What is the requirement of dike walls for large LPG storage vessels?
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You need to quantify your use of the term "large" and state if your tank is "refrigerated". Only refrigerated tanks require "dikes" (e.g. officially called impoundments in NFPA 58) as the spilled liquid will remain a liquid when you have a LOPC event. When we have a non-Refrigerated tank (most typical) we do not want any liquid to be trapped under the tank as this creates a pool fire scenario which can lead to a BLEVE event. See NFPA 58, 184.108.40.206 (2018 edition) and/or 13.5 Refrigerated LP-Gas Container Impoundment.
You might find some useful information in API Standard 2510 Design and Construction of LPG Installations
Note Section 1.5 and 5.5
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