Chemical foam cleaning of turbine rotors and other rotary equipment
Current world economic situation and natural challenges arising from the competitiveness in all industry branches force the companies to introduce careful monitoring of their production processes. New ideas and methods are constantly sought after to maintain the maximum efficiently of these processes. Generating profits is often achieved by reducing costs rather than raising prices for products. Improving methods of maintenance is a great opportunity for achieving long-term savings, as well as increase the reliability of production assets. Chemical cleaning with foam of steam and gas turbines is one of the most frequent practices of operability management, rewarding in measurable benefits in a short time.
Power industry faces even more serious challenges every day. Operational parameters of a power plant, such as produced power, efficiency (heat consumption intensity), or production flexibility have never been so important as before. It seems obvious that each piece of equipment in the facility adds up to the total operational efficiency, yes some of theme require more attention. The turbines are at the top position of such a list. Contaminated flow systems in turbines generate expenditures for the owners of the facility due to production losses resulting from a decrease in efficiency and flexibility.
Chemical cleaning with foam is a recognised and economically attractive method of sediment removal from the flow systems in turbines, without a need of removing the system. The cleaning process can be performed within 24 hours in a downtime lasting just a few days (usually 4 or 5 days) necessary to cool down the turbine to a proper temperature and set the foam-cleaning unit. After the cleaning process is over, the turbine is quickly recommissioned.
The article presents the methodology of cleaning process, as well as the samples of fulfilment.
A competitive market requires low-cost production. Power plant operators try to maximally lengthen the periods between overhauls. Working turbines should generate as much powers as possible, as its drop by several or several dozens of megawatts leads, in longer perspective, can be losses worth millions. That is why, the efficiency needs to be regained economically, in-between the planned overhaul. If the drop of efficiency results from worsening operation parameters of the turbine (and the monitoring does not show any mechanical failures), the drop will frequently be a result of sediments gathered on the flow elements of the turbine. A traditional solution to the problem requires a shutdown, when the turbine is removed, the rotor removed, and the elements contaminated with sediments are jet-abraded. This process although effective, is also long and costly.
An excellent alternative, allowing fast and effective restoration of nominal operation parameters of the turbine, is chemical cleaning with active foam. Such method does not require the turbine to be removed, which, compared to the traditional methods, allows considerable savings. And even though, the technique in question has been declared the optimal solution in case of the capital overhaul works, its advantages are applied more and more frequently to restore nominal parameters during turbine operation. Such an approach allows a larger flexibility in the planning of operational periods, as the times between the overhaul works are longer, and the turbine work becomes more stable without a complete overhaul. The method in question of contamination removal with active foam is 100% efficient and restores nominal power in just a few days.
Issue of contamination inside a turbine
In steam turbines, part of contamination going through the boiler with steam, deposited on elements of the flow system. Turbine blades have a properly designed shape, guaranteeing more efficient energy conversion. Turbine efficiency is characterised with, amongst others, heat consumption intensity and generated power. As the sediment rests on the turbine blades, their shape changes and the parameters of steam flow between the blades suffer. The process leads to a reduction in generated power and if the nozzle channels are clogged, the steam flow is also reduced. In the situation when the flow in the turbine is restricted, the power loss can be compensated to some extent with the pressure increase at the turbine inlet. Nevertheless, such a solution poses a threat for the pressure equipment in the boiler, and has a negative impact on the unit efficiency.
Dissolved copper oxide and other metal oxide compounds created in the process of corrosion of the elements of the steam-and-water unit system (e.g. the tubes in the condensers, regeneration heaters, boiler tubes, etc.), as well as contamination introduced into the system with the oiler water, and transported by steam, are deposited in various parts of the circuit. In many cases, that contamination keeps depositing in the turbine’s flow system, changing its aerodynamics and thus lowering its efficiency and gained power.
Solution to the problem
The best method of avoiding the problem of sediment deposition is to prevent them from arising during water-and-steam operation. It is very difficult to achieve this in practice. Properly conducted operation significantly reduces the amount of contamination carried through the system, yet from our experience, it does not eliminate the problem completely. That is why periodic cleaning of the flow systems is necessary for maintaining proper operation parameters of the turbine.
Contamination in turbines are a frequent condition in utility units, where the water-and-steam systems are made of copper alloys (in regenerative exchangers and condensers). Such sediments deposited during operation, when copper oxides are soluble in the boiler water and are transported within the system. Rinsed out copper can accumulate in metallic form as well as in the form of phosphorates and other salts. Such contamination has negative impact on the steam flow in the turbine, resulting in lower power production as well as changing the heat consumption intensity. Copper, in its metallic form, deposits on steel elements and can speed up electrochemical corrosion (copper acts as a cathode, whereas the iron-steel alloy is corroded). Chemical substances used to neutralise copper compounds are usually based on HCl, chelates, and alkaline oxidising agents. Technology uses solvent and foam-forming agent. An oxidation of a foamed solution with pure oxygen or hydrogen peroxide, transforms metallic copper to copper oxide, which is then dissolved. Depending on the type of sediments, the technology may require adding or removing some other oxidants to make the process more efficient.
Copper residues and deposits may appear on the outlet nozzles, on blades and other turbine elements. As the process of deposit decomposition may vary in each case, it is necessary that the active foam made contact with each part of cleaned surface. When the chemical foam is injected to the turbine, the flow system—up to the outlet of the cleaned turbine part—is cleansed by the active foam.
When the turbine cools down to the desired temperature, 65–75⁰C, the process of chemical cleaning takes about 24–36 hours, including the process of plugging the foam unit to the system, active foam flushing, neutralising foam flushing, and steam flushing. When the turbine remains on the turning gear, and its temperature kept at a level of 65–75⁰C, foam is injected directly to the nozzle chest through the removal of control valves from the S-and-W turbine. After removing copper compounds from the turbine, water foam (water + foam-formation agent) is introduced to the turbine to flush out the remaining chemical compounds. At this stage, the turbine is flushed with saturated steam at a low temperature until the conductivity of the condensate downstream of the turbine is equal to the conductivity of the steam entering the turbine. Cleanliness parameters are determined by the chemical department of the power plant supervising the project. Typical, the time to achieve the correct parameters is not more than 24 hours.
See also: Hydrodynamic cleaning and flushing of lubrication systems