Modeling and Control

I have seen two control loops that did not go digital during the migration to a DCS. These electronic analog controllers stick out like a sore thumb in a modern day control room. The user would like to get rid of them along with the parts, maintenance, and operator interface issues. What keeps these relics from the 1970s hanging around?

The two analog control holdouts I have seen had 4 things in common: a variable speed drive, zero process dead time, a critical process impact, and an inability to run in manual.

If you don’t have time to read on to get details on the particular loops, the most important “insights” are:

(1) Controller tuning tools and methods that rely on an open loop test cannot be used
(2) Digital adaptive controllers that identify tuning from set point changes are needed
(3) Must faster measurement update, communication, and controller scan and execution time intervals must be developed for valid holdouts to go digital
(4) If a loop has a control valve, it is rarely a valid holdout

The first application was polymer melt pressure controlled by the manipulation of melt pump speed. The melt pressure was important for throughput and relative viscosity control. An analog trend chart recorder showed what would appear to be a lot of noise. However, if the loop was taken out of auto, the amplitude of the fluctuations got so much worse you could not afford to stay in manual for more than a few seconds. The loop was reacting and compensating for incredibly fast disturbances. The process time constant can be estimated from the fluid inertia and viscosity and typically varies between 50 and 500 milliseconds. The process dead time can be estimated as the time it takes a pressure wave traveling at the speed of sound in the fluid to propagate from the final element to the first major resistance to change the pressure difference that is the driving force for the acceleration of a basically incompressible column of fluid. In other words, the process dead time was essentially zero. The dead time in the loop was all due to the automation system. The dead time of a variable speed drive (VSD) is nearly zero if the following conditions are met in the VSD application.

(1) The change in speed is larger than the resolution limits of the VSD A/D card
(2) The change in speed is larger than any dead band introduced by the user into the VSD configuration to suppress reaction to noise
(3) The rate of change of speed is smaller than any rate limiting introduced into the VSD configuration to reduce motor load and upsets to down stream equipment
(4) The rate of change of speed is smaller than any rate limiting from rotor inertia

These conditions are met for a well designed VSD for liquid pressure control, which leaves the measurement and controller as the sources of dead time. The process is self-regulating but it takes a high speed recorder to see any sort of time constant unless a signal filter is added. Note that I am not advocating replacing control valves with a VSD. There are practical problems when a control valve is omitted, such as the reversal of flow and the creation of incredibly fast flow upsets to other loops and unit operations.

There is an important exception to zero process dead time for liquid flow control. For highly viscous flows, a “ketchup bottle effect” has been observed where there is a huge dead time to initially start a flow through a small injection orifice as described in the first chapter of my book titled A Funny Thing happened on the Way the Control Room available soon on http://www.EasyDeltaV.com/ControlInsights/

We all know about aliasing from digital communication, even more important here is introduction of a delay into a control loop that has essentially no process dead time.

Why am I obsessed with dead time? The ultimate performance (IAE) achievable for unmeasured disturbances with the fastest tuning is proportional to the dead time squared and the ultimate period for this dead time dominant loop is twice the dead time.

The second application was incinerator pressure controlled by the manipulation of an induced daft (ID) fan speed. The loop behaved like an integrator. If the controller was put in manual, the pressure could ramp and hit the interlock trip point in less than a second. Since open loop testing for exact quantification was not reasonable, a dynamic simulation was used to show it could occur in 0.25 seconds. While the residence time was on the order of 0.1 minute, the process gain was incredibly large because the measurement scale span was just a few inches of water column. The simulation also showed the decoupler between the forced draft (FD) fan and ID fan speed (air flow feedforward) was doing more harm than good because of the inverse response associated with the cold air flow. After elimination of the decoupler and retuning, the frequency of furnace trips was reduced but trips still occurred every couple of days. This process was controllable only because the process dead time associated with the furnace volume was essentially zero. For gas pressure systems the process dead time originates from gas volumes in series separated by flow resistances. The pressure sensor was seeing and the ID fan was acting on the same gas volume. The dead time in the loop was all due to the automation system.

Summarizing, a digital controller with a 0.1 second execution time was tried on startup but the furnace trips were excessive despite the best tuning and strategy. In 0.1 second, the pressure was almost half way to the trip point. When the digital controller was replaced with an analog electronic controller the pressure trips were eliminated.

This application and a phosphorous furnace application are discussed in chapter 3 titled “Pressure Control – Without Dead Time I would be out of a Job” in the aforementioned book A Funny Thing happened on the Way the Control Room.

Next week I will share my experiences with making control valves respond faster. With this under our belts, I will offer how fast digital devices and communication and data historians need to be once you get these valves to “turn on a dime or at least a quarter”.

Note that the equations for computing process dead time and time constants for these systems is in Tuning and Control Loop Performance – 3rd edition, but is out of print.

