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Troubleshooting problems in control system

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1

Questions

Question 1

Consider this control system, set up to maintain the temperature of a chemical reactor vessel at aconstant (“setpoint”) value. The reactor’s source of heat is a steam “jacket” where hot steam is admittedthrough a motor-operated (M) control valve (TV) according to the temperature inside the reactor sensed bythe temperature transmitter (TT):

Reactor

Steam jacket

To condensate return

TT

From steamsupply (boiler)

MTV

TICSP TI

You arrive at work one day to find the operator very upset. The last batch of product emptied fromthe reactor was out of spec, as though the temperature were too cold, yet the controller (TIC) displays thetemperature to be right at setpoint where it should be: 175 oF.

Your first step is to go to the reactor and look at the temperature indicating gauge (TI) mounted nearthe same point as the temperature transmitter. It registers a temperature of only 137 oF.

From this information, determine what is the most likely source of the problem, and explain how youmade that determination.

Suggestions for Socratic discussion

• Why was it a good decision to consult the temperature gauge (TI) on the reactor as a first diagnosticstep?

• Suppose a fellow instrument technician were to suggest to you that the problem in this system could bea controller configured for the wrong action (e.g. direct action instead of reverse action). Do you thinkthis is a plausible explanation for the symptoms reported here? Why or why not?

• Could the problem be that someone left the controller in manual mode rather than automatic mode asit should be? Explain why or why not.

• Based on the P&ID shown, are the instruments pneumatic or electronic?• Given the fact that we know this reactor is steam-heated, is it possible to conclude that the chemical

reaction taking place inside it is either endothermic (heat-absorbing) or exothermic (heat-releasing)?• Safety shutdown systems often use a “two-out-of-three” (2oo3) voting algorithm to select the best

measurement from three redundant transmitters. Explain how this same concept may be applied by theinstrument technician in the course of troubleshooting the problem.

file i00137

2

Question 2

On the job, you are sent to troubleshoot a brand-new control system, consisting of a pneumatic liquidlevel transmitter connected to a pneumatic controller, which in turn drives a pneumatic control valve. Theprocess vessel, piping, control valve, controller, and level transmitter are all brand-new: they even sport afresh coat of paint.

LTLG

LIC74 74

According to the unit operator, this level control system has never worked. As she shows you, the liquidlevel inside the vessel is so low that the level gauge (LG) registers empty, yet the controller is commanding thevalve 100% open, which of course continues to drain the vessel and prevent any liquid level from accumulating.

Being versed in process control theory, you decide to check how the controller is configured. Lookinginside the controller case, you notice the controller is set for direct action: an increasing PV results in anincreasing output signal (MV), which will move the air-to-close valve more toward the “closed” state.

Realizing how to fix the problem, you reach inside the controller and move a lever that switches it intoreverse action mode.

Explain why this fixes the problem.


• Explain the significance of the “newness” of this process. How would your assumptions differ if you sawthis process vessel was old and rusted instead of shiny-new?

• How do you suppose the controller got to be mis-configured in the first place?• What would have to be different in this control system to permit a direct-acting controller instead of a

reverse-acting controller?• Suppose you did not discover the controller’s action set for direct action. If the controller had been

left in manual mode instead of automatic mode, could this account for the problems exhibited by thissystem?

file i00140

3

Question 3


Reactor

Steam jacket


TT


MTV

TICSP TI

You arrive at work one day to find the operator very upset. The last batch of product emptied fromthe reactor was out of spec, and the temperature displayed by the indicating controller (TIC) shows it to be197 oF. The setpoint is set at 175 oF, and the controller is in the automatic mode as it should be.

Your first step is to look at the indication on the controller showing the output signal going to the motor-actuated steam valve (TV). This output signal display (the “manipulated variable”) shows 0 %, which means“valve fully closed.”

Next, you decide to check the temperature shown at the temperature indicator (TI) located near thetemperature transmitter (TT) on the reactor. There, you see a temperature indication of 195 oF.

From this information, determine what is the most likely source of the problem, and explain how youmade that determination.


• Why is it important for us to know that the controller is in automatic mode? Would it make a differenceif it were in manual mode instead?

• Explain why the first two diagnostic steps were to check the controller’s output display, then to checkthe TI on the reactor. What do each of these checks tell us about the nature of the problem?

• Suppose a fellow instrument technician were to suggest to you that the problem in this system could bea controller configured for the wrong action (e.g. direct action instead of reverse action). Do you thinkthis is a plausible explanation for the symptoms reported here? Why or why not?

file i00138

4

Question 4


Reactor

Steam jacket


TT


MTV

TICSP TI

While doing some clean-up work near the reactor, you receive a frantic call from the operator on yourtwo-way radio. He says that the controller (TIC) is registering a temperature of 186 oF, which is 11 degreeshigher than the setpoint of 175 oF. A temperature this high could ruin the product inside the reactor. Hewants you to check the temperature indicator on the side the reactor (TI) and let him know what it reads.

You look at the TI, and see that it registers a temperature of 172 oF, which is a bit too cold if anything,not too hot. You immediately report this to the operator using your radio, who then asks you to check outthe system to see why he’s getting a false reading on the controller display.

Fortunately, you have your multimeter and tool set with you, so you proceed to the temperaturetransmitter to measure the milliamp signal it is outputting. Removing a cover from a round junctionbox on the conduit where the transmitter’s wires are routed, you see a terminal block inside with a 1N4001rectifying diode placed in series with the circuit:

Conduit

Con

duit

To transmitter

To controller

Setting your multimeter to measure milliamps, you connect the red and black test leads across the diode.

5

This shorts past the diode, forcing all the current to go through the meter instead of the diode, allowingyou to “break in” to the 4-20 mA circuit without having to physically break a wire connection anywhere.Making a mental note to thank your instrumentation instructor later for showing you this trick, you see thatyour multimeter registers 15.683 mA.

