OverrideControl

Title: Override control: What is it? When useful? When not? Common usage.

Authors: Jason Bourgeois, Michael Kravchenko, Nicholas Parsons, Andrew Wang

Date Presented: 10/31/06 Date Revised:


 * First round reviews for this page
 * Rebuttal for this page

Introduction
Override control is a control method that is used to select the most pertinent information from a set of indicators to control the output. This situation usually arises when two or more variables in a system have to be controlled so that they do not surpass certain limits. These constraints usually arise from safety concerns, issues with efficiency, and economics of the process. Override controls do not replace other type of dynamic controllers (e.g. PID). Instead these controllers are used in case where a choice must be made between inputs, such as emergencies that fix problems. Limits (maximums or minimums) are used in order to select an output.

Disclaimer
All images displayed in this article were produced in Microsoft Visio 2003 and are copyright of ChE 466 Team T.

Selectors
In order to accomodate for constraints in a control process, a selector is used. The two basic types of these override controllers are high selectors and low selectors. They are generally available as both electronic and pneumatic selectors. Selectors are also available in a number of different versions that will accomodate for varying amounts of input signals. Two or more inputs are placed into the selector and one output comes out depending on the selector.

High Selectors
High selectors are designed so that they filter out all but the highest value from a multiple input feed stream. The selector then sends this single highest value through to the output signal.



Low Selectors
Low selectors are designed so that they filter out all but the lowest value from a multiple input feed stream. The selector then sends this single lowest value through to the output signal.



Selection of Median Value
Often times we will not want to select just the highest or lowest value from a set of inputs. Instead, we might want to the select the median value. A combination of high and low selectors can be used in this case in order to select this value. In order to do this, we will first need to compare each input signal to the others in a one-on-one fashion, using either all high selectors or all low selectors. The outputs from these selectors will then all be sent to one final selector, which will be of the opposite type than of the intial selector (i.e. a low selector will be used if the first filter of signals was done by a parallel arrangment of high selectors). An example of this application of override control can be seen below, in which we have 3 input signals and wish to select the median value.



A simplified version of the median selector for use in a P&ID can be seen below.



Applications of Override Control
The use of override control can be divided into four main categories, which are discussed below.

Equipment Protection
An important application of override control is to protect a piece of equipment in a process. Often times, equipment can be damaged or destroyed when it exceeds its optimal operating boundaries. In order to keep this from happening, high and low selectors are used to make sure that the output signal to the control system does not damage the equipment in the process. The selectors are set up in such a fashion that there will be multiple inputs from various sensors that are all supposed to control the same object (i.e. a valve), yet only one output stream.

An example of using override control to protect equipment would be when you have a compressor that you cannot allow to exceed a certain temperature $$T_{max}$$. This compressor also cannot be allowed to exert a certain pressure $$P_{max}$$ in the pipe line. In order to effectively protect this piece of equipment, a temperature sensor must be placed on the compressor, the signal of which is sent to a temperature controller. A pressure sensor should also be placed further down the line and its signal sent to a pressure controller. The signals from these two controllers would then be fed to a high selector, which would then send the highest output (which would also be the most dangerous signal) to the valve. Thus, the valve will shut off the valve in the pipeline and the compressor turned off if the temperature becomes greater than $$T_{max}$$ or if the pressure becomes greater than $$P_{max}$$. The input values coming into the selector are usually electrical signals. The selector is able to convert mV into proper units and can compare this with the limits that are setup. Thus if one of the limits is reached before the other, the selector will send that value as the output signal. In this case it would check for the temperature and pressure to see which limit is reached first.



Auctioneering
Auctioneering is the process of choosing one output signal from a set of multiple input signals. In order to use auctioneering in your control process, you will first need to have multiple signals all measuring the same variable. The signals will then all be sent to a set of selectors aligned in series. For each selector, there will be two inputs. For the first selector, the two inputs will be the first two signals from the device being controlled. For each subsequent selector, one signal will be the output signal from the previous selector, while the other input signal will be the next signal from the device.

For example, suppose that you have a pressurized storage tank holding a deadly gas, such as chlorine. On this tank, you have attached three pressure sensors, each measuring the pressure of the tank. In order to maintain the maximum amount of safety in your plant, you will want to choose the highest pressure read by the three sensor signals. The reason for choosing the highest pressure is because there is a chance of the tank exploding if the pressure inside gets too high. In order to select the highest pressure, an auctioneering system will be used, in which the first two signals will be sent through a high selector, and then this output value will be sent to a second high selector that compares it to the third input signal. This second selector then sends the output signal to control the pressure release valve.



Instrumentation Redundancy
To maintaining continuous operation in a process, the use of redundancy in the design of the process is essential. However, it is important to consider the additional costs of the redundant controller. There are instances where it is not worth the additional cost. In order to compensate for any sensor failures that may occur in the process, redundant sensors are often placed along the same line. There are two main types of sensor failures that may occur in a process:


 * Downscale failure: In downscale failure, one of the sensors measuring your process fails to zero. In this case, the other sensor is automatically selected in order to allow for uninterrupted operation of your process.
 * Upscale failure: In upscale failure, one of the sensors measuring your process fails to full scale. In this case, this sensor is automatically selected and the process will compensate in order to bring down the variable being measured.

