Pressure Conservation Vents

Overpressure relief for storage tanks.

What It Is & How It Works

Pressure conservation vents are weight-loaded relief devices that protect storage tanks from overpressure, releasing vapor when internal pressure exceeds a calibrated set point. They guard against shell deformation or roof failure from pressure buildup during thermal expansion, product filling, or inert gas blanketing overshoot. Pressure-only models carry a single pressure pallet and no vacuum relief mechanism.

The pallet sits on a machined seat at the top of the vent body, held closed by calibrated weights. When internal pressure exceeds the weight load, the pallet lifts and vapor escapes to atmosphere; when pressure drops below the set point, it reseats under its own weight, cycling automatically with each event.

Vents mount on standard tank roof nozzles, sized per API 2000 for the tank’s maximum out-breathing rate. Set points adjust by adding or removing weights, and construction comes in carbon steel, stainless steel, or aluminum to match the product’s vapor characteristics.

Pressure Vent vs. Pressure & Vacuum Conservation Vent

A combined Pressure & Vacuum conservation vent provides both pressure and vacuum relief in one device. A pressure-only vent omits the vacuum pallet, relying on a separate blanketing valve or dedicated vacuum vent for vacuum conditions. Specify pressure-only when another device already manages vacuum relief at the same tank.

When to Specify Pressure Conservation Vents

Pressure conservation vents are specified when a tank requires overpressure relief at a dedicated nozzle while vacuum relief is handled separately:

Inert Gas Blanketed Tanks: Nitrogen-blanketed storage where a blanketing valve supplies vacuum makeup and the pressure vent relieves overpressure from filling, thermal expansion, or regulator overshoot without venting blanket gas.
Tanks with Separate Vacuum Relief: Installations where a dedicated vacuum vent or pilot valve handles vacuum at another nozzle, leaving the pressure vent to provide independent pressure-only protection per API 2000.
Directional Vapor Control: Chemical tanks where pressure and vacuum paths route to different destinations, such as vapor recovery for pressure and filtered intake for vacuum, requiring separate single-function devices.
Petroleum Storage Under EPA 40 CFR 60/63: Fixed-roof tanks where the vent holds a vapor-tight seal below set point to minimize fugitive emissions, with vacuum managed by the blanketing system.

Why It Excels

Blanketed Tanks Where Blanket Gas Conservation Matters

Relieving pressure without a vacuum pallet lets blanket gas escape only on overpressure, while a separate blanketing valve holds the inert atmosphere.

Operations Needing Enhanced Sealing

The Expanda-Seal™ diaphragm option holds leakage to no more than 0.5 SCFH at 95% of set point, reducing blowdown and vapor loss.

Simple, Reliable Mechanical Protection

The single-pallet, weight-loaded design has no springs to fatigue and no pilots to maintain, giving years of service between rebuilds.

Tanks Where Vapor Recovery Requires a Dedicated Pressure Outlet

When pressure-relieved vapors must be routed to a vapor recovery unit, scrubber, or thermal oxidizer, a dedicated pressure vent provides a single-purpose outlet that can be directly piped to the collection system without routing vacuum air intake through the same path.

Breathing Vent Configurations — Selection Guide

Attribute	Pressure & Vacuum Conservation Vent	Pressure-Only Vent	Vacuum-Only Vent
Primary Function	Both pressure and vacuum relief in one device for bidirectional breathing	Overpressure relief only; vacuum handled by a separate device or not required	Vacuum relief only; pressure handled by a separate device such as a blanketing valve
Pallet Configuration	Two pallet stacks, pressure and vacuum, in one housing	Single pressure pallet; no vacuum pallet	Single vacuum pallet; no pressure pallet
Typical Set-point Range	Pressure 0.5 to 16 oz/sq in.; vacuum 0.5 to 8 oz/sq in. (weight-adjustable)	0.5 to 16 oz/sq in. pressure (weight-adjustable)	0.5 to 8 oz/sq in. vacuum (weight-adjustable)
When to Specify	Default for most atmospheric fixed-roof tanks needing both reliefs at one nozzle	Blanketed tanks where a separate valve handles vacuum makeup and only pressure relief is needed	Tanks where a separate pressure vent or pilot valve handles pressure relief
Regulatory Driver	API 2000, EPA 40 CFR 60/63, NFPA 30	API 2000 pressure sizing, often with blanketing valves	API 2000 vacuum sizing, API 650 structural vacuum limits
Common Pairing	Flame arrester at outlet; emergency vent for fire case	Blanketing valve for vacuum makeup; flame arrester at outlet	Pressure vent or blanketing valve for pressure relief
Recommendation	Standard breathing device for fixed-roof tanks storing volatile or flammable liquids needing both reliefs	Specify when a blanketing system provides vacuum makeup and only overpressure relief is needed	Specify when pressure relief is handled independently and a dedicated vacuum device is needed

What to Consider Alongside Pressure Conservation Vents

Consider an alternative when:

The tank needs both pressure and vacuum relief at one nozzle. A combined Pressure & Vacuum conservation vent handles bidirectional breathing in a single device and reduces roof penetrations. See Pressure & Vacuum Conservation Vents.
Tighter emission control or higher relief capacity is required. Pilot-operated relief valves provide snap-action sealing that eliminates the seat leakage weight-loaded vents allow near the set point. See Pilot Operated Relief Valves.
The tank needs large-volume relief for external fire exposure. Pressure conservation vents are sized for normal breathing, not fire-case loads.

How Pressure Conservation Vents Fit Into a Larger System

Pair with a blanketing valve for vacuum makeup, a flame arrester at the outlet for ignition prevention, and an emergency vent for fire-case relief to fully protect a blanketed tank. See Tank Blanketing & Vapor Recovery, Flame, Deflagration & Detonation Arresters.
Combine with L&J Technologies level and temperature instrumentation feeding Clairvoyance to log breathing events with inventory movements, supporting EPA emission and API 2350 records. See Level Alarms and FuelsManager®.