Operating a steam turbine with steam of insufficient quality or purity leads to oversaturation of impurities during expansion, which are then separated from the steam and form deposits on the turbine blades.[1]
Depending on the composition, deposits can form on the high-pressure, medium-pressure and/or low-pressure sections of a steam turbine.[2]
In a gas turbine, deposits have a similar, if not greater, effect on compressor blades. For gas turbines, in most cases, it is the compressor that suffers from the fouling that can finally result in a reduction of air intake and thereby reduced power output, leading to stall of the compressor that even might destroy the machine.
For gas turbines, the deposits are caused by impurities that enter the compressor with the intake air. The water that is injected for cooling or to increase performance (fogging / high fogging) can also lead to the formation of deposits in the compressor or turbine if it is not clean enough.
Scale and deposits on blades can be detected by the effects they cause. The consequences are reduced swallowing capacity of the turbine or compressor, increased axial thrust, and higher axial bearing temperatures and, increased vibrations of the rotor shaft due to imbalances. This finally reduces the overall efficiency of the power plant, which sums up to remarkable monetary losses compared to a turbine without such problems.[3]
To remove such deposits or scale, different methods can be applied.[4] The methods that are applied in most cases are:
- mechanical cleaning;
- water wash;
- wet steam washing; and
- foam cleaning.
These methods will be described and their advantages and disadvantages will be highlighted. As foam cleaning is not very well known in Europe, a few more words will be devoted to it in the article.
Mechanical cleaning by blasting
Mechanical removal of deposits requires a relatively long standstill and the opening of the turbine or compressor. Due to the long time and high cost involved, mechanical deposit removal is often only carried out during a major overhaul. As a consequence, efficiency loss in the period between major overhauls is accepted, resulting in a financial loss that is less than the cost of mechanical scale removal with weeks of non-production.
For mechanical cleaning by blasting, a variety of blasting materials is available. This allows selection of the blasting material based on the deposit to be removed and the surface finish to be achieved.
Table 1 gives an overview of blasting materials commonly used for cleaning of turbine blades.
In addition to turbine blade cleaning, as mentioned above, blasting can be used to prepare surfaces for non-destructive testing and to induce compression stresses into the surface of cleaned parts for local structural reinforcement.
Blasting enables all deposits from accessible surfaces to be completely removed, leading to full restoration of turbine performance.
However, blasting methods require opening of the turbine casing, as already mentioned, and rotor removal. This makes mechanical cleaning by blasting very time consuming due to pre- and post-blasting work (turbine opening, decoupling, rotor lifting, reinstallation of rotor, rotor realignment, reassembly, closing of turbine casing, balancing, etc).
In a nutshell, mechanical cleaning by blasting:
- Is very expensive due to high level of workmanship required.
- Is the method of choice when major overhaul is anyway due.
- Is used in the event of an incident that requires turbine opening.
- Will usually not be applied just for scale removal in the case of performance degradation that does not jeopardise turbine operation.
- Will restore turbine efficiency.
Entails separate cleaning of rotor and casing. The upper part of the split casing can be cleaned easily in a workshop, while the lower part of the casing remains in place and all cleaning activities including removal of the blasting material and the removed deposits need to be collected manually with vacuum cleaners or similar methods.
Water wash
Water wash offers no big advantage compared to mechanical cleaning with respect to the time taken because the turbine casing also needs to be opened.
Washing the turbine internals with demineralised water is effective for water soluble substances that have come into the turbine with the steam.
Sea water intrusion into the water steam cycle might be a case where water wash is a good method for dissolving and removing deposits from the turbine. Such a case was reported by Monica Nielsen at the vgbe conference in Ingolstadt in 2023.[5]
A second potential application is the cleaning of the turbine in a situation where lube oil has been spilled in the turbine during flushing of the lube oil system, for example. In such a case, degreasing chemicals and detergents can be added to the demineralised water together with an antifoam additive.
Water wash requires the opening of the turbine casing, as noted, but the rotor remains in place. The turbine support springs need to be blocked because the springs are not designed to hold the additional weight of a water fill. The lower casing is then filled with demineralised water (with additives) up to the level of the shaft bushings. The turbine is put into turning gear operation, whilst the demineralised water is continuously refreshed (see picture 1) or filled, soaked with turning gear in operation, and drained. This sequence needs to be repeated until the checking parameters (for example, conductivity, sodium concentration, oil content) are below preset values.
To avoid corrosion, the pH of the water can be adjusted and/or corrosion inhibitors can be added. Surface active substances (detergents) can also be added for increased cleaning efficiency in crevices and cavities.
This cleaning method consumes a large quantity of demineralised water. Dependent on the kind and quantity of the additives, large volumes of liquid waste might need to be treated before disposal. Applying water wash for removal of salt deposits from the turbine carries the risk that dissolved salts, and other forms of contamination, might be carried over into crevices and bucket suspensions.
Wet steam washing
Wet steam washing can be done “on the fly” during start-up or shutdown of the machine. The big advantage is that it entails almost no extra time for cleaning. But the cleaning is likely to be incomplete, leaving some of the deposits in place.
