Nitrogen sealing for methanol storage tanks
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For methanol storage tanks with internal floating roofs, as is well known, both the tank top and the tank walls have ventilation holes that are connected to the atmosphere. According to the fire protection standard GB50160-2008, methanol is classified as a Class A B medium, which requires the use of a floating roof. However, the client insists on having a nitrogen seal between the internal floating roof and the tank top. In that case, nitrogen would be released into the atmosphere through the ventilation holes in the tank top and walls, making it impossible to achieve a proper nitrogen seal. If those holes are to be blocked, it would be better to use a fixed roof (without a floating roof), but this again does not comply with the fire protection standards. I’m not sure what to do in this situation. I’ve indeed heard of many cases where such designs are used, with internal floating roofs along with a nitrogen seal, but I’ve never seen them in person and am not aware of how this is achieved structurally. If anyone has seen such systems, please share their insights
I need some help with this; I hope experienced professionals will kindly offer their guidance!
An internal floating roof combined with a nitrogen seal is a very common design. To use a nitrogen seal, it is not possible to have vents on the top and sides of the tank; the area above the floating roof must be a sealed space.
I just built one, with a fixed roof and an internal floating roof, along with a vent valve and a safety valve; in addition, a pressure relief manhole was installed
Thank you for your answer. If that’s the case, then what’s the need for an internal floating disk? It’s sufficient to simply fix the top part and use a nitrogen seal
Internal floating roof + nitrogen seal + vent valve on the tank top is the most common approach used here.
If a nitrogen seal is added to the internal floating roof tank, the floating disk can be removed, as there is no longer a need to consider sealing issues related to oil and gas. Meanwhile, the breather vents on the tank top and walls should be sealed, and the breather valve should be replaced with a slightly positive pressure breather valve; it is also advisable to install an emergency relief valve on the tank top.
I also worked on a similar project recently. The storage tank originally had an internal floating roof together with vent holes, but due to environmental regulations, we designed to seal those vent holes (it’s best to use a removable sealing method so that once nitrogen is no longer used, the tank can be reverted to its normal internal floating roof configuration, thereby enhancing its resistance to risks). In addition, a nitrogen purge system, a roof vent valve, and an emergency relief hatch were installed, along with oil and gas recovery pipelines. This approach not only meets environmental requirements but also reduces losses caused by oil and gas evaporation. The purpose of the internal floating disk is to reduce the evaporation losses of oil; combined with a fixed roof and a nitrogen seal, further reduction in oil vapor loss is achieved. The nitrogen seal also helps maintain a certain pressure inside the tank, preventing it from deforming. This is relatively advantageous for large storage tanks.
How are the nitrogen replenishment amount and exhaust gas discharge amount for internal floating roof tanks calculated? Please give me some advice!
Seal the vent holes, keep the floating disc, and add a nitrogen sealing system.
Let’s start by explaining the basic concepts: an internal floating roof is used to reduce the evaporation of oil products; in other words, it is a requirement driven by environmental considerations, as explicitly stipulated in GB31571. Nitrogen sealing is a safety requirement aimed at ensuring that the gas phase space does not contain explosive mixtures that reach the explosive limit. To achieve nitrogen sealing, the tank must be modified to have a fixed roof, along with a vent valve, nitrogen sealing system, and an emergency release manhole. Many of us make such modifications here; just consult the design institute, and they will usually agree to it. Be sure to recheck the strength of the tank top. As for the method mentioned by @yy510232542 of sealing the ventilation windows/holes in a detachable manner, it is theoretically possible as well, but it is necessary to ensure the reliability of such sealing. This combination of internal floating roof, nitrogen sealing, vapor recovery, breather valve, and breathing manhole clearly results in better environmental benefits; as for the cost, each party should decide according to their own financial capabilities.