May I ask how everyone determines the set pressure for the nitrogen seal?
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Some people say that the lower the set pressure of the nitrogen sealing system, the better, as it can reduce nitrogen consumption. But I checked, and some netizens also said that the set point cannot be lower than the saturated vapor pressure of the material. For a container containing styrene, whose saturated vapor pressure is around 1.33 Kpag, should the nitrogen purge pressure be set higher than that value? Is 1.5 kPa a bit too high? Thank you!
I just checked – the saturated vapor pressure of hexane is 17 Kpa. It’s impossible for the nitrogen seal valve to be set at such a high value, right?
Generally speaking, storing materials at room temperature means storing them in a saturated state, which is the most economical approach. I wonder what the corresponding temperatures are that were mentioned by the two people upstairs? The set pressure for a typical nitrogen seal valve refers to the pressure of the nitrogen gas supplied, in order to prevent low pressures from occurring inside the tank due to large amounts of material being discharged from it (or changes in temperature, etc.), which could affect the safety of the tank itself. This set pressure has little to do with the saturation pressure; it generally depends on the design pressure (negative pressure) of the storage tank. If the designed negative pressure for the storage tank is -500 Pag, then a set pressure of generally +200 to +500 Pag will suffice. Of course, it depends on the specifics of the project. If the saturated vapor pressure of the material at room temperature (typically 20°C) is too high, dome tanks are not recommended; instead, internal floating roof storage tanks are advised. If the material is toxic or requires air isolation, a configuration with an internal floating roof and nitrogen blanketing can be considered.
Thank you to the moderator for the help! I don’t quite understand the concept of saturated state storage. Could you explain it again? Saturation state refers to 100% pure material with no other components, such as water content? The storage temperature is at room temperature, around 20-30 degrees.
I roughly understand it, but I’m not sure if it’s correct; please ask the moderator to correct me. The main purpose of the nitrogen seal valve is to maintain a slight positive pressure inside the storage tank; this serves to prevent the tank from collapsing and also to stop air from entering the tank (which is the primary function of the nitrogen seal). Its main function is not to reduce the evaporation of materials, so it does not need to be set above the saturated vapor pressure. The most effective way to reduce material evaporation is to install an internal floating roof. One more question: how do we determine whether an internal floating roof is needed? Should it be determined by a saturated vapor pressure higher than how many Kpa? Thank you!
Regarding whether to use an internal floating roof and whether to install a nitrogen blanketing system, reference can be made to the \"SHT3007-2014 Design Code for Tank Areas in Petrochemical Storage and Transportation Systems\", which provides detailed information on these topics.