Monotube Boiler Design – Radical Thinking

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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fredrosse
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Re: Monotube Boiler Design – Radical Thinking

Post by fredrosse » Sun Jun 07, 2020 9:47 am

Stean Captain, there are technical problems with the diagram you have presented. As I see it drawn, the feed pump will indeed need to pump all excess flow capacity against full boiler pressure. As you mention, say the feedwater pump flow rate provides 400% of the steam generator flow. For this case, if the recirculation flow averages 300%, then at the pump suction there will be (on average) a mix of 1 unit feedwater flow from the hotwell (or feed tank, or lake water, whatever) and 3 units of hot saturated water from the recirculation. This mixture results in very high temperature water at the pump suction, which will flash to a steam/water mixture at the pump suction, and that is an unacceptable condition for pump suction.

The monotube arrangement described previously (within this forum thread)delivers a solution that is relatively efficient, keeps all the steam generator tubes always flooded with liquid water/steam mix, and uses no moving parts to provide the control , giving steam only at the output to the steam engine.
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Re: Monotube Boiler Design – Radical Thinking

Post by Steam Captain » Sun Jun 14, 2020 8:01 pm

I see about the suction evaporation. I didn't think of that. But how do recirculation pumps actually avoid that issue? What would the solution be?

But the plumbing schematics I presented do mean that the pump does not in fact work against all the boiler pressure, as the pipe on the suction side is also under boiler pressure, since it directly connects the pump with the steam dryer. So, the boiler pressure stands on both pump sides(plus/minus the frictional pressure losses of the recirculation cycle). The pump thus does indeed only need the work to overcome the recirculation flow resistance while working on recirculation mode.

Say the steam dryer stands under 150psi pressure and the pump moves 50lb of water. In analogue to weights, it means the weight of 50lb of water is already on ~100 meters height (the "height" - read equivalent of pressure of the steam dryer)
To move these 50lb of water through the coils back to the steam dryer, the water needs to be lifted only, lets arbitrarily say 20 more meters for 30psi more pressure to end up in the steam dryer, back at 100 meters/150psi, only for those 20 meters/30psi to have been lost to the resistance of the recirculation. But the pump does not in fact need to "lift" the water from the 150psi the water is already in to another additional 150psi plus friction losses while recirculating.
The pump only works against 30psi in this example and this example is already using very tiny tubes/fast flow. Sucking off of the hotwell is of course the counter example. That water is on 10 meters height/atmospheric pressure only.

Here is a very simple diagram of the pressure changes along the system for both feed water and recirculation mode: (Remember the pressures used are just for using some rough numbers for readability)

Image

But the question remains how a recirculation pump avoids the evaporation issue. Their existence means there is a way, but is a way applicable for us? I've read that Lamont boiler test page from the Stanley steamer site again and the designer uses a turbo pump for the circulation. Can one say turbo pumps are less prone to suction evaporation/cavitation than reciprocating pumps?
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Re: Monotube Boiler Design – Radical Thinking

Post by Steam Captain » Wed Jun 17, 2020 4:15 pm

The cavitation issue was keeping me occupied and I gathered some thoughts:
I don't think reciprocating pumps are by design unsuitable for a recirculation. I am still just not sure about the proper definition of a monotube and its destinction to Lamont/Benson boilers. If monotube boilers are defined by a once-through operation, the whole recirculation subject might be spread out somewhere else. Leave alone the whole "flash" steam boiler discussion. I think a flash is not possible as a continuous operation and can only and exclusively appear as a momentary event, like the famous example of a steam locomotive running too low on water, leaving parts of the heating surface dry and red hot. A change in the incline of the railway then covers those red hot surfaces with water and the water suddenly flashes into steam. This is flash steam.
For example there seem to be comments here and there about those small steam hydroplane "flash" boilers and some even saying the tubes are red hot while operating. I don't have made up my mind, but one youtube video for example is in deed showing glowing tubes. But when does it happen? The moment the engine and pump aren't running and the tubes have no work to do. The moment the engine is started, the red glow immediately ceases as the water/steam is taking in the energy. No flash. A continuous energy transfer.
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Re: Monotube Boiler Design – Radical Thinking

Post by fredrosse » Thu Jun 18, 2020 2:03 am

OK, now I understand you are thinking of a system where in one moment the pump is a feedwater pump, then it becomes a recircualtion pump, then reverts back to a feedwater pump, etc. I think there would be difficulties, as the duty requirements are so different, while feeding the boiler from the hotwell (or feedwater tank,etc.) the pump must push against full boiler pressure, then in recirculation mode, only push against a moderate pressure drop, but at a much larger flow rate.

This is very similar to the Lamont boiler setup, except the Lamont arrangement uses two seperate pumps, each appropriate to its own pressure and flow requirements. I think that arrangement is more practical.

"But the question remains how a recirculation pump avoids the evaporation issue. Their existence means there is a way, but is a way applicable for us?" ANSWER: Some large utility boilers use centrifugal pumps to force circulation in the boiler evaporating circuits, and to avoid pump cavitation, what you call "the evaporation issue". All pumps, be they centrifugal or reciprocating, require some NPSH (Net Positive Suction Head) to avoid internal cavitation. In the utility industry, these pumps get the required NPSH by placing the pump suction at a lower elevation than the boiling water area of the boiler, therefore gravity does the job of increasing the pump suction pressure. Usually the pumps require several feet lower elevation, about 25 feet is typical. Obviously this is not an option for a small steamboat plant! Reciprocating pumps usually require even more NPSH.

"I've read that Lamont boiler test page from the Stanley steamer site again and the designer uses a turbo pump for the circulation." ANSWER: That arrangement may work for a limited time, but with such a low NPSH available in an automobile, the pump is very probably cavitating severely (steam flashes in the pump impeller, then vapor collapses when pressure increases in the impeller, causing damage) and will not last for a long time. Also the pumping performance of a turbo pump is very much reduced if it is cavitating.
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Re: Monotube Boiler Design – Radical Thinking

Post by Steam Captain » Fri Jun 19, 2020 10:39 pm

Ah, I see. Now I understand why Lamont boilers in our dimensions seem to live a niche existence. Thanks a lot for your ever so invaluable information.

Yes, I seem to sometimes use own word creations. I hope they are clear enough to understand.
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