On Tue, 19 Aug 2003 04:34:12 GMT, Gerald Miller
wrote:

On Mon, 18 Aug 2003 07:10:33 -0500, Jim Kovar
wrote:



Hunting snowshoes *on* snowshoes. That's fun!
Not when you dip the muzzle of the 20 gauge in the snow just before
you see the bunny!
Gerry :-)}
London, Canada

Thats what condoms or electrical tape are for.

Gunner

"The French are a smallish, monkey-looking bunch and not dressed any better, on average, than the citizens of Baltimore. True, you can sit outside in Paris and drink little cups of coffee, but why this is more stylish than sitting inside and drinking large glasses of whiskey I don't know." -- P.J O'Rourke (1989)
--------------------------------------------------------------------------------

Jon Elson wrote:

Jon Elson wrote:

It might be worth while developing some large power store system to
be kept on-site at a nuclear reactor... like the thing we Brits have
in Wales, a large lake up a mountain which water is pumped to when
the grid is underloaded, and used for hydroelectric power when it's
not.


Oconee Nuclear Station (Duke Power) with 3 reactors has the only
system with the back up being a hydro plant. The hydro plant
only has two turbines. There is also a dedicated line to a steam
plant 30 miles away for secondary backup.




Ah. Well, that's a LOT of power. I think the problem is that these
large stations don't operate
properly, over the long term, under very light load. Also, most
mid-size nukes have a single
turbo-alternator set.

Very true. If the load is removed, they do NOT dump it to the
cooling towers and try to maintain the reactors on line. This
calls for the reactors to go to minimum power or to be shut down.
In a multiple reactor plant, one unit may stay up with minimum
power providing power to maintain cooling water flow to all the
systems.

Daniel Haude wrote:

On Mon, 18 Aug 2003 16:23:10 -0500,
Jon Elson wrote
in Msg.



Up to this last tiny bit, everything you have replied is absolutely
perfect. The large
medical MRIs, due to the weight of the magnet, dewars and cryogens
alone, as well
as the magnetic fields, the requirement that things be incredible stable
for the MRI
imaging process to work, etc. require the magnet housings to be massive
structures.
I know of several medical MRIs in my area that have been hit by Oxygen
bottles of
various sizes, as well as carts, instruments, gurneys and a floor
polisher! As far as
I know (I have a very good friend who works on many of them) all of the
magnets basically
survived the assault.



But they did quench, no?


Oh, yeah! Any large iron-containing object that moves within a meter or
so of the
magnet is likely to make the field jump around, and a quench is likely
to happen.
There have been cases where a truck moved in the alley behind the NMR
facility,
and the magnet quenched.

Some newer magnets use external field cancelling coils to reduce the
volume of the
strong external field, and it also reduces the magnet's susceptibility
to external
influences. So, these are supposed to be more resistant to outside
influences causing
a quench.



All of them suffered total destruction of the
plastic "beauty"
covers, and a great mess to the shim and gradient coils that are inside
the main magnet
coil. But, as far as I remember, he said no magnet itself needed to be
replaced.
These incidents still can cause several hundred thousand Dollar repair
bills, as it
is almost always hospital negligence that caused the incident.



Indeed. But you're right: A heavy iron object accidentally brought near a
medical NMR is too likely an event in a hospital to permit the complete
annihilation of the single most expensive piece of equipment around. My
observation was based on my experience with scientific magnets which are
supported by rather scanty stainless-steel and plastic rods to minimize
heat transfer into the dewar.


Well, the insides of the medical ones have superinsulation, and fancy
low-thermal
conductivity supports, too, but the outer housing is pretty beefy. They
have to
vacuum test the dewars and the outside housing after a quench to detect
any leaks
that may have developed. That right there requires some heavy
structure. After
the vacuum checks, the outer housing remains evacuated.

Jon

Alaric B Snell wrote:

Jon Elson wrote:

No, it is really just hundreds of amps. They have to "ramp up" such
a magnet with a non-superconducting power
supply, with non-superconducting cables, so the currents outside
can't be enormous. Therefore, there's no
way to get enormous currents INSIDE the superconducting part. But,
it doesn't matter, as they just add more
turns of the superconducting wire. The flux can add up to millions
of amp-turns, however.


Ok, cool.

