Steam turbine control was the subject of the last technology evening presented by Moore Process Controls.
A steam turbine is a device that extracts thermal energy from pressurised steam and uses it to do mechanical work on a rotating output shaft. The modern manifestation was invented by Sir Charles Parsons back in 1884.
There are two main types: impulse and reaction turbines. The main difference is the manner in which the steam is expanded through the turbine. Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse to improve the efficiency.
For any steam turbine to operate there must be a pressure difference between the steam supply and exhaust. There are different classifications of turbines depending on their purpose and working pressures. The different classes are:
• Condensing steam turbine.
• Back-pressure steam turbine.
• Reheat steam turbine.
• Extraction turbine (steam extraction).
Turbine control is important to prevent catastrophic failures. The typical IO control inputs used for turbines are speed and steam valve output. The following IO are optional to improve the controls of the speed of the turbine: steam valve feedback; upstream steam pressure and temperature; downstream steam pressure and temperature; steam flow; and turbine casing temperature.
Up to three speed measurements can be used for speed control and will most commonly be implemented as follows:
• When three inputs are available – use the average.
• When two inputs are available – use highest of the two.
• When one input is available – use that measurement.
• When zero inputs are available – stop the turbine.
It is important to start and stop a turbine in auto to ensure that each start-up or shutdown is identical for the same process conditions. A turbine typically has three start-up sequences: hot, warm and cold. A turbine typically has two shutdown sequences: normal and fast stop. Data for these sequences is typically provided by the OEM.
The presentation also included a demonstration of steam turbine speed control.
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