Quantcast Chapter 6. Instrument and Controls

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CHAPTER 6.
INSTRUMENTS AND CONTROLS
MEASUREMENT AND CONTROL OF FLUIDS
S e c t i o n 1.
1 IMPORTANCE OF MEASUREMENT AND CONTROL.  E f f i c i e n t and safe operation of a
d i s t r i b u t i o n system requires measuring and controlling flows, temperatures,
a n d pressures of the fluids handled.  Several types of Instruments and
c o n t r o l s are used for that purpose, a s described in the following paragraphs.
The amount and type of instruments and controls used depends on the complexity
a n d importance of the installation.
2 INDICATING INSTRUMENTS.  These instruments give Immediate information on
f l u i d condition for purposes of regulation and control, and warning of unusual
c o n d i t i o n s , such as abnormal pressures, temperatures, or flows.  Pressure
gauges, thermometers, a n d flowmeters are the indicating instruments most
commonly used in distribution systems.
2 . 1 Indicating Pressure Gauge.
When employing a Bourdon gauge to measure steam pressure, use a
gauge siphon or water leg to ensure that hot steam does not
c o m e into contact with the tube.
The more common type of pressure gauge is the Bourdon type, whose Internal
m e c h a n i s m is illustrated in figure 6-1.  The measuring element of the Bourdon
t y p e gauge is a thin-walled, oval-shaped tube bent into an arc. The tube is
closed at one end and connected to the pressure source at the other.  The
o v a l - s h a p e d tube changes its shape with a change in pressure resulting in the
following conditions.
( a ) When the pressure within the tube Increases, the oval cross section
t e n d s to become circular.
( b ) The above action causes the tube to straighten. This is similar to
t h e action of a paper tube toy which is normally curled until air pressure is
a p p l i e d to the opening.
( c ) The movement of the free end of the Bourdon tube is transmitted
t h r o u g h a toothed gear sector and pinion to a pointer which indicates the
p r e s s u r e on a uniformly graduated dial.
2 . 2 Pressure Snubbers.  Pressure snubbers are sometimes used to protect the
gauge from damage to the mechanism by cushioning shock and pulsation. A gauge
cock must be Installed next to the gauge, and provisions are often made for
t h e Installation of a test gauge for testing purposes.
2 . 3 Thermometers.  These instruments are used to measure the temperature of
f l u i d s .  There are principally three types: liquid, analog, and digital.
2 . 3 . 1 Liquid Thermometers.  The more common type of liquid thermometer
c o n s i s t s of a capillary glass tube from which air has been removed. The upper
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