API STD 2000:2014 pdf download.Venting Atmospheric and Low-pressure Storage Tanks
1 Scope
This standard covers the normal and emergency vapor venting requirements for aboveground liquid petroleum or petroleum products storage tanks and aboveground and underground refrigerated storage tanks designed for operation at pressures from full vacuum through 103.4 kPa (ga) (15 psig). Discussed in this standard are the causes of overpressure and vacuum; determination of venting requirements; means of venting; selection and installation of venting devices; and testing and marking of relief devices. This standard is intended for tanks containing petroleum and petroleum products, but it can also be applied to tanks containing other liquids; however, it is necessary to use sound engineering analysis and judgment whenever this standard is applied to other liquids. This standard does not apply to external floating-roof tanks.
3.2.5.4 Abnormal Heat Transfer
Steam, tempered water, and hot oil are common heating media for tanks that contain materials that must be maintained at elevated temperature. Electrical heating elements are also used for the same purpose. Failure of a tank’s heat medium supply control valve, temperature-sensing element, or control system can result in tank overpressure due to increased heat input and liquid vaporization. Heated tanks that have two liquid phases present the possibility of a rapid vaporization if the lower phase is heated to the point where its density becomes lower than the density of the liquid above it. It is recommended to specify design and operating practices to avoid these conditions. If a tank maintained at elevated temperatures is empty, excessive feed vaporization can result when the tank is filled. If the temperature control system of the tank is active with the temperature sensing element exposed to vapor, the tank’s heating medium can be circulating at maximum rate with the tank wall at maximum temperature. Filling under such conditions can result in excessive feed vaporization. The excessive feed vaporization stops as soon as the walls have cooled and the fluid level covers the sensing element. For a tank with a cooling jacket or coils, liquid vaporization as a result of the loss of coolant flow shall be considered.
3.2.5.6 Vent Treatment Systems
If vapor from a tank is collected for treatment or disposal by a vent treatment system, the vent collection system can fail. This failure shall be evaluated. Failures affecting the safety of a tank can include back-pressure developed from problems in the piping (liquid-filled pockets and solids build-up), other equipment venting, or relieving into the header or blockage due to equipment failure. An emergency venting device that relieves to atmosphere, set at a higher pressure than the vent treatment system, may be used if appropriate.
API STD 2000:2014 pdf download
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