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I was trying to find out what would happen to Supercritical Water (3200psi/374C) if the pressure was suddenly reduced to 1 atm (14.7 psi). [EDIT: My actual plan is to reduce it to 2000psi]. My guess is that the water would immediately flash to steam. But I've had many people tell me that "what happens" depends entirely on HOW the Supercritical Water was made. Was it heated in a boiler to supercritical state, compressed mechanically to supercritical state, found on the moon, etc. Does anyone know definitively what the answer is?

Kyle Borghini
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  • I don't understand the nuances--but I do know you have described a boiler explosion, which are so deadly b/c when the pressure containment fails and immediately become 1 atm (0 gauge)--the water is now steam and is in way too small of a volume. – JEB Apr 19 '18 at 01:30
  • Suppose the Supercritical Water were flowing through an adiabatic porous plug, across which the pressure was being dropped from 3200 psia to 14.7 psia. This would basically be a Joule Thompson flow, in which the enthalpy change is zero. What do the steam tables tell you about the final state? – Chet Miller Apr 19 '18 at 01:44
  • @Chester Miller I know very little about thermodynamics but I desperately want to learn. I had an idea for creating supercritical water through compression, and I was told that it could never become steam (despite any pressure drop) simply because it had not been HEATED to a supercritical state and therefore enthalpy was 0. – Kyle Borghini Apr 19 '18 at 04:55
  • My answer here provides a link to a website that tells you more than you would ever want to know about the phases of water: https://physics.stackexchange.com/a/381615/170832 - 3rd link, P10. – Rob Apr 19 '18 at 05:46
  • You had an idea of creating supercritical water through compression? Compression starting from what temperature and pressure? – Chet Miller Apr 19 '18 at 11:51
  • @Chester Miller Compressing water (and maintaining it) through mechanical means is no simple task, but's lets say we start with water that is 1 atm and room temperature. Once it is compressed, we heat it to 800F with an electric heating coil. There are textbook examples on how to do this to make an end-run around the critical point. My original question asked if, upon expanding the water to some subcritical pressure, would it become steam, or would it Not vaporize simply because it hadn't been heated from start and had no latent heat stored in it? I've been repeatedly told the latter. – Kyle Borghini Apr 19 '18 at 13:36
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    It doesn’t matter how it arrived at its initial state above the critical temperature. – Chet Miller Apr 19 '18 at 14:07
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    That's what I thought. How it arrived at critical state is irrelevant... Only thing that matters is that it IS at critical state, and what I do after that is all that matters. Taking the supercritical water down to some subcritical state will give me whatever result the steam tables dictate. Thanks for the clarification @Chester Miller – Kyle Borghini Apr 19 '18 at 15:57

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If it started out at the initial state you identified in your first post (basically the critical point), and you passed it passed it through an adiabatic porous plug down to atmospheric pressure of 14.7 psia, the water would emerge at 100 C, and as a combination of about 25% liquid water and 75% water vapor (steam).

Chet Miller
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  • I'm a bit rusty. adiabetic = no heat loss. enthalphy = H = U + PV. So we're assuming U is constant? – Mohammad Athar Apr 19 '18 at 15:07
  • No. We're assuming H is constant (for flow through an open system). – Chet Miller Apr 19 '18 at 15:24
  • @Chester Miller The reduction down to 1 atm was just an example in the original post. I really intend to reduce it only to 2000 psi, reduction probably taking place through an expansion valve. I'm hoping the result will approximate the steam table's prediction – Kyle Borghini Apr 19 '18 at 21:17