The lowest cost and quickest implementation of optimization simply adds a PID for valve position control (VPC). This is not to be confused with valve positioners or digital valve controllers (DVC). The VPC for optimization is a PID whose process variable (PV) is typically the largest output or implied valve position of the process PID controllers that limits an increase in production rate or a decrease in energy use. Actual valve position readback is not necessary in the traditional implementation of the VPC since the VPC is not trying to keep the furthest open valve at an exact position but is simply preventing the process PID from running out a valve to do its job of controlling a process. The output of the VPC is normally the setpoint of an upstream process PID who’s PV can be optimized. The process PID setpoint to be optimized is typically a compressor discharge pressure, refrigeration unit temperature, or a feed rate. The VPC setpoint is the maximum desired throttle position of the limiting valve.
For minimizing compressor pressure or maximizing refrigeration unit temperature, there is typically just one VPC whose PV is the furthest open valve of the downstream flow controllers that are users of the compressed gas or chilled water. The highest of the user flow controller outputs is the PV of the VPC. The VPC lowers pressure or raises temperature until one of the valves exceed its maximum desired throttle position.
For maximizing reactor feed, there may be several VPC to prevent a jacket water valve, condenser water valve, vent valve, and reactant valve from exceeding the maximum desired throttle position for good control. The lowest output of the multiple VPC is the set point for reactant feed rate. The multiple VPC pushes to increase feed rate until one of the valves exceeds its maximum desired throttle position. For maximizing column feed the limiting valves might be for steam, reflux, distillate, and bottoms flow.
The maximum desired throttle position is where the slope of the installed characteristic of the valve is getting too flat indicating the valve gain is too low and lacks the muscle to correct for process disturbances. Butterfly valves are notorious for having characteristics that get too flat above 45 degrees. If the portion of the system pressure drop available to the control valve is low for energy conservation or a result of an increase in piping frictional losses from production rates beyond plant design, excessive flatness occurs at much lower rotations.
There is an interaction between the VPC and the process PID. The interaction is minimized by making the VPC much slower than the process PID. A rule of thumb sets the VPC reset time to be greater than 10x the product of the primary process PID reset time and gain. For example, if there is a cascade control of reactor temperature to jacket temperature to makeup coolant flow, the VPC reset time must be 10x larger than the product of the reactor temperature PID reset time and gain. Another rule of thumb sets the VPC reset time 10x greater than the residence time of the equipment. This rule is more conservative and must be modified for batch operations because residence time is not applicable since there is no throughput flow as there is with continuous operations.
The slowing down of the VPC is consistent with the concept that optimization is gradual to minimize disruptions to regulatory loops and the changes that lead to changes in optimization such as day to night temperature and raw material composition are often slow. However, faster disturbances can occur. For these cases, an error squared algorithm could be used to ignore small corrections but take care of large corrections. A thought experiment similar to what I did for using the enhanced PID for wireless (DeltaV PIDPlus) for eliminating oscillations from stick-slip, backlash, split range discontinuities, and feedforward timing errors leads to the possibility that the use of the PIDPlus as a VPC could provide faster corrections when necessary and reduce oscillations from stick-slip and backlash and interactions. In this case, actual valve position would be used. The readback of position as a HART secondary variable would be fast enough. The next step is to use the virtual plant to test out various scenarios of VPC.
VPC is also used to eliminate the split range of small and big valves in parallel. The VPC provides the rangeability but also delivers the sensitivity of the small valve which split ranged control does not provide at high demands when only the big valve is throttled. The VPC also eliminates the discontinuity of the split range point. Here again the PIDPlus may have advantages in suppressing oscillations and providing a faster response to large disturbances. Smart feedforward control can also be used for measured disturbances to judiciously split the correction needed between the large and small valve.