over-speed in munti-shaft gas turbines


Sometimes that's very difficult to do--prevent an overspeed during a load rejection.

Other than telling us the turbine is a multi-shaft gas turbine, you really didn't give us anything else to work with.

Is the HP/gas generator is tripping on overspeed or the LP/power turbine is tripping on low pressure

Is it a heavy duty gas turbine, or an air-craft derivative?

What fuel is it burning when it overspeeds on load rejection?

What does the turbine drive--a generator or a compressor or a pump or ???

What kind of control system does the turbine have?

If the turbine drives a compressor, is the compressor control well-integrated with or a part of the turbine control system?

Is this overspeeding on load rejection new behaviour or has it always been occurring during a load rejection?

Does this happen on loss of any amount of load, or just full load, or any time load is lost when running at 80% or above, or some other load?

How long has the turbine been in service?

How long has the turbine/compressor control system been in service?

Those are the kinds of questions we need answers to and the information we need to be able to help understand the situation.

We are also presuming you want the turbine to stay running (at least the HP/gas generator section) so the LP/power turbine section can be re-loaded very quickly. You might just want the unit to go into shutdown without tripping--but you didn't tell us that.

Obviously, the key is to reduce the fuel flow-rate to the HP/gas generator section as quickly as possible. How does the turbine control system know the load has been lost? AND, WHEN does the turbine control system get notified the load has been lost?

Also, when the load rejection occurs, does the turbine control system go immediately to speed control, or does it remain on some other kind of control scheme (pressure control; flow control; etc.)?

Lots of variables, as you probably already know. The biggest one--in my personal opinion--is: How quickly does the turbine control system (the one controlling the fuel flow-rate to the HP/gas generator section) get "notified" of the loss of load? Followed VERY closely in importance by: How quickly does the turbine control system switch to speed control when the load rejection occurs?

Again, LOTS of variables here. Also important is the type of turbine--because sometimes some turbine control systems are not really suited for some types of turbines. For example, aircraft-derivative turbines tend to be very light (low inertia) and require very fast fuel control valves and valve controls to be able to respond to load rejection without tripping the turbine on overspeed.

So, without the answers to the questions above--most all of them, really--we can't answer your question with any certainty or solid recommendations. Other than to say: The fuel flow-rate must be reduced very quickly--AND reduced very quickly without extinguishing the flame if you want to prevent an overspeed AND keep the turbine running so it can be re-loaded quickly.

Best of luck with your problem, sir. When you write for help in the future, if would be really good if you can provide a lot more information about the turbine and control system(s) and application and particulars of the situation.