What is a vapor recovery unit (VRU)?

A vapor recovery unit (VRU) is a compression system that captures hydrocarbon vapors from storage tanks, loading operations, and process vessels that would otherwise be vented to the atmosphere or flared. The VRU compresses the vapors and routes them to a sales gas line, fuel gas system, or back into the process. VRUs typically achieve 95-98% vapor recovery efficiency and are required at many oil and gas facilities by EPA NSPS OOOO/OOOOa regulations.

When is a VRU required by EPA regulations?

Under EPA NSPS Subpart OOOO (2012) and OOOOa (2016), storage vessels with potential VOC emissions of 6 tons per year or more must reduce emissions by 95% through a VRU, combustion device, or equivalent control. NSPS OOOOb (2024) further expanded requirements. State regulations like Texas TCEQ and Colorado AQCC may have stricter thresholds. A VRU is the preferred control because it recovers product revenue rather than destroying the vapors.

How do you calculate flash gas from a storage tank?

Flash gas generation from a storage tank is calculated using the Vasquez-Beggs correlation or API MPMS Ch. 19.4. The key inputs are oil throughput (BPD), API gravity, Reid vapor pressure (RVP), separator pressure, and tank pressure. Higher RVP and higher separator pressure produce more flash gas. Typical flash gas rates for Permian Basin crude are 200-800 SCF/bbl, depending on the crude oil characteristics and upstream separator conditions.

What types of VRU compressors are used?

Three main compressor types are used in VRU service: rotary vane (most common for small-medium VRUs, 5-100 HP, simple operation), rotary screw (medium-large VRUs, 50-500 HP, higher efficiency), and ejector (no moving parts, uses motive gas, suitable for remote locations). Rotary vane compressors dominate the upstream oil and gas market due to their tolerance of liquid carryover and simple maintenance.

Vapor Recovery Unit (VRU) Sizing Calculator

Production Parameters

Number of Storage Tanks

tanks

Oil Throughput

BPD

API Gravity

°API

Light crude: 35-45, Medium: 25-35, Heavy: <25

Reid Vapor Pressure (RVP)

psia

Typical: 4-12 psia. Higher RVP = more flash gas.

Flash Gas & Vapor

Flash Gas Rate Input

Known Flash Gas Rate

SCFH

Ambient Temperature

°F

Altitude

VRU Configuration

VRU Compressor Type

Discharge Pressure (sales line)

psig

Natural Gas Price

$/MCF

Estimated VRU Capital Cost

Understanding Vapor Recovery

Flash Gas Sources
Flash gas is generated when crude oil or condensate enters a storage tank at atmospheric pressure after being separated at higher pressure. The dissolved gas comes out of solution, generating VOC-rich vapors.

NSPS Requirements
EPA NSPS OOOOa: 95% reduction for tanks with ≥ 6 tpy VOC potential. TCEQ: permit-specific limits. Colorado AQCC: 95% control for ≥ 2 tpy sites.

VRU Benefits:
Recover saleable gas, reduce VOC/HAP emissions, avoid flaring and combustor operating costs, improve ESG profile, and maintain regulatory compliance with NSPS OOOO/OOOOa.

Learn the Theory

Understand flash gas generation, VRU compressor selection, emissions calculations, and NSPS compliance requirements

Read Engineering Guide →

Formula

Q_flash = GOR × Q_oil / 24

Q_flash = Flash gas rate (SCFH)

GOR = Gas-oil ratio from flash (SCF/bbl)

Q_oil = Oil throughput (BPD)

HP = Q × P_s × [(rⁿ-1)/n] / (33000 × η)

VOC = Q_flash × MW / 379.5 × VOC_frac

Standards & References

EPA 40 CFR 60 Subpart OOOO
NSPS for Crude Oil and Natural Gas Facilities (2012)
EPA 40 CFR 60 Subpart OOOOa
Updated NSPS for Oil and Gas (2016)
TCEQ 30 TAC Chapter 115
Texas Control of Air Pollution from VOCs
GPSA Engineering Data Book
Chapter 7: Separation Equipment
API MPMS Ch. 19.4
Evaporative Loss Measurement

Engineering Notes

NSPS threshold: 6 tons/yr potential VOC emissions triggers 95% control requirement
Rotary vane: Most common VRU type. Tolerates liquid carryover. 5-100 HP typical.
Screw compressors: Better efficiency at higher HP. Less liquid-tolerant.
Suction pressure: Typical 2-8 oz/in² (tank vent pressure) for atmospheric tanks
Safety factor: Size VRU for 125-150% of calculated vapor rate for turndown
Liquid knockout: Install scrubber upstream of VRU to remove entrained liquid

Vapor Recovery Unit (VRU) Sizing Calculator

Production Parameters

Flash Gas & Vapor

VRU Configuration

Understanding Vapor Recovery

Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Related Calculators

Flash Calculations

Compressor HP

Relief Valve Sizing

Pipeline Tariff