Chemical Engineering October 2014 - 39

Cover Story
mechanically, via a wet-leg/dry-leg
or capillary (Figure 6). This is useful
for tall tanks because it eliminates
the need for long impulse lines. A
caveat to this method is that accuracy
is impacted in tanks with high
blanket pressures relative to the DP
measurement for level. When considering
such an arrangement, it is
best to consult a factory expert to
help select the best technology.
Environmental considerations
Operating temperature is a vital
consideration. If using a remote
seal system, be sure to choose a fill
fluid compatible with both the process
temperature and the ambient
temperature. At low temperatures,
impulse lines can freeze, or the fill
fluid can gel. At high temperatures,
the fill oil in a remote seal system
can boil or degrade. There are fill
fluids that can withstand high temperatures,
but some of these have
limited low-temperature ranges.
Will there be significant mechanical
vibration? Transmitters should
always be installed to minimize
vibration, shock and temperature
fluctuations.
Does the process involve significant
pressure pulsation (for example,
at the discharge end of a positive-displacement
pump)? Rapid
pulsation degrades measurement
accuracy and, if it continues for
long periods, can wear out the pressure-sensing
element. A pulsation
damper or snubber may be required.
This can be as simple as a porous
metal filter or an adjustable needle
valve inserted in the impulse piping.
In general, strain gage and capacitive
sensors are more resistant than
other types to the wear and tear
from highly pulsating processes.
Will the device be used in a hazardous
area? If so, what approvals
will be required? Relevant approvals
can include ATEX, IECEx, CSA
and Factory Mutual (FM), for starters.
Many devices are available
with combination approvals that
make them suitable for a variety of
hazardous areas.
In food-and-beverage and pharmaceutical
applications, remember
that the transmitter may be
Figure 4. A biplanar connection has
two ports on the side of the lower part of
the unit. It supports gage, differential and
absolute pressure measurements. It is
heavier and more challenging to connect
than in-line or coplanar designs
Process
connection
Emerson Process Management
subjected to spraydown/washdown
with hot and aggressive chemicals,
and must be rated for such service.
Maintenance considerations
It is important to know how often the
transmitter will need recalibration,
and what will be involved in doing
that. It may require that the device
be physically removed and sent to
an instrument laboratory, which,
depending on the way it is installed,
may require a process shutdown.
If the transmitter can be isolated
with shutoff valves, the disruption
will be considerably reduced. In addition,
some transmitters now can
offer longer stability specifications,
thus reducing maintenance costs
in some applications since they require
fewer calibrations.
Intelligent transmitters
Many of today's intelligent process
transmitters contain a great deal
more information than simply the
pressure reading; this can include
temperature, the device history,
range-setting information and more.
An intelligent transmitter can detect
internal problems, allow the unit to
be recalibrated and re-ranged remotely
and in some cases even spot
process anomalies and plugging of
the impulse lines. Some interesting
work is being done based on using
the raw, high-speed data coming directly
from the transmitter's sensing
element before it is smoothed and
filtered for delivery to the control
system. There are a number of ways
this additional information can be
made available, as discussed below.
Choices of transmission media
Analog. The simplest and most
common way for a transmitter to
deliver its output is analog, via a
4-20-mA current loop: 4 mA indicates
the bottom of the device's
range and 20 mA the top; the 4 mA
value is used to ensure that zero
current will be seen as an indication
of loop failure, not a reading of
zero; the 4 mA is also sufficient to
power the transmitter. There have
been, and are, some competing
analog standards, like 10-50 mA
(now obsolete) and 0-20 mA, but
they are seldom seen. There are
1-5-V systems, but they are used
in only a few applications, such as
low-power installations.
Use of a current loop requires
that each transmitter have its own
wire pair leading back to the control
room (a distance that can reach a
mile or more, in some facilities), plus
its own input point in the control
system, which adds significantly to
the cost of adding additional measurement
points to a facility.
HART. There is much more information
available in today's transmitters
than just the process variable
output, and in the 1980s the
HART (Highway Addressable Remote
Transducer) protocol became
ChemiCal engineering wwww.Chemengonline.Com oCToBer 2014 37
http://wwww.Chemengonline.Com
Chemical Engineering October 2014

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