American Petroleum Institute

Evaporation Loss from Low-Pressure Tanks - First Edition
 N MPMS 19.1A

Annotation

Introduction

The term low-pressure tank, as used in this evaporative 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 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 15-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 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 type 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.