API Bull 2516:2006 pdf download

API Bull 2516:2006 pdf download.Evaporation Loss From Low-Pressure Tanks.
Introduction
The term low-pressure tank, as used in this evaporation 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 gasoline, 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 appreciable 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 specified range. Low-pressure tanks are constructed 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 increasing the tank design pressure, liquids of highir 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 tosses are considered: breathing loss, boiling loss, working loss, and leakage loss. Factors are discussed that affect the performance of low-pressure tank storage.
Vent Pressure Required to Prevent Breathing Loss
Breathing loss occurs when vapors are vented from the vessel as a result of thermal expansion of the vapors in the vapor space and by the vapor pressure increase resulting from the increase in the liquid-surface temperature. No breathing loss occurs unless the pressure rise resulting from these two variables exceeds the vent setting.
For an apparent liquid-surface temperature up to
100 F, experience has shown that a pressure of 2.5 psig
will substantially prevent breathing losses from motor
gasolines having a Reid vapor pressure up to 14 lb.
A small annual loss may result from seasonal changes
in storage temperature.
By the use of equation (1) (derived in Appendix I), the theoretical pressure (P,) required to prevent breathing losses may be calculated:
P, has been calculated to be 2.5 psig for 14-lb-RVP gasoline, using equation (1) and sea-level atmospheric pressure. By using the temperature assumptions demonstrated to work for motor gasolines, required storage pressures may be calculated for liquids of higher volatility.
The relation in equation (1) applies only when the vapor pressure of the liquid at minimum surface temperature (Pi) is less than the absolute pressure (P, + P1) 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 (P1) calculated to eliminate breathing 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 discussed in a subsequent section. The Fig. 5 plot of equation (1)is for the condition where Pg = 0.0 psig. The value of Pi 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 Pi=8 to S= I 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 pressures other than 14.7 psia may be made by substituting the proper atmospheric pressure (P0) in equation (1). Table I lists the atmospheric pressure existing at various altitudes.