Guide to Low-Pressure Tank Vent Datasheets - WittyWriter

Guide to Low-Pressure Tank Vent Datasheets

This guide provides best practices for process engineers filling out datasheets for low-pressure relief devices, specifically Breather Valves and Emergency Vent Valves, as covered by API-2000. Its purpose is to ensure all necessary data is defined for proper vendor sizing and procurement.

General Considerations

Common Abbreviations

Breather Valve (Conservation Vent) Datasheet Guide

A breather valve handles normal inbreathing (vacuum) and outbreathing (pressure) to protect the tank.

Parameter Guidance & Best Practices
General Data Specify basic identifiers: Tag Number, P&ID Number, and Protected Equipment tag and description. For Quantity, it is common to mark 'VTS' (Vendor to Specify) initially, as different models may require different numbers to meet the required capacity.
Service Select the valve's function:
  • • Pressure Relief Only
  • • Vacuum Relief Only
  • • Pressure & Vacuum Relief (most common)
Flame Arrestor A flame arrestor may be required if:
  • • Stored liquid flash point is < 60°C (140°F).
  • • Storage temperature can exceed the flash point.
  • • The tank can otherwise contain a flammable vapor space.
Note: While API-2000 suggests arrestors, be aware of potential clogging/choking issues. API RP 2210 notes it is not always mandatory for above-ground tanks. Evaluate the risk vs. operational concerns.
Vendor Data Mark Manufacturer, Model Number, and Drawing Number as 'VTS' during the quotation phase.
Process Conditions
  • Barometric Pressure: Specify site atmospheric pressure, as this is critical for differential pressure calculations.
  • Fluid: Name the vapor (e.g., Nitrogen, Air, Hydrocarbons) and note any special characteristics (corrosive, hazardous).
  • Operating Temperature: Specify the normal operating temperature.
Set Pressure & Overpressure This is a critical parameter. The overpressure for standard breather valves is high, often 30% to 60% of the set pressure. The set pressure must be low enough to protect the tank.

Tank Design Pressure ≥ Set Pressure × (1 + %Overpressure/100) + Inlet Line Loss

Valves with low overpressure (e.g., 10%) exist. They allow a higher set pressure (reducing venting losses) but may have commercial (cost) or vendor limitations.
Total Allowable Backpressure This is the sum of constant and built-up backpressure. Built-up backpressure is caused by flow through outlet piping and/or a flame arrestor.
  • • For piped-away valves, this value should typically be ≤ 10% of the set pressure.
  • • For valves venting to atmosphere (with a hood), specify backpressure as 0 mbarg (Atmospheric).
Set Vacuum & Under-vacuum Similar to set pressure, the set vacuum must protect the tank's vacuum rating. A typical under-vacuum value is 10%.

Tank Design Vacuum ≥ Set Vacuum × (1 + %Undervacuum/100) + Inlet Line Loss
Equivalent Air Capacity Specify the required Inbreathing and Outbreathing flow rates based on API-2000 calculations (thermal, pumping, etc.).
Crucial: These rates must be in Equivalent Air Capacity at Normal Conditions (e.g., 1.033 kg/cm²a & 0°C). The outbreathing rate due to fire is handled by the Emergency Vent, not the breather valve.
Valve Outlet Select the outlet type:
  • • Flanged: For a piped-away system.
  • • Hood: For direct discharge to the atmosphere.
Mechanical & Material
  • Protected Equipment Data: List the tank's Design Pressure, Design Vacuum, and Design Temperature.
  • Sizes/Ratings: Specify tank connection, flange facing, etc. Inlet/Outlet sizes should be marked 'VTS' initially.
  • Materials (MOC):
    • Body: Minimum as per inlet piping specification.
    • Pallet & Seat: SS316 minimum. (Aluminum may be used for very low set pressures).
    • Hood: CS, SS316, or Aluminum are common.
Tests Specify Hydrotest Pressure as 'VTS'. Add any other project-specific test requirements.

Emergency Vent Valve Datasheet Guide

An emergency vent provides protection against excessive overpressure from abnormal events, typically an external fire.

Parameter Guidance & Best Practices
General & Vendor Data Same as the breather valve: specify Tag, Quantity (VTS), P&ID, Equipment, and mark Manufacturer/Model as 'VTS'.
Process Conditions Specify Site Barometric Pressure, Fluid (e.g., Hydrocarbon vapor), Operating Pressure, and Operating Temperature.
Set Pressure & Overpressure Typical overpressure for emergency vents is around 20%. The set pressure is set higher than the breather valve but below the tank's design pressure.

Tank Design Pressure ≥ Set Pressure × (1 + %Overpressure/100)
Equivalent Air Capacity This is the Outbreathing rate calculated for the emergency fire scenario per API-2000.
Crucial: This flow rate must be specified in Equivalent Air Capacity at Normal Conditions (e.g., 1.033 kg/cm²a & 0°C).
Mechanical & Material
  • Protected Equipment Data: List the tank's Design Pressure, Design Vacuum, and Design Temperature.
  • Sizes/Ratings: Specify tank connection, flange facing, etc. Inlet size should be marked 'VTS'.
  • Materials (MOC): Generally the same as the breather valve.
    • Body: As per piping spec.
    • Pallet & Seat: SS316 minimum.
    • Hood: CS, SS316, or Aluminum.
Tests Specify Hydrotest Pressure as 'VTS'. Add any other specific test requirements.
🍪 We use cookies to improve your experience. Learn more