CHE 185 – PROCESS CONTROL AND DYNAMICS

OPTIMIZATION AND PRIMARY LOOP ELEMENTS

TYPES OF CONTROL

• OPTIMIZATION– INTENTION IS TO GET THE “BEST”

ECONOMIC/QUALITY CONDITIONS– CONTROL IS THE MORE GENERAL FORM

OF RESPONDING TO CHANGE

CONTROL & OPTIMIZATION

• FOR A BIOREACTOR

http://www.automation.siemens.com/wcmsnewscenter/details.aspx?xml=/content/10001666/en/as/Pages/PN-200201-06-Alles_unter_Kontrolle.xml?NoRedirect=true&xsl=publication-en-www4.xsl

EXAMPLE OF OPTIMIZATION

• OPTIMIZATION AND CONTROL OF A CSTR

EXAMPLE OF OPTIMIZATION

• ECONOMIC OBJECTIVE FUNCTION

• ECONOMIC VALUES , VB > VC, VA, OR VAF

• AT LOW T, LITTLE FORMATION OF B• AT HIGH T, TOO MUCH OF B REACTS TO

FORM C• SO THERE EXISTS AN OPTIMUM REACTOR

TEMPERATURE, T*

AFACCBBAA VCQVCQVCQVCQ 0

OPTIMZATION EXAMPLE

• METHOD OF SOLUTION • 1. SELECT INITIAL GUESS FOR

REACTOR TEMPERATURE, T*• 2. EVALUATE CA, CB, AND CC

• 3. EVALUATE F• 4. CHOOSE NEW REACTOR

TEMPERATURE AND RECYCLE THROUGH STEP 2 UNTIL T* IS IDENTIFIED.

EXAMPLE OF OPTIMIZATION

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EXAMPLE OF OPTIMIZATION• GRAPHICAL SOLUTION OF OPTIMUM

REACTOR TEMPERATURE, T*

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OTHER TYPES OF CONTROL

• SUPERVISORY– RESPONDS TO THE SIGNAL FROM THE

OPTIMIZATION CONTROLLER– SENDS A SIGNAL TO THE REGULATORY

CONTROL LOOP

TYPES OF CONTROL

• REGULATORY– RECEIVES A SIGNAL FROM THE

SUPERVISORY CONTROLLER– ACTUALLY ADJUSTS A PROCESS

VARIABLE TO MAKE NECESSARY CHANGES FOR OPERATION.

REGULATORY CONTROL EXAMPLE

• TEMPERATURE CONTROL FOR A HEAT EXCHANGER

TT

Condensate

Steam

Feed

TCProductStream

Setpoint

CATEGORIES OF LOOP COMPONENTS

• FEEDBACK LOOP THE PRIMARY COMPONENTS ARE SHOWN SCHEMATICALLY

LOOP COMPONENTS - SENSORS

• MEASURE THE VALUE OF THE TARGET AND MANIPULATED VARIABLES– TYPICALLY P, T, L, D, ρ,Μ, COMPOSITION,

MASS– LOCATED “IN-LINE”, EVEN IF THEY DO

NOT CONTACT PROCESS FLUIDS

LOOP COMPONENTS - SENSORS

• TYPICALLY ANALOG DEVICES WITH FULL SCALE RANGES:– 20 MA CURRENT SIGNAL– mV VOLTAGE SIGNAL– 3 - 15 PSIG PNEUMATIC SIGNAL

LOOP COMPONENTS - TRANSMITTERS

• ANALOG DEVICES– CONVERT SIGNAL INTO A VALUE THAT

CAN BE TRANSMITTED WITHOUT SIGNIFICANT LOSS IN VALUE

– TYPICAL OUTPUT IS 4 - 20 MA CURRENT FULL SCALE RANGE

LOOP COMPONENTS - TRANSDUCERS

• CONVERT THE ANALOG SIGNAL FROM THE TRANSMITTER INTO A DIGITAL SIGNAL– LABELED A/D FOR ANALOG TO DIGITAL– LABELED D/A FOR DIGITAL TO ANALOG

