Expansion theory

steamfitter

The expansion of water turning into steam by 1700 X its volume is an amazing phenomenon. When a student asked me this question, I was stumped:
Is the expansion relative to the pressure? In other words, when water is pressurized to let's say psi and heat is added enough to boil it, will it still expand 1700 X?
I am embarrassed to admit that I was stumped. Immediately I said yes and then had to say I need to research that question. I feel inclined to think that water under pressure will not expand as much. A little help???

steamfitter

Sorry guys, I meant to write: ...100 psi, and heat is added...

Fred

I would think that the boiling point of the water would change by pressure but it would probably still expand at the same rate????

JStar

I'm going to say that yes, it will remain the same, because the pressure is all relative.

bob_46

you need to look at steam tables. Remember pressure has to be in psi absolute. Steam at atmospheric pressure 14.696 absolute is at 212º F and the saturated liquid occupies .016719 cu.ft. per pound(weight). The saturated vapor occupies 26.799 cu. ft. per lb. so it expands 1,602 times. Steam at 117.992 psi absolute and 340ºF the sat liquid occupies .01787 cu.ft. per lb. . The sat vapor occupies 3.7878 cu. ft. per lb.. So it expands 211.96 times.

DanHolohan

Bob is right.

JStar

I was wrong. Woohoo! But now we know.

vaporvac

Yet another reason to keep those pressures down (and a very good explanation of how it all works.) Now we just need to explore the numbers in a vacuum!!!

Harvey Ramer

So what is the expansion rate when you boil the water under pressure and release the steam to atmospheric pressure? Gotta be some condensation.

vaporvac

Here's a great little pdf with all the info in one place: http://cincinnatitriplesteam.org/documents/SteamTables.pdf
It speaks directly to @ChrisJ 's? (I think it was you chris!) comments on the possibly corrosive effects of condensate, and so many other issue constantly addressed on this forum.

And how did I miss this gem right in my back yard? https://www.youtube.com/watch?v=Qa3luVezzq4

DanHolohan

Harvey, if you release it to atmosphere its 1,700 to 1 again. Stand back.

JUGHNE

That explains how water heater tanks and boilers can fly.

KC_Jones

I think I have mentioned before that my father worked in power plants for 32 years. He used to take me there anytime they had anything "cool" disassembled for maintenance. He was the maintenance supervisor. Anyway we were walking around on the turbine deck once and he points up at the main steam pipe feeding the turbine and says "if that line ruptures we are all dead....instantly. That pipe was carrying 3600PSI super heat steam at I THINK 1100 F. If memory serves it was a 10" line, but I can't remember for sure. So vent that to atmosphere and it returns to 1700:1...it's a lot of steam.

Larry_52

3600psi and 1100deg super heated steam had to be a critical universal boiler. The critical point being when steam and water have the same density so there is no way to get natural circulation. If the main steam line ruptured on a unit as such yes your father was correct, most people in the plant are dead, thank god for ASME. What wouldn't happen is the whole 1700 time expansion as you envision it. Any water would flash due to rapid depressurization but the convection path of the super heaters would be a choke path.

KC_Jones

It was as you say a "super critical" boiler. Can't remember all the details (he retired 10 years ago), but I remember him talking about how different that one was from the other 3 units in the plant....less forgiving for sure. I would say the "choke point" is relative considering the size of those boilers. Can't remember if it was that unit or another, but if memory serves the feed pump was something like 400HP....it could flow a lot of steam. Those weren't even "big" units 4 units, total output was 1000 MW. Biggest unit was 415 MW. There are single units bigger than that whole plant. It's a fascinating site to see for sure, the scale of the mechanics is hard to wrap your brain around even when you see it.

Larry_52

I imagine you left a zero out of that feed pump. 4000hp is more in tune to feed such a boiler. Consider that I use multiple 2000hp feed pumps for a 200MW unit.

Larry_52

If the biggest unit was 400mw the boiler probably flowed steam at psi and temp mentioned in the low 3 million pound per hour range.

KC_Jones

Now that I think of it you are correct...it's been a LONG time since I was around it. I am probably thinking of the salt water pumps for the condensers.

steamfitter

Great discussion and interesting information!
Thanks Bob for referring me to the Steam Table. I should have known to check the table as there is a Volume (cu. ft./lb) column. The table I have is very basic and rounds off the numbers; from 0 psig/14.7 psia @ 27 cu.ft./lb of Steam to 125 psig/140 psia @ 3 cu. ft./lb of Steam.
I forgot that volume is included in the Properties of Saturated Steam Table.
I really appreciate the knowledge you guys have on the subject and this valuable resource: heatinghelp.com! Thanks Dan!

KC_Jones

Larry said:

I imagine you left a zero out of that feed pump. 4000hp is more in tune to feed such a boiler. Consider that I use multiple 2000hp feed pumps for a 200MW unit.

Talked to my Dad today I forgot those feed pumps are run off the turbine not a separate electric motor. He said they are about 15k HP. My memory is really failing me lately. This is the plant he used to work at:
https://en.wikipedia.org/wiki/Herbert_A._Wagner_Generating_Station