Control Loop Hardware and Troubleshooting

Chapter 2

Control Loop Hardware and Troubleshooting

Overall Course Objectives

• Develop the skills necessary to function as an industrial process control engineer.– Skills

• Tuning loops

• Control loop design

• Control loop troubleshooting

• Command of the terminology

– Fundamental understanding• Process dynamics

• Feedback control

Control Relevant Aspects of Control Loop Hardware

• Necessary for control loop troubleshooting:– To determine if each subsystem (control

computer, actuator system, and sensor system) is functioning properly

– To understand the proper design and operation of all the components that make-up each of the subsystems of a control loop

Control Diagram of a Typical Control Loop

Controller

F1

T1

TF

F2

T2

TC

ActuatorSystem

TT

SensorSystem

Components and Signals of a Typical Control Loop

TF

F2

T2

Thermocouplemillivolt signal

Transmitter4-20 mADCS

ControlComputer

3-15 psig

4-20 mA

OperatorConsole

Tsp

I/PAir

F1

T1

Thermowell

Actuator System

Controller Sensor System

D/A

A/D

Controllers/Control Computers

• Pneumatic controllers

• Electronic analog controllers

• Supervisory control computers

• Distributed Control Systems (DCS)

• Fieldbus technology

Pneumatic Controllers - Phase I

• Introduced in the 1920’s

• Installed in the field next to the valve

• Use bellows, baffles, and nozzles with an air supply to implement PID action.

• Provided automatic control and replaced manual control for many loops

Pneumatic Controllers - Phase II

• Transmitter type pneumatic controllers began to replace field mounted controllers in the late 1930’s.

• Controller located in control room with pneumatic transmission from sensors to control room and back to the valve.

• Allowed operators to address a number of controllers from a centralized control room.

Pneumatic Controller Installation

TransmitterPnuematicController

3-15 psig

Air

3-15 psig Air

Thermowell

F1

T1

F2

T2

FT


Tsp

Electronic Analog Controllers

• Became available in the late 1950’s.

• Replaced the pneumatic tubing with wires.

• Used resistors, capacitors, and transistors based amplifiers to implement PID action.

• Out sold pneumatic controllers by 1970.

• Allowed for advanced PID control: ratio, feedforward, etc.

Electronic Controller Installation

TransmitterElectronicController

3-15 psig

Air

4-20 mA

Thermowell

F1

T1

F2

T2

FT


Tsp

4-20 mA

I/P

Computer Control System

• Based upon a mainframe digital computer.

• Offered the ability to use data storage and retrieval, alarm functions, and process optimization.

• First installed on a refinery in 1959.

• Had reliability limitations.

Supervisory Control Computer

PrinterVideo Display

Unit

InterfacingHardware

AnalogControl

Subsytem

AlarmingFunctions

Supervisory Control Computer

Data StorageAcquisition

System

...

Distributed Control System- DCS

• Introduced in the late 1970’s.

• Based upon redundant microprocessors for performing control functions for a part of the plant. SUPERIOR RELIABILITY

• Less expensive per loop for large plants.

• Less expensive to expand.

• Facilitates the use of advanced control.

DCS Architecture

Process Transmitters and Actuators

Data Highway(Shared Communication Facilities)

......

DataStorage Unit

HostComputer

SystemConsoles

PLC

4-20 mA

LocalConsole

LocalControl

Unit

4-20 mA

LocalControl

Unit

LocalConsole

DCS and Troubleshooting

• The data storage and trending capability of a DCS greatly facilitate troubleshooting control problems. That is, the sources of process upsets can many times be tracked down through the process by trending a group of process measurements until the source of the process upset is located.

Control Relevant Aspects of a DCS

• The most important control aspect of a DCS is the cycle time for controller calls. The shortest cycles times are typically around 0.2 seconds while most loops can be executed every 0.5 to 1.0 seconds. These cycle times affect flow control loops and other fast control loops.

PLCs

• PLCs can withstand has industrial enviroments.

• PLCs are used for discrete and continuous control.

• Discrete control is used for startup and shutdown and batch sequencing operations.

• Ladder logic is used to program PLCs.

PLCs vs. DCSs

• Advantage of PLCs:– Better to withstand harsh operating enviroments,

faster cycle time are possible, easier to maintain due to modular nature and lower cost for small and medium sized applications.

• Advantage of DCSs:– Lower cost per loop for applications involving a

large number of control loops.

PLC Architecture

ProcessorPowerSupply

Data Highway

PLC Cabinet

ProgrammingInterface

I/O Modules

InputDevices

OutputDevices

Fieldbus Technology

• Based upon smart valves, smart sensors and controllers installed in the field.

• Uses data highway to replace wires from sensor to DCS and to the control valves.

• Less expensive installations and better reliability.

• Can mix different sources (vendors) of sensors, transmitters, and control valves.

• Now commercially available and should begin to replace DCSs.

