To keep gas turbines operating at their optimum requires regularly scheduled shutdowns and maintenance. But the major source of power loss, and the most easily correctable, is contaminant fouling of the compressor.
‘Carbon, oils and waxes, anything that is in the ambient air will get into the axial compressor,’ says Klaus Brun, manager, Rotating Machinery and Measurement Technology for the Southwest Research Institute in San Antonio, Texas. Not only does this add to the drag, it also leads to major problems suchà‚ as corrosion pitting of the blades. ‘Many millions of research dollars are spent to tickle out another percent or twoà‚ of turbine efficiency,’ says Oliver H. Platz, manager of FP Turbomachinery Consultants GmbH of Emmendingen, Germany. ‘All that seems almost in vain when these machines are then run at five (or more) percent fouling-based losses afterà‚ commissioning.’
Those losses, however, can be easily recovered through a proper programme of compressor washing.
Gradually lowered efficiency
Overall turbine efficiency is limited by the amount of work produced by the turbine that is consumed by the compressor. Typically a bit more than half the output goes to driving the compressor, with aero-derivatives being less efficient than the larger frame units. As the compressor becomes fouled with minute particles, the efficiency drops even further. This fouling, if not remediated on a regular basis, can cause 70%à‚—85% of a turbine’s output loss over time.
The air doesn’t have to be particularly dirty to have a profound effect. With impurities running at a mere 10 ppm, a GE Model 7FA would still bring 153 tons (139 metric tonnes) of impurities per year into the compressor. Industrial turbines have far smaller air intake requirements, but that is offset by the quality of the environment they work in.
Figure 1. Effects of cleaning on engine performance
‘All sites are different depending on the air filtration system, wind speed, wind direction and environmental conditions, such as whether you have a cement factory or an oil refinery next door,’ says Andrew Bromley, Vice President of Operations for Turbotect Ltd.’s (Baden, Switzerland) US subsidiary.
The particular contaminants one needs to be concerned with vary with location. At rural installations, soil, dust, sand, fertilizers, pesticides, insects and plant matter can all make it into the compressor. Airborne salt is a problem for offshore oil platforms or facilities located near the coast. In urban areas there is smog. In industrial areas there is coal dust and material from evaporative coolers and cooling towers. Then there are the bearing oil leaks and GT exhaust that can make their way into the compressor.
‘Oil and grease act as ‘glue’ on the compressor blading, and will trap and hold other foulants entering through the air filters,’ says Cyrus Meher-Homji, Bechtel Fellow and Senior Principal Engineer for Bechtel Corporation in Houston.
Whatever the source, the deposits on the rotating and stationary blades affect their aerodynamic profile and reduceà‚ the air mass flow, gradually reducing the efficiency and output. Other effects include erosion and corrosion, higher emissions, clogging of hot-section cooling passages, and lowered reliability. The effect is most dramatic on the early stages and inlet guide vanes, as each stage’s performance depends on the performance of the earlier stages. When contaminants reach the deeper, higher-temperature stages, however, they can get baked onto the blades and are harder to remove.
Three steps to cleaner compressors
The solution to reduced maintenance is to remove the deposits on a regular basis, but the best way to do it varies from one location to another. Generally speaking, there are three methods of compressor washing à‚— on-line, off-line (‘crank washing’) and hand washing. The three are not exclusive; all three will be used at different times.
‘On-line washing just extends the interval between the times off-line washing is done,’ says Brun. On-line washing is the answer to keeping the turbine running 24/7, without allowing the performance to degrade excessively. With on-line washing, an array of nozzles in the inlet area injects water droplets into the compressor while the turbine is running. ‘On-line washing should be performed frequently to avoid deposit build-up; for example daily or weekly,’ says Meher-Homji. ‘If the interval between on-line washing is too long, the benefits will not be seen. Frequent on-line washing also avoids sending ‘slugs’ of foulant into the combustion section.’
Brun says that the on-line washes don’t have to last long, since most of the washing occurs in the first 30 seconds, but since water is cheap, you can keep it running till the water tank empties. The main problem with on-line washing, he says, is that it removes material from the early stages and then re-deposits later as the water evaporates. To avoid adding to the deposits, he advises not to add any detergents to the spray, but to use pure, demineralized water à‚— and lots of it.
