Basic Process Control System

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Basic Process Control System Guidelines
Colin S. Chip Howat Ph.D.
Kurata Thermodynamics Laboratory Department of
Chemical Petroleum Engineering University of
Kansas
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BPCS Guidelines
Lecture Two Class Periods Title Guidelines
for Process Control Thought What keeps you
going isnt some fine destination but just the
road you are on, and the fact that you know how
to drive. Animal Dreams Barbara
Kingsolver Question What equipment ensures
that the process operates the way that it was
intended to operate? Purpose Introduce general
guidelines for establishing control over a
process. Objective 2. Specifications 5.
Ambiguity 6. Review Glossary Manipulated
Variables Controlled Variables Measured
Variables
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Processes do not automatically run at steady
state or follow a prescribed time pattern.
Process Flow Diagram Examples
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What do we control? How do we know it
is controlled?
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Essentially every independent stream in the
process needs to be controlled. Examples of what
we might control are Flow FC for Flow
Controller Temperature TC for Temperature
Controller Pressure PC for Pressure
Controller Composition AC for Composition
(Analytical) Controller pH Density Weight We
may also want alarms associate with the above.
These alarms can represent high or low
conditions. An example is TA for temperature
alarm. On a process flow diagram, our standard
symbol is
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What is the common representation of
an installation on the PI?
BPCS Guidelines
The installation, as you would anticipate, is
substantially more complicated than the symbols
implied on the PFD. While we are most interested
in the PFD representation in Design I, we are
interested in the mechanical installation in
Plant Environmental Safety. The installation
may will include a transmitter (FT), an indicator
(FI), a controller (FC) and a control valve
(FCV). As implied with the discussion of the
alarms, there may also be alarms. There may also
be current to pneumatic converters, positioners,
etc. The valve installation representation on
the PI is given below.
Blocks for Control Valve Isolation
Bleeds to Relieve Pressure
Bypass for Hand Control
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Examples of Control Valve Installations
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BPCS Guidelines
Example of Valve Installations
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How many valves are required to control
this installation?
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A typical distillation tower schematic
Distillation is used to effect a composition
change and, typically, recover a certain
percentage of a component(s). This requires
stable operation.
Upsets?
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Identify possible upsets. Estimate the number of
control valves.
BPCS Guidelines
Identify possible upsets to which the control
system must respond.
Estimate the number of control valves required to
respond to these possible upsets.
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Manipulated variables are used to effect control,
i.e. bring back to set point.
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The flowsheet at left shows the independent
streams that need a control valve. The streams
are manipulated variables. The flows will be
manipulated to control some variable.
What do we measure?
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Measured variables must have some relation to the
desired controlled variable.
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How?
Measured Variables Flow Pressure Composition
Level Temperature Interface (level)
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So, manipulated and measured whats left?
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Manipulated variables are adjusted based on the
deviation of the Measured variables from the set
point. While the Manipulated variables and the
Measured variables might actually be the variable
that we wish to control, they dont need to be.
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So, manipulated and measured whats left?
BPCS Guidelines
As example, if we want to control the feed flow
to a distillation column, we might identify the
flow rate as the controlled variable. We might
also measure it making the feed flow the measured
and controlled variable. Further, to effect
control, we might place a control valve in the
feed line making the feed flow the manipulated
variable. In this case, then, the feed flow is
the controlled, measured and manipulated variable.
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So, manipulated and measured whats left?
BPCS Guidelines
As an alternative, consider composition.
Measurement of composition typically has a long
lag time making it an inappropriate variable for
measurement even though we might want to make it
the controlled variable. We would need to
specify another variable to be the measured one.
This one would need to have some direct relation
to the desired measured variable. Further, we
may not be able to manipulate composition
directly. We may need to manipulate some other
variable, e.g. steam flow, to control composition.
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All we have to play with are material and energy
balances, dont we?
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Its clear that we need to have some measure of
sump level control Steam Bottoms We need some
stream for reflux drum level control Reflux Di
stillate Typically, try to keep feed constant
(but not a requirement) Feed Flow Now, we have
to control the separation
Direct or indirect
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We need a language to which we agree to
adhere. This overrides the usual use of these
terms.
BPCS Guidelines
We need some terms
Controlled Variables
Ex. Variables selected to be kept at certain
values which, if done, indicate that the
process is operating as intended.
Manipulated Variables
Ex. Variables which will be adjusted to keep
controlled variables at set point.
Measured Variables
Ex. Variables which will be measured and are
indicative of the state of the process.
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Are these guidelines requirements?
BPCS Guidelines

• Guidelines Controlled Variables
• Select Variables which arent self-regulating
• Select Variables that may exceed equipment and
  operating constraints
• Select Variables that are a direct measure of
  product quality or that strongly
• affect it
• Select Variables that seriously interact with
  other controlled variables
• Select Variables that have favorable dynamic and
  static characteristics

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BPCS Guidelines

• Guidelines Manipulated Variables
• Select Variables that have large effects on the
  controlled variables
• Select Variables that rapidly affect the
  controlled variables
• Select Variables that have a direct affect on the
  controlled variable
• Select Variables that avoid recycling or
  propagating disturbances

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BPCS Guidelines

• Guidelines Measured Variables
• Select Variables where measurements are reliable
  and accurate
• Select Variable measurement points with adequate
  sensitivity
• Select Variable measurement points that minimize
  time delays/constants

Typically, controlled variables are measured
on-line and used for feedback control. However,
it is possible to control a process variable
using a theoretical mathematical model to
calculate the value of unmeasured controlled
variable.
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Examples for Controlled Variables
BPCS Guidelines
Selection of Controlled Variables

• Select variables that arent self-regulating.
• Select variables that may exceed equipment and
  operating constraints.
• Select variables that are a direct measure of
  product quality or that strongly affect it.

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Examples for Controlled Variables
BPCS Guidelines

• Select variables that seriously interact with
  other controlled variables.
• Choose variables that have favorable dynamic and
  static characteristics.

Remember that above guidelines are not hard and
fast rules. They may be conflicting. The above
is NOT hierarchical.
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Examples for Manipulated Variables
BPCS Guidelines
Selection of Manipulated Variables

• Select variables that rapidly affect the
  controlled variables
• Select variables that rapidly affect the
  controlled variables

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Examples for Manipulated Variables
BPCS Guidelines

• Select variables that have a direct affect on the
  controlled variable
• Select variables that avoid recycling or
  propagating disturbances

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Examples for Measured Variables
BPCS Guidelines
Selection of Measured Variables

• Select variables where measurements are reliable
  and accurate

Proper length orifice runs Temperature probes at
sensitive points Avoid measuring saturated
flow Small delay in sample loops
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Examples for Measured Variables
BPCS Guidelines

• Select variable measurement points with adequate
  sensitivity
• Select variable measurement points that minimize
  time delays/constants

Now, back to distillation.
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BPCS Guidelines
Now consider our distillation tower