Given a calibrated temperature transmitter range of 100 to 200 degrees F, determine what this currentmeasurement tells you about the location of the problem in this temperature control loop, and explain howyou made that determination.


• Why is it important for technicians to be able to easily convert milliamp signal values into correspondingprocess variable (PV) values?

• How does the diode perform this useful function of allowing current measurement without breaking thecircuit?

• Supposing there were no diode in this loop circuit, how would you suggest we measure the transmitter’soutput current?

• Is it possible that the fault in this system could be something to do with the control valve? Why orwhy not?

file i00139

6

Question 5

In this process, two chemical streams are mixed together in a reactor vessel. The ensuing chemicalreaction is exothermic (heat-producing) and must be cooled by a water cooling system to prevent overheatingof the vessel and piping. A temperature transmitter (TT) senses the reaction product temperature and sendsa 4-20 mA signal to a temperature indicating controller (TIC). The controller then sends a 4-20 mA controlsignal to the temperature valve (TV) to throttle cooling water flow:

Feed A Feed B

Reaction product out

Cold water

Reactor

in

Hot water out

TT

TIC

TV

TIR

New recorder

Suppose an instrument technician adds a temperature-indicating chart recorder (TIR) to thetemperature transmitter circuit, necessitating the addition of a 250 ohm resistor to the 4-20 mA circuitto provide a 1-5 volt voltage signal which the recorder can read. Now the 4-20 mA temperature circuit hasmore resistance in it than it did before.

Describe in detail the effect this circuit modification will have on the performance of the cooling system.file i02931

7

Question 6

This water filter level control system uses an ultrasonic level transmitter to sense the level of water inthe filter, and a controller to drive a motor-actuated valve introducing raw water to be filtered:

Influent

Filteringmedia

Filter

LT

Ultrasonic

LIC

M

Effluent

Setpoint

LIRH

L

Assuming a direct-acting level transmitter (increasing filter level = increasing signal), and a signal-to-open control valve (increasing controller output signal = wider open valve), determine whether the levelcontroller needs to be configured for direct-action or reverse-action, and explain your reasoning. Annotatethe diagram with “+” and “−” symbols next to the PV and SP controller inputs to show more explicitlythe relationships between the controller inputs and output.

Next, determine the response of the controller to the following situations. In other words, determinewhat the controller’s output signal will do when this water level control system is affected in the followingways:

• A sudden increase in effluent flow rate (clean water demand)

• Level transmitter fails high (indicating 100% full water level)

• Control valve actuator fails, driving valve fully open (ignoring controller signal)


• Re-draw the diagram for this water filter level control system, replacing the controller (circle) with anop-amp symbol (triangle), determining the “+” and “−” input assignments on the opamp for PV andSP.

• Explain why level control is important in a water filter such as this.• What do the “H” and “L” symbols near the LIR represent?

file i02370

8

Question 7

There is a problem somewhere in this liquid flow control system. The controller is in automatic mode,with a setpoint of 65%, yet the flow indicator and the flow controller both register 0.3%: (nearly) zero flow.A P&ID of the loop appears here:

I/P

FY

FIC

FT

FIR

Pump

Explain how you would begin troubleshooting this system, and what possible faults could account forthe controller not being able to maintain liquid flow at setpoint.


• Explain how you could divide this control system into distinct areas or zones which you may then beginto refer to when “dividing and conquering” the problem.

file i02518

9

Question 8


Feed A Feed B


Cold water

Reactor

in

Hot water out

TT

TIC

TV

TT

TIR

Suppose operators decide to increase production in this process reactor. This means the incoming feedflow rates will be increased, producing more heat.

Describe in detail how the cooling system will respond to this change in process operations.file i02933

10

Question 9

In this process, maple syrup is heated as it passes through a steam heat exchanger, then enters anevaporator where the water boils off. The purpose of this is to raise the sugar concentration of the syrup,making it suitable for use as a food topping. A level control system (LT, LIR, LIC, and LV) maintainsconstant syrup level inside the evaporator, while an analytical control system (AT, AIR, AIC, and AV)monitors the sugar concentration of the syrup and adjusts steam flow to the heat exchanger accordingly.

PV = 34%SP = 34%

LIR

Out = 22%

PV = 52%SP = 50%

Out = 86%

85% open

24% open

PV = 34%

PV = 52%

at 66% concentration

50% level in evaporator

LG

Level gauge shows

Laboratory tests syrup

Evaporator

Steamsupply

Condensatereturn to boiler

LT

LIC

LV

Syrup in

Heatexchanger

AT

AIR

AV

Concentrated

FT

Water vapor out

syrup out

Liquid pump

Vapor compressor

AIC

Examine the live variable values shown in the above diagram, and then determine where any problemsmay exist in this syrup concentrating system.


• A valuable principle to apply in a diagnostic scenario such as this is correspondence: identifying whichvariables correspond at different points within the system, and which do not. Apply this comparativetest to the variables scenario shown in the diagram, and use the results to defend your answer of wherethe problem is located and what type of problem it is.

file i02934

11

Question 10

Examine this P&ID for a level control system in a vessel where two different fluids (Feed A and FeedB) are mixed together:

LT

LIC

Mixingvessel

Feed A Feed B

LGLevelgauge

M

LV

Motor

Determine the effect on the control system’s regulation of liquid level inside the vessel if an instrumenttechnician accidently mis-calibrates the control valve such that it opens 2% more than it should (e.g. whenthe controller sends a 50% signal to the valve, it actually opens to 52% stem travel). Assume all other loopcomponents are properly configured and that the controller is well-tuned.

file i04391

12

Question 11

Examine this P&ID for a level control system in a vessel where two different fluids (Feed A and FeedB) are mixed together:

LT

LIC

Mixingvessel

Feed A Feed B

LGLevelgauge

M

LV

Motor

Determine the effect on the control system’s regulation of liquid level inside the vessel if an instrumenttechnician accidently mis-configures the controller for the wrong type of action (e.g. direct action when itshould be reverse, or vice-versa). Assume all other loop components are properly configured and that thecontroller is well-tuned.

file i04393

13

Question 12


Feed A Feed B


Cold water

Reactor

in

Hot water out

TT

TIC

TV

Suppose something fails in the control valve mechanism to make it incapable of opening further than80%. From 0% to 80% position, however, the valve responds normally.

