Chemical Engineering August 2015 - 48

FIGURE 2. This image shows a cold-temperature
qualification test for a pressure transmitter whose
LCD is clearly readable at a temperature of -40°C
FIGURE 3. These modern wireless temperature transmitters have been successfully operating in Fort
McMurray, in Alberta, Canada, for several years, without the need for enclosures
operating, as older liquid crystal display
(LCD) designs do not update at temperatures
below -20°C
* To ensure performance, as accuracy, drift,
ambient temperature effects and response
time tend to be compromised at temperatures
below -40°C
* To meet the transmitter's agency approvals;
for instance, Canadian Standards Association
(CSA) may not be valid at temperatures
below -40°C
Total installed cost of the heated enclosure
- including procuring the enclosure and
heater(s), factory assembly and connection
in the field - typically exceeds the cost of
the transmitter itself. After startup, the user
also incurs ongoing electricity costs.
And there are other considerations: Heaters
require frequent maintenance, seals can
leak, and a thermostat that fails on or off can
result in damage to the transmitter. Finally,
any fugitive emissions from the threaded
connections can become trapped and build
up inside the sealed enclosure, creating a
potential safety risk when it is opened by
maintenance or operations personnel.
Some newer transmitters contain modern
electronics that are designed and approved
for extended operation in very cold tempera400
350
300
250
200
150
100
50
tures,
without significant degradation of performance,
and can even withstand multiple
restarts at -50°C. Figure 2 shows a coldtemperature
qualification test for a modern,
smart pressure transmitter - note that the
LCD is clearly readable to -40°C.
A modern transmitter that has been designed
for operation and startup in very
cold temperatures does not require or benefit
from installation in an enclosure, heated
or unheated. Eliminating the need for an
enclosure and installing the transmitter
directly in the environment yields savings
in capital and operating costs - often exceeding
the cost of the transmitter - and
yields improvements in performance, safety
and reliability.
Figure 3 shows one of the hundreds of
smart transmitters that have been operating
outdoors without enclosures in northern Alberta,
Canada, over multiple winters with no
failures. Although the photograph shows
a wireless temperature transmitter,
similar installations are also operating
for wired and wireless pressure, flow, level
and other transmitters.
No wind
24 km/h wind
Eliminating line heating
In pressure or level applications, the pressure
transmitter is connected to the process
using sensing or " impulse " lines. These are
called " wet legs " if filled with liquid, or " dry
legs " if filled with vapor. Where differential
pressure is measured - to obtain level in
a closed vessel or pressure drop across a
flow element or a filter - the pressure taps
are in different locations. Sensing lines allow
the transmitter to be installed between the
two taps.
Even where only pressure is being meaDistance
from process, mm
- Insulated tube (no wind) - Insulated tube (24 km/hr)
FIGURE 4. Heat dissipation at 0°C ambient (shown here) is six times faster than at 50°C, but
is three times slower than at -40°C. This varying heat dissipation complicates the design of
sensing lines in environments with widely varying ambient temperatures
46
sured with a single tap, the sensing line allows
the user to locate the transmitter at a
location that may be more convenient for
maintenance. Finally, the sensing line is used
with a very hot or very cold process. The
sensing line is sized to ensure that the transmitter
sensor is not exposed to a temperature
outside of its safe operating range (-40
to 120°C is typical).
ChemiCal engineering www.Chemengonline.Com august 2015
Temperature, °C
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Chemical Engineering August 2015

Table of Contents for the Digital Edition of Chemical Engineering August 2015