Override Control is a control architecture that allows a process to run within a natural process or safety limit, without interlocking or shutting down the process.
It is important to examine the overall impact/cost of shutting your process down. Once this has been done, you should determine if an alternate control scheme or an application of override control can help to keep your process online.
Although Interlocking is often necessary for safety reasons, multiple shutdowns effect onstream efficiencies, and may also affect process quality. Some processes just naturally run close to a limit. Override control is employed to safely run inside of the process limit, to avoid the consequence of exceeding the limit. For continuous processes, (e.g. separation columns, extruders, boilers) an interlock that may be otherwise avoided, represents costly downtime, stress on equipment, and often off specification products or waste during transition on the way down and in restarting the process.
An Example of Pressure Override of Feed Control
Suppose that you are feeding a raw material that is naturally near its Vapor-Liquid-Equilibrium point and you need to keep the material in a liquid state to accurately measure and control its rate. Although it is important to maintain a process rate of flow, if the feed pressure drops and the material vaporizes in the feed line, you may lose the measurement, which could lead to a mischarge or even a safety incident. Certainly, an interlock may be designed to prevent mishap or stop the process, but shutting down and restarting the feed of the material adversely affects the quality and yield of your process. This is when it is important to use an override control scheme as depicted below.
This control architecture is classically known as “High Select” or “Low Select” override control, where a choice of two or more controller signals is “selected” to one or more final elements. In the above diagram, the flow controller and pressure controller outputs are low selected to limit the control valve position and maintain sufficient pressure (without interlocking near the flash point). If the pressure doesn’t approach the pressure setpoint, you may feed at a desired rate. But, if the pressure goes too low, the pressure controller will close the valve to protect against vaporizing and losing the ability to measure. It is important that the controller that is NOT selected is able to track the selected output to prevent windup and to a control bump on transfer between the controllers. Here’s another thought – you will need to determine the pressure setpoint to ensure the feed will stay liquid. But, you also need to know if it is dependent on the other process variables that you can measure.
With modern control platforms, alternative control architectures may be used to more elegantly provide an override control - as shown below.
In this architecture, the pressure controller writes to the High Output Limit parameter of the primary flow controller in order to prevent a low-pressure condition. There is no need to worry about handling windup during override, because the flow controller has that function covered for its own limits. In addition, the operator may monitor the override limitation to flow from his primary flow control faceplate. Further logic may be included in this scheme to provide more flexibility or to refine exactly when the override is desired.
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