Turbine technology|Maintenance
Picture 1. Water wash at Avedøre power station[5]
. Photo courtesy Monica Nielsen
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
80 70 60 50 40 20 10 0
1800 1600 1400 1200 1000 800 600 400 200 0
Picture 2. Result of first wet steam washing sequence after cooling water inleakage. Image courtesy Christiane Holl[6]
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. 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
Turbine condensate, second washing sequence
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08:00
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10:00
11:00
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Picture 3. Result of second wet steam washing sequence after cooling water inleakage. Image courtesy Christiane Holl[6]
24 | July/August 2025|
www.modernpowersystems.com
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