• CAN TRANSFER ELECTRONIC TO PNEUMATIC

LOOP COMPONENTS - CONTROLLERS

• RECEIVES DIGITAL SET/POINT AND MEASURED SIGNALS FOR A VARIABLE

• MAKES A COMPARISON BETWEEN THE SIGNALS USING A BRIDGE

• PRODUCES A DIGITAL OUTPUT SIGNAL TO ADJUST THE MEASURED VALUE TO THE SET/POINT VALUE

LOOP COMPONENTS - ACTUATORS

• RECEIVES THE OUTPUT SIGNAL FROM THE CONTROLLER, VIA THE TRANSDUCER

• ADJUSTS THE POSITION OF A DEVICE (FINAL CONTROL ELEMENT) TO CHANGE A PROCESS VARIABLE

LOOP COMPONENTS - FINAL CONTROL ELEMENT

• CHANGES A PROCESS VARIABLE TO OBTAIN A CORRECTION TO THE MEASURED VARIABLE

• FOR CHEMICAL PLANTS, MOST OF THE TIME THESE DEVICES ARE VALVES

SOURCE OF SET/POINTS

• FOR A SIMPLE PROCESS, THESE MAY BE ENTERED MANUALLY

• FOR CONTEMPORARY SYSTEMS, THESE SIGNALS COME FROM A COMPUTER– PLC - PROGRAMMABLE LOGIC

CONTROLLER– DCS COMPUTER - DISTRIBUTED

CONTROL SYSTEM COMPUTER

DCS FUNDAMENTALS

• DCS HAS THE FOLLOWING GENERAL COMPONENTS– LOCAL CONTROLLERS– INPUT/OUTPUT PANELS (I/O PANELS) TO

RECEIVE PROCESS VARIABLE VALUES FROM THE CONTROLLER AND SEND SET/POINT VALUES

– DATA HIGHWAY/MULTIPLEXER TO SAMPLE THE VALUES ON A REGULAR FREQUENCY

DATA HIGHWAY/MULTIPLEXER

DCS FUNDAMENTAL COMPONENTS

• HOST COMPUTER TO ENTER SET/POINTS

• ALARM COMPUTER - PLC’S TO INITIATE ALARMS AND/OR INTERLOCKS

• DATA STORAGE COMPUTER– KEEPS TREND DATA– LOGS ALARMS– OTHER COMPUTERS THAT HAVE ACCESS

FINAL CONTROL ELEMENT CHARACTERIZATION - VALVES

• VALVES ARE USED FOR EITHER ON/OFF OR THROTTLING OPERATION - SEE PERRY’S 7TH PAGE 8-64

• ON/OFF VALVE CHARACTERISTICS– HAVE A COMPLETELY OPEN AND

COMPLETELY CLOSED POSITION– HAVE A TIGHT SHUT-OFF CAPABILITY– HAVE LIMITED VALUE FOR THROTTLING

PLUG VALVES AND BALL VALVES

• 90° RANGE FROM OPEN TO CLOSED• USE PNEUMATIC OR MOTOR

ACTUATORS

GATE VALVES

• TYPICALLY USED FOR ISOLATION VALVES - FOR MAINTENANCE AND AS BACK-UP FOR CLOSED THROTTLING VALVES

GATE VALVES

• MAY HAVE SEVERAL TURNS BETWEEN OPEN AND CLOSED

• MAY HAVE MOTOR OPERATORS – BUT MORE FREQUENTLY USE MANUAL OPERATION

THROTTLING (CONTROL) VALVES• VARIOUS FLOW CHARACTERISTICS BASED ON THE

SHAPE OF THE PLUG AND THE SEAT• FLOW THROUGH THE VALVE DEPENDS ON THE

FRACTION OPEN AND THE NET PRESSURE DROP• HAVE COMPLETELY OPEN AND COMPLETELY

CLOSED POSITION, BUT DESIGNED FOR INTERMEDIATE POSITION FOR CONTINUOUS OPERATION - NOT TIGHT SHUT OFF

• GENERAL REFERENCE - HTTP://WWW.DOCUMENTATION.EMERSONPROCESS.COM/GROUPS/PUBLIC/DOCUMENTS/BOOK/CVH99.PDF

GLOBE (THROTTLING) CONTROL VALVE

CROSS-SECTION OF A GLOBE VALVE

THROTTLING (CONTROL) VALVES

• BASIC FLOW EQUATION IS THE SAME AS FOR AN ORIFICE:

• CV IS SIMILAR TO AN ORIFICE COEFFICIENT, HOWEVER IT VARIES WITH VALVE POSITION

• K IS EQUAL TO UNITY WHEN IS EXPRESSED AS SPECIFIC GRAVITY, FLOWRATES ARE IN gpm AND PRESSURE DROP IS IN psi

GLOBE VALVE TYPES

• QUICK OPENING- FOR SAFETY BY-PASS APPLICATIONS WHERE QUICK OPENING IS DESIRED

• EQUAL PERCENTAGE- FOR ABOUT 90% OF CONTROL VALVE APPLICATIONS SINCE IT RESULTS IN THE MOST LINEAR INSTALLED CHARACTERISTICS

