HRSG Health: Efficiency and Performance

FROM THE FEBRUARY ISSUE: Plants built in the ethanol construction peak are approaching 10 to 15 years of operation, and many are exploring whether to repair or replace. HRST Inc. has some advice.
By Evan Almberg | January 18, 2019

The thermal oxidizer (TO) heat recovery steam generator (HRSG) is a staple system for many ethanol plants built during the mid-2000s construction boom. As plants near the 10- to 15-year operating mark, performance and efficiency might be the deciding factor between repairing or replacing equipment. Routinely inspecting and quantifying performance can improve the reliability and operation of an existing HRSG, extending the life of the system.

Inspection Priorities
Assessing the condition of the HRSG is a critical first step in determining its overall health. Having a comprehensive water- and gas-side inspection helps establish a baseline condition of the HRSG that can be compared to future inspection findings. Comparing can be qualitative and simple, “The tubes appear more oxidized than last year;” or more quantitative and complex, “The material thinning in this location is 1/16 of an inch and last year it was at 1/32 of an inch.”

A variety of locations should be observed on the water and gas side. Knowing at-risk areas can help determine where to look and when. Inspection areas for a typical TO/HRSG configuration include the HRSG inlet from the TO, the evaporator section of the HRSG, steam drum connections and internals and the packaged economizer. Important items and components to observe in these locations are: liner and baffle condition; tube-to-header connections/welds; external tube fouling, corrosion and oxidation; internal tube deposits or material loss; steam separation equipment and configuration; and overall as-built design (pipe size, configuration, material).

Common Component Issues
Temperature is a key factor in how efficiently the HRSG components operate. In HRSGs that are fired harder (e.g., a TO burner that is pushed to its maximum firing rating), metal components often exceed the recommended design limit temperature. Liners, baffles and tubes are all susceptible to overheat.

Overheated liners might oxidize, warp or break off studs when thermal expansion causes them to distort. As the liner sheets distort and expose insulation, the high temperature turbulent gas flow can wear away the insulation and cause hot spots on the HRSG.

Baffles become brittle and degrade over time as they overheat, eventually allowing the flue gas flow to bypass the tube bank, resulting in lower HRSG performance. In the case of bypass along the sidewalls of the evaporator, the increased flow along the panel end tubes will disproportionately cause tubes to overperform and generate more steam than the tubes toward the center of the panels.

The evaporator tubes are less susceptible to overheat damage, but the fin material will begin to oxidize and break off until the fin tip temperature is at the material threshold. While this is not a reliability concern, it can lower the performance of the evaporator. Tube leaks or failures often occur at the tube-to-header connection. This can be caused by high stress at the weld joint, often related to the geometry of the tube-to-header connection, but sometimes because of flow conditions on the water or gas side. Performing a gas-side inspection at the beginning of a shutdown can allow more time for tube leaks to be identified and repaired. Depending on the location within the bundle, this could be a time-consuming process. When tube failures occur, a root cause failure analysis (RCFA) should be performed to determine the failure mechanism, in addition to implementing a preventative operation or maintenance procedure to mitigate future failures. Don’t just weld the crack; involve someone who can review the location, failure and maintenance history.

Fouling is also a performance concern. As fouling buildup occurs, the tube surfaces become insulated from the gas flow, which decreases steam generation, increases backpressure and reduces thermal performance. In a TO/HRSG system behind a DDG dryer, acid dew point (sulfur dew point) corrosion might be a bigger issue than just tube-side cleaning during a shutdown. Exhaust flow with sulfur content can dramatically increase the dew point corrosion from condensation on the tubes, specifically in the cookwater economizer and preheater sections of the HRSG. This can be mitigated by process changes to the feedwater temperature or by upgrading to a corrosive-resistant tube material.

Thermal Performance
Getting the most from the HRSG can be a driving factor for plants that need process steam. HRSG thermal performance is affected by both gas- and water-side parameters, as well as the mechanical condition of the HRSG. Degradation and fouling reduce the steam generation rate, whereas upstream process changes—such as dryer optimization or harder firing of the TO burner—can cause the HRSG to exceed rated boiler capacity.

Performance can be determined through a thermal model of the HRSG by using mechanical and process data to identify areas of under and overperformance. A thermal model can be used to compare the as-is performance to the original equipment manufacturer’s predicted values, as well as to quantify the effects of degradation and changes to process operating conditions.

Quantifying Underperformance
Underperformance is often a result of gas-side fouling, flue gas bypass around heat transfer surfaces, as well as degradation and typical wear and tear of the HRSG. Gas-side fouling is a common issue, particularly in process plant conditions, that reduces the heat absorption effectiveness. Fouling can depend on both the flue gas composition and the process conditions of the HRSG.

Gas-side cleaning, such as dry ice blasting, is often performed during an annual shutdown. But it might not be the best approach. A thermal model and a performance degradation tracking procedure can help determine when cleaning is necessary or economically advantageous to gain back the heat absorption of the HRSG. If cleaning is warranted, it’s important to clean all the way down to surfaces deep into the tube bank. A thermal model can show before and after.

Predicting Upgrade Performance
An overperforming HRSG can impact the HRSG and downstream equipment such as a letdown turbine, and raise safety concerns. Overperformance can occur because of a process change on upstream equipment or an upgrade of HRSG components themselves. Examples of this include increasing the feedwater temperature to prevent preheater dew point corrosion or max firing the TO burner to increase steam generation.

Before a change is made, a thermal performance assessment of the HRSG system should be conducted to evaluate what the possible effects could be. Key checkpoint areas should include: tube metal and nonpressure part temperatures; design pressures and safety valve capacity; material selection and operating environment; changes in gas-side backpressure; steam separator capacity; and boiler feed pump capacity.

Upon evaluation of the upgrade performance, codes often allow rerating the HRSG to the higher steam generation rate, and in some cases, higher pressures or temperatures, although the latter two require extensive pressure part evaluation. Rerating can involve new nameplate documenting, the new limits to the steam flow, plus modifications to satisfy standards of the American Society of Mechanical Engineers and National Board Inspection Code.

Whether overperforming or underperforming, the HRSG affects many parts of the plant, and vice versa. Inspect and quantify HRSG performance to keep everything running smoothly.

Author: Evan Almberg
Analysis Engineer, HRST Inc.