Processing, August 2016

Steam purity and shutdown conditions are two of Steam purity and shutdown conditions are two of the parameters that lead to corrosion damage The environmental conditions produced during shutdown can be another important factor These are the conditions that occur during unprotected shutdown when oxygenated moist and liquid films form on steam path surfaces as a result of hygroscopic effects These films are directly caused by inadequate shutdown practices adopted by a steam turbine operation maintenance team or overall team They can lead to pitting which is most often the precursor to the corrosion mechanisms Adequate material properties such as composition structure and internal stresses and design temperature stresses and crevices also play essential roles Pitting may also initiate during operation in crevices such as blade attachments The purity of steam controls most corrosion processes and is vital to steam turbine reliability Mechanical blockages might occur as a result of deposits Although rare blockages at sensitive locations usually have severe consequences For example even small deposits on the stem of a steam turbine check valve can interfere with its function In the event of a steam turbine trip a malfunctioning check valve may lead to continued steam flow and some damage to the turbine Also deposits on stationary parts if thick and strong enough may impede blade movement presenting a particular risk of mechanical damage to small blades Plugging of the steam flow path changes the pressure relations in the steam turbine in a way that can cause an axial shift of the shaft This can lead to contact between rotating and stationary parts with the potential for severe failure Such conditions are often detected and avoided by monitoring the steam turbine pressures A more frequent but less consequential result of a steam flow blockage is a reduction of the flow passing ability swallowing capacity of the steam turbine and the change in effective steam flow profile on the steam turbine blades These changes result in steam flow reduction reduced power output and degradation of the turbines efficiency Common examples are deposits of copper and aluminum in high pressure steam turbines and silica deposits in medium pressure and low pressure turbines Amin Almasi is a senior rotating machinery consultant in Australia He is a chartered professional engineer of Engineers Australia and IMechE and holds bachelors and masters degrees in mechanical engineering and RPEQ He is an active member of Engineers Australia IMechE ASME and SPE and has authored more than 100 papers and articles dealing with rotating equipment condition monitoring offshore subsea and reliability CREATING THE FUTURE OF WATER WEFTEC is the one event for professionals industry experts and the most innovative companies from around the world Learn from the very best thought leaders in water quality REGISTRATION IS NOW OPEN JOIN US IN NEW ORLEANS New Orleans Morial Convention Center September 24 28 2016 32 Processing AUGUST 2016 www weftec org 250 WEFTEC_ Std_ FP_ 0816 indd 1 7 20 16 3 31 PM 3 STEAM TURBINE DRIVERS Condensing The primary type of steam turbine is the condensing steam turbine which has been used for large drive applications above a certain power rating limit roughly 1 above 8 MW These steam turbines exhaust directly to one or more condensers that maintain vacuum conditions at the discharge of the steam turbine An array of tubes with cooling water condenses the steam into water liquid in the condenser Back pressure Back pressure steam turbines are the most suitable equipment for mechanical drive applications such as 2 the drivers of compressors or pumps The term back pressure refers to steam turbines that exhaust steam at above atmospheric pressures The discharge pressure is usually established by the specific application of the steam in a plant Lower pressures are frequently used in small and large low pressure applications such as heating systems and higher pressures are often used when supplying steam to industrial processes Extraction An extraction turbine has one or more openings in its casing for extraction of a portion of the steam at some 3 intermediate pressure The extracted steam may be used for process purposes The steam extraction pressure may or may not be automatically regulated depending on the steam turbine design Plugging of the steam flow path changes the pressure relations in the steam turbine in a way that can cause an axial shift of the shaft This can lead to contact between rotating and stationary parts with the potential for severe failure