Pressure and Vacuum Conservation Vents

Controlled breathing for storage tanks.

What It Is & How It Works

Conservation vents, also called pressure/vacuum (P/V) vents, are weight-loaded relief devices mounted on storage tank roofs that allow the tank to breathe in a controlled manner. They protect the tank shell from overpressure and vacuum damage caused by thermal expansion, product transfer, and ambient temperature changes while minimizing the release of vapors to atmosphere. Every fixed-roof atmospheric storage tank must breathe, and the conservation vent ensures that breathing occurs only when internal pressure or vacuum exceeds safe thresholds.

The vent contains two independent pallet assemblies, one for pressure relief and one for vacuum relief, each held closed by calibrated weights. When internal tank pressure exceeds the pressure set point, the pressure pallet lifts to release vapor. When vacuum exceeds the vacuum set point, the vacuum pallet lifts to admit air. Both pallets reseat automatically when the condition normalizes. This weight-loaded design requires no external power, control signals, or operator intervention.

Conservation vents mount on standard tank roof nozzles and are sized per API 2000 based on the tank’s thermal breathing rate and maximum pumping-in and pumping-out flow rates. Set points are adjusted by adding or removing weights from the pallet stacks, and construction materials are selected to match the stored product’s vapor corrosivity.

Conservation Vent vs. Emergency Vent

Conservation vents handle normal tank breathing during routine operations, pumping, and temperature changes. Emergency vents handle abnormal overpressure from external fire exposure or equipment failure, providing large-volume relief that conservation vents are not sized to deliver. Most tanks require both: a conservation vent for daily breathing and an emergency vent for fire-case protection. Conservation vents control emissions; emergency vents prevent catastrophic failure.

When to Specify Conservation Vents

Conservation vents are specified on virtually every fixed-roof atmospheric storage tank that must control vapor emissions while preventing structural damage from pressure and vacuum:

Fixed-Roof Petroleum Storage Tanks Crude oil, refined product, and intermediate storage tanks where API 2000 requires controlled pressure and vacuum relief, and where EPA 40 CFR 60/63 mandates vapor-tight closures to limit fugitive hydrocarbon emissions to atmosphere.
Chemical and Solvent Storage Requiring Emission Control Tanks storing volatile organic compounds where the conservation vent’s sealed design reduces vapor losses compared to open venting, helping the facility meet Title V emission limits and LDAR reporting thresholds.
Tanks with Inert Gas Blanketing Systems Nitrogen or other inert gas blanketed tanks where the conservation vent must maintain a minimum positive pressure to preserve the blanket while relieving excess pressure during filling operations without venting the blanket gas unnecessarily.
Terminal and Tank Farm Operations Subject to NFPA 30 Multi-tank facilities storing flammable or combustible liquids where NFPA 30 requires pressure/vacuum relief on each tank, and where the conservation vent provides the required protection at the lowest installation and maintenance cost per tank.
Water and Wastewater Treatment Chemical Storage Tanks storing treatment chemicals such as sodium hypochlorite, ferric chloride, or caustic soda where corrosion-resistant conservation vents in FRP or stainless steel prevent both vapor release and tank vacuum damage during batch deliveries.

Why Shand & Jurs Conservation Vents Excel

Tanks Requiring Reliable Protection Without External Power

Weight-loaded pallets operate purely on the pressure differential between the tank vapor space and atmosphere, requiring no electricity, pneumatics, control signals, or operator intervention to provide continuous protection through power outages and control system failures.

Facilities Where Emission Reduction Drives Regulatory Compliance

The sealed pallet design keeps the tank vapor space closed during normal conditions, releasing vapor only when pressure exceeds the set point, reducing the cumulative fugitive emissions that contribute to Title V inventories compared to open venting or loose-fitting thief hatches.

Installations Requiring Simple, Low-Cost Maintenance

Set-point adjustment requires only adding or removing calibrated weights from the pallet stack. Field rebuilds use standard gaskets and pallet assemblies without specialized tools, keeping maintenance within the capability of on-site operations staff.

