The pandemic has been challenging for many small brewers. Although draft beer sales in the US were down more than 40% in 2020, the number of craft breweries increased to an all-time high of 8,764. The popularity of these smaller breweries shows their resilience to weathering the worst of economic conditions. Although each brewery has a unique brewing style and operation, they all require the use of steam to brew. Proper boiler operation and maintenance is important to ensure an uninterrupted steam supply during brews. However, due to how small breweries operate, effectively managing oxygen corrosion can be very challenging.
Steam Use in Breweries
Steam is an essential utility in breweries. Its primary use is in the brewing process, but steam is for other functions such as sanitizing the equipment between brews, keg washing, and pasteurization. Steam demand varies with the brewing schedule, which means that boilers are often sized for peak demand and can sit offline when no brewing operations take place. Boilers that sit offline for even short period of time can experience serious corrosion problems due to dissolved oxygen.
Oxygen Corrosion in Boilers
Dissolved oxygen in boiler systems can cause localized pitting corrosion on carbon steel metallurgy that not only reduces the life of expensive capital equipment but can also lead to unexpected failure and production losses. The potential for oxygen corrosion problems is a function of the steam system design, operating schedule, and water treatment practices.
The makeup water added to replace condensate and blowdown water losses is an ongoing source of dissolved oxygen in steam systems. At 60ºF, the makeup water can contain up to 10 ppm of dissolved oxygen, which must be completely removed to prevent oxygen pitting corrosion problems in the feedwater, boilers, and condensate return equipment. The associated corrosion by-products can also form insulating iron deposits inside the boiler that will increase the fuel demand and can even lead to metal failure due to overheating.
The majority of this dissolved oxygen is removed by heating the feedwater in either a deaerator (a pressurized vessel specifically designed for oxygen removal) or a simple feedwater tank. Regardless of whether a deaerator or feedwater tank is used, an oxygen scavenger such as sulfite must also be added to chemically remove the remainder of the dissolved oxygen. The smaller boiler systems found in craft breweries are usually designed with a simple vented feedwater tank rather than a deaerator. Because feedwater tanks are much less efficient for oxygen removal, the boiler system can be more prone to oxygen corrosion problems.
The intermittent brewing schedule also poses challenges for corrosion control. The vacuum created as the water in an offline boiler cools can introduce large amounts of dissolved oxygen into the boiler water. This oxygen can quickly consume any residual oxygen scavenger left in the boiler, resulting in pitting and corrosion inside the feedwater tank and boiler. Breweries that only require steam for a short duration during the day and leaving the boiler idle can be prone to severe corrosion problems.
Strategies to Prevent Oxygen Corrosion
Proper boiler design and operation can go a long way in improving corrosion control inside a boiler. On the design front, installing a deaerator can reduce the feedwater dissolved oxygen to less than below 10 parts per billion. A chemical oxygen scavenger such as sulfite is still required to completely remove dissolved oxygen from the feedwater, but pairing a deaerator with an oxygen scavenger will greatly reduce the amount of chemical used. In systems with feedwater tanks, a thermostatically controlled steam sparge should always be installed to keep the feedwater temperature > 180ºF. This will allow more efficient oxygen removal and reduce oxygen scavenger usage.
Operational and water treatment practices must also be considered to address oxygen corrosion problems. This is especially important in small breweries, since steam is only necessary for a few hours during the week, and during which the remainder of the time the boiler is offline filled with water. The vacuum that is created when a hot boiler is taken offline can result in high concentrations of corrosive dissolved oxygen in the boiler water that can concentrate at water/metal interfaces, depleting oxygen scavenger residuals near the boiler tubes while maintaining adequate levels in the rest of the bulk water.
Operators should ensure that the proper boiler shutdown and startup procedures are strictly followed. Increasing the sulfite oxygen scavenger concentration to a minimum of 100 ppm prior to shutdown can increase protection against oxygen corrosion while a boiler is offline for short periods of time. Where oxygen corrosion in offline boilers cannot be addressed with higher sulfite residuals alone, consider keeping the boilers hot and pressurized by operating on low fire or by installing an external pump with piping connections to prevent stratification of the sulfite in offline boilers.
Although maintaining high oxygen scavenger residuals can help mitigate corrosion inside an offline boiler, it may not be a complete solution. The scavenging of oxygen by sulfite is not an instantaneous reaction and requires a minimum residence time for complete oxygen removal. Since sulfite is non-volatile, it also does not prevent corrosion from occurring due to oxygen in the steam distribution and condensate return piping.
New innovative treatment products based on filming amine technology are available to address the shortfalls of traditional water treatment approaches, especially in boiler systems that operate intermittently. Contact your local Chem-Aqua rep to determine the best approach to protecting your boiler system and whether these new methods may be a solution for you.
Factors Contributing to Oxygen Corrosion in Steam Boilers
Written by: John Bychkowski