TESCORP VRU Best Practices Series
Choosing the right machine starts with the gas at hand
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With a personal history of designing and manufacturing Vapor Recovery Systems since 1976, and specifically creating TESCORP, a company dedicated to vapor emission recovery technology in 1987, I have seen many applications and as many misapplications of Vapor Recovery Units due to the lack of focus on the produced gas and conditions of service.
There are many conversations proliferating within the industry that often imply that one type of compressor or system fits all. This is a huge misnomer. The application process and component selection are not that simple. There are many variables associated in the proper selection of components for the gases, process requirements for pressures, temperatures and “dew points’ within the actual process. Unlike “dry gas’ applications, stock tank vapors are “wet” and “unstable” gases that have associated design considerations that must be addressed to properly design a good working Vapor Recovery Unit. It is more than just a type of unit; it is a process.
The Dynamics of the Vapor Gas Stream
In order to discuss the application and function of a Vapor Recovery Unit, it is important to first define and understand the source of these gases – their origin and composition. The following is a very brief explanation of the petroleum crude oil origins and composition that is the basic source for the off gases that are the feed stock of vapor recovery systems.
The feed source for the stock tank vapor recovery units is a mix of hydrocarbon components that are released from the petroleum crude oil when the pressure(s) are relieved on the oil at different process points. These components vary in composition and vapor pressures. When this crude oil is under pressure as found in the reservoirs, these components are in a liquid state or a solid. Once the pressure is relieved to that of the “Flash Gas Separator” pressure and again to the “Stock Tank” pressure that is equal to the site atmospheric pressure, these components evaporate into gases that vary in their composition. These are normally heavy gases with high economic value that if not captured would enter the atmosphere as pollutants. These gases vary in composition at each phase and pressure.
***The Vapor Recovery Unit is the applicable technology to capture these vapor emissions and send them onto a process or market.***
Throughout the process from wellhead to sales, the crude oil and its entrained components go through pressure/temperature changes that create “thermally unstable” to “stable” and then “unstable” conditions at various pressure points. At each “thermally unstable” condition, different entrained components within the crude oil vaporize to create an off gas.
The onsite crude oil stock tank is the final storage and liquid/gas separator prior to the crude oil being shipped for sale. At this point, the final “thermal stabilization” of this product occurs. The entering oil product does still have entrained hydrocarbon condensates in solution with the oil. These entrained components were stabilized at separator pressure and temperature. But again, at this point the pressure is reduced to near atmospheric pressure where the “decanting” process occurs again with those components that have a vapor pressure greater than that of the site atmospheric pressure becoming unstable. This instability creates a heavy gas product that is a saturated wet gas very near/at dewpoint. This volume of gas in comparison to the oil flow is the “Gas Oil Ratio” (GOR) and the feedstock for the Vapor Recovery Unit (VRU).
It is not often noted or understood that the gases vary greatly in their composition. And variances also greatly affect the types of compressors and their ancillary components that may be required to properly design, construct, and operate a good working Vapor Recovery System. To properly compensate for these gas variances is to understand their impact to the compressor and the system required to properly recover and transport them. That is why we focus on and specialize in saturated “wet” gases associated with stock tank and flash gas vapor emissions and the problems and idiosyncrasies in working with those gases.
There are some basic facts concerning crude oil and their associated gases that must be considered in the application of a “Stock Tank Vapor Recovery System”. They are:
An example of pressure dewpoint would be a steam kettle.
Water boils at: 212oF @ Sea level =14.7 Psia
203oF @ 5,000 ft. = 12.2 Psia
32oF (freezing) @ a vacuum of 29.7 in-Hg
Inversely, it condenses just below these same temperatures or when the pressures are slightly elevated.
All natural gas components have specific boiling and condensing pressures & temperatures. All vary in both. The stock tank vapors are a combination of these components that exist in a vapor state at the site pressure & temperature. As conditions change i.e. temperature or back pressure, this composition may change.
This same effect of evaporation and condensing continues throughout the compression process. Therefore, the stock tank vapors must be monitored as pressures and temperatures of the compression cycle may achieve the dewpoint of some of the components in that process.
The stock tank vapors are comprised of gases that are heavy in their molecular weight and have low vapor pressures. Therefore, this gas has a high weight flow and is 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.
There is no common stock tank gas composition. 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. In a follow-up post, the issues of working with and compressing of these gases will be discussed.
-President and Founder, Vince Thomas