Chemical Engineering October 2014 - 36

Cover Story
Feature Report
Part 1
Pressure Transmitter Basics:
Selection Guidelines
Climbing the decision
tree to pick the right
pressure sensor
Wally Baker
Emerson Process Management
W
hat are the steps to finding
the best pressure
sensor for a particular
application? That's a challenge
faced by many a young engineer
just out of school. Engineering
school teaches a great deal of
theory, but what is the best way to
choose when so many things that
affect the decision are not obvious,
and not necessarily part of the formal
curriculum?
This article reviews a series of
questions and other considerations
that help guide the engineer in
making critical decisions regarding
pressure applications. Also, the
basic principles of pressure measurement
are provided in the box
on p 35.
Why measure pressure?
The first question is the most
basic: What is the purpose of the
measurement? There are generally
three reasons for taking a pressure
measurement: monitoring,
control and safety. While much of
the same hardware is used in all
three, there are significant differences.
The purpose of a monitoring
point is to keep the operator
informed, but not to close a control
loop; this is the least critical of the
three. A control point provides an
input to a control system, and requires
the greatest accuracy, while
for a safety point the most important
parameters are reliability and
keeping users safe.
Emerson Process
Management
Figure 1. Pressure transmitters are
supplied in many configurations
What pressures are involved?
An important question to ask early
on is, what is the normal working
pressure? That range of pressure
should fall within the device's most
accurate range. What are the highest
and lowest pressures expected
during normal operation? Accuracy
can generally degrade somewhat at
the extremes, but the device must
remain repeatable under these conditions,
and must not suffer any
damage, nor require recalibration.
It is also vital to know the maximum
pressure the device will experience,
for safety reasons. It must be
able to withstand a pressure as high
as the pressure rating of the vessel
or pipe to which it is attached without
bursting - and that means all
attached piping, flanges manifolds
and other accessories that will be
exposed to pressure must be rated to
a minimum threshold. Some devices
can continue to operate after such an
overpressure incident and retain accuracy,
while others may need recalibration
or even replacement, but the
primary consideration here is safety.
34 ChemiCal engineering www.Chemengonline.Com oCToBer 2014
What accuracy is required?
Control applications generally need
more accuracy than monitoring applications,
while, with safety applications,
reliability is key. The exact
level of accuracy needed depends
entirely on the needs of the process
and application being measured.
How to connect to process?
Because pressure transmitters
are also used in flow, level or pressure
applications, they are often
integrated with other components
(Figure 1). An in-line mounted
transmitter (Figure 2) has a single
connection to the process (for gage
or absolute pressure) at the bottom
of the unit. An in-line mounted
transmitter is light in weight and
may require no mounting bracket.
A coplanar mounted transmitter
(Figure 3) has two process connections
for differential pressure
(DP) on the bottom of the unit.
This transmitter is light in weight
and is installed on a single process
flange. This connection type
is more modern than the biplanar
http://www.Chemengonline.Com
Chemical Engineering October 2014

Table of Contents for the Digital Edition of Chemical Engineering October 2014
