How to control and monitor emissions in venting systems

Venting systems are critical infrastructure in industrial facilities that store and process hydrocarbons, chemicals, and other volatile organic compounds.

The effective control of emissions generated by these systems is a challenge for facility and environmental managers, as it is necessary to comply with increasingly stringent regulations without compromising profitability or halting production.

In this article, we will explain how to monitor and control emissions in venting systems in the safest and most effective way.

What is a venting system and what is its function?

A venting system is a critical safety device used to relieve pressure in industrial infrastructure (such as tanks and reactors), preventing their collapse.

These systems are designed to manage the controlled exchange of gases between different enclosed systems and the atmosphere.

Their fundamental purpose is to maintain the delicate balance between controlling gas emissions—ensuring structural integrity and operational safety by preventing dangerous pressure or vacuum build-up—and, at the same time, minimising polluting emissions into the atmosphere to avoid penalties and protect the environment.

Vent systems are most commonly found in tanks and storage tanks, but they also need to be installed in wellheads, compressors, reactors and pipelines, among other industrial infrastructure.

Why are vent gases emitted?

Although there may be other reasons (technological reasons and economic decisions), between 50 and 60% of vent gas emissions into the atmosphere are caused by thermodynamic and operational physics.

Liquids continuously evaporate in enclosed spaces and these vapours expand with temperature, causing pressures that can deform the structure or even cause it to rupture completely.

On the other hand, when liquid is extracted, the empty volume must be refilled to prevent a progressive vacuum from forming and causing the structures to collapse.

This is why industrial equipment cannot remain airtight without a regulated relief system, such as venting systems.

What types of venting do exist?

Based on the most common standard classification according to API 2000, harmonised with ISO 28300, venting is divided into:

• Normal venting, which includes emissions caused by temperature/pressure changes during day/night cycles (‘standing losses’) and emissions resulting from operations or processes (‘working losses’).

• Emergency venting, which refers to extraordinary pressure relief due to exposure to fire or system failure.

Image: diagram showing how ‘breathing’ or venting works during day/night cycles (standing losses)

On the other hand, if we are looking for a more regulatory approach, the EPA (US Environmental Protection Agency) distinguishes between:

• Process Venting (regulated under 40 CFR Part 60 Subpart RRR),

• Storage Tank Venting (detailed in AP-42 Chapter 7)

• Pressure Relief Devices or Safety Venting (40 CFR Part 63)

The EPA makes this distinction because different control strategies are required for each.

Main gases emitted in vents

Although they are not the only ones, most of the gases emitted in vents that require monitoring and control belong to the family of Volatile Organic Compounds (VOCs).

Volatile Organic Compounds (VOCs) are carbon-based pollutants that easily convert into vapours at room temperature and require strict monitoring in vents because they are precursors to tropospheric ozone and carcinogens.

VOCs are the undisputed protagonists in venting in the chemical, petrochemical and pharmaceutical industries; some, such as benzene, toluene and xylene (BTX), require the most control.

Additionally, although methane (CH₄) is not technically classified as a VOC for regulatory purposes (even though it literally is one), it is one of the most sought-after gases in venting due to its significant climate impact as a greenhouse gas.

So much so that it has its own legislation at European level, Regulation (EU) 2024/1787, in force since October 2024, which prohibits routine methane venting, allowing it only in genuine emergency situations.

In addition, it establishes stricter requirements for LDAR inspections (quarterly for high-emission facilities) and requires independent verification of emissions data.

Relationship between types of venting, pollutants and industries

Image: LNG tanker venting gas into the atmosphere

What is the difference between fugitive emissions and venting?

It is very common to confuse the terms venting emissions and fugitive emissions, but the fundamental distinction lies in the design, the route of exit, and the intentionality.

Based on the explicit definitions in the BREF documents for gas treatment, we can establish the following classification:

Channeled Emissions

These are emissions that are captured and released into the atmosphere in a controlled manner through a defined duct, chimney, or stack. They are the main target for measurement by CEMS (Continuous Emissions Monitoring Systems) because, as they are confined, extractive or in-situ probes can be inserted to measure the flow and concentration of gases.

Within this type of emissions, we can differentiate between:

• Combustion & Process Stacks

This is the largest category in industry. These are not ‘venting’ emissions that involve relief or breathing, but continuous exhaust streams derived from the burning of fuels to generate energy or from chemical reactions inherent in production.

These types of emissions are, for example, those produced in the main chimney of a biomass boiler, in a cement kiln or in a cogeneration turbine, where combustion products (NOx, SO2, CO and PM particles) are constantly evacuated.

• Channeled Vents

These are outlet points designed to release gases derived from process operations (reactors, distillations) or safety operations (pressure relief), which have been connected by pipes to a final treatment system or an elevated discharge point.

An example of these channeled vents can be found in the battery of chemical reactors connected to a common collector that sends the gases to a Thermal Oxidiser (RTO) before they exit through the chimney.

