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In Control - Considerations for
Part One: Process Parameters
by Dan Capano, DTS, Inc.
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Instrumentation is a vital component of any
plant. The measurement and control of the various processes is
paramount to the proper performance of the plant.
Several key parameters are regularly monitored and controlled:
- Flow: The rate of flow of a fluid or material past a certain
point is measured to control the amount of a fluid or material
to be added to a process. This fluid may be wastewater, raw
drinking water, chemicals, slurries, and/or sludge. Each
material presents different and unique characteristics that
must be dealt with to accurately measure the amount of
material passing through a flowmeter. Flow can be measured
in many different ways, each method having advantages and
disadvantages over another. The proper choice and application
of flowmeters allows for maximum accuracy and efficiency.
- Level: The amount of material in tanks and other vessels is
important for many reasons. Chemical storage tank level must
be constantly monitored to avoid spillage from over-filling or
for maintaining a required supply of the chemical. Also, many
processes, such as sequential batch reactors (SBR), rely on accurate measurement of level
to perform the proper process sequences. Protection of pumping
equipment is also an important application for level
measurement. Level measurement instruments are available using
a variety of measurement techniques. Material characteristics
and cost are the major determining factors in the
specification and application of level measurement
- Pressure: Pumps and plumbing are rated to the maximum
pressure sustainable. When the pressure of a material within
some vessel falls outside of a specified range of pressures,
equipment is prone to inefficient operation, improper
operation and failure. Excessively high or low pressure is
known to cause catastrophic equipment failure, and in some
cases, injury to personnel. Pressure is also used to determine
level, especially in large storage tanks.
- Temperature: Process temperature is among the most common
and important parameters controlled by instrumentation. Many
processes require heat for proper operation and must be
precisely controlled in many cases. Many options are available
for temperature measurement and control. Each method presents
good and bad points. Some processes involving extremely high
temperatures can only use specialized non-contact types of
measurement, such as infra-red, to measure and control
- Dissolved Oxygen: DO is a crucial parameter used to control
the common treatment process known as "activated sludge". In this process, "bugs" are grown in
order to digest organic material and make it more "settle-able".
The predominant method of this parameter is done using a
membrane-type sensor, which is either floating or immersed in
the process fluid. The proper application and maintenance of
these sensors allows significant cost reductions in their use.
- Oxidation-Reduction Potential (ORP): ORP is an indicator of
the state of a process involving oxidation-reduction. A
familiar example is the application of chlorine to water and
wastewater for disinfection. While it is possible to
accurately control the application of chlorine using this
parameter, Newer and specialized chlorine sensors perform the
task more efficiently. Processes where several chemical
components are being mixed and are reacting dynamically, such
as in the sump of a packed column, require the continuous
response of an ORP instrument to allow precise control of the
- pH: This is another process parameter which is commonly
monitored. Many processes will operate in a very narrow range
of pH while others rely on the ability of the operator or
control system to dynamically manipulate process pH. An
example of such a process is the removal of metals by
precipitation. Understanding pH and it's effects is crucial to
effective process control
These parameters are the most common among many different
types that may characterize a process. Other familiar process
parameters are: alkalinity, turbidity, density, viscosity, mass
flow, weight, relative humidity, the concentration of specific
chemicals or reagents; there are countless others. Our preliminary
discussions will involve the theory behind the various parameters
and the methods for measuring and controlling them.
The second part of the equation is the methods by which a
parameter is controlled. The point of measurement can be
considered the start of the measurement and control process,
control action the end. In between, many operations can take
place, depending on the process and the desired results. Many
processes rely on dynamic controllers to allow superior and
precise control, resulting in high quality end products. Other
processes are much more simple, being of the mostly repetitive
type which would otherwise require human intervention, leaving the
end-product subject to the vagaries and unpredictability of human
error. In both cases, control systems can be as simple or
sophisticated as desired.
Simple Control Systems
The simplest systems consist of a single monitoring device,
which directly drives a controlling device. A good example would
be a flow-control system consisting of a magnetic flowmeter and a
single loop controller, and a control valve. A flow set-point is
entered into the loop controller, which obtains actual flow data
from a magmeter.
The controller will compare the actual flow
rate to the desired flow rate and generate an error signal,
assuming the flow is not at the desired rate. This error signal is
processed and applied to the control valve, which either opens or
closes, proportionately, as the situation dictates. This is basic
closed loop control, so called because of the use of an active
measurement point providing "feedback" to the system.
Another type of system is a chemical feed system that adds
a chemical to a process based upon another parameter without relying
on the use of feedback. The chemical is added according to a
pre-determined formula, or proportionately. A good example would
be the addition of chlorine to plant effluent. If the strength of
the effluent is well known, a pump may be set to "pace"
the injection of chlorine based on flow rate. This method does not
allow for any variation in the strength of the wastewater, so
these systems will consistently over- and under-dose the effluent.
These systems are known as "feed-forward", or
Fine Tuning Control Systems
Within each system, several control settings are used to
fine-tune the response of the control system. The most common
tuning parameters are Proportional Band, Integral, and Derivative,
collectively known as "PID". Some controllers provide expanded
capabilities such as logic operations, ramping and profiling. Briefly:
Proportional Band is the most commonly used tuning parameter.
Proportional Band is simply the range around a set point which will cause a
0-100% output from the controller. It is a "proportional response"
to the variation of a process parameter around the set-point and
controls the rate of the "corrective response".
Integral or Rate: This is the amount of time, usually in seconds,
that a controller performs corrective action. This means that if a
controller has a setting of 0.01seconds, a corrective change will occur,
if required, every 0.01 seconds. This feature is useful for very
fast processes. Derivative is not commonly used, but is useful for
improving control response in fast processes. Derivative acts as an anticipatory response to a rapidly
Next month: Methods of Flow
Measurement for the Liquid Phase.
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