Describe in detail the effect this fault will have on the performance of the cooling system.file i02932

14

Question 13

In this process, maple syrup is heated as it passes through a steam heat exchanger, then enters anevaporator where the water boils off. The purpose of this is to raise the sugar concentration of the syrup,making it suitable for use as a food topping. A level control system (LT, LIC, and LV) maintains constantsyrup level inside the evaporator, while an analytical control system (AT, AIR, AC, and AV) monitors thesugar concentration of the syrup and adjusts steam flow to the heat exchanger accordingly.

Evaporator

Steamsupply


LT

LIC

LV

Syrup in

Heatexchanger

AC

AT

AIR

AV

Concentrated

FT

Water vapor out

syrup out

Liquid pump

Vapor compressor

Suppose the syrup analyzer (AT) suffers a sudden calibration problem, causing it to register too low(telling the analytical controller that the sugar concentration of the syrup is less than it actually is).

Describe in detail the effect this calibration error will have on the performance of the analytical controlsystem.


• What economic effect will this mis-calibration have on the process? In other words, does the processbecome more or less profitable as a result of this change?

• Suppose someone shuts the manual block valve on the steam line just a little bit, so that it is about 80%open instead of 100% open. How will this process change affect the control systems in this process?

file i02936

15

Question 14

In this process, maple syrup is heated as it passes through a steam heat exchanger, then enters anevaporator where the water boils off. The purpose of this is to raise the sugar concentration of the syrup,making it suitable for use as a food topping. A level control system (LT, LIC, and LV) maintains constantsyrup level inside the evaporator, while an analytical control system (AT, AIR, AC, and AV) monitors thesugar concentration of the syrup and adjusts steam flow to the heat exchanger accordingly.

Evaporator

Steamsupply


LT

LIC

LV

Syrup in

Heatexchanger

AC

AT

AIR

AV

Concentrated

FT

Water vapor out

syrup out

Liquid pump

Vapor compressor

Suppose the steam tubes inside the heat exchanger become coated with residue from the raw maplesyrup, making it more difficult for heat to transfer from the steam to the syrup. This makes the heatexchanger less efficient, which will undoubtedly affect the process.

Describe in detail the effect this heat exchanger problem will have on the performance of the analyticalcontrol system.


• Suppose the operations personnel of this maple syrup processing facility wished to have an automatic

method for detecting heat exchanger fouling. What variable(s) could be measured in this process toindicate a fouled heat exchanger?

• What economic effect will this fouling have on the process? In other words, does the process becomemore or less profitable as a result of the heat exchanger fouling?

file i02937

16

Question 15

Pictured here is a P&ID (Process and Instrument Diagram) of a liquid flow control “loop,” consisting of aflow transmitter (FT) to sense liquid flow rate through the pipe and output an electronic signal correspondingto the flow, a flow controller (FC) to sense the flow signal and decide which way the control valve should move,a current-to-air (I/P) transducer (FY) to convert the controller’s electronic output signal into a variable airpressure, and an air-operated flow control valve (FV) to throttle the liquid flow:

I/P

FT

FCFY

FV

Pipe

4-20 mA signal 4-20 mA signal

3-15 PSI signal

Pump

The actions of each instrument are shown here:

• FT: increasing liquid flow = increasing current signal• FC: increasing process variable (input) signal = decreasing output signal• FY: increasing current input signal = increasing pneumatic output signal• FV: increasing pneumatic signal = open more

Describe what will happen to all signals in this control loop with the controller in “automatic” mode(ready to compensate for any changes in flow rate by automatically moving the valve) if the pump were tosuddenly spin faster and create more fluid pressure, causing an increase in flow rate.

Also describe what will happen to all signals in this control loop with the controller in “manual” mode(where the output signal remains set at whatever level the human operator sets it at) if the pump were tosuddenly spin faster and create more fluid pressure, causing an increase in flow rate.


• Explain the practical benefit of having a “manual” mode in a process loop controller. When might weintentionally use manual mode in an operating process condition?

file i00124

17

Question 16

An operator reports a high level alarm (LAH-12) displayed at the control room for the last 13 hours ofoperation, in this sour water stripping tower unit (where sulfide-laden water is “stripped” of sulfur compoundsby the addition of hot steam). Over that time period, the sightglass (level gauge LG-11) has shown the liquidlevel inside vessel C-406 drifting between 2 feet 5 inches and 2 feet 8 inches:

LG11

1"

1"

1 1/2"

1 1/2"

1"

1"

LT12

TI340

3/4"

1/2"

PG422

ST

ST

ST

E-2

PG463

C-7

LLL

NLL

HLL

1’-3"

2’-6"

4’-1"

E-9

ST

To water treatmentDwg. 45772

LIC

12

LV12

LSH

12 12

LSL

12 12

LAH LAL

P-102P-101

LP cooling waterDwg. 31995 ST

P-103

FT27

27

27I/P

FY

FIC

TIC21 21

FI97

FI98

V-10

10’ packed bed

10’ packed bed

LP cooling waterDwg. 31995

Set @50 PSI

75 PSISet @

From 50 PSIsteam headerDwg. 13301

I/P

FY28

FIC

FT28

28

FIR28 L

From nitrogenheaderDwg. 13322

2" thickinsul

Set @2" vac.2" press.

