Boilers used for steam and energy production, or operations such as washing, are sometimes taken offline for inspection and maintenance, or kept for backup use. Another common time for boilers to be out of service is during shipment or while in storage at a new plant still under construction.
In each of these cases, proper boiler lay-up and preservation is critical to ensuring a good startup experience. Without adequate protection, corrosion can begin speedily and result in many unpleasant surprises for the person who comes to restart the system. Lay-up of previously used boilers can be especially difficult because it is hard to completely drain the water from a system and keep it moisture-free. This increases the chance that some form of corrosion will appear on interior metal boiler surfaces before the boiler is put back into use.
Simple oxidation and rusty film formation, or more invasive pitting, flaking and large tubular rust growths may be the result of unprotected lay-up. Corrosive debris like this is harmful to a boiler system and threatens to clog equipment and cause further problems. For that reason, it is important to have a good corrosion protection system in place to guard against unwelcome surprises.
Problems with Traditional Lay-Up
Even though the need for corrosion protection is well known, traditional lay-up methods are often clumsy, labor intensive or inadequate. Because moisture is one of the key instigators of boiler lay-up corrosion, a common approach is to drain the system and place desiccant or silica inside. This can be a labor intensive and costly process, and silica can be a contaminant for boilers. Desiccant and silica must be cleaned out before restarting the boiler, thereby incurring additional time and cost during reactivation.
Additionally, desiccant has a threshold to the amount of moisture it can absorb. If saturated, the silica creates a corrosion initiation site. When this capacity is reached, it must be replaced by new desiccant to avoid corrosion and allow continued moisture absorption. The higher the environment’s humidity, the more often this cycle must be repeated.
Another well-known lay-up option is nitrogen blanketing. The idea of this method is to displace all of the oxygen in the boiler so that oxidation cannot occur. The system must be evacuated to a low dewpoint before the blanket is applied. Regular inspection also is needed to ensure that the system remains perfectly sealed. If the seal is broken, all of the nitrogen will escape and protection will be lost. The nitrogen must be reapplied in order for protection to continue.
Nitrogen blanketing presents another risk. If the nitrogen has leaked out of a boiler that is in an enclosed area, personnel walking into that room may be overcome by asphyxiation and collapse before they ever realize there is a problem. Sadly, this situation happens too often. Between August 2007 and February 2014, more than 25 people died or were injured in connection with the use of nitrogen. Of those unfortunate workers, 10 lost their lives to asphyxiation while seven more were killed from explosions related to pressurized nitrogen. Because of the risks related to nitrogen, extra steps must be taken to remove the nitrogen blanket before work is done inside the boiler, and then to recharge the system once work is done.
Sometimes, conditions and convenience dictate that a wet lay-up is to be performed. This can be as simple as filling a boiler with water and leaving it. The results can be mixed, perhaps avoiding the growth of large red rust tubules but nevertheless leaving a thin film of oxidation on the inside of the boiler.
One typical water treatment chemical that can be added for protection during wet lay-up is hydrazine, an oxygen scavenger that is dangerous and does not protect the boiler’s interior surfaces above the water level. Hydrazine scores high on the Hazardous Materials Identification System (HMIS) for health, flammability and reactivity. An inside joke among some chemists refers to this as the “laboratory fun scale.” When adding up the value of each component, hydrazine scores 11 out of 12 on this scale. As fun as that may sound, it is no joke when personal safety is concerned. Additionally, wet lay-up with various chemicals like hydrazine requires periodic monitoring to ensure stable pH levels.
Another challenge with wet lay-up is the freezing temperatures experienced by boilers in cold, northern climates. Avoiding the headaches that can come from frozen and bursting lines in these situations requires the boiler to be in a heated room. This may add unwanted expenses or be impractical for systems with associated steam lines that go outside. These factors are important to consider when choosing a lay-up system.
Vapor-Phase Corrosion Inhibitor Technology
An alternative to these challenges is the use of vapor-phase corrosion inhibitors (VPCIs). For instance, dry lay-up of a boiler can be as simple as draining the boiler, throwing in a water-soluble pouch of vapor-phase corrosion inhibitors, and leaving it for up to two years. When it is time to restart the boiler, there is no need to remove the powder. In fact, leaving the powder in the boiler as it is refilled with water can offer further protection as the powder is dissolved and begins to protect in liquid phase.
