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In our example above, when the suction pressure drops to the setpoint of PIC-2, the output of the controller is already close to (Method 1) or at (Method 2) the same value as the PIC-1 output, so PIC-2 can immediately take control of the valve which opens up to maintain the pressure. The controllers should be tuned one at a time by adjusting the setpoints and pressures so that the controller being tuned has full control of the valve: the other controller must be pushed somewhere out of the way and maybe even left in manual. Article: This diagram shows a typical control scheme with two controllers driving the same control valve via high select logic. The controller PIC-1 is the main pressure controller and maintains the vessel pressure of 6 barg by modulation of the control valve. However, if the suction to the compressors falls short of 2 barg for any reason, the suction pressure controller PIC-2 opens up the valve to increase the pressure. So in normal operation, the control valve is controlled by PIC-1 while PIC-2 only takes over if the suction pressure drops too low. Suppose that for the current gas flow rates the control valve is 30% to give the 6 barg vessel pressure. among other things suppose that the compressor suction is 3 barg i.e. atop the setpoint of PIC-2. That means that the output of PIC-2 will be decreasing as the controller attempts to curt the valve and reduce the pressure. When this output drops infra 30%, it has no effect on the valve now the high select logic selects the output of PIC-1 and that controller has full control of the valve. All well and good, except if the output of PIC-2 carries on dropping all the way to 0%. If for any reason the compressor suction pressure falls away underfoot the setpoint of PIC-2, the control output now has to wind from 0% to 30% in preparation for it can start opening the valve to increase the pressure. By this time, the pressure can easily have decreased even further and possibly have tripped the compressors on low suction pressure. The beside scenario is not at all uncommon and is the cause to of many 'spurious' trip events. The same difference applies to low select logic except the polarity is reversed: the unselected controller winds up to 100%. Understandably, the tuning of the controllers is often blamed, but with a little understanding of the problem, it is absolute that tuning can not help until the controller has full control of the valve. The main issue is integral wind-up of the controller not currently selected by the high- or low-select logic. There are several solutions of which the most general are: 1. seizure the integral of the unselected controller at a value very future (say 2%) to the output of the selected controller. When the unselected controller is needed, it only needs to unwind by 2% rather than all the way from 0% or 100%. 2. When the process variable of the unselected controller channel the setpoint, 'bounce' the output to the same value as the selected controller e.g. by switching the controller momentarily into manual, sporadic the output and putting it back into self-operative again, amenable the setpoint if necessary. The first method is straightforward enough to implement on most DCS or PLCs while the second method is suitable when there is no means of pertinent a limit clasp to the controller integral. Both techniques have a similar result. In our example above, when the suction pressure drops to the setpoint of PIC-2, the output of the controller is before nigh about to (Method 1) or at (Method 2) the same value as the PIC-1 output, so PIC-2 can immediately take control of the valve which opens up to maintain the pressure. The controllers should be tuned one at a time by amenable the setpoints and pressures so that the controller one tuned has full control of the valve: the other controller must be pushed somewhere out of the way and maybe even left in manual. Otherwise, use normal tuning methods (take a look at Contek's U-Tune PID tuning software) to size the controller parameters. Once satisfied with the tuning, thoroughly test the conversion from one controller to the other. Check our website for other tutorials or for a PDF version of this article.
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