Water Column for API 2000 Calibration
In API 2000 (venting requirements for atmospheric and low-pressure storage tanks), pressure
and vacuum settings are often specified in inches of water column (WC). When calibrating
pressure relief or vacuum relief valves, ensuring accurate measurement in inches WC is critical.
Water column vacuum (often written as "inches of water column" or "WC") is a unit of
pressure measurement used to describe very low pressures, typically in ventilation, gas flow, and
vacuum applications.
Understanding Water Column Vacuum
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1 inch of water column (WC) = 0.0361 psi (pounds per square inch)
1 inch WC = 2.49 mmHg (millimeters of mercury)
1 inch WC = 249 pascals (Pa)
How It Relates to Vacuum:
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A vacuum measured in inches of water column means the pressure is lower than
atmospheric pressure.
It is often used in low-pressure venting systems, such as those referenced in API 2000 for
storage tanks.
Common Applications:
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Tank Venting: API 2000 specifies venting requirements in inches WC to protect storage
tanks from overpressure or vacuum collapse.
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Typical Pressure & Vacuum Set Points in API 2000
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Normal Pressure Relief: 0.5 in WC to 24 in WC
Normal Vacuum Relief: -0.5 in WC to -24 in WC
Emergency Pressure Relief: Higher values (depends on tank design)
Emergency Vacuum Relief: Typically around 2 to 6 in WC
Calibration Procedure for API 2000 Valves
1. Equipment Needed:
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Digital or analog manometer (e.g., Dwyer, Ashcroft, Testo)
Calibrated pressure source (air or nitrogen)
Vacuum pump (for vacuum relief valves)
Deadweight tester or reference gauge (for higher accuracy)
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API 2000 standard reference document
2. Steps for Calibration:
1. Connect the valve to the test setup with a controlled pressure or vacuum source.
2. Slowly apply pressure or vacuum and observe at what WC level the valve starts to
open.
3. Compare the measured value against the API 2000 specifications for the valve type.
4. Adjust the setpoint if necessary (some valves allow spring tension adjustment).
5. Document the calibration results for compliance and quality assurance.
Calibration Procedure for API 2000 Valves
1. Equipment Needed:





Digital or analog manometer (e.g., Dwyer, Ashcroft, Testo)
Calibrated pressure source (air or nitrogen)
Vacuum pump (for vacuum relief valves)
Deadweight tester or reference gauge (for higher accuracy)
API 2000 standard reference document
2. Steps for Calibration:
1. Connect the valve to the test setup with a controlled pressure or vacuum source.
2. Slowly apply pressure or vacuum and observe at what WC level the valve starts to
open.
3. Compare the measured value against the API 2000 specifications for the valve type.
4. Adjust the setpoint if necessary (some valves allow spring tension adjustment).
5. Document the calibration results for compliance and quality assurance.