The “secret” behind wet steam washing is that condensing steam has a very good cleaning effect as it intrudes into even the tiniest crevices and pores. During condensation, water droplets are formed, in which salts show excellent solubility. The droplets take up the salt deposits and transport them away from the turbine blades. In addition to that dissolving cleaning effect, the droplets in the wet steam also have a mechanical effect similar to blasting or water jet cleaning. The best cleaning effect is limited to removal of water-soluble scale and at HP and IP inlet rows. Polluted wash condensates are collected in the condenser hot well and need to be removed from the water/steam cycle during operation.
Wet steam washing is less effective in the middle and end sections. Deposits of mixed composition are only partly removed. During the washing, there is an increased demand for clean make-up water and the boiler and other equipment in the water steam cycle (for example feedwater pumps) need to be operated outside their normal characteristics during the wet steam washing process. If wet steam washing is to be used to remove deposits from the turbine that originate from malfunctioning of the make-up water plant, from the condensate polishing plant or was caused by cooling water intrusion, it might be necessary to repeat the cleaning process several times to remove all salt from the system. This is shown in pictures 2 and 3. In picture 2 the decreases in conductivity and sodium concentration in the turbine condensate are shown during the first washing sequence.
inleakage (Image courtesy Christiane Holl[6])
The sodium concentration (Na turbine cond, brown line) in the turbine condensate remained at some 11 mg/l during the entire washing process. The acid conductivity (AC turbine cond, red line) and the direct conductivity (DC turbine cond, green line) declined significantly, from 80 µS/cm to 8 µS/cm AC and from 1600 µS/cm to some 180 µS/cm (DC). The degas conductivity (Degas turbine cond, blue line) remained more or less stable all the time around 2 µS/cm.
Based on these results, it was clear that at least a second wash sequence needed to be executed to reach low concentrations of sodium and low conductivity levels that allow restart of the turbine. Before the second cleaning cycle could be started, the turbine and the boiler needed to cool down again to be able to provide wet steam of sufficient amount and for a sufficient time span.
During the second wash cycle, the conductivities dropped to 8 µS/cm for the direct conductivity, 1.6 µS/cm for the acid conductivity, less than 1.0 µS/cm for the degassed conductivity, and sodium concentration was less than 0.4 mg/l (see picture 3).
The analytical results for conductivity and the sodium concentration in the wet steam condensate were good enough to allow restart of the turbine. The restart of the turbine can be regarded as a third or final washing cycle. After an extended bypass operation, the turbine went back to service. The overall efficiency had increased from 78% to 87% for the turbine.
Foam cleaning
Removing deposits with the help of active foam is a very elegant method for removal of deposits without having to uncover the machine. The time required for such cleaning, during which the turbine is not available, is about one week including cool-down time. The foam cleaning itself normally needs less than one day.
Foam cleaning has proven to be more effective than saturated steam washing and online or offline compressor washing, and is also suitable for removing stubborn deposits that are insoluble in water and steam, as the mixture of the cleaning foam can be adapted to the expected deposit composition. Oil spills in the turbine can also be removed with foam.
The method has been used in the United States on steam turbines for more than 40 years and has proven itself several hundred times over without any impairment of turbine operation or corrosive attack on turbine components being observed after foam cleaning. This includes the most critical areas of the turbines, like rotor steeples and blade dove tails. Some of these steam turbines have been cleaned several times during their lifetime for the purpose of deposit removal. The technology originally was developed for removal of deposits from steam turbine blades. In the beginning the focus was on copper and on deposits of iron and its oxides. In later years, foam cleaning has been applied to removal of deposits of other compositions.
Foam cleaning has also been successfully applied to gas turbines.[7] The usual method for removing fouling layers on GT compressor blades is online and offline washing. Offline cleaning is done with turning gear in operation and the cleaning mixture is sprayed into the compressor. With this method, the front rows of the compressor can be cleaned very well, but the high-pressure stages of the compressor are only partly cleaned or even not at all.
With online washing, the cleaning agent is sprayed into the running machine. This has an additional effect similar to water jet cleaning. However online washing carries the risk that dirt loosened in the low-pressure section of the compressor is transferred to the high-pressure section of the compressor and cakes there due to the high temperatures that occur when the combustion air is compressed.
There is also a risk that some of the loosened dirt will become caked in the burner cans and the turbine section or in the cooling channels of the turbine blades. Clogged cooling channels lead to reduced cooling air flow, overheating of the blades and ultimately to irreparable blade damage.
The foam acts as a gas and liquid, it fills the entire area to be cleaned and is great for cleaning systems not designed to hold the weight of a liquid. The foam enters the smallest spaces for total cleaning and due to its razor-soap-like texture it can carry particles in suspension. A big advantage compared to wet steam washing and water washing is that even though the cleaning volume is large, the generated volume of liquid waste to be disposed of is small.
Depending on conditions and the nature of deposits, the foam composition can be altered over a wide range to maximise cleaning effectiveness. Best cleaning results are achieved if the composition of the deposit is known in advance.
