Maintenance|Turbine technology
Table 2. Foam cleaning process steps Step
Initial
First active phase: mixture is adapted based on expected composition of deposit
Rinse
Second active phase: mixture is further adapted based on expected composition of deposit (NH4
OH, EDTA, ABF…) Rinse Neutralisation/passivation Rinse Chemicals used Foaming agent; demin water
Foaming agent; demin water; active component I; inhibitor
Foaming agent; demin water; active component I; active component II; inhibitor
Foaming agent; demin water; ammonium hydroxide; hydrogen peroxide
Purpose
Check for tightness of all hoses, bindings, temporary installations…
Removal of water soluble deposits and scale
Foaming agent; demin water Push out acidic foam
Removal of metal oxides and silicates
Foaming agent; demin water Push out acidic foam
Neutralise acidic residues; passivate metal surfaces by formation of oxide layers
Foaming agent; demin water Push out acidic foam
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
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
83.00 82.00 81.00 80.00 79.00 78.00 77.00
5/90 9/91 1/93
Grit blasting during major outage
6/94 10/95 Time
3.97 7/98 12/99 4/01
Foam cleaning requires a one week long outage
Picture 6. Effect of different cleaning methods on performance
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).
loss due to ageing
washing
washing
Operation hours
Picture 7. Efficiency recovery: offline and online washing compared with foam cleaning
www.modernpowersystems.com | July/August 2025 | 29
cleaning
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