TECHNICAL DESCRIPTION
ULTRAFLOW
®
54 DN150-250
5512-876 GB/04-2012/Rev. F1
33
8.7
Guidelines for dimensioning ULTRAFLOW
®
In connection with installations it has proved practical to work with higher pressures than those mentioned below:
Nominal flow q
p
Recommended back
pressure
Max. flow q
s
Remommended back
pressure
[m³/h]
[bar]
[m³/h]
[bar]
150
1
300
2
250
1.5
500
2.5
400 (DN150)
1.5
800
2.5
400
1
800
2
600
1.5
1200
2.5
1000
1.5
2000
2.5
Table 12
The purpose of recommended back pressure is to avoid measuring errors as a result of cavitation or air in the
water.
It is not necessarily cavitation in the sensor itself, but also bubbles from cavitating pumps and regulating valves
mounted before the sensor. It can take some time until such bubbles have been dissolved in the water.
Furthermore, water can include air dissolved in the water. The amount of air which can be dissolved in water
depends on pressure and temperature. This means that air bubbles can be formed due to falling pressure, e.g.
caused by a velocity rise in a contraction or above the sensor.
The risk of these factors affecting accuracy is reduced by maintaining a fair pressure in the installation.
In relation to above table, the steam pressure at current temperature must also be considered.
Table
12 Applies to
temperatures up to approx. 80° C. Furthermore, it must be considered that the above-mentioned pressure is the
pressure at the sensor, and that the pressure is lower after a contraction than before one (among other things
cones). This means that the pressure, when measured elsewhere, might be different from the pressure at the
sensor.
This can be explained by combining the continuity equation and Bernoulli’s equation. The total energy from the
flow will be the same at any cross section. It can be reduced to: P + ½
ρ
v
2
= constant.
Steam pressure
0
1
2
3
4
5
6
80
90
100
110
120
130
140
150
[°C]
[b
a
r]
Diagram 10
Dimensioning a flow sensor you must take the above into consideration, especially if the sensor is used within the
scope of EN 1434 between q
p
and q
s
, and in case of heavy contractions of the pipe.