Diffuse Emissions

Diffuse emissions can be considered to be any emissions that are NOT released through a confined source (chimney) and that are dispersed into the ambient air.

As they cannot be measured in a pipe, these types of emissions require perimeter monitoring (immission) or calculation by estimation to detect whether they are affecting the environment.

• Storage Emissions / Atmospheric Vents (Non-channeled Vents)

These are intentional and design emissions that are released directly into the atmosphere without passing through a collection duct. They occur because the equipment is designed to ‘breathe’ freely, such as the breather valves on the roof of a diesel storage tank, which allow vapours to escape when the tank is filled or heated by the sun.

These atmospheric vents are permitted within the limits set out in the IEP.

Image: storage tank with internal floating roof for vent emission reduction

• Fugitive Emissions

Unlike atmospheric venting, these are unintentional and accidental emissions resulting from loss of tightness or sealing failures in equipment. These emissions should not occur if the equipment is in perfect condition.

Examples of fugitive emissions include a loose flange on a gas transport pipe, a worn seal on a centrifugal pump, or leaks caused by a control valve stem.

Unlike atmospheric vents, fugitive emissions are subject to mandatory detection through LDAR (Leak Detection and Repair) programmes with established repair deadlines.

Summary table: main differences between venting and fugitive emissions

Solutions for monitoring and controlling gas emissions in venting systems based on legislation

The choice of monitoring technology is not purely a technical decision, but a strategic one. It depends strictly on the classification of the emission source (channelled vs. diffuse) and the legal requirements imposed by the plant’s Integrated Environmental Permit (IEP).

The IEP is the administrative document that specifies, for each facility, which sources require continuous monitoring (mandatory CEMS) and which can be controlled by indicative measurements or periodic inspections.

At Envira, we address this issue by dividing the strategy into two operational scenarios:

Channeled Emissions

When regulations (such as Spanish Royal Decree 815/2013 or the Industrial Emissions Directive) classify venting as a main source due to its pollutant load, the installation of an Continuous Emission Monitoring System (CEMS) is mandatory.

In these cases, it is not enough to install a sensor; a comprehensive solution is required to guarantee the legal validity of the data before the Administration:

• Compliance with the UNE-EN 14181 standard: Envira’s CEMS systems are designed to meet the most demanding quality assurance levels: NGC1 (QAL1) for equipment certification, NGC2 (QAL2) for calibration and NGC3 (QAL3) for operational monitoring.

• Data availability: to avoid penalties, our systems guarantee data availability of over 98%, thanks to robust designs and specialised maintenance.

• Intelligent Validation (ANEMO Software): beyond the hardware, we integrate our own ANEMO platform. This software automatically validates data, detects errors in real time and generates environmental reports ready to be sent to the competent authority, minimising the administrative burden.

Diffuse emissions or operational control vents

On the other hand, there are numerous points in the plant, such as atmospheric tank vents or areas prone to fugitive emissions, where the law does not require a CEMS, but where the plant needs control to prevent product losses, safety risks or complaints from the surrounding environment.

For these cases, the optimal solution is indicative measurement using compact or perimeter stations, such as the NANOENVI EQ line or compact immission stations.

• Perimeter Surveillance: as there is no chimney in which to insert a probe, these stations are deployed around the perimeter of the tank or process area.

• Cost-Effectiveness: they allow real-time monitoring of VOCs and other gases at a significantly lower investment than a regulatory CEMS.

• Early Detection: they are ideal tools for identifying leaks in safety vents or sealing failures before they become a major environmental problem.

Key features of the main solutions for venting emission monitoring

Frequently asked questions about industrial venting

When is it mandatory to install a CEMS in a vent?

In Spain, it is mandatory when the Integrated Environmental Permit (IEP) requires it due to exceeding certain pollutant mass flow thresholds, in accordance with RD 815/2013.

What is the difference between a process vent and an emergency vent?

A process vent is a routine emission resulting from operation (e.g. tank breathing), while an emergency vent is a one-off relief for safety reasons (e.g. fire or critical overpressure).

Can vents and leaks in ATEX zones be monitored without stopping the plant?

Yes. Envira uses certified instrumentation and safety protocols to carry out installations and maintenance in hot ATEX zones without stopping production.

Conclusion: technical precision for compliance and safety in vent monitoring

Emissions control in venting systems is a challenge that combines legal requirements, industrial safety and economic efficiency. As we have analysed, the key does not lie in applying a single solution, but in the correct characterisation of the emission source: rigorously distinguishing between channelled and diffuse emissions is the decisive step in selecting the appropriate technology.

Effective control of emissions in vents not only avoids penalties, but also optimises processes, recovers valuable product and prepares the plant for future, more restrictive regulations.

Given the technical and regulatory complexity, the most prudent approach is to design a tailor-made monitoring strategy with companies that have extensive experience in emissions and air quality monitoring, such as Envira, ensuring that the plant operates within the law and to the highest safety standards.