API 616 overspeed trip setpoint specification

[2012TECH]

API 616 5.4.4.6 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)?

 

[stgrme]

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.

Best of luck!

 

[2012TECH]

Thanks for your reply stgrme; What reference are you susing for the speed? Are those refering to the percentage of rated speed?

 

[abeltio]

The OEM typically specifies what is the OS setting of their protection devices.

Also, classification societies (Lloyd's, ABS, Bureau Veritas, etc) and insurance companies have a lot of experience on this matter.

The over speed protection sttings for mechanical drives (e.g. compressors) is typically different than for power generation.

In power generation i've always seen the electrical overspeed protection setting at 110% (10% above rated speed) and the mechanical over speed bolt setting at 112.5%
Since the early 1990's the classification societies, insurance companies and construction codes have accepted the elimination of the mechanical overspeed bolt replaced with a second, independent, set of speed pick-ups for overspeed protection.

saludos.
a.

 

[2012TECH]

Thanks for your input Albetio!
The number 110% seems to be very frequently evoked as overspeed shutdown setpoint!
I have a litle bit of confusion between the different terms used in the standard; what is the relation between the MCOS (maximum continuous speed) and the Rated Speed?
Stgrme, in view of API 614 section 1.1 (The vendor may offer alternative designs), how do you evaluate the compliance to the API614 standard?

 

[2012TECH]

Read API 616 instead of API 614 in the previous post

 

[stgrme]

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.

As abeltio noted, an overspeed trip setting of 110% of rated speed is quite common for large indutrial 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.

Best of luck!

 

[snoopnoon]

Normal speed = 100% rated speed. that is for the rated or normal point of all driven equipment

Maximum Cont Operating speed (MCOS)= 105% of normal speed, This is the speed that would be required by any of the specified operating conditions.

Trip speed = 110% of of MCOS for steam, 105% MCOS for gas. in relation to normal it is 115.5% (of 100% normal)=105*1.1 for steam and 110.3% or normal = 105x1.05 for gas

The whole idea behind API machines is the ability to meet the process needs at all off design points,

 

[2012TECH]

Thanks to all for your input!