What Is Vapor Recovery?

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Where the vapors come from

Crude oil and condensate held in atmospheric storage tanks give off vapor for several reasons. Flashing occurs when liquid arriving from higher-pressure separation drops to tank pressure and releases dissolved light ends. Working losses come from filling and emptying, which pushes vapor out as liquid level rises. Standing or breathing losses come from day-night temperature and pressure swings. Together these produce a continuous stream of light hydrocarbons and volatile organic compounds.

Left uncontrolled, this vapor is either vented to the atmosphere or sent to a flare. Venting releases methane, a potent greenhouse gas, and VOCs that contribute to smog and can carry hazardous air pollutants. Flaring destroys most of the hydrocarbons but wastes the energy and still emits combustion products. Vapor recovery captures the stream instead.

How vapor recovery works

A vapor recovery system collects vapors from the tank vapor space at very low pressure, typically ounces above atmospheric, through a manifold. The captured vapor is drawn into a compressor package that boosts it to a pressure high enough to inject into the gas gathering line for sale, or to use as on-site fuel. A control system holds tank pressure in a tight band so tanks neither over-pressure nor pull a vacuum.

Because tank vapor generation is variable, systems are sized and controlled to follow the load, and they often work alongside a flare or enclosed combustor that handles surges or takes over if the recovery unit is down. Instrumentation on tank pressure, compressor status, and flow is essential so that a loss of recovery is caught immediately and vapors are safely directed to backup control rather than released.

Why it matters

Vapor recovery delivers two benefits at once. Environmentally, it prevents the release of methane and VOCs, helping operators meet tightening federal and state air rules and voluntary methane-reduction commitments. Economically, the recovered vapor is saleable gas or free fuel, so a well-run system can pay back its cost through captured product that would otherwise have been lost.

The concept also appears beyond production tanks, at truck-loading racks and terminals, where vapor recovery captures the displaced vapors as tankers are filled. In every case the principle is the same: keep the hydrocarbons in the pipe, out of the air, and turn a would-be emission into recovered value.

The ounces-wide pressure band

What makes tank vapor control genuinely hard is the width of the operating window. The vapor space has to stay above atmospheric pressure so the tank never breathes air in, yet below the settings of the relief devices so nothing vents, and the whole usable band sits in ounces per square inch on a vessel whose vapor generation swings with every truckload, slug of production, and afternoon of sun. Air ingress is the underrated failure: oxygen pulled in during a vacuum event contaminates the recovered gas, can put it off sales specification, and creates a combustible-mixture concern that site safety procedures exist to prevent.

That is why the compressor's suction control is the heart of the system - it must take exactly as much vapor as the tanks are making, no more and no less, following a load that never sits still. The mechanics of holding that balance are covered in VRU suction pressure control; the short version is that recycle and turndown keep the machine matched to the tanks across the whole swing.

Leak paths that quietly defeat recovery

A recovery system only captures what reaches it, and a tank battery offers plenty of paths that never do. A thief hatch left unlatched after gauging, or sealing on a worn gasket, vents preferentially because it is the lowest-resistance opening on the tank. Pressure relief vents can weep below their nominal settings as seals age. Vapor piping that is not balanced between tanks lets one tank over-pressure while another sits idle. None of these announce themselves; the compressor keeps running, just on less gas than the tanks are actually making.

The tell is in the trends: recovered-gas flow falling while production holds steady, or tank pressure riding lower than the control setpoint implies, both point at vapor escaping upstream of the machine. Optical gas imaging surveys find the leaks directly; the trend data tells you when to go looking between surveys.

Commissioning and surveillance checklist

A practical checklist for putting a system in service and keeping it honest:

1. Verify the tank pressure measurement location and range actually resolve the narrow band being controlled.
2. Confirm the compressor follows a falling load - recycle or turndown - without short-cycling.
3. Prove the backup path: on a simulated shutdown of the vapor recovery unit, vapors must route to the combustor or flare, not the atmosphere.
4. Trend tank pressure, compressor status, and recovered flow from day one, so the healthy baseline is on record.
5. Investigate any sustained drop in recovered volume against steady production as a probable leak path.

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