I have seen some of the best and the worst of control valves. I have also found out that not all positioners are created equal and responsible responsiveness depends upon the whole package.

Here is a checklist for making sure your valve is nimble enough to respond to every demand of your loop if you don’t have time to read my positioner horror stories.

Checklist for Responsive Responsible “Right On” Control Valves

(1) Digital positioner with high gain and rate action that is properly tuned
(2) Low friction packing that is properly tightened
(3) Low seating and sealing friction at closure
(4) Diaphragm or floating piston actuator that is properly sized
(5) Spline shaft connections for rotary valves
(6) Ball, disk, or plug stem (not actuator shaft) position feedback for rotary valves

If you are an optimist and don’t really want to know whether a control valve is really moving as the controller output changes, then make sure there is no positioner and check the valve stroke manually in the field for 0%, 50%, and 100% signals. This is a time honored tradition that started in the 1980s when the old timer instrument engineers who knew the importance of a positioner and how to adjust them had retired. I have seen a large plant responsible for the most profitable herbicide in modern times demand that projects not buy control valves with positioners because they were too troublesome to calibrate and maintain. If there is no position feedback in the control room and the smallest change made by the instrument technician is 50%, it may seem like the right conclusion. The project manager is not going to argue ($500 or more per valve savings adds up). It is amazing to me no one ever reasoned that hopefully your loop is not making 50% changes but more like 0.5% changes from execution to execution even during the most upsetting times (except for surge control).

Today we have position feedback from Hart or Fieldbus positioners and we are much more enlightened or are we? On a 1999 project, the plant insisted on using a valve that was great as an isolation valve that was also relatively inexpensive, which for a 18 inch valve was certainly appreciated by the project. Other loops with these valves oscillated continually but this was attributed to some strange behavior in the process. As a concession to the fact this on-off valve really needed to be a throttling valve, a digital positioner with all types of internal analysis and plotting capability and a customer witnessed acceptance test at the manufacturer was specified.

When I arrived for the test, days of fancy plots and data had been gathered to show the positioner feedback for changes as small as 0.5%. Wow, it looked great until I noticed the disk wasn’t moving. A travel gauge installed on the disk itself revealed that the disk would not budge until the change in signal was 8% or more. The positioner feedback was actuator shaft position. Even though the shaft had two pin connections to the disk stem position there was enough play that the high sealing (rubbing) friction for the tight shutoff valve meant the when the shaft moved the disk did its own thing, which was usually nothing. Also, there was a tendency for the stem to eventually jump form shaft windup. The application here was air so there were plenty of choices for real control valves.

In another application, a similar thing occurred but it was even worse in that the ball seal was so tight that the ball stem connection didn’t even follow the actual ball position. The digital positioners in the control room said everything was OK but the loop PV said otherwise. Unfortunately, since this was liquid phosphorous, it was difficult to find a throttling control valve that would not seize up. However, one plant did get a special v-notch ball valve to work after I configured a periodic pulse to flush it out. In other applications I have avoided solids accumulation, fouling, and plugging from low flow throttling by a transition to pulse duration control for throttle positions less than 25%. In other words, there often ways to make a control valve do its job without having to go to valves that piping has specified as isolation or block valves. Next week we will offer the top signs your control valve is really an on-off valve in disguise.

In my younger days I was presented with the critical need to make air compressor surge and electrical phosphorous furnace pressure control loops able to handle very abrupt and extreme disturbances. I vastly prefer the present to that present. These applications offered more excitement than engineers should be allowed to have.

The compressors provided the air feed to multiple exothermic reactors whose flow could drop enough on a reactor trip to trigger a surge in about 2 seconds. A surge every month would cause the other reactors to trip and cause accumulating damage and loss of efficiency in the compressor besides reactor downtime and a subsequent challenging startup of reactors and the associated waste oxidation boilers.

The phosphorus furnaces had to deal with what was called “controlled explosions” from sudden shifts in the ore around the electrodes (slag slides) that caused bursts of water vapor and CO2 besides phosphorus vapors and particles. The slag slides caused a pressure spike large enough each shift to blow the water out of the electrode seals. There were tubs of water around the furnaces to jump in if the hot phosphorous landed on you. Little fires would break out when you walked by the furnaces from your shoes scraping the phosphorous residue on the floor.

These were big problems in terms of both size (18 to 24 inch pipelines) and the safety implications besides the process efficiency and capacity considerations.

High speed recorder measurements of the of the compressor flow and furnace pressure response confirmed that the process dead time in both cases was essentially zero and the observed dead time was due entirely to the components in the automation system. I installed transmitters with a sensor response time constant of less than 0.1 seconds and controllers with a special scan rate of 0.05 seconds. I had to take some special precautions in the configuration to insure the controller loading would never have negative free time (a lesson as well for our personal lives).

