Breather Valves, Conservation Vents and PVRVs: A Practical Guide

Understand how pressure/vacuum relief vents work, what information to prepare for selection, and how they differ from emergency relief equipment.

What is a breather valve?

A pressure/vacuum relief valve (PVRV), often called a breather valve or conservation vent, helps control pressure and vacuum in the vapour space of a storage tank during normal operating conditions. As liquid enters or leaves a tank, or as temperature changes affect the vapour space, the tank may need to discharge vapour or admit air (or another permitted gas) to remain within its allowable operating limits.

In a typical weight-loaded design, a pallet stays seated until the pressure or vacuum reaches the force set by its weight and the effective area. Other designs use springs or a combination of loading methods. The actual construction and available settings depend on the selected model.

Pressure relief and vacuum relief do different jobs

Pressure side

When tank pressure rises to the selected setting, the pressure pallet lifts to release vapour and limit further pressure accumulation, subject to the vent’s capacity and operating conditions.

Vacuum side

When tank pressure falls below atmospheric pressure by the selected amount, the vacuum pallet opens to admit air or another specified gas. This helps reduce the risk of tank damage from excessive vacuum.

A combined PVRV provides both functions in one assembly. Some applications need pressure-only or vacuum-only equipment, an in-line or pipe-away arrangement, spring-loaded settings, or a configuration integrated with a flame arrester. Do not assume every vent offers every configuration.

Why are these valves also called conservation vents?

Storage tanks can lose product through vapour emissions during breathing. A properly selected conservation vent is intended to keep the tank within its allowable pressure and vacuum range while reducing unnecessary vapour loss compared with an unrestricted opening. The result depends on the vent design, set points, tank breathing requirements and the wider vapour-control system; it is not a guarantee of zero emissions.

How to prepare for selection

Before requesting a quotation or specifying a model, assemble the information needed for engineering review:

Tank type, design limits and applicable tank standard.
Required pressure and vacuum set points, including the units and reference basis.
Normal and maximum liquid filling and withdrawal rates, plus relevant thermal breathing requirements.
Fluid and vapour properties, operating temperature, and whether vapours must be routed away.
Required venting capacity and acceptable pressure drop, supported by the appropriate calculations.
Materials, corrosion considerations, flange or connection details, and environmental exposure.
Any flame-arrester, blanketing, emissions-control or other system interface requirements.
Inspection, maintenance access and documentation requirements.

Important selection checks

The nominal connection size alone is not enough to select a vent. Required capacity must be checked against the tank’s normal and applicable emergency scenarios, using verified manufacturer flow data and the governing design basis. Set points must remain compatible with the tank’s allowable pressure and vacuum limits. Also consider fouling, icing, condensate, corrosion and access for inspection, as applicable to the service.

A normal-service PVRV is not a substitute for a dedicated emergency pressure relief vent where emergency relief is required. Fire exposure and other abnormal scenarios require a separate assessment. Likewise, a PVRV with an integrated flame arrester should only be used where the complete configuration is suitable for the identified flame hazard and process conditions.

Questions to include in an RFQ

What tank and process data were used to select the vent?
What are the specified pressure and vacuum settings, capacities and flow curves?
Which materials, connections and options are included?
Are current product datasheets, test information and installation/maintenance instructions available?
What inspection and maintenance actions are required for the intended service?

Providing this information at the enquiry stage helps the supplier and manufacturer confirm a suitable configuration instead of selecting by size or product name alone.

This article is general guidance, not a substitute for engineering review or current manufacturer documentation. Final selection and sizing must be confirmed for the actual tank, fluid, operating conditions and applicable standards.

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