API MPMS 19.1A-2006 pdf download.Manual of Petroleum Measurement Standards Chapter 19—Evaporative Loss Measurement Section 1A—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 I 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 specifed 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 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 fxed-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 afect 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 (p,) 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 2 ) 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 P, = 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 = I for the condition p, = 17 .5, because the higher vapor pressure stocks tend to have a smaller slope. Altitude will afect the required storage pressure. Adjustment of storage pressures for atmospheric pres sures other than 14.7 psia may be made by substituting the propr atmospheric pressure (P a ) in equation (1 ). Table 1 lists the atmospheric pressure existing at various altitudes.
Working loss wil1 occur during fUing if the pressure of the vapor space exceeds the vent setting and vapors are expelled. If the pressure at the start of flling is less than the pressure vent setting, the air-hydrocarbon mixture will be compressed during fUing. The hydro carbon condenses maintaining nearly a constant partial pressure. A .. certain fraction of .vapor space may be fUed with liquid before the vent opens, thus decreasing working Joss.