Gas-side deposits and corrosion of HRSG heat-transfer surfaces are inevitable and are a common cause of reduced steam production, low steam temperatures, and degraded gas-turbine performance. These effects contribute to reduced electricity production and lost revenue. Thus it makes good business sense to include removal of HRSG gas-side deposits in your plant’s annual maintenance program.
President Chris Norton and VP Randy Martin of Environmental Alternatives Inc (EAI) told the editors that cost-effective cleaning is guided by the monitoring and trending of such key performance indicators as GT backpressure, steam production and temperature (for each pressure level), and stack temperature, and then comparing the data collected to corrected plant design conditions. Plant heat rate and output also should be tracked, they said.
Careful review of the data can provide advance warning regarding the location and amount of fouling, and the rate of deposit formation within the HRSG. This information allows the owner to determine precisely when an outage for tube cleaning is economically justified. Norton and Martin believe HRSG cleaning should be done when the gas-path pressure drop across the HRSG reaches 3 to 4 in. H2O over the “new and clean” condition—generally speaking, that is.
Cleaning media: the options. Once the need for cleaning has been established and an outage date determined, the next step is to select the optimal cleaning technology. The standard options are high-pressure water blasting, grit blasting, acoustic cleaning, and CO2 blast cleaning. The experts recommended that the plant owner carefully consider the pros and cons associated with each cleaning option before making a final selection.
High-pressure water can be effective but may also have the undesirable side effect of a water/deposit interaction that creates an acidic environment and accelerates tube corrosion. It also could turn the water/deposit mixture into a concrete-like substance when the plant is restarted. Important to remember is that this cleaning technique is limited to line-of-sight deposits and the high-pressure water may push removed deposits further back into inaccessible regions of the HRSG.
Unless carefully performed, high-pressure water blasting also can quickly damage insulation that is extremely difficult to access for repairs and/or may erode some tubes or damage tube fins. Finally, contaminated water is difficult to contain and may require expensive waste disposal, if determined to be a hazardous waste.
Grit blasting, also limited to line-of-sight cleaning, can quickly thin the tube wall or damage fins if not carefully performed by experienced technicians. Unfortunately for the plant owner, thinning of tube walls is not obvious during cleaning but will become apparent when the rate of tube leaks increases in the future. Like high-pressure water blasting, a large amount of waste material is produced, some of which may be classified as hazardous and require special (read “expensive”) handling and disposal.
Owner/operators report mixed results when using sonic horns for deposit removal, particularly in the cold end of the HRSG. Acoustic energy is ineffective for removing ammonia salts and baked-on deposits.
CO2 pellet blasting, the remaining option for HRSG cleaning, is the only alternative that is non-destructive and produces no secondary waste products. This dry process does not contribute to corrosion or erosion of heat-transfer surfaces. Just as important to the owner, deep cleaning between tubes can be performed.
CO2 blasting with small dense pellets (Fig 1) penetrates deposits and completely cleans modules located deep within the HRSG, eliminating the time and expense of mechanically spreading tubes to access tubes not in the technician’s line-of-sight. It has been proven by over 20 years of industry experience and is recognized by HRSG manufacturers as a cleaning best practice.
The general cleaning process is illustrated in Fig 2. CO2 pellets are fed into a portable machine that is connected to a high-pressure compressor. The pellets are educted into the air stream and propelled through a hose to a specially designed nozzle that propels them at speeds up to 1000 ft/sec. The pellets exit the nozzle and penetrate the debris layer on the surface being cleaned.
CO2 sublimates once the pellets penetrate the deposit. During sublimation at atmospheric conditions, the CO2 pellets undergo a transformation from a solid directly to a gas, unlike ice that must first melt into liquid water before evaporating into vapor form. When CO2 sublimates from a solid to a vapor, it expands 750 times in volume creating a “mushroom” effect inside the deposit that lifts and removes deposits from metal surfaces. A HEPA vacuum system collects the debris. Typical HRSG tubes shown prior to (left) and following (right) CO2 blast cleaning illustrate cleaning effectiveness (Fig 3).
High-density CO2 pellet production is the cornerstone of EAI’s cleaning process, Norton and Martin said. Manufacturing pellets onsite assures the quality and density for maximum cleaning effectiveness. Pre-made pellets from an offsite dry-ice vendor usually are 24 to 48 hours old before they are used and will have already experienced a loss in density. Lower-density pellets begin to sublimate in the hose and “soften.” Soft pellets are a less-effective cleaning medium because they are less able to reach the bundle interior.
Onsite production of high-density CO2 pellets is possible by using a completely self-contained mobile support trailer (Fig 4). The trailer houses a 350-psig air compressor, air dryer/after-cooler (for clean instrument grade air with low moisture content), a liquid CO2 storage tank, a pellet conversion unit, and all necessary support systems for direct connection to onsite power. The trailer also carries all the necessary tools, personal protection equipment, and other safety gear to the plant.
The true effectiveness of high-density CO2 pellet blast cleaning becomes evident when comparing pre- and post-cleaning plant performance data—such as pinch points, steam flow, heat rate, fuel consumption, pressure drop, and unit power. In the case study below, the performance restoration experienced after cleaning is presented.
Case history. Monitoring performance data at a nominal 500-MW combined cycle in the Northeast is part of the plant’s ongoing HRSG maintenance and cleaning program. The data collected are used to develop performance trends and to estimate the power output that can be restored by cleaning. A simple economic analysis compares the value of lost power sales revenue when running with a fouled HRSG, with the lost revenue incurred for an outage and the cost of an HRSG cleaning. This analysis helps the plant owner decide when a cleaning should be scheduled.
Data collected from the plant historian before and after an HRSG cleaning is shown in the table. The capacity restored as a direct result of the cleaning was 1120 kW. This plant normally operates at a 90% capacity factor and sells power into the market at 3.5¢/kWh off-peak, a conservative price. Assuming the plant can sell the additional power generated, the gross savings resulting from the restored power is about $309,000 annually. The owner’s payback for the HRSG cleaning is a matter of weeks.
HRSG tube cleaning shows positive impact on plant performance
HRSG performance data
HP pinch point, F deg
Another approach to determining the value of an HRSG cleaning is to calculate the fuel saving that occurs when a plant runs at a fixed power output. In that situation, the fuel saving is a function of the plant’s improved plant heat rate. If the HRSG gas-side pressure drop increases by 4 in. H2O because of fouling, the resulting heat-rate increase can be determined from plant-specific design data.
For this case, the heat-rate improvement is approximated as proportional to the power restored (1120 ÷ 500,000) or 0.22%. Assuming a typical 500-MW combined cycle has a gross heat rate of about 7000 Btu/kWh, the heat-rate restoration is about 16 Btu/kWh. If fuel is purchased at $3.50/million Btu then the annual fuel savings for the improved heat rate is about $220,000.