A few questions regarding power generation by steam

[pranj5]

I have a few questions. Let's assume that we have steam at 200 barA pressure and 400°C
temperature. If the temperature fluctuates but remains above the saturation temperature (365.746°C), does
that affect power generation and turbine performance badly? Do the pressure too will vary with the
temperature?
My common sense tells me that the pressure shouldn't vary until the temperature will reach the saturation
level and the power generation along with turbine performance shouldn't vary too. But just want to be
assured from others here.

 

[russ_watters]

I'm not following your question. You haven't specified what states you have at each step of the process. Please do. In particular, I don't see a pressure and temperature before and after a turbine. I just seen one state with temperature that "fluctuates" a little. There is no power generation implied by what you describe.

For the part at the end, though; pressure has to be constant between processes where there aren't any obstructions - such as a boiler and condenser, has to rise across a pump/compressor and has to drop across a turbine or throttling valve. Steam cycles are driven by pressure, not temperature.

 

[pranj5]

The pressure before entering the turbine is already mentioned and after the turbine let's consider that to be 1 barA and 100C. I simply want to know if the output will be affected enough if the degree of superheating changes a bit; as for example by 20-30C. Kindly remember t hat despite this fluctuation, the steam will still be in superheated state.
In the Boiler the water is converted into steam and superheated in a isobaric manner. If steam can be heated isobarically, that means that can be cooled too in isobaric way.

 

[JBA]

Any fluctuation in the temperature will proportionally raise or lower the boiler section pressure and if not controlled that would vary the turbine speed; however, in these systems there is a pressure control valve between the boiler and the turbine that controls the steam feed pressure required at the turbine inlet. On electric power systems that valve is controlled by sensing the turbine speed so as to maintain the generator's 60 cps output required by the power distribution system.

 

[Nidum]

Power turbines are normally supplied with steam at a pressure and temperature which allows them to run at best efficiency for a given loading . If the pressure and temperature of supplied steam are significantly out of the tolerance band for normal running then efficiency will usually drop and the turbine could be damaged .

Changing the condition of the steam supplied to a steam turbine affects what happens at every stage in the turbine .This is not something which can be properly analysed in abstract .

 

[pranj5]

Power turbines are normally supplied with steam at a pressure and temperature which allows them to run at best efficiency for a given loading . If the pressure and temperature of supplied steam are significantly out of the tolerance band for normal running then efficiency will usually drop and the turbine could be damaged .

Kindly go through my first post where I have clearly said that the input steam is superheated and fluctuation isn't that much that it will lost its superheated status. Superheated means the temperature of the steam is above the boiling point at that pressure. Let's come back to my first post again. The steam is at 200 bara pressure and the temperature is 400°C. The saturation temperature at 200 bara pressure is 365.746°C. I want to mean that the temperature fluctuate but never goes below 365.746°C. As for example, if the temperature falls to 380°C from 400°C. I want to know whether such fluctuation can drastically affect the output and performance of the turbine. My common sense tells me that as the temperature is still above the saturation level at that pressure, therefore the pressure shouldn't vary much with that fall.

Changing the condition of the steam supplied to a steam turbine affects what happens at every stage in the turbine .This is not something which can be properly analysed in abstract .

That can be said with a little common sense. I need some more precise answer.

 

[JBA]

The boiler pressure must be maintained at a level sufficiently above the control valve design inlet pressure required for the valve to maintain a turbine inlet pressure that assures the turbine can maintain the required 60 cps rotation speed as the generator loading varies with the power distribution system load demand variations.

As long as the boiler temperature is high enough to maintain the boiler pressure above that critical minimum pressure and above the saturation temperature, variations in the boiler temperature/pressure will have no effect on the turbine.

 

[pranj5]

As long as the boiler temperature is high enough to maintain the boiler pressure above that critical minimum pressure and above the saturation temperature, variations in the boiler temperature/pressure will have no effect on the turbine.

Thanks! That's what I want to be clarified. In short, you want to mean that until and unless the temperature will go below the saturation level, the fluctuation in temperature wouldn't effect (or effect very little) to the turbine performance, right?

 

[Nidum]

In short, you want to mean that until and unless the temperature will go below the saturation level, the fluctuation in temperature wouldn't effect (or effect very little) to the turbine performance, right?

That's not correct . Think about the mass flow of steam and the enthalpy drop across the turbine when it is producing constant power .

 

[JBA]

As I stated above: It must also be maintained above the temperature needed to provide the minimum turbine inlet pressure required for desired turbine performance as well.
Superheated steam is a gas and it obeys all of the standard gas laws i.e. P2/P1 = T2/T1 and k= 1.3; and, pressure is the controlling factor in a steam turbine driven system's performance.

 

[pranj5]

That's not correct . Think about the mass flow of steam and the enthalpy drop across the turbine when it is producing constant power .

I can understand that reduction in temperature means drop in enthalpy and that means drop in output. But, I just want to know whether the change will be enough so that turbine can be harmed?

As I stated above: It must also be maintained above the temperature needed to provide the minimum turbine inlet pressure required for desired turbine performance as well.
Superheated steam is a gas and it obeys all of the standard gas laws i.e. P2/P1 = T2/T1 and k= 1.3; and, pressure is the controlling factor in a steam turbine driven system's performance.

I have stated that the inlet pressure is 200 bara and gas temperature can be changed both at constant volume and at constant pressure. I want to know whether the pressure will remain constant during the fluctuation until the saturation level is reached or both pressure and volume will reduce along with the temperature.

 

[Nidum]

reduction in temperature means drop in enthalpy and that means drop in output.

Output of a real turbine is set by the load .

As JBA explained there is a monitoring and control system on a real turbine to adjust the steam flow so as to deliver the required power and maintain the required rpm .

Normally supply pressure and temperature of steam is closely regulated . If in an abnormal condition - like you are asking about - the pressure and temperature go outside the acceptable limits then steam flow would have to be further adjusted to compensate .

Usually though an abnormal condition would trigger a shut down while the problem was investigated and before any damage was done .

 

[Nidum]

Think about the mass flow of steam and the enthalpy drop across the turbine when it is producing constant power

Try doing the analysis . How do mass flow and efficiency vary with changing steam inlet conditions when turbine is producing constant power ?

 

[pranj5]

Well, then just tell me if such condition can arrive if the steam temperature is still above the saturation level or not. In short, how much temperature fluctuation can be termed as "abnormal condition".

Try doing the analysis . How do mass flow and efficiency vary with changing steam inlet conditions when turbine is producing constant power ?

You want to mean that the power can be kept constant by increasing the mass flow of steam, right?

 

[JBA]

Actually, that the generator rotation speed can be maintained constant by increasing the mass flow of the steam as the generator load is increased, and visa-versa.

 

[pranj5]

In short, you are saying that a control valve that can adjust the flow rate is enough to handle the issue, right?

 

[JBA]

That is correct, as long as the temperature remains above its critical minimum as discussed above.

It is the minimum required temperature that is an important boundary, if you had stated your question as: "...a minor increase of the temperature of xx degrees will not affect turbine performance..." this thread would probably have been much shorter; but, you would not have learned as much about all of issues involved.