‘It has to be done with the highest possible water to air ratio,’ he says. ‘By increasing the water amount, by having a very high water to air ratio, you are reducing the re-depositing.’
Off-line, or crank washing, requires shutting down the turbine and letting it cool. Once the temperature has dropped sufficiently, water is again sprayed into the turbine inlet while the turbine slowly rotates. ‘Off-line washing is still the best way to completely clean the compressor and give maximum power recovery,’ says Meher-Homji. ‘Therefore, operators should perform off-line crank washing whenever convenient, during all scheduled outages.’
Detergent should be used for off-line washing, followed by a thorough rinse with clean water to ensure all the salts and grease get fully removed from the compressor casing. Heà‚ recommends performing conductivity testing on the effluent water and continuing to rinse until those measurements stabilize.
‘Material that is not removed during the rinse cycle will be redistributed on the blading when the unit is started,’ he says. ‘Usually more than one rinse cycle is needed.’ Unlike on-line washing, you don’t have to worry about evaporation; but since the turbine is only spinning slowly, there isn’t enough air velocity to pull the water down into the later stages and clean them. ‘If you have a 16-stage compressor your off-line washing will not be very effective in the later stages,’ says Brun. ‘If you have a heavy dirt or salt deposit in the late stages of your compressor, you will have a hard time getting those out with off-line washing.’
Finally, there is hand washing, sending in a team during a scheduled shutdown to wash the blades manually. ‘With most GTs, there is a way to reach in and hand-clean the inlet guide vanes,’ says Bromley. ‘Most fouling occurs in the early stages of the compressor, and hand cleaning is recommended in all cases.’ The rest of the blades should be hand cleaned any time the casing is opened.
Given the benefits obtained from regular cleaning of compressors, most GTs now come with some sort of cleaning system built in. Operators may, however, want to investigate other options.
‘The design and performance of these wash systems varies tremendously between the OEMs and between GT models,’ says Meher-Homji. ‘Many end-users have installed improved on-line wash systems as retrofits à‚— but operators under a long-term service agreement are often prevented from doing these upgrades.’
There is currently broad agreement that cleaning should be done, but there are widely diverging opinions as to the best approach. This includes the type of nozzles, how much water to use, when and whether to use detergents and so on.
‘Compressor washing has not been consistently handled by the OEMs over time,’ says Bromley. ‘The end result is that the end users get a bit confused and lose confidence in the subject.’
The key to thorough cleaning lies in evenly distributing the right quantity and size of droplets across the entire air path. If the droplets are too small, they will evaporate in the early stages or are deflected by the air mass flow and so do not make an adequate job of cleaning. ‘Large droplets will tend to remove the deposits better than the small droplets, but very large ones have the potential to cause a little bit of erosion of the blades,’ says Brun, ‘especially if you have some really expensive coatings à‚— you want to ponder this before you start throwing buckets of water in there.’
Most systems produce droplets in the 80à‚—250 micron range, which is adequate for on-line washing. For off-line washing, large droplets are not a major issue since you are not concerned with impact caused by the high rotational velocities of the blades. Higher pressures, however, are usually used for off-line systems. Since you don’t have the high-speed air flow to pull the droplets into the turbine, you need to rely on the water pressure to do the job.
There is some disagreement on whether you can use a single set of spray nozzles for both on-line and off-line washing. Gas Turbine Efficiency AB (GTE) of Stockholm, Sweden uses a single set. ‘GTE uses a high pressure system that, by atomization, produces a soft mist of water droplets just the correct size to travel with the airflow in order to penetrate the total gas path of the compressor and turbine,’ says GTE managing director Pàƒ¤r Krossling. ‘On- and off-line washing can be done with the same set of nozzles from one location.’
Most firms, however, say that separate arrays are needed. ‘Some people say you can use the off-line and on-line systems interchangeably, which we don’t agree with’, says Gregory Labas, president of Conntect, Inc. in Brookfield, Connecticut. ‘They are completely different applications.’
Engine Cleaning Technology, Inc. of Bridgeport, Pennsylvania, takes a middle approach. It uses a single nozzle array, but offers interchangeable nozzle tips à‚— one set for on-line and the other for off-line washing.
Down to specifics
The bottom line, when selecting and operating a compressor washing system, is that there is no ‘one size fits all’ approach. This applies to nozzle design, placement, amount of water and use of cleaning fluids.
