[time-nuts] Line Frequeny Stablity

Peter Reilley preilley_454 at comcast.net
Thu Apr 6 06:31:26 EDT 2017


Another purpose of the reheat process is to control moisture.   The term 
"moisture" here
describes the amount of liquid water in the steam as it flows through 
the system.   The spec
for this is generally about .25% moisture.

The temperature of the steam exiting the boiler is very close to the 
boiling point of water
at that pressure.   The energy to boil water is 970 BTU per lb. The 
energy to change
the temperature of water by 1 degree F is 1 BTU per lb.   Thus there is 
very little to be
gained by heating the steam hotter than it's boiling point.   You can't 
add that much
additional energy and have a temperature that you can handle.   Of 
course the boiling point
of water is quite high at these pressures.

The steam enters the turbine at very high temperature and pressure.   As 
it passes
through the many stages of the turbine you would like the pressure and 
temperature
of the steam to remain just above the boiling point.   You want to 
maintain that
balance of pressure and temperature to insure less than .25% moisture.

The reason that the steam must remain "just above" the boiling point is 
any condensation
produces liquid water.    This liquid water is very destructive to the 
turbine blades.
The velocity of the steam in the turbine approaches supersonic speeds 
and any little
drops of water hitting the blades at those speeds erodes the steel.   
The areas of the
blades most at risk for this have a very hard material braised onto 
their surface.
But even this gets eroded.   I remember it being called "Stellite".

Reheat is used to raise the temperature of the steam to keep it above the
boiling point at certain stages in it's flow through the turbine.

There is a limit to how hot you can go.   Water has a critical point at 
705 degrees F
at 3200 PSI.    Above this point there is no longer a sharp distinction 
between vapor
and water.   There have been super-critical power plants built. But when 
I was in
the business, in the 1970's, they were experimental and not reliable.   
I don't know
if that is true now.

Pete.

On 4/5/2017 5:44 PM, Bryan _ wrote:
> There is a pretty nice "How it Works" video on steam turbines. As Pete mentions they use valving to control the speed of the turbines, interesting how they reheat the steam for the high/medium/low stages.
>
>
> https://youtu.be/SPg7hOxFItI
>
>
> -=Bryan=-
>
>
> ________________________________
> From: time-nuts <time-nuts-bounces at febo.com> on behalf of Peter Reilley <preilley_454 at comcast.net>
> Sent: April 5, 2017 9:34 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Line Frequeny Stablity
>
> The response time in a large plant is very slow.   Large steam plants
> running at steady
> state are running with their steam valves wide open.   A partially
> closed valve is an energy
> loss and is only used when changes occur.
>
> The power control for a plant running at a steady load is the amount of
> fuel thrown into
> the boiler.   When you want more power you shoot more gas, oil, or coal
> into the boiler.
> For a nuke you pull the control rods.   Behind all of this is a lot of
> thermal mass.   Things
> don't change quickly.
>
> Pete.
>
> On 4/5/2017 9:01 AM, jimlux wrote:
>> On 4/4/17 2:28 PM, Thomas D. Erb wrote:
>>> Thanks for the info.
>>>
>>>
>>> So that tells me how data is recorded - but not how the frequency is
>>> kept stable ?
>>>
>>> Is the line frequency now directly tied to GPS clock - with no drift ?
>> The line frequency is adjusted, for the most part, by adjusting the
>> prime power (steam valves, dam penstocks, etc.) on the generators at
>> power stations. That changes the speed, slightly, although as
>> generator 1 of N starts to get ahead, the electrical load increases,
>> and it slows down.
>>
>> It's actually a pretty complex system, since there are a whole raft of
>> "spring constants" in between the multiple generators in a system,
>> there's phase shifts due to transmission line inductance and capacitance.
>>
>> "Stabilizing" a system in the face of changing demand is a non-trivial
>> task.
>>
>>
>>
>>
>>
>>> Thomas D. Erb
>>> tde at electrictime.com<mailto:tde at electrictime.com> /
>>> Electric Time Company, Inc.
>>> Office: 508-359-4396 x 117 / Fax: 508-359-4482
>>> 97 West Street Medfield, MA 02052 USA
>>> www.electrictime.com<http://www.electrictime.com<http://www.electrictime.com<http://www.electrictime.com>>
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>>>
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