Optimisation of a dual circuit induced draft cooling water system

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Override selector control

When there is more than one controlled variable sharing a manipulated variable, override selector control can be applied to allow for transition of control from one controller to another.
The most common variants of override selector control are high, low and middle selection. In most cases, override selector control is used for constraint handling, where one variable is the normally controlled variable and another is a constraint, that has to be honoured under certain conditions (at the temporary expense of the primary controlled variable).
An important consideration in the use of override selector control is the prevention of wind-up in the non-selected controller(s). This is usually achieved by allowing the output on the nonselected controller to float only a calculated distance away from the selected control output.
The distance is calculated as the product of the gain and error of the non-selected controller. Therefore, the further the non-selected controller is from set-point, the larger the allowed deviation is. On the other hand, if the non-selected controller is approaching its set-point, it means that it should prepare for selection to be transferred to it and the allowed deviation becomes progressively less until eventually it is equal to zero at the point of transfer (when the non-selected controller is at set-point). This allows for bump-less transfer of control from one controller to the next.
An example of the application of override selector control is in the control of fuel gas to a multi-burner furnace. To maintain the desired duty in the furnace, the flow of fuel gas would have to be maintained at set-point (assuming there is no change in calorific value of the gas) regardless of whether burners are being brought on-line or taken off-line. The constraint to be managed during these events is preventing the pressure from dropping below a point where the flame could be lost. Therefore, the normal controller would be the flow controller whereas the constraint handling controller would be a pressure controller with a set-point slightly above the minimum allowed pressure. When a burner is suddenly brought on-line, the flow resistance of the system decreases which would cause a temporary decrease in burner inlet pressure. At the same time the flow rate would temporarily increase causing the flow controller to cut back on the control valve. This causes a further decrease in burner inlet pressure with the possibility of losing the flame(s). Therefore, the override selector control would temporarily transfer control to the pressure controller which would prevent the control valve from cutting back too far (at the temporary expense of a higher flow rate). As soon as the transient response has settled out and the pressure starts increasing again, control would be transferred back to the flow controller.

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Valve position/throttling control

Valve position control refers to using the output of one controller as the controlled variable of another controller with the ability of manipulating the operating point of the first controller.
This is used for example where two control valves are installed in series in order to accurately control the flow in the line over a wide operating range. The first controller will manipulate one valve in order to control the flow-rate at set-point whereas the second controller will manipulate the position of the second throttling valve to ensure that the first controller’s valve position remains close to a desired value (for example 50%). This technique is also useful for handling throughput constraints on systems, where the valve position of the bottleneck control valve is used as an indication of when a throughput constraint is hit, and a valve position controller throttles back on the input to the system accordingly, or moves the point at which the production rate is set (typically through the use of an override selector). Therefore, this technique may be used to eliminate the need for complex logic for reconfiguring the control structure when certain constraints become active.

1.1 Problem statement
1.2 Research objective and questions
1.3 Hypothesis and approach
1.4 Research goals
1.5 Research contribution
1.6 Overview of study
2.1 Chapter objectives
2.2 Energy management
2.3 Process control
2.4 Optimisation
2.5 System modelling
2.6 Hybrid systems
2.7 Process equipment
2.8 Cooling water systems
2.9 Conclusion
3.1 Chapter overview
3.2 Process description
3.3 System modelling
3.4 Control and optimisation
3.5 Conclusion
4.1 Chapter overview
4.2 Control and optimisation results
5.1 Concluding remarks
5.2 Future scope


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