Vapor Recovery Unit (VRU) FAQ
Frequently Asked Questions (FAQ) About VRUs
What is a Vapor Recovery Unit (VRU)?
A Vapor Recovery Unit (VRU) is a compression system used to capture and compress low volume natural gas streams for injection into a fuel gas system, larger compressor or natural gas sales line. VRUs are most commonly used to recover tank vapor and vent gas and then route it to the gas gathering system for sale.
How does a VRU work?
VRUs are primarily used to capture tank vapors that collect inside the headspace of a sealed above ground fluid storage tank containing crude oil, condensate, or water. VRUs may also be used in combination with Vapor Recovery Towers (VRTs) to capture vapors separated by VRTs.
VRUs work on the principle of the ideal gas law, which states that the pressure, volume, and temperature of a gas are directly proportional to each other, as long as the number of particles and the mass of the gas remain constant. A VRU compresses vent gas and tank vapors into a smaller volume, which increases the pressure of the gas so it can be injected into a natural gas sales line or a larger compressor.
VRUs are mechanically driven by a gas or diesel engine or an electric motor, depending on the fuel source that is available at the well site or facility.
There are three primary steps to VRU operation:
The gas compressor pulls vapor into the scrubber.
The scrubber removes any debris, water vapor, and other liquids from the stream.
The recovered vapor is compressed, and then injected into natural gas lines for fuel gas or natural gas sales line, and the liquids captured by the scrubber are sent to a pipeline for sale.
What components does a VRU have?
A VRU typically consists of four major components:
Compressor. The vapor recovery compressor collects the low pressure gas and compresses it into a smaller volume. Compressor types used in oil and gas applications are typically one of three options, including flooded rotary screw, rotary sliding vane, or reciprocating piston.
Variable Frequency Drive (VFD). The VFD controls the speed of the compressor, adjusting for the flow rate of gas into the VRU.
Switching Device. A sensor (typically a transmitter/transducer) mounted to the liquid storage tanks that activates the compressor automatically, depending on tank pressure.
Scrubber. Scrubbers are needed to remove contaminants from the incoming natural gas stream. They are especially critical when compressing “wet” gas, gas containing hydrogen sulfide (H2S), or when capturing BTU-rich tank vapors.
What are the sources of gas and vapors VRUs capture?
Sources of gas captured by VRUs include tank vapors accumulating in the headspace of storage tanks containing crude oil, condensate, or produced water, as well as flash gas generated by Vapor Recovery Towers (VRTs).
What size VRU do I need?
Correctly sizing a VRU for an oil and gas production site or facility is a function of several variables, most importantly the volume of vent gas from the crude oil storage tanks. In some cases, it is possible to estimate or simulate the vent gas volumes, but the best practice is direct measurement of actual volumes.
Directly measuring vent gas flow volumes helps determine the minimum and maximum flowrates generally over a 24-hour period. Ideally, additional analysis is performed to determine the composition of the vapor or flash gas, including BTU content. This data can be used for air permitting, system design, and economic analysis.
More information is available on our blog article How to Right Size a Vapor Recovery Unit (VRU).
Does the size of VRU needed change over time?
Keep in mind, that the ideal VRU size will change as a well ages. Wells further down the decline curve produce less flash gas and tank vapor, therefore require less compressor capacity and a smaller VRU. A common strategy is to move larger VRUs off older well sites and re-deploy them to new well sites, and then backfill the older site with a smaller unit. This helps align operating costs with revenue over the life of a well.
See our blog article Coping with the Decline Curve: When and How to Downsize Your VRU for more information on downsizing your VRU to align costs with lower production levels.
Are VRUs required?
Vapor recovery requirements vary by country and regulatory agency. In the United States, the U.S. Environmental Protection Agency (EPA) has set emissions requirements for oil and gas production operations, transportation, crude oil refining, and natural gas processing.
EPA requires oil and gas operators to employ vapor removal to mitigate harmful air emissions from natural gas processing plants, crude oil storage facilities, and natural gas wells under the New Source Performance Standard 40 CFR, Part 60, Sub-part OOOO, also known as ‘Quad O.’
Quad O mandates 95% capture or elimination of vapors produced from oil storage tanks. In general, VRUs are the preferred method for achieving compliance because they both provide vapor capture and generate incremental revenue as a result. Although flaring tank vapor gas may be permitted or even required by some states, burning off BTU-rich vapors generates additional emissions and results in the loss of a finite resource and revenue.
Do I need to use a scrubber with a VRU?
Yes, all VRUs should be equipped with a scrubber/scrubbers. Scrubbers are critical when capturing and compressing wet gas containing high levels of natural gas liquids (NGL), water vapor, and tank vapors that typically contain high levels of NGL.
What is the capture efficiency of VRUs?
EPA assumes a capture efficiency of 95%, although operators can claim a higher capture rate if their production system meets certain criteria.
Texas Commission on Environmental Quality (TCEQ) allows operators to take a 95% credit against Potential to Emit (PTE) by using a Vapor Recovery Unit to capture tank vapor gas. That is often a reduction sufficient to qualify for the less stringent Permit by Rule requirements for air permitting and avoid expensive and time-consuming Title V permits.
Read our blog article How VRUs Help You Take Advantage of Permit By Rule in Texas for more information.
How long should a VRU last?
A VRU should provide many years of trouble-free service if maintained to the manufacturer’s preventative maintenance schedule and process. Like all mechanical equipment, however, VRUs will wear out with use but can be refurbished to extend their useful lives.
See our blog article VRU Refurbishment for Improving Environmental Performance at Older Wells for more information on VRU refurbishment.
The Platinum Advantage
Platinum VRUs punch above their power ratings. Platinum VRUs typically move 30% more gas volume for the same horsepower rating as competing units, because of our innovative design and quality materials. Our efficient, class-leading design means that in most cases a smaller and less expensive Platinum unit will do the same job of a larger and more expensive unit to significantly capture and sell more rich-BTU gas and put out the flare.
Flexible Fleet™ lease agreements. Our Flexible Fleet lease agreements allow you to move VRUs from one site or facility to another at will and without burdensome paperwork. That allows you to move larger units from an older site further down the decline curve that produces less tank vapor to a newer drill. That allows you to tailor VRU capacity to the individual site and to your entire field.
Contact us today at firstname.lastname@example.org to learn more about our solutions and how we can help you harness the full potential of your well site.