21
ECNO: 622
7. Q & A
7.5 Is it possible to measure the flow in piping that is not full?
In horizontal piping, if the pipe is filled with liquid up to 2/3 of inside diameter D as shown below,
the flow velocity can be measured. In this case, the flow rate indicated is the assumed one under
filled pipe conditions.
Therefore, the flow indicated is larger than the actual flow.
If sludge is accumulated on the bottom of the piping, the flow velocity can be measured up to 1/3 of
inside diameter D. In this case, the flow rate indicated is the assumed one under filled pipe condi-
tions without any sludge.
7.6 What happens when the liquid contains air bubbles ?
When liquid contains excessive air bubbles, no measurement can be made because of transmis-
sion failure of the ultrasonic waves. When air bubbles enter the liquid momentarily, the output is
retained by the self-check function, thereby causing no problems. Air bubbles easily enter liquid in
the following cases.
(1)
Suction of air due to low liquid level of pump well
(2) Occurrence of cavitation
(3) Pressure in the piping becomes negative and air enters from piping connection.
D
1D/3
D
2D/3
P
P
q
e
Negative pressure
