In 2014, a styrene storage tank at a chemical plant suddenly deformed inward during operation. Polymerized material had caused the breather valve to stick and blocked the flame arrester.
As product was pumped out, vacuum developed inside the tank and air could not enter fast enough. The pressure difference eventually caused a tank implosion. This article explains how breather valves protect storage tanks, why blockage happens, and what should be considered when selecting the right storage tank venting solution.
CONTENT:
- What Is a Breather Valve and How Does It Protect a Storage Tank?
- Why Can a Storage Tank Collapse Under Vacuum?
- Why Do Styrene and Similar Chemicals Block Breather Valves?
- How to Select a Breather Valve for Difficult Storage Conditions?
- FAQ About Breather Valves for Storage Tanks
- Conclusion
What Is a Breather Valve and How Does It Protect a Storage Tank?
A breather valve is a pressure-control device installed on atmospheric or low-pressure storage tanks. It is also commonly called a vacuum and pressure relief valve, pressure vacuum relief valve (PVRV), PV valve, or conservation vent. Its main job is to keep the tank pressure within a safe operating range by allowing vapor to leave or air to enter when needed.
When liquid enters the tank or the vapor space heats up, internal pressure rises and the valve opens outward to release vapor. When liquid is withdrawn or the tank cools, internal pressure falls and the vacuum side opens to admit air or blanketing gas. This inbreathing function works like a vacuum relief valve and helps prevent tank deformation or implosion.
The valve may look like a small tank accessory, but its capacity and pressure settings must match the tank’s actual operating conditions. Once the breathing path is restricted, the problem is no longer limited to the valve. The tank itself begins to carry the pressure difference between the inside and outside.
Why Can a Storage Tank Collapse Under Vacuum?
Large atmospheric storage tanks are strong, but their shells are relatively thin compared with their diameter. Internal liquid pressure mainly puts the shell in tension. Vacuum does the opposite. It creates external compressive loading, which can make the cylindrical shell buckle inward once the tank’s vacuum resistance is exceeded. API 650 therefore treats external pressure as a separate design case and uses factors such as tank diameter, shell thickness, height, and stiffening rings to evaluate buckling resistance.
In the 2014 styrene tank case, continued product withdrawal allowed the internal vacuum to exceed what the tank could safely resist, causing the shell to buckle inward. The immediate problem was loss of venting, but the underlying cause was polymer buildup inside the breathing system. The deposits stuck the breather valve and restricted the flame arrester, cutting off the air path the tank depended on.
Why Do Styrene and Similar Chemicals Block Breather Valves?
Breather valve blockage is more common in petrochemical plants, monomer and resin plants, coating and adhesive plants, chemical tank farms, and other facilities that store reactive or temperature-sensitive liquids.
For styrene, vinyl acetate, acrylic acid, methacrylic acid, acrylates, and methacrylates, vapor can condense inside vent nozzles, valve seats, vacuum breakers, or flame arresters. The condensate may contain little or no effective inhibitor. Polymer can then form in these areas, creating sticky deposits that reduce the available flow area and may eventually prevent the breather valve from opening normally.
Some media also create crystallization or solidification risks. Acrylic acid and methacrylic acid can face both polymerization and crystallization concerns, while glacial acetic acid can freeze at about 16.35°C and phenol can solidify at around 41°C. If crystals or solids form near the vent system, they can restrict the breathing path in the same way.
How to Select a Breather Valve for Difficult Storage Conditions?
For difficult chemical service, a breather valve should not be selected by connection size alone. The valve must match both the stored medium and the tank operating conditions.
1. Check the stored medium first.
For styrene, acrylic monomers, vinyl acetate, and similar media, consider polymerization, crystallization, solidification, corrosion, and deposit formation. If fouling is expected around the valve seat or flame arrester, the vent design should reduce areas where material can collect.
2. Match the pressure and vacuum settings to the tank.
The PVRV set points must remain within the tank’s allowable pressure and vacuum limits. Low-pressure storage tanks have limited tolerance for vacuum, so the valve setting cannot be selected independently from the tank design.
3. Size for the actual breathing demand.
The required breather valve capacity depends mainly on maximum filling and withdrawal rates, thermal breathing, and other credible pressure or vacuum scenarios. API Standard 2000, 8th Edition, published in August 2026, is the current API standard for venting atmospheric and low-pressure storage tanks. AIChE also emphasizes that liquid movement and thermal changes should be included when sizing PVRVs.
4. Consider fouling and flame protection together.
For polymerizing or crystallizing service, a conventional valve plus flame arrester may create additional narrow passages where deposits can accumulate. Anti-fouling or liquid-loaded designs can be considered for these conditions, together with suitable flame-protection and maintenance arrangements.
In our storage tank projects, we normally start with the medium, operating temperature, filling and withdrawal rates, and tank pressure limits. Gelan then designs the tank and matches the appropriate pressure vacuum relief valve, flame-protection arrangement, and venting configuration to the actual service conditions.
FAQ About Breather Valves for Storage Tanks
What is the difference between a breather valve and a pressure vacuum relief valve?
In storage tank service, the terms are often used for the same type of device. A pressure vacuum relief valve (PVRV) controls both positive pressure and vacuum by allowing vapor out or air or blanketing gas in.
Can a blocked breather valve cause a tank to implode?
Yes. If liquid continues to leave the tank while the vent path is blocked, the internal pressure can fall below the tank’s allowable vacuum. The resulting external pressure can buckle the shell or roof and cause a tank implosion.
How is a breather valve sized for a storage tank?
Sizing depends on the maximum filling and withdrawal rates, thermal breathing, tank pressure and vacuum limits, and other credible operating scenarios. API Standard 2000 is commonly used for venting atmospheric and low-pressure storage tanks.
Does every storage tank need a flame arrester?
No. The requirement depends on the stored medium, vapor flammability, vent arrangement, ignition risk, and applicable project standards. Some tanks use a separate flame arrester, while others use an integrated flame-arresting PVRV.
What should be checked for polymerizing or crystallizing chemicals?
Check the operating temperature, inhibitor requirements, deposit formation, possible crystallization or solidification, and the risk of blockage around the valve seat, vent nozzle, and flame arrester.
Conclusion
A breather valve is a small part of the storage tank, but its failure can affect the entire vessel. For difficult chemical service, valve selection should consider the stored medium, temperature, venting demand, fouling risk, and the tank’s allowable pressure and vacuum.
Gelan designs and manufactures complete storage tanks for chemical and petrochemical service. Based on the actual medium and operating conditions, we can coordinate the tank structure, materials, venting requirements, breather valve, and flame-protection arrangement as one system.