Compliance

Vapor Recovery Units: Catching Tank Vapors Before They Vent

Every tank battery gives off vapors. Oil sits in a tank at near atmospheric pressure, and lighter hydrocarbons keep trying to escape into the air. A vapor recovery unit, usually called a VRU, catches those vapors before they vent and sends them into a gas line where they can be sold or used. This article explains where tank vapors come from, how a VRU works, and what it takes to keep one operating day after day.

Where tank vapors come from

Tank vapors come from three places: flashing, breathing, and loading. Each one moves hydrocarbons out of the liquid and into the vapor space above it, and from there they are one vent open away from the atmosphere.

Flashing happens when produced fluids drop from wellhead or separator pressure down to tank pressure. Gas that was dissolved in the liquid comes out of solution, the way carbonation leaves a soda after the bottle is opened. Breathing is the daily cycle of the vapor space itself. Tanks warm in the afternoon sun and cool overnight, so the vapor inside expands and pushes out through the vents, then contracts and pulls air back in. Every tank breathes, and every breath can carry hydrocarbons out with it.

Loading adds a third source. When a truck takes liquid from a tank, the incoming flow pushes vapor out ahead of it, and the growing empty space fills with vapor as the tank drains. A tank being loaded or filled is at its most likely to vent. Each of these releases is a loss of product and a release of methane and other hydrocarbons to the air. A tank that vents freely is a tank giving its gas away.

What a VRU does

A VRU is a small compressor package tied into the vapor space of one or more tanks. Its job is simple: pull the vapors out of the tanks and push them somewhere useful instead of letting them escape.

A compressor draws a slight vacuum on the tank vapor space, so vapors flow toward the unit instead of out the vents. The recovered gas goes into the sales line, a gas gathering system, or sometimes into fuel gas for equipment on site. What used to be a vent becomes a product that the operator gets paid for.

When the unit is running, tank pressure stays at or slightly below atmospheric pressure, and the thief hatches and vents stay shut. That is the whole point of the system. A tank under slight vacuum cannot push vapors to atmosphere, because the flow is moving in the other direction.

How the system is arranged and operated

The heart of a VRU is the compressor. Screw compressors are common on smaller units because they tolerate dirty gas and occasional liquid better than some other designs. Reciprocating compressors show up too. The whole package sits on a skid with the compressor, a driver, and the controls.

Before gas reaches the compressor it passes through a scrubber, sometimes called a knockout. The scrubber catches liquid and slugs carried in the vapor stream so they do not damage the machine. A level control dumps the collected liquid back into the tanks or into a line.

Controls run the unit automatically. A pressure switch reads the tank vapor space pressure and starts the compressor when it builds, then stops it once the pressure is drawn down. Pressure relief devices and shutdowns protect the system when something goes wrong. Automatic does not mean unattended, though. Operators still check the unit, watch the alarm log, and confirm the set points are where they belong.

Why regulators care

Storage tanks are a known source of methane and volatile organic compound, or VOC, emissions in oil and gas production. Methane is a potent greenhouse gas, and VOCs react in the air to form ozone. Tank vents are one of the most visible ways those emissions reach the atmosphere, so regulators have put tanks squarely in their sights.

EPA air rules for oil and gas, including the New Source Performance Standards in 40 CFR 60 Subparts OOOOa and OOOOb, require controls on storage vessels above certain size and throughput thresholds. In many cases the practical answer is a vapor recovery unit or an equivalent control that keeps the tank from venting to atmosphere.

The same rules push operators to find and fix leaks in the equipment around the tanks. Hatches, flanges, and fittings all develop small leaks over time. A leak detection and repair program, or LDAR, catches them early, and our article on LDAR explains how one is run. The tanks themselves, and the piping that ties them together, are covered in our storage tanks article.

Monitoring, maintenance, and recordkeeping

A VRU is only useful while it is actually running. A unit that is down turns a controlled tank battery back into a venting one, so operators watch it the way they watch any key piece of production equipment.

Routine checks cover compressor pressures and temperatures, oil level, scrubber liquid level, and the alarm panel. Scheduled maintenance follows the manufacturer's recommendations: oil and filter changes, belt checks, and compressor service. Small problems found early stay small, and a tank battery that stays under control is one that is not venting.

Records matter because they prove the unit was operated and maintained. Keep a log of run time, checks, maintenance, and any downtime, and keep it current. VRU maintenance logs run through the same document flow as OpsFlo, so the paperwork stays with the work orders and tickets. When an inspector asks, the record is the answer.

Sources and further reading

• EPA: Oil and Natural Gas Air Pollution Standards
• eCFR: 40 CFR 60 Subpart OOOOa
• eCFR: 40 CFR 60 Subpart OOOOb