25
Flow profile distortions result from upstream
disturbances such as bends, tees, valves,
pumps and other similar obstructions.
To ensure a uniform profile the transducers
must be mounted far enough away from any
cause of distortion such that it no longer has
an effect.
Figure 14
The minimum length of upstream straight pipe
is 20 Diameters and 10 Diameters downstream
to ensure accurate results will be achieved.
Flow measurements can be made on shorter
lengths of straight pipe down to 10 Diameters
upstream and 5 Diameters downstream, but
when the transducers are sighted this close to
any obstruction errors can be considerable.
It is not possible to predict the amount of error
as this depends entirely upon the type of
obstruction and the configuration of the
pipework.
The message therefore is clear: Do not expect
to obtain accurate results if the transducers are
positioned closer than allowed to any
obstruction that distorts the uniformity of the
flow profile.
MOUNTING THE TRANSDUCERS
It will be impossible to achieve the accuracy of
measurement specified for the Portaflow 300 if
the transducers are not clamped to the pipe
correctly and if the data - I.D. O.D., Pipe
Material - are not accurate.
Apart from the correct positioning and
alignment of the transducers, of equal
importance is the condition of the pipe surface
in the area under each of the transducers.
An uneven surface that prevents the
transducers from sitting flat on the surface of
the pipe can cause Signal Level and Zero
Offset problems. The following procedure is
offered as a guide to good practice with
respect to positioning and mounting the
transducers.
1)
Select the site following the rules on
page 23 - Transducer Positioning.
2)
Inspect the surface of the pipe to ensure
it is free from rust or is not uneven for
any reason. Transducers can be
mounted directly on painted surfaces as
long as the surface is smooth and that
the underlying metal surface is free from
rust bubbles. On bitumen or rubber
coated pipes the coating must be
removed in the area under the
transducers as they should be mounted
directly on to the base metal.
3) Transducers can be mounted on both
Vertical and Horizontal Pipe Runs.
4) Apply Interface couplant to the face of the
transducers. The amount of couplant used
is extremely important particularly on pipes
of less than 89mm bore.
Figure 15
For all small pipes below 89mm, using 2MHz
transducers, the bead of couplant used must
be approximately 20mm long and 2mm
maximum diameter for the moveable sensor
and 30 long and 2mm diameter for the fixed
sensor. Using more couplant could cause wall
signals to be generated which cause errors in
measurement.
On Stainless Steel Pipes the amount of
couplant applied should never exceed the
amount indicated on page 3. On large Plastic
and Steel Pipes the amount of couplant
applied is less critical, however do not use
more than is absolutely necessary.
Transducer set ‘B’ and ‘C’.The main difference
between transducer set ‘B’ and ‘C’ is the angle
that the crystal has been inserted in the sensor
block itself. The maximum amount of couplant
required is a bead 30mm long by 5mm wide.
Figure 16
INCORRECT
> 20 D
FLOW
CORRECT
FLOW
FLOW
10
20
FLOW
5
20
< 20 D
