Tuesday, April 24, 2018

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Other Types of Instruments

So far we have just looked at instruments that sense, control, and influence process variables. Transmitters, controllers, and control valves are respective examples of each instrument type. However, other instruments exist to perform useful functions for us.

1. Indicators

One common “auxiliary” instrument is the indicator, the purpose of which is to provide a human-readable indication of an instrument signal. Quite often process transmitters are not equipped with readouts for whatever variable they measure: they just transmit a standard instrument signal (3 to 15 PSI, 4 to 20 mA, etc.) to another device. An indicator gives a human operator a convenient way of seeing what the output of the transmitter is without having to connect test equipment (pressure gauge for 3-15 PSI, ammeter for 4-20 mA) and perform conversion calculations. Moreover, indicators may be located far from their respective transmitters, providing readouts in locations more convenient than the location of the transmitter itself. An example where remote indication would be practical is shown here, in a nuclear reactor temperature measurement system:




No human can survive near the nuclear reactor when it is in full-power operation, due to the strong radiation flux it emits. The temperature transmitter is built to withstand the radiation, though, and it transmits a 4 to 20 milliamp electronic signal to an indicating recorder located outside of the containment building where it is safe for a human operator to be. There is nothing preventing us from connecting multiple indicators, at multiple locations, to the same 4 to 20 milliamp signal wires coming from the temperature transmitter. This allows us to display the reactor temperature in as many locations as we desire, since there is no absolute limitation on how far we may conduct a DC milliamp signal along copper wires.

A numerical and bargraph panel-mounted indicator appears in this next photograph:



This particular indicator, manufactured by Weschler, shows the position of a flow-control gate in a wastewater treatment facility, both by numerical value (98.06%) and by the height of a bargraph (very near full open – 100%).

A less sophisticated style of panel-mounted indicator shows only a numeric display, such as this Red Lion Controls model shown here:



Indicators may also be used in “field” (process) areas to provide direct indication of measured variables if the transmitter device lacks a human-readable indicator of its own. The following photograph shows a Rosemount brand field-mounted indicators, operating directly from the electrical power available in the 4-20 mA loop:



2. Recorders

Another common “auxiliary” instrument is the recorder (sometimes specifically referred to as a chart recorder or a trend recorder), the purpose of which is to draw a graph of process variable(s) over time. Recorders usually have indications built into them for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values, and for this reason are usually designated as indicating recorders. A temperature indicating recorder for the nuclear reactor system shown previously would be designated as a “TIR” accordingly.

A circular chart recorder uses a round sheet of paper, rotated beneath a pen moved side-to-side by a servomechanism driven by the instrument signal. Two such chart recorders are shown in the following photograph:




Two more chart recorders appear in the next photograph, a strip chart recorder on the right and a paperless chart recorder on the left. The strip chart recorder uses a scroll of paper drawn past one or more lateral-moving pens, while the paperless recorder does away with paper entirely by drawing graphic trend lines on a computer screen:



Recorders are extremely helpful for troubleshooting process control problems. This is especially true when the recorder is configured to record not just the process variable, but also the controller’s setpoint and output variables as well. Here is an example of a typical “trend” showing the relationship between process variable, setpoint, and controller output in automatic mode, as graphed by a recorder:



Here, the setpoint (SP) appears as a perfectly straight (red) line, the process variable as a slightly bumpy (blue) line, and the controller output as a very bumpy (purple) line. We can see from this trend that the controller is doing exactly what it should: holding the process variable value close to setpoint, manipulating the final control element as far as necessary to do so. The erratic appearance of the output signal is not really a problem, contrary to most peoples’ first impression. The fact that the process variable never deviates significantly from the setpoint tells us the control system is operating quite well. What accounts for the erratic controller output, then? Variations in process load. As other variables in the process vary, the controller is forced to compensate for these variations in order that the process variable does not drift from setpoint. Now, maybe this does indicate a problem somewhere else in the process, but there is certainly no problem in this control system.

Recorders become powerful diagnostic tools when coupled with the controller’s manual control mode. By placing a controller in “manual” mode and allowing direct human control over the final control element (valve, motor, heater), we can tell a lot about a process. Here is an example of a trend recording for a process in manual mode, where the process variable response is seen graphed in relation to the controller output as that output is increased and decreased in steps:


Notice the time delay between when the output signal is “stepped” to a new value and when the process variable responds to the change. This sort of delay is generally not good for a control system. Imagine trying to steer an automobile whose front wheels respond to your input at the steering wheel only after a 5-second delay! This would be a very challenging car to drive, because the steering is grossly delayed. The same problem plagues any industrial control system with a time lag between the final control element and the transmitter. Typical causes of this problem include transport delay (where there is a physical delay resulting from transit time of a process medium from the point of control to the point of measurement) and mechanical problems in the final control element.

