Questions regarding the calculation of the discharge volume of the tower top safety valve?
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For a stripping tower or distillation tower equipped with a reboiler, if a safety valve is installed at the top of the tower, what operating conditions need to be taken into account when calculating the discharge capacity of this safety valve? I hope those with experience in calculations will come and discuss this. First, regarding fire conditions: when performing calculations, how should the wet surface area of the tower be determined? Should it be the total surface area of the tower, the surface area of the tower bottom, or the surface area of the tower bottom plus a certain margin? Secondly, in the case of reflux interruption, how is the leakage rate calculated under such conditions? Should the feed rate and the reboiler load be considered? If there are any other important operating conditions, please feel free to share your thoughts so we can discuss them together.
The maximum discharge rate is usually calculated based on fire conditions. The wet surface area of the tower should be the area of the liquid layer on the tray. Additionally, it is necessary to take into account that, when calculating under fire conditions, the gas phase inside the tower expands due to heat. The discharge volume is calculated based on liquid phase vaporization + gas phase expansion on the tray. I haven’t seen any discussions on this topic in relevant books; this is purely my personal opinion, and I hope everyone will share their views.
The leakage rate calculated under fire conditions is not necessarily the highest. In my opinion, the leakage caused by thermal expansion of the gas phase is quite small. As for the vaporization of the liquid phase, I feel that in the event of a fire, since the tower is well-designed, the part that is most affected by heat should be the liquid in the bottom of the tower; the trays also need to be taken into consideration, but to a lesser extent. I’m curious to know what others think
That’s how our company does it; the most severe discharge scenario is likely one resulting from a fire, but backflow failures or ruptured heat exchange tubes cannot be ruled out either. The maximum wetting area is determined in accordance with API521; if I remember correctly, **the wetting area within a certain height must be taken into account, including the surface area at the bottom of the tower, the surface area of the pipes from the bottom of the tower to the reboiler, as well as possibly part of the reboiler’s surface area and the surface area of any other connecting pipes. Other heat loads are not considered. Additionally, the fire calculation formula in GB150 is more conservative than that in API521; cost considerations are involved, so the original poster should study this matter further on their own. Our company uses API521; the formula is Q=43100*F*A^(0.82), where F is set to 1, and a margin is left for A.
1. The maximum discharge capacity of the top safety valve is equal to the amount of vapor rising from the first tray at the top of the tower. 2. The maximum leakage rate for containers used to store liquefied gas is determined based on fire conditions, with two options regarding insulation; it is calculated using the wet surface area of the container and its latent heat. 3. For other various situations, you can determine it by referring to the discharge capacity of the safety valve specified in the process manual.
When calculating the discharge capacity of the top safety valve, the discharge volume at the pressure level reached by the safety valve in the event of a water loss failure in the top condenser must also be taken into account. Since this calculation is rather complex, it is possible to use as a reference the gas volume exiting the tower under normal operating conditions, multiplied by a safety factor.
Two scenarios should be considered: fire and cooling water failure. The calculation for the former is relatively simple, as explained in 4F, while the latter is generally calculated based on the VAPOR LOAD at the top of the tower (the feed rate to the condenser). See which one is greater. Just take that one.
Please refer to the \"Petrochemical Design Manual\", which contains detailed explanations.