Fieldbus Architecture

Plant Optimization

.................

SmartSensors

Smart ControlValves andControllers

LocalArea

Network

SmartSensors

Smart ControlValves andControllers

LocalArea

Network

H1 Fieldbus Network H1 Fieldbus Network

H1 Fieldbus H1 Fieldbus

Data Storage

PLCs

High Speed Ethernet

Actuator System

• Control Valve– Valve body– Valve actuator

• I/P converter

• Instrument air system

Typical Globe Control Valve

Cross-section of a Globe Valve

Types of Globe Valves

• Quick Opening- used for safety by-pass applications where quick opening is desired

• Equal Percentage- used for about 90% of control valve applications since it results in the most linear installed characteristics

• Linear- used when a relatively constant pressure drop is maintained across the valve

Inherent Valve Characteristics

0

0.5

1

0 20 40 60 80 100Stem Position (% Open)

f(x)

=%

QOLinear

Use of the Valve Flow Equation

Given: water as the fluid; 16psig; 5

Determine the flow rate through this control

valve. Using Equation 2.3.3,

/ . . (1)(5) 16 /1 20 gpm

v

V v

P C

F KC p s g

Typical Flow System

C.W.

FT

Pressure Drop vs. Flow Rate

0

5

10

15

20

25

0 50 100 150 200Flow Rate (GPM)

Pre

ssur

e D

rop

(psi

)

Line Losses

Pump Head

Valve P

Installed Flow Characteristic

0

50

100

150

200


Inst

alle

d F

low

Rat

e (G

PM

)

Linear Valve

=% Valve

Slope of Installed Characteristic

0

12

34

56

7


Slo

pe o

f th

e In

stal

led

Flo

w C

hara

cter

isti

c Linear Valve

=% Valve

Effect of Linearity in the Installed Valve Characteristics

• Highly nonlinear installed characteristics can lead to unstable flow control or a sluggish performance for the flow controller.

Flow System with Relatively Constant Valve Pressure Drop

30 ftFT

Pressure Drop vs. Flow Rate

0

5

10

15

0 100 200 300 400 500 600Flow Rate (GPM)

Pre

ssur

e D

rop

(psi

)

Line Losses

Valve P

Hydrostatic Head

Installed Valve Characteristics

0

100

200

300

400

500

600


Inst

alle

d F

low

Rat

e (G

PM

) Linear Valve

=% Valve

Analysis of These Examples

• Note the linear installed valve characteristics over a wide range of stem positions.

• If the ratio of pressure drop across the control valve for the lowest flow rate to the value for the highest flow rate is greater than 5, an equal percentage control valve is recommended.

Control Valve Design Procedure• Evaluate Cv at the maximum and minimum flow

rate using the flow equation for a valve (Eq 2.3.3).• Determine which valves can effectively provide

the max and min flow rate remembering that, in general, 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.

• Choose the smallest valve that meets the above criterion for the minimum capital investment or choose the largest valve to allow for future throughput expansion.

Additional Information Required to Size a Control Valve

• 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.

Valve Sizing Example

• Size a control valve for max 150 GPM of water and min of 50 GPM.

Determine CV at Max and Min FV

• Use the valve flow equation (Equation 2.3.3) to calculate Cv

• For P, use pressure drop versus flow rate (e.g., Table on page 82)

max min

( )/

150 5028.9; 9.1

27 /1 30 /1

mv

v v

FC x

K P

C C

Valve Position for Max and Min Flows for Different Sized Valves

Max flow Min flow

1-inch valve not large enough 75%

1.5-inch valve not large enough 68%

2-inch valve 67% 45%



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)

Valve Deadband

• It is the maximum change in instrument air pressure to a valve that does not cause a change in the flow rate through the valve.

• Deadband determines the degree of precision that a control valve or flow controller can provide.

• Deadband is primarily affected by the friction between the valve stem and the packing.

For Large Diameter Lines (>6”), Use a Butterfly Valve

Valve Actuator Selection

• Choose an air-to-open for applications for which it is desired to have the valve fail closed.

• Choose an air-to-close for applications for which it is desired to have the valve fail open.

Cross-section of a Globe Valve

Optional Equipment

• Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position. Greatly reduces the deadband of the valve. Positioners are almost always used on valves serviced by a DCS.

• Booster relay- provides high capacity air flow to the actuator of a valve. Can significantly increase the speed of large valves.

Photo of a Valve Positioner

Adjustable Speed Pumps

• Used extensively in the bio-processing industries (better 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.

Control Relevant Aspects of Actuator Systems

• The key factors are the deadband of the actuator and the dynamic response as indicated by the time constant of the valve.

• Control valve by itself- deadband 10-25% and a time constant of 3-15 seconds.

• Control valve with a valve positioner or in a flow control loop- deadband 0.1-0.5% and a time constant of 0.5-2 seconds.

Control Loop Hardware and Troubleshooting

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