scope:

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.