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 Tmax. This compressor also cannot be allowed to exert a certain pressure Pmax 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 Tmax or if the pressure becomes greater than Pmax. 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.
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 Tmax. This compressor also cannot be allowed to exert a certain pressure Pmax 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 Tmax or if the pressure becomes greater than Pmax. 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.
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