16-01-2013, 04:28 PM
INSTRUMENTATION EQUIPMENT
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Introduction
• Instrumentation is used in almost every industrial process and generating
system, where consistent and reliable operations are required.
• Instrumentation provides the means of monitoring, recording and controlling a
process to maintain it at a desired state.
• A typical industrial plant such as an electric generating station (Figure 1) yields
many process variables that have to be measured and manipulated.
• Variables such as boiler level, temperature and pressure, turbine speed,
generator output and many others have to be controlled prudently to ensure a
safe and efficient station operation.
• Because of the continuous interactive nature of most industrial process
systems, manual control is non-feasible and unreliable. With instrumentation,
automatic control of such processes can be achieved.
Pneumatic Signals
• A pneumatic process sensor coupled to a transmitter is used to monitor a
process variable; such as level in a tank or pressure in a pipe.
• The output signal of the pneumatic transmitter is air pressure, the magnitude of
which is directly proportional to the process variable being monitored.
• The standard industrial range for pneumatic signals is 20 to 100 kPa(g)
(kPa(g) = kPa above atmospheric), which corresponds to a 0% to 100%
process condition.
Advantages and Disadvantages of Pneumatic Transmission.
• One advantage of a pneumatic system (over an electronic system) is that
sparks will not be produced if a transmitter malfunction occurs, making it much
safer when used in an explosive environment.
• Furthermore, there is no electric shock hazard.
• On the other hand, a pressurized system can be dangerous if a line ruptures.
• Also, pneumatic signal lines are bulky and difficult to install.
• The biggest problem with pneumatic systems is that air is compressible. This
means that a pressure transient representing a process change will only travel
in the air line at sonic velocity (approximately 300 m/sec.). Long signal lines
must therefore be avoided to prevent substantial time delays a serious
drawback when you consider the size of our nuclear generating stations.
Electronic Signals
• For large industrial process applications such as generating stations where
central control rooms are used, electronic signals are preferred and in many
cases are used exclusively.
• The process condition is monitored and an electronic signal that is proportional
to that process condition is produced by an electronic transmitter.
• The accepted industrial standard for electronic signals is now a 4 to 20 mA
current signal that represents the 0% to 100% process condition.
• Again, a live zero (4 mA) is used to distinguish between 0% process (4 mA)
and an interrupted or faulted signal loop (0 mA).
The 4 - 20 mA Current Loop
• The simplified electronic transmitter can be considered as a variable resistor
where the resistance is altered by the process condition. When used in series
with a constant voltage power supply, a 4 - 20 mA current can be produced in
the loop.
• Recall Ohm's Law, I = V/R
• By varying the resistance while the power supply voltage is kept at a constant
value, the amount of current in a loop can be proportionally manipulated.
• The same two wires that power the transmitter also carry the signal. Using a
current signal minimizes the number of wires needed and the effect of
background noise, which is essentially induced voltage.
• Readout devices which can either be current or voltage sensitive can be used
to provide a signal indication. To get a direct indication of the current signal, a
milli-ammeter can be connected in series in the loop. Alternatively, a voltage
sensitive device such as a voltmeter can be connected in parallel with a
dropping resistor to sense the potential developed across the resistor by the
current flow. (Figure 4).