Discharge of saturated steam into atmosphere 2

[Giskard]

I have doubt regarding the discharge of saturated steam under pressure (250 psig) into the atmosphere, after a rupture of a pipeline.

Do I have to suposse that this process is adiabatic (isoenthropic) and that part of its enthalpy is transformed into kinetic energy, so the steam can condensate partially?

Or should I suposse that, as there is no work exchange, the process is isoenthalpic, getting a super-heated steam but at a low temperature than the initial?

Maybe someone can help.

Ferran Tarrasa

 

[TD2K]

The process is isenthalphic, just as is if it expanded across a control valve. You will wind up up with an atmospheric stream of superheated steam.

 

[vpl]

Could I question this a bit (for my own education)? When I've seen steam leaks at plants, there is definitely condensation going on. I always figured it was an adiabatic process. Why would the steam stay superheated rather than losing temperature and pressure to the surroundings?

 

[packdad]

As TD2K said, the process is best represented as that of constant enthalpy. Depending on the quality of the original steam and the pressure drop involved, you may end up with superheated discharge steam.

 

[TD2K]

The expansion of the steam across the leak or fitting is at constant enthlaply from a pure thermodynamic point of view.

However, there are complicating factors in the real world. Once the steam is in the atmosphere, there is heat transfer and vapor pressure considerations as it mixes with the 'cold' ambient air. That will cause some steam to condense into 'fog' as the air can't hold all of the steam in vapor form. Note, while I've seen some water dripping off the piping at a steam leak, I haven't seen a spray of water droplets coming out which is more what I'd expect IF a constant entrophy expansion (versus enthalphy) was occuring.

In the real world, the cloud of condensed steam simply mixes with more ambient air until the condensed water is all picked up again. This assumes there is enough ambient air (ie. not inside) and enough physical room before it hits the surrounding (ground, beam, vessel) where some of the condensed steam could 'stick' to the solid object (note the highly technical term 'stick', there must be a better term, can't think of it though). Plus, if the leak is small, heat transfer from the escaping steam through the 'fitting' to the surrounding piping/fitting and then to atmosphere might cause some condensation, that would cause the drips you sometimes see. Any significant steam leak I've seen from a valve packing or a fitting more resemblances a steam jet than a mixture of steam and water droplets.

I've got a really nice little steam program from Archon Engineering (highly recommended BTW) that I'll run some numbers on tonight to show what the outlet temperature temperature would be for a constant enthalphy versus constant entropy.

However, I can assure you it is an constant enthalphy expansion, this is the principle used in a throttling steam calorimeter to quantify steam quality or its initial temperature.

 

[TD2K]

I went back and did some digging through my old thermodynamic books since this was a case where I 'knew' the right answer but I couldn't explain why it was right. Luckily, this wasn't an oral exam.

To answer VPL's question, an adiabatic process does not automatically mean you have an isentropic process. An adiabatic process that is reversible IS isentropic but you can also have a irreversible, adiabatic process. In this case, the change is enthalphy is zero (not totally true for gases but is for the purposes of this discussion).

Essentially, if you have turbulence, you do NOT have a reversible process. So, while steam leaking to atmosphere can properly be considered to be adiabatic, it is not reversible as it is highly turbulent and therefore not an isentropic expansion. Steam flowing through a turbine on the other hand is much less turbulent and therefore closer to an isentropic expansion.

Turbulent here is a quantative term since the flow of all gases and liquids involve some degree of turbulence. Almost makes me want to take thermodynamics over again.