How to “Right Size” a Vapor Recovery Unit

“Don’t Supersize it – Right Size It”

In order to know how to properly size a vapor recovery unit, you must understand the compressor operational parameters. For starters, you need to understand what the suction pressure, discharge pressure and the total throughput the unit is expected to handle. Not sure you say? Typically, vapor recovery units that are pulling emissions off of production tanks are holding the tank pressure in ounces and discharging into a sales line of 20-100 psig. It’s the expected volume that seems to always be the mystery.
One of the best practices to finding the actual vent gas emission volume is to have your storage tanks tested. OTA Compression utilizes an approved methodology called Direct Measurement and Simulation Testing. Direct Measurement is the preferred method for compression sizing. This procedure involves metering the vent gas directly off of the tanks through a common vent line for a period of 24 hours. By actually metering the real-time flow of vent gas, it allows the operators to know precisely the flow (mscfd) and volume (mscf) of hydrocarbon vapors that would have been captured and rerouted to the process by a compressor.

GTO 20 Performance Curves_WeekIV

Not only is it a requirement by the TCEQ that a vapor recovery unit be able to handle the highest peaks and lowest valleys of their VOC emissions, it is important that the compressor keep up with the crude oil volatility to prevent pressure vacuum relief valves from opening and leaking out a new revenue source that was once lost.
When you comprehend the operational demands on the VRU, you will want to take a look at the type of compressor needed in order to effectively handle the dynamic nature of the produced liquids. Along with electrical availability, you will need to know the BTU, the constituents that are present in the gas stream and the elevation of where this unit will be operating. The above mentioned will help you choose the right type of compressor for the job and the horsepower required to drive it. Remember to also look to what the future holds at the facility. Will you be producing more wells in the near future, thus increasing your emissions footprint? If this is the case, you may want to choose a VRU similar to OTA’s GTO 20E that utilizes a patented KS40 compressor. This proprietary reciprocating compressor is exceedingly efficient, versatile and has the ability to grow with your VOC increases.
Understanding, evaluating and “right sizing” your vapor recovery unit can also help reduce your operating costs by lowering your lease rates or purchase price. Larger horsepower VRU’s are typically more expensive, harder to maintain and will also consume more fuel or electricity. Don’t Supersize it – Right Size it!

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