Section 5 . Systems Integration

8/13/2019 Section 5 . Systems Integration

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.Process Control: Loo Tunin and Anal sis


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Understand the process

Why does this process exist?

What are the Safety, process, and quality constraints?

How does this process relate to the main product beingproduced?

What would happen if it were bypassed?

What are the cost impacts of this process? What are the quality impacts of this process?

How does it impact on other loops?

Does the SP remain constant?

How fast are disturbances?

Slide 2


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Review P&IDs

process engineers

operators

.Does it exist to:

reduce variation?

blend?

Slide 3


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You may find that there are conflicting descriptions ofthe process objectives.

This is paydirt!

Get everyone together and have a discussion aboutthe real process objectives.

Gaining alignment on the purpose of the process will

problems.

Slide 4


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Top 20 Mistakes

Greg McMillan and Stan Weiner, two of the worlds great,

mistakes made in the field of process instrumentation.

Slide 5


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Mistake No. 1

Control valve pressure drops that are not engineered. .

This is accentuated by the tendency for the mechanical,process, and instrument engineer to each put in some

.

Slide 6


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Mistake No. 2

The use of on-off valves for control valves. The primary culprits of the supposed transition from on-off to

throttling are rotary valves. Taking a ball or butterfly valve, slapping a rack and pinion or

scotch yoke piston actuator and a spool type positioner andcalling it a control valve will create lots of repetitive cycles andquestions like what is wrong.

Also beware of names that mean the opposite. For example, thehigh performance isolation valve is really the lousyperformance throttling valve because the high sealing frictionand shaft windup cause excessive stick-slip.

Remember that tight shutoff means high friction.

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Mistake No. 3

The placement of differential head or insertion type (e.g.

vortex, annubar, thermal mass) flow meters downstream

of the control valve. It screws up the velocity profile and the next performance

review of you and your meter. Yet websites, newsletters, and books have this arrangement

proudly displayed as a logo for process control.

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Mistake No. 4

Recalibration as the solution to instrument problems. This ives the a earance of doin somethin .

The real problem is usually associated with the operatingconditions, application, or the installation., ,

have boiled out the liquid in the sensing lines, connections

could be plugged, condensate could have collected in sensing,(density and viscosity) could have changed.

Slide 9


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Mistake No. 5

Tight control of level in a surge tank. The ur ose of this volume was to absorb chan es in flow and

for the level to roll with the punches. Keeping the level within a few per cent of set point passes

. Most oscillations in a process can often be traced back to an

over zealous person tuning a level controller and an .

Slide 10


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Mistake No. 6

Hydro test and flush of control valves and instruments.

wrench can make the valve packing; whether the vendor reallymeant the pressure over range limit, or whether your valve trim,

,pipe wrenches travelling at 3 m/s.

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Mistake No. 7

Equalization lines without a purge or tracing. s no a case w e er you v ew e ne as a u or a

empty. You need to choose full or empty and then install a purge or

ea rac ng o guaran ee . Trusting technicians to periodically empty or fill lines is an

exciting exercise in wishful thinking. Vapours will condense and fill up an empty line, or changes in

pressure and temperature will empty a full line at the leastopportune times.

Slide 12


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Mistake No. 8

The over use of differential head meters to measure

flow.

Orifices stink. To use them for mass flow, borders on the absurd. A arentl en ineers are oblivious to measurement noise

poor rangeability from the square root relationship, the shift inmeter coefficient with wear and Reynolds number, the change

, . There are too many better choices with less noise, more

rangeability, and better long term accuracy and reliability, such, , .

Insertion versions are available for large pipelines.

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Mistake No. 9

The installation of tem erature sensors in baffles.

This is a good idea if you want a smooth and gradual responsein temperature while your reaction runs away. The insertion of sensors in baffles coated with lass or full of

cigarette butts and sand makes trends so slow you will think

you are in the Caribbean.

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Mistake No. 10

The over-use of thermocouples for control. If you like to adjust set points to compensate for drift and

increase trend display scales to flatten out noise and scatter,

dont want to use derivative action, then save the $200 and gowith the thermocouple (TC).

Just tell the process engineer the TC is faster and strongerthan the RTD, and dont mention that it is inside a thermowell

whose design dominates the time response and reliability of theinstallation.

Be sure to save an extra $800 by omitting the smart transmitterand take the TC directly into a DCS.

Just remember to set the trend compression high to make theA/D noise disappear and turn off the rate action.

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Mistake No. 11

The use of anything other than the most simple, reliable,

.

If you pride yourself on your intelligence and creativity and areinto extreme sports while on the job, go for the most exotic andcomp ex ns rumen s an ana ysers you can ream up.

Forget about Coriolis density meters.

They are just too accurate and reliable to be anything butor ng a er ey are ns a e .

Instead pick an analyzer that likes a well heated, ventilated,and air conditioned environment.

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Mistake No. 12

Installation of pressure gauges and electrodes before start-

.

This is a great idea if you have stock in these companies. Just think of all the gauges and electrodes that get busted

.

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Mistake No. 13

Steam tracing left on during the summer. This is great for conducting chemistry experiments, such as

how hot does the process fluid need to get before its getssticky, solidify, or polymerize.

Also, it provides a good check on sensor temperature ratings

and safety programs for technicians to wear gloves.

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Mistake No. 14

The use of field mounted switches.

This is lots of fun if ou like to uess the tri settin s,

especially for field pressure and temperature switches. Limit switches on valves may be unavoidable but make

components.

The higher amp rating of other types doesnt do you muchgoo ey ge ung up or corro e .

Just remember that limit switches, which are designed totell you if a valve has failed, fail more often than the valve.

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Mistake No. 15

The use of home-made instruments, analysers, and

al orithms. The use of special fabricated or designed stuff instead of off the

shelf standard solutions is exciting for the innovator but not forever one else who has to su ort or use it.

The testing is prejudiced and the documentation is nonexistent,but dont let this stop you. After all that next promotion or

algorithm away.

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Mistake No. 16

The placement of all your eggs in one basket.

o a goo ea un ess you are g or s up enoug o

absorb the loss. The placement of interlocks or control system that can

disable an entire plant in one computer or controller or onone power supply or circuit is risky business.

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Mistake No. 17

The use of too much integral action (too small integral

me n reac or an co umn empera ure con ro ers.

Integral has no sense of direction and will only make acorrection after it crosses the line much like a 90 year olddriver.

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Mistake No. 18

The use of not enough derivative action in temperature

an p con ro ers. Turn off the rate in these applications and experience first hand

the exhilaration from acceleration. Just make sure the trend recording shows the new highs

reached by exothermic reactors or steep titration curves.

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Mistake No. 19

The omission of positioners on fast loops.

,

packing friction, shaft windup, high seating friction, and thecapabilities and diagnostics of digital smart positioners

person forced to guess what the valve position is for every

valve without a positioner.

Slide 24


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Mistake No. 20

The use of steam traps in large or critical applications. Why not make the performance of millions of dollars worth of

process equipment and instrumentation hinge on a $100 streamtrap?

One reason is that the use of a small vessel level sensor andcontroller to hold-up condensate might cost a few thousanddollars.

Who cares if the stream tra sta s shut and li uid condensatebacks up into the process equipment and reduces the heattransfer area?

through to the condensate system? Engineers and operators have gotten sick to their stomach

Slide 25

, ,columns with cheap traps.

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Section 5 . Systems Integration