Common Mistakes not Commonly Understood - Part 1

There are many mistakes but some are repeated over and over again even though the automation engineer is attentive and experienced and has the best intentions. Part of the problem is overload in terms of tasks and the time crunch. It is highly unlikely engineers today read even a smattering of the thousands of pages in books, handbooks, white papers and articles. The knowledge to prevent the following mistakes may be buried in this literature but I am not so sure of even this. In any case, one probably could not find it. Here is my effort to get straight to the point of realizing and fixing mistakes.

(1) Reset time set too large for deadtime dominant processes. Most tuning algorithms don’t recognize what Shinskey found is that the reset time could be decreased by a factor of 8 or more from 3 to 4 times the dead time to 0.4 to 0.5 times the dead time for the same controller gain setting for severely dead time dominant processes. Lambda tuning can accomplish a dramatic reduction in reset time since the reset time is the time constant that for dead time dominant processes is by definition less than the dead time. The controller gain is also proportionally reduced providing stability despite a much smaller reset time, which is generally good since these processes are more likely to have noise and a jagged not so smooth response due to the lack of a significant time constant. The criticism that the controller reset time and gain becomes too small leading to integral-only type of control for severely dead time dominant processes is avoided by simply putting a limit of ¼ the dead time on the reset time that is then used in the equation for the controller gain as discussed in the June 2017 Control Talk Column “Opening minds about controllers, part 1”. This column is also a good resource for understanding the next common mistake where the reset time is set too small.

(2) Reset time set too small for lag dominant (near-integrating) processes, integrating and runaway processes. These processes lack self-regulation in the process and depend more upon the gain action in the PID to provide the negative feedback missing in the process. Since engineers are not comfortable with controller gains greater than 5 and operators object to sudden movements of PID output, the controller gain is often an order of magnitude or more too small. Since the product of the reset time and gain must be greater than twice the inverse of the integrating process gain to prevent the start of slow oscillations, the reset time is an order of magnitude or more too small. Since we are taught in control theory classes how too high a PID gain causes oscillations, the PID gain is typically decreased making the problem worse. For more on this pervasive problem and the fix, see the 9/14/2017 Control Talk Blog “Surprising Gains from PID Gain” which leads us to the next common mistake.

(3) PID gain set too small for valves with poor positioner sensitivity and excessive dead band from backlash and variable frequency drives with a large dead band setting. Not only is the PID gain too small per last mistake but is also too small to deal with valve problems as seen in my ISA 2017 Process Control and Safety Symposium slides ISA-PCS-2017-Presentation-Solutions-to-Stop-Most-Oscillations.pdf that details a lot of cases where a counterintuitive increase in PID gain reduces or stops oscillations.

(4) Split ranged valves used to increase valve rangeability. The transition from the large to small valve is not smooth since the friction and consequently stiction is greatest near shutoff as plugs rub seats and balls or disks rub seals. Since the stiction in percent stroke translates to a larger abrupt change in flow and amplitude in the limit cycle, small smooth changes in flow are not possible especially near shutoff but also whenever the large valve is open.  The better solution is a large and small  valve stroked in parallel either where a Valve Position Controller manipulates the large valve with directional move suppression to keep the small valve near an optimum position or by simultaneous manipulation as detailed in the November 2005 Control feature article  “Model Predictive Control can Solve Valve Problem.”  

(5) Ignoring effect of meter velocity on flow measurement rangeability. The maximum velocity for a given meter size rarely corresponds to the velocity at the maximum flow in a process application. Often the maximum process velocity is less than half the maximum meter velocity for line size meters. Thus, the velocity at the minimum process flow is so far below the minimum flow for a good meter response that the actual rangeability is less than half what is stated in the literature. 

(6) Ignoring the effect of noise on flow measurement rangeability. The signal to noise ratio often deteriorates before the meter reaches the low flow corresponding to its rangeability limit. The flow measurement can essentially become unusable for flow control making the actual rangeability much less than what is stated in the literature.

(7) Ignoring the effect of stiction, backlash and pressure drop on valve rangeability. There are many definitions of valve rangeability that are erroneous, such as those that define rangeability as the ratio of maximum to a minimum flow coefficient (Cv) where the closeness of the actual to theoretical inherent flow characteristic determines the minimum Cv leading to the conclusion that a rotary valve offers the greatest rangeability. The real rangeability should be the ratio of maximum to minimum controllable flow. Deadband from backlash and resolution from stiction near the shutoff position should determine the minimum position that gives a controllable flow. Since stiction is greatest as the plug moves into the seat or ball or disk moves into the seal particularly for tight shutoff valves, the minimum controllable position can be quite large (e.g., 2% to 20%). The flow at this position needs to be computed based on the installed flow characteristic. A ratio of valve to pressure drop less than 0.5 will cause a linear characteristic to distort to quick opening increasing the flow at the minimum controllable position causing a significant loss in rangeability. For equal percentage valves there is also a loss in the minimum controllable flow due to excessive flattening of the installed characteristic. There may also be significant flattening of the installed flow characteristic for rotary valves resulting in any rotation past 50 degrees being ineffectual due to the flatness of the installed flow characteristic, which shows up as the controller output through integral action wandering about above 50 degrees. My book Tuning and Control Loop Performance Fourth Edition published in 2014 by Momentum Press gives the equations to compute the real rangeability. It turns out sliding stem valves with diaphragm actuators and smart positioners have the best rangeability.    