*thinks*

How does one define the energy content of a ramped-up magnet? The
problem with superconductors is that I'm familiar with nice ohmic
devices. Big current round the coil, but no voltage, so no power
transfer - that's fine, it's a uniform energy density and it's not
changing.

But how does one work out an energy from that? I feel the inductance
of the coil must be involved somewhere...

The energy is in the field, not the winding. So, winding current
doesn't mean much. It is the total
number af amp-turns, and the volume of the interior of the solenoid. If
it has an external iron
shield, that lowers the energy, I think. (I'm more used to working with
iron-core inductors with
small air gaps, so these things are a bit far from my general experience.)

Of course, you can just measure the inductance!

Jon

Howard R Garner wrote:


Very true. If the load is removed, they do NOT dump it to the cooling
towers and try to maintain the reactors on line. This calls for the
reactors to go to minimum power or to be shut down. In a multiple
reactor plant, one unit may stay up with minimum power providing power
to maintain cooling water flow to all the systems.

But, even if you trip the reactor immediately, the fission reaction
takes an hour or more to subside,
unless you poison the reactor by dumping thousands of gallons of borated
water into it, which will
require about a 6 month shutdown to recover from. So, even if you trip
the reactor immediately,
the thermal output will go to about 50% in 12 hours, and take 2 weeks
more to go to about 20%!
That is the fission tail of the fission daughters breaking down
spontaneously, and the spontaneous
fissions don't pay any attention to control rods.

Jon

On Tue, 19 Aug 2003 09:39:47 GMT, Gunner
wrote:

On Tue, 19 Aug 2003 04:34:12 GMT, Gerald Miller
wrote:

On Mon, 18 Aug 2003 07:10:33 -0500, Jim Kovar
wrote:



Hunting snowshoes *on* snowshoes. That's fun!
Not when you dip the muzzle of the 20 gauge in the snow just before
you see the bunny!
Gerry :-)}
London, Canada

Thats what condoms or electrical tape are for.

It was my dad who did this about 80 years ago, and your suggested
precautions were not that readily available. I still have the single
shot Stevens with the little bulge about 5" back from the muzzle, Last
time I fired it, I got to eat the bunny.
Gerry :-)}
London, Canada

The container that the RF coil adapted to was test tube size.
The field coils were much larger.

The RF had an RF coil - e.g. 20 turns of 16 gauge wire attached
to a coax. The sample was placed into the tube and away we went.

No LOX no LN03 - none of that expensive exotic stuff needed.
It could be done in the home shop actually - not that exotic.

Martin
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

On Sat, 16 Aug 2003 02:07:30 -0500, Jon Elson
wrote:


Eastburn wrote:

No, actually, the big turbo-alternator sets can't ever stop turning.
The alternator
rotors weigh 50 + tons, and the shafts will bend if they stop turning.
They have
a small motor called the turning gear that keeps the machine rotating slowly
when no steam is available. The air gap on the rotors is incredibly small
for such a huge machine, about .001" per side. The rotor on an 800 MVA
alternator is about 18-20" diameter and about 12 feet long. it is solid
steel,
with the shaft and rotor one piece. Grooves are cut into the face of the
rotor, and solid rectangular copper bars are fitted into the grooves, with
insulating paper surrounding them. They run some fantastic current through
these bars, something like 10,000 Amps at 100 V DC! They'd melt if it
wasn't for the Hydrogen cooling.

Jon

I have had the opportunity to be in a station (coal fired, Port Neal
on the Misouri River near Sioux City, IA) while a turbine was shut
down for maintenece and saw the drive system used to keep it turning,
that said, they were working on the turbine and it was stationary.
They had the bearing cap off it, big babbit style bearings. I've also
seen pictures of them slinging the rotors by big hoists to lift ehm in
and out of their cradles. So here is my question, do they actually
bend from their own weight or do they jsut keep them rotated to keep
the bearings lubrication working or something? If they do bend when
stationary then I assume this must take a while because I doubt they
have a good way of supporting the center while turning before stopping
rotation.

Dave

David L Peterson wrote:

I have had the opportunity to be in a station (coal fired, Port Neal
on the Misouri River near Sioux City, IA) while a turbine was shut
down for maintenece and saw the drive system used to keep it turning,
that said, they were working on the turbine and it was stationary.
They had the bearing cap off it, big babbit style bearings. I've also
seen pictures of them slinging the rotors by big hoists to lift ehm in
and out of their cradles. So here is my question, do they actually
bend from their own weight or do they jsut keep them rotated to keep
the bearings lubrication working or something? If they do bend when
stationary then I assume this must take a while because I doubt they
have a good way of supporting the center while turning before stopping
rotation.


I don't think the turbine, itself, is the problem. The complete turbine
rotor to
power an ~ 800 MW alternator is not all that large. Although it is
somewhere
near the size of the alternator rotor, it is mostly spindly blades on a
heavy
shaft. The low-pressure section does have some pretty large rotor
discs, though.

But, I think it is the alternator that is the problem. It is about
10-12 feet long,
and 18 - 24" or so in diameter. Using the largest of those dimensions, the
rotor (not including the shaft) weighs 9 tons (amazing, I thought it
would be
way more than that) and only clears the stator of the alternator by
about .001"!
If you let it sit, it will sag and crash into the stator. I'm not sure
what you do
to get it free again once that happens. The babbit bearings are
hydrostatic,
I think, and as long as the lube pump is on, there's no problem of it
setting
the shaft down onto the babbit.

Yes, I assume it takes some time for the steel to sag enough to crash
the rotor.

Jon

On Wed, 20 Aug 2003 23:10:27 +0100, Mark Rand
wrote:

On Wed, 20 Aug 2003 00:59:00 -0500, David L Peterson wrote:


Problem is that the inlet end of the HP rotor and the IP rotor are at approx
1000 deg F during normal running. When the turbine is shut down the rotors are
more than hot enough to sag under their own weight. Either sagging or hogging
can occur with uneven cooling of any part of the rotor.

OK, now that makes a lot of sense, thanks for clearing that up.

Typically the barring
gear will be used for 48 hours after a shut down. If the electrical barring
gear is unavailable or trips due to a bent rotor, then the hand barring gear
will be used and the rotor will be turned by 1/4 turn every five minutes to
reduce the problems.


Mark Rand
RTFM

Cool, didn't know they had provisions for barring it over by hand
while assembled. How much does it take? (length of bar, number of
people, or is it through the reduction so it's just a lot of pulls)
The electric drive seemed to have a pretty big motor on it when I saw
it.

Howard R Garner scribed in
:

DejaVU wrote:
so, Gunner and his survivalist buddies have a point after all!

but I still fail to understand why the nuclear plants shut
themselves down? anyone care to explain?

Easy.

Remove the load and you have no need for the power they could
generate. It is also part of their safty system.

All nuclear plants have a secondary source of power and also have
emergency generation capabilities. Never a concern about
radioactive releases. Just a safty shutdown lake a steam plant
would do.

Thanks Howard

Howard
20 years of Navy nuclear experience

note I am not anti nuclear, nor do I fear it. I asked because the
news was full of it

lies, damn lies, and the 8 o clock news

--
swarf, steam and wind

--
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Eastburn scribed in
:

Without a load, one doesn't want to generate G Watts - in fact
where does it go. The pop the lines and shut down the core.

hmmmm.... when my bike lamp disconnects from the alternator the
alternator happily spins and does nothing (-:
a voltage spike, but no current.... since it's not connected,it
doesn't matter.
but I guess big turbines are a different matter...

Gee haven't you ever been in a power house ? - been near ? I have
been and worked inside as a consultant.

nope, not been inside one. been inside one of the cooling water
ponds at the local steamplant which was shut down in the late 70's.
All machinery is gone, but the cooling ponds and pipework is still
around. 12 inch pipes iirc.... a friend of mine hired one of the
pump houses a a workshop for a while, pumps gone but floor still had
the mounting points and pipes in the walls....

about 130km from here is an emergency plant that has gas turbines
that would come online to power essential services for that big city.
their problemis that the thing was built 'outside town' but now town
has enveloped it, and the noise is scary when it runs and the people
complain.

Martin

--
swarf, steam and wind

--
David Forsyth -:- the email address is real /"\
http://terrapin.ru.ac.za/~iwdf/welcome.html \ /
ASCII Ribbon campaign against HTML E-Mail - - - - - - - X
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