NC

NC

NC

11

Mag

PG401

PC

PV

PCV10

Set @60 PSI

PG405

To incineratorDwg. 13319

ST

ST

Slope

PG406

ST

STST

From acid gasseparatorDwg. 25311

From sour waterflash drumDwg. 25309 S

T

ST

PG402

ST

ST

FI37

PG316

ST

24" MH

20

115

115

Liquid dist.

Set @100 PSI

PG461

PG459

Slope

Strainer

ST

V-10SOUR WATER TANK8’-0" Dia 12’-0" SidewallDP AtmosphereDT 190 oF

P-201

P-201

85 ACFM @ 1" H2O

P-102SOUR WATER PUMP

5 GPM @ 80 oFRated head: 75 PSI

P-101COOLING WATER PUMP


P-103STRIPPED WATER PUMP


C-7SOUR WATER STRIPPER

12" x 40’ SSDP 55 PSIGDT 350 oF

Each bed 10’ of 1" pall rings

E-2SOUR WATER HEATER

Rated duty: 300 MBTU/HRShell design: 70 PSI @ 360oFTube design: 125 PSI @ 360 oF

E-9STRIPPED WATER COOLERRated duty: 50 MBTU/HRShell design: 150 PSI @ 350 oFTube design: 150 PSI @ 350 oF

PG441

PG438

To flare headerDwg. 13320ST

Slope

NC

Set @100 PSI

343

2" thickinsul

Steam dist.

FV27

FV28

TV21

NC

TIR

21

TT

PG315

PG312

FQ

27

H

L

PG300

PG299

ST

ST

ST

TG477

TG478 TG

479

TG480

PSV355

PSV354

PSV353

PSV352

PSV351

LG19

3/4"

LY

12

LIR12a 12b

P/I

LR

LT18

18

18LI

LIR

344TG

TG

FT29

FIR29

29FI

LSH

18

18

LAH18

LAL18

LLL

1’-0"

HLL

10’-6"

TG

TG345

TG346

26TG

AIT342

342

AIT341 341

M

FT30

M

FT31

FIR30

FIR31 L

L

H

AIT347 347

AIRpH

AIT

AIR

pH

348

348L

L

AAH

AAH

Cond

Cond

PSL201

201

PAL

PSHPAH

202 202

LAL

LSL

11

LSLL203

I

PSLL204

SOUR WATER TANK EJECTOR

LSL

Identify the likelihood of each specified fault in this process. Consider each fault one at a time (i.e. nocoincidental faults), determining whether or not each fault could independently account for all measurementsand symptoms in this process.

Fault Possible Impossible

LT-12 miscalibratedLG-11 block valve(s) shut

LSH-12 switch failedLSL-12 switch failed

Leak in tubing between LT-12 and LIC-12LIC-12 controller setpoint set too high

LV-12 control valve failed openLV-12 control valve failed shut

file i03540

18

Question 17

In this process, steam is introduced into “stripping” vessel C-7 to help remove volatile sulfur compoundsfrom “sour” water. The temperature of the stripped gases exiting the tower’s top is controlled by a pneumatictemperature control loop. Unfortunately, this loop seems to have a problem.

Temperature indicating recorder TIR-21 registers 304 degrees Fahrenheit, while temperature indicatingcontroller TIC-21 registers 285 degrees Fahrenheit. The calibrated range of TT-21 is 100 to 350 degreesFahrenheit. A technician connects a test gauge to the pneumatic signal line and reads a pressure of 12.8PSI:

LG11

1"

1"

1 1/2"

1 1/2"

1"

1"

LT12

TI340

3/4"

1/2"

PG422

ST

ST

ST

E-2

PG463

C-7

LLL

NLL

HLL

1’-3"

2’-6"

4’-1"

E-9

ST


LIC

12

LV12

LSH

12 12

LSL

12 12

LAH LAL

P-102P-101


P-103

FT27

27

27I/P

FY

FIC

TIC21 21

FI97

FI98

V-10

10’ packed bed

10’ packed bed


Set @50 PSI

75 PSISet @


I/P

FY28

FIC

FT28

28

FIR28 L


2" thickinsul


NC

NC

NC

11

Mag

PG401

PC

PV

PCV10

Set @60 PSI

PG405


ST

ST

Slope

PG406

ST

STST



T

ST

PG402

ST

ST

FI37

PG316

ST

24" MH

20

115

115

Liquid dist.

Set @100 PSI

PG461

PG459

Slope

Strainer

ST


P-201

P-201

85 ACFM @ 1" H2O













PG441

PG438


Slope

NC

Set @100 PSI

343

2" thickinsul

Steam dist.

FV27

FV28

TV21

NC

TIR

21

TT

PG315

PG312

FQ

27

H

L

PG300

PG299

ST

ST

ST

TG477

TG478 TG

479

TG480

PSV355

PSV354

PSV353

PSV352

PSV351

LG19

3/4"

LY

12

LIR12a 12b

P/I

LR

LT18

18

18LI

LIR

344TG

TG

FT29

FIR29

29FI

LSH

18

18

LAH18

LAL18

LLL

1’-0"

HLL

10’-6"

TG

TG345

TG346

26TG

AIT342

342

AIT341 341

M

FT30

M

FT31

FIR30

FIR31 L

L

H

AIT347 347

AIRpH

AIT

AIR

pH

348

348L

L

AAH

AAH

Cond

Cond

PSL201

201

PAL

PSHPAH

202 202

LAL

LSL

11

LSLL203

I

PSLL204


LSL

Which instrument is faulty: the transmitter, the recorder, or the controller, or is it impossible to tellfrom what little information is given here?

file i03541

19

Question 18

This P&ID shows an incinerator stack used to safely burn poisonous gases. The high temperature of thegas flame reduces the poisonous compounds to relatively harmless water vapor, carbon dioxide, and oxidesof sulfur and nitrogen.

