API STD 2000:2014 pdf download.Venting Atmospheric and Low-pressure Storage Tanks.
3.5.3 Inert-gas-blanketed Tanks
An inert-gas system may be used to avoid drawing air into the tank during vacuum conditions. The use of inert-gas systems instead of a vacuum-relief device is beyond the scope of this standard. For tanks that use an inert-gas supply system, the likelihood of a potentially explosive atmosphere is reduced and there can be benefits related to a less severe hazardous area classification. See Annex F for a discussion of other benefits and for Informative guidance for Inert-gas blanketing of tanks for flashback protection. The venting devices shall be sized for the case where the inert gas is unavailable (see 3.3.1).
3.5.4 Flame Propagation Through PressureNacuum Valves
Testing has demonstrated that a flame can propagate through a pressure/vacuum valve and Into the vapor space of the tank Tests have shown that ignition of a PV valves relief stream (possibly due to a lighting strike) can result in a flashback to the PV valve with enough overpressure to lift the vacuum pallet, allowing the flame to enter the tank’s vapor space. Other tests show that, under low-flow conditions, a flame can propagate though the pressure side of the PV; see Reference (23).
Flashbacks through PV valves are rare in the petroleum industry. The following are some factors that may explain this
— The materials stored in most cone roof tanks often do not result in a flammable atmosphere in the tank.
— A lightning strike Is likely to occur under conditions of cloud cover, so there Is a reduced likelihood that the tank is out-breathing. However, it can still be out-breathing if liquid is entering.
A lightning strike Is almost always preceded by winds, which keeps the size of the flammable cloud near the PV valve to a minimum
3.6 Relief-device Specification
3.6.1 Sizing Basis
The pressure- and vacuum-relief device(s), induding open vents, shall be suitable to pass the venting requirements for the largest single contingency or any reasonable and probable combination of contingencies (see 3.2.5 and 3.3.1).
When evaluating the overpressure scenarios in 3.2.5. the user should determine if the relief load should be handled using normal out-breathing relief devices or emergency venting. This can be an important consideration if the emergency venting is via a frangible roof or a nonreclosing relief device (e.g. rupture-disk or blow-off hatch).
A tank inerling system as described In 3.5.3 may be specified to avoid pulling air into the tank during vacuum conditions. No credit for these inerting systems shall be taken for the purpose of sizing the vacuum-relief device.
The inlet and outlet hydraulics can affect the relief-device sizing, which can be an iterative design process. The basis for the sizing equations is explained in Annex D.
API STD 2000:2014 pdf download
