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Example: Boiler Water Level Control System

Steam boilers are very common in industry, principally because steam power is so useful. Common uses for steam in industry include doing mechanical work (e.g. a steam engine moving some sort of machine), heating, producing vacuums (through the use of “steam eductors”), and augmenting chemical processes (e.g. reforming of natural gas into hydrogen and carbon dioxide).

The process of converting water into steam is quite simple: heat up the water until it boils. Anyone who has ever boiled a pot of water for cooking knows how this process works. Making steam continuously, however, is a little more complicated. An important variable to measure and control in a continuous boiler is the level of water in the “steam drum” (the upper vessel in a water-tube boiler). In order to safely and efficiently produce a continuous flow of steam, we must ensure the steam drum never runs too low on water, or too high. If there is not enough water in the drum, the water tubes may run dry and burn through from the heat of the fire. If there is too much water in the drum, liquid water may be carried along with the flow of steam, causing problems downstream.

In this next illustration, you can see the essential elements of a water level control system, showing transmitter, controller, and control valve:

 

Boiler_Water_Fig_002.JPG

 

The first instrument in this control system is the level transmitter, or “LT”. The purpose of this device is to sense the water level in the steam drum and report that measurement to the controller in the form of an instrument signal. In this case, the type of signal is pneumatic: a variable air pressure sent through metal or plastic tubes. The greater the water level in the drum, the more air pressure output by the level transmitter.Since the transmitter is pneumatic, it must be supplied with a source of clean, compressed air on which to run. This is the meaning of the “A.S.” tube (Air Supply) entering the top of the transmitter.

This pneumatic signal is sent to the next instrument in the control system, the level indicating controller, or “LIC”. The purpose of this instrument is to compare the level transmitter’s signal with a setpoint value entered by a human operator (the desired water level in the steam drum). The controller then generates an output signal telling the control valve to either introduce more or less water into the boiler to maintain the steam drum water level at setpoint. As with the transmitter, the controller in this system is pneumatic, operating entirely on compressed air. This means the output of the controller is also a variable air pressure signal, just like the signal output by the level transmitter. Naturally, the controller requires a constant supply of clean, compressed air on which to run, which explains the “A.S.” (Air Supply) tube connecting to it.

The last instrument in this control system is the control valve, being operated directly by the air pressure signal generated by the controller. This particular control valve uses a large diaphragm to convert the air pressure signal into a mechanical force to move the valve open and closed. A large spring inside the valve mechanism provides the force necessary to return the valve to its normal position, while the force generated by the air pressure on the diaphragm works against the spring to move the valve the other direction.

When the controller is placed in the “automatic” mode, it will move the control valve to whatever position it needs to be in order to maintain a constant steam drum water level. The phrase “whatever position it needs to be” suggests that the relationship between the controller output signal, the process variable signal (PV), and the setpoint (SP) can be quite complex. If the controller senses a water level above setpoint, it will take whatever action is necessary to bring that level back down to setpoint. Conversely, if the controller senses a water level below setpoint, it will take whatever action is necessary to bring that level up to setpoint. What this means in a practical sense is that the controller’s output signal (equating to valve position) is just as much a function of process load (i.e. how much steam is being used from the boiler) as it is a function of setpoint. Consider a situation where the steam demand from the boiler is very low. If there isn’t much steam being drawn off the boiler, this means there will be little water boiled into steam and therefore little need for additional feedwater to be pumped into the boiler. Therefore, in this situation, one would expect the control valve to hover near the fully-closed position, allowing just enough water into the boiler to keep the steam drum water level at setpoint.

If, however, there is great demand for steam from this boiler, the rate of evaporation will be much higher. This means the control system will have to add feedwater to the boiler at a much greater flow rate in order to maintain the steam drum water level at setpoint. In this situation we would expect to see the control valve much closer to being fully-open as the control system “works harder” to maintain a constant water level in the steam drum.

