TESCORP VRU Best Practices Series
Choosing the Right Machine Starts with the Gas at Hand
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We left off in our first post in the VRU Best Practices series – The Dynamics of the Vapor Gas Stream – with the concept there is no common stock tank gas composition. Following are just a few brief highlights to kick off our follow-up conversation about Stock Tank Vapors.
There may be slight variances within an area processing a common zone, but each source should be considered before applying to a Vapor Recovery Unit. Here, we’ll be addressing the issues of working with and compressing these gases.
Stock Tank Vapors
Stock tank vapors are comprised of gases that are heavy in their molecular weight and have low vapor pressures. Therefore, these gases have a high weight mass flow, a low Cp / Cv (specific heat ratio) and are quite condensable into a liquid state when temperatures and pressures equal the “dewpoint” of some or all of the gas components. Special consideration should be applied to the choice of compression type, and the amount of condensate condensing between stages.
In-order to properly elevate (compress) these gases to pressures and temperature that meet a required system parameter, the Vapor Recovery Unit must comply with the following process requirements:
Note: As explained, the gas composition does dramatically vary by the type of crude oil, down hole pressures and temperatures, and the site environment and elevation. Therefore, it has to be assumed that the flow volumes into the Vapor Recovery Unit can vary dramatically throughout the area, during the day, and by the seasons.
And, that the gas as processed within the system, will see changes of state (condensation) as they reach dew point(s) and condense. This condensation within the system does change the flow volumes and in some cases condense almost all of the product with little gas to sales and more important little flow capacity control utilizing recycle gas.
Therefore, the following design considerations must be considered:
A Vapor Recovery System is not a generic gas compression system as is normally applied to dry gas applications. It is a complex system that must address the various gas compositions, varying gas flows, varying gas dewpoint(s) as well as that of the produced condensates with varying bubble points.
All design considerations start with the crude oil processed and the produced vapor that will be emitted from the crude at various pressure & temperature conditions.
-President and Founder, Vince Thomas