The incinerator was recently out of service for three full weeks being rebuilt. Following the rebuild,operations personnel have attempted to start the incinerator’s burner on plant fuel gas with no success.They can get it started with natural gas, but the burner management system keeps tripping whenever theyswitch to fuel gas. They call you to investigate.

F-1

2" thick

Waste stream #1Dwg. 12022

Dwg. 12022

Dwg. 12022

Waste stream #2

Waste stream #3

Vent(Note 1)

ET

FC

TE37 37

TTTETT3636

TAL TAH36 36

From fuel gas headerDwg. 32915

gas headerDwg. 32915

From natural

Vent(Note 1)

FT

AE35

FV

3838

FY38a

I/P

FY38b

38

FIC38

TIC37

RSP

FIQ38

FY

38c

HART to analog

TIR38

DIRHARTZS38

(Note 2)

PSH

PSL

BMS

NE

D

AE34

90o apart atelev. 50’ 6"

67’ abovegrade

PSH

PSL

BE

BE

BMS

(Note 3)

PG

ST

ST

ST

AT

AT

34

35

AIR34/35

O2

SO2

from gradeto 24’ 0"

34’ 6"above grade

AAH

BAL

PG

PG

PG

2"x1" 2"x1"

6"x2"

6"x2" 6"x2"

Rain shieldfrom 24’ to 67’

F-1

DP AtmosphereDT 1650 oF

INCINERATOR

(3) - 3" nozzles

35

ET

ET

33

AT

AIR

AY

33

Gateway

RS-485

Ethernet

Modbus

AIRAIR AIR33a 33b 33c 33d

HNO3

NOTES:

1. Gas safety vent pipes to extend 10 feet above grade,situated at least 30 feet from any source of ignition.

(Note 2)

drawing 17003 for wiring details.

3. Gas chromatograph supplied by vendor, located inanalyzer shack at base of incinerator tower. See drawing 17059 for wiring and tubing details.

24" MW

GC

AIR33e

2. Burner management system supplied by vendor, locatedin NEMA4X enclosure at base of incinerator tower. See

SV101

SVSV102

103

SV

SV SV

104105

106

107 108

109110

111 112

113

114H2S C2H2 NH3

PCV40

PCV39

41

42

43

PG44

1/2"

2"

2"

SV115

CH4

2"x3/4"2"

2"

2"

2"

2"

2"

2"

2"

3/4"3/4"

1" 1"

1"

1"1"

1"x1/2"

1"1"

TIR36

Res Time 1.5 sec minimum

20



SV-115 leaking airPSL-105 failedPSL-114 failed

PCV-39 pressure setpoint too lowPCV-39 pressure setpoint too highPCV-40 pressure setpoint too lowPCV-40 pressure setpoint too high

ZS-38 failedBlind inserted in natural gas header

Blind inserted in fuel gas header

file i03500

21

Question 19

The compressor emergency shutdown system (ESD) has tripped the natural gas compressor off-linethree times in the past 24 hours. Each time the operator goes to reset the compressor interlock, she noticesthe graphic display panel on the interlock system says “Separator boot high level” as the reason for the trip.After this last trip, operations decides to keep the compressor shut down for a few hours until your arrivalto diagnose the problem. Your first diagnostic test is to look at the indicated boot level in the sightglass(LG-93). There, you see a liquid level appears to be normal:

V-65

M

To gas coolingDwg. 10921

From natural gas

Dwg. 38422

From natural gas

Dwg. 38422

From natural gas

Dwg. 38422

P-8

FT

TERTD

PDT

LSHH LGLT

LIC

FC

H

L

Vent stacks 20’ above grade

PT

TERTD

1:1

I/P

P

Anti-surge

VXE VYEVXE VYE

VZE

VXE VYE

Vibration monitor

TIR PIR

Rod out

IAS

I

To motor controlsDwg. 52331

Bently-Nevada 3300 series

(See dwg. 58209 for wiring details)

V-65COMPRESSOR INLET SEPARATOR

DP 450 PSIGDT 100 deg F

Size 3’ 5" ID x 12’ 0" length

PSV PSV PSV

Set @405 PSIG 410 PSIG408 PSIG

Set @ Set @

P-8COMPRESSOR

50 MSCFH @ 315 deg F dischand 175 PSID boost pressure

OWS

TERTD

TSH

TT

Set @325 deg F

TT

ETET

vent

NDE

DE

ESD

HS

LPDT

ETET

Slope

Slope

PDIR H

FIR

FY

PT

FSL

I

PDSH

HS

AND

RTD

TE

RTD

TE

M

JT

JIRJAHH

H

L

IAS

IAS

M

Set @

Set @30 MSCFH

12"x6"

12"x6"

12"x6"

12" 12"4" 4" 4" 1"

1" 1"

2"

2"

2"

2"

2"

2"

2"

2"12" 8"

12"x8"

12"x8"

220

220220

221

222 223224

225 226 227 228

229

230

75

75

75

75

93

93

93

91

88

88

88 89

89

92

LV

92

92

93231

232 232

232

11 12 13

SV92

XAXC

XY

XY

74

73

76 76

76a

76b

77

0.9 PSID

1"x1/2" 1"x1/2"

NLL = 1’ 4"

LLL = 0’ 7"

HLL = 1’ 11"

HHLL = 2’ 6"(ESD)

LIR92

H

L

PG131

PG

1/2"

132

PG133

PG134

PG135

source A-3

source A-2

source A-1

72TG

First, explain why this first diagnostic test was a good idea. Then, identify what would your next

diagnostic test be.

