Relief case: Critical pressure : Latent heat~0
Relief case: Critical pressure : Latent heat~0
(OP)
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..
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..





RE: Relief case: Critical pressure : Latent heat~0
thread1203-197733: PRV Set Pressure above Critical Pressure
Did you search the forum before asking?
Good luck,
Latexman
RE: Relief case: Critical pressure : Latent heat~0
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.
RE: Relief case: Critical pressure : Latent heat~0
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
RE: Relief case: Critical pressure : Latent heat~0
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