PSIA to PSIG Converter

Pressure vessel certification: design calculations use absolute pressure (psia), but operating procedures and gauge readings use gauge pressure (psig).

Compressor discharge pressure conversion from thermodynamic calculations (absolute) to operating setpoints (gauge).

Safety valve settings verification.

In the design of pressure vessels and boilers per ASME BPVC Section I (Power Boilers) and Section VIII (Pressure Vessels), the maximum allowable working pressure (MAWP) is stamped on the nameplate in PSIG (or kPa, bar) for the US market, but the stress calculations in the design report are done with absolute pressure because the design fatigue cycle is referenced to the absolute lower bound.

A boiler designed for 1,000 PSIG operating pressure (1,014.7 PSIA) and a 20,000-cycle fatigue life requires the stress range calculation to use the full absolute pressure range, with the conversion applied at the stress calculation step.

In the design of steam turbines, gas turbines, and large centrifugal compressors, the performance map uses pressure ratio (a dimensionless quantity) plotted against corrected speed and flow, with the inlet pressure explicitly in PSIA.

The discharge pressure gauge reading (PSIG) is then derived from the PSIA discharge and the local atmospheric pressure.

Safety relief valve sizing per API 520 and API 521 is done using the set pressure in PSIG (or kPa, bar) at the valve inlet, but the superimposed back pressure and the atmospheric pressure must be accounted for in the calculation using absolute pressures, with the conversion applied to the set pressure plus the atmospheric pressure to get the absolute pressure at the valve inlet.

The set pressure of a 100 PSIG safety valve on a vessel at 5,000 ft elevation with P_atm = 12.23 PSIA means the valve opens at 112.23 PSIA, a value that must be used in the mass flow calculation for the required orifice area.

In the SCADA and DCS systems of refineries, chemical plants, and power plants, pressure transmitters typically measure PSIG (or the differential pressure) and the control system displays the value in the operator's preferred unit — the underlying HART, Foundation Fieldbus, or PROFIBUS configuration must be aware of the atmospheric pressure reference, especially at elevated plant sites or at plants with barometric pressure variations, and the calibration certificate of the transmitter must specify the reference conditions.