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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?

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  • $\begingroup$ 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. $\endgroup$
    – JEB
    Apr 19, 2018 at 1:30
  • $\begingroup$ 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? $\endgroup$ Apr 19, 2018 at 1:44
  • $\begingroup$ @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. $\endgroup$ Apr 19, 2018 at 4:55
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    $\begingroup$ It doesn’t matter how it arrived at its initial state above the critical temperature. $\endgroup$ Apr 19, 2018 at 14:07
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    $\begingroup$ 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 $\endgroup$ Apr 19, 2018 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).

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  • $\begingroup$ I'm a bit rusty. adiabetic = no heat loss. enthalphy = H = U + PV. So we're assuming U is constant? $\endgroup$ Apr 19, 2018 at 15:07
  • $\begingroup$ No. We're assuming H is constant (for flow through an open system). $\endgroup$ Apr 19, 2018 at 15:24
  • $\begingroup$ @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 $\endgroup$ Apr 19, 2018 at 21:17

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