Pressure Storage Tank

Key Takeaways

• The distinction between atmospheric and pressure storage is determined by the vapor pressure of the stored fluid at storage temperature — a fluid with low vapor pressure (stabilized crude oil, produced water, diesel fuel) can be stored in an atmospheric tank because its tendency to vaporize is insufficient to build significant pressure in the vapor space above the liquid; a fluid with high vapor pressure (raw NGL, LPG, propane, light condensate) would rapidly build pressure in an atmospheric tank, potentially exceeding the tank's allowable working pressure and causing catastrophic failure; Reid Vapor Pressure (RVP), measured at 100 degrees Fahrenheit using ASTM D323 or ASTM D6377, is the standard metric used to classify storage requirements — crude oil with RVP above 11 psi requires internal floating-roof or pressure storage, and NGL fractions with RVP above 14.7 psia (the atmospheric pressure boundary at sea level) must be stored in pressure vessels to prevent evaporation loss and pressure buildup; the RVP of produced hydrocarbons varies significantly by formation and treatment level, requiring measurement on each produced stream before storage equipment is specified.
• Bullet tanks are the most common pressure storage vessel in oilfield surface facilities design, used to store LPG and NGL at gathering stations, compressor stations, gas processing plants, and fractionation facilities — named for their elongated cylindrical shape with hemispherical or ellipsoidal heads (resembling a bullet cross-section), these horizontal vessels are available in standard sizes from 125-gallon propane tanks for remote wellsite heating fuel to 30,000-gallon and larger tanks for NGL product storage at gathering plants; bullet tanks are designed to the ASME BPVC Section VIII Division 1 standard, with maximum allowable working pressures (MAWP) typically in the range of 150-250 psig for LPG storage (sufficient to contain propane at summer ambient temperatures up to approximately 110 degrees Fahrenheit); the tank pressure rating must account for the highest vapor pressure of the stored fluid at the maximum expected ambient temperature, with a safety margin — a propane bullet in a Texas Gulf Coast location, where summer temperatures can reach 105 degrees Fahrenheit, requires a higher MAWP rating than the same tank installed in northern Alberta where ambient temperatures rarely exceed 90 degrees Fahrenheit.
• Pressure relief valves (PRVs) and rupture disks are mandatory safety devices on pressure storage tanks, designed to prevent tank overpressure from fire exposure, vapor generation from ambient temperature increase, or overfilling scenarios — API 520 and API 521 specify the methodology for sizing pressure relief devices on pressure storage vessels, including calculating the required relief capacity for fire case scenarios (where external fire heats the tank contents and generates vapor at a rate that the relief device must handle without the tank pressure exceeding 1.1 times the MAWP); pressure relief valves on LPG storage tanks are sized to handle the fire case vapor generation rate at the maximum expected fire exposure, and the relief valve set pressure must be equal to or less than the MAWP of the vessel; the discharge from pressure relief valves on pressure storage tanks is typically piped to a flare or vapor recovery system rather than vented to atmosphere to prevent uncontrolled hydrocarbon releases at ground level near the tank farm; the combination of properly sized relief devices and adequate drainage under the tank (to remove flammable liquid in a spill scenario) is the primary engineered safeguard against pressure storage tank failures.
• Mounded pressure storage tanks are an alternative to aboveground bullet tanks in which the pressure vessel is covered with a compacted earth mound that provides passive fire protection, reduces vapor heating from solar radiation (reducing vapor pressure in the tank and therefore reducing venting losses), and mitigates BLEVE (Boiling Liquid Expanding Vapor Explosion) risk in a fire scenario — a BLEVE occurs when a pressure storage vessel is exposed to external fire that weakens the tank shell through localized overheating while the internal vapor pressure simultaneously increases; if the shell fails catastrophically, the superheated liquid flashes instantaneously to vapor, creating an overpressure wave that can extend hundreds of feet from the tank; mounding provides passive cooling that prevents the localized overheating that initiates BLEVE, and is mandated by some national codes for LPG storage above certain capacity thresholds in populated areas; mounded storage requires significantly more land area and earth-moving cost compared to aboveground bullet tanks, making it more commonly used for large strategic reserve storage (tens of thousands of cubic meters of LPG) rather than for small-to-medium gathering facility tank farms.
• Inspection requirements for pressure storage tanks under API 510 specify the maximum allowable intervals between external visual inspections (typically 5 years for aboveground vessels) and internal inspections (up to 10 years, or risk-based intervals determined by a fitness-for-service assessment) — the principal degradation mechanisms for pressure storage tanks in oilfield service are corrosion (from water accumulation at the tank bottom, particularly in wet crude or NGL service where free water settles below the hydrocarbon), stress corrosion cracking (in sour service environments where H2S reacts with steel under stress, a mechanism covered by NACE MR0103 materials selection requirements), and fatigue from pressure cycling (most relevant in tanks that are frequently filled and emptied as part of batch transfer operations); ultrasonic thickness measurements taken during inspection verify that corrosion has not thinned the tank wall below the minimum required thickness, and the remaining corrosion allowance determines the acceptable service life until the next inspection or replacement is required; pressure storage tanks that have been derated (operated at less than their original MAWP because of wall thinning) must be clearly labeled with the current MAWP and tracked in the facility's mechanical integrity management system.