C068 – ISO 5167:1991 – Orifice Flow Calculation

This calculation can be set to calculate the flow rate through an orifice plate metering system, the differential pressure setting of the metering system or the orifice size to use for specified flowing conditions according to the 1991 revision of the ISO 5167 standard.


ISO 5167-1:1991 – Measurement of fluid low by means of pressure differential devices – Part 1: Orifice plates, nozzles and venturi tubes inserted in circular cross-section conduits running full

BS 1042: Part 1. Pressure differential devices – Section 1.2 Specification for square-edged orifice plates and nozzles (with drain holes, in pipes below 50mm diameter, as inlet and outlet devices) and other orifice plates and Borda inlets (1984)

Kelton calculation reference C068

FLOCALC calculation reference F068

Kelton calculation C270 – ISO 5167 Orifice Mass Flow Calculation

Kelton calculation C273 – ISO 5167 Differential Pressure Calculations

Kelton calculation C274 – ISO 5167 Sizing Calculation

Kelton calculation C261 – Gas Flowrate Conversion from Qm

Kelton calculation C262 – Liquid Flowrate Conversion


Solve for

  • Flow Rate
  • Orifice Size
  • Differential Pressure

This option is used to select what type of calculation the user wishes to implement determined by the necessary output.

If solving for flow rate the options for this calculation are the same as for C270 ISO 5167 Orifice Plate Calculations however the year of the standard will be set to 1991.

When solving for orifice size or differential pressure the options available will be the same as C274: ISO 5167 Sizing Calculation or C273: ISO 5167 Differential Pressure Calculation respectively with the standard year set to 1991 and the meter type set to orifice.


The calculations available to the user here all revolve around the mass flow rate equation as can be seen in C270. The 1991 standard uses the Stolz equation to calculate discharge coefficient. The equations used to calculate expansibility factor, pressure loss and upstream temperature can be found here and the upstream density calculation can be found here. Note that to comply with the 1991 ISO 5167 standard the isentropic corrections must be applied to temperature and density (if such a correction is necessary).