The control valves were the largest source of dead time. The pre-stroke dead time and stroking time for the big actuators were estimated as the fill or exhaust factor for the actuator supplied by the valve manufacturer divided by effective fill or exhaust flow coefficient of the existing positioner. This yielded pre-stroke dead times ranging from 1.0 to 2.0 seconds and stroking times of about 10 to 20 seconds. A booster had a fill and exhaust flow coefficient that was 10 times larger than the positioner and therefore offered dead times and stroking times that were 10 times faster. However, I knew the actuator connection and air tubing would then become the restricting limitation, so I had these sizes judiciously enlarged in the field, I also added a position transmitter (before the days of Hart and Fieldbus positioners with position read back).

Armed with the rule “boosters instead of positioners should be used on fast loops” and a copy of the theoretical frequency response studies to back it up I arrived onsite for the compressor application and boldly insisted against the advice of the well seasoned instrument maintenance technician to replace the positioner with a booster on the compressor vent valves.

My confidence was shattered the morning the first surge valve was put in service. The flow transmitter showed the impending surge and the controller asked the valve to open. The valve responded by doing the worst possible thing. It slammed shut before the forward flow to any of the reactors had been established.

The technician who wanted the positioner took me to the surge valve and showed that he could move it to any desired position by tugging on the actuator shaft. Obviously, the buffeting action of the turbulent flow could cause the disc to wander and eventually close. The actuator size was checked and found to be adequate; the spring rate was increased but the results remained the same. Subsequent tests showed that the stem resisted movement considerably better if the actuator was fed directly from an I/P transducer and that it could not be budged at all if a positioner was installed.

We still needed speed, so I installed the booster on the outlet of the positioner. Unfortunately the positioner looking into the small inlet port volume of the actuator can change the pressure here much faster than the booster can change the actuator pressure. The consequence is an audibly and visually impressive 1 cps limit cycle. The booster had a built in bypass whose restriction was then adjusted so that the positioner could see part of the actuator volume. Of course, the more you bypassed the booster, the slower the valve got so the restriction was opened just enough to reliably prevent the limit cycle.

On the furnace pressure control application, I put my pre-stroke dead time and stroking time requirements on the control valve specifications along with a test to be witnessed by me at the valve manufacturer. When I arrived at the valve factory, the control valves each had a booster instead of a positioner. I walked up to the valves and showed them how I could stroke these big butterflies by grabbing the shaft. Needless to say they were astonished. The actuator sizing and spring rating was rechecked. We put on the same booster and positioner combination with a tuned bypass and the problem was solved.

There is no official explanation but obviously since neither one of us had the strength to move the shafts of these big boys at will, the booster was doing something to assist us. Possibly the extreme outlet port sensitivity of the booster (fractional inch of water column) provided positive feedback in that a slight change in the diaphragm actuator volume would cause a change in the booster outlet port pressure and hence booster flow.

These valves were designed for throttling with minimal packing and sealing friction so the dead time from deadband and resolution limits were small and in fact less than the booster because the inlet sensitivity of the booster was reduced by design to work better on piston actuators. Thus, the positioner had less dead band than a booster and the combined use of them meant that source of the most of the loop dead time was relegated to actuator pre-stroke dead time. This is not the case for isolation (block and interlock) valves masquerading as control valves so here as promised last week is the help you need for this bigger problem discussed in my upcoming Control Talk column (May 2007 issue of Control magazine).

Top Ten Signs Your Control Valve is an On-Off Valve in Disguise

(10) Valve body looks suspiciously like the block valve next to it
(9) Actuator looks suspiciously like the one on the interlock valve
(8) Process engineer is seen going out to lunch with the on-off valve supplier
(7) The valve deal is a steal
(6) Your flow is on-off
(5) Positioner measures actuator shaft instead of ball or disk stem position for feedback
(4) Limit cycle amplitude exceeds largest data historian compression setting
(3) 360 degree feedback in your loop becomes 360 degree feedback in your performance review
(2) The valve package is nicknamed “Sloppy Joe”
(1) No leakage till the controller output is greater than 40%

From the 1980s to the present I decided to have some fun and add humor to the world of process control literature. The result was a series of books

How to Become an Instrument Engineer - The Making of a Prima Donna
A Funny Thing Happened on the Way to Control Room
How to Become an Instrument Engineer - Part 1.523
Logical Thoughts at 4:00 am
Dispersing Heat Through Conviction - The Funnier Side of Process Control
The Life and Times of an Automation Professional - An Illustrated Guide

A Funny Thing Happened on the Way to Control Room is my favorite because it presented detailed results of solving tough process control problems in a creative way to help open minds to new possibilities and concepts. This book is out of print but thanks to Deborah Franke at Emerson Process Management, it can be viewed for free in its electronic form via the following link.

http://www.easydeltav.com/controlinsights/FunnyThing/default.asp