Finally, comment on the decision by operations to leave the compressor shut down until your arrival.Do you think this was a good idea or a bad idea, from a diagnostic perspective? Why or why not?

file i03502

22

Question 20

The overhead pressure control system in this fractionator seems to have a problem. The controller(PIC-33) indicates the pressure being over setpoint by a substantial margin: the pressure reads 48 PSI whilethe setpoint is 37 PSI:

C-5

RO

PG

PG

RO

PG

PG

M

PG

NC

NC

NC

NC

NC

C-5MAIN FRACTIONATION TOWER

PT

PIC

HP cooling waterDwg. 11324

Dwg. 11324

Cooling waterreturn

PY

FT

FT IAS

P

FC

LIC

LT

Overhead productDwg. 28542

Distillate productDwg. 28543

Bottoms productDwg. 28544

LG

LGLT

LSH

LSL

FT

LIC FIC

P

Sidedraw productDwg. 28545

NC

FT

FIC FY

FV

FT

FYFYLead/LagLead/Lag

AT

AIC

FY

Dwg. 10957

FT

P

IAS

IAS

AIC FY

Dwg. 10957Condensate return

FOUNDATION Fieldbus

FOUNDATION Fieldbus

FOUNDATION Fieldbus

IAS

P

FC

IAS

IAS

V-13

V-13OVERHEAD ACCUMULATOR

P-10 P-11

Dwg. 10957

Fractionator feed

Dwg. 27004from charge heater

E-5

E-6

E-7

E-8

E-9

FO

FC

FO

FC

FO

FO

PSL

PSL

SS

R

IAS

I

HC

RO

PG

PG

RO

PG

M

FO

PSL

PSL

SS

R

I

IAS

HC

PG

RO

PG

PGM

FO

PSL

PSL

SS

R

I

IAS

HC

RO

PG

PG

Dwg. 10957Condensate return

PG

PG

PG

PG

PG

PG

TT

TT

TT

TT

TT

TT

TIR

TIR

TIR

TIR

TIR

TIR

AITTT TIR

TTTIR

3131 30

3032

33

33

34

34 34

35

35 35

36

36

37

3738

39

40

40a 40b

40c

FFC

41

41

42

42

50 50

5151

5252

53 53

54 54

55 55

56 56

57

58

57

LAL

58

LAH

LLL = 3’-8"

NLL = 5’-4"

HLL = 7’-2"

33PR

33aPY

33b

106

107

60

61

62

63

64

65

59 59

108

PG

PG

109

110

111

112

113

114

115

116

117

118119

120

121

122

123 124

125

127

35

FV34

PV

FV31

FV41

FV37

Dwg. 62314

To LP flareFO

NC

33a

33bPV

3 to 9 PSI

9 to 15 PSI

PG130

PG131

PG132

PG133

PG134

PG135

PG136

PG137

PG138

PG139

P-12 P-13 P-14 P-15

P-15P-10 P-11 P-12 P-13 P-14MAIN OVERHEAD PRODUCT PUMP BACKUP OVERHEAD PRODUCT PUMPMAIN BOTTOMS PRODUCT PUMP BACKUP BOTTOMS PRODUCT PUMPMAIN CHARGE FEED PUMP BACKUP CHARGE FEED PUMP

E-5, E-6, E-7FEED HEAT RECOVERY EXCHANGERS

H

L

PSH

PAH

66

66

H

L

H

L

Note 1Note 1Note 1

NOTES:

1. Backup (steam-driven) pumps automatically started by 2oo2 triplogic, where both pressure switches must detect a low-pressurecondition in order to start the backup pump.

FOUNDATION Fieldbus

FOUNDATION FieldbusFOUNDATION Fieldbus

FOUNDATION Fieldbus

M

FT67

FOUNDATION Fieldbus

FIR

67

FT68 PT

68

68TT

FY

68

Modbus RS-485 FIQ

68

PT

TTFT69

69

69

FY

69

FIR

69

RTD

RTD

Note 2

2. Transit-time ultrasonic flowmeter with pressure and temperaturecompensation for measuring overhead gas flow to flare line.

Dia 10’-3" Height 93’DP 57 PSIG

Set @ 55 PSISet @ 55 PSI

Set @ 52 PSI Set @ 52 PSI

DT 650 oF top, 710 oF bottom

DP 81 PSIGDT 650 oF

E-9BOTTOMS REBOILER

E-8OVERHEAD PRODUCT CONDENSER

2100 GPM @ 460 PSID 1900 GPM @ 460 PSID 2880 GPM @ 70 PSID 2880 GPM @ 70 PSID 2350 GPM @ 55 PSID 2350 GPM @ 55 PSID80 MM BTU/hrShell 500 PSIG @ 650 oF

Tube 165 PSIG @ 400 oFTube 660 PSIG @ 730 oFShell 120 PSIG @ 650 oF

55 MM BTU/hr 70 MM BTU/hrShell 630 PSIG @ 800 oFTube 600 PSIG @ 880 oF

Set @410 PSI

Set @500 PSI

Set @100 PSI

Set @73 PSI

600 PSI steam

1000 PSI steam

PG140

PG141

LT

38a

38bLT38c

LY

38

Median

142 143 144

Radar

select

Magnetostrictive (float)



PT-33 calibration errorPY-33a calibration errorPY-33b calibration errorPV-33b block valve closedPV-33b bypass valve open

Instrument air supply to PY-33b failedInstrument air supply to FV-34 failed

file i03533

23

Question 21

Inspecting the trends of PV and SP on a process chart recorder, you notice the poor quality of control:

Time0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

%SP

PV

The “wandering” of the process variable (PV) around setpoint may be due to excessive action on thepart of the controller, or it may be due to load fluctuations in the process itself. In other words, the instabilitymay be the fault of the controller reacting too aggressively, or it may be that the controller is not workingaggressively enough to counter changes in process load.

Identify a simple method to determine which scenario is true. Hint: the way to check is as simple aspushing a single button, in most cases.

file i01646

Question 22

A very useful technique for testing process control loop response is to subject it to a “step-change” incontroller output. In other words, the process is perturbed (the highly technical term for this is “bumped”)and the results recorded to learn more about its characteristics.

