Tips for HRSG Maintenance and Upgrade Success

The heyday of combined cycle plant construction (especially in the U.S.) was at the start of the millennia. Between 2000 and 2005, about 130 GW of natural gas–fired combined cycle plants opened in the U.S., according to the U.S. Energy Information Administration (EIA). Since then, the annual totals have been more modest. But the average per year has remained steady in the 4 GW to 8 GW range, with some years surpassing 15 GW. In 2023, for example, 16 U.S. natural gas–fired power plants came online totaling 8.6 GW. That’s up from 11 in 2022, amounting to 5.6 GW. The EIA expects another 20 new natural gas–fired power plants to come online in 2024 and 2025.

A steady built-out of new facilities is good news for heat recovery steam generator (HRSG) manufacturers. And with the bulk of the combined cycle fleet now aged 20 years or more, there are plenty of plants around that are in urgent need of maintenance, repair, and upgrade work. Here are some key points to consider in the maintenance and upgrading of HRSGs.

HRSG Logistics Are Challenging

HRSGs are big. Transporting them to the site requires careful consideration to ensure that they can be taken to the facility and moved to where they will be installed in the plant. To reduce transportation difficulties and make it easier to get the HRSG where it needs to be, some manufacturers deliver the different HRSG modules and components in stages to reduce clutter and avoid taking up too much space on the ground. After all, space is almost always an issue in any plant. Designs are typically compact. There is limited space to integrate the HRSG into the building or site layout. Advance planning and plenty of homework is needed to scope out the layout of the facility and determine how much room is available for the HRSGs. The design must be tailored to fit the available space and minimize plant modifications.

1. Space can be tight at plants. Plan appropriately for staging and movement of components on job sites. Courtesy: Rentech Boiler Systems

Unforeseen problems often crop up once the equipment arrives onsite if planning is perfunctory. Large parts may not clear entrances, tight turns, or other headway restrictions. When the equipment arrives at the combined cycle location, there needs to be enough space (Figure 1) to stage and assemble the HRSG. Those working on HRSG installations, upgrades, or repairs need to have enough room allocated to them to do the work and sufficient space for components to be stored and workers to use as an onsite office.

Other Items to Stay Abreast Of

Having a solid plan is not only important during HRSG installation, upgrade, and repair projects, it’s also essential throughout the life of a unit. Consider placing a keen focus in the following areas.

Maintenance. HRSGs must deal with high temperatures and high steam pressures. They are pounded by hot, pressurized water and steam constantly. Components must be able to withstand extremes between hot and cold conditions. Not surprisingly, the most poorly maintained part of any HRSG tends to be the pressure vessel. The pressure vessel needs to be well maintained to preserve performance and longevity. Failure to do so could lead to unscheduled plant shutdowns.

Testing and Chemistry. Go to any HRSG user event and a lot of the sessions concern HRSG chemistry. Water quality and water chemistry play an essential role in HRSG health and output. Operators need to take great care to maintain their HRSGs within the parameters laid out in manufacturer guidelines. That means regular testing, analysis, and tuning to counter any degradation issues detected. Corrosion, deposition, and other problems can be found early if regular inspections are being done and the right water treatment steps are being taken as part of HRSG maintenance schedules.

Hardware Inspections. As well as maintaining water chemistry, plant operators also need to keep a close eye on all HRSG components, piping, valves, vents, and fittings. Components will degrade over time. Even if well maintained, they will eventually need repair or replacement. Look out for rust, leaks, and other damage. Poorly operated units sometimes need tube replacements in as little as a couple of years. Properly maintained units, on the other hand, can run well for decades.

Once inside the unit, those maintaining HRSGs should ensure that heat transfer surfaces are cleaned to remove calcium, magnesium, silica, rust, and any other material coating the water sides of the heat transfer tubes. Failure to do so lowers thermal conductivity, retards heat transfer, produces overheating, and eventually leads to tube failures. All it takes is a 1/64-inch layer of iron and silica scale to produce a 3.5% fuel loss in an HRSG. Yet, a poorly maintained HRSG with poor water chemistry can build up that amount of scale quickly.

Complying with Emissions Regulations. HRSGs are often upgraded as part of a plant’s efforts to adhere to stricter environmental regulations. HRSG maintenance actions should pay attention to how well the facility is dealing with the emissions limitations imposed upon it by local air quality districts. Those HRSGs that already utilize supplemental firing or wish to add this feature need to carefully monitor emissions levels and understand how changes may impact NOx and CO. In some cases, selective catalytic reduction (SCR) for NOx and oxidation catalysts to reduce CO may be needed to fulfill permitted emissions requirements. For other HRSGs, burners should be considered that are designed to lower NOx output. The right burners can sometimes mean that not as much reagent is needed for SCR. When space is too tight for a full-sized SCR, a burner retrofit may make it possible to use a more compact SCR because it doesn’t need to deal with the higher mass flow or higher NOx levels.

These are just a few pointers that can help an HRSG to run more efficiently and that can avoid unforeseen difficulties during HRSG maintenance and upgrade projects. By adhering to them and other maintenance best practices, operators can look forward to long life for their HRSGs.

Kevin Slepicka is vice president of Heat Recovery Boilers at Rentech Boiler Systems.

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