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-boun...@febo.com> on behalf of Peter Reilley
<preilley_...@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
t...@electrictime.com<mailto:t...@electrictime.com> /
Electric Time Company, Inc.
Office: 508-359-4396 x 117 / Fax: 508-359-4482
97 West Street Medfield, MA 02052 USA
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