This next example shows another type of problem revealed by a trend recording during manual mode testing:




Here, we see the process quickly responding to all step-changes in controller output except for those involving a change in direction. This problem is usually caused by mechanical friction in the final control element (e.g. sticky valve stem packing in a pneumatically-actuated control valve), and is analogous to “loose” steering in an automobile, where the driver must turn the steering wheel a  little bit extra after reversing steering direction. Anyone who has ever driven an old farm tractor knows what this phenomenon is like, and how it detrimentally affects one’s ability to steer the tractor in a straight line.

3. Process switches and alarms

Another type of instrument commonly seen in measurement and control systems is the process switch. The purpose of a switch is to turn on and off with varying process conditions. Usually, switches are used to activate alarms to alert human operators to take special action. In other situations, switches are directly used as control devices.

The following P&ID of a compressed air control system shows both uses of process switches:



The “PSH” (pressure switch, high) activates when the air pressure inside the vessel reaches its high control point. The “PSL” (pressure switch, low) activates when the air pressure inside the vessel drops down to its low control point. Both switches feed discrete (on/off) electrical signals to a logic control device (signified by the diamond) which then controls the starting and stopping of the electric motor-driven air compressor.

Another switch in this system labeled “PSHH” (pressure switch, high-high) activates only if the air pressure inside the vessel exceeds a level beyond the high shut-off point of the high pressure control switch (PSH). If this switch activates, something has gone wrong with the compressor control system, and the high pressure alarm (PAH, or pressure alarm, high) activates to notify a human operator.

All three switches in this air compressor control system are directly actuated by the air pressure in the vessel. In other words these are process-sensing switches. It is possible to build switch devices that interpret standardized instrumentation signals such as 3 to 15 PSI (pneumatic) or 4 to 20 milliamps (analog electronic), which allows us to build on/off control systems and alarms for any type of process variable we can measure with a transmitter.

For example, the chlorine wastewater disinfection system shown earlier may be equipped with a couple of alarm switches to alert an operator if the chlorine concentration ever exceeds predetermined high or low limits:



The labels “AAL” and “AAH” refer to analytical alarm low and analytical alarm high, respectively. Since both alarms work off the 4 to 20 milliamp electronic signal output by the chlorine analytical transmitter (AT) rather than directly sensing the process, their construction is greatly simplified. If these were process-sensing switches, each one would have to be equipped with the capability of directly sensing chlorine concentration. In other words, each switch would have to be its own chlorine concentration analyzer, with all the inherent complexity of such a device!

An example of such an alarm module (operating off a 4-20 mA current signal) is the Moore Industries model SPA (“Site Programmable Alarm”), shown here:



Like all current-operated alarm modules, the Moore Industries SPA may be configured to “trip” electrical contacts when the current signal reaches a variety of different programmed thresholds. Some of the alarm types provided by this unit include high process, low process, out-of-range, and high rate-of-change.

Process alarm switches may be used to trigger a special type of indicator device known as an annunciator. An annunciator is an array of indicator lights and associated circuitry designed to secure a human operator’s attention1 by blinking and sounding an audible buzzer when a process switch actuates into an abnormal state. The alarm state may be then “acknowledged” by an operator pushing a button, causing the alarm light to remain on (solid) rather than blink, and silencing the buzzer. The indicator light does not turn off until the actual alarm condition (the process switch) has returned to its regular state.

1D.A. Strobhar, writing in The Instrument Engineer’s Handbook on the subject of alarm management, makes the interesting observation that alarms are the only form of instrument “whose sole purpose is to alter the operator’s behavior.” Other instrument devices may automatically respond to process changes by directly influencing the process, but only alarms work to control the operator.

This photograph shows an annunciator located on a control panel for a large engine-driven pump. Each white plastic square with writing on it is a translucent pane covering a small light bulb. When an alarm condition occurs, the respective light bulb flashes, causing the translucent white plastic to glow, highlighting to the operator which alarm is active:


Note the two pushbutton switches below labeled “Test” and “Acknowledge.” Pressing the “Acknowledge” button will silence the audible buzzer and also turn any blinking alarm light into a steady (solid) alarm light until the alarm condition clears, at which time the light turns off completely. Pressing the “Test” button turns all alarm lights on, to ensure all light bulbs are still functional.

Opening the front panel of this annunciator reveals modular relay units controlling the blinking and acknowledgment latch functions, one for each alarm light:



This modular design allows each alarm channel to be serviced without necessarily interrupting the function of all others in the annunciator panel.

A simple logic gate circuit illustrates the acknowledgment latching feature (here implemented by an S-R latch circuit) common to all process alarm annunciators:



Panel-mounted annunciators are becoming a thing of the past, as computer-based alarm displays replace them with advanced capabilities such as time logging, first-event recording, and multiple layers of ackowledgement/access. Time logging is of particular importance in the process industries, as the sequence of events is often extremely important in investigations following an abnormal operating condition. Knowing what went wrong, in what order is much more informative than simply knowing which alarms have tripped.


Go back to the first part of Introduction to Industrial Instrumentation

Go Back to Lessons in Instrumentation Table of Contents

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