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Mar
16

What Have I Learned? – Manipulation of Multiple Flows (Part 2 – Valve Position Control)

If you have manipulated flows with counteracting effects (application 5), such as steam and coolant or acid and base reagents, your most straightforward solution is split range control because split ranged control prevents a loss in efficiency from both streams flowing at the same time if there is no overlap at the split range point and no low limits in the manipulated flows.

What about applications to increase plant turndown and capacity (application 1), reduce process variability (application 2), and improve plant efficiency (applications 3 and 4)?

(1) Extend rangeability

(2) Improve resolution

(3) Enable preferential use of flows based on cost

(4) Send flows to multiple destinations possibly based on priorities

(5) Provide counteracting effects

If the manipulated flows had perfect valves and no discontinuity at the split range point, we could use split range control for applications 1-4 if we addressed the tuning considerations for the different dynamics of the manipulated flows. If the manipulated flows had the same time constant and deadtime, compensation would reduce to setting the split range point to compensate for the different process gains for each manipulated flow as mentioned in Part 1. When the speed of response is different, a more effective technique may be to schedule controller tuning settings based on which flow is being manipulated. Scheduling of the gain, reset, and rate time will take into account the changes in the process time constant and deadtime as well as process gain. For example, if a loop is manipulating waste bark feed and natural gas flow to a boiler, the response of steam generation to waste bark flow will be much slower than to natural gas flow. Often the less expensive manipulated flow is the one with the slowest and most variable response. An adaptive controller, such as DeltaV Insight, can continuously update the scheduling of the tuning of the settings for a manipulated flow with variability, such as the heating value of waste fuels, the acid and/or base concentrations of waste reagents, the composition of recycle flows, and the temperature of heat recovery streams.

What are the options for dealing with the specific problem of a single critical process controlled variable and two manipulated flows with different costs, dynamics, stick-slip, and backlash? Can we mitigate the consequences of non-ideal valves? Can we avoid the nasty discontinuity of the split range point and limit the need to schedule PID settings to the effect of just one manipulated flow on the critical process variable?

A solution in the regulatory control world is to continuously manipulate the flow with the faster and fixed dynamics (FFD) for tight control of the critical process variable and only move the flow with the slower and variable dynamics (SVD) when absolutely necessary.

This strategy uses a PID to tightly control the critical process variable by directly and rapidly manipulating the FFD flow. A valve position controller (VPC) keeps the FFD flow from getting too high or low by slowly manipulating the SVD flow. The valve position control (VPC) is an integral-only controller that is optimizing the FFD flow. Proportional and rate action are not used in the optimizing VPC because fast and abrupt changes create interaction and disruption. A description of VPC starts on slide 25 in ControlUsingTwoManipulatedVariables.pdf

Control valves, particularly rotary valves, lose their sensitivity at high positions (installed valve characteristic flattens). Consequently, there is a maximum throttle position for good control. At the other end, it is undesirable to ride the seat of any control valve. Many develop more stick-slip and backlash as you approach the closed position (< 20%). As a result, there is a minimum throttle position for good control.

The VPC set point is the optimum desired FFD flow. If the FFD flow is more costly, the VPC set point is a minimum FFD flow that still enables good control. A minimum FFD flow may also exist for stability, such as a minimum gas natural flow for flame stability. If the FFD flow is less costly (less common case), the VPC set point is a maximum FFD flow that still enables good control. If there is no cost difference between the FFD and SVD, the VPC set point is the mid throttle range of the FFD (e.g. 50%). Whenever, small and large valves are used on the same stream to increase rangeability and resolution, the small valve is considered the FFD flow because the smaller valve generally has a faster response and a finer resolution in terms of total flow. In this case, the critical PID directly throttles the small valve (fine adjustment) and the VPC throttles the big valve (coarse adjustment). The VPC set point is the best mid throttle position of the small valve. The best mid throttle position is a function of the room to roam on the best part of the installed valve characteristic and keeping away from the seat.

The VPC process variable is the FFD flow. Typically, the critical PID controller output is used. Since the VPC response is intentionally slow and the optimum VPC set point knowledge is rarely better than 1%, the use of actual flow or valve position read back is unnecessary as the PV of the VPC. There might be some advantage in using actual flow in terms of linearization, but there are bigger issues like what is the ball park for tuning? The good news is we have only one VPC tuning setting, integral time. The bad news is this integral time tuning is not defined for applications. We know the VPC should be slow enough to prevent interaction with the PID but fast enough to allow the PID to do its job. The best paper I have seen on VPC tuning is “Analysis of Valve-Position Control for Dual-Input Processes” by Cheng-Ching Yu and William L. Luyben published in the American Chemical Society journal in 1986 (0196-4313/86/1025-0344$01.50/0). The conservative tuning in this paper appears to me to be the best and simplifies to the integral time setting being approximately the ratio of the SVD process time constant to the FFD process time constant for stable (self-regulating) processes. For unstable (runaway) processes, a satisfactory integral time is about half the ratio. For the exothermic reactor example cited, the integral time is about half of the ratio of SVD heat removal time constant to the FFD heat removal time constant. This article implies an independence of the VPC integral time from other process dynamics. This independence should be confirmed through more analysis and testing. The VPC integral time might also be a function of the ratios of process gains and dead times in the response of the critical process variable to the manipulated flows.

It is important that the critical PID be tuned first for tight control. For unstable processes this PID must have enough gain and rate action to prevent a runaway. The VPC is then tuned next and any fighting between the loops or oscillations created in the PID loop for a set point change in the VPC loop must be prevented by increasing the VPC reset time. For large and fast disturbances that drive the FFD flow out of the good control range, it is important to add feedforward control to put the valves in the right position without having to wait for the slow VPC loop to respond. If we are doing the small valve PID and big valve VPC control deal, it may be useful to turn off integral action in the VPC when the fine valve is within an acceptable throttle range (e.g. 40-60%) so the big valve (“Mr. Big”) with its big problems is only asked to move for a big disturbance. This eliminates a big limit cycle from the big stick-slip and big backlash of “Mr. Big.”

Stay safe. Always monitor and test any new strategy or tuning for worst case scenarios.