API 616 126.96.36.199 states: An overspeed trip protection shall operate at 105% of maximum continuous speed. Multiple shaft turbines shall have individual overspeed trip protection for each shaft.
API 616 3.22: maximum continuous speed: The speed (revolutions
per minute) at least equal to 105% of the highest speed required of any of the operating conditions.
API 616 3.38:rated speed: The speed (revolutions per minute) of
the gas turbine output shaft at which site rated power is developed.
For a turbine with a rated speed of 10300 rpm, is it okay to have the overspeed trip at 11330 rpm (110% of rated speed)?
Please include references used for speed.
Section 1.1 of API 616 does allow a vendor to offer alternative designs. Therefore, an overspeed trip setting higher than 5% may be acceptable. However, please be certain that the turbine is designed for an appropriate maximum speed. For example, a turbine with an overspeed trip setting of 110% may need to be designed for a maximum speed of 120%. I would also suggest requiring an overspeed test at the maximum speed in the factory.
API 616 covers both mechanical drive and generator drive applications. For mechanical drive applications, the gas turbine will operate over a range of speeds; therefore, the overspeed trip setting would be in relation to the upper limit of the operating speed range (MCOS). For a generator drive, the speed of the gas turbine will be constant and will be referred to as the rated speed. Therefore, for a generator drive, the overspeed trip setting would be in relation to the rated speed.
Generally, the gas turbine will be designed for a particular maximum speed. This maximum speed, determined by the gas turbine vendor, is selected based on the speed attained on a loss of full load event (perhaps, with a small margin added). The gas turbine vendor chooses an overspeed trip setting to avoid reaching this maximum speed. Setting the overspeed trip too low may result in spurious trips.
An overspeed trip setting of 110% of rated speed is quite common for large industrial gas turbines in power generation service.
For vendor’s alternative designs, you must evaluate each deviation from the API 616 standard. If you are not confident in making those judgments, hire a qualified consultant.
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