Frequently Asked Questions about Vapor Recovery Units (VRUs)
August 24, 2016

What is a VRU?

A VRU, or vapor recovery unit, is a compression system used to collect and compress low volume gas streams for injection into the suction of a larger compressor, a meter run, a local site fuel gas system or directly into a gas gathering line. Mechanical VRUs consist of a driver motor or engine that supplies the power to the compressor. They are often used by oil and gas operations to recover vent gas.

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Why use a VRU for oil and gas operations?

  • VRUs can make money for a facility. Sending the natural gas recovered to the sales pipeline increases the facility’s total volume of gas sold.
  • VRUs reduce air pollution emissions since they recover vent gas that would be emitted to atmosphere or burned in a flare.
  • Assist in meeting air permit limits.
  • Using VRUs to capture vent gas can reduce current and future risks and liability associated with greenhouse gas emissions.

What are typical sources of natural gas recovered by VRUs?

Typical sources include:

  • Casing head gas
  • Crude oil/condensate storage tanks
  • Produced water storage tanks
  • Heater treater flash gas
  • Glycol dehydrator still column vent gas

What are the types of compressors typically used as VRUs for oil and gas operations?

The most commonly used mechanical compressors used for VRUs include:  

Less commonly used VRUs include:

  • Vapor Jet Pump – non mechanical method using pressurized water to recover and compress gas.
  • Reciprocating mechanical compressor used more often in dry gas applications but some specific type of reciprocating compressors are better suited to wet gas and can be used in VRU service.

Are VRUs considered process equipment under NSPS OOOO/OOOOa?

Yes, VRUs considered process equipment under NSPhttps://hy-bon.com/index.php/downS OOOO/OOOOa and are not considered emission control devices.

This is important in OOOO/OOOOa because the emissions recovered (and not emitted to the atmosphere) are not included in the potential to emit (PTE) calculations for a storage tank. This helps ensure a company can meet the 6 tons VOC per year emissions limit.

What is used to drive mechanical VRUs?

  • Electric motors are used if electricity is available.
  • Natural gas engines are used when electricity not available.

How do VRUs work?

Mechanical VRU compressors work by squeezing the gas into a smaller volume thus increasing the pressure. Based on the ideal gas law the pressure of a gas is inversely proportional to the volume; decrease the volume of the container holding the gas will increase the pressure of the gas.

The Vapor Jet System is used for recovery of vent gas from oil and gas production facility storage tanks. The process utilizes a pump to pressurize a stream of produced water to serve as the operating medium for a jet pump. The potential energy (pressure) of the produced water stream is converted to kinetic energy (velocity) in the jet pump. The high velocity water stream entrains the near atmospheric vent gas vapors and returns them to the facilities’ low pressure system for separation and sale. The water is then returned to the water storage tanks for further de-gassing and reuse in the Vapor Jet System process, disposal or injection.

What is the purpose of Variable Frequency Drives?

Variable frequency drives (VFD), also called variable speed drives, adjust the speed of the compressor driver based on the flow of gas to the VRU. This increases the turndown ratio and so increases the operational range of gas the VRU can effectively recover. As VRU applications have relatively large fluctuations in volumes and pressures, the VFD (or VSD) can provide the overall system a larger degree of operating flexibility.

The VFD allows the compressor to more quickly respond and recover increases and decreases in the flow of gas to the VRU over a short period of time.

When should direct measurement of vent gas flowrate be used?

Where and when possible, consider measuring the flowrate of vent gas from the emission source. HY-BON/EDI uses its IQR Emission Survey to measure vent gas flowrate using turbine meters or thermal mass flow meters. Optical Gas Imaging (OGI) cameras are used to ensure no leaks occur from tank hatches and PSVs during measurement.

Measurement of actual flowrate of vent gas is valuable for sizing, design and operation of a VRU. This the following reasons:

  • Determine the minimum and maximum flowrate of vent gas over a 12 to 24-hour period.
  • Use to properly design and operate the VRU system.
  • Samples of the vent gas taken during measurement can supply data on the chemical makeup (molecular weight, VOC, methane) and BTU content of the gas. This is useful data for air permitting, emission inventories and calculating the value of the vent gas recovered by the VRU.

Are scrubbers needed for VRUs?

Applications for natural gas vent gas sources that are “wet” (containing natural gas liquids) require a scrubber vessel to collect free liquids that might condense out upstream of the compressor inlet.

Crude oil/condensate storage tanks typically have high BTU vent gas (greater than 1500 BTU/SCF) and will have more free liquids that should be removed prior to sending the gas to the compressor.

What are considerations for the piping from the storage tank to the VRU?

The piping should slope (20 degrees or greater) downward from the storage tank(s) to the scrubber of the VRU.

How do VRUs handle gas containing H2S?

Depending on metallurgy of the VRU, the following ranges can be handled.

  • Flooded Rotary Screw compressors: 2 to 4% H2S
  • Rotary vane compressors: Up to 40% by volume
  • Vapor Jet Pumps: No upper limit on the H2S concentration

What are some VRU design considerations for crude oil/condensate storage tanks?

