Ingenieria

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CHAPTER SEVEN

Loop Diagrams
Overview

The Loop Diagram is surely the most recognizable document used in the instrumentation and
controls field. It is also the most common document, except for perhaps the Specification Form
(instrument data sheet). The popularity of the well-executed Loop Diagram stems from its universal utility. Loop Diagrams are found on designers’ desks, inconstruction trailers, in notebooks
in the maintenance shop, stuffed in the pockets of engineers and maintenance technicians, and
they can even be found littering the bottom of field junction boxes. They are spread throughout
industrial facilities because people use them. Loop Diagrams tell us what components comprise
our control system and how they are interconnected in a stylized but logical andsomewhat elegant fashion. The presentation of information on Loop Diagrams, and, indeed the information
itself, is undergoing some change and development as the systems we connect to change as well.
The ISA dictionary definition of a Loop Diagram is: “A schematic representation of a complete
hydraulic, electric, magnetic or pneumatic circuit.”1 The complete circuit is generally called a
loop.Two definitions of a loop, also from the ISA dictionary, are:
“A combination of one or more interconnected instruments that are arranged to measure
and/or control a process variable…
“All the parts of a control system: the process, sensor(s), transmitter(s), the controller, and
the final control element.”2
These definitions are broad and cover any interconnection between two devices.
The keyconcept in loop definition is
Interconnecting a Single Instrument?
that all the devices in the loop monitor
The authors, like many of you savvy readers, have had a problem
or control a single process variable. A
visualizing what is meant by “one interconnected instrument”. We
simple loop might be a field mounted
suggest the “interconnected” single device assumes interconnection
pressureswitch that illuminates a light
to some other device, perhaps a process control computer, or a
device such as a now dated mag meter which consisted of two interon a local panel when the pressure in a
connected components, a flow element FE and a transmitter FT. This
pipeline is too high or, for you purists,
makes sense since, traditionally, a stand-alone or self-contained instruwhen theprocess pressure is above the
ment like a pressure regulator or a rotameter does not call for its own
switch set point. This switch may reside
Loop Diagram.
on its own loop or it may be included
on the loop that controls the line pressure. A “pipeline” was chosen for this example since it is
common for the pressure control loop to be in another operating unit, or off site. The single elementpressure switch loop is familiar.

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Chapter 7: Loop Diagrams

The most common electronic control loop consists of a transmitter, a controller, an I/P transducer, and a control valve. A more complex electronic
system that is still a single loop includes a transmitter with two or more control
valves with I/P transducers, as long as the valves exist to influence a single
process variable.Figure 7-1 and Figure 7-2 show typical loops.
Figure 7-1: A Pneumatic Loop
PNEUMATIC
CONTROLLER

PIC
100
PNEUMATIC
TRANSMITTER

PT
100
PV
100

CONTROL VALVE
FO

A PNEUMATIC LOOP - CONTROLLING PRESSURE

Figure 7-1 shows a pneumatic loop (PIC 100) controlling pressure in a
pipeline. All signal transmission is pneumatic at 3-15 psig.
Figure 7-2: An Electronic Loop
AN ELECTRONICCONTROL LOOP - CONTROLLING FLOW
FIC
100

ELECTRONIC
CONTROLLER
I

FY
101
ELECTRONIC
TRANSMITTER

P

TRANSDUCER

FT
101

FLOW ELEMENT
ORIFICE PLATE

FV
101

FE
101

FO
CONTROL VALVE

Figure 7-2 is an electronic loop (FIC 101). The transmitter and controller outputs are electronic signals, most commonly 4-20 mA. The transducer (FY 101)
changes the electronic signal...