Relief case: Critical pressure : Latent heat~0

[jamesbanda]

Dear sirs.

i'd like some guidenace on two cases i've encounted.. (and validate my methods)..I appreciate this is a bit techy.. i've not show all the methods i've used as i dont want to bias responsces..


1. Relief pressure close to critical pressure.
On this cases the latent heat approaches zero near the ciritical prsesure so you end up with a very large relief valve. This does not seem quite right to me. am i missing something in the anaylsis ?
to date, on projects i've elected to take a lower relief pressure and small valves on some occasions.. but that does not quite sit right..

2. Relief pressure above critical pressure but normal case below critical pressure..

on this case i'm struggling to size valve.. do
A) Treat it as a vapour only for determining as if the system is just above ciritical temp
b) treat as a liquid ?
and is there good guideance on this..

 

[Latexman]

The reference in this post is cited frequently when near or above the critical point:

thread1203-197733

Did you search the forum before asking?

Good luck,
Latexman

 

[don1980]

Are you talking about fire cases? It sounds like it but I can't be sure. If so, then I think these cases are effectively the same as all-vapor vessels, which can't adequately be protected from fire exposure by using a PSV, regardless of the size of that PSV. PSVs protect against fire exposure by establishing a heat equilibrium; heat-in = heat-out. Unless the heat-out comes from vaporizing liquid (Hvap), then the PSV can't prevent the vessel temperature from rising, and failing.

So, if there's no Hvap then sizing a PSV for fire exposure is just an academic exercise with little real-world value. In such cases I size the PSV for one of the other scenarios, and rely on other measures (de-pressurization, automatic water spray, fire resistant insulation) to protect the vessel from fire.

 

[MortenA]

1) No you are absolutely right latent heat approaches zero as you approach critical pressure - or there would be a disconitunity once you entered the dense phase - where condensation cant take place.

So the API formulas just dont work so well near critical pressure...

2) Getting hold on the reference article seems like a good idea. I actually have it. Its actually very common - natural gas above 80 barg is "super critical" - so quite common i would say

In the case - its a gas, and always a gas - i dont think you would have much doubt - heat influx comes from API "dry vessel". So its a "natural convection" case. The reference article also deal withthis in some details. So if you fluid starts out as a two phase - why should it be any different? HYSYS assigns a "vapour fraction" even to critical fluids - but if you read the manual its clear that its "pseudo". And it can be only 0 or 1 and transition does not require any energy - so dont pay attention to this.

Best regards

Morten

 

[knapee]

The method performs a volume/energy balance around the vessel for supercritical fluid as unbalanced mass or energy enters the system.
Starting from normal operating cases, the unbalanced mass or energy enters the vessel, at constant volume, therefore the pressure in the vessel rises.

Once relieving pressure has been reached, the vessel pressure is held constant, the temperature, density, and mass
of fluid in the vessel vary with time.

So the vessel’s relief valve must discharge
the necessary mass/volume from the system to maintain the fluid volume equal to the system volume and thereby prevent the vessel pressure from rising above the relief pressure.

After determining the conditions at which relief pressure has first been got, calculate the unbalanced mass and/or energy input to the vessel about 10-minute interval.


Hope this will help