When VRUs are used to recover vent gas from storage tanks, the vent line from the storage tank or the common manifold suction line for multiple storage tanks, is piped to the suction scrubber on the VRU.  The suction scrubber is a 2-phase separator that removes some of the free oil in the vent gas.  Oil recovered in the suction scrubber is pumped back to the storage tank(s).  The gas from the suction scrubber flows to the VRU for compression.  The discharge piping from the VRU can be routed to the gas gathering line, a meter run and/or the suction of a gas compressor. 

Piping connecting the storage tanks and VRU or vapor recovery towers (VRT) and VRUs, (referred to as the Closed Vent System (CVS) by NSPS OOOO/OOOOa) should operate with no detectable emissions.

CVS piping from the storage tank to the VRU should slope downward toward the VRU.  This applies to piping from a vapor recovery tower and a VRU.  This ensures that liquids that condense in the piping can be gravity-fed back to the VRU inlet scrubber to help keep the piping clear. 

VRUs are designed to turn on and off based on the pressure of the gas in the storage tank vapor space.  To detect the pressure in the storage tanks, an independent sensing line is run from the top of one tank to a small drip pot on the control panel. Location of the sensor should be in the most active storage tank or located at a distance as far as practical from the VRU suction line if all tanks are equally active in receiving oil.  Some systems use a tank mounted pressure transducer (transmitter) with an analog line to the control panel to detect pressure in the storage tank vapor space. 

The VRU is configured to stop and start automatically, depending on the pressure in the tanks. A properly designed VRU must include a bypass system that will initiate automatically and divert the discharge volume back to the suction scrubber. This process allows tank pressure to build back to the point at which compression occurs. If the pressure continues to decrease while in the bypass mode, the unit will shut down and wait in standby for the start pressure to be obtained.

Properly designed VRUs are also configured to shut down before any type of vacuum is reached to avoid pulling oxygen into the tanks, or imploding them. If air (oxygen) is pulled into the system, it is typically caused by an improperly designed package (no bypass system or improper pressure settings), improperly sealed tank hatches, or leaking relief valves. These units actuate on pressures as small as 0.5 inches of water column (0.019 psi), and most packages used in typical oil field compression are simply not able to calibrate to these minute pressures. With a correctly designed package, the addition of a properly designed gas blanketing system on the tanks can alleviate the majority of any other issues that could cause oxygen ingress.

Either rotary screw or rotary vane compressors are strongly recommended for crude oil/condensate storage tank application because of their ability to effectively handle wet gas. Reciprocating compressors are good machines in dry gas applications, but they are always not recommended for VRU service for wet gas. Wet gas tends to foul the valves and seats in reciprocating most compressors, and condensate can fall out in the crankcase and compromise the lubricating oil, resulting in component failure.

Vapor recovery process flow with key components indicated

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Should a facility use a Vapor Recovery Tower?

Vapor recovery towers (VRTs) are tall, vertical separators used to recover flash gas emissions that would be generated in a crude oil storage tank. 

Flash gas results from natural gas coming out of solution when crude oil or condensate is transferred from higher pressure production separators (or other pressure vessels) into storage tanks.  The higher the pressure drop between a pressure vessel and a fixed roof storage tank, the higher the amount of flash gas generated in the storage tank. The VRT can recover most of the flash gas from the oil and then route lower pressure oil to the storage tank. Gas recovered by the VRT is routed to the VRU. Since lower pressure oil is sent to the storage tank, the flash gas generated in the storage tank(s) is dramatically reduced.

When should backup combustion devices (enclosed combustion device or flare) be used with a VRU application?

Some operators use an enclosed combustion device (VCU) or flare as a backup control device with VRUs. The combustion device can combust the gas during times the gas is not recovered by the VRU.

Backup emission controls help ensure the facility can continuously meet NSPS OOOO/OOOOa limits for storage tanks and facility State site specific air permit limits. They can also help avoid the need to report upset, bypass and emergency air emissions that would occur if the gas was not combusted.

What environmental regulations affect the piping (closed vent system) for VRUs?

NSPS OOOO and OOOOa require initial and annual inspection for leaks from the closed vent system of VRUs (and enclosed combustors and flares) used to control an affected storage tank.

NSPS OOOOa requires that a qualified, registered professional engineer design and certify the closed vent system for affected storage tanks that require a NSPS OOOOa control device.

What are the operating ranges for VRUs?

VRU Type

Horsepower Range

Maximum Discharge Pressure

Minimum Volume (MSCFD)

Maximum   Volume (MSCFD)

Flooded Rotary Screw

5 to 1000

350 psig

15

5000

Rotary Sliding Vane

5 to 600

55 psig

2

2000

Vapor Jet Pump

Not applicable

55 psig

2

75

Reciprocating Compressor

5 to 2000

4500 psig

2

50,000+

Based on natural gas with specific gravity of 0.65, inlet gas temperature of 60°F and suction pressure of 0 psig.

 

HY-BON/EDI’s Complete Solution

HY-BON/EDI’s engineered vapor recovery units (VRU), vapor recovery towers (VRT) and enclosed vapor combustion units (VCU) along with our IQR and Leak Detection and Repair (LDAR) services offer a complete package for operators to comply with the storage vessel emission control requirements. We design and size our VRUs and VCUs meet and exceed the requirements of this EPA guidance.