Multi-Tank Terminals Where Standardization Reduces Inventory Costs

A common vent platform across dozens or hundreds of tanks in a terminal allows standardized spare parts inventory, consistent maintenance procedures, and uniform operator training across the entire facility.

Tanks Storing Products with Varying Vapor Pressures

Adjustable weight stacks allow the same vent body to be recalibrated for different set points when the stored product changes, accommodating seasonal product switches and terminal flexibility without replacing the vent device.

Breathing Vent Configurations — Selection Guide

Attribute	P/V Conservation Vent	Pressure-Only Vent	Vacuum-Only Vent
Primary Function	Provides both pressure and vacuum relief for tanks requiring bidirectional breathing control in a single device	Relieves overpressure only, used where vacuum relief is provided by a separate device or is not required	Relieves vacuum only, used where pressure relief is provided by a separate device such as a blanketing valve
Pallet Configuration	Two independent pallet stacks: one pressure pallet and one vacuum pallet in a common housing	Single pressure pallet stack; no vacuum pallet installed	Single vacuum pallet stack; no pressure pallet installed
Typical Set-point Range	Pressure: 0.5 to 16 oz/sq in.; Vacuum: 0.5 to 8 oz/sq in. (adjustable by weight selection)	0.5 to 16 oz/sq in. pressure relief (adjustable by weight selection)	0.5 to 8 oz/sq in. vacuum relief (adjustable by weight selection)
When to Specify	Default choice for most atmospheric fixed-roof tanks where both pressure and vacuum protection are needed at one nozzle	Tanks with inert gas blanketing where a separate blanketing valve handles vacuum makeup and only pressure relief is needed at this nozzle	Tanks where pressure relief is handled by a dedicated pressure vent or pilot-operated valve at a separate nozzle
Regulatory Driver	API 2000 (combined thermal breathing), EPA 40 CFR 60/63 (emission control), NFPA 30 (flammable liquids storage)	API 2000 (pressure relief sizing), often paired with blanketing valves for nitrogen conservation	API 2000 (vacuum relief sizing), API 650 (tank structural vacuum limits)
Common Pairing	Flame arrester at the vent outlet for ignition prevention; emergency vent for fire-case protection	Blanketing valve for vacuum makeup on the same tank; flame arrester at the vent outlet	Pressure vent or blanketing valve for pressure relief on the same tank
Recommendation	Specify as the standard breathing device for any fixed-roof tank storing volatile or flammable liquids where both pressure and vacuum relief are required	Specify when a separate gas blanketing system provides vacuum makeup and only overpressure relief is needed at the vent nozzle	Specify when pressure relief is handled independently and the tank needs a dedicated, precisely weighted vacuum relief device

What to Consider Alongside Conservation Vents

Tighter Emission Control or Higher Relief Capacity is Required Pilot-operated relief valves provide a snap-action seal that eliminates the seat leakage conservation vents allow near the set point, and deliver higher effective capacity through the same nozzle size. See Pilot-Operated Relief Valves.
The Tank Requires Only Atmospheric Open Venting With Weather Protection When the stored product is non-volatile and the tank operates at atmospheric pressure without emission concerns, a free vent provides open breathing at lower cost. See Free Vents.
Corrosive Service Requiring Full FRP Construction Throughout When the vapor environment attacks metallic components, FRP vents provide complete corrosion resistance from housing to pallet. See FRP Vents & Hatches.
Complete Tank Roof Protection Assembly Pair conservation vents with flame arresters at the vent outlet for ignition prevention, emergency vents for fire-case overpressure relief, gauge hatches for vapor-tight access, and internal safety valves for pipeline isolation to address every protection requirement at the tank. See Flame & Detonation Arresters, Emergency Vents & Manway Covers, Gauge Hatches & Manway Covers, and Internal Safety Valves.
Integrated Overfill Prevention and Emission Monitoring Combine conservation vents with L&J Technologies level alarm probes and automatic tank gauging feeding Clairvoyance to correlate breathing events with inventory movements, providing the data trail needed for API 2350 overfill prevention and EPA emission calculations.