Crude oil can sit quietly inside a storage tank while valuable hydrocarbons escape from it.
You may not see anything.
There may be no visible smoke.
But light hydrocarbons dissolved in the oil can leave the liquid, enter the vapor space above it, and eventually escape through the tank vent.
A vapor recovery unit gives those vapors somewhere else to go.
Instead of venting useful hydrocarbon gas to the atmosphere, the VRU collects it, raises its pressure, and sends it back into a usable system.
That can reduce emissions while recovering product that would otherwise be lost.
What Does VRU Mean?
VRU means vapor recovery unit.
In oil and gas production, a VRU commonly collects low pressure hydrocarbon vapors from equipment such as:
Crude oil storage tanks
Condensate tanks
Flash vessels
Some compressor related vents
Other low pressure vapor sources
The collected gas can be routed to sales gas, fuel gas, compression, or another process destination depending on the facility.
Why Do Oil Tanks Produce Vapor?
Produced crude is not made entirely of heavy liquid hydrocarbons.
It can contain lighter hydrocarbons dissolved in the liquid at pressure.
As pressure falls through the production system, some of those components leave the liquid phase.
More gas can flash when oil enters an atmospheric storage tank.
Temperature changes can also change the amount of vapor above the liquid.
The tank therefore acts as more than a simple container.
It can become a source of hydrocarbon vapor.
What Is Flash Gas?
Flash gas is gas released from a liquid as pressure decreases.
Imagine oil leaving a separator at elevated pressure and entering a storage tank operating near atmospheric pressure.
The oil experiences a large pressure reduction.
Light hydrocarbons that remained dissolved at separator pressure may no longer remain dissolved at tank pressure.
They flash into vapor.
This can create a substantial vapor load.
What Are Working and Breathing Losses?
Tank vapor is not produced only by flashing.
Working losses can occur when liquid enters a tank.
As the liquid level rises, vapor in the space above the liquid is displaced.
Breathing losses occur because vapor expands and contracts as tank temperature and pressure change.
Warm daytime conditions can increase vapor pressure.
Cooling at night can reduce it.
A VRU may therefore see changing vapor flow even when liquid production appears steady.
How Does a Basic VRU Work?
The process usually starts with a vapor collection header connected to the tank vapor spaces.
The VRU draws vapor from that header.
Before the vapor reaches the compressor, a suction scrubber or separator removes entrained liquid.
The compressor then raises the vapor pressure.
After compression, the recovered gas can be routed to an appropriate destination.
The unit starts and stops or loads and unloads based largely on vapor header pressure.
This sounds simple, but controlling very low tank pressure is one of the main challenges.
Why Is VRU Suction Pressure So Low?
Storage tanks are not pressure vessels in the same sense as separators or pipelines.
Many atmospheric storage tanks can tolerate only small pressure differences.
The VRU therefore cannot simply pull as hard as possible.
If suction pressure becomes too low, the unit can create excessive vacuum in the tank.
That can damage a tank.
Pressure control must be sensitive enough to recover vapor without pulling tank pressure outside its safe operating range.
What Happens If a VRU Pulls Too Much Vacuum?
A tank designed for very low pressure can be damaged by excessive vacuum.
The roof or shell can deform.
Air may also be pulled into the vapor system through openings or pressure vacuum equipment.
That can create additional operating and safety concerns.
This is why VRU control is not simply about maximizing suction.
The system has to maintain a safe vapor pressure.
Why Is There a Suction Scrubber Before the Compressor?
Compressors are intended to compress gas.
Tank vapor lines can also carry liquid.
Liquid may come from:
Condensation
Tank carryover
Slugging in the vapor header
Poor piping drainage
Changing temperatures
The suction scrubber provides a place for liquid to separate before gas enters the compressor.
A high liquid level in the scrubber can shut the VRU down to protect the machine.
What Type of Compressor Is Used on a VRU?
The compressor type depends on vapor rate, pressure, gas composition, and facility design.
VRUs commonly use machines capable of handling low suction pressure and variable vapor loading.
Some systems use rotary compressors.
Others may use different compression technologies.
What matters operationally is that the compressor must respond to a vapor source that can change considerably throughout the day.
Why Does Tank Pressure Rise When the VRU Stops?
The tanks continue producing vapor.
Oil may still be entering them.
Flash gas may still be coming out of solution.
Temperature may still be increasing.
If the normal vapor recovery path disappears, pressure in the vapor header rises.
The tank pressure control system may eventually route vapor somewhere else.
Depending on the facility, backup handling can involve a flare, combustor, vent control device, or another recovery path.
Is a VRU the Same as a Flare?
No.
A VRU attempts to recover hydrocarbons.
A flare destroys combustible gas by burning it in a controlled manner.
A producer generally prefers to recover useful gas when practical.
However, the flare may provide an important backup when vapor generation exceeds recovery capacity or the VRU is unavailable.
The two systems can therefore serve different roles at the same facility.
Why Can a VRU Help Make Money?
The gas leaving a crude storage tank can contain valuable hydrocarbons.
Instead of losing that material, a VRU can recover it.
