Provide redundant speed protection for gas turbine

Hi every body,

My question is related more to designers than maintenance staff. But I want to share this with every body, may be somebody can help depending on his field experience.

We have GE gas turbines Frame 9E controlled by mark IV control system. The over-speed protection is provided by 3 speed pick-ups and mechanical over speed bolt. In the last period, this bolt is causing many problems and tripping for the units. We are requested by our managers to bypass (cancel) this bolt and provide a new set of speed sensors as a redundant protection.

But we don't want to make any modification in Mark IV software. So a separate panel and sensors should be provided to achieve this function with 2oo3 logic. The output then can be connected in series with emergency push button or any other way.

Any way, I don't want to guide your mind in some direction, if any body has any suggestion or idea, please tell me.

Thanks and regards.
Bently-Nevada and Woowdward both make suitable overspeed protection units which have been used on many similar units.

However, the problem with most overspeed bolts is that they don't work when they should (they get gummed up and stick), not that they trip when they shouldn't.

So, what's different about the overspeed bolt mechanisms at your site? Is it that the bolts are actually operating, or that the trip latches aren't working properly or aren't reset properly?

The overspeed bolt itself is just a spring-loaded device which, when actuated by centrifugal force, "pops out" and unlatches a mechanism which opens a valve to port Trip Oil pressure to drain (depressurizing the Trip Oil system).

I have personally never heard of a problem with a mechanical overspeed bolt actuating before it should, unless it was improperly adjusted and it would have to be severely improperly adjusted.

I have heard of trip latches getting smoothed out over time and not holding very well. But, another part of this problem was that the accessory coupling vibrations were very high and this, coupled with the trip latch not being held very well, caused the problem; not the actuation of the overspeed bolt.

I think the managers are not understanding the problem properly and are doing the typical controls "band-aid" (plaster) for a mechanical problem that hasn't been very well diagnosed.

Bruce Durdle

My first recommendation would be to find out the problems with the original mechanical overspeed bolt and fix them. A mechanical bolt is about as simple and user-friendly as you can get, and my concern is that if the current problems are caused by eg inadequate maintenance or similar management-related issues, then there is every possibility that these will carry over to any replacement system.
you can't bypass the Mecanical Overspeed (Bolt) by an Electronic Overspeed protection. If you are sure that the M.O.S occurs, then you have to fix the problem.

> in new gas turbine there is no over-speed trip both frame 5 and 7 why?? <

MarkV has a Protective core that has it's own set of three speed pickups and trips on the level of those independant of <RST>. No speed bolt required. I also agree with previous posts about looking into the issue with the bolt operation. Most MarkIV that get upgraded, do away with the speed bolt but it's basically done to fit the new control system and not because the bolt is a defective protective device.
> in new gas turbine there is no over-speed trip both frame 5 and 7 why?? <

I don't know of any GE gas turbine lacking over-speed protection. Codes require it. While the mechanical bolt is no longer used, there are triple redundant probes and monitoring with 2oo3 voting for the trip protection.

You're going to learn quickly that people don't always type what they mean.

It's probably that this respondent meant to say that a lot of new machines aren't including mechanical over-speed bolts these days, and are opting for a second (protective or "emergency") electrical over-speed trip.

But every turbine GE sells will have two "independent" over-speed detection means for tripping the turbine. One is usually electrical or electronic, and the other is either a mechanical over-speed bolt or a second electrical/electronic over-speed detection method.

Most retrofits don't include deleting the mechanical over-speed bolt, unless the Customer has lots of money to spend. It's not a simple job to add the tapped holes to the speed sensing ring for additional speed pick-ups, and to run new conduit and wiring from the speed pickups, in some cases it has to be run all the way back to the turbine control panel location. Most retrofits just jumper the existing speed pickups to the protective or emergency electrical over-speed trip speed pickup inputs.

There is no "standard" method; it's all determined by Customer requirements and budgets.