Flow metering based on differential pressure

Throttling of the medium increases the differential pressure. By measuring this differential pressure and correlating it to the density of the medium and the pipe size used, the flow rate can be calculated with the help of the differential pressure.
This measurement is suitable for liquids, gases and steam.

Simple design as an advantage

Differential pressure measurement is an old and a relatively simple method for flow measurement.

The BOPP&REUTHER and METRA differential pressure units we represent can be delivered for separate installation or as compact models in which the differential pressure transmitter has been installed directly in the measurement flange.

For invoicing-related steam consumption measurement, methods according to the standard SFS EN 5167 are mainly used.

The measuring methods according to the standard SFS-EN ISO 5167 are as follows:

  • measuring flange or orifice flange
  • flow nozzle (ISA-1932)
  • Venturi nozzle
  • Venturi tubes

The standard SFS-EN ISO 5167 specifies the following:

  • differential pressure unit structure and manufacturing method
  • flow rate calculation equation
  • differential pressure unit discharge factor for different bore ratios and measurement methods
  • expansion factor for gaseous media
  • straight sections before and after the differential pressure unit

Process pressure and temperature are rarely stable. Because of this, the medium density ρ needs to be compensated for based on temperature and pressure. Compensation can be arranged either directly in the differential pressure transmitter or by using a separate compensation unit.

Metal expands when heated. In invoicing-related measurements, temperature variations are taken into account by modifying the size of the flow port based on temperature measurements and a pre-programmed thermal expansion coefficient.

As an option, METRA differential pressure transmitters can be equipped with an automatic zero point calibration module. This significantly prolongs the retention of measuring accuracy and reduces the need for unit calibration.