Introduction to flow measurement
based on differential pressure

A given mass of a media (liquid, gas or steam) contains a given
amount of energy. 

Bernoulli ' s equation states that energy cannot be lost. If the flow speed is increased by a restriction in the pipe, pressure energy is converted into speed energy. This means that the pressure just ahead of the restriction is higher than at the restriction, i.e. there is a pressure difference. 

If this differential pressure is measured and its square root taken, the resulting signal is flow-proportional.
Care must be taken of with low values of flow, as a 10% flow will only give a different pressure signal of 1 %. 

An additional source of error can be caused by different temperatures at the plus and minus pressure taps.
More details on flow-calculation are given in
DIN1952/ISO5167.


Restrictor devices

The most common restrictor devices are

ORIFICE PLATES
ORIFICE PLATES WITH CARRIER RINGS
VENTURI-NOZZLES
CLASSICAL VENTURITUBES


ORIFICE PLATES

These cost-effective devices are used tor large nominal pipe diameters of ON 200 (8") and larger. Normally they will not delivered with pressure taps.
They are mounted between flanges e. g. acc. OIN 19214
or ANSI 16.36 or other standards.


ORIFICE PLATES WITH CARRIER RINGS

Available for nominal pipe diameters of ON 50 (2")
to ON 400 (16"), these devices consist of a metal ring holding the orifice plate and fitted completely with pressure taps welded to the carrier ring. They are mounted between flanges and should be used with clean media.


VENTURI-NOZZLES

Venturi-Nozzles are best suited for applications in which only a limited pressure drop is acceptable or where dirty liquids or dust y gases must be measured. The flow nozzle is the most cost-effective version. They are mounted between flanges or be welded into the pipe.


CLASSICAL VENTURITUBES

Classical Venturi-tubes are suited far application where a limited
pressure drop is acceptable and a short inlet pipe is necessary
or for large pipes over DN 400 (16"). Venturi-tubes can be
constructed for flanging or far welding.