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# Thermal Relief - two phase fluid

## Thermal Relief - two phase fluid

(OP)
Thermal relief is considered for liquid filled systems. What if the fluid contains a few percent gas? Does the presence of the gas cause the fluid to no longer be considered a liquid, and therefore discount the need to provide thermal relief protection. I am making the assumption that the density of the gas phase increases faster than the density of the liquid phase as temperature increases.

### RE: Thermal Relief - two phase fluid

Agreed, the relief rate may be gas expansion dominated in comparison to the liquid expansion. However, as press / temp rise in this 2 phase system, strange things can sometimes happen. In some multicomponent fluids, the gas may all condense into liquid. See if you can quantify the % gas in this system, model this in a process simulator and see what happens as pressure and temp rise in batch mode ( fixed total volume and fixed total mass) to approach relieving pressure.

### RE: Thermal Relief - two phase fluid

(OP)
George, my question was generic in nature, but you make a good point about the condensation. I could make a case that the gas condenses, providing for a liquid only system. Then if temperature increases enough, the specific volume might change enough to cause a relief. So in the case that condensation happens with a mild increase in pressure, then temperature increases fairly substantially, I could see that the overall specific volume might increase. Thanks for the input.

### RE: Thermal Relief - two phase fluid

I get the impression that you believe there is no case for relief if the system remains as 2 phase at relieving press/ temp. But if this were the case, then you'd still have gas expansion relief to account for initially, followed by liquid relief once all the gas has gone past the RV.

### RE: Thermal Relief - two phase fluid

Thermal relief is normally only required for wholly liquid filled systems due to the large rise in pressure per degree, e.g. exposed to solar heating.

The issue here as I see it is how reliable your "few percent" is. That only you can answer, but if the gas can be swept out or absorbed into the liquid under various conditions, then no it doesn't allow you to avoid thermal relief protection.

It's all about the worst case because thermal expansion pressures unrelieved can be very high.

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