A technical question: Turbine run back.

[Nomy-the wanderer]

Our "safety analysis for nuclear power plants" professor asked for an FMEA worksheet and i used some of the malfunctions mentioned http://www.cti-simulation.com/ctisimulation/PWR.htm" [Broken], but i don't really get what was meant by the turbine runback, and hence i don't get the fault here. And I've searched google, but it was really tiring cause i didn't manage to extract some definition.

I've been searching all the morning for malfunctions, circumstances, causes, effects...etc. And I'm almost done.

Also I have another question that might be regarded as an OT question, it's about the criticality rank in an FMEA sheet, the professor asked us to rate it as L, M, H...I was wondering if the c.r. norm corresponded to the environment?? Because I think it could be relative to personnel, system & the environment but that's not what is demanded obviously.

Thank you in advance.

 

[Astronuc]

The term 'turbine runback' refers to a reduction of the no-load speed setpoint at a fixed preset rate of between 1 and 10% of full power per second. The rate varies from station to station.

http://canteach.candu.org/library/19930205.pdf [Broken] (214 pages)

But this definition doesn't seem correct. I was recently involved with a situation where a plant has a turnbine runback at about 75% power and the core power was reduced to 50% rather quickly.

As far as I know the runback has to with the inability of the system to provide sufficient steam (mechanical energy/force) to the turbine when load it applied on the generator. Remember, there load provides a resistance and backward torque on the generator.


With regard to 'criticality' - do you mean severity?

The 'criticality' or 'severity' has to do with the consequences or outcome of the anomalous condition or accident.

How are L, M, H classified/defined?

 

[Nomy-the wanderer]

First of all thanks for the reply.

A runback happens when it's operating at a certain fraction of its power, and a turbine trip happens when there's not enough steam(could be a closed valve or something), which makes the runback a power related issue, while trips and spurious trips are related to the equipments and the components used.

The mal function of a runback would be the inability of the system to provide enough steam and the reaction of the turbine might be damaging to the generator, right??

As for the criticality issue, he asked us to provide a low, medium or high rank for that slot, but i doubt he specified criticality or severity regarding what??

I mean if the runback happens, it would not be ranked high regarding environment or personnel, but might be critical for providing the required electrical power(did i understand that correctly??).

And that's why i was asking about the norm, if it wasn't mentioned that this failure was critical regarding what?

Also for example if i said that the turbine tripped because the valves that controlled the steam were closed, would that have a bad effect in the pressure of the pipes and leads to leakage for example. The steam isn't radioactive so i wouldn't consider it as critical regarding the environment. Also would it be correct that the damage will be limited for the carrying pipes, would it mess the circuit and nearby components or not??

But the crriticality rank seem so vague without specifications indeed.

 

[Astronuc]

It's been a long time since I worried about balance of plant issues.

At low power, I would imaging the criticality would be low. I don't see this as being a severe event. At higher power it might reach medium if it damages the plant requiring shutdown.

If there were sufficient torque on the shaft or coupling or bearings on other the turbine or generator, that could present a significant disruption in operation.

I guess I am wondering about the criteria for selecting L, M, or H. What does each mean in terms of consequences, such operability/reliability and safety of the plant.

I mean if the runback happens, it would not be ranked high regarding environment or personnel, but might be critical for providing the required electrical power(did i understand that correctly??).

The plant should not be the single generator. In theory, a utility has capacity margin from other plants, which would pick up the load.

 

[Nomy-the wanderer]

Thank you astronuc, that pdf was helpfull too (it was a good read).

 

[turbo]

I am not familiar with nuclear power generation, but steam is steam, and it can be made by burning wood, coal, oil, etc. My understanding of turbine run-back control is that it is necessary to protect the turbine-generator set in the event of a disruption in the system - whether it be a sudden loss of load, loss of feedwater upstream, etc. The criticality of such an event should probably be evaluated on the basis of its impact on the steam generator, and not necessarily on the basis of its impact on the T-G set. For instance, a turbine in a Kraft pulp mill may be tripped into a run-back by the loss of feedwater to a Kraft chemical recovery boiler, but the primary danger is not to the T-G set, but to the chemical recovery boiler, which must experience a rapid, but controlled shutdown in order to avoid a catastrophic explosion.

 

[Nomy-the wanderer]

Thanks for your help Turbo, I finally got to meet my professor today, showed him my worksheet and he was very happy, frowned(he never smiles) and told me "Excellent work, i didn't expect that."And then he added: "But there are of course technical errors." I said: "Ofcourse." ( The less mistakes I make the more suspicious he should get, if I were him at least)

For the criticality rank, he told me to rank failures with effects that are related to SG as M or H depending on its severity, if there's an explosion threat i guess that would be H.

I think I'm done with that, next term I'll study some of these issue with more details.

 

[Astronuc]

Any event which causes rupture of the pressure system boundary, either primary or secondary (in a PWR) would be considered H.

If a failed component, e.g. pump or valve, can be isolated and the plant can still operate safely, perhaps derated, then the event is less severe.

Breach of the primary system, which contains contaminated (radioactive) water is consider a severe situation, IMO.

Somewhere the should be a standardized ranking system. I would imagine IAEA or WANO has published such as system.