Overpressure is a term that can bring nightmares to tank owners, operators and engineers alike. Vacuum failure is the same. Too much pressure buildup in a tank can cause it to rupture, resulting in loss of containment, increased risk of fire and potentially loss of life. Too little pressure can create a vacuum within a tank, causing it to implode and resulting in similar but generally less severe effects. To alleviate these pressure changes, storage tanks are required to have vents that can allow internal vapors to escape (to alleviate overpressure) as well as allow air to fill in the tank (to prevent vacuum). Recently, API 2000 — the governing standard for tank venting requirements — introduced a more precise means of determining the flow requirements. However, these new provisions provide additional levels of complexity in the computations and remain an enigma to those who are accustomed to the old venting calculations (now moved to Annex A). This article sheds light on these new requirements in order to prevent catastrophic failures in the future.
Both overpressure and vacuum failure can be caused by a variety of different sources; however, two primary sources are accounted for in API 2000: liquid movement in/out of the tank and thermal effects. For out-breathing flow requirement, the amount of liquid getting pumped into the tank causes a displacement of the vapors within the tank. As such, this displacement of vapor needs to be vented out of the tank. The equations in API 2000 convert the pumping rate into the tank — in units of barrels/hour — into an equivalent amount of air flow — in units of standard cubic feet per hour — needed to leave the tank. The constants merely represent a unit conversion. However, there is an additional consideration for volatile liquids. Volatile liquids, when pumped into the tank, can flash and produce additional vapors, which can cause a greater buildup of pressure. API 2000 accounts for this vapor generation by doubling the flow requirements; this only occurs when pumping into the tank and not out.
Liquid movements are not the only considerations for tank venting. Overpressure and vacuum failure can occur as a tank is heated up/cooled down, respectively. These thermal effects of the tank depend on the size of the tank, the location of the tank — hotter versus colder climates can change the vapor in the tank — and also the amount of insulation the tank has. Insulated tanks are more resistant to temperature changes, which thus reduces the amount of flow required by the vents. This reduction factor depends on the properties of the insulation — such as the thermal conductivity and heat-transfer coefficient with typical values included in API 2000 — as well as the thickness of the insulation material. In the case of in-breathing (or vacuum relief), tanks kept at higher temperatures can be cooled by the environment at a faster rate than they can be heated. The design case for cooling and inbreathing is when a warm tank on a hot day is rapidly cooled by a sudden rainstorm. Condensable product vapors can also increase the cooling effect on storage tanks; both of these are accounted for in API 2000. These thermal effects, in addition to the liquid movements of the tanks, are considered minimum requirements for the venting flow rates. The total venting requirement assumes these two to act concurrently and sums the flow requirements of each.
These are the considerations made when determining the venting requirements for a storage tank. The previous provisions, now in Annex A, do not sufficiently account for thermal cooling effects for larger tanks. As such, this provision tends to underestimate the vacuum requirement for storage tanks, which if used improperly can cause tanks to implode. The tank industry needs to make a shift toward the main body of API 2000 in order to properly determine the venting requirements for storage tanks. In general, the cost of additional venting protection is very small compared to the potential damage of an overpressure event. Keeping up with venting requirement standards reflects the quality and integrity of everyone in the tank industry.
For more information, email Phil Tran at phil.tran@pemyconsulting.com.