Steam Turbine Overspeed Calculations Based on Steam Flow

K

Thread Starter

KRManish

Hi all,

Basically I am interested in the over speed protection setting calculations for steam turbines, though the same may be relevant in some ways to gas turbines also. OK, here goes:-

A steam turbine trip [stop valve closing], not leading to a generator trip [Generator circuit breaker opening], since the generator megawatt transducer is showing a value higher than the low forward power trip setting --> indicating heavy passing of the stop valves. Now, I want to know what calculations we do so as to arrive at this low forward power setting? gas turbines have reverse power sensing, generator also has slight motoring capability making it much safer without risk of over speeding. What is the minimum steam flow that can cause over speeding? What are some rough values when it is expressed as the percentage of steam flow for 1] FSNL? 2] base load?
 
KRManish,

If the stop valves are passing that badly, wouldn't the prudent course of action be to resolve this problem?

Since most steam turbines are different from most other steam turbines (with the exception of the Combined Cycle steam turbines being produced to day which are very similar to each other (by manufacturer and model/size), you would probably have to contact the manufacturer to find out how much steam flow is required to just reach the overspeed setpoint--or you could do a little trial and error testing on your own, especially since you probably aren't going to be wanting to actually overspeed the turbine in any case.

It seems you're proposing to open the generator breaker on some positive power if the steam turbine stop valves are closed and hope that the amount of leakage from the stop valves doesn't overspeed the turbine? Because, if the stop valves are already closed, then, how would you "trip" the turbine anyway?

Again, it would seem the most prudent course of action would be to resolve the passing stop valves. Most steam turbines I know of have "stem-free" tests to make sure they will close in the event of an emergency. I would presume the reason the stop valves aren't closing is because of scale/build-up on the valve stems, preventing full travel--which is what the stem-free test does, in essence, is to try to prevent an excessive build-up of scale/deposits by regularly exercising the valves to make sure they do actually close. And, if they don't, then the turbine is usually shut down and the valves opened and cleaned/refurbished.

So, something about the operation and maintenance of these stop valves is definitely amiss if they're passing (leaking) that badly. And, if the stop valves have already been closed (either by operator intent or as the result of some emergency), then is it your intent to keep generating power for some period of time if the steam flow doesn't exceed that which would result in an overspeed if the generator breaker opened? That seems extremely dangerous.

To my way of thinking, if the stop valves are closed (either by operator intent or as the result of some emergency) and the generator breaker doesn't open in a very short period of time, then the generator breaker should be opened.

It's just extremely dangerous to operate a steam turbine under these conditions.

I won't even comment on gas turbine stop valve(s) passing so badly.
 
A long standing recommendation for GE steam turbine valve leakage is that with the stop valves closed the leakage shall be such that the settled speed is no more than 2/3 of rated speed. This is very liberal compared to anything other steam turbine OEM's recommend. Within TVA, when it has been necessary to draw a line in the sand with respect to steam turbine valve leakage, that is where it has been drawn, this has been the case for many decades. Operating a machine with leakage as large as you seem to be experiencing is a serious personnel safety hazard, not to mention the risk of loss of the machine.
 
CSA,

You seem to have mistaken this for an actual scenario --- no it isn't. I am just interested in the theoretical calculation part. I have seen control valve passing but not stop valve passing. But I am interested in the speed calculations of steam turbine based on steam flow since I have not seen it anywhere in the manuals. Though I am mentioned just the stop valve passing case, there are other scenarios where this can happen like valve mechanical jamming, trip oil not draining, RTS malfunctioning, etc. Though Siemens designed steam turbines have only combined trip signal [both turbine trip + low forward power relay] for opening generator circuit breaker, I have heard that MHI designed steam turbines do not have the same protection logic, & I recently read about an actual catastrophic case of overspeeding on such a machine. That's why I am interested in these calculations.
 
mhwest,

No, this is not a situation I am facing --- it is just for the theoretical know-how. You mentioned 2-3% of rated speed --> how much % of MCR steam flow does it convert to?
 
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