Hydrocarbon Processing - June 2021 - 79

Process Controls, Instrumentation and Automation
2
-2
-4
-6
-8
-10
-270
-200
-100
Temperature, °C
FIG. 6. Thermocouples are better than RTDs for low temperature, but
it is critical to compensate for changes in linearity.
devices-such as the A/D converters and other elements of
field device transmitters-move out of a moderate range. Fortunately,
many transmitters have more robust circuits, and the
internal components are less affected by colder temperatures.
Many transmitters can operate at temperatures down to
-40°C (-40°F), but this limit can be extended to guarantee
startup down to -60°C (-76°F). However, the transmitter
may not deliver full accuracy if the temperature remains below
-40°C (-40°F).
In these environments, more extreme measures are often
used, including the liberal use of heat tracing and heated instrumentation
cabinets. Fortunately, specialized instruments are
available to handle the most extreme cold temperature ranges
without the need for extra protection.
Measuring cryogenic temperatures. Conventional temperature
sensors can often be used even at the low end of the
spectrum; however, there are limitations. Whereas RTDs are
often a first choice for many normal applications, when reading
below -50°C (-58°F), it is critical to know the sensor's rating.
Different classes of RTDs have different low limits, ranging
from -50°C to -200°C (-58°F to -328°F). Thermocouples are
better adapted to handling low temperatures, provided an appropriate
type is selected. Some-such as type B-simply are
not suitable for low temperatures. Types E, K, N and T have
ranges down to -270°C (-450°F), with Type T especially popular
in cryogenic applications.
Once below -100°C (-150°F), most thermocouples begin
to lose linearity (FIG. 6). This is not necessarily a problem since
it is a known characteristic and can be corrected in the transmitter.
However, not all temperature transmitters or controllers are
set up to work at the low end, so users must ensure any device
intended for these applications has the required capabilities.
FIG. 7. DP flowmeters can be used with LNG and other cryogenic
liquids, but the transmitter should be placed above the pipe.
DP flowmeters for low temperatures. Out of the many potential
flowmeter technology choices, dP flowmeters using appropriate
dP transmitters are well suited for cryogenic liquids, such as
LNG. Users must pay particular attention to the placement of the
various devices and the material selection for components.
For dP flowmeters in conventional liquid applications, the
typical mounting position places the transmitter and impulse
lines below the pipe to prevent air and gas entrapment. LNG
applications reverse this, positioning the transmitter above the
primary element (FIG. 7) so there is an insulating gas barrier preventing
contact of the cold liquid directly with the transmitter
diaphragm. Brief direct contact should not cause the instrument
to fail, but it will slow responsiveness.
The transmitter has a stainless-steel body that can handle
the cold, but it is important to look at other parts, such as the
impulse lines. Designers must ensure gaskets, O-rings and bolts
are also compatible with these low temperatures, as the wrong
materials can become brittle and lead to leakage. DP flowmeters
equipped with welded impulse lines reduce the potential for
sealing material embrittlement by moving gaskets away from the
frost line. Impulse tubing should have a small diameter-typically
0.25 in. (6 mm)-to help maintain the gas barrier.
Isolating the transmitter. Treatment of high- and low-temperature
applications uses similar techniques as those employed
for handling aggressive fluids. A diaphragm seal can be used
for pressure or level measurements, but extreme temperatures
call for specialized fill fluids designed to retain their fluidity at
the operating temperature without freezing or boiling. In cases
Hydrocarbon Processing | JUNE 2021 79
Type E
Type K
Type N
Type T
mV
Hydrocarbon Processing - June 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - June 2021
