Why not just take grab samples on a VRU?

Air ingress on a VRU is often intermittent. A hatch, a temperature swing or a compressor pulling vacuum can start and stop between samples. A continuous analyzer shows the event when it happens, and the signal can drive an alarm.

Which oxygen range should I use on a VRU?

Pick the range around your alarm point. The 0–50 ppm range suits low-ppm limits, and the percent ranges (0–1 %, 0–10 %, 0–21 %) suit watching larger air ingress.

Can it sample a line at or below atmospheric pressure?

The optional sample system includes a pressure regulator, a filter and a calibration valve, but moving sample through the analyzer at near-atmospheric pressure needs a plan for your site.

Can the signal go to the VRU controller or SCADA?

Yes. The analyzer has a 4–20 mA loop (current sink) and RS-485 Modbus RTU output, so it can feed a controller, RTU, PLC or SCADA system where you set your own alarm points.

Does the analyzer have a cell to replace?

No. The analyzer measures oxygen optically, so there is no electrochemical cell to replace.

VRU Oxygen Analyzer for Vapor Recovery

How does air get into a vapor recovery unit?

A VRU draws vapor from tanks at or near atmospheric pressure. If it pulls faster than the tanks make vapor, the vapor space can go slightly negative, and air enters through any open path: an unlatched thief hatch, a relief valve that has not reseated, a worn seal or a loose fitting. That air is about 21 % oxygen.

EPA's Natural Gas STAR guidance on installing VRUs on storage tanks describes the design answer: a control pilot that shuts the compressor down and lets vapor flow back into the tank, so a vacuum does not form when the oil level drops. An oxygen reading tells you when that protection was not enough.

The events are often brief. A hatch is open during a load-out, a tank cools overnight, or the compressor outruns the vapor supply for a few minutes. A grab sample captures one moment and can miss all of them. A continuous reading shows when it happens and how often.

Where the analyzer mounts

The analyzer mounts on the vapor line between the separator and the VRU compressor. That position puts it ahead of the compressor, so it reads the vapor before the vapor is compressed into the recovered gas, and it catches air before the air reaches the sales line.

What the installation involves

The sample tap uses a 1/8 in tube compression fitting. The analyzer measures 9 × 7 × 2.5 in, weighs 4 lb, and runs on 9–36 VDC at 35 mA plus the current used by its analog outputs. The analyzer sends 4–20 mA or RS-485 Modbus RTU to your controller or SCADA. The optional sample system adds a pressure regulator and gauge, a secondary filter and a 3-way valve for calibration gas or process sample, and an optional flowmeter regulates vent flow to atmosphere if required.

Tank vapor is pulled at very low pressure, so moving sample through the analyzer needs a plan for each site.

What to look for in the trend

Three patterns are worth separating. A short spike during loading or gauging can point to a hatch or a transfer event. A rise that repeats at the same time each night can point to temperature-driven contraction. A slow climb that stays up can point to a leak that stays open, such as a relief valve, a seal or a fitting. These are places to start looking. The analyzer reports oxygen, not the cause.

Why oxygen in recovered gas matters

Each pipeline sets its own oxygen limit in its tariff or gas sales agreement, and gas over that limit can be rejected, so spec risk is the first reason to measure. The guide to oxygen in natural gas gives a published example and shows what the limit means in air.
Oxygen speeds up most corrosion mechanisms, and it does so even more when CO₂ or H₂S are present.
Oxygen and hydrocarbon vapor can form a combustible mixture inside the VRU and connected tankage, so a continuous reading is a process safety input as well as a quality check.

Optical sensing in tank vapor

Tank vapor often carries H₂S and CO₂. OxyScout has no cross-sensitivity to them and no poisoning of the optical sensor, and there is no electrochemical cell to wear out. Typical response time is under 10 seconds.*

*Based on controlled lab conditions.

Specifications for a VRU

Ranges	The Ultra-Trace range is 0–50 ppm, Trace range is 0–1000 ppm and the Percent ranges are 0–1 %, 0–10 % and 0–21 % by volume.
Detection limit*	The limit of detection is ≤ 0.5 ppm O₂ on the Ultra-Trace range (0–50 ppm), 50 ppm O₂ on the Trace range (0–1000 ppm), 0.01 %vol O₂ on the Percent 0–1 %vol range and 0.1 %vol O₂ on the Percent 0–10 % and 0–21 %vol ranges.
Accuracy*	Accuracy is ±2 ppm or ±5% of reading, whichever is greater, on the Ultra-Trace range (0–50 ppm). Accuracy is ±0.5% of full scale on the 0–1 %vol Percent range, and ±5% of full scale on the Trace range (0–1000 ppm) and the 0–10 % and 0–21 %vol ranges.
Response time*	Typical response time is under 10 seconds.
Pressure	Up to 250 PSI input pressure†
Outputs	The outputs are a 4–20 mA loop (current sink) and RS-485 Modbus RTU (2-wire half-duplex).
Power	The analyzer runs on 9–36 VDC and draws 35 mA plus the current used by its analog outputs.
Area classification	The analyzer is engineered for Class 1 Division 2 (Zone 2) hazardous areas.
Environment	Process and ambient temperature is 0 to 50 °C, the enclosure is IP67, and extreme-heat and extreme-cold enclosure options are available.

*Based on controlled lab conditions.

†Without Flowmeter installed. Higher ratings if regulator installed upstream.

Bring this to the quote meeting

Bring the oxygen limit or alarm point you care about, in ppm or percent.
Bring the pressure at the sample point, and tell us whether it ever goes below atmospheric.
Tell us what else the vapor carries besides hydrocarbons, such as H₂S, CO₂ or liquids.
Tell us what power is available at the tank battery, and whether the signal goes to a VRU controller, RTU or SCADA.

Oxygen Analyzer for Vapor Recovery Units (VRU)