Selection of safety valves for liquefied gas desulfurization towers
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Which marine engineer is familiar with the issue of selecting safety valves for situations where the fluid is in liquid state when it reaches the valve, but vaporizes immediately after exiting it? For example, in the case of safety valves used at the top of liquefied gas desulfurization towers, the liquefied gas enters the valve in a liquid state and begins to vaporize once it exits the valve. What considerations should be taken into account here? This includes choosing the appropriate type of valve, determining the amount of gas that will be emitted and vaporized, and deciding whether pipeline heating is necessary after the valve to provide the latent heat required for vaporization.
This is closely related to the choice of discharge conditions; even for liquefied gas, if the discharge volume is calculated based on fire conditions with liquid vaporization, the liquefied gas enters the safety valve in gaseous form and is also discharged as a gas. For the liquefied gas desulfurization tower, I personally believe that the leakage volume should be the highest when calculating under fire conditions; of course, this is just a preliminary estimate, and further discussion on this issue is possible.
If expansion is a concern, choose a slightly open-type liquid safety valve. Depending on your situation, whether it’s regarding the process or fire relief, it involves gas release; in such cases, a full-open safety valve should be chosen. As mentioned above, when the installation height of your tower is not high, the fire conditions are generally severe; however, both scenarios need to be considered, and the higher value should be adopted.
"The liquefied gas in liquid state enters the safety valve, and after passing through it, it begins to vaporize” – this type of operating condition seems to occur very rarely! No matter what the liquid is or how large the container is, it is impossible for the container to be completely filled with liquid – especially in towers and containers, there must always be a gas phase space at the top. So, unless it is on a pipeline, safety valves rarely have “liquid in, gas out”!
How to perform calculations for the safety valve of liquefied petroleum gas
I now basically understand what’s going on; it’s sufficient for the liquid to enter the safety valve and then the receiving system, and a normal micro-opening safety valve for liquids will do.
Select based on the maximum liquid leakage rate, and verify according to the fire conditions.
Regarding the calculation of the safe venting volume for liquefied gas, it is first necessary to determine the venting condition; formulas for calculating this in case of a fire are provided in both the \"Regulations on Pressure Vessels\" and \"GB150\"； For more detailed information, refer to HG 20570.2 or API 520, API 521 ； Full-open safety valves are used, often equipped with a bellows.