• LINEAR- WHEN A RELATIVELY CONSTANT PRESSURE DROP IS MAINTAINED ACROSS THE VALVE

GLOBE VALVE FLOW CHARACTERISTICS

GLOBE VALVES - LINEAR

• PLUGS/SEATS DESIGNED TO HAVE THE FRACTION OF FLOW BE LINEAR WITH THE VALVE STEM POSITION FOR A CONSTANT Δp

• f(x) = x WHERE x IS VALVE POSITION • DESIGN POSITION IS ABOUT 50%

OPEN

GLOBE VALVES - EQUAL PERCENTAGE

• EQUAL PERCENTAGE CHANGE IN VALVE POSITION RESULT IN EQUAL PERCENTAGE CHANGE IN FLOW

• AND x IS VALVE OPENING • DESIGN POSITION IS ABOUT 70%

OPEN

GLOBE VALVES - QUICK OPENING

• TRIM IS DESIGNED TO HAVE LARGE INCREASE IN FLOW WITH SMALL CHANGE IN VALVE OPENING

• DESIGN POSITION IS OPEN OR CLOSED

GLOBE VALVES - OTHER TYPES• ECCENTRIC PLUG VALVES

– COMBINATION OF PLUG AND GLOBE IN THEIR CHARACTERISTICS

– CLAIM TO BE TIGHT SHUT OFF

– REFERENCE:

http://www.millikenvalve.com/pdf/plug2011.pdf

CHARACTERIZATION OF CONTROL ELEMENTS

• TIME CONSTANTS– TIME IT TAKES FOR A UNIT TO RESPOND

TO A SIGNAL AND COMPLETE A CHANGE– FOR VALVES THIS IS THE TIME TO

STROKE TO A NEW POSITION• DEADBAND

– RANGE OF SIGNAL THAT REQUIRED TO INDICATE AN ACTUAL CHANGE

CHARACTERIZATION OF CONTROL ELEMENTS

• POSITION– DEADBAND– RANGE OF SIGNAL THAT REQUIRED TO

INDICATE AN ACTUAL CHANGE– FOR THERMOCOUPLES THIS MIGHT BE +1

C.

CHARACTERIZATION OF CONTROL ELEMENTS

• TURNDOWN RATIO– SPECIFIES THE RANGE OF STABLE

OPERATION FOR THE DEVICE, MINIMUM TO MAXIMUM

– NORMAL OPERATING RANGE SHOULD NOT BE AT EITHER EXTREME POSITION

CONTROL VALVE DESIGN

• EVALUATE CV AT THE MAXIMUM AND MINIMUM FLOW RATE

• DETERMINE WHICH VALVES CAN EFFECTIVELY PROVIDE THE MAX AND MIN FLOW RATE– THE VALVE POSITION SHOULD BE GREATER

THAN ABOUT 15% OPEN FOR THE MINIMUM FLOW RATE

– AND LESS THAN 85% OPEN FOR THE MAXIMUM FLOW RATE.

CONTROL VALVE DESIGN

• CHOOSE– SMALLEST VALVE THAT MEETS THE RANGE CRITERION

FOR THE MINIMUM CAPITAL INVESTMENT– THE LARGEST VALVE TO ALLOW FOR FUTURE

THROUGHPUT EXPANSION.

• CV VERSUS % OPEN FOR DIFFERENT VALVE SIZES.

• AVAILABLE PRESSURE DROP ACROSS THE VALVE VERSUS FLOW RATE FOR EACH VALVE. NOTE THAT THE EFFECT OF FLOW ON THE UPSTREAM AND DOWNSTREAM PRESSURE MUST BE KNOWN.

http://www.norriseal.com/files/comm_id_47/Valve_Size_Manual.pdf

CONTROL VALVE DESIGN EXAMPLE

• SIZE A CONTROL VALVE FOR MAX 150 GPM OF WATER AND MIN OF 50 GPM.

• USE THE VALVE FLOW EQUATION TO CALCULATE CV

• FOR DP, USE PRESSURE DROP VERSUS FLOW RATE (E.G., TABLE ON PAGE 82)

CONTROL VALVE DESIGN EXAMPLE

• EQUATION AT LIMITS

VALVE POSITION FOR MAX AND MIN FLOWS BASED ON SIZE

ANALYSIS OF RESULTS

• 2-INCH VALVE APPEARS TO BE BEST OVERALL CHOICE: LEAST EXPENSIVE CAPITAL AND IT CAN PROVIDE UP TO A 50% INCREASE IN THROUGHPUT.

• 3-INCH AND 4-INCH VALVE WILL WORK, BUT NOT RECOMMENDED BECAUSE THEY WILL COST MORE TO PURCHASE. THE 2-INCH VALVE WILL PROVIDE MORE THAN ENOUGH EXTRA CAPACITY (I.E., SOMETHING ELSE WILL LIMIT CAPACITY FOR IT)

ADJUSTABLE SPEED PUMPS FOR FLOW CONTROL

• USED EXTENSIVELY IN THE BIO-PROCESSING INDUSTRIES TO MAINTAIN STERILE CONDITIONS AND RELATIVELY LOW FLOW RATES.

• FAST AND PRECISE.• DO REQUIRE AN INSTRUMENT AIR SYSTEM

(I.E., 4-20 MA SIGNAL GOES DIRECTLY TO PUMP).

• MUCH HIGHER CAPITAL COSTS THAN CONTROL VALVES FOR LARGE FLOW RATE APPLICATIONS.