(8) Ignoring the effect of static head, motor and frame type, and inverter type and control algorithm on VFD rangeability. Since the inverter waveform is not purely sinusoidal, it is important to select motors that are designed for Pulse Width Modulation (PWM). These “inverter duty” motors have windings with a higher temperature rating (class F). Another option that facilitates operation at lower speeds to achieve the maximum rangeability offered by the PWM drive is a higher service factor (e.g. 1.15).  To help prevent motor overheating at low speeds, larger frame sizes and line powered ventilation fans are used. In the process industry, totally enclosed fan cooled (TEFC) motors are used to provide protection from chemicals and ventilation by a fan that is run off the same power line as the motor. The fan speed decreases as the motor speed decreases. To reduce the problem from motor overheating at low speeds, an AC line power constant speed ventilation fan and a larger frame size to provide more ventilation space can be specified. Alternately, a separate booster fan can be supplied. For very large motors (e.g. 1000 HP), totally enclosed water cooled (TEWC) motors are used to deal with the extra heat generation. For low static head pump applications, the overheating at low speeds is not a problem because the torque load decreases with flow. Turndown also depends upon the control strategy in the variable frequency drive. All of the control strategies discussed here use pulse width modulation to manipulate the frequency and amplitude of voltage and current to each phase. Open loop voltage (volts/hertz) control has the simplest algorithm but is susceptible to varying degrees of slip. Most of the drives provided for pump control use this strategy in which the rate of change of flux and hence speed is taken as proportional to voltage. At low speeds the motor losses are larger making the difference between the computed and actual speed (slip) much larger. Some drives make a correction to the voltage to account for estimated motor losses. Ultimately these drives depend upon the DCS to correct for dynamic slip through proportional action and to correct for steady state slip through integral action in process controller(s). The rangeability is normally 40:1 with 0.5% speed regulation. Closed loop slip control has a speed loop cascaded to a torque loop. Speed (tachometer) and torque feedback are shown to be from sensors. The torque feedback may be calculated from a current sensor. A DCS process controller output is the speed set point for the speed controller whose output is the set point to a torque controller. PI rather than P-only controllers can be used since sticktion and resolution limits are negligible, eliminating any concern about limit cycles from integral action. The control system in the VSD is analogous to the cascade control system in a digital positioner. The speed controller plays a role similar to the valve position controller and the torque controller serves a similar purpose as the relay controller. However, in the digital positioner the relay response is inherently much faster than the valve position response. In the VSD, the torque controller can have a relatively sluggish response. To prevent a violation of the cascade rule that requires the secondary loop (torque) to be 5x faster than the primary loop (speed), the speed loop is slowed by decreasing the speed controller gain and integral time. Since the speed set point comes from process controller in the DCS, there is at least a triple cascade. In many cases there is a quadruple cascade control system, for vessel temperature to jacket temperature to coolant flow to speed cascade. The detuning of the speed controller causes detuning of the flow controller, which in turn may cause detuning the temperature controller. As a result, the ability to reject fast process disturbances may be compromised. The rangeability is normally 80:1 with 0.1% speed regulation. However, if the static head approaches the total pressure rise, the rangeability can deteriorate by an order of magnitude for all VFDs with a resulting installed flow characteristic that is quick opening.

 (9) Ignoring changes in fluid composition in thermal mass flow meters. Changes in fluid composition cause a change in the assumed thermal conductivity and specific heat capacity of the fluid and the viscosity for liquids introducing a significant error. If you are trying to use a thermal flow meter on air/gas/vapors never install it in a service where it can ever see a gas/vapor approaching dew point. Fouling also causes an error due to thermal lags. Thermal mass flow meters are generally only successfully used on small dry pure gas flows such as oxygen or air for lab or pilot plant bioreactors (very controlled environment) where the fluid is clean single phase and composition is fixed and the remaining 1% or 2% error is corrected by a primary dissolved oxygen  controller manipulating the secondary oxygen or air flow controller setpoint.

(10) Ignoring changes in emissivity in optical pyrometers. Two-color or ratio pyrometers measure the radiation at two wavelengths. If the change in emittance at each wavelength with temperature is identical (gray-bodies), the effect of emittance can be cancelled out by ratio calculations. In reality, the change in emittance with temperature varies with wavelength (non-gray-bodies). Additionally, the change in emittance with changes in surface, operating conditions, and the composition of the intervening space may vary with wavelength. In one comparison test on a blackbody, a single-color and two-color pyrometers exhibited errors of 2 and 30 degrees C, respectively. Equal changes in emittance due to surface and operating conditions and intervening gases, particles, and vapors may make a two-color ratio pyrometer more accurate than a single-color pyrometer but it puts into question any accuracy statements for two-color pyrometers that are much better than 30 degrees C.

Please take my advice. I am not using it anyway. I have mostly retired into the virtual world.