Page 223 - Dynamics and Control of Nuclear Reactors

222 CHAPTER 16 Nuclear plant instrumentation

16.2.4 Flow sensors
Flow measurements are very important in nuclear reactors and other process indus-
tries. Consequently, there have been major efforts to develop flow sensors for various
process conditions, resulting in numerous ways to measure fluid flow in processes.
Some of the methods are old and widely used and some are recently developed.
Described below are several methods with applicability in current or future reactors.

16.2.4.1 Flow vs. pressure drop
A common reactor fluid flow measurement involves measurement of the pressure
drop across a constriction in the flow path. This method has seen widespread use
in process industries for many years. The constriction can be an orifice or a venturi
section (see Fig. 16.11) or even just a bend in a pipe.

16.2.4.2 Advanced flowmeters
Advanced flow measurement technology is also available, and some will likely be
used in advanced reactors. These include ultrasonic sensors, magnetic flowmeters
for liquid metal flow measurement, and eddy current sensors for liquid metal flow
measurement. These sensors exist in various configurations. Examples that illustrate
the technology are described briefly below:
One type of ultrasonic flowmeter employs two sound transducers mounted at an
angle to the pipe and mounted so as to face one another. One transducer is aimed in
the direction of flow and one aimed in opposition to the flow in a pipe. When the
upstream transducer emits an ultrasonic pulse, the signal reaches the downstream
transducer after passing through fluid traveling in the same direction as the pulse.
When the downstream transducer emits an ultrasonic pulse, the signal reaches the
upstream transducer after passing through fluid flowing in the opposite direction
of the pulse. The transit times of the pulses depends on the velocity of the fluid

FIG. 16.11
A venturi device for flow measurement.