Process control fundamentals
A manipulated input is one that can be adjusted by the control system (or process
A disturbance input is a variable that affects the process outputs but that cannot be
adjusted by the control system. Inputs may change continuously or at discrete intervals of
Output variables—classify these as (a) measured or (b) unmeasured variables;
measurements may be made continuously or at discrete intervals of time. The output
variable is the variable which is being controlled.
Constraints—classify these as (a) hard or (b) soft.
Any process has certain operating constraints, which are classified as hard or soft. An
example of a hard constraint is a minimum or maximum flow rate—a valve operates
between the extremes of fully closed or fully open. An example of a soft constraint is a
product composition—it may be desirable to specify a composition between certain
values to sell a product, but it is possible to violate this specification without posing a
safety or environmental hazard.
Operating characteristics are usually classified as continuous, batch, or semi-continuous
Continuous processes operate for long periods of time under relatively constant operating
conditions before being “shut down” for cleaning, maintenance, and so forth. For
example, some processes in the oil-refining industry operate for 18 months between
shutdowns. Batch processes are dynamic in nature—that is, they generally operate for a
short period of time and the operating conditions may vary quite a bit during that period
of time. Example batch processes include beer or wine fermentation, as well as many
specialty chemical processes. For a batch reactor, an initial charge is made to the reactor,
and conditions (temperature, pressure) are varied to produce a desired product at the end
of the batch time. A typical semi-batch process may have an initial charge to the reactor,
but feed components may be added to the reactor during the course of the batch run.
Safety, environmental, and economic considerations.
In a sense, economics is the ultimate driving force. An unsafe or environmentally
hazardous process will ultimately cost more to operate, through fines paid, insurance
costs, and so forth. In many industries (petroleum refining, for example), it is important
to minimize energy costs while producing products that meet certain specifications.
Better process automation and control allows processes to operate closer to “optimum”
conditions and to produce products where variability specifications are satisfied. The
concept of “fail-safe” is always important in the selection of instrumentation. For
example, a control valve needs an energy source to move the valve stem and change the
flow; most often this is a pneumatic signal (usually 3–15 psig). If the signal is lost, then
the valve stem will go to the 3-psig limit. If the valve is air-to-open, then the loss of
instrument air will cause the valve to close; this is known as a fail-closed valve. If, on the