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EXPERTS DISCUSS CAUSES AND CURES: FOULING, AIR FILTRATON, ONLINE AND OFFLINE WATER WASHING
Compressor degradation is the major cause of output and efficiency loss in a gas turbine. “Degradation reduces the air mass flow and pressure ratio, thus reducing power output,” said Jean-Pierre Stalder, head of new product development for Turbotect Ltd.
Fouling is a major contributor to degradation. According to Stalder; it accounts for much higher losses than aging, for example. He pointed out that enough air passes through the compressor on a 172 MW turbine in one year to make up a column the size of a football field rising 1,320 miles high. Also, that massive volume of air ingested into the compressor would contain 139 metric tons of foulant, assuming 10 ppm average ambient concentration.
But fouling is not the only element to consider when compressor performance declines. Erosion, corrosion, abrasion and damage all play their part (Figure 1). This includes viscous hydrocarbons from flares and exhausts, dust, sand, cement dust, shot blast, iron oxides, and salt crystals as well as cyclic wet and dry conditions. Degradation can also be caused by changes in airfoil and flow path geometry, altered surfaces and shift of machine clearances.
Much of this performance loss is recoverable, and the available tools include online and offline washing, engine adjustments, filtration, component replacement and repairs, said Rainer Kurz, Manager of Systems Analysis at Solar Turbines (and a Myth Buster columnist for Turbomachinery International).
“The particle size most responsible for fouling is 1 to 5 microns,” said Kurz. “Therefore, fouling can be controlled by the quality of the air filtration system.”
As gas turbines consist of many interacting turbomachinery units, the picture can become complex due to various contaminants attacking different parts. Bad air filtration, for example, tends to have more of an impact on the compressor while bad fuel affects the hot section.
“The amount of power or efficiency lost for a given amount of component degradation differs for various ambient conditions,” said Kurz. “Therefore, it is not possible to establish a universal rate of engine degradation that is valid for any condition.”
Kurz offered tips on how to deal with component degradation:
• The impact is more severe on the full load power output than on the full load efficiency or heat rate of the engine
• Except for a reduction in compressor flow capacity or a reduction in gas generator turbine flow, all other component degradations have a larger impact on engine performance at higher ambient temperatures
• Airflow reacts distinctly to most types of degradation
• Monitoring compressor discharge pressure is the most effective way to keep track of fouling
• Traditional safeguards, such as air filtration, fuel, water, steam treatment, process gas strainers, knock-out drums, online and offline washing and cleaning should be applied.
Meanwhile, conventional aging curves describing overall gas turbine performance degradation may be losing their usefulness in the face of the modern trend toward more frequent OEM upgrades and roll-in, roll-out maintenance. According to Andrea Silingardi of Ansaldo Energia, the new approach consists of an evaluation of each aging component; then overall GT performance degradation is analyzed factoring in each single contribution.
“The impact of roughness on compressor performance degradation is higher at reduced aerodynamic speeds,” said Silingardi. “The performance decrease becomes steeper with more than 10 microns of roughness.”
This factor affects blades more than vanes. Further, the mass flow through the compressor decreases mainly in the first three stages.
“Mass flow reduction is driven mainly by front-stage roughness while lowered compressor efficiency is equally driven by all stages of the machine,” he said. “Ansaldo, which has 46 units in its AE94-3A fleet operating around the world, has developed tools to evaluate performance based on observed degradation levels.”
Turbotect studies indicate that inlet guide vanes (IGVs) are the highest contributor to power loss from fouling as they have the longest blade surface and are positioned at the inlet. As water vapor progresses through the stages, water evaporates and the blades become smaller so there is much less impact from fouling. At approximately the fourth stage, the amount of deposition falls to almost nothing. Keeping the IGVs and early stages clean, therefore, is critical.
Stalder made the observation that there are differences in blade geometries between different types and different sizes of gas turbine, and this can also have an influence on fouling rates and the impact of fouling. But in general, smaller engines have greater sensitivity to fouling than larger ones.
“Each compressor fouling situation is unique as the nature and quantity of fouling material is site specific,” he said (Figure 2).
Air filtration is one of the time-honored defenses against fouling which consists of material that is either insoluble, or water and oil soluble. Filter media are effective at collecting solid particles, and insoluble material is largely gathered on the leading edge of the filter.
But analysis of media demonstrates that different amounts of soluble foulants migrate through the filter toward the clean air end. In one study performed by Turbotect, approximately 24% of total material retained in the front portion of the filter media was water soluble, compared to 66% in the middle. However, 77% of total foulants accumulated at the clean air end were found to be water soluble — thus confirming migration of soluble salts through the filter. Once the media become saturated, they no longer retain any watersoluble material.
Further research shows that airflow across the filter does not have a uniform speed. Where air flow is greater, saturation happens more rapidly. The picture is further complicated by the presence of SOx, NOx and salts. High salt concentration makes the water more viscous, said Stalder, and NOx and SOx increase solubility and salt absorption.
“The engineering of the water washing system as well as cleaning and rinsing procedures (timing, process used, water quality, detergent selection) play an important role in recovering compressor performance,” said Stalder.
