Deposition, Corrosion, and Erosion in Steam Turbines
Power generation plants are widely preferred by manufacturing industries; which require extensive power and steam for operational purposes. Power generation or cogeneration plants utilize steam to produce power or electricity. It uses external mechanical energy to convert into electrical energy that drives the electric charges to produce electricity. Steam generated in boilers is used for the production of electricity through turbines. In turbines, the steam, liquid, or combustion air pushes a series of blades on a rotor. The force compels the blade to spin or rotate that converts the mechanical energy of the rotor into electrical energy. Therefore, turbines are indispensable for generating power for large-scale industries.
Factors such as high-strength steel, contaminated steam, etc., in the turbine, affect its overall performance. Deposition, corrosion, and erosion are the most commonly found issues in steam turbines.
Deposition in Turbine:
Depositions form and settle in the steam passage and negatively impact the shape of turbine blades and nozzles. Various factors cause deposit formation, but the effects are usually the same. Depositions increase the resistance of the steam flow. The hindrance in steam passage affects steam pressure and pressure drops, thereby reducing the steam turbine efficiency. In severe conditions, deposition can result in excessive rotor thrust that unbalances the rotor and causes vibration issues. With poor steam purity, turbine deposits can accumulate on turbine blades in a short time.
Causes of Deposition in Steam Turbine:
Water Impurity:
Utilization of impure water for attemperation of steam or leakages in closed heat exchangers causes turbine depositions. Examining the attemperating system should be ensured if the turbine deposit persists after using pure water. It should be ensured that the attemperation water and steam are equally pure.
Boiler Water Salts Vaporization:
Vaporization of salt in the boiler water is another cause of steam turbine deposition, specifically silica, as it can vaporize into the steam at low operating pressure. It causes deposition issues in the turbine due to the increased solubility of silica at high temperatures. When the steam cools through the turbine, it results in reducing the solubility and forming depositions. It is essential to control the quantity of silica in the steam, and maintain it below 0.02 ppm to reduce the steam turbine deposition.
Localized silica saturation occurring and condensing from the steam is also a source of turbine deposition. Partial evaporation of precipitated silica can result due to a portion of silica dissolved with the continuous steam flow. It leads to deposits remaining in the steam turbine.
Entrainment:
Entrainment of drops of boiler water in steam is inevitable. However, excessive boiler water carryover leads to steam-carried solids producing steam turbine deposits. Some of the common causes of high boiler water carryover are priming and foaming. A high level of water carryover is a usual occurrence that also results in superheater tube failures.
Removal of Deposits:
When the turbine is affected with boiler water salts due to water carryover or attemperating water impurity, the water washing can retain the turbine capacity. It requires special attention and should be conducted under a boiler manufacturer or turbine vendor guidance to avoid severe damage to turbine performance. In case the turbine is contaminated with compounds not water-soluble, it is recommended to remove the deposits by blasting with aluminum oxide.
Erosion:
Erosion of turbine blades leads to rough surfaces that hinder the path of steam flow that reduces the efficiency and capacity of the steam turbine. Solid particles such as iron oxide present in steam cause erosion at a high-pressure end of the turbine. Water in steam causes erosion of intermediate and low-pressure blades. Erosion is also caused, due to operating below design inlet steam temperature or low load that leads to condensation. The presence of carbon dioxide or other acidic components in the condensate can increase the damage. Low distribution ratio amines for neutralizing the acid level and increasing the pH of the condensate can control erosion in the steam turbine
Corrosion:
Some of the issues of steam turbines include pitting, stress corrosion, and corrosion fatigue, usually due to chloride, sulfate, sodium hydroxide, and sulfides. The contaminant solubility in steam decreases as the steam expands through the turbine. It condenses on the surface at higher solution concentrations that cause corrosion. Chloride deposits lead to pitting on rotors, buckets, and disks. Corrosion can be controlled, by maintaining an oxygen-free or condensate-free atmosphere.
Methods to Clean Steam Turbine:
Blast Cleaning:
Blast cleaning is an ideal method for the removal of deposits from the rotating areas of the turbine. The method enables a Nondestructive test (NDT) that aids in removing persistent deposits.
Water washing:
Although water washing dissolves the soluble deposits, the insoluble deposits require mechanical force with high-pressure jet and sandblasting. The water washing method is advantageous for removing the chemically water-soluble aggressive compound.
In close water cleaning, the turbine is cooled or heated at 100°C and filled with condensate through a turbine drain. The rotor is manually turned by hand, and the condensate is drained after 2 to 4 hours. The process is repeated numerous times.
Manual cleaning with solvent:
Cleaning with the hand is a slow process, best suited for inaccessible surfaces and the blade parts that may get damaged with blast cleaning.
Wet steam cleaning:
Soluble salt deposits can be removed effectively from turbine blades through plasma-activated wet steam. The method is usually used during the steam turbine operation.
Rakhoh Boilers have successfully manufactured highly efficient and reliable steam boilers and cogeneration boilers for more than 20 process industries in India and overseas. With 38+ years of expertise in thermal solutions, Rakhoh is a globally trusted name for industrial steam boilers and boiler services.
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- Published in Boiler