How to Defeat Dew Point Corrosion in Heat Recovery Steam Generators

The experience of South Humber Bank Power Station in the UK is just one example of overcoming corrosion challenges at power generation facilities. Facing issues during routine maintenance shutdown of heat recovery steam generators (HRSGs), South Humber began using VCI (Vapor Corrosion Inhibitor) technology to solve their dew point corrosion problem in 2009. Although it relies on a longstanding chemistry applied in industrial layup and preservation situations around the globe, this highly effective and efficient technology is still unfamiliar to some. Indeed, many power facilities are unaware of the advantages they miss by opting for default layup methods of nitrogen blanketing or dehumidification instead. Comparing these layup methods with Cortec VpCI-337 fogging fluid in light of the last decade of successful use at South Humber Bank Power Station (Figure 1) presents a clear case to recommend VCI technology to other power plants, as well.

Maintenance Shutdown and Dew Point Corrosion

Dean Kenny is a corrosion engineer at Lake Engineering Solutions, supplier of VCI to South Humber Bank Power Station. He explained the main corrosion issue for the five HRSGs installed at South Humber: “One problem they were experiencing was failure of the top tube banks on the HRSGs in service as an effect of dew point corrosion. The HRSGs’ tube banks at South Humber Bank Power Station … run horizontally, and this made the top row of tube banks [Figure 2] on each of the five HRSGs only reach around 80 degrees Celsius [176F]. This would cause these tube banks to fail in service due to dew point corrosion.” South Humber periodically needs to shut down the HRSGs for offline maintenance or extended layup and required a solution to avoid corrosion problems at startup.

Comparing the Preservation Options

There are several ways that power plants can address dew point corrosion or other causes of corrosion during layup. One option is to apply a dehumidification system, which removes moisture to eliminate a key cause of corrosion by keeping the boiler internals dry. Kenny explained that while this method is known to be effective, one of the downsides is the potential for corrosion when the system either fails or is turned off. Corrosion can also happen during draw-down, the period of time in which the system is gradually drying out.

Nitrogen blanketing is another option that can also be effective, but with costly and potentially dangerous side effects. The basic theory is to eliminate corrosion by filling a void completely with nitrogen to eliminate all oxygen, thus taking out a critical ingredient of the corrosion cycle (oxygen + iron + electrolyte). However, there are a number of drawbacks. For one reason, nitrogen blanketing requires an airtight seal. If the opening is breached and the nitrogen leaks, the nitrogen must be replaced all over again.

Kenny pointed out that these leaks could be extremely dangerous because nitrogen is a heavy gas that settles and is a silent killer. If a person enters an area filled with nitrogen, unconsciousness and asphyxiation could overtake them before they know what is happening. Nitrogen is also very costly, especially for large volumes. According to Kenny, the monthly cost for nitrogen blanketing of a steam-side system at one UK cogeneration plant was £2,000 per month, adding up to £48,000 for two years of preservation.

A third option is Vapor-phase Corrosion Inhibitor (VCI/VpCI) technology, based on salts of amine carboxylates. These corrosion inhibitors adsorb onto metal surfaces, forming a protective molecular film that inhibits the natural corrosion reaction from occurring even in the presence of oxygen and an electrolyte, such as moisture (Figure 3). Unlike dehumidification and nitrogen blanketing, these Vapor-phase Corrosion Inhibitors do not require the complete absence of moisture or oxygen in order to be effective, vastly improving the practicability of this protection method.

Another beneficial characteristic of VCI technology is the ability of these molecules to vaporize and disperse themselves throughout an enclosed space to reach even small or difficult to access surfaces. They can be applied in many formats, such as powders or liquids, but the most commonly recommended form for large voids such as HRSGs is a waterborne fogging fluid, which is able to travel a significant distance on its own but can also be assisted by placing a fan at the opposite end of the void to draw the vapors through.

The HRSG should remain closed during the preservation period, but constant pressure and an airtight seal is not required as for nitrogen blanketing. Further, VCI technology often does not need to be removed before equipment operation, making startup simple. Although VCI technology can be used in conjunction with dehumidification, if desired, it continues to work independently even if the dehumidification system fails or is turned off.