A preliminary study is an important part of the foam cleaning package. This includes an on-site inspection, which takes 1-2 days. The site-specific features are evaluated. The installation locations for the foam cleaning equipment are determined and inspected. The necessary media supply is addressed and extraction points for the foam are inspected and determined.
The next step is the design and planning of the process and the manufactuinrg of necessary temporary equipment. Picture 4 schematically shows the structure of the foam cleaning system.
In the case of gas turbine cleaning, the foam is injected at the air intake bell mouth. Operation of the turning gear pushes the foam through the entire compressor, combustion chamber and turbine. At the turbine exhaust diffusor, an antifoam liquid is sprayed onto the foam to convert it back into liquid, which is then collected and pumped into a waste collection tank (see pictures 5a – 5c).
A control system with CCTV and continuous online recording of various process data are installed to monitor and steer the cleaning process. A mobile laboratory is used to determine progress during the whole cleaning process. The laboratory is also used to ensure that the turbine materials are not attacked by the foam. This is done by analysing the iron and the chromium content in the liquified foam at the turbine outlet.
The cleaning itself is partitioned into a number of process steps. These are listed in Table 2.
The start of each step takes into account laboratory results.
If necessary, a second active foam phase follows. This might be necessary if a larger portion of the deposit is known or expected to be of non-water-soluble nature, like silica, metal oxides, etc.
After removing the temporarily installed connections and blind flanges, the turbine can be put back into service. The first step then is a quick wet steam washing and clean run of the turbine in the same way as is usual after a major overhaul.
The outcome of the foam cleaning shows efficacy comparable to mechanical cleaning (see picture 6) but with way shorter outage time and much lower costs.
In contrast, wet steam washing of gas turbines in most applications cannot restore full efficiency because some of the deposits can remain on the turbine blades (picture 7) or are simply shifted from the front stages to high pressure stages of the compressor. That results in the accumulation of deposits which cannot be removed with online or offline washing. As a consequence, the deterioration of compressor efficiency is slowed but not completely halted.
Examples of successful foam cleaning projects
Two GE 7A gas turbines suffered from heavy fouling of their compressors because of dust intake via leaks and bypasses in the air filter system and due to accidental use of raw water instead of demineralised water for fogging and high fogging.
The deposit on the blades therefore was assumed to be composed of hardness scale (mainly CaCO3) and silica compounds. Foam cleaning was executed with two active foam stages. In the first active phase, citric acid was used to dissolve and remove hardness scale. For dissolution of silicates, in the second active foam phase, ABF was mixed with the foam. After the foam cleaning was finished, the gas turbines went back into service. It turned out that the efficiency of the turbines was recovered almost completely (see Table 3).
The performance of the gas turbines was measured in April 2024 prior to cleaning and May 2024 after the foam cleaning.
In summary, large masses of deposits were removed from the gas turbine blades (see Table 4).
The small accumulated amount of iron and chromium detected in the liquified foam outlet is a strong indication that gas turbine materials were not attacked by the foam.
Similar results were observed during the foam cleaning of the turbine of a biomass fired district heating power plant.
The deposits there were composed of hardness scale (principally calcium carbonate), silica and mainly iron oxides.
After cleaning, the efficiency of the steam turbine returned from 90% of nominal to design values, and the swallowing capacity, which had been reduced by 20% also went back to 100% (see pictures 8 and 9).
In this case, 7.3 kg of silicates (as SiO2), 5.9 kg of hardness scale (as CaCO3) and 4.3 kg of iron (as Fe2O3) was removed from the steam turbine by the foam cleaning process.
A proven method
In summary, foam cleaning is a proven method for removing deposits from gas and steam turbine blades without jeopardising turbine integrity. The method has been used for over 40 years in the United States. It can compete with mechanical cleaning in terms of efficiency restoration. However, foam cleaning requires much less time because the turbine casing does not need to be opened and the rotor does not need to be removed from the machine. Compared with water wash and wet steam washing, foam cleaning can do a much more thorough job of removing scale and deposits because the foam reaches all parts of the machine and because the foam mixture can be adapted to the expected or known composition of the deposits.
Literature:
[1] F U Leidich, Some basics of power plant chemistry – corrosion and deposition, PPChem 2023, 25(01), 38 [2] State-of-knowledge on deposition, Part 2: Assessment of Deposition Activity in Fossil Plant Units, 2003, Electric Power Research Institute, Palo Alto, CA, USA, EPRI 1004930 [3] F U Leidich, The economic impact of power plant chemistry in regard to maintenance and lifetime, presented at IMechE Steam Turbine and Generator User Group meeting, 2016, Manchester [4] F U Leidich, Steam turbine deposits – how they occur, their effects and how they can be eliminated, PPChem 2024, 26(01), 4 [5] Monica Nielsen, Handling sea water ingress cases at Avedøre power station 2022, presented at vgbe conference 2023, Ingolstadt [6] C Holl, Hydro Engineering GmbH, Mülheim an der Ruhr [7] F U Leidich, Foam cleaning of gas turbine compressors – a fast and efficient method to reestablish performance and efficiency, PPChem 2024, 26(01), 18 [8] W Majka, Ecol SP. z.o.o., Rybnik
