PSV Relief Requirements

[Lijantropo]

Good morning,

I have to size an PSV that will be located on a air drum. I have been reading the API 521, and from the list of contingencies, the applicable is "external pool fire".

The drum is 26ft (height) and 11 ft (diam.) and it will operate at 105 psig & 105°F. The design consider a 3ft water level in the drum.

The API 521 said that you should take the height of liquid equal or less than 25 ft above the source of flame, in this case the 3ft, and use it determine the wetted surface, the heat absorbed and the relief requirements (using the heat absorbed and the enthalpy of vaporization).

The mix air-water is in the two phase region at the operation conditions, so any heat flow to the drum will evaporate an defined amount of water.

The cp of air is low, I mean, it takes a small amount of energy to increase the temperature of the air. And if I take into account the area in contact with the air and exposed to fire (the area above the liquid level but under 25 ft) probably the heat flow form fire will increase the temperature of the air, and increase the pressure inside the drum.

This effect, and the vaporization of water will occur at the same time, but for me, the geometry of the drum (and the height of liquid level) will made that the increase of the air's temperature have a mayor influence in the relief requirements of the PSV than the vaporization.

What do you think?


Regards,

Lij.

 

[tickle]

Firstly is a pool fire under the drum a possible scenario

It is usual to use just the wetted area. In a fire scenario it is assumed that the process is isolated to lock in the pressure, therefore the two phases will be separated

Heat that goes to the vapour space will transfer to the water vapour that is subsequently being relieved through the PSV.

 

[BenThayer]

if a fire is valid scenario, you might want to consider a rupture disc.

with the relatively small level in the tank, the water will be evaporated rather quickly and then you will have a fairly large vessel with ~100 psig on it that will likely fail due to rupture.

 

[Lijantropo]

Thank you tickle & Ben for your responses.

I am trying to do a dynamic calculation to determine what is most important: the vaporization or the increase in the temperature of the air. If anyone have extra information I will appreciate it.

Regards,

Lij

 

[robsalv]

Why is there a water level in an air receiver drum?

At our plant, drums with no liquid service automatically get exemption from having a fire case considered for SV sizing.

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[Lijantropo]

Good evening,

There is a water level because the drums is located next to compressors, and the air is saturated at that point. It will occur some condensation in the drum and if you take into account that we are handle a really big flow, it is possible to reach the 3 ft of water.

Regards,

Lij