Chemical Engineering December 2016 - 61

Feature Report
Pressure Measurement:
Handling Difficult
Process Applications
A better understanding of what pressure instruments do and how they work can help improve
measurement performance, especially in troublesome applications
E
ffective use of process
instrumentation
is critical
in any chemical
manufacturing environment.
These instruments
are strategically located
throughout various units
and measure all types of
process variables, and are
the operators' eyes and
ears to the world inside
the vessels and pipes. Ensuring
good performance
requires careful selection,
proper installation and ongoing
maintenance.
These concepts apply to
1. Pressure is applied to flexible isolating
diaphragms
2. Pressure is transferred from diaphragms to
sensor via internal oil-filled system
3. Sensor and electronics board convert pressure
readings into an electrical signal
4. Transmitter electronics convert electrical signal
into required output protocol as well as perform
additional diagnostics and functionality
5. Pressure reading is sent to control system via
analog, bus or wireless technology
IN BRIEF
WHAT IS A PRESSURE
INSTRUMENT?
THE TRANSMITTER
FUNCTION INFLUENCES
APPLICATION
IMPULSE LINES
FIGURE 1. This is a representative illustration of the working internals of a differential
pressure instrument
all types of instrumentation, including flow,
level, temperature, pressure and other measurement
categories. We will concentrate on
pressure in this article because these instruments
are at the heart of many techniques
for measuring not only pressure, but also
flow and level. Getting reliable and accurate
pressure measurements can be a challenge,
often due to installation difficulties and other
issues. But modern industrial pressure instruments
address many of these issues and are
exceptionally stable, accurate and durable.
Spending a few moments thinking about
how pressure is measured can help a user
sort through the mechanics of an application
and ensure that it performs exactly as desired.
The simplest pressure instrument is a
manometer, a U-shaped tube partially filled
with a liquid of known density such as water.
If both ends are open to the atmosphere, the
two sides of the liquid slug will be of equal
height. If pressure is applied on one end but
not the other, the liquid will go down on the
side where pressure is applied and up on the
other. The difference in height can be measured,
and this unit of measure is commonly
used to determine pressure.
If unequal pressure above atmospheric
pressure is exerted on both ends, the greater
pressure will overcome the lesser. This is differential
pressure (DP), since there are two
dynamic sources of pressure on opposite
sides of the slug of liquid.
Pressure measurements can be thought of
as differential measurements referenced to
different points. These reference points can
be tied to atmospheric pressure in the case
of a gage transmitter, vacuum in the case of
an absolute pressure transmitter, or other
process points in terms of a DP reading.
Most applications use gage or DP, although
for some applications, absolute pressure
readings are necessary.
What is a pressure instrument?
In the manometer tube, the liquid indicates
the pressure visually. With a pressure instrument,
a diaphragm essentially replaces
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2016
55
DIFFERENTIAL
PRESSURE
CHALLENGES
CHALLENGES WITHIN
THE CPI
NEW INSTRUMENTS IN
OLD ENVIRONMENTS
WEATHER AND
ENVIRONMENTAL
ISSUES
LISTENING TO THE
DIAGNOSTICS
WORKING WITH FLOW
MEASUREMENTS
Wally Baker
Emerson Automation
Solutions

http://WWW.CHEMENGONLINE.COM

Chemical Engineering December 2016

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