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API Bull 2516-2000 pdf free download

API Bull 2516-2000 pdf free download.Evaporation Loss From Low-Pressure T anks.
The term low-pressure tank, as used in this evapora- tion loss bulletin, refers to vessels having a maximum pressure vent setting in the range from just above atmospheric pressure to 15 psig and a vacuum vent setting normally 1 to 2 oz per sq in. The tanks are used for the storage of products, such as motor gaso- line, pentanes, and natural gasolines, having a Reid vapor pressure up to 30 lb. Although a storage pressure of less than 2.5 psig may be used for some products, the type of vessel construction does not permit appre- ciable economy by using lower design pressures. The loss principles applying to 2.5-psig to 1 5-psig pressure will also apply for higher or lower working pressures than the specifed range. L ow-pressure tanks are con- structed in many sizes and shapes, depending upon the operating pressure range. Fig. 1, 2, 3, and 4 show typical types of construction. Pressure tanks difer from other conservation tanks in: that they have neither moving parts nor a variable vapor space. The principle of operation is the same as that for the conservation vented fixed-roof tank. The basic diference is the ability of low-pressure tanks to withstand higher pressure variations. Because of this, venting loss due to boiling and breathing loss due to daily temperature changes are prevented. By increas- ing the tank design pressure, liquids of higher volatility may be stored without breathing loss. The amount of loss from pressure storage tanks has been considered by users and tank manufacturers, but few data are available. Therefore, a theoretical basis has been used to estimate losses resulting from various storage conditions and types of products. Four types of losses are considered: breathing loss, boiling loss, working loss, and leakage loss. Factors are discussed that affect the performance of low-pressure tank storage.
The relation in equation (1) applies only when the vapor pressure of the liquid at minimum surface tem- perature (p1) is less than the absolute pressure (P,+P.) at which the vacuum vent opens. Under this condition air is always present in the vapor space. The breathing curve shown in Fig. 5 is a plot of equation (1) and gives the pressure (P,) calculated to eliminate breath- ing losses for products ranging up to 17.5 psia TVP at 100 F with storage at sea-level atmospheric pressure. Products having a true vapor pressure above 17.5 psia are subjected to boiling losses; these products are dis- cussed in a subsequent section. The Fig. 5 plot of equation (1) is for the condition where P1= 0.0 psig. The value of p: corresponding to P: was obtained from the vapor pressure chart, Fig. 6. A range of distillation slopes was used ; i.e. S=3 for the condition p:=8 to S=1 for the condition p,= 17.5, because the higher vapor pressure stocks tend to have a smaller slope. Altitude will affect the required storage pressure. Adjustment of storage pressures for atmospheric pres- sures other than 14.7 psia may be made by substituting the proper atmospheric pressure (Pa) in equation (1). Table 1 lists the atmospheric pressure existing at various altitudes.
Leakage Lo8s The efficiency of a low-presure vessel is directly dependent on maintaining a vapor-tight vessel. Defec- tive operating equipment which allows vapors to escape at pressures below the vent setting results in an inef- cient operating unit. Leakage loss is not subject to estimation by means of a correlation. Conclusion The loss correlations are based on theory and as- sumed conditions. In order to correlate the theoretically calculated results with actual service, it would be necessary to obtain test data under actual operating conditions. Because of the limited use recently of low-pressure tanks for products having a Reid vapor pressure of less than 15 lb, a field test program is not warranted. In the case of products having a Reid vapor pressure from 15 lb to 30 ]b, data are needed for a wide range of pump-in rates and vessel sizes under summer conditions to establish the working pressure which will effectively prevent working loss.

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