power boiler analysis 1

[insinyur02]

please explain why:

- in a power plant system, steam line from a power boiler (140 bar) is connected to one 100 MW turbine (1), and the other line is connected to 134 bar header to supply 100 MWe turbine (2) and to supply 82 bar header by using PRV for other turbines .

question: when turbine (1) trip, why steam drum level and pressure decreases and suddenly increases?

your idea would be appreciated..

many thanks

 

[rmw]

I don't know where you are observing the pressure reading but I doubt that the steam drum pressure decreases. With the level on the other hand, the level decrease is very predictable because when the turbine trips the pressure rises in the boiler and the boiling stops or greatly slows down so the volume of water that was occupied by steam bubbles is now occupied by water and the level drops.

Then the other turbines try to pick up the load and when their throttle valves move open the steam drum pressure drops and boiling begins again vigerously causing the water to swell raising the level until pressures stabilize.

rmw

 

[MintJulep]

insinyur,

You may have noticed that you are getting relatively few replied to the questions that you have posted here on Eng-Tips. Here is why:

The questions you are asking are very basic AND you do not appear to be making any attempt to answer your own questions, or even demonstrate that you have a fundamental understanding of the area in question. Also, your questions are generally poorly phrased, and lacking in detail.

Eng-Tips is not a substitute for the things that you should have learned in school.

Please take some time to read some of the threads here on Eng-Tips that have many replies. You will see the types of questions that generate interest.

It appears that English may not be your first language. Do not worry about that. Take the time to be more descriptive and provide more details in your posts. That will also spur more interest. Additionally, writing skills are important for engineers. Any writing practice is good.

 

[rmw]

Gee Mint,

I answered him within 19 minutes of his post....

Except for the fact that he needs to substantiate what he perceives as a pressure drop when he sees the level drop (those two happening simultaneously are incongruous to me) I thought his question was reasonably understandable. I've lived through his very scenario white knuckled while watching the level drop waiting for the low water cut out to take the boiler out too only to do it all over again when the boiling starts again worrying that the high water cut out was going to get us (and many times it did.)

rmw

 

[MintJulep]

I was thinking about the pressure drop.

Now I'm not a turbine guy, but I assume that when the turbine trips the throttle valve is supposed to close, right?

But imagine if the throttle doesn't close, or closes slowly. They turbine's load has suddenly gone away, and now you've got this spinning thing with lots of inertia - acting like a pump.

Maybe?

 

[insult2injury]

I suppose it is possible that when the generator trips and the turbine overspeeds that the corresponding lower flow resistance is admitting more flow dropping the pressure until the stop valves close. Once the stop valves close, less steam flow is required, but the boiler may maintain the same heat input rate. The pressure will increase in this scenario; however, this may be nothing like the real scenario.

I2I

 

[rmw]

When the turbine trips (normal trip) the stop valves close as in slam shut right now. The steam flow instantly stops. The boiler meanwhile has no way to know this instantaneosly and keeps operating at the firing rate which was producing enough flow for the turbine. Not until the pressure rises (which affects the level too) does the boiler master sense the error signal and back the firing rate down all of which doesn't occur anywhere near as fast as the turbine stop valves slamming shut.

So the pressure in the boiler rises, which changes the saturation temprature/pressure relationship of the steam in the boiler internals and the water and the boiling is curtailed, causing the water to shrink as the bubbles either stop being made or even collapse due to the higher pressure.

Since he had more than one turbine on the header, and other users which I took to be power turbines, the other turbine generator(s) how ever many there are, immediately reach out and try to grab the load that was dumped when the first turbine tripped. Its (their) control valve(s) open rapidly to try to load up and until their control systems realize that they have overshot, they can actually cause the boiler demand to be higher than it was (assuming of course that the other turbines have enough capacity to reach out and grab the load) until they overshoot and are brought back and stabalize.

That is what I see in the description given, first a drop in the level (I still dispute a drop in the pressure) and then a rise in the level as all the others reach out and try to grab the load all at once, each of them overshooting somewhat.

rmw

 

[thickhead]

I’ve seen drum bouncing because of safety’s popping caused by incorrect functioning of valves; turbine by-pass or dump to atmosphere.
Boiler 40 bar; 30 ton per hour steam; turbine using all produced steam on pressure control; turbine trip; safety valves lifting. It certainly got everyones attention

 

[vpl]

Sometimes, it's an indication problem. Indicated pressure could appear to drop, if the response time on the control valves for the other turbine exceeded the response time for the pressure instrument.

As rmw said, if the control valves for the other turbines opened rapidly to reach out and "grab the load" they may have overreacted and opened further than acutally needed before closing back down and restabilizing things. So in reality, pressure may have gone up when the first turbine went off line, gone down when the control valves for the second turbine opened, and then stabilized when the transient flattened out.

But if the pressure sensor is relatively slow, it could have easily missed the first pressure rise and only caught the pressure downturn making it appear that there was a pressure decrease on loss of the first turbine.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[taffcad]

Hi

I am no expert, just 36 years in a boiler house so don't all jump dowm my throat. Isn't this just basic PRIMING? i.e. Sudden significant load change. If the changes can't be slowed with say anticipatory control because of turbine location is too close to the boiler what about a pressure surge vessel?

If that is all rot. Just say so. I am thick skinned.

 

[stgrme]

Since there appears to be one boiler, two turbines connected directly to the boiler, and a pressure letdown to other turbines, this is a complex problem.

If one turbine trips, its stop valve closes in about 0.2 second or less. This fast closing sends a pressure wave upstream toward the boiler. If the other turbines are not at full capacity, they may pick up some or all of the first turbine's load. This load pick-up by the other turbines may tend to reduce the pressure in the upstream line.

You would need to run a transient analysis to determine the pressure characteristics at any point in the piping system.

Best of luck!