Transcript Chapter 2

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
Actuator
System
F1
F2
T1
T2
Sensor
System
Controller
T
F
TC
TT
Components and Signals of a
Typical Control Loop
Actuator System
F1
F2
T1
T2
Thermowell
3-15 psig
T
F
Air
I/P
Operator
Console
4-20 mA
Tsp
D/A
DCS
Control
Computer
Controller
Thermocouple
millivolt signal
A/D
4-20 mA
Transmitter
Sensor System
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
F1
F2
T1
T2
Thermowell
3-15 psig
F
T
Thermocouple
millivolt signal
Air
Tsp
Pnuematic
Controller
3-15 psig
Transmitter
Air
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
F1
F2
T1
T2
Thermowell
3-15 psig
Air
F
T
I/P
Thermocouple
millivolt signal
4-20 mA
Tsp
Electronic
Controller
4-20 mA
Transmitter
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
Vide o Displ ay
Un i t
Alarm i n g
Fu n cti on s
Pri n te r
S u pe rvi sory C on trol C om pu te r
An alog
C on trol
S u bsyte m
...
In te rfacin g
Hardware
Data S torage
Acqu i si ti on
S yste m
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
System
Consoles
Host
Computer
Data
Storage Unit
PLC
Data Highway
(Shared Communication Facilities)
Local
Console
Local
Control
Unit
......
4-20 mA
Local
Control
Unit
4-20 mA
Process Transmitters and Actuators
Local
Console
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
Programming
Interface
PLC Cabinet
Power
Supply
Processor
Data Highway
I/O Modules
Input
Devices
Output
Devices
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
Data Storage
Plant Optimization
High Speed Ethernet
PLCs
Local
Area
Network
.................
H1 Fieldbus
Smart
Sensors
Local
Area
Network
H1 Fieldbus
Smart Control
Valves and
Controllers
H1 Fieldbus Network
Smart
Sensors
Smart Control
Valves and
Controllers
H1 Fieldbus Network
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
f(x)
1
QO
0.5
Linear
=%
0
0
20
40
60
80
Stem Position (% Open)
100
Use of the Valve Flow Equation
Given: water as the fluid; P  16 psig; Cv  5
Determine the flow rate through this control
valve. Using Equation 2.3.3,
FV  KCv p / s.g.  (1)(5) 16 /1  20 gpm
Typical Flow System
C.W.
FT
Pressure Drop vs. Flow Rate
Pressure Drop (psi)
25
Pump Head
20
Valve  P
15
10
Line Losses
5
0
0
50
100
150
Flow Rate (GPM)
200
Installed Flow Rate
(GPM)
Installed Flow Characteristic
200
Linear Valve
150
=% Valve
100
50
0
0
20
40
60
80
Stem Position (% Open)
100
Slope of the Installed
Flow Characteristic
Slope of Installed Characteristic
7
6
5
4
3
2
1
0
Linear
Valve
=% Valve
0
20
40
60
80
Stem Position (% Open)
100
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
FT
30 ft
Pressure Drop vs. Flow Rate
Pressure Drop (psi)
15
Hydrostatic Head
Valve  P
10
5
Line Losses
0
0
100
200 300 400
Flow Rate (GPM)
500
600
Installed Valve Characteristics
Installed Flow Rate
(GPM)
600
500
400
Linear Valve
300
200
100
=% Valve
0
0
20
40
60
80
Stem Position (% Open)
100
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)
Cv ( x) 
max
v
C
K
Fm
P / 
150
50
min

 28.9; Cv 
 9.1
27 /1
30 /1
Valve Position for Max and Min
Flows for Different Sized Valves
Max flow
Min flow
not large enough
75%
1.5-inch valve not large enough
68%
2-inch valve
67%
45%
3-inch valve
4-inch valve
55%
47%
30%
22%
1-inch valve
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., 420 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.