A Comprehensive Guide to Accurately Size Pressure and Vacuum Relief Devices for Atmospheric and Low Pressure Storage Tanks

摘要

Increased environmental concerns and regulation have made proper Pressure and Vacuum Relief Valve (PVRV) design a priority. PVRVs protect tanks against rupture due to increases in internal pressure or implosion due to vacuum. They also minimize emissions, loss of product, equipment corrosion due to emissions, and decrease maintenance costs. The use of Pressure and Vacuum Relief Valves is mandated by almost all of the recognized regulatory institutions involved in tank safety, such as API, OSHA, NFPA, etc. API 12F, Section 6, further stipulates the use of Emergency Pressure Relief Vents, in addition to PVRV’s when storage tanks containing flammable liquids are exposed to fire, unless the tanks in question are installed in remote locations. The venting system must be designed in accordance to API 2000 which supplies the rigorous formulas to calculate both normal venting and emergency (fire case) venting. Annex B and C of API 12 F provide “short cut” tables with values for thermal venting and emergency venting for different tank sizes, respectively. Most API 12F tanks come furnished with a standard size thief hatch, usually 8 inches. It is often mistakenly assumed that this will be adequate to handle the emergency venting requirement. This paper presents a guide to ensuring the tank venting system is designed to handle the maximum relieving requirements in keeping with the most stringent stipulations of the codes. It discusses topics such as: common mistakes made when using API 12 F, inconsistencies between API 2000 and API 12F, use of a dynamic method to calculate the worst case venting scenarios, and the effective reduction in flow thru a standard 8” opening when a thief hatch is installed.