3 replies to this topic

[ChemEng01]

Hey Guys

 

 

I am sizing a relief valve for a fire case.

 

 

At the relieving conditions (In my case 282 barg and 295 degC) all the Hydrocarbon contents of my system will be in the vapour phase. There is however still liquid water present (Critical P & T of water 373.9degC & 219 barg).

 

I have used the API equations for vessels filled with vapour / supercritical fluids however I am interested to understand the effects of the water in the system.

 

The relieving pressure is above the critical pressure of water (219 barg) so as the temperature increases above 373.9 degC the water will go from being a liquid to supercritical (dense phase). Would this change have a latent heat in which I could figure out the relief rate? ( Relief = heat input / latent heat). I'm presuming not as the change is not going through vapour phase?

 

HYSYS gets very confused when trying to pass a water stream at these conditions through a heater and setting the downstream vapour fraction to 1 in order to get the liquid / vapour mass enthalpies. It states that a temperature at specified conditions cannot be specified (presumably because its in dense phase and cannot figure it out).

 

Any advice would be much appreciated

 

 

Thanks

 

 

[frpe]

I have a different software (PRODE PROPERTIES)  and

my knowledge is mainly for Excel applications with Prode, 

however, for your problem, if you know

(or can estimate, there are different procedures for that)

the relieving conditions you can

solve a multiphase flash operation

to calculate each phase at equilibria,

then you can calculate the enthalpy (specific or total)

of each phase and from that the heat required

to vaporize a certain amount, 

that could be a possible solution in your case.

[TS1979]

If you can find latent heats for each components, you don't need software to do the calculation. For fire case, you need only to consider the liquid inside the vessel, mixture or not. Based on liquid level inside the vessel, you only need to follow API 521 to calculate the wetted surface and heat transferred to the liquid using the formula provided in API 521. Then you get the vapor flow rate and you can size you PSV based on the vapor flow rate

[aroon]

Hi ProcessNo1,

 

First thing you need to understand is that the vessel relieving pressure is itself very high, which is above critical pressure of many of the normal liquids. In the event of fire, the temperature will first start to increase to liquid saturation point. However, relief pressure itself is in critical region. Relieving temperature will first reach its critical temperature (where simulation for mixtures will show 100% vapor). This means liquid inside vessel will be in critical region at relief condition. For this super-critical fire as guided in API is one of the option to calculate relieving requirement.

 

There is no need to consider separate water phase as at relief all mixture will be in critical zone. However, presence of water (as component) will affect on the fluid properties such as molecular weight (decrease), which will eventually affect on the size of the required PSV.

 

Fire by considering liquid is not at all practical for such a situations because fluid phase itself is not clear at relief.

 

Hope this may be helpful for you.

 

- Aroon