"Purpose and Operation Reduction of product loss is the major purpose of variable-vapor-space systems. In these systems@ the usual daily outbreathing of vapor and inbreathing of air are minimized. API Bulletin 2513 defines breathing loss as ""Vapors expelled from a tank because of the thermal expansion of existing vapors@ and/or expansion caused by barometric pressure changes@ and/or an increase in the amount of vapor from added vaporization in the absence of liquid level change@ except that which results from boiling."" It is this type of loss that the variable-vapor-space system is designed to prevent. This breathing loss is minimized by passing the vapors to a central vanab1e-volume reservoir for temporary storage. The vapor spaces of all the tanks in a system are connected together (called manifolding) by large-diameter@ lightweight pipe. Therefore@ expanding vapors and new vapors rising oft the product surface during the heating cycle of a day flow to the storage reservoir. When the tanks cool@ the vapors return and prevent an intake of air. It is possible for products to fail to meet specification as a result of improper interconnection of vapor spaces. This must be considered in designing a system. If one or more tanks with interconnected vapor spaces are filled while simultaneously others are emptied@ all or part of the expelled vapors will be transferred to the tank@ or tanks@ being emptied. This is called balanced pumping. Filling losses are reduced in these systems whenever balanced pumping occurs. Also@ there may be unused expansion capacity available during the filling of tanks which will further reduce loss."