Breather Valve & Emergency Vent Valve - Guideline 🛡️

1.0 Introduction

This guideline defines the input values for low-pressure relief devices, specifically 'Breather Valves' and 'Emergency Vent Valves' as covered by API-2000. It is intended to help Process Engineers fill datasheets (G-PROC-055.022-F & G-PROC-055.026-F). If a project-specific specification is not provided by the client, the template in Annexure-3 should be used.

3.0 General Considerations

API-2000 covers venting for storage tanks operating from vacuum to 15 psi (1.034 barg). These tanks must be protected from both over-pressurization and under-vacuum.

• Breather Valves (Conservation Vents): These are typically combined valves that provide protection for normal operation (thermal inbreathing/outbreathing).
• Emergency Vent Valves: These provide protection against over-pressurization during abnormal scenarios, most typically fire.

Process engineers are responsible for defining the relieving conditions for both types of valves.

Service & Flame Arrestor

• Service (6): Specify if the valve is for 'Pressure Relief', 'Vacuum Relief', or 'Pressure & Vacuum Relief'.
• Flame Arrestor (7): A flame arrestor is considered if the liquid flash point is < 60°C, if storage temp can exceed flash point, or if the tank can contain a flammable vapor space. However, API-2000 and API RP 2210 note concerns about clogging and conclude it's not a must for above-ground petroleum tanks. The engineer must review this requirement with the client.

Pressure Relief Sizing (Pressure)

• Set Pressure (15): Must be set so that the relieving pressure does not exceed the tank design pressure.
• Overpressure (16): Typically high (30-60%). Some vendors offer 10% overpressure models, which allow for a higher set pressure and thus reduce venting losses, but may be more costly.
• Formula: Tank design pressure >= Breather valve set pressure x (1+% Overpressure/100) + line loss.
• Total Allowable Backpressure (17): Should typically not exceed 10% of the set pressure. For valves venting to atmosphere with a hood, backpressure is 0 mbarg.

Vacuum Relief Sizing (Vacuum)

• Set Vacuum (20): Must be set so the relieving vacuum does not exceed the tank design vacuum.
• Under-vacuum (21): Typically 10% is considered.
• Formula: Tank design vacuum >= Breather valve set vacuum x (1+% under vacuum/100) + line loss.

Capacity & Mechanical Data

• Equivalent Air Capacity (24, 25): Specify the required inbreathing and outbreathing rates as calculated per API-2000. Use the TANKVENT-CALC spreadsheet. Flowrates must be in air equivalent at Normal conditions (1.033 kg/cm²a & 0°C).
• Fire Case: Outbreathing due to fire is an emergency case and should NOT be included in the Breather Valve calculation.
• Valve Outlet (26): Select 'Flanged' (if piped connection) or 'Hood' (if discharging to atmosphere).
• Materials (37-40): Body material should match the inlet piping spec. Pallet & Seat should be SS316 minimum. Hood is typically CS.

Sizing & Capacity

This valve is for emergency over-pressurization, primarily the fire scenario.

• Set Pressure (14): Must be set below the tank design pressure.
• Overpressure (15): Typically around 20%.
• Formula: Tank design pressure >= Emergency vent set pressure x (1+% Overpressure/100).
• Equivalent Air Capacity (17): Specify the estimated outbreathing rate based on the API-2000 methodology for the Fire Scenario. This must be specified as an air equivalent flowrate at Normal conditions.

Mechanical & Material Data

• Materials (27-31): Body material should match the inlet piping spec. Pallet & Seat should be SS316 minimum. Hood is typically CS.
• Protected Equipment Data (19-21): Specify the tank's design pressure, vacuum, and temperature. This helps the vendor optimize the design.