Depending on gas quality and facility configuration, recovered vapor can contribute to:
Sales gas
Fuel gas
Natural gas liquids recovery
Reduced product loss
The economic value depends on vapor volume, composition, equipment cost, operating cost, and whether there is a practical destination for the recovered gas.
Why Are VRUs Important for Emissions?
Tank vapors can contain methane and volatile organic compounds.
Methane is a greenhouse gas.
Volatile organic compounds can contribute to air quality concerns.
Recovering vapor instead of releasing it can therefore reduce emissions while keeping hydrocarbons in the process.
This combination of product recovery and emissions reduction is one reason VRUs are common at modern production facilities.
Why Does VRU Capacity Matter?
The average vapor rate is not always the most important number.
A tank system can produce short periods of much higher vapor flow.
For example, several wells may send liquid into tanks at the same time.
A truck loading operation may change vapor movement.
Temperature may rise.
Separator conditions may change.
If the VRU cannot handle the peak vapor rate, header pressure rises and the backup vapor control system may open.
Correct sizing must therefore consider maximum expected vapor generation, not simply yesterday’s average flow.
What Causes a VRU to Run Constantly?
A VRU that previously cycled on and off may begin operating almost continuously.
Possible reasons include:
Higher oil production
Higher separator pressure
Warmer crude entering the tank
More volatile crude
Additional tanks connected to the header
Changed gas composition
Leaking tank thief hatches
Control problems
Aging compressor performance
The first step is to determine whether vapor generation increased or VRU capacity decreased.
What Causes a VRU to Short Cycle?
Short cycling means the unit starts and stops too frequently.
This can happen when:
Pressure control settings are too close together
The vapor system volume is small
The compressor is oversized for the normal vapor load
Pressure sensing is unstable
Valves are responding poorly
Vapor generation is intermittent
Repeated cycling can increase mechanical wear.
A stable unit should respond to tank vapor demand without constantly starting and stopping unnecessarily.
Why Would the VRU Suction Scrubber Fill With Liquid?
Possible sources include:
Condensation in the vapor header
Oil carryover from tanks
Poor line slope
Liquid trapped in low points
Sudden temperature changes
Upstream process problems
A separator upset can also increase the volatility of liquid entering storage, indirectly changing VRU behavior.
A high scrubber level trip should therefore be investigated beyond simply draining the vessel and restarting the compressor.
Can Separator Pressure Affect VRU Load?
Yes.
Higher separator pressure can keep more light hydrocarbons dissolved in the liquid until that liquid reaches the tank.
When the oil enters the much lower pressure tank, more gas may flash.
Changing separator pressure can therefore change storage tank vapor generation.
This is a good example of why facility systems should not be troubleshot independently.
A change upstream can appear later as a tank or VRU problem.
Why Do Thief Hatches Matter?
A thief hatch provides access to a tank and can also form part of the tank pressure protection arrangement.
If the hatch does not seal correctly, vapor can escape instead of flowing through the recovery header.
Air can also enter under certain pressure conditions.
A VRU may appear to be operating normally while vapor is being lost through a poorly sealed tank opening.
Tank vapor recovery depends on the entire collection system being reasonably tight.
Why Does Vapor Header Design Matter?
VRU suction pressure differences can be very small.
A restrictive vapor line can therefore have a large effect.
Problems can include:
Undersized piping
Long vapor runs
Liquid accumulation
Poor line slope
Blocked flame arresting equipment where installed
Restrictions
Improper valve positions
The compressor can only recover vapor that actually reaches its suction.
What Happens During a Power Failure?
If the VRU loses power, tank vapor generation does not automatically stop.
The facility needs another way to protect tanks from excessive pressure.
Pressure vacuum relief equipment and alternate vapor control paths can provide protection depending on the design.
Operators should know where vapor goes when the VRU is unavailable.
That question becomes particularly important during extended outages.
How Do Operators Know a VRU Is Working Properly?
Useful observations include:
Vapor header pressure
Compressor suction pressure
Discharge pressure
Scrubber liquid level
Compressor temperature
Runtime
Start frequency
Recovered gas flow
Tank pressure
Backup flare or combustor activity
The best indication is not one reading.
It is whether the entire vapor system is behaving normally over time.
Frequently Asked Questions
What does VRU stand for?
VRU means vapor recovery unit.
What does a VRU do in oil and gas?
It captures low pressure hydrocarbon vapor and raises its pressure so the gas can be recovered rather than released.
Where does tank vapor come from?
It can come from flashing as crude pressure decreases, liquid movement inside the tank, and temperature related breathing.
Why does a VRU have a suction scrubber?
The scrubber removes liquid before vapor enters the compressor.
Can a VRU damage a tank?
Improper suction control can create excessive vacuum, which is why tank pressure control and protection are important.
Is a VRU the same as a flare?
No. A VRU recovers gas. A flare burns gas when it needs to be safely disposed of.
Why does a VRU shut down on high scrubber level?
The shutdown helps prevent liquid from entering and damaging the compressor.
Why would tank pressure increase when the VRU stops?
Vapor continues to be generated while the normal recovery path is unavailable.
What is the most important thing to understand about a VRU?
The VRU and storage tanks are one pressure system. Compressor operation, vapor piping, tank pressure, liquid carryover, and backup vapor handling all affect one another.