What practical concerns might surround “bumping” a process such as this? Remember, the processvariable (PV) is a real physical measurement such as pressure, level, flow, temperature, pH, or any numberof quantities. What precautions should you take prior to perturbing a process to check its response?

file i01652

24

Question 23

In this process, sulfur-laden water is “stripped” of sulfur compounds by the addition of hot steam. Alevel control system is supposed to maintain a constant level of liquid at the bottom of the stripping tower,but it seems to have a problem:

LG11

1"

1"

1 1/2"

1 1/2"

1"

1"

LT12

TI340

3/4"

1/2"

PG422

ST

ST

ST

E-2

PG463

C-7

LLL

NLL

HLL

1’-3"

2’-6"

4’-1"

E-9

ST


LIC

12

LV12

LSH

12 12

LSL

12 12

LAH LAL

P-102P-101


P-103

FT27

27

27I/P

FY

FIC

TIC21 21

FI97

FI98

V-10

10’ packed bed

10’ packed bed


Set @50 PSI

75 PSISet @


I/P

FY28

FIC

FT28

28

FIR28 L


2" thickinsul


NC

NC

NC

11

Mag

PG401

PC

PV

PCV10

Set @60 PSI

PG405


ST

ST

Slope

PG406

ST

STST



T

ST

PG402

ST

ST

FI37

PG316

ST

24" MH

20

115

115

Liquid dist.

Set @100 PSI

PG461

PG459

Slope

Strainer

ST


P-201

P-201

85 ACFM @ 1" H2O













PG441

PG438


Slope

NC

Set @100 PSI

343

2" thickinsul

Steam dist.

FV27

FV28

TV21

NC

TIR

21

TT

PG315

PG312

FQ

27

H

L

PG300

PG299

ST

ST

ST

TG477

TG478 TG

479

TG480

PSV355

PSV354

PSV353

PSV352

PSV351

LG19

3/4"

LY

12

LIR12a 12b

P/I

LR

LT18

18

18LI

LIR

344TG

TG

FT29

FIR29

29FI

LSH

18

18

LAH18

LAL18

LLL

1’-0"

HLL

10’-6"

TG

TG345

TG346

26TG

AIT342

342

AIT341 341

M

FT30

M

FT31

FIR30

FIR31 L

L

H

AIT347 347

AIRpH

AIT

AIR

pH

348

348L

L

AAH

AAH

Cond

Cond

PSL201

201

PAL

PSHPAH

202 202

LAL

LSL

11

LSLL203

I

PSLL204


LSL

25

Here is what the trend recording from LR-12b looks like during the time an operator placed the controllerin manual mode and then back to automatic mode:

Time0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

% SPPV

Output

A fellow technician tells you he thinks the controller is over-tuned (having too much gain). The operator,who just did the manual-mode test, disagrees. Based on the information seen in the trend, what do youthink the source of the oscillation is, and how would you go about testing your hypothesis?

file i01902

26

Answers

Answer 1

Answer 2

Answer 3

Answer 4

Answer 5

This circuit modification will have absolutely no effect on the performance of the system, as long as theloop-powered transmitter receives its minimum terminal voltage for proper operation.

27

Answer 6

This controller needs to be reverse-acting:

Influent

Filteringmedia

Filter

LT

Ultrasonic

LIC

M

Effluent

Setpoint

Reverse-acting

This re-drawing of the control system uses an opamp symbol in place of the ISA-standard circle usedto represent a loop controller:

Influent

Filteringmedia

Filter

LT

Ultrasonic

LIC

M

Effluent

Setpoint

LIRH

L

−

+

• A sudden increase in effluent flow rate (clean water demand): controller output increases

• Level transmitter fails high (indicating 100% full water level): controller output decreases

• Control valve actuator fails, driving valve fully open (ignoring controller signal): controller output

decreases

28

Answer 7

One possible fault has to do with the control valve: perhaps something has happened to make it failclosed (loss of air supply, signal, etc.). Other possible problems include the following:

• Pump not running (no source of fluid power to motivate flow)• Very poor controller tuning• Wrong controller action• Valve failed closed (loss of air supply, signal, etc.)• Transmitter failed, showing no flow when in fact there is

A good “first test” for troubleshooting the loop is to check the controller output: is it trying to openup the valve?

Answer 8

The controller should still be able to maintain the process temperature at setpoint, but it will have toopen the cooling water valve further than usual to do so.

Answer 9

The one glaring discrepancy we see here is between the laboratory’s measurement of syrup concentrationand what the AIC and AIR indicate. Given that both the AIC and AIR agree with each other on PV value,we may conclude that the signal to both of these instruments corresponds to a 34% measurement. Theproblem is either the transmitter (AT) mis-measuring the syrup concentration, or else it is sensing theconcentration okay but outputting the wrong 4-20 mA signal nonetheless, or else the laboratory made ameasurement error of their own and incorrectly reported a syrup concentration that is too high.

We also see some minor discrepancies between controller output indications and actual valve stempositions, but these are small enough to ignore. Likewise, the discrepancy between the level gauge (LG)indication and the level controller/recorder indications is small enough that it does not pose a serious problem.

Answer 10

There will be no adverse effect resulting from this mis-calibration, unless the valve is unable to achievea full-closed position when required. In such a case, the liquid level will slowly fall below setpoint.

Answer 11

The liquid mixing vessel will either drain empty or overflow, depending on which side of setpoint theprocess variable was on at the time of the mis-configuration.

Answer 12

There will be no effect on the performance of this cooling system, except in circumstances where thecontroller tries to open the valve further than 80%. In those cases, the process temperature will exceedsetpoint.

Answer 13

The syrup’s sugar concentration will eventually become excessive as the analytical controller (AC)attempts to maintain setpoint.

