VEGA VEGADIF 34 … 51 - Manual (Page 54)

54 VEGADIF 34 … 51
20094-EN-030731
5.5 Flow measurement
Principle of operation
The implemented effective pressure method is
based on the fact that the flow and the energy
content of a flowing medium are constant at all
positions in a closed system. This total energy
consists of a dynamic element (due to the flow
rate) and a static element (due to the pres-
sure). A restriction of the flow causes an in-
crease of the flow speed and a reduction of
the pressure. This pressure difference is
called effective pressure and is a clear meas-
ure of flow. The following correlation applies
here:
q = c •
√ ∆
P
q: Flow
c: Influence factor of the effective pressure transmit-
ter (restriction)
∆
P
: Measured effective pressure (differential pressure)
The flow is thus proportional to the square root
of the differential pressure across the restric-
tion.
Installation connections
Measurement set-up for liquids
For measurement of liquids, the impulse lines
must be completely filled with liquid. Gas
pockets in the impulse line or in the measuring
cell cause measurement errors. By mounting
the transmitter below the pressure tapping
points, the impulse lines can degas automati-
cally.
Hence the recommended installation position
of the transmitter is below the pressure tap-
ping points.
1 Inlet valve
2 Equalization valve
3 Transmitter
4 Vent or drain valve
5 Blow down valve (if needed)
6 Block valve
7 Effective pressure transmitter
The transmitter can be also installed above the
pressure tapping points. With this arrange-
ment, however air or gas pockets can form in
the highest part of the measuring system and
influence the measurement. Therefore a vent is
necessary. This arrangement should be used
only when there is sufficient static pressure. It
is very advantageous when buildup or sludge
is expected: measuring cell and impulse lines
clean themselves automatically.
5 5
2
1
6 6
7
6
- +
4
3