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Why Do Most Vessel Control Loops Need the Reset Time Increased by Two or More Orders of Magnitude?

The post Why Do Most Vessel Control Loops Need the Reset Time Increased by Two or More Orders of Magnitude? first appeared on the ISA Interchange blog site.

This excerpt from InTech magazine was written by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemicals).

The proportional, integral, derivative (PID) controller is the common key component of all control loops. Basic control systems depend upon the PID to translate the measurement signals into set points of secondary loop controllers, digital valve controllers, and speed controllers for variable frequency drives. The success of advanced control, such as model predictive control, depends upon the basic control system foundation and hence the PID.

vessel-control-loops-near-integrating-process

Elmer Sperry developed the first example of the PID in 1911, and Nicolas Minorsky published the first theoretical analysis in 1922. Ziegler and Nichols published papers on the ultimate oscillation method and reaction curve method for controller tuning in 1942 and 1943. While the parameters chosen as factors in the tuning settings provided overly aggressive control, the basic premise of an ultimate gain and ultimate period is essential to the fundamental understanding of the limits of stability. The identification of the slope in the reaction curve method is a key to the use of the near-integrator concept that we will find here to be critical for most composition, temperature, and pH loops to improve tuning settings and dramatically shorten test times.

Bohl and McAvoy published a paper in 1976 that showed the PID can provide nearly optimal control for unmeasured load disturbances. Shinskey wrote many books detailing the knowledge of process dynamics and relationships essential to the best application of PID control. Shinskey developed the original equation for integrated error from disturbances as a function of tuning settings, as detailed in the January/February 2012 InTech article “PID tuning rules.” Shinskey also published a book dedicated to PID controllers that showed the simple addition of a dead time block in the external reset feedback path could further enhance the PID performance by dead time compensation.

Internal model control (IMC) and lambda tuning rules were developed based on pole and zero cancellation to provide a good response to set points and disturbances at the process outlet. However, most of the improvement in set point response could have been achieved by a set point lead-lag or PID structure. Also, these tuning rules do not perform well for the more common case of disturbances on the process input (load upsets), particularly for lag dominant processes. Skogestadt developed significant improvements to IMC tuning rules. Bialkowski showed that always using lambda rather than lambda factors, relating lambda to dead time, and treating lag dominant processes as near-integrators enable the PID to provide good nonoscillatory control for load upsets besides dealing with the many different difficulties and objectives for which lambda tuning was originally designed. Not realized is that most methods converge to the same basic expression for the PID gain and reset time when the objective is load disturbance rejection and that a tuning parameter that is the closed loop time constant or arrest time is set relative to dead time. Also not recognized is how PID features, such as structure, external reset feedback, enhanced PID for analyzer and wireless, simple calculation of a future value, valve position controller, and “full throttle” set point response, can increase process efficiency and capacity, as noted in the ISA 2013 book 101 Tips for a Successful Automation Career.

vessel-control-loops-runaway-process

Overload

The user is confronted with considerable disagreement of tuning rules as seen in the 400 pages of tuning rules in the 2006 book by O’Dwyer, not realizing most of them can be adjusted by factors or a near-integrator concept to achieve good control.

The modern PID has many more options, parameters, and structures that greatly increase the power and flexibility of the PID, but most are underutilized due to insufficient guidance. Additionally, ISA standard form used in most modern control systems is not the parallel form shown in most textbooks or the PID series form pervasively used in the process industry until the 1990s.

All of this can be quite overwhelming to the user, particularly because tuning is often done by a generalist faced with rapid changes in technology and with many other responsibilities. My goal in my recent articles, books, and columns (including blogs), which are more extensive and less supplier-specific than white papers, is to provide a unified approach and more directed guidance based on the latest PID features that are missing in the literature. The recently completed Good Tuning: A Pocket Guide, Fourth Edition seeks to concisely present the knowledge needed and to simplify the tuning by switching between just two sets of tuning rules, largely depending upon whether the PID is a primary or secondary controller. A primary PID for vessel or column composition, gas pressure, level, pH, and temperature control uses integrating process tuning rules where the lambda arrest time is set. A secondary PID for liquid pressure, flow, inline pH, and heat exchanger temperature control, uses self-regulating process tuning rules where the closed loop time constant is set. In both situations, lambda rather than a lambda factor is used and chosen relative to the dead time to provide the degree of tightness of control and robustness needed.

