The ISO 5167 Story


ISO 5167: a measurement standard that delivers

The money involved in differential-pressure flow measurement is enormous.  Most flows can be measured using an orifice plate or a Venturi tube.  An orifice plate has the special and valuable property that it does not require calibration in a flowing fluid, but the flow can be calculated from ISO 5167.  So the actual equations in ISO 5167 are of immense importance: the measured flow is directly proportional to both the discharge coefficient and the expansibility factor, for which the equations were both derived at NEL.  In the last few years both equations have been shown to deliver what they promise: the data for the discharge coefficient equation are given in Figure 1.  Fulfilment of the promise depends both on the equations and on the design of the orifice meter being in accordance with ISO 5167 (and ISO 5167 being correct).

Discharge Coefficient Data

Figure 1: Percentage mean deviation in discharge coefficient from the Reader-Harris/Gallagher (1998) Equation (in ISO 5167) from more than 50 orifice plates not used to fit the Equation (almost all the data were published in 2001 or later)

A deep understanding

A systematic record and an analysis of the experimental data on which ISO 5167 is based have been undertaken and published by Michael Reader-Harris of NEL as ‘Orifice Plates and Venturi tubes’ (Springer, 2015).  The analysis is based on an understanding of the physics of the flow gained from experiment and computational fluid dynamics over a career at NEL particularly devoted to this topic.

Impact and relevance

‘Orifice Plates and Venturi tubes’ enables the reader to estimate the effect of a deviation from ISO 5167.  Moreover, the book will be particularly useful for members of standards working groups and committees when it comes to revise ISO 5167: it shows where the data can be found to justify each clause and how the data have been analysed.  At present the author chairs BSI CPI 30/2 (differential pressure meters) and ISO/TC 30 (flow measurement), but he has now ensured that in due course his successors can build on what has been achieved.

In the UK most natural gas is measured at least once using an orifice plate: each year perhaps £25 billion of natural gas is so metered with an uncertainty of about £250 million.  NEL and others have done work over decades to ensure that ISO 5167 is correct, that is fully justified, neither inadequately restrictive, causing error, nor too restrictive, causing unnecessary expense.

ISO 5167 is not restricted to measurements of natural gas; it is referred to in standards of many ISO Technical Committees: in a survey a few years ago there were references in the standards of the following 13 Technical Committees (in addition to Measurement of fluid flow in closed conduits and Natural Gas): Refrigeration and air-conditioning, Compressors and pneumatic tools, machine and equipment, Road vehicles, Fire safety, Industrial fans, Welding and allied processes, Acoustics, Cleanrooms and associated controlled environments, Equipment for fire protection and fire fighting, Gas turbines, Hydrometry, Pumps, and Valves (TCs with references in many standards are given first).  Such wide use demonstrates the value of years of effort.