Page 54 - Instrumentation Reference Book 3E

Point velocity measurement 39
The field coil develops an electromagnetic field
in the region of the sensor and the electrodes pick
up a voltage generated which is proportional to
the point velocity. The probe system can be used
for either open-channel or closed-pipe flow of
conducting liquids. It should be noted, however,
that the accuracy of a point-velocity magnetic
flow meter is approximately similar to that of a
paddle-wheel or other point-velocity meter.
Although it shares a measurement technology
with a highly accurate flowmeter, it IS not one.
Recently, a combination of the multiple port con-
cept of an Annubarm-type meter with the point
Figure 1.54 Multiple SensorAveraging Insertion velocity magnetic flowmeter has been released,
Magmeter, courtesy of Marsh-McBirney Inc with excellent results. See Figure 1.54.

1.5.5 Insertion turbine
ures the static pressure less the suction pressure.
In this way a differential pressure representing the The operating principle for this device is the same
mean velocity along the tube is obtained enabling as for a full-bore pipeline flowmeter. It is used
the flow to be obtained with an accuracy of normally for pipe-flow velocity measurement in
&1 percent of actual flow. liquids and consists of a small turbine housed in a
protective rotor cage as shown in Figure 1.55. In
normal application the turbine meter is inserted
1.5.4 Electromagnetic velocity probe through a gate valve assembly on the pipeline;
This type of device is basically an inside-out ver- hence it can be installed under pressure and can
sion of the electromagnetic pipeline flowmeter be precisely located for carrying out a flow tra-
discussed earlier, the operating principle being verse. Also, given suitable conditions it can be
the same. The velocity probe consists of either a used as a permanent flowmetering device in the
cylindrical or an ellipsoidal sensor shape which same way as the pitot tube. The velocity of the
houses the field coil and two diametrically turbine is proportional to liquid velocity but a
opposed pick-up electrodes.

Electrical
connector

Clamp
chamber 4 0 Pressure
I so lation
release valve

Figure 1.56 Propeller-type current meter. Courtesy,
Figure 1.55 Insertion turbine flowmeter. Nixon Instrumentation Ltd.