Vapor Recovery Unit Monitoring and Control

Why Vapor Recovery Matters

Storage tanks at oil production facilities emit volatile organic compounds (VOCs) and methane as crude oil flashes from reservoir conditions to atmospheric pressure. Without vapor recovery, these emissions represent both lost revenue (the gas has commercial value) and environmental liability. The EPA and Texas Commission on Environmental Quality (TCEQ) regulate tank emissions through New Source Performance Standards (NSPS Subpart OOOO and OOOOa) that require vapor recovery or emission controls on tanks with potential emissions exceeding 6 tons per year of VOCs.

A vapor recovery unit (VRU) is a small compressor system that captures these vapors, compresses them to gathering system pressure, and delivers them for sale as natural gas. A well-maintained VRU operating at 95%+ uptime can capture gas worth $50,000-200,000+ per year per tank battery, while simultaneously ensuring emissions compliance. Automation is the key to maintaining high VRU uptime and documenting emissions capture for regulatory purposes.

VRU System Components

Compressor Types

VRU compressors must handle variable suction pressures (near atmospheric), variable gas compositions (including heavier hydrocarbon fractions), and often intermittent flow as tanks breathe with temperature changes and production fluctuations:

• Rotary vane compressors: Most common VRU compressor type. Oil-flooded design handles wet, dirty gas well. Suction pressures down to 2-4 oz (inches of water column above atmospheric). Capacity range of 20-500 MSCFD.
• Rotary screw compressors: Used for larger volume applications (500-3,000 MSCFD). Better efficiency at higher compression ratios. More sensitive to liquid slugging than rotary vane designs.
• Reciprocating compressors: Used when higher discharge pressures are required to reach the gathering system. More maintenance-intensive but handle higher compression ratios efficiently.

Instrumentation Requirements

Effective VRU monitoring requires several key measurements:

• Suction pressure: Measured in inches of water column (inWC) or ounces per square inch (oz/sq in). Low suction pressure (below atmospheric) indicates inadequate vapor supply. High suction indicates the VRU is undersized or has a mechanical problem.
• Discharge pressure: Must match the gathering system operating pressure. High discharge indicates downstream restrictions or gathering system problems.
• Oil pressure and temperature: For oil-flooded compressors, these indicate lubrication system health and cooling effectiveness. Low oil pressure or high oil temperature triggers shutdown.
• Gas flow rate: An orifice or thermal mass flow meter on the VRU discharge measures captured gas volume. Essential for emissions reporting and revenue tracking.
• Motor current and VFD status: Motor current indicates compressor loading. VFD integration allows speed adjustment to match variable vapor generation rates.
• Thief hatch position/pressure: Pressure sensors on tank thief hatches detect when tanks are venting past the VRU, indicating the VRU is not keeping up with vapor generation.

PLC/RTU Control Logic for VRU Systems

Automatic Start/Stop Sequencing

VRU control logic manages the compressor through its operating states:

• Pre-start checks: Verify oil level adequate, discharge valve open, suction valve open, no active shutdown alarms. Only allow start if all permissives are satisfied.
• Start sequence: Open suction valve, start lubrication oil pump (if separate), start compressor motor. Monitor for successful start (suction pressure pulldown within 30 seconds).
• Running optimization: Adjust VFD speed based on suction pressure to maintain a target suction pressure setpoint. As vapor generation increases (hot afternoons, truck loading), the VRU speeds up. As vapor generation decreases (cool nights), the VRU slows down.
• Protective shutdowns: High discharge pressure, high oil temperature, low oil pressure, high motor temperature, vibration alarm, or low suction pressure (indicating no gas to compress) trigger automatic shutdown with alarm notification.
• Auto-restart: After a recoverable shutdown (low suction, high discharge), the controller waits a configurable delay period and attempts an automatic restart. After three failed restarts, the unit locks out and requires manual intervention.

Tank Pressure Management

The primary job of the VRU is to maintain tank vapor space pressure below the thief hatch relief pressure (typically 0.5-2.0 oz/sq in). The control system coordinates VRU operation with tank activities:

• During truck loading (tanks filling), vapor generation increases dramatically. The VRU must speed up or a second VRU must start to prevent venting.
• During oil transfers out of tanks, vapor space increases and the VRU may need to slow down to prevent pulling vacuum on the tank.
• Temperature swings between day and night cause tanks to breathe (expand and contract), creating cyclical vapor loads that the VRU must track.

Emissions Compliance Monitoring

TCEQ and EPA Requirements

Operators must demonstrate that VRU systems achieve the required emission reduction efficiency (typically 95% or greater). Automated monitoring supports compliance by:

• Continuous gas capture measurement: Flow meters on VRU discharge quantify captured gas. This data directly supports emissions inventory calculations.
• Downtime tracking: The SCADA system logs every VRU shutdown event with start time, stop time, and cause. Emissions during VRU downtime must be calculated and reported.
• Thief hatch monitoring: Pressure or position sensors detect when thief hatches open (indicating tank venting). This data documents periods when emissions bypassed the VRU.
• Flare monitoring: If a backup flare is installed, flow measurement and pilot flame detection document combustion of vapors when the VRU is offline. Flare destruction efficiency (typically 98%) differs from VRU capture efficiency in emissions calculations.

Revenue Recovery and ROI

VRU automation pays for itself through increased gas capture and reduced maintenance costs. A typical automated VRU system with SCADA monitoring costs $15,000-30,000 for the automation package (excluding the compressor). At current natural gas prices, capturing an additional 20-50 MSCFD (by maintaining 95%+ VRU uptime versus 80% uptime without automation) generates $20,000-60,000 per year in incremental gas sales revenue. Additionally, avoiding TCEQ emissions violations (penalties of $25,000+ per day per violation) provides substantial risk reduction.

Predictive Maintenance with VRU Data

Trending VRU operational data reveals maintenance needs before failures occur. Rising discharge temperature indicates worn vanes or valves. Increasing oil consumption suggests seal wear. Declining flow rate at constant speed indicates internal bypass or worn compression elements. Automated alerts based on these trends enable planned maintenance during scheduled site visits rather than emergency call-outs that result in extended VRU downtime and uncontrolled emissions.