By Anand Gopa Kumar, HRST Inc
Editor’s note: Gas-baffle failures are common yet often overlooked during HRSG inspections and maintenance programs. Most failures are from operational wear, particularly coil-to-coil baffles in the middle of the unit. Failures in evaporators and economizers have the largest negative impact on performance.
Although cycling adds to thermal stress throughout the combined-cycle system, other equipment issues can contribute to thermal stress and decrease performance regardless of the operating mode. One category is gas-pass baffling within the HRSG.
To make a distinction, the gas-turbine exhaust first encounters flow distribution devices, such as perforated plates or guide vanes, within the HRSG inlet duct. They direct the gas flow primarily for optimal performance of duct burners and emissions control equipment.
Properly designed gas-pass baffles are arranged to prevent gas flow from bypassing the tube bundles (superheater and reheater, for example) and to maximize heat transfer.
Baffles are simple yet effective devices. They must also be sturdy and properly arranged to accommodate thermal expansion.
Even small gaps impact performance
Small gaps allowing bypass of turbine exhaust gas are sometimes overlooked in HRSG inspection and maintenance programs, or perhaps in the original design. Properly placed baffles redirect the gas into the intended path thereby allowing the heat-transfer surfaces to absorb the maximum amount of thermal energy from the gas flow (Fig 1). Improper gas baffling can cause bypass around the tube bundle and create damage that decreases performance (Fig 2).
The lack of gas baffles can leave gaps between the tube bundle and the casing wall or between the left and right hand side of the tube bundles (coil to coil). The lack of resistance allows the gas to flow faster through the openings and reduce heat transfer.
The panels with heat-transfer performance most affected by gas bypass are evaporators and economizers. Additionally the thermal energy not absorbed by the evaporators is absorbed by the economizers thereby increasing the potential of economizer steaming and other damage, including flow-accelerated corrosion (FAC) and wall thinning.
The table compares the performance characteristics of an unfired HRSG operating behind a GE 7FA gas turbine in a 1 × 1 baseload combined cycle, both with and without baffling in the gas path. The results were obtained with HRST’s in-house HRSG performance modeling software Performance ProTM.
The performance model was set up for an ideal case where an HRSG with proper gas baffling was compared to the same HRSG with a two-inch gas bypass between the panel and casing wall, and a four-inch gap between the coils throughout the HRSG.
Certain modules, such as the HP evaporators, experience a significant improvement in thermal duty with the addition of baffles to prevent gas bypass. Additionally, modules such as the HP economizer would extract more thermal duty without proper gas baffling as hotter exhaust gas bypassing from the upstream module has more energy available.
These higher gas temperatures could increase the risk of economizer steaming. However, the presence of gas baffles also could cause the superheaters to over-perform, thereby requiring increased attemperator spray for steam temperature control. Overall, the power output of the steam turbine will be increased by a factor of 1.08% or approximately 1 MW.
A similar comparative performance analysis was performed at a 1 × 1 combined cycle in Bangladesh powered by an MHI 701F. Visual inspection of the HRSG revealed significant gas bypass in many of the modules. A comparative performance model was built to determine the effects of adding gas baffles to the bypass areas.
A 1.2-MW increase in power output was predicted for the steam turbine using the performance model with the addition of gas baffles to access lanes near the evaporators. Upon installing the necessary baffles, the customer reported an increase in excess of 1 MW by ensuring proper heat transfer between the tubes and turbine exhaust gas without any bypass.
Potential flow-induced damage
Poor gas baffling in IP and LP evaporator tube banks can lead to an elevated risk of FAC wear in the upper headers of the evaporators. The gas bypass between the casing wall, tubes, and coil-to-coil gaps would lead to tubes in these locations experiencing a higher temperature.
This results in a higher amount of steam being produced in these tubes. The mass flow rates of the steam/water mixture in these tubes will be higher than those in the remainder of the tubes in the same row. A high-velocity steam/water mixture in the LP and IP evaporator will exponentially increase the wear-rate of carbon steel attributed to FAC.
Fig 3 shows a sidewall upper header that was subjected to increased mass flow in the tubes close to the end of the panel. The elevated mass flow of the steam/water mixture created a hole in the header because of FAC wear.
The risk of wear is further increased with the presence of a steam/water mixture discharging into the upper header. Operation at such instances can lead to unforeseen damage to the HRSG and potentially lead to a forced outage because of leakage in the evaporator. Preventive measures—such as proper gas baffling repair and ultrasonic testing readings at the end of upper-header panels—are recommended.
Offline inspection opportunities
Most common offline inspections begin with distribution plates and guide vanes, liner plates and components, tubes and fins, and tube ties. Baffles should be an equally important inspection target in all ducts, access lanes between tube bundles, and crawl spaces above and below headers.
Their condition should be documented and recorded through both photography and measurement. CCJ