‘Comparative tests can only be performed in the field on actual GT units operating under real, local environmental conditions,’ says Meher-Homji. ‘Field tests should run for a sufficient length of time and all collected performance data must be corrected to ISO conditions.’
Nozzle placement should start with using computational fluid dynamics (CFD) to model the air flow and water flow through the compressor. Labas says that the models sometimes show the nozzles should be placed far from where you would expect. ‘With an LM6000, the initial CFD we did using one manifold showed that all the water hit the cone,’ he says. ‘We determined that we had to have a second manifold outside to get coverage from the root to the tip of the blade.’
Once the modelling is done for one type of turbine, it doesn’t necessarily apply to others of that same model. He cites the case of a frame unit where, because of the inlet plenum arrangement, the nozzles had to be loaded on one side to get proper coverage.
The same principle applies to issues such as whether to use detergents in on-line washing. Meher-Homji recommends using water only, when possible, but says that detergents should be used when grease and oil are present. ‘This decision can only be made at the specific power plant, by conducting comparative test programmes over an adequate time period,’ he says. ‘It is wrong and misleading to make blanket recommendations and pronouncements for all power plants.’
Drew Robb is a US-based writer on energy.
Choosing a cleaner
In addition to selecting the compressor washing equipment, one also has to decide what type of cleaner to use for off-line and on-line washing.
‘If you are in the middle of farm country you will have completely different kinds of soils that will get on your blades than if you are near a refinery,’ says Gregory Labas, president of Conntect, Inc. in Brookfield, Conn. ‘What product works best and how often you need to clean is a site-specific thing.’
For years, the standard had been to use solvent-based cleaners. These did a good job of removing any oil or grease from the compressor, and reducing the build-up of particulates à‚— while these are still available, environmental and safety regulations have caused a shift to waster-based detergents. ‘Solvent-based cleaners are classified as hazardous and their low flash points make them more expensive to transport,’ says Andrew Bromley, Vice President of Operations for Turbotect Ltd.’s (Baden, Switzerland) US subsidiary. ‘Also, with off-line washes where you have an effluent water stream to dispose of, you have to treat that effluent like oil. If you use a water-based detergent, however, that effluent can go into a holding tank and can be less expensive to dispose of.’
Early detergents did not do as good a job with oil and grease as their predecessors, but vendors have released a newer generation of water-based products that are designed specifically for addressing this problem.
‘If you do head-to-head comparisons, particularly with the crank-wash chemicals we offer, we can’t measure a difference,’ says Bruce Tassone, president of Engine Cleaning Technology, Inc. in Bridgeport, Penn. ‘If you can’t measure the difference, with all the liability and cost impact of solvents, why use them?’
Whatever type of cleaner is used, however, timing can be important for meeting emission limits. ‘When you wash on-line with either a solvent- or water-based cleaner, the NOx goes down, but the CO goes up,’ says Labas. ‘If you have a permit with a CO requirement, you have to make sure the CO doesn’t exceed your limit.’
He says this gives you two options. One is to get a variance for the permit, but this can be difficult. The other, if you have a 24-hour time limit, is to start the wash shortly before midnight and split the increased emissions over the two days. This same strategy can be used with VOC limits.
Size of turbine also affects the choice of cleaner. ‘For large machines, we have found it is important to have better foam dissipation properties,’ says Bromley. ‘In an off-line wash, if you are not careful, the whole plenum fills up with foam. As machines get larger, it takes more and more rinse cycles to clean it out. It requires a balancing act. The foam acts as the transport mechanism for the dirt, so you don’t want it to collapse too soon, and redeposit the material on the blades. On the other hand, stronger foam will require additional rinse time, and more effluent water to manage and dispose of.’
It is critical, both in selecting and using a cleaner, that the cleaner doesn’t exacerbate the problem by leaving its own residue to add to the foulant build-up. This leads some to recommend not using detergents at all à‚— just letting demineralized water do the job à‚— for on-line washes. Labas says this is a fallacy, particularly for turbines that operate nearly continuously.
‘In some cases, the guy can’t shut down, so the only thing they can do to clean their engine is on-line washing,’ he says. ‘Some will tell you that water works as good as anything. I say, try telling that to your wife when she’s washing the dishes.’