A human operator running this boiler has the option of placing the controller into “manual” mode. In this mode, the control valve position is under direct control of the human operator, with the controller essentially ignoring the signal sent from the water level transmitter. Being an indicating controller, the controller faceplate will still show how much water is in the steam drum, but it is now the human operator’s sole responsibility to move the control valve to the appropriate position to hold water level at setpoint.

Manual mode is useful to the human operator(s) during start-up and shut-down conditions. It is also useful to the instrument technician for troubleshooting a misbehaving control system. When a controller is in automatic mode, the output signal (sent to the control valve) changes in response to the process variable (PV) and setpoint (SP) values. Changes in the control valve position, in turn, naturally affect the process variable signal through the physical relationships of the process. What we have here is a situation where causality is uncertain. If we see the process variable changing erratically over time, does this mean we have a faulty transmitter (outputting an erratic signal), or does it mean the controller output is erratic (causing the control valve to shift position unnecessarily), or does it mean the steam demand is fluctuating and causing the water level to vary as a result? So long as the controller remains in automatic mode, we can never be completely sure what is causing what to happen, because the chain of causality is actually a loop, with everything affecting everything else in the system.

A simple way to diagnose such a problem is to place the controller in manual mode. Now the output signal to the control valve will be fixed at whatever level the human operator or technician sets it to. If we see the process variable signal suddenly stabilize, we know the problem has something to do with the controller output. If we see the process variable signal suddenly become even more erratic once we place the controller in manual mode, we know the controller was actually trying to do its job properly in automatic mode and the cause of the problem lies within the process itself.

As was mentioned before, this is an example of a pneumatic (compressed air) control system, where all the instruments operate on compressed air, and use compressed air as the signaling medium. Pneumatic instrumentation is an old technology, dating back many decades. While most modern instruments are electronic in nature, pneumatic instruments still find application within industry. The most common industry standard for pneumatic pressure signals is 3 to 15 PSI, with 3 PSI representing low end-of-scale and 15 PSI representing high end-of-scale. The following table shows the meaning of different signal pressures are they relate to the level transmitter’s output:

 

Transmitter air signal pressure Steam drum water level
3 PSI   0% (Empty)
6 PSI   25%
9 PSI   50%
12 PSI   75%
15 PSI  100% (Full)

 

 Likewise, the controller’s pneumatic output signal to the control valve uses the same 3 to 15 PSI standard to command different valve positions:

 

Controller output signal pressure Control valve position
3 PSI 0% open (Fully shut)
6 PSI 25% open
9 PSI 50% open
12 PSI 75% open
15 PSI 100% (Fully open)


It should be noted the previously shown transmitter calibration table assumes the transmitter measures the full range of water level possible in the drum. Usually, this is not the case. Instead, the transmitter will be calibrated so it only senses a narrow range of water level near the middle of the drum. Thus, 3 PSI (0%) will not represent an empty drum, and neither will 15 PSI (100%) represent a completely full drum. Calibrating the transmitter like this helps avoid the possibility of actually running the drum completely empty or completely full in the case of an operator incorrectly setting the setpoint value near either extreme end of the measurement scale.

An example table showing this kind of realistic transmitter calibration is shown here:

 

Transmitter air signal pressure Actual steam drum water level
3 PSI 40%
6 PSI 45%
9 PSI 50%
12 PSI 55%
15 PSI 60%

 



Go back to the first part of Introduction to Industrial Instrumentation

Go to the next Example: Wasteswater Disinfection

Go Back to Lessons in Instrumentation Table of Contents




Comments (1)Add Comment
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Automatic Water Level Controller
written by ceilajohny, February 17, 2013
Thanks for nice information!

Automatic Water Level Controller
http://goo.gl/qhqui
Skayvon Electronics Pvt. Ltd. is a leading trader and service-provider engaged in offering customers with wide range of Home and Office Automation Products. Skayvon electronics Pvt. Ltd.
Automatic Water Level Controller

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