He provided several water washing pointers:
• Washing often takes the salt off the center of the blade but misses the root and tip
• Calcium salts are not very water soluble so water does not clean them
• Oily deposits make salt and other contaminants more adhesive so detergents are required to remove them.
As to the question of whether online or offline water washing is best, Stalder advocates that both methods should be applied — but with online washing carried out more frequently to reduce deposit accumulation until the next offline wash. Offline washing, he said, reaches all stages, can bring about virtually a full recovery of power and can help to reduce the risk of blade corrosion. On the downside, it takes 12 to 36 hours, which can be expensive in terms of lost revenue.
Additionally, online washing uses demineralized water and much smaller droplets in order to avoid the possibility of damage. But it requires no shutdown, extends the operational period between offline washes if done correctly, produces no effluent for disposal and engenders around 0.2% to 1% gain in power output per wash.
Online washing primarily cleans the IGVs, and Stalder suggested it should be performed daily or at least every two to three days in order to be effective. Frequent online washing also minimizes deposit accumulation so that high-concentration “slugs” of salts and other foulants are not dislodged and sent through the combustion section. Depending on the particular fouling situation, detergent use can be optimized and may not be needed at each online wash. For example, it is best to use detergent on alternate online washes or at least every three days.
“Detergent should be used during offline washing, but thorough rinsing must then be done to remove detergent and foulants” said Stalder. “Varying crank speed RPM during offline water injection can improve spray distribution.”
He wrapped up with helpful operational tips:
• A high water injection rate does not equate to more effective online washing
• Low-pressure, low-flow online systems are preferred
• For online washing a droplet size range of about 50 to 250 microns is preferred and should be varied based on different trajectories
• Nozzle design and positioning can reduce water consumption, while maintaining a cleaner compressor (Turbotect has designed an air-assisted nozzle to improve online wash results in large-sized high-output gas turbines).
• Some offline water washing uses too much water which can cause downstream problems and overload drainage and effluent systems
• Water flow rates must be optimized for the engine
• During offline washing, insufficient rinsing will redistribute foulant material on blades at startup
• A wash audit can determine how many rinses should be performed by monitoring the appearance and conductivity of effluent water samples.
Effective filtration is one of the best means of minimizing degradation in gas turbine compressors, according to Herman Ruijsenaars, R&D Manager-Service at Siemens, Lincoln, UK. He stated that anecdotal feedback and evidence at overhaul from sites fitted with HEPA filtration confirm that enhanced filtration is the most cost effective means of improving product reliability and availability while reducing O&M costs.
“This precludes in some cases, or drastically reduces in others, the need for compressor washing and cleaning while maintaining optimum compressor performance,” said Ruijsenaars.
Siemens is more interested in the quality of the filtered air downstream than which filtration systems is used. (See the Turbomachinery Handbook for a list of major filter media and system providers).
Ruijesenaars prefers multi-stage filter systems with the final stage being a polishing filter that delivers to H13 (or better), but he acknowledged that in some instances (particularly sites with single-stage pulse clean filters) for retrofit applications, a single- stage replacement filter cartridge may need to be a compromise or the only acceptable (economically viable) option.
To avoid the adverse effect of erosion pitting, blade tip rubs, foreign object damage and fouling, Dresser-Rand advocates regular inspections of various kinds such as visual, penetrant, borescope and even eddy current inspections (See p. 24) which can go a long way towards eliminating degradation and forced outages.
Monitoring, too, plays a vital role in spotting indicators of fouling, clearance changes and other forms of performance degradation. In the presence of hot, humid, coastal or corrosive industrial site conditions, monitoring becomes even more critical.
Dresser-Rand’s Envision Compressor Performance Module (CPM), for example, incorporates computational capabilities to enable prompt corrective action to avoid excessive energy and fuel use and reduced compressor wear. It identifies anomalies such as fouling, seal damage, internal labyrinth seal damage, balance piston seal leakage and process-related surge, and predicts probable fouling locations.
The API standard for industrial centrifugal compressors calls for 20+ years of operation. OEMs supplying API machines meet that expectation through proper material selection for centrifugal components.
Over a 20-year period, however, operating environments are subject to changes that can impact compressor performance. The shale gas boom provides a good example of this. Process operators have the opportunity to lower their costs by changing their feedstocks to the more economical natural gas liquids, condensates or oil abundantly available through the shale recovery process.
The challenge, however, is that when new feedstocks are introduced, off-spec chemical reactions in the process can cause fouling, said Bill Blair, Senior Consulting Engineer at Elliott Group (Figure 3).
“System degradation becomes less predictable, and preventive measures may be required to mitigate the risk of corrosion, erosion or fouling,” he said.
“Performance degradation caused by fouling can also affect the mechanical integrity of the compressor by causing increased vibration over time,” added Blair. “Properly selected protective coatings can be used to restore and maintain performance levels after an efficiency loss caused by corrosion, erosion or fouling.”
Elliott’s materials science group documented a case of fouling that resulted in an efficiency loss of five points over a 17-month period. A properly selected coating restored and maintained compressor performance levels. Most recently, the company has developed a coating technology to protect fully assembled rotors. The alternative is to disassemble the rotor and remove the impellers. The capability to coat assembled rotors makes it possible to add the coating within the window of a scheduled plant shutdown, said Blair.