This multi-phase activity is one of the advantages of vapor-phase corrosion inhibitor chemistry. In the vapor phase, the moderate vapor pressure of the inhibitor molecules allows them to disperse until they are distributed throughout an enclosed space. The molecules are attracted to metal surfaces, where they physically adsorb and form a molecular layer of protection against the attack of moisture and oxygen. In the liquid phase, these ions dissolve and provide protection for metal surfaces in contact with the water treated with vapor-phase corrosion inhibitor. When these liquids evaporate, they carry a measure of corrosion inhibitors along with them. This can lend further protection to metal surfaces in upper void spaces where the treated water vapor condenses. Vapor-phase corrosion inhibitors also have shown protection at the critical interface area where the water and air meet. These characteristics enable vapor-phase corrosion inhibitors to protect both hard-to-reach spaces and areas that may not have been fully drained.
Boiler Lay-Up and Startup Processes
Dry boiler lay-up is usually the easiest preservation route and allows the most vapor action of vapor-phase corrosion inhibitors. As mentioned, it is especially desirable in cold winter climates to avoid the risk of freezing. The vapor-phase corrosion inhibitor pouches can be placed in industrial boiler water drums or on tubes and in mud drums. From there, the vapor-phase corrosion inhibitors will vaporize and disperse to adjacent void spaces, protecting thousands of gallons of boiler area per bag. Vapor-phase corrosion inhibitors also can be fogged as a liquid and drawn through piping and inaccessible superheater systems by vacuum force. The choice will depend on the intricacy of the system.
While leaving a water-soluble pouch of vapor-phase corrosion inhibitors in an empty boiler is the easiest option, the liquid is preferred for especially large and intricate systems such as those at power plants. With a little preparation and the use of a blower to create airflow, vapor-phase corrosion inhibitors can be effectively drawn through larger boilers and associated piping for more effective protection.
Sometimes, wet lay-up is preferred so that backup boilers will be ready to go online at a moment’s notice. While vapor-phase corrosion inhibitor dry lay-up requires little to no preparation before putting a boiler back into service, boilers using vapor-phase corrosion inhibitors for wet lay-up can theoretically be started immediately without even draining the feedwater. If drainage is necessary, the use of environmentally friendly vapor-phase corrosion inhibitors in the water eases disposal.
Vapor-phase corrosion inhibitors in wet or dry lay-up are a good substitute for the use of hydrazine because they create a protective layer that guards against the corrosive elements that attack metal. Hydrazine generally is used to reduce the amount of oxygen in water so that it is not free to produce oxidation in the first place. However, it is possible to replace hydrazine with a low toxicity oxygen scavenger combined with a volatile neutralizing amine that speeds up the passivating process. This type of treatment can be used alone or combined with vapor-phase corrosion inhibitors to work on two fronts of corrosion prevention.
Beyond Seasonal Lay-Up
Vapor-phase corrosion inhibitors are able to protect much longer than is typically needed for seasonal layup. For example, a commercial laundry that wanted to take one of its two firetube boilers offline in a humid environment found the boiler was still free of corrosion and deposition a year after performing dry lay-up with a vapor-phase corrosion inhibitor pouch. Another time, two package boilers and a heat recovery steam generator were treated with vapor-phase corrosion inhibitors at a mothballed power station. Eighteen months later, they were opened, and the steel indicator discs inside were still in good condition.
Vapor-phase corrosion inhibitor protection is a huge advance in the available options for boiler lay-up and the quality of the results. It is highly efficient where swift lay-up and startup are concerned. Vapor-phase corrosion inhibitors turn what can be a complicated process into a simple operation. Harnessing the power of vapor-phase corrosion inhibitor chemistry is therefore a great way to take it easy on boiler lay-up.
United States Department of Labor, Occupational Safety & Health Administration. “Accident Search Results: Nitrogen.” <https://www.osha.gov/pls/imis/AccidentSearch.search?acc_keyword=%22Nitrogen%22&keyword_list=on>. 2 August 2016.
Cortec Corp. “Case History 500.” Cortec case histories. October 2015.
Cortec Corp. “Case History 232. Boiler Lizard Protecting Boiler for Dry Lay-Up.” Cortec case histories. September 2003.
Cortec Corp. “Case History 438. HRSG & Package Boiler Preservation.” Cortec case histories. August 2013.