29

Answer 14

The analytical control system should still be able to maintain sugar concentration at setpoint, unlessthe heat exchanger fouling is so extreme that even a wide-open steam valve does not heat the incoming syrupenough to sufficiently concentrate it.

Follow-up question: suppose the heat exchanger fouling really is this bad, but we cannot fix the heatexchanger with the tools we have available. What would you recommend the operator do to make this systemproduce on-spec syrup?

Answer 15

In automatic mode:

Process flow rate (increase) → FT output signal (increase milliamps) → FC output signal (decreasemilliamps) → FY output signal (decrease PSI) → FV position (moves further closed, pinching off liquidflow).

In manual mode:

Process flow rate (increase) → FT output signal (increase milliamps) → FC output signal (remainssteady) → FY output signal (remains steady) → FV position (holds position).

The important part of this question is the difference in response between “automatic” and “manual”controller modes. In automatic control mode, the controller takes action to bring the process back to setpoint.In manual control mode, the controller just lets the process drift and takes no action to stop it.

At first, having a “manual” mode in a control system seems pointless. However, giving human operatorsthe ability to manually override the otherwise automatic actions of a control system is important for start-up,shut-down, and handling emergency (unusual) conditions in a process system.

Manual mode is also a very important diagnostic tool for instrument technicians and operators alike.Being able to “turn off the brain” of an automatic control system and watch process response to manualchanges in manipulated variable (final control element) signals gives technical personnel opportunity to testfor unusual control valve behavior, process quirks, and other behaviors in a system that can lead to poorautomatic control.

Answer 16


LT-12 miscalibrated√

LG-11 block valve(s) shut√

LSH-12 switch failed√

LSL-12 switch failed√

Leak in tubing between LT-12 and LIC-12√

LIC-12 controller setpoint set too high√

LV-12 control valve failed open√

LV-12 control valve failed shut√

Answer 17

We know the indicating controller (TIC-21) must be miscalibrated, because the pneumatic signalpressure of 12.8 PSI agrees with the recorder’s indication of 304 degrees F.

30

Answer 18


SV-115 leaking air√

PSL-105 failed√

PSL-114 failed√

PCV-39 pressure setpoint too low√

PCV-39 pressure setpoint too high√

PCV-40 pressure setpoint too low√

PCV-40 pressure setpoint too high√

ZS-38 failed√

Blind inserted in natural gas header√

Blind inserted in fuel gas header√

Answer 19

Given the fact that the ESD system keeps indicating a high boot level, you know that it “thinks” theliquid level inside the boot is higher than it should be. The next logical step is to determine whether or nota high liquid level condition does indeed exist. If so, the trip is legitimate and there may be a problem withthe liquid level control system. If not, the LSHH-231 or its associated wiring may have a fault that sends afalse trip alarm to the ESD system.

However, the decision to leave the compressor idle for a few hours until your arrival was not a good onefor diagnosis. If indeed there is a problem with excessive liquid collecting in the boot, this would only beevident during running operation. With the compressor idle and no new gas entering the separator vessel,there will be no new liquid collecting in the boot, which will give the boot level control system ample timeto empty that liquid down to a normal level and make it appear as though there is no level problem. Inother words, leaving the compressor idle for a few hours “erases” the evidence, making it more difficult totroubleshoot.

Aside from re-starting the compressor and watching it run, you could perform a test on the liquid levelcontrol system by simulating a high-level condition inside the boot (e.g. applying pressure to one side ofLT-92) and observing how fast or slow the actual liquid drains out (as indicated by LG-93). If there is aproblem with the level control valve LV-92 or its associated components, you should be able to tell in theform of a long (slow) drain time. The fact that the blind flange at the bottom of the boot drain line says “Rodout” on the P&ID suggests this line is prone to plugging with debris, which could explain a slow-drainingcondition and consequently the frequent high-level trips.

Answer 20


PT-33 calibration error√

PY-33a calibration error√

PY-33b calibration error√

PV-33b block valve closed√

PV-33b bypass valve open√

Instrument air supply to PY-33b failed√

Instrument air supply to FV-34 failed√

Answer 21

Place the controller in manual mode and observe the PV trend!

31

Answer 22

Some processes may not take well to “bumps,” especially large bumps. Imagine “bumping” the coolantflow to a nuclear reactor or the fuel flow to a large steam boiler: the results could be catastrophic! Not onlyis it a potential problem to exceed an operating limit (PV too high or too low) in a process, but it may bedangerous to exceed a certain rate of change over time.

Short of catastrophe, unacceptable variations in product quality may result from perturbations of theprocess. Again, these may be functions of absolute limit (PV too high or too low), and/or rates of changeover time.

Remember, the purpose of regulatory control systems is to maintain the PV at or near setpoint. Anytime the control system is disabled and the process purposely “bumped,” this purpose is defeated, if onlymomentarily. It is essential that operations personnel be consulted prior to manually perturbing a process,so that no safety or operating limit is exceeded in the tuning process.

Answer 23

This oscillation is clearly not the result of an over-tuned controller, because it persists even when thecontroller is in manual mode. The source must be coming from somewhere else in the process.

At this point in time, it would be a good thing to note the frequency of this oscillation, and beginsearching for anything that could cause the level to go up and down at this frequency, or perhaps somethingthat could “fool” level transmitter LT-12 into thinking the level is oscillating at this frequency. If thefrequency is relatively high, local machine vibration could be the cause of it. This hypothesis makes a lotof sense, based on the fact that the controller’s action in automatic mode doesn’t seem to be correctingthe oscillations at all: the oscillation amplitude seems to remain unchanged between automatic and manualmodes. This is what we might expect from a vibration-induced oscillation, where the frequency of theoscillation is much faster than the liquid level can possible change, and therefore faster than the level controlsystem can physically compensate.

32