Compared to nitrogen blanketing, VCI technology is also extremely cost effective. Kenny reported that the cost to protect a full system with VCI for two years at a UK cogeneration plant was less than half the cost of only protecting the steam side with nitrogen blanketing. VCI protection of the steam side only was estimated at approximately 6% of the nitrogen blanketing cost (Table 1).

History of VpCI Use at South Humber

Starting in 2009, South Humber began treating their HRSGs with VCI fogging fluid (Figure 4). Whenever any of their five HRSGs were going offline, they would contact Lake Engineering/HITEK-nology to fog VpCI-337 into the top tube banks. Kenny said that this solved the problem, and “they now experience zero failure because of corrosion.” Mark Cresswell, operations director at HITEK-nology, said they have applied VCI inside South Humber HRSGs on 13 separate visits from 2009 to October 2021, typically laying up two HRSGs per visit.

While VCI fogging is much safer, simpler, and less costly than a technique such as nitrogen blanketing, it does involve careful preparation for project success when working with large industrial equipment and confined spaces. The first step HITEK-nology takes is to do an onsite survey of the HRSGs and look at equipment drawings to determine proper product dosage. Cresswell, who has worked on the project multiple times, explained, “We have to provide a full job pack, including method statement, COSHH [Control of Substances Hazardous to Health] assessments, manual handling assessments and general risk assessments, and where applicable, confined space entry forms.” He said that confined space entry is required at South Humber (Figure 5), though not at all plants, as some boilers can be treated from the outside through access doors.

For cases like South Humber, Cresswell and others are specially trained to safely work in confined spaces. This involves wearing standard personal protective equipment (PPE), personal gas monitors, and anti-explosive radios. They have throat microphones and earpieces in order to stay in constant communication with their outside contact. Fortunately, they do not have to worry about inadvertent nitrogen gas exposure at the same time.

Since the upper banks of the HRSGs are approximately 10–12 stories high, the first half-day of the project is required simply to haul the product and equipment up to that level (Figure 6). The rest of the application takes about one day. Other than confined space concerns, application is relatively simple using airless spray equipment. HITEK-nology also installs corrosion coupons inside and outside the HRSGs for periodic corrosion monitoring. The good news is that protection is effective, and no removal is needed at startup.

South Humber has been very happy with the service and the results. After a recent 12-month layup of two HRSGs (in combination with a redundant dehumidification system at South Humber’s request), their mechanical engineer, Malcolm Fleming, had this to say: “This technique had been used by our station some years ago during another long-term preservation period and was very successful in reducing any corrosion within the unit and eliminating … any corrosion dust … at start-up. The service engineers from HITEK-nology who applied the solutions have worked in this field for many years and as per usual worked very safely, with well-controlled chemical control, and completed the works in a very safe and timely manner, providing well-documented safety information.”

Positive responses like these are very common among VpCI-337 users, who are often pleasantly surprised at its ease of use and effectiveness (Figure 7). Kenny commented, “A lot of people think it’s a myth, and it’s not.” He said that many people mistakenly think VpCI-337 is a new technology or something that will not really work. However, once they try it, he finds that they do not go back to the old ways.

Freedom from Constraints of Dehumidification and Nitrogen Blanketing

Power plants will always have temporary layup needs for routine maintenance or other purposes. The threat of corrosion remains, but what can change is the methodology used to lay up HRSGs or other equipment with large internal voids. Power plants are no longer bound by the constraints or downsides of dehumidification or nitrogen blanketing. Instead, they can opt for the cost, safety, and functionality benefits of VCI technology as South Humber Bank Power Station has done to successfully address HRSG dew point corrosion.

—Julie Holmquist is marketing content writer at Cortec Corp., manufacturer of VpCI-337 and many other VCI technologies. Special thanks to Dean Kenny (Lake Engineering Solutions) and Mark Cresswell (HITEK-nology Solutions) for sharing their experiences and photos of HRSG preservation at South Humber Bank Power Station.