Vapor Recovery Unit Product

Overview

Crude oil and condensate keep giving off gas after they reach the stock tank. Each fresh barrel dumped into an atmospheric tank flashes its remaining light ends — methane through pentanes — and daily heating and cooling breathes more vapor out of the tank roof. Historically that gas went to a vent or flare. A vapor recovery unit captures it instead: a small compressor package that pulls vapor off the tank at nearly atmospheric pressure and pushes it into the gas gathering or sales line. The recovered stream is valuable — tank vapor runs 1,200–2,500 Btu/scf, far richer than pipeline gas — and the capture is increasingly mandatory, since EPA rules under 40 CFR 60 Subpart OOOOb effectively prohibit routine venting from new tank batteries.

The defining engineering problem is the suction condition. A welded stock tank tolerates only ounces per square inch of pressure and even less vacuum before it buckles, so the unit must hold suction inside a band of roughly ±1 oz/in² while vapor make swings from near zero at night to surges every time a heater-treater dump valve fires.

How it works

Tank vapor enters through the fail-closed ESD Inlet Valve and the modulating Suction Control Valve, then passes into the Suction Scrubber. Rich vapor condenses readily, and a slug of liquid entering a compressor is destructive, so the Scrubber Shell knocks out free liquid with a tangential inlet and a Scrubber Mist Pad catches the fines. Accumulated condensate is returned by the Condensate Pump; a high-high Level Switch trips the unit if the pump cannot keep up.

Compression is almost always an oil-flooded rotary screw. The Screw Compression Element handles a 20:1 pressure ratio in one stage because injected lubricant absorbs the heat of compression and seals the rotor clearances — a reciprocating machine would need two or three stages and intercoolers for the same lift. The lubricant loop runs through the Oil Cooler and back, and the Oil Separator coalesces it out of the discharge gas to below 5 ppm so the sales meter sees dry gas. The Discharge Cooler then drops the discharge from 90–100 °C to near ambient before the Discharge Check Valve and the sales tie-in.

Capacity control

Matching a compressor to a vapor source that varies 10:1 over a day is the control challenge, and the VFD Control Panel solves most of it. A Pressure Sensor on the tank vapor line feeds the Unit Controller, which commands the Variable Frequency Drive to speed the Drive Motor up as tank pressure rises and slow it as pressure falls, tracking vapor make across a 25–100 % speed range. Below minimum stable speed, the Recycle Valve opens and loops cooled discharge gas back to suction so the machine stays loaded without pulling vacuum on the tank; if pressure keeps falling the unit simply stops and restarts on a rising-pressure permissive. This stop-start-modulate hierarchy keeps the tank inside its ounces-wide window without a flare ever lighting.

Shutdowns are conventional compressor protections: high discharge temperature from the Temperature Transmitter pair, low oil pressure, scrubber high-high level, and the Vibration Switch. On any trip the Blowdown Valve vents trapped gas to a safe location and the ESD valve isolates the tank.

Installation and classification

Everything on the skid lives in a hazardous area — Class I Division 1 or 2 depending on layout — so the motor is explosion-proof, instruments are rated for the zone, and the VFD Control Panel is either purged, rated, or mounted at a distance. The Skid Frame carries the package as one crane lift, with the Drip Pan satisfying spill-containment rules and Process Piping terminating at flanged battery limits.

Sizing starts from a flash calculation: barrels per day of oil, separator pressure ahead of the tank, and oil composition fix the standard cubic feet of vapor per barrel, typically 15–60 scf/bbl. Designers add tank breathing and a margin for dump-valve surges, then pick the frame. A common Permian tank battery producing 1,000 bbl/day might flash 40 Mscfd of vapor — a 40 hp unit — and pay for itself in months at typical gas and NGL prices, since the vapor's natural-gas-liquids content often matters more than its methane.

A companion device, the vapor recovery tower, is often installed upstream: a tall, low-pressure vessel between separator and tank that flashes oil at a few psig so the tanks themselves generate less vapor, shrinking the VRU duty and smoothing its load. Together they routinely capture more than 95 % of tank emissions that would otherwise vent.