CO2: The versatile non-hazardous cleaner
Dry ice in pellet form is used extensively to remove rust, sulfur deposits, ammonia salts, and insulation debris from HRSG heat-transfer surfaces (finned tubes). It’s been used to prepare catalyst beds for new blocks. It’s non-hazardous with no cleanup or no residuals beyond the materials removed. So what about other areas of a combined-cycle plant? How versatile is this technology?
We brought that question to Plant Manager Ric Chernesky at Frederickson 1 in Tacoma, Wash, who had seen great results with his tight-tubed HRSG. He also uses CO2 at reduced pressure to clean the gas turbine, and says he has heard about all kinds of other plant applications, although not yet tested at “Freddie.”
So just how versatile (and useful) is this non-toxic cleaner? With a note that oil-based deposits might still need extra attention, some typical examples follow.
Generator stator cleaning with CO2 is becoming increasingly common. With field (rotor) removed, crews will generally blast the core and get any debris or contamination off the laminations. If they feel that solvents have been used in the past, especially on the end windings, they will use CO2 first to get everything clean, then follow with hand wiping as needed, perhaps using alcohol and Positron.
The point is this entire process was done by hand in the past with potentially harmful cleaners that could leave a residue. Using CO2 has removed most of the hand work and is chemical-free. The dry ice sublimates directly to a gas.
Rotor work is generally done offsite; exciters have been cleaned on location.
Gas turbines. During hot-gas-path and major inspections, the upper half of the turbine casing is removed to access, inspect, and remove (as needed) any blades (buckets). GT power augmentation (steam or water) and extended time on turning gear can add to the risk of rust and other deposits accumulating in the rotor slots.
Often, because of accumulation, removing some blades can be challenging, as can be reinstalling the blades. The rotor slots must be completely clean, and this is becoming an ideal application for dry ice. In the past this was done by hand—a slow and interruptive process with possible chemical exposure. CO2 can do a better job in less time, without interrupting overhaul activities. More than one site reported that all turbine rotor slots can be blasted clean in one shift (Fig 1).
Compressor section cleaning schedules are traditionally site-specific and vary from daily or near-daily water washing to offline cleaning during outages. The outage cleaning methods also differ: from water wash with soap to hand cleaning with an abrasive sponge and cleaner. Some use ice, or if deposits are oily, bead blasting. It seems to be across the board, and inspections determine the extent of compressor-stage cleaning based on dirt buildup and black film.
Ice blasting is a quick and easy option for compressor blades (Fig 2). When CO2 is used on the compressor stator, cleaning is reported as thorough and efficient. Cleaning of inlet scrolls in place is another application and has been done successfully in the field. Inside, outside, and beyond. So what about plant electrical equipment: transformers, high- and medium-voltage switchgear, insulators, and other components that need cleaning?
CO2 has been used on virtually all distribution equipment including pad-mounted switchgear and transformers, and owner/operators report both unexpected surprise and high satisfaction with the results. Substation cabinets can become contaminated with dust and dirt or, in an extreme case, flood damage with water carrying silt, salts, and other contaminants. Potentially, all of this can interfere with proper operation. Cabinets also carry the risk of fire. Again, restoration seems to be a logical application for dry ice. One crew member reports removing all soot and debris without any harm to the paper stickers used to identify the wires.
One substation manager reported very dirty 220-kV bushings and lightning arresters and opted for dry ice blasting. Normally, this equipment would be cleaned by hand and this job would have taken two technicians two shifts to complete the work. Ice blasting was completed in half a shift with excellent results. All contaminants were removed and there was virtually no cleanup, the operations manager said, and even that was done by the contractor. Transformer test results showed improvement. Another utility end user called for ice blasting after a breaker fire. The unit was cleaned and rebuilt at less than one-quarter of the cost of a new unit. Prior to giving ice blasting a try, the breaker likely would have been scrapped. Reactors, insulators, and a host of other equipment become logical applications as well. As one operations manager put it, “whether it’s a fragile surface or non-fragile, whether you are trying to remove paint or contaminant, or just trying to clean it, all things become candidates for ice blasting.”
And of course you never know what you will find further out in the field. Take for example Fig 3. This open-air step-down transformer facility included a finned-tube heat exchanger that had probably never been cleaned. Using CO2, the tubes were cleaned of all condensation, general slime, moss, and other heat-transfer obstructions after many years of operation. Versatility, safety, and scheduling. Looking back at Chernesky’s workhorse HRSG, ice blasting that began at around 235 psig is now up to 350 psig. Backpressure is low, and he keeps coming back to this cleaning technology.
There are, of course, limits. In these non-HRSG applications, oil and grease (if wet) tend to be moved around rather than removed, and cleaning by hand will still be required in some situations. High-pressure blasting also requires care and attention. Stator insulation, for example, can be damaged when the pressure is too high. But if you know the right variables, you can adjust CO2 feed rate and jet pressure. An experienced operator can also play with distance. So even in a sensitive environment, as one owner/cleaner put it, “ice blasting becomes perhaps the only choice.” You can just move forward and apply the variables (think School Zone).
JLN Associates LLC, Old Lyme, Ct, reminds us of some unique safety requirements. The jet-cleaning process can be quite loud, and dry ice is extremely cold (-109F), calling proper personal protective equipment. Also, CO2 released in high concentrations can decrease oxygen content in the work area, meaning strict attention to air quality. Any loose objects must be removed or secured, and static electricity is always a safety concern, calling for proper grounding.
On the positive side, one interesting account described a 150-ft stack that required cleaning to remove peeling paint. In a good climate, water blasting would be ideal. But the work was performed in the north, in December. For both cleanliness of equipment surrounding the stack, and safety of the crew (don’t freeze the workers), they chose ice blasting. During this review the editors also found crew mobilization (usually two), clean up, and demobilization of both crew and equipment to be heralded as “painless.” CO2 itself is clean, and the process does not make a big mess. It therefore seems logical that if an ice-blast crew is on site for an HRSG, the owner/operator might consider other tasks, even those hidden from daily view.