An example of instrumentation redundancy would be when three temperature sensors are placed on the outflow of a CSTR reactor. The purpose of these sensors is to ensure that the temperature of the exit stream from the CSTR is not above a certain safety temperature. In order to do this, the signals from the three sensors are sent to a high selector, in order to ensure that the maximum temperature read is sent to the cooling water controller. The reason for using a high selector to select the maximum temperature read is so that our system is overcautious instead of undercautious, which is for obvious reasons.



When using multiple controllers in a line, it is important to use three or more controllers. If there are only two controllers, it is impossible to know which value is correct. Thus the third controller shows which value is correct.

Another application of instrument redundancy is to use multiple sensors on a part of your process, and then send the signals to a medium selector as discussed earlier. By using this setup, the process will always be able to select the medium value of the sensors, regardless of whether one of the sensors fails. This is due to the ability to upscale or downscale the selectors, and thus maintain seamless operation of your process.

Finally, different types of sensors may be used in conjunction with each other for instrumentation redundancy. Since, temperature and pressure are inter-related if 3 temperature sensors are required any combination from 1-3 temperature sensors and 0-2 pressure sensors may be used.

Artificial Measurements
Artificial measurements are certain minimum or maximum operating constraints that are set by the use of selectors. In order to accomplish this, we can use low selectors to create an upper operating limit and high selectors to create a lower operating limit. These two selectors are cascaded such that the output from the first is the input to the second, and the second sends the signal to the controlled device. Thus, if the signal to the first selector does not exceed the value of the filter, it will be sent on the second selector. However, if this signal exceeds the limit of the second selector, the second selector will pass on the value of the upper or lower limit as its output signal, depending on if it is a high or low selector. By predetermining these minimum and maximum values, the selectors prevent the process from exceeding its high and low limits.

For example, suppose we have a process where we want to control the feed to cooling water ratio. For this particular process, we want to maintain a ratio that lies between 20% and 80%. In order to accomplish this, we can set a high selector on the feed stream that has a minimum value of 20%, thereby always maintaining the minimum 20% ratio or higher. The high selector will then feed its output signal to a low selector that has a maximum value of 80%, thereby always keeping the ratio at 80% or less. This low selector will then use its signal to control the valve on the cooling water, changing the flow of the water as necessary to maintain our designated operating range.



Worked out Example 1
There is a reaction that normally goes from A --> B under extremely high pressures. However, when the pressure in the reactor goes below P = 10 atm, the reaction becomes a runaway reaction and A degrades into multiple components. This runaway reaction will also occur if the temperature in the reactor goes below 300 C. What type of a selectors would be used to protect this equipment?

Solution to Example Problem 1
The answer to this problem is done by acknowledging that the reactor needs to be kept above 10 atm and 300 C. Thus, to ensure the safety and protection of the equipment, a pressure sensor should be placed on the reactor and its signal sent to a pressure controler with a $$P_{min}$$ = 10 atm. A temperature sensor should be placed on the reactor, and its signal should be sent to a temperature controller with a $$T_{min}$$ = 300 C. The signals from these two controllers should then be sent to a low selector that will select the lowest signal, which would be the the more dangerous one. This would then control the feed of A into the reactor. For improved safety and redundancy, you may also wish to add another pressure and temperature sensor.



Worked out Example 2
You have a CSTR that carries out the reaction A + B --> C. However, if there is too much A fed, a highly exothermic side reaction, A + C -> D occurs and will quickly melt the entire reactor. You are told you need two sets of redundant controls on this CSTR to meet safety regulations and to ensure continuous operation. Which controls do you add? And where?

Solution to Example Problem 2
There are two sets of redundancies that need to be added:


 * First, a set of flow sensors on feed stream A that feed into a high selector to ensure that the highest flow rate of A is being read to prevent too much A in the CSTR.


 * Second, because there is threat of a runaway exothermic reaction, a set of temperature sensors that both feed into a high selector as we would want the highest possible reading to prevent the case of high temperature explosion.

The temperature sensors would then feed into a temperature control that would operate a valve on the cooling water to the CSTR. The flow sensors would then feed into a flow control that would operate a valve on feed stream A. Adjustments to the two flow rates would be made accordingly.



Multiple Choice Question 1
What type of selectors are used in equipment protection?

A. Median Selectors Only

B. High Selectors Only

C. Mean Selectors Only

D. Both Low and High Selectors

Multiple Choice Question 2
A high selector is represented by what type of sign?

A. Less than

B. Greater than

C. Equal

D. Does not equal

Submitting answers to the multiple choice questions

 * Authors of this wiki, please email the correct answers to 466answers@umich.edu (and please remember to indicate which wiki article the answers correspond to).


 * Everyone else, the deadline for submitting your answers is the start of class on Tuesday, 10/31. You are expected to work on these multiple choice questions under the Honor Code.  Please use the following link to submit your answers to the above mutiple choice questions: https://lessons.ummu.umich.edu/2k/che_466/OverrideControl