
If there is not a ready heat source for hot (wet) lay-up, the injection of inert nitrogen gas can be utilized to keep a positive pressure on the boiler. As with hot lay-up, the system pH should be elevated (10.0 – 11.0). A source of nitrogen (purchased tanks or an on-site nitrogen generator) is required and only about 5 psi of pressure, controlled by a pressure regulator, is needed. Usually the regulator is set to inject the nitrogen at 3 psi with a halt at 5 psi. As experience has shown, if the regulator is set above 5 psi, the nitrogen will find a way out of the system, making the nitrogen requirement much higher.
The main disadvantage of cold lay-up is that natural circulation cannot be achieved. It is recommended that a small, low pressure circulating pump be installed to provide the needed circulation. This pump must be isolated prior to start-up as it will most likely not be designed for the normal system operating pressures.
The final widely used, though not always recommended, method of cold (wet) lay-up is to allow the system to drop to ambient pressure and attempt to chemically scavenge the dissolved oxygen from the boiler water. The issue with this method of lay-up is that the levels of oxygen scavenger required to chemically scavenge ambient water is very high, and at lower temperatures the reaction is very slow. In addition, most chemical oxygen scavengers are acidic, so additional pH adjustments are usually required. Finally, EPRI research has shown that reducing conditions can increase the solubility of the protective magnetite layer which could lead to unprotected internal surfaces.