Siemens 6DR2410, Manual

Page: 60 / 276

1 Technical Description
1.5 Functional Description
1.5.8 Complex Functions (Arithmetic blocks c, d, h)
Manual
58
SIPART DR24 6DR2410
C79000-G7476-C153-03
Physical notes
The active pressure measuring method is based on the law of continuity and Bernoulli’s energy
equation.
According to the law of continuity the flow of flowing material in a pipe is the same at all places.
If the cross--section is reduced at one point, the flow speed at this point should increase. Ac-
cording to Bernoulli’s energy equation the energy content of flowing material is made up of the
sum of the kinetic energy (due to the speed) and the potential energy (of the pressure).
An increase in speed therefore causes a reduction in pressure.
This drop in pressure, the so--called ”active pressure”
Δ
p is a measure of the flow q.
q
=
c ·
Δ
p

The following applies:
with c as a factor which depends on the dimensions of the pipe, the shape of the constriction,
the density of the flowing medium and some other influences.
The equation states that the active pressure generated by the constriction is in the same ratio
as the square of the flow.
0
0.81
Δ
p
0.25
0.49
1.00
0.09
0.3 0.7 0.5 0.9 1.0 q
Figure 1--26 Relationship between flow q and active pressure
Δ
p
To measure the flow, a choke is installed at the measuring point which constricts the pipe and
has two connections for tapping the active pressure.
If the properties of the choke and the measuring material are known to the extent that the equa-
tion specified above can be calculated, the active pressure is a measure of the flow.
If you have chosen a certain choke, the flow can be described in the calculation state or opera-
tion state.
q
B
=
K ·
ρ
B

·
Δ
p

or q
=
K ·
ρ

·
Δ
p

Since the density is included in the measuring result according to the above equation, measur-
ing errors occur when the density in the operating state differs from the value based on the cal-
culation of the choke. Therefore a correction factor F is introduced for the density.