API Bull 2516-2006 pdf download.Evaporation Loss From LOW- Pressure Tanks.
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 15-psig pressure will also apply for higher or lower working pressures than the specified range. Low-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 differ 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 difference 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 (P1+ 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 (P2) 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 distilation 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.G
When a product is stored in a tank having a higher pressure stting than required by Fig. 5, the working ioss may be estimated for this condition by use of values obtained from Fig. 5 and Fig. 8. By deducting the pressure required, as determined from Fig. 5, from the actual vent setting, the balance, designated as sP, may then be considered to be the effective pressure available for reducing working loss as determined from Fig. 8. Fig. 8 is based on equation (3a) except that P; is considered to be sP. The loss values are repre- sentative for normal turnovers (12 per year) experi- enced with low-pressure storage. The values do not apply for low-pressure vessels which have rapid or frequent throughputs. Theoretically, stocks which may be stored in the absence of air (minimum Po equals or exceeds P;+Pa) should not incur working loss. In practice, working loss does occur and depends upon the rate of flling, dissipation of heat, and vent pressure. Data are not available to determine the vent setting which would prevent working loss when air is absent.