Vapor Recovery Unit

Photo of Leslie Esparza, Krisha Mehta, Sean Swearingen Students: Leslie Esparza, Krisha Mehta, Sean Swearingen

Sponsor: Chevron

Date: Fall 2009

Requirements:
The first function requirement was to reduce carbon emissions by taking low pressure gas that would normally be burned off or "flared" and increasing its pressure so that it can be added to the sales gas stream and sold or rerouted back to the platform for use on-site. Another significant requirement for this project is to bring the low pressure gas from the new bulk surge tank and add it to the gas from the existing bulk surge tank. This recovered low pressure gas comes from a damaged facility and will be integrated with a functional facility. Other key requirements for the VRU include making easy installation and access, and minimum size and weight due to the limited space available on the platform. The design will be reliable and efficient, keeping in mind that repairing and replacing production equipment offshore is costly. The VRU designs must also provide a cost-benefit analysis and improve the overall efficiency of the gas production system. Constraints include operating pressures, temperatures, and flow rates, equipment sizes, and gas properties.

Problem:
Currently on an offshore platform, low pressure natural gas is being burned off into the atmosphere rather than recapturing it for sales onshore. For environmental and financial concerns, the team configured a Vapor Recovery Unit (VRU) that recaptures and increases two low pressure natural gas flows, one from the current offshore platform and another from a damaged facility, to a sufficient pressure to enter the sales gas compressor.

Solution:
Overall VRU and detailed individual component designs have been developed through research and analysis. Our final VRU design recommendation incorporates the following key components: gas compression and cooling, water and gas separation, piping systems for condensate water removal, equipment drivers, and key valve systems. Reciprocating compressors, powered by natural gas engines, are used for gas compression. Hot gas at the compressor outlet is cooled using air-cooled heat exchangers, which are powered by electric motors. Vertical liquid-vapor separators use gravity to separate condensate water from dry gas after cooling, where the water is piped from the scrubbers to a water collection point. The key valve systems include valves for the compressor inlet and discharge, control valves for the scrubber, and a three way valve to combine gas flows. Because of the extremely low gas pressures, multiple compression stages are needed. As a result, three sets of compressors, coolers, and scrubbers are modeled in our overall VRU design. The VRU is designed to handle two inlet streams, taking into account liquid removal from the separators, and staying within temperature constraints for the compressors.

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