When storing or handling hydrocarbon liquids, it is important to maintain a constant pressure in the storage tank. A vapor-recovery system prevents leakage of vapors into the atmosphere. It also prevents pressure buildup that could cause structural damage to a storage tank. A slight positive pressure in the tank must be maintained to prevent inward collapse or entry of air.
Storage tanks used for storing volatile hydrocarbon liquids are connected together through a manifold. A vapor-recovery system is used to maintain the tank battery vapor pressure at the design pressure. These applications require compressor systems that can compress vapors with entrained liquids. Typical compressors used are the sliding-vane, liquid-ring, and reciprocating types.
These compressor systems usually have very low suction pressures and can operate at a slight vacuum. The service is normally not continuous duty. The compressor needs to be designed to have liquid-handling characteristics. This is dictated due to the condensation occurring in the compressor during frequent shutdowns.
A vapor recovery tower (VRT) is a tall pressure container installed between the production separator(s) and the liquid storage tanks. VRTs should be engineered for proper retention time to allow gas to separate from the liquid. They should have no potential for liquid traps in gas vapor piping to the vapor recovery unit (VRU). VRTs are normally rated for pressures between 50 and 175 psig. Typically they gravity feed to the liquid storage tanks at very low pressure (~1 psig). In most installations, the flash gas from the liquids in the VRT flows to a VRU for compression.
A VRU is a specialized compression package designed to capture low pressure, wet gas streams from oil and condensate storage tanks.
Hydrocarbon liquids of all types are stored in specially designed storage tanks and transported in tank cars. During storage or transit, the liquids are constantly generating hydrocarbon vapors. As the liquid level in the tank fluctuates, these vapors are often vented to the atmosphere. It is necessary to have a vapor-recovery compression system. VRU collects hydrocarbon vapors boiled off the hydrocarbon liquids, compresses these vapors to a specified pressure, and then cools the vapors. VRUs are relatively simple systems capturing about 95% of the Btu-rich vapors for sale or for use onsite as fuel. During the cooling process, liquids condense, and are collected and pumped back into a storage tank or pipeline.