The best thing a user can do is to use good tuning software, attend supplier schools, and get a consultant in the plant for on-site solutions and practice. It is also important to take responsibility for avoiding common tuning mistakes. Here we step back to make sure we are not susceptible to oversights and misunderstandings. The following compilation has the most common, disruptive, and potentially unsafe mistakes first, but all can come into play and be important.

vessel-control-loops-true-integrating-process

Mistakes

1. Using the wrong control action: In analog controllers and in many early distributed control systems (DCSs) and programmable logic controllers (PLCs), the valve action affected only the display of the output on the station or faceplate. The specification of an “increase-to-close” valve action for a fail-open valve reversed the display but not the actual output. Consequently, the control action had to take into account the valve action besides the process action. If the valve was “increase-to-open” (fail close), the control action was simply the reverse of the process action (direct control action for reverse-acting process and vice versa). If the valve was “increase-to-close” the control action was the same as the process action (direct control action for direct-acting process and vice versa) if not reversed in the current-to-pneumatic (I/P) transducer or positioner. In today’s systems, the user can specify “increase-to-close” in the PID block or analog output block besides the digital valve controller, enabling the control action to be set as the opposite of the process action. The challenge is realizing this and making sure the increase-to-close valve action is only set in one place. If you do not get the control action right, nothing else matters (the PID will walk off to its output limit).

2. Using PID block default settings: The settings that come with a PID block as it is dragged and dropped into a configuration must not be used. When first applying PID to dynamic simulations of new plants, typical settings based on process type and scale span can be used as a starting point. However, tuning tests must be done and settings adjusted before operator training and loop commissioning.

3. Using parallel form and series tuning settings in the ISA standard form: A parallel form that uses integrator gain and derivative gain settings that are put into the ISA standard form as reset time and rate time settings can be off by orders of magnitude. A series form can provide good control with the rate time equal to or greater than the reset time. This is because interaction factors inherently reduce the PID gain and rate time and increase the PID reset time to prevent oscillations from the derivative mode contribution being greater than the contribution from the other modes. Using a rate time equal to or greater than the reset time in an ISA standard form can cause severe fast oscillations.

4. Using the wrong units for tuning settings: Here we consider just the series form and ISA standard form. Controllers can have a gain or proportional band setting for the proportional mode. The gain setting is dimensionless and is 100 percent divided by the proportional band. Some PID algorithms in control studies and actual industrial systems have the gain setting in engineering units, which leads to a very bizarre setting. The integral mode setting can be repeats per second, repeats per minute, minutes per repeat, or seconds per repeat. The units of these last two settings are commonly given as just minutes or seconds. The omission of the “per minute” can cause confusion in the conversion of settings. The conversion of the rate time is simpler, because the units are simply minutes or seconds.

5. Using the wrong units for output limits and anti-reset limits: In analog controllers and in many early DCS and PLC systems, the output and consequently the output limits and anti-reset windup limits were in percent. In modern control systems, the output is in engineering units, and the limits must be set in engineering units. For valves, the units are usually percent of valve stroke. For a primary (upper) PID that is sending a set point to a secondary (lower) PID, the primary PID output is in the engineering units of the secondary PID process variable.

6. Tuning level controllers: If you calculate the product of the valve, gain, process gain, and measurement gain, where the process gain is simply the inverse of the product of the fluid density and vessel cross-sectional area, you realize the open loop integrating process gain is very small (e.g., 0.000001 1/sec), leading to a maximum PID gain for stability that is more than 100. For surge tank level control, a PID gain closer to unity is desired to absorb fluctuations in inlet flows without passing them on as changes to a manipulated outlet flow that will upset downstream users. Users do not like a high PID gain even when tight level control is needed. Decreasing the level controller gain without a proportional increase in the reset time will cause nearly sustained slow rolling oscillations. Further decreases in the PID gain only make the oscillations worse. Most oscillations in production plants and poor performance of distillation columns can be traced back to poorly tuned level controllers. The solution is to choose an arrest time (lambda for integrating processes) to either maximize the absorption of variability (e.g., surge tanks level control or distillate receiver level control where distillate flow is manipulated) or maximize the transfer of variability (e.g., reactor level for residence time control or distillate receiver level control where reflux flow is manipulated for internal reflux control). The integrating process tuning rules prevent the violation of the window of allowable PID gains by first setting the arrest time and using this time to compute the reset time and finally the PID gain.

7. Violating the window of allowable controller gains: We can all relate to the fact that too high of a PID gain causes oscillations. In practice, what we see more often is oscillations from too low of a PID gain in primary loops. Most concentration and temperature control systems on well-mixed vessels are vulnerable to a PID gain that violates the low PID limit, causing slow rolling, nearly undamped oscillations. These systems have a highly lag dominant (near-integrating), integrating, or runaway process response. All of these processes benefit from the use of integrating process tuning rules to prevent the PID gain from being less than twice the inverse of the product of the open loop integrating process gain and reset time, preventing the oscillations shown in the figures. The oscillations in the figures could have been stopped by increasing the reset time. In industrial applications, the reset time in vessel control loops often needs to be increased by two or more orders of magnitude. Note that the oscillations get worse as the process loses internal self-regulation, going from a near-integrating (low internal negative feedback) to an integrating (no internal feedback) and to a runaway (positive feedback) open loop response. For runaway processes, there is also a minimum gain setting independent of reset time that is the inverse of the open loop runaway process gain. The identification of the open loop integrating process gain can generally be done in about four dead times, greatly reducing the test time and reducing the vulnerability to load upsets.

Click here to continue reading Greg McMillan’s article at InTech magazine.

Source: ISA News