The most disturbing disturbances, the ones that are frequent, fast and furious (FFF), are self-inflicted either in the design, installation, maintenance or operation of the process control system. The good news is that through better application of advances in technologies and the better education of everyone responsible for the implementation, maintenance, operation and technical support of the plant's systems, we can eliminate most of these disturbances.
This is an opportunity for the synergy between modeling and control to identify, prototype, test, install and continuously improve solutions not forgetting that training plant people is the key for long term effectiveness. We can revitalize the automation profession by finding and implementing process control improvements that goes beyond a simple migration project. Keep in mind as we go through the sources of these most disturbing disturbances that this dialog is not meant as criticism. It is intended to open minds and provide insights as to opportunities.
Disturbances most often originate from the Advanced Process Control (APC), Basic Process Control Systems (BPCS), Sequential Operations, Safety Instrumented Systems (SIS), maintenance and operators. The disturbances from outside influences (e.g., raw materials and weather) and internally (fouling of surfaces and deactivation of catalysts) tend to be slower, smaller and less often. The disturbances originating from people, valves, and PID controllers tend to be FFF.
Cyclic disturbances are the epitome of frequent disturbances. Cyclic disturbances pose additional problems in terms of the disturbance frequency being much faster than a loop’s natural frequency becoming effectively noise or even worse, the disturbance frequency being around the loop frequency causing resonance. Cyclic disturbances tend to originate from PID controllers due to poor tuning or poor choice of structure, throttle valves due to backlash, stiction, and positive feedback from recycle steams, on-off valves due to sequential operations, regeneration of catalysts, defrosting of crystal coated surfaces and cleaning of fouled surfaces. Cycles can appear that coincide with shift changes due to operators moving the process to their preferential operating points. SIS actions, shutdown and startup are extremely disruptive and can appear to be cyclic if there are many parallel unit operations that are going up and down. If you go a little deeper, you realize nearly all the disturbances begin with valve movement and speed changes since these are ultimately what are manipulated to affect the process. If the valve positions and speeds were not changed, the process would not be at the right operating conditions and possibly unsafe but there would be few cyclic disturbances. We should not lose sight of this fundamental when trying to track down and eliminate a disturbance. We should always seek to find out what valve moved first and what happens if the valve does not move (loop momentarily in manual). Finally, we should ask does that valve need to move and can the movement be slower? For a short term disturbance whose amplitude is not going to cause a trip or damage equipment, the best correction may be no correction. Any feedback correction may be late creating a second disturbance.
We will focus in this blog on the BPCS and Sequential Operations since they are the primary source of the above cyclic disturbances. Well-designed Model Predictive Control (MPC) takes into account interactions and dynamics and tends to move the process slowly through proper adjustment of penalty on move (move suppression) and penalty on error. However, if the MPC in the process of pushing the process closer to constraints causes an activation of the SIS, the consequences are severe. MPC by nature is pushing the limits so there needs to be caution exercised. Valve Position Control (VPC) also pushes the limits with the added risk of a VPC not providing decoupling and the PID being tuned improperly. We will address minimizing the disruption by a VPC as part of the discussion of PID control.
The overuse of integral action is the primary culprit since it is has no sense of direction and is never satisfied continually moving the process. Integral action is often used in lieu of proportional action. Before we even go any further, let’s make sure that any concern about excessively abrupt changes by proportional action for setpoint changes is alleviated by the use of PID options such as structure or a setpoint filter as noted in the ISA Mentor Program 1/26/2106 post “Equivalent Methods to Eliminate proportional Step and Derivative Kick”. For all types of changes, external reset feedback along with up or down rate limits on setpoints can provide directional move suppression, even more flexible than what we get from an MPC. This move suppression prevents upsets to other loops from more proportional action, whether a speed or valve is being manipulated. Simply turning on external reset feedback suspends integral action that would cause the manipulated loop, valve or speed setpoint to change faster than the loop, compressor, fan, pump or valve can respond. This eliminates the confusing burst of oscillations for large disturbances or setpoint changes that occurs from violation of the cascade rule. External reset feedback eliminates the limit cycle from valves that have excessive backlash, a slow response of a large actuator or an insensitive poor positioner design. External reset feedback also enables a VPC to more effectively do its job to give a gradual optimization but a fast recovery for upsets. External reset feedback is also the key feature of an enhanced PID that suppresses oscillations from the excessive dead time of analyzers as discussed in the 7/6/2015 Control Talk Blog “Batch and Continuous Control with At-Line and Offline Analyzers Tips”. I could go on and on about all the benefits I have found. Simply turning on external reset feedback and making sure the feedback signal is fast and representative of the response of what is manipulated can prevent oscillations. The PID gain can be increased eliminating the slow oscillations from too low of a PID gain in counterintuitive situations where oscillations get worse as we decrease the PID gain. We are taught that a high PID gain causes oscillations not realizing there many important situations where too low of a PID gain causes even more disruptive oscillations because the amplitude is larger and the cycling is perpetual.
We have the counterintuitive situation where for composition, gas pressure, level, pH and temperature control loops on vessels and columns, we have oscillations that get worse as we decrease the PID gain. The process response in these loops is near-integrating, true integrating or runaway. Another common non-intuitive situation is where the limit cycle from backlash and stiction in these processes is reduced by increasing the PID gain. In self-regulating processes, external reset can stop an oscillation from stiction. The offset can be corrected by a setpoint change from an upper loop or the operator. The oscillations from insensitive positioners (e.g., most single stage and spool positioners from the last century) and from the slow response of large actuators (aggravated by low bleed positioners) can be eliminated in most cases by increasing the PID gain after turning on external reset feedback.
Manual actions are abrupt, subjective, non-repeatable and usually late and are guesses at best. All manual actions by operators including those during startup, transitions, maintenance activities and abnormal operations should be automated preferably by the use of PID control. Feedforward control, principally ratio control, should be used to keep the process unit operations working together in unison.
The scheduled addition of batch feed, reagent, substrate, air and utility flows, should be made more continuous by ratio control and by fed batch control of composition, dissolved oxygen, pH or temperature. The effect of batch operations on downstream equipment should be moderated by intervening tank volumes whose level control is tuned for maximum absorption of flow variability being careful the counterintuitive situation is not created by too small of a PID gain and reset time. The product of the PID gain and reset time should be larger than twice the inverse of integrating process gain. This can be assured by lambda tuning with the arrest time set to prevent the maximum rate of change of level from activating a low or high level alarm.
Turning batch, manual, discrete, and sequential operations over to a PID loop intelligent options and tuning with move suppression provides better control and smoother more repeatable manipulation of flows leading to less FFF and better recognition and attainment of process control improvement.
I have run out time here and I suspect we are bordering on overload , so let’s simply say that mitigating and even eliminating the FFF disturbances comes down to slowing down the movement of on-off valves for sequences and of throttle valves and speed setpoints in loops by the use of move suppression and external reset feedback. The stroking of on-off valves can be slowed down by needle valves in the actuator air lines but a more effective solution is a smart positioner to provide precise intelligent adjustment and monitoring of the stroking time. The installed flow characteristic curve of these valves can be used to set the valve signal rate of change to slow down and speed up on the steep and flat part of the curve, respectively to provide a more constant less disruptive rate of change of flow.
For a concise presentation of the concepts and details on PID control see my ISA book Good Tuning: A Pocket Guide, 4th Edition. If you are up for a more comprehensive view, see my Momentum Press book Tuning and Control Loop Performance - 4th Edition.