[Solved] How do steam turbines work considering that the steam force should place equal pressure on both pipe ends?

Thread Starter

ballsystemlord

Joined Nov 19, 2018
76
Hello,
So I understand that locomotives have boilers that are filled with water and then they seal the boiler shut and boil the water. Therefore, the steam has only one direction to travel to relieve the pressure. That is to the load side and thus to the outside air.
But when you're running a continuous operation, like a CSP (Concentrated Solar Reactor, picture below) -- or more so, a nuclear reactor, how do the designers get the steam generators to work?
The amount of water pressure should be equal on both the intake to the heat source and the output to the generator. And at least nuclear reactors must have a continuous source of cool water. So the amount of power needed to push the water towards the heat source should be equal to the amount of resistance in the pipeline + the resistance of turning the generator. in other words, you should loose power instead of generating it.

I've tried to read up on the subject, but I suspect that the diagrams I've seen are oversimplified.

Thanks!
 

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boostbuck

Joined Oct 5, 2017
183
In a closed cycle steam system, the pressure drop across the extractor turbine is provided by the steam return repressuring pump following condensation.
 

Reloadron

Joined Jan 15, 2015
6,584
Years ago, maybe 30 years ago, I had the pleasure of working with some torpedo programs. The MK 46, MK 48 and the MK 48 ADCAP all ran on otto fuel which is a sort of diesel fuel with its own oxidant. When a faster deeper torpedo was in demand it was the birth of the MK 50. The MK 50 was driven by a steam turbine including power generation. We had a boiler which was fired by spraying Sulphur hexafluoride on a block of lithium. Serious violent reaction generating tremendous heat. Once the high pressure steam ran the turbine system. The steam propels the torpedo in a closed Rankine cycle,[5] supplying power to a pump-jet. This propulsion system offers the very important deep-water performance advantage in that the combustion products—sulfur and lithium fluoride—occupy less volume than the reactants, so the torpedo does not have to force these out against increasing water pressure as it approaches a deep-diving submarine. The stream returns in the closed loop as condensed water and the process repeats. Really a cool system. When Westinghouse bought the rights they also bought me on loan from my company. That was sweet for me. Eventually I left NAVORD (Naval Ordinance) and went to Naval Reactors and propulsion where I remained till retirement. I always saw the principal of operation for thr MK 50 as interesting. Collecting data on those turbines was just plain interesting.

Ron
 

crutschow

Joined Mar 14, 2008
29,772
The amount of water pressure should be equal on both the intake to the heat source and the output to the generator.
No.
As boostbuck noted, it's a closed system, but there is a break in the loop.
All the air is taken out of the system and the turbine exhaust goes to a water cooled condenser which generates a near vacuum at the exhaust.
So the pressure across the turbine equals the relative steam pressure plus about 14psi (or the absolute steam pressure).

The condensed water is returned to the boiler using a high pressure pump.
 
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Thread Starter

ballsystemlord

Joined Nov 19, 2018
76
Call me stupid, but I'm not understanding how this works at all. Even after reading the wikipedia page linked.
I've taken the liberty of editing the wikipedia picture to show what I don't understand. When you heat the water up the steam could go both directions with equal pressure (relative to the pressure the water was pumped to). When you cool it back down again, the generated vacuum would put near equal suction on both sides.

Feel free to annotate the image to show me where I've misunderstood.

Thanks!
 

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boostbuck

Joined Oct 5, 2017
183
I'm not sure where you get your +/- 100 psi figures from, but in your diagram the point 3 is high pressure, falling across the turbine to point 4. The condenser reduces pressure by condensing steam to water (delivering energy as heat to the cooling system) and the pressure drop across the condenser is maintained by the return pump, which has creates pressure differential.
 

boostbuck

Joined Oct 5, 2017
183
It is the return pump maintaining the primary pressure differential that creates the flow of energy, since without it pressure would reach equilibrium with an (limited) energy flow using both paths from the boiler to the condenser.
 

boostbuck

Joined Oct 5, 2017
183
here you go. Your misunderstanding seems to lie in the behaviour of the return pump and it's preventing reverse flow. Some numbers to illustrate:

1647482413070.png
 

MrSalts

Joined Apr 2, 2020
1,762
The red color represents steam (water vapor - a gas). Steam is not like the aerosol above your tea kettle, it is pure gaseous water. That steam can be at 1200 psi or more. You can see above how the turbine is designed to allow that high pressure gas (steam) expands through the turbine as pressure drops to near zero psi at the heat-exchanger (showing Q(out). The speed snd momemtum of the steam turns the turbine (much like the blades on a jet engine (or, simplistically, like a windmill)) - which turns the generator.

The blue is steam with much reduced pressure (reduced temp) and, at the bottom of the heat exchanger, you see the drops as steam condensed to water and the whole loop starts again at the boiler (Q(in).
 

crutschow

Joined Mar 14, 2008
29,772
In an actual power plant the boiler pressure would be several thousand psi, so the feedwater pump obviously has to be a very high pressure type.
One reference states that a large (700MW) power plant has a feedwater pump that requires a 17.5kW steam turbine to drive the pump at 6000 RPM, transferring 1200 t/h of water (about 900k US gallons) at a 5500psi pressure difference from the condenser to the boiler (!).
 

strantor

Joined Oct 3, 2010
5,962
the amount of power needed to push the water towards the heat source should be equal to the amount of resistance in the pipeline + the resistance of turning the generator. in other words, you should loose power instead of generating it.
If this feels like an overunity problem to you, consider that the steam is only a medium. Like the magnets and wires in a motor which are electrically connected to the wires and magnets in a generator. The power comes not from the steam (magnets) but from the heat source (generator).

The reason why it takes less energy to pump the condensed water back to the heat source than what is generated, is because the water has much less volume than the steam. That's how the whole thing works. Take water and turn it into steam with heat, then its volume increases many fold, and with that increase in volume and pressure it has incentive to go somewhere.
Screenshot_20220316-231451_Google.jpg

Pumping 1 cubic foot of water at 100psi requires much less energy than pumping 1700 cubic feet of water 100 psi.
 

Thread Starter

ballsystemlord

Joined Nov 19, 2018
76
I'm not sure where you get your +/- 100 psi figures from, but in your diagram the point 3 is high pressure, falling across the turbine to point 4. The condenser reduces pressure by condensing steam to water (delivering energy as heat to the cooling system) and the pressure drop across the condenser is maintained by the return pump, which has creates pressure differential.
It was a made-up number for illustrative purposes.
 

Thread Starter

ballsystemlord

Joined Nov 19, 2018
76
If this feels like an overunity problem to you, consider that the steam is only a medium. Like the magnets and wires in a motor which are electrically connected to the wires and magnets in a generator. The power comes not from the steam (magnets) but from the heat source (generator).

The reason why it takes less energy to pump the condensed water back to the heat source than what is generated, is because the water has much less volume than the steam. That's how the whole thing works. Take water and turn it into steam with heat, then its volume increases many fold, and with that increase in volume and pressure it has incentive to go somewhere.

Pumping 1 cubic foot of water at 100psi requires much less energy than pumping 1700 cubic feet of water 